ALBERT

Description: The Atlantic meridional overturning circulation (AMOC) carries large amounts of heat into the North Atlantic influencing climate regionally as well as globally. Palaeo-records and simulations with comprehensive climate models suggest that the positive salt-advection feedback may yield a threshold behaviour of the system. That is to say that beyond a certain amount of freshwater flux into the North Atlantic, no meridional overturning circulation can be sustained. Concepts of monitoring the AMOC and identifying its vicinity to the threshold rely on the fact that the volume flux defining the AMOC will be reduced when approaching the threshold. Here we advance conceptual models that have been used in a paradigmatic way to understand the AMOC, by introducing a density-dependent parameterization for the Southern Ocean eddies. This additional degree of freedom uncovers a mechanism by which the AMOC can increase with additional freshwater flux into the North Atlantic, before it reaches the threshold and collapses: an AMOC that is mainly wind-driven will have a constant upwelling as long as the Southern Ocean winds do not change significantly. The downward transport of tracers occurs either in the northern sinking regions or through Southern Ocean eddies. If freshwater is transported, either atmospherically or via horizontal gyres, from the low to high latitudes, this would reduce the eddy transport and by continuity increase the northern sinking which defines the AMOC until a threshold is reached at which the AMOC cannot be sustained. If dominant in the real ocean this mechanism would have significant consequences for monitoring the AMOC.

Description: In this study we present a comparative quantification of CaCO3 production rates by rhodolith-forming coralline red algal communities situated in high polar latitudes and assess which environmental parameters control these production rates. The present rhodoliths act as ecosystem engineers, and their carbonate skeletons provide an important ecological niche to a variety of benthic organisms. The settings are distributed along the coasts of the Svalbard archipelago, being Floskjeret (78◦180N) in Isfjorden, Krossfjorden (79◦080N) at the eastern coast of Haakon VII Land, Mosselbukta (79◦530N) at the eastern coast of Mosselhalvøya, and Nordkappbukta (80◦310N) at the northern coast of Nordaustlandet. All sites feature Arctic climate and strong seasonality.

Description: The eastern Mediterranean transient (EMT) was caused by a combination of high‐salinity waters intruding into the Aegean Sea and the two particularly strong winters of 1991–1992 and 1992–1993. The approach in this chapter is to search for specific signatures in the historic hydrographic observations, which date back to 1910. To deal with the problem that up into the 1950s the data not only are of limited precision but also have gaps of about 20 years, it is advantageous to consider the fact that the evolution of the actual EMT is rather well documented over a similar time span. The chapter begins by outlining the characteristics of the current EMT. Thereafter, a selection of suitable hydrographic observations among the available historic data is provided to compare these with signatures expected from the evolution of the actual EMT.

Description: This study presents a new estimate of the oceanic anthropogenic CO2(Cant) sink over the industrial era (1780 to present), from assimilation of potential temperature, salinity, radiocarbon, and CFC-11 observations in a global steady state ocean circulation inverse model (OCIM). This study differs from previous data-based estimates of the oceanic Cant sink in that dynamical constraints on ocean circulation are accounted for, and the ocean circulation is explicitly modeled, allowing the calculation of oceanic Cant storage, air-sea fluxes, and transports in a consistent manner. The resulting uncertainty of the OCIM-estimated Cant uptake, transport, and storage is significantly smaller than the comparable uncertainty from purely data-based or model-based estimates. The OCIM-estimated oceanic Cant storage is 160–166 PgC in 2012, and the oceanic Cant uptake rate averaged over the period 2000–2010 is 2.6 PgC yr−1 or about 30% of current anthropogenic CO2 emissions. This result implies a residual (primarily terrestrial) Cant sink of about 1.6 PgC yr−1 for the same period. The Southern Ocean is the primary conduit for Cant entering the ocean, taking up about 1.1 PgC yr−1 in 2012, which represents about 40% of the contemporary oceanic Cant uptake. It is suggested that the most significant source of remaining uncertainty in the oceanic Cant sink is due to potential variability in the ocean circulation over the industrial era.

Description: Correlations between particulate organic carbon (POC) and mineral fluxes in the deep ocean have inspired the inclusion of “ballast effect” parameterizations in carbon cycle models. A recent study demonstrated regional variability in the effect of ballast minerals on the flux of POC in the deep ocean. We have undertaken a similar analysis of shallow export data from the Arctic, Atlantic, and Southern Oceans. Mineral ballasting is of greatest importance in the high-latitude North Atlantic, where 60% of the POC flux is associated with ballast minerals. This fraction drops to around 40% in the Southern Ocean. The remainder of the export flux is not associated with minerals, and this unballasted fraction thus often dominates the export flux. The proportion of mineral-associated POC flux often scales with regional variation in export efficiency (the proportion of primary production that is exported). However, local discrepancies suggest that regional differences in ecology also impact the magnitude of surface export. We propose that POC export will not respond equally across all high-latitude regions to possible future changes in ballast availability.

Description: Linking lower and higher trophic levels requires special focus on the essential role played by mid-trophic levels, i.e., the zooplankton. One of the most relevant pieces of information regarding zooplankton in terms of flux of energy lies in its size structure. In this study, an extensive data set of size measurements is presented, covering parts of the western European continental shelf and slope, from the Galician coast to the Ushant front, during the springs from 2005 to 2012. Zooplankton size spectra were estimated using measurements carried out in situ with the Laser Optical Plankton Counter (LOPC) and with an image analysis of WP2 net samples (200 μm mesh size) performed following the ZooScan methodology. The LOPC counts and sizes particles within 100–2000 μm of spherical equivalent diameter (ESD), whereas the WP2/ZooScan allows for counting, sizing and identification of zooplankton from ~ 400 μm ESD. The difference between the LOPC (all particles) and the WP2/ZooScan (zooplankton only) was assumed to provide the size distribution of non-living particles, whose descriptors were related to a set of explanatory variables (including physical, biological and geographic descriptors). A statistical correction based on these explanatory variables was further applied to the LOPC size distribution in order to remove the non-living particles part, and therefore estimate the size distribution of zooplankton. This extensive data set provides relevant information about the zooplankton size distribution variability, productivity and trophic transfer efficiency in the pelagic ecosystem of the Bay of Biscay at a regional and interannual scale.

Description: In the vast Low Nutrient Low-Chlorophyll (LNLC) Ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be one order of magnitude greater over short timescales. The short turnover time of atmospheric Fe and N supply (〈1 month for nitrate) further supports deposition being an important source of nutrients in LNLC regions. Yet, the extent to which atmospheric inputs are impacting biological activity and modifying the carbon balance in oligotrophic environments has not been constrained. Here, we quantify and compare the biogeochemical impacts of atmospheric deposition in LNLC regions using both a compilation of experimental data and model outputs. A metadata-analysis of recently conducted field and laboratory bioassay experiments reveals complex responses, and the overall impact is not a simple “fertilization effect of increasing phytoplankton biomass” as observed in HNLC regions. Although phytoplankton growth may be enhanced, increases in bacterial activity and respiration result in weakening of biological carbon sequestration. The application of models using climatological or time-averaged non-synoptic deposition rates produced responses that were generally much lower than observed in the bioassay experiments. We demonstrate that experimental data and model outputs show better agreement on short timescale (days to weeks) when strong synoptic pulse of aerosols deposition, similar in magnitude to those observed in the field and introduced in bioassay experiments, is superimposed over the mean atmospheric deposition fields. These results suggest that atmospheric impacts in LNLC regions have been underestimated by models, at least at daily to weekly timescales, as they typically overlook large synoptic variations in atmospheric deposition and associated nutrient and particle inputs. Inclusion of the large synoptic variability of atmospheric input, and improved representation and parameterization of key processes that respond to atmospheric deposition, is required to better constrain impacts in ocean biogeochemical models. This is critical for understanding and prediction of current and future functioning of LNLC regions and their contribution to the global carbon cycle.

Description: Knowledge of calcium phosphate (Ca-P) solubility is crucial for understanding temporal and spatial variations of phosphorus (P) concentrations in water bodies and sedimentary reservoirs. In situ relationships between liquid-and solid-phase levels cannot be fully explained by dissolved analytes alone and need to be verified by determining particular sediment P species. Lack of quantification methods for these species limits the knowledge of the P cycle. To address this issue, we (i) optimized a specifically developed conversion-extraction (CONVEX) method for P species quantification using standard additions, and (ii) simultaneously determined solubilities of Ca-P standards by measuring their pH-dependent contents in the sediment matrix. Ca-P minerals including various carbonate fluorapatite (CFAP) specimens from different localities, fluorapatite (FAP), fish bone apatite, synthetic hydroxylapatite (HAP) and octacalcium phosphate (OCP) were characterized by XRD, Raman, FTIR and elemental analysis. Sediment samples were incubated with and without these reference minerals and then sequentially extracted to quantify Ca-P species by their differential dissolution at pH values between 3 and 8. The quantification of solid-phase phosphates at varying pH revealed solubilities in the following order: OCP〉 HAP〉 CFAP (4.5% CO3)〉 CFAP (3.4% CO3)〉 CFAP (2.2% CO3)〉 FAP. Thus, CFAP was less soluble in sediment than HAP, and CFAP solubility increased with carbonate content. Unspiked sediment analyses together with standard addition analyses indicated consistent differential dissolution of natural sediment species vs. added reference species and therefore verified the applicability of the CONVEX method in separately determining the most prevalent Ca-P minerals. We found surprisingly high OCP contents in the coastal sediments analyzed, which supports the hypothesis of apatite formation by an OCP precursor mechanism.

Description: Dedicated to the memory of our colleague Klaus Hochheim, who tragically lost his life in the Arctic expedition in September 2013. A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 18 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although we cannot substantiate our conclusions by direct observation-based estimates of mixing rates in the area, we hypothesize that the enhanced vertical mixing along the St. Anna Trough eastern flank favors the upward heat loss from the intermediate warm Atlantic water layer. Modeling results support this hypothesis. The upward heat flux inferred from hydrographic data and model simulations is of O(30–100) W m−2. The region of lowered sea ice thickness and concentration seen both in sea ice remote sensing observations and model simulations marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin. In fact, the sea ice shows a delayed freeze-up onset during fall and a reduction in the sea ice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.

Description: Following the launch of ESA's Soil Moisture and Ocean Salinity (SMOS) mission, it has been shown that brightness temperatures at a low microwave frequency of 1.4 GHz (L-band) are sensitive to sea ice properties. In the first demonstration study, sea ice thickness up to 50 cm has been derived using a semi-empirical algorithm with constant tie-points. Here, we introduce a novel iterative retrieval algorithm that is based on a thermodynamic sea ice model and a three-layer radiative transfer model, which explicitly takes variations of ice temperature and ice salinity into account. In addition, ice thickness variations within the SMOS spatial resolution are considered through a statistical thickness distribution function derived from high-resolution ice thickness measurements from NASA's Operation IceBridge campaign. This new algorithm has been used for the continuous operational production of a SMOS-based sea ice thickness data set from 2010 on. The data set is compared to and validated with estimates from assimilation systems, remote sensing data, and airborne electromagnetic sounding data. The comparisons show that the new retrieval algorithm has a considerably better agreement with the validation data and delivers a more realistic Arctic-wide ice thickness distribution than the algorithm used in the previous study (Kaleschke et al., 2012).

Description: The first long-term aerosol sampling and chemical characterization results from measurements at the Cape Verde Atmospheric Observatory (CVAO) on the island of São Vicente are presented and are discussed with respect to air mass origin and seasonal trends. In total 671 samples were collected using a high-volume PM10 sampler on quartz fiber filters from January 2007 to December 2011. The samples were analyzed for their aerosol chemical composition, including their ionic and organic constituents. Back trajectory analyses showed that the aerosol at CVAO was strongly influenced by emissions from Europe and Africa, with the latter often responsible for high mineral dust loading. Sea salt and mineral dust dominated the aerosol mass and made up in total about 80% of the aerosol mass. The 5-year PM10 mean was 47.1 ± 55.5 μg m−2, while the mineral dust and sea salt means were 27.9 ± 48.7 and 11.1 ± 5.5 μg m−2, respectively. Non-sea-salt (nss) sulfate made up 62% of the total sulfate and originated from both long-range transport from Africa or Europe and marine sources. Strong seasonal variation was observed for the aerosol components. While nitrate showed no clear seasonal variation with an annual mean of 1.1 ± 0.6 μg m−3, the aerosol mass, OC (organic carbon) and EC (elemental carbon), showed strong winter maxima due to strong influence of African air mass inflow. Additionally during summer, elevated concentrations of OM were observed originating from marine emissions. A summer maximum was observed for non-sea-salt sulfate and was connected to periods when air mass inflow was predominantly of marine origin, indicating that marine biogenic emissions were a significant source. Ammonium showed a distinct maximum in spring and coincided with ocean surface water chlorophyll a concentrations. Good correlations were also observed between nss-sulfate and oxalate during the summer and winter seasons, indicating a likely photochemical in-cloud processing of the marine and anthropogenic precursors of these species. High temporal variability was observed in both chloride and bromide depletion, differing significantly within the seasons, air mass history and Saharan dust concentration. Chloride (bromide) depletion varied from 8.8 ± 8.5% (62 ± 42%) in Saharan-dust-dominated air mass to 30 \textpm 12% (87 ± 11%) in polluted Europe air masses. During summer, bromide depletion often reached 100% in marine as well as in polluted continental samples. In addition to the influence of the aerosol acidic components, photochemistry was one of the main drivers of halogenide depletion during the summer; while during dust events, displacement reaction with nitric acid was found to be the dominant mechanism. Positive matrix factorization (PMF) analysis identified three major aerosol sources: sea salt, aged sea salt and long-range transport. The ionic budget was dominated by the first two of these factors, while the long-range transport factor could only account for about 14% of the total observed ionic mass.

Description: An earth system model of intermediate complexity (CLIMate and BiosphERe – CLIMBER-2) and a land surface model (JSBACH), which dynamically represent vegetation, are used to simulate natural fire dynamics through the last 8000 yr. Output variables of the fire model (burned area and fire carbon emissions) are used to compare model results with sediment-based charcoal reconstructions. Several approaches for processing model output are also tested. Charcoal data are reported in Z-scores with a base period of 8000–200 BP in order to exclude the strong anthropogenic forcing of fire during the last two centuries. The model–data comparison reveals a robust correspondence in fire activity for most regions considered, while for a few regions, such as Europe, simulated and observed fire histories show different trends. The difference between modelled and observed fire activity may be due to the absence of anthropogenic forcing (e.g. human ignitions and suppression) in the model simulations, and also due to limitations inherent to modelling fire dynamics. The use of spatial averaging (or Z-score processing) of model output did not change the directions of the trends. However, Z-score-transformed model output resulted in higher rank correlations with the charcoal Z-scores in most regions. Therefore, while both metrics are useful, processing model output as Z-scores is preferable to areal averaging when comparing model results to transformed charcoal records.

Description: Satellite observations of microwave brightness temperatures between 19 GHz and 85 GHz are the main data sources for operational sea-ice monitoring and retrieval of ice concentrations. However, microwave brightness temperatures depend on the emissivity of snow and ice, which is subject to pronounced seasonal variations and shows significant hemispheric contrasts. These mainly arise from differences in the rate and strength of snow metamorphism and melt. We here use the thermodynamic snow model SNTHERM forced by European Re-Analysis (ERA) interim data and the Microwave Emission Model of Layered Snowpacks (MEMLS), to calculate the sea-ice surface emissivity and to identify the contribution of regional patterns in atmospheric conditions to its variability in the Arctic and Antarctic. The computed emissivities reveal a pronounced seasonal cycle with large regional variability. The emissivity variability increases from winter to early summer and is more pronounced in the Antarctic. In the pre-melt period (January–May, July–November) the standard deviations in surface microwave emissivity due to diurnal, regional and inter-annual variability of atmospheric forcing reach up to Δε = 0.034, 0.043, and 0.097 for 19 GHz, 37 GHz and 85 GHz channels, respectively. Between 2000 and 2009, small but significant positive emissivity trends were observed in the Weddell Sea during November and December as well as in Fram Strait during February, potentially related to earlier melt onset in these regions. The obtained results contribute to a better understanding of the uncertainty and variability of sea-ice concentration and snow-depth retrievals in regions of high sea-ice concentrations.

Description: Epibenthos communities play an important role in the marine ecosystems of the Weddell Sea. Information on the factors controlling their structure and distribution are, however, still rare. In particular, the interactions between environmental factors and biotic assemblages are not fully understood. Nachtigaller Hill, a newly discovered seabed structure on the over-deepened shelf of the northwest Weddell Sea (Southern Ocean), offers a unique site to study these interactions in a high-latitude Antarctic setting. Based on high-resolution bathymetry and georeferenced biological data, the effect of the terrain and related environmental parameters on the epibenthos was assessed. At Nachtigaller Hill, both geomorphological and biological data showed complex distribution patterns, reflecting local processes such as iceberg scouring and locally amplified bottom currents. This variability was also generally reflected in the variable epibenthos distribution patterns although statistical analyses did not show strong correlations between the selected environmental parameters and species abundances. By analysing the interactions between environmental and biological patterns, this study provides crucial information towards a better understanding of the factors and processes that drive epibenthos communities on the shelves of the Weddell Sea and probably also on other Antarctic shelves.

Description: Increasingly large climate model simulations are enhancing our understanding of the processes and causes of anthropogenic climate change, thanks to very large public investments in high-performance computing at national and international institutions. Various climate models implement mathematical approximations of nature in different ways, which are often based on differing computational grids. These complex, parallelized coupled system codes combine numerous complex submodels (ocean, atmosphere, land, biosphere, sea ice, land ice, etc.) that represent components of the larger complex climate system.

Description: The response of coccolithophore calcification to ocean acidification has been studied in culture experiments as well as in present and past oceans. The response, however, is different between species and strains, and for the relatively small carbonate chemistry changes observed in natural environments, a uniform response of the entire coccolithophore community has not been documented so far. Moreover, previous palaeo-studies basically focus on changes in coccolith weight due to increasing CO2 and the resulting changes in the carbonate system, and only few studies focus on the influence of other environmental factors. In order to untangle changes in coccolithophore calcification due to environmental factors such as temperature and/or productivity from changes caused by increasing pCO2 and decreasing carbonate ion concentration, we here present a study on coccolith calcification from the Holocene North Atlantic Ocean. The pre-industrial Holocene, with its predominantly stable atmospheric CO2, provides the conditions for such a comprehensive analysis. For an analysis on changes in major components of Holocene coccolithophores under natural conditions, the family Noelaerhabdaceae was selected, which constitutes the main part of the assemblage in the North Atlantic. Records of average coccolith weights from three Holocene sediment cores along a north–south transect in the North Atlantic were analysed. During the Holocene, mean weight (and therefore calcification) of Noelaerhabdaceae (Emiliania huxleyi and Gephyrocapsa) coccoliths decreased at the Azores (Geofar KF 16) from around 7 to 6 pg, but increased at the Rockall Plateau (ODP site 980) from around 6 to 8 pg, and at the Vøring Plateau (MD08-3192) from 7 to 10 pg. The amplitude of average weight variability is within the range of glacial–interglacial changes that were interpreted to be an effect of decreasing carbonate ion concentration. By comparison with SEM assemblage counts, we show that weight changes are not only partly due to variations in the coccolithophore assemblage but also an effect of a change in calcification and/or morphotype variability within single species. Our results indicate that there is no single key factor responsible for the observed changes in coccolith weight. A major increase in coccolith weight occurs during a slight decrease in carbonate ion concentration in the late Holocene at the Rockall Plateau and Vøring Plateau. Here, more favourable productivity conditions apparently lead to an increase in coccolith weight, either due to the capability of coccolithophore species, especially E. huxleyi, to adapt to decreasing carbonate ion concentration or due to a shift towards heavier calcifying morphotypes.

Description: Four sediment cores from the central and northern Greenland Sea basin, a crucial area for the renewal of North Atlantic deep water, were analyzed for planktic foraminiferal fauna, planktic and benthic stable oxygen and carbon isotopes as well as ice-rafted debris to reconstruct the environmental variability in the last 23 kyr. During the Last Glacial Maximum, the Greenland Sea was dominated by cold and sea-ice bearing surface water masses. Meltwater discharges from the surrounding ice sheets affected the area during the deglaciation, influencing the water mass circulation. During the Younger Dryas interval the last major freshwater event occurred in the region. The onset of the Holocene interglacial was marked by an increase in the advection of Atlantic Water and a rise in sea surface temperatures (SST). Although the thermal maximum was not reached simultaneously across the basin, benthic isotope data indicate that the rate of overturning circulation reached a maximum in the central Greenland Sea around 7 ka. After 6–5 ka a SST cooling and increasing sea-ice cover is noted. Conditions during this so-called "Neoglacial" cooling, however, changed after 3 ka, probably due to enhanced sea-ice expansion, which limited the deep convection. As a result, a well stratified upper water column amplified the warming of the subsurface waters in the central Greenland Sea, which were fed by increased inflow of Atlantic Water from the eastern Nordic Seas. Our data reveal that the Holocene oceanographic conditions in the Greenland Sea did not develop uniformly. These variations were a response to a complex interplay between the Atlantic and Polar water masses, the rate of sea-ice formation and melting and its effect on vertical convection intensity during times of Northern Hemisphere insolation changes.

Description: We investigate the respective role of variations in subpolar deep water formation and Nordic Seas overflows for the decadal to multidecadal variability of the Atlantic meridional overturning circulation (AMOC). This is partly done by analysing long (order of 1000 years) control simulations with five coupled climate models. For all models, the maximum influence of variations in subpolar deep water formation is found at about 45° N, while the maximum influence of variations in Nordic Seas overflows is rather found at 55 to 60° N. Regarding the two overflow branches, the influence of variations in the Denmark Strait overflow is, for all models, substantially larger than that of variations in the overflow across the Iceland–Scotland Ridge. The latter might, however, be underestimated, as the models in general do not realistically simulate the flow path of the Iceland–Scotland overflow water south of the Iceland–Scotland Ridge. The influence of variations in subpolar deep water formation is, on multimodel average, larger than that of variations in the Denmark Strait overflow. This is true both at 45° N, where the maximum standard deviation of decadal to multidecadal AMOC variability is located for all but one model, and at the more classical latitude of 30° N. At 30° N, variations in subpolar deep water formation and Denmark Strait overflow explain, on multimodel average, about half and one-third respectively of the decadal to multidecadal AMOC variance. Apart from analysing multimodel control simulations, we have performed sensitivity experiments with one of the models, in which we suppress the variability of either subpolar deep water formation or Nordic Seas overflows. The sensitivity experiments indicate that variations in subpolar deep water formation and Nordic Seas overflows are not completely independent. We further conclude from these experiments that the decadal to multidecadal AMOC variability north of about 50° N is mainly related to variations in Nordic Seas overflows. At 45° N and south of this latitude, variations in both subpolar deep water formation and Nordic Seas overflows contribute to the AMOC variability, with neither of the processes being very dominant compared to the other.

Description: Methyl iodide (CH3I}, bromoform (CHBr3) and dibromomethane (CH2Br2), which are produced naturally in the oceans, take part in ozone chemistry both in the troposphere and the stratosphere. The significance of oceanic upwelling regions for emissions of these trace gases in the global context is still uncertain although they have been identified as important source regions. To better quantify the role of upwelling areas in current and future climate, this paper analyzes major factors that influenced halocarbon emissions from the tropical North East Atlantic including the Mauritanian upwelling during the DRIVE expedition. Diel and regional variability of oceanic and atmospheric CH3I, CHBr3 and CH2Br2 was determined along with biological and meteorological parameters at six 24 h-stations. Low oceanic concentrations of CH3I from 0.1–5.4 pmol L-1 were equally distributed throughout the investigation area. CHBr3 of 1.0–42.4 pmol L-1 and CH2Br2 of 1.0–9.4 pmol L-1 were measured with maximum concentrations close to the Mauritanian coast. Atmospheric mixing rations of CH3I of up to 3.3, CHBr3 to 8.9 and CH2Br2 to 3.1 ppt above the upwelling and 1.8, 12.8, respectively 2.2 ppt at a Cape Verdean coast were detected during the campaign. While diel variability in CH3I emissions could be mainly ascribed to oceanic non-biological production, no main driver was identified for its emissions in the entire study region. In contrast, oceanic bromocarbons resulted from biogenic sources which were identified as regional drivers of their sea-to-air fluxes. The diel impact of wind speed on bromocarbon emissions increased with decreasing distance to the coast. The height of the marine atmospheric boundary layer (MABL) was determined as an additional factor influencing halocarbon emissions. Oceanic and atmospheric halocarbons correlated well in the study region and in combination with high oceanic CH3I, CHBr3 and CH2Br2 concentrations, local hot spots of atmospheric halocarbons could solely be explained by marine sources. This conclusion is in contrast with previous studies that hypothesized the occurrence of elevated atmospheric halocarbons over the eastern tropical Atlantic mainly originating from the West-African continent.

Description: Marine ecosystem models used to investigate how global change affects ocean ecosystems and their functioning typically omit pelagic plankton diversity. Diversity, however, may affect functions such as primary production and their sensitivity to environmental changes. Here we use a global ocean ecosystem model that explicitly resolves phytoplankton diversity by defining subtypes within four phytoplankton functional types (PFTs). We investigate the model's ability to capture diversity effects on primary production under environmental change. An idealized scenario with a sudden reduction in vertical mixing causes diversity and primary-production changes that turn out to be largely independent of the number of coexisting phytoplankton subtypes. The way diversity is represented in the model provides a small number of niches with respect to nutrient use in accordance with the PFTs defined in the model. Increasing the number of phytoplankton subtypes increases the resolution within the niches. Diversity effects such as niche complementarity operate between, but not within PFTs, and are constrained by the variety of traits and trade-offs resolved in the model. The number and nature of the niches formulated in the model, for example via trade-offs or different PFTs, thus determines the diversity effects on ecosystem functioning captured in ocean ecosystem models.

Description: Two consecutive summer upwelling events, each lasting for less than 24 h, were surveyed in high temporal and vertical resolution close to the Boknis Eck time-series station (BE) in the western Belt Sea (Baltic Sea) in summer 2010 with an autonomous glider. Driven only by moderate offshore winds both events resulted in more than 5 K cooling of surface waters, while only for the second event were significant irreversible changes in the vertical stratification observed. Generalizing the glider survey observations with hourly wind data from nearby meteorological stations, it is found that upwelling in the BE area occurs for wind directions between 190 to 260° and wind speed exceeding 4 m s−1. Based on these thresholds the wind-induced summer (June to September) upwelling conditions in the BE area for the period 1982 to 2012 are reconstructed. On average about 18 days of upwelling favourable wind conditions are found for the four summer months, with significant interannual variability ranging from 7.7 days (2006) to more than 28 days (1985). By aligning upwelling favourable wind conditions with the monthly BE surveys it is found that extreme anomalies in BE surveys follow extended periods of upwelling favourable winds.

Description: The Surface Ocean CO2 Atlas (SOCAT), an activity of the international marine carbon research community, provides access to synthesis and gridded fCO2 (fugacity of carbon dioxide) products for the surface oceans. Version 2 of SOCAT is an update of the previous release (version 1) with more data (increased from 6.3 million to 10.1 million surface water fCO2 values) and extended data coverage (from 1968–2007 to 1968–2011). The quality control criteria, while identical in both versions, have been applied more strictly in version 2 than in version 1. The SOCAT website (http://www.socat.info/) has links to quality control comments, metadata, individual data set files, and synthesis and gridded data products. Interactive online tools allow visitors to explore the richness of the data. Applications of SOCAT include process studies, quantification of the ocean carbon sink and its spatial, seasonal, year-to-year and longerterm variation, as well as initialisation or validation of ocean carbon models and coupled climate-carbon models.

Description: Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM-HAMOCC (MPIOM - Max Planck Institute Ocean Model HAMOCC - HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air-sea flux that ranges between 70 and 260 Gg yr(-1). On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).

Description: In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies", www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of water-column oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts.

Description: The potential for a dynamical impact of Saharan mineral dust on the North Atlantic Ocean large-scale circulation is investigated. To this end, an ocean general circulation model forced by atmospheric fluxes is perturbed by an idealized, seasonally varying, net shortwave flux anomaly, as it results from remote sensing observations of aerosol optical thickness representing Saharan dust load in the atmosphere. The dust dynamical impact on the circulation is assessed through a comparison between perturbed and an unperturbed run. Results suggest that, following the dust-induced shortwave irradiance anomaly, a buoyancy anomaly is created in the Atlantic offshore the African coast, which over the course of the time propagates westward into the interior Atlantic while progressively subducting. Changes in the large-scale barotropic and overturning circulations are significant after 3 years, which coincides with the elapsed time required by the bulk of the buoyancy perturbation to reach the western boundary of the North Atlantic. Although small in amplitude, the changes in the meridional overturning are of the same order as interannual-to-decadal variability. Variations in the amplitude of the forcing lead to changes in the amplitude of the response, which is almost linear during the first 3 years. In addition, a fast, but dynamically insignificant, response can be observed to propagate poleward along the eastern boundary of the North Atlantic, which contributes to a nonlinear response in the subpolar region north of 40°N.

Description: Late Miocene tectonic changes in Mediterranean–Atlantic connectivity and climatic changes caused Mediterranean salinity to fluctuate dramatically, including a ten-fold increase and near-freshening. Recent proxy- and model-based evidence suggests that at times during this Messinian Salinity Crisis (MSC, 5.96–5.33 Ma), highly saline and highly fresh Mediterranean water flowed into the North Atlantic Ocean, whilst at others, no Mediterranean Outflow Water (MOW) reached the Atlantic. By running extreme, sensitivity-type experiments with a fully coupled ocean–atmosphere general circulation model, we investigate the potential of these various MSC MOW scenarios to impact global-scale climate. The simulations suggest that although the effect remains relatively small, MOW had a greater influence on North Atlantic Ocean circulation and climate than it does today. We also find that depending on the presence, strength and salinity of MOW, the MSC could have been capable of cooling mid–high northern latitudes by a few degrees, with the greatest cooling taking place in the Labrador, Greenland–Iceland–Norwegian and Barents seas. With hypersaline MOW, a component of North Atlantic Deep Water formation shifts to the Mediterranean, strengthening the Atlantic Meridional Overturning Circulation (AMOC) south of 35° N by 1.5–6 Sv. With hyposaline MOW, AMOC completely shuts down, inducing a bipolar climate anomaly with strong cooling in the north (mainly −1 to −3 °C, but up to −8 °C) and weaker warming in the south (up to +0.5 to +2.7 °C). These simulations identify key target regions and climate variables for future proxy reconstructions to provide the best and most robust test cases for (a) assessing Messinian model performance, (b) evaluating Mediterranean–Atlantic connectivity during the MSC and (c) establishing whether or not the MSC could ever have affected global-scale climate.

Description: Atlantic multidecadal variability (AMV) is known to impact climate globally, and knowledge about the persistence of AMV is important for understanding past and future climate variability, as well as modeling and assessing climate impacts. The short observational data do not significantly resolve multidecadal variability, but recent paleoproxy reconstructions show multidecadal variability in North Atlantic temperature prior to the instrumental record. However, most of these reconstructions are land-based, not necessarily representing sea surface temperature. Proxy records are also subject to dating errors and microenvironmental effects. We extend the record of AMV 90 years past the instrumental record using principle component analysis of five marine-based proxy records to identify the leading mode of variability. The first principal component is consistent with the observed AMV, and multidecadal variability seems to persist prior to the instrumental record. Thus, we demonstrate that reconstructions of past Atlantic low-frequency variability can be improved by combining marine-based proxies.

Description: A link between atmospheric variability in the Tropics independent of ENSO and the Southern Annular Mode (SAM) is found based on seasonal mean data for austral summer. Variations associated with El Niño Southern Oscillation (ENSO) are removed usinga linear method and a Tropics Index (TI) is defined as the zonal average of the ENSO-removed 500 hPa geopotential height between 10°S and 10°N. Since the detrended TI shows no link to SST variability in the Tropics, it appears to be related to internal atmospheric variability. We find that the TI can explain about 40% variance of the SAM interannual variability and about 75% of the SAM long term trend between 1957/58 and 2001/02, where here the SAM includes the ENSO signal. Positive/negative values of the TI are associated with the positive/negative SAM. A possible link between the TI and global warming is noted.

Description: The upper ocean circulation of the Pacific and Indian Oceans is connected through both the Indonesian Throughflow north of Australia and the Tasman leakage around its south. The relative importance of these two pathways is examined using virtual Lagrangian particles in a high-resolution nested ocean model. The unprecedented combination of a long integration time within an eddy-permitting ocean model simulation allows the first assessment of the interannual variability of these pathways in a realistic setting. The mean Indonesian Throughflow, as diagnosed by the particles, is 14.3 Sv, considerably higher than the diagnosed average Tasman leakage of 4.2 Sv. The time series of Indonesian Throughflow agrees well with the Eulerian transport through the major Indonesian Passages, validating the Lagrangian approach using transport-tagged particles. While the Indonesian Throughflow is mainly associated with upper ocean pathways, the Tasman leakage is concentrated in the 400–900 m depth range at subtropical latitudes. Over the effective period considered (1968–1994), no apparent relationship is found between the Tasman leakage and Indonesian Throughflow. However, the Indonesian Throughflow transport correlates with ENSO. During strong La Niñas, more water of Southern Hemisphere origin flows through Makassar, Moluccas, Ombai, and Timor Straits, but less through Moluccas Strait. In general, each strait responds differently to ENSO, highlighting the complex nature of the ENSO-ITF interaction.

Description: Oceanic uptake and long-term storage of atmospheric carbon dioxide (CO2) are strongly driven by the marine “biological pump,” i.e., sinking of biotically fixed inorganic carbon and nutrients from the surface into the deep ocean (Sarmiento and Bender, 1994; Volk and Hoffert, 1985). Sinking velocity of marine particles depends on seawater viscosity, which is strongly controlled by temperature (Sharqawy et al., 2010). Consequently, marine particle flux is accelerated as ocean temperatures increase under global warming (Bach et al., 2012). Here we show that this previously overlooked “viscosity effect” could have profound impacts on marine biogeochemical cycling and carbon uptake over the next centuries to millennia. In our global warming simulation, the viscosity effect accelerates particle sinking by up to 25%, thereby effectively reducing the portion of organic matter that is respired in the surface ocean. Accordingly, the biological carbon pump's efficiency increases, enhancing the sequestration of atmospheric CO2 into the ocean. This effect becomes particularly important on longer time scales when warming reaches the ocean interior. At the end of our simulation (4000 A.D.), oceanic carbon uptake is 17% higher, atmospheric CO2 concentration is 180 ppm lower, and the increase in global average surface temperature is 8% weaker when considering the viscosity effect. Consequently, the viscosity effect could act as a long-term negative feedback mechanism in the global climate system.

Description: The abyssal warming around Antarctica is one of the most prominent multidecadal signals of change in the global ocean. Here we investigate its dynamical impacts on the Atlantic Meridional Overturning Circulation (AMOC) by performing a set of experiments with the ocean-sea ice model NEMO-LIM2 at 1/2 degrees horizontal resolution. The simulations suggest that the ongoing warming of Antarctic Bottom Water (AABW), already affecting much of the Southern Hemisphere with a rate of up to 0.05 degrees C decade(-1), has important implications for the large-scale meridional overturning circulation in the Atlantic Ocean. While the abyssal northward flow of AABW is weakening, we find the upper AMOC cell to progressively strengthen by 5-10% in response to deep density changes in the South Atlantic. The simulations suggest that the AABW-induced strengthening of the AMOC is already extending into the subtropical North Atlantic, implying that the process may counteract the projected decrease of the AMOC in the next decades.

Description: Most of the short-lived biogenic and anthropogenic chemical species that are emitted into the atmosphere break down efficiently by reaction with OH and do not reach the stratosphere. Here we show the existence of a pronounced minimum in the tropospheric column of ozone over the West Pacific, the main source region for stratospheric air, and suggest a corresponding minimum of the tropospheric column of OH. This has the potential to amplify the impact of surface emissions on the stratospheric composition compared to the impact when assuming globally uniform OH conditions. Specifically, the role of emissions of biogenic halogenated species for the stratospheric halogen budget and the role of increasing emissions of SO2 in Southeast Asia or from minor volcanic eruptions for the increasing stratospheric aerosol loading need to be reassessed in light of these findings. This is also important since climate change will further modify OH abundances and emissions of halogenated species. Our study is based on ozone sonde measurements carried out during the TransBrom cruise with the RV Sonne roughly along 140-150 degrees E in October 2009 and corroborating ozone and OH measurements from satellites, aircraft campaigns and FTIR instruments. Model calculations with the GEOS-Chem Chemistry and Transport Model (CTM) and the ATLAS CTM are used to simulate the tropospheric OH distribution over the West Pacific and the transport pathways to the stratosphere. The potential effect of the OH minimum on species transported into the stratosphere is shown via modeling the transport and chemistry of CH2Br2 and SO2.

Description: The nitrogen cycle is fundamental to Earth's biogeochemistry. Yet major uncertainties of quantification remain, particularly regarding the global oceanic nitrogen fixation rate. Hydrogen is produced during nitrogen fixation and will become supersaturated in surface waters if there is net release from diazotrophs. Ocean surveys of hydrogen supersaturation thus have the potential to illustrate the spatial and temporal distribution of nitrogen fixation, and to guide the far more onerous but quantitative methods for measuring it. Here we present the first transect of high resolution measurements of hydrogen supersaturations in surface waters along a meridional 10,000 km cruise track through the Atlantic. We compare measured saturations with published measurements of nitrogen fixation rates and also with model-derived values. If the primary source of excess hydrogen is nitrogen fixation and has a hydrogen release ratio similar to Trichodesmium, our hydrogen measurements would point to similar rates of fixation in the North and South Atlantic, roughly consistent with modelled fixation rates but not with measured rates, which are lower in the south. Possible explanations would include any substantial nitrogen fixation by newly discovered diazotrophs, particularly any having a hydrogen release ratio similar to or exceeding that of Trichodesmium; under-sampling of nitrogen fixation south of the equator related to excessive focus on Trichodesmium; and methodological shortcomings of nitrogen fixation techniques that cause a bias towards colonial diazotrophs relative to unicellular forms. Alternatively our data are affected by an unknown hydrogen source that is greater in the southern half of the cruise track than the northern.

Description: Continuous high-resolution underway measurements of dimethyl sulfide (DMS) and isoprene in the ocean surface were conducted from Germany to South Africa in November 2008. DMS, total dimethylsulfoniopropionate (DMSPt), isoprene and 19'-hexanoyloxyfucoxanthin (19'-hex) correlated in nitrogen-depleted regions when they were clustered by nitrogen to phosphorous ratio (N:P). The 19'-hex-containing algae groups might be a common source of DMS, DMSPt, and isoprene in the low N: P regions. Additionally, DMS and isoprene correlated in nitrate-depleted regions when they were clustered against nitrate concentrations. Correlations between DMS and isoprene were also found within nitrate-depleted eddies encountered along the cruise track. Eddies with N: P of similar to 2.8 showed the highest positive correlations between DMS and isoprene. We conclude that the DMS/isoprene relationships in the eastern Atlantic Ocean were influenced by nutrient availability, with implications for using nutrients to predict the DMS and isoprene concentrations over a range of oceanographic areas depleted in nitrogen

Description: The relationship between soil moisture (SM) and evaporative fraction (EF) is an important component of land-atmosphere interactions. Frequently, land-atmosphere studies are based on land-surface models and not on observations. This study examines SM-EF interactions over the United States Southern Great Plains using both in situ observations and simulations from the Variable Infiltration Capacity hydrologic model. Specifically, we evaluate how the relationship between SM and EF varies by season, we determine why these variations occur, and we compare model-derived and observed SM-energy flux relationships. Data from four sites (2004-2008) that are part of the United States Department of Energy's Atmospheric Radiation MeasurementSouthern Great Plains network are used in this study. Results show that SM-EF interactions in both the model and observations are in general agreement with the evaporative regime theory described in past studies. That is, EF is a linear function of SM when SM is between the wilting point and the critical value, and when SM is above the critical value, EF is not dependent on SM. However, SM-EF relationships vary substantially from year to year. EF is a linear function of SM only when daily net radiation is above normal. Our results suggest that the strength of SM-EF interactions is not solely controlled by soil wetness but is also strongly influenced by daily net radiation and meteorological conditions.

Description: Precisely quantifying the current climate-related sea level change requires accurate knowledge of long-term geological processes known as Glacial Isostatic Adjustments (GIA). Although the major postglacial melting phase is likely to have ended ∼6–4 ka BP (before present), GIA is still significantly affecting the present-day vertical position of the mean sea surface and the sea bottom. Here we present empirical rsl (relative sea level) data based on U/Th dated fossil corals from reef platforms of the Society Islands, French Polynesia, together with the corresponding GIA-modeling. Fossil coral data constrain the timing and amplitude of rsl variations after the Holocene sea level maximum (HSLM). Upon correction for isostatic island subsidence, we find that local rsl was at least ∼1.5 ± 0.4 m higher than present at ∼5.4 ka. Later, minor amplitude variations occurred until ∼2 ka, when the rsl started dropping to its present position with a rate of ∼0.4 mm/yr. The data match with predicted rsl curves based on global ice-sheet chronologies confirming the role of GIA-induced ocean siphoning effect throughout the mid to late Holocene. A long lasting Late Holocene highstand superimposed with second-order amplitudinal fluctuations as seen from our data suggest that the theoretical predicted timing of rsl change can still be refined pending future calibration.

Description: It has been proposed that increasing levels of pCO(2) in the surface ocean will lead to more partitioning of the organic carbon fixed by marine primary production into the dissolved rather than the particulate fraction. This process may result in enhanced accumulation of dissolved organic carbon (DOC) in the surface ocean and/or concurrent accumulation of transparent exopolymer particles (TEPs), with important implications for the functioning of the marine carbon cycle. We investigated this in shipboard bioassay experiments that considered the effect of four different pCO(2) scenarios (ambient, 550, 750 and 1000 mu atm) on unamended natural phytoplankton communities from a range of locations in the northwest European shelf seas. The environmental settings, in terms of nutrient availability, phytoplankton community structure and growth conditions, varied considerably between locations. We did not observe any strong or consistent effect of pCO(2) on DOC production. There was a significant but highly variable effect of pCO(2) on the production of TEPs. In three of the five experiments, variation of TEP production between pCO(2) treatments was caused by the effect of pCO(2) on phytoplankton growth rather than a direct effect on TEP production. In one of the five experiments, there was evidence of enhanced TEP production at high pCO(2) (twice as much production over the 96 h incubation period in the 750 mu atm treatment compared with the ambient treatment) independent of indirect effects, as hypothesised by previous studies. Our results suggest that the environmental setting of experiments (community structure, nutrient availability and occurrence of phytoplankton growth) is a key factor determining the TEP response to pCO(2) perturbations.

Description: Four carbonate system variables were measured in surface waters during a cruise aimed at investigating ocean acidification impacts traversing northwestern European shelf seas in the summer of 2011. High-resolution surface water data were collected for partial pressure of carbon dioxide (pCO2; using two independent instruments) and pH using the total pH scale (pHT), in addition to discrete measurements of total alkalinity and dissolved inorganic carbon. We thus overdetermined the carbonate system (four measured variables, two degrees of freedom), which allowed us to evaluate the level of agreement between the variables on a cruise whose main aim was not intercomparison, and thus where conditions were more representative of normal working conditions. Calculations of carbonate system variables from other measurements generally compared well with direct observations of the same variables (Pearson's correlation coefficient always greater than or equal to 0.94; mean residuals were similar to the respective accuracies of the measurements). We therefore conclude that four of the independent data sets of carbonate chemistry variables were of high quality. A diurnal cycle with a maximum amplitude of 41 μatm was observed in the difference between the pCO2 values obtained by the two independent analytical pCO2 systems, and this was partly attributed to irregular seawater flows to the equilibrator and partly to biological activity inside the seawater supply and one of the equilibrators. We discuss how these issues can be addressed to improve carbonate chemistry data quality on future research cruises.

Description: Our understanding of the processes driving the patterns of dissolved iron (DFe) in the ocean interior, either in observations or models, is complicated by the combined influences of subduction from the surface mixed layer, notable subsurface sources, regeneration, and scavenging loss. We describe a ventilation-based framework to quantify these processes in a global ocean biogeochemical model including diagnostics along potential density surfaces. There is a prevailing control of subsurface DFe by the subduction of surface DFe as preformed DFe augmented by benthic sources of DFe from hydrothermal activity and sediments. Unlike phosphate, there is often a first-order balance with a near cancelation between regeneration and scavenging with the remaining “net regeneration” controlled by the ventilation of surface excesses in Fe-binding ligands. This DFe framework provides a more stringent test of how the total DFe distribution is mechanistically controlled within a model and may be subsequently used to interpret observed DFe distributions.

Description: Nitrate (NO3-) is the major nutrient responsible for coastal eutrophication worldwide and its production is related to intensive food production and fossil-fuel combustion. In the Baltic Sea NO3- inputs have increased 4-fold over recent decades and now remain constantly high. NO3- source identification is therefore an important consideration in environmental management strategies. In this study focusing on the Baltic Sea, we used a method to estimate the proportional contributions of NO3- from atmospheric deposition, N-2 fixation, and runoff from pristine soils as well as from agricultural land. Our approach combines data on the dual isotopes of NO3- (delta N-15-NO3- and delta O-18-NO3-) in winter surface waters with a Bayesian isotope mixing model (Stable Isotope Analysis in R, SIAR). Based on data gathered from 47 sampling locations over the entire Baltic Sea, the majority of the NO3- in the southern Baltic was shown to derive from runoff from agricultural land (33-100 %), whereas in the northern Baltic, i.e. the Gulf of Bothnia, NO3- originates from nitrification in pristine soils (34-100 %). Atmospheric deposition accounts for only a small percentage of NO3- levels in the Baltic Sea, except for contributions from northern rivers, where the levels of atmospheric NO3- are higher. An additional important source in the central Baltic Sea is N-2 fixation by diazotrophs, which contributes 49-65% of the overall NO3- pool at this site. The results obtained with this method are in good agreement with source estimates based upon delta N-15 values in sediments and a three-dimensional ecosystem model, ERGOM. We suggest that this approach can be easily modified to determine NO3- sources in other marginal seas or larger near-coastal areas where NO3- is abundant in winter surface waters when fractionation processes are minor.

Description: One decade of time-variable gravity field observations from the GRACE satellite mission reveals low-frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year-long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea-level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA-Interim sea-level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE-based OBP observations are indeed sensitive to changes in the prevailing sea-level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Description: Previous studies have shown that ENSO's anomalous equatorial winds, including the observed southward shift of zonal winds that occurs around the event peak, can be reconstructed with the first two Empirical Orthogonal Functions (EOFs) of equatorial region wind stresses. Using a high-resolution ocean general circulation model, we investigate the effect of these two EOFs on changes in warm water volume (WWV), interhemispheric mass transports, and Indonesian Throughflow (ITF). Wind stress anomalies associated with the first EOF produce changes in WWV that are dynamically consistent with the conceptual recharge oscillator paradigm. The ITF is found to heavily damp these WWV changes, reducing their variance by half. Wind stress anomalies associated with the second EOF, which depicts the southward wind shift, are responsible for WWV changes that are of comparable magnitude to those driven by the first mode. The southward wind shift is also responsible for the majority of the observed interhemispheric upper ocean mass exchanges. These winds transfer mass between the Northern and the Southern Hemisphere during El Niño events. Whilst water is transferred in the opposite direction during La Niña events, the magnitude of this exchange is roughly half of that seen during El Niño events. Thus, the discharging of WWV during El Niño events is meridionally asymmetric, while the WWV recharging during a La Niña event is largely symmetric. The inclusion of the southward wind shift is also shown to allow ENSO to exchange mass with much higher latitudes than that allowed by the first EOF alone.

Description: Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology and climate change. Although the relationship between hydrates, gas chimneys and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study we present the analysis of new three-dimensional high-resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e. protothrusts, normal faults from flexural extension, and shallow (〈1 km) arrays of oblique shear structures) implicated in fluid migration within the gas hydrate stability zone in an area of 2x7 km. In addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault, the data show seismic evidence for sub-seafloor gas-rich fluid accumulation associated with proto-thrusts and extensional faults. In these latter systems fluid pressure dissipation through time has been favored, hindering the development of gas chimneys. We discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins.

Description: Marine calcareous sediments provide a fundamental basis for paleoceanographic studies aiming to reconstruct past oceanic conditions and understand key biogeochemical element cycles. Calcifying unicellular phytoplankton (coccolithophores) are a major contributor to both carbon and calcium cycling by photosynthesis and the production of calcite (coccoliths) in the euphotic zone and the subsequent long-term deposition and burial into marine sediments. Here we present data from controlled laboratory experiments on four coccolithophore species and elucidate the relation between the divalent cation (Sr, Mg and Ca) partitioning in coccoliths and cellular physiology (growth, calcification and photosynthesis). Coccolithophores were cultured under different seawater temperature and carbonate chemistry conditions. The partition coefficient of strontium (DSr) was positively correlated with both carbon dioxide (pCO2) and temperature but displayed no coherent relation to particulate organic and inorganic carbon production rates. Furthermore, DSr correlated positively with cellular growth rates when driven by temperature but no correlation was present when changes in growth rates were pCO2-induced. The results demonstrate the complex interaction between environmental forcing and physiological control on the strontium partitioning in coccolithophore calcite. The partition coefficient of magnesium (DMg) displayed species-specific differences and elevated values under nutrient limitation. No conclusive correlation between coccolith DMg and temperature was observed but pCO2 induced a rising trend in coccolith DMg. Interestingly, the best correlation was found between coccolith DMg and chlorophyll a production suggesting that chlorophyll a and calcite associated Mg originate from the same intracellular pool. These results give an extended insight into the driving factors that lead to variations in the coccolith Mg / Ca ratio and can be used for Sr / Ca and Mg / Ca paleoproxy calibration.

Description: In the tropical eastern South Pacific the Stratus Ocean Reference Station (ORS) (∼20°S, 85.5°W) is located in the transition zone between the oxygen minimum zone (OMZ) and the well-oxygenated subtropical gyre. In February/March 2012, extremely anomalous water mass properties were observed in the thermocline at the Stratus ORS. The available eddy oxygen anomaly was −10.5 × 1016 µmol. This anomalous water was contained in an anticyclonic mode-water eddy crossing the mooring site. This eddy was absorbed at that time by an anticyclonic feature located south of the Stratus mooring. This was the largest water property anomaly observed at the mooring during the 13.5 month deployment period. The sea surface height anomaly (SSHA) of the strong mode-water eddy in February/March 2012 was weak, and while the lowest and highest SSHA were related to weak eddies, SSHA is found not to be sufficient to specify the eddy strength for subsurface-intensified eddies. Still, the anticyclonic eddy, and its related water mass characteristics, could be tracked backward in time in SSHA satellite data to a formation region in April 2011 off the Chilean coast. The resulting mean westward propagation velocity was 5.5 cm s−1. This extremely long-lived eddy carried the water characteristics from the near-coastal Chilean water to the open ocean. The water mass stayed isolated during the 11 month travel time due to high rotational speed of about 20 cm s−1 leading to almost zero oxygen in the subsurface layer of the anticyclonic mode-water eddy with indications of high primary production just below the mixed layer.

Description: In a pilot study conducted in October and November 2011, air–sea gas transfer velocities of the two sparingly soluble trace gases hexafluorobenzene and 1,4-difluorobenzene were measured in the unique High-Speed Wind-Wave Tank at Kyoto University, Japan. This air–sea interaction facility is capable of producing hurricane strength wind speeds of up to u10=67 m s−1. This constitutes the first lab study of gas transfer at such high wind speeds. The measured transfer velocities k600 spanned two orders of magnitude, lying between 11 cm h−1 and 1180 cm h−1 with the latter being the highest ever measured wind induced gas transfer velocity. The measured gas transfer velocities are in agreement with the only available dataset at hurricane wind speeds (McNeil and D'Asaro, 2007). The disproportionately large increase of the transfer velocities found at highest wind speeds indicates a new regime of air–sea gas transfer, which is characterized by strong wave breaking, enhanced turbulence and bubble cloud entrainment. It was found that tracers spanning a wide range of solubilities and diffusivities are needed to separate the effects of enhanced surface area and turbulence due to breaking waves from the effects of bubble and spray mediated gas transfer.

Description: Mass deposition fluxes of mineral dust to the tropical northeast Atlantic Ocean were determined within this study. In the framework of SOPRAN (Surface Ocean Processes in the Anthropocene), the interaction between the atmosphere and the ocean in terms of material exchange were investigated at the Cape Verde atmospheric observatory (CVAO) on the island Sao Vicente for January 2009. Five different methods were applied to estimate the deposition flux, using different meteorological and physical measurements, remote sensing, and regional dust transport simulations. The set of observations comprises micrometeorological measurements with an ultra-sonic anemometer and profile measurements using 2-D anemometers at two different heights, and microphysical measurements of the size-resolved mass concentrations of mineral dust. In addition, the total mass concentration of mineral dust was derived from absorption photometer observations and passive sampling. The regional dust model COSMO-MUSCAT was used for simulations of dust emission and transport, including dry and wet deposition processes. This model was used as it describes the AOD's and mass concentrations realistic compared to the measurements and because it was run for the time period of the measurements. The four observation-based methods yield a monthly average deposition flux of mineral dust of 12–29 ng m−2 s−1. The simulation results come close to the upper range of the measurements with an average value of 47 ng m−2 s−1. It is shown that the mass deposition flux of mineral dust obtained by the combination of micrometeorological (ultra-sonic anemometer) and microphysical measurements (particle mass size distribution of mineral dust) is difficult to compare to modeled mass deposition fluxes when the mineral dust is inhomogeneously distributed over the investigated area.

Description: The Indian-Atlantic water exchange south of Africa (Agulhas leakage) is a key component of the global ocean circulation. No quantitative estimation of the paleo-Agulhas leakage exists. We quantify the variability in interocean exchange over the past 640,000 years, using planktic foraminiferal assemblage data from two marine sediment records to define an Agulhas leakage efficiency index. We confirm the validity of our new approach with a numerical ocean model that realistically simulates the modern Agulhas leakage changes. Our results suggest that, during the past several glacial-interglacial cycles, the Agulhas leakage varied by ~10 sverdrup and more during major climatic transitions. This lends strong credence to the hypothesis that modifications in the leakage played a key role in changing the overturning circulation to full strength mode. Our results are instrumental for validating and quantifying the contribution of the Indian-Atlantic water leakage to the global climate changes.

Description: Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from ten field campaigns in different parts of the world's oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 pptv with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the Differential Optical Absorption Spectroscopy (DOAS) method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore it could play a more important role in the MBL than previously considered.

Description: We present a new classification of geological domains at the Africa-Eurasia plate boundary off SW Iberia, together with a regional geodynamic reconstruction spanning from the Mesozoic extension to the Neogene-to-present-day convergence. It is based on seismic velocity and density models along a new transect running from the Horseshoe to the Seine abyssal plains, which is combined with previously available geophysical models from the region. The basement velocity structure at the Seine Abyssal Plain indicates the presence of a highly heterogeneous, thin oceanic crust with local high-velocity anomalies possibly representing zones related to the presence of ultramafic rocks. The integration of this model with previous ones reveals the presence of three oceanic domains offshore SW Iberia: (1) the Seine Abyssal Plain domain, generated during the first stages of slow seafloor spreading in the NE Central Atlantic (Early Jurassic); (2) the Gulf of Cadiz domain, made of oceanic crust generated in the Alpine-Tethys spreading system between Iberia and Africa, which was coeval with the formation of the Seine Abyssal Plain domain and lasted up to the North Atlantic continental breakup (Late Jurassic); and (3) the Gorringe Bank domain, made of exhumed mantle rocks, which formed during the first stages of North Atlantic opening. Our models suggest that the Seine Abyssal Plain and Gulf of Cadiz domains are separated by the Lineament South strike-slip fault, whereas the Gulf of Cadiz and Gorringe Bank domains appear to be limited by a deep thrust fault located at the center of the Horseshoe Abyssal Plain.

Description: Enhanced permafrost warming and increased arctic river discharges have heightened concern about the input of terrigeneous matter into Arctic coastal waters. We used optical operational satellite data from the Ocean Colour sensor MERIS onboard the ENVISAT satellite mission for synoptic monitoring of the pathways of terrigeneous matter in the southern Laptev Sea. MERIS satellite data from 2006 on to 2011 were processed using the Case2Regional Processor, C2R, installed in the open-source software ESA BEAM-VISAT. Since optical remote sensing using Ocean Colour satellite data has seen little application in Siberian Arctic coastal and shelf waters, we assess the applicability of the calculated MERIS parameters with surface water sampling data from the Russian-German ship expeditions LENA2010 and TRANSDRIFT-XVII taking place in August and September 2010 in the southern Laptev Sea. The surface waters of the southern Laptev Sea are characterized by low transparencies, due to turbid river water input, terrestrial input by coastal erosion, resuspension events and, therefore, high background concentrations of Suspended Particulate Matter, SPM, and coloured Dissolved Organic Matter, cDOM. The mapped calculated optical water parameters, such as the first attenuation depth, Z90, the attenuation coefficient, k, and Suspended Particulate Matter, SPM, visualize resuspension events that occur in shallow coastal and shelf waters indicating vertical mixing events. The mapped optical water parameters also visualize that the hydrography of the Laptev Sea is dominated by frontal meanders with amplitudes up to 30 km and eddies and filaments with diameters up to 100 km that prevail throughout the ice-free season. The meander crests, filaments and eddy-like structures that become visible through the mapped MERIS C2R parameters indicate enhanced vertical and horizontal transport energy for the transport of terrigenous and living biological matter in the surface waters during the ice-free season.

Description: Volcanic eruptions have been hypothesized as an iron supply mechanism for phytoplankton blooms; however, little direct evidence of stimulatory responses has been obtained in the field. Here we present the results of twenty-one 1-2day bottle enrichment experiments from cruises in the South Atlantic and Southern Ocean which conclusively demonstrated a photophysiological and biomass stimulation of phytoplankton communities following supply of basaltic or rhyolitic volcanic ash. Furthermore, experiments in the Southern Ocean demonstrated significant phytoplankton community responses to volcanic ash supply in the absence of responses to addition of dissolved iron alone. At these sites, dissolved manganese concentrations were among the lowest ever measured in seawater, and we therefore suggest that the enhanced response to ash may have been a result of the relief of manganese (co)limitation. Our results imply that volcanic ash deposition events could trigger extensive phytoplankton blooms, potentially capable of significant impacts on regional carbon cycling.

Description: The ocean is responsible for up to a third of total global nitrous oxide (N2O) emissions, but uncertainties in emission rates of this potent greenhouse gas are high (〉100%). Here we use a marine biogeochemical model to assess six major uncertainties in estimates of N2O production, thereby providing guidance in how future studies may most effectively reduce uncertainties in current and future marine N2O emissions. Potential surface N2O production from nitrification causes the largest uncertainty in N2O emissions (estimated up to ~1.6 Tg N yr-1, or 48% of modeled values), followed by the unknown oxygen concentration at which N2O production switches to N2O consumption (0.8 Tg N yr-1, or 24% of modeled values). Other uncertainties are minor, cumulatively changing regional emissions by 〈15%. If production of N2O by surface nitrification could be ruled out in future studies, uncertainties in marine N2O emissions would be halved.

Description: Sea ice thickness information is important for sea ice modelling and ship operations. Here a method to detect the thickness of sea ice up to 50 cm during the freeze-up season based on high incidence angle observations of the Soil Moisture and Ocean Salinity (SMOS) satellite working at 1.4 GHz is suggested. By comparison of thermodynamic ice growth data with SMOS brightness temperatures, a high correlation to intensity and an anticorrelation to the difference between vertically and horizontally polarised brightness temperatures at incidence angles between 40 and 50° are found and used to develop an empirical retrieval algorithm sensitive to thin sea ice up to 50 cm thickness. The algorithm shows high correlation with ice thickness data from airborne measurements and reasonable ice thickness patterns for the Arctic freeze-up period.

Description: Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and a methodology to quantify all major components of the global carbon budget, including their uncertainties, based on the combination of a range of data, algorithms, statistics and model estimates and their interpretation by a broad scientific community. We discuss changes compared to previous estimates, consistency within and among components, alongside methodology and data limitations. CO2 emissions from fossil-fuel combustion and cement production (EFF) are based on energy statistics, while emissions from land-use change (ELUC), mainly deforestation, are based on combined evidence from land-cover change data, fire activity associated with deforestation, and models. The global atmospheric CO2 concentration is measured directly and its rate of growth (GATM) is computed from the annual changes in concentration. The mean ocean CO2 sink (SOCEAN) is based on observations from the 1990s, while the annual anomalies and trends are estimated with ocean models. The variability in SOCEAN is evaluated for the first time in this budget with data products based on surveys of ocean CO2 measurements. The global residual terrestrial CO2 sink (SLAND) is estimated by the difference of the other terms of the global carbon budget and compared to results of independent dynamic global vegetation models forced by observed climate, CO2 and land cover change (some including nitrogen–carbon interactions). All uncertainties are reported as ±1σ, reflecting the current capacity to characterise the annual estimates of each component of the global carbon budget. For the last decade available (2003–2012), EFF was 8.6 ± 0.4 GtC yr−1, ELUC 0.9 ± 0.5 GtC yr−1, GATM 4.3 ± 0.1 GtC yr−1, SOCEAN 2.5 ± 0.5 GtC yr−1, and SLAND 2.8 ± 0.8 GtC yr−1. For year 2012 alone, EFF grew to 9.7 ± 0.5 GtC yr−1, 2.2% above 2011, reflecting a continued growing trend in these emissions, GATM was 5.1 ± 0.2 GtC yr−1, SOCEAN was 2.9 ± 0.5 GtC yr−1, and assuming an ELUC of 1.0 ± 0.5 GtC yr−1 (based on the 2001–2010 average), SLAND was 2.7 ± 0.9 GtC yr−1. GATM was high in 2012 compared to the 2003–2012 average, almost entirely reflecting the high EFF. The global atmospheric CO2 concentration reached 392.52 ± 0.10 ppm averaged over 2012. We estimate that EFF will increase by 2.1% (1.1–3.1%) to 9.9 ± 0.5 GtC in 2013, 61% above emissions in 1990, based on projections of world gross domestic product and recent changes in the carbon intensity of the economy. With this projection, cumulative emissions of CO2 will reach about 535 ± 55 GtC for 1870–2013, about 70% from EFF (390 ± 20 GtC) and 30% from ELUC (145 ± 50 GtC). This paper also documents any changes in the methods and data sets used in this new carbon budget from previous budgets (Le Quéré et al., 2013). All observations presented here can be downloaded from the Carbon Dioxide Information Analysis Center (doi:10.3334/CDIAC/GCP_2013_V2.3).

Description: A critical question regarding the organic carbon cycle in the Arctic Ocean is whether the decline in ice extent and thickness and the associated increase in solar irradiance in the upper ocean will result in increased primary production and particulate organic carbon (POC) export. To assess spatial and temporal variability in POC export, under-ice export fluxes were measured with short-term sediment traps in the northern Laptev Sea in July-August-September 1995, north of the Fram Strait in July 1997, and in the Central Arctic in August–September 2012. Sediment traps were deployed at 2–5 m and 20–25 m under ice for periods ranging from 8.5 to 71 h. In addition to POC fluxes, total particulate matter, chlorophyll a, biogenic particulate silica, phytoplankton, and zooplankton fecal pellet fluxes were measured to evaluate the amount and composition of the material exported in the upper Arctic Ocean. Whereas elevated export fluxes observed on and near the Laptev Sea shelf were likely the combined result of high primary production, resuspension, and release of particulate matter from melting ice, low export fluxes above the central basins despite increased light availability during the record minimum ice extent of 2012 suggest that POC export was limited by nutrient supply during summer. These results suggest that the ongoing decline in ice cover affects export fluxes differently on Arctic shelves and over the deep Arctic Ocean and that POC export is likely to remain low above the central basins unless additional nutrients are supplied to surface waters.

Description: A large-scale multidisciplinary mesocosm experiment in an Arctic fjord (Kongsfjorden, Svalbard; 78°56.2′N) was used to study Arctic marine food webs and biogeochemical elements cycling at natural and elevated future carbon dioxide (CO2) levels. At the start of the experiment, marine-derived chromophoric dissolved organic matter (CDOM) dominated the CDOM pool. Thus, this experiment constituted a convenient case to study production of autochthonous CDOM, which is typically masked by high levels of CDOM of terrestrial origin in the Arctic Ocean proper. CDOM accumulated during the experiment in line with an increase in bacterial abundance; however, no response was observed to increased pCO2 levels. Changes in CDOM absorption spectral slopes indicate that bacteria were most likely responsible for the observed CDOM dynamics. Distinct absorption peaks (at ~ 330 and ~ 360 nm) were likely associated with mycosporine-like amino acids (MAAs). Due to the experimental setup, MAAs were produced in absence of ultraviolet exposure providing evidence for MAAs to be considered as multipurpose metabolites rather than simple photoprotective compounds. We showed that a small increase in CDOM during the experiment made it a major contributor to total absorption in a range of photosynthetically active radiation (PAR, 400–700 nm) and, therefore, is important for spectral light availability and may be important for photosynthesis and phytoplankton groups composition in a rapidly changing Arctic marine ecosystem.

Description: We use new gas-hydrate geochemistry analyses, echosounder data, and three-dimensional P-Cable seismic data to study a gas-hydrate and free-gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of similar to 3.3 degrees C result in a shallow top of the hydrate stability zone (THSZ) at similar to 340 m below sea level (mbsl). Therefore, hydrate-skinned gas bubbles, which inhibit gas-dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to similar to 150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas-hydrate and free-gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice-covered Arctic environments.

Description: Factors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H2O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, Öræfajökull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off-rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high-temperature hydrothermally altered crust (δ18O 〈 2‰) at shallow depths (≥1.8 km). These rhyolites are hot (935–1008°C), relatively dry (H2O 〈 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874–902°C), wet (H2O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (Öræfajökull, Hekla) originate through differentiation deeper in the crust (≥4 km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal δ18O values (5.2–6‰). Although off-rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.

Description: We provide a time series of Agulhas leakage anomalies over the last 20-years from satellite altimetry. Until now, measuring the interannual variability of Indo-Atlantic exchange has been the major barrier in the investigation of the dynamics and large scale impact of Agulhas leakage. We compute the difference of transport between the Agulhas Current and Agulhas Return Current, which allows us to deduce Agulhas leakage. The main difficulty is to separate the Agulhas Return Current from the southern limb of the subtropical "supergyre" south of Africa. For this purpose, an algorithm that uses absolute dynamic topography data is developed. The algorithm is applied to a state-of-the-art ocean model. The comparison with a Lagrangian method to measure the leakage allows us to validate the new method. An important result is that it is possible to measure Agulhas leakage in this model using the velocity field along a section that crosses both the Agulhas Current and the Agulhas Return Current. In the model a good correlation is found between measuring leakage using the full depth velocities and using only the surface geostrophic velocities. This allows us to extend the method to along-track absolute dynamic topography from satellites. It is shown that the accuracy of the mean dynamic topography does not allow us to determine the mean leakage but that leakage anomalies can be accurately computed.

Description: Large explosive volcanic eruptions can inject massive amounts of sulfuric gases into the Earth's atmosphere and, in so doing, affect global climate. The January 1835 eruption of Cosigüina volcano, Nicaragua, ranks among the Americas’ largest and most explosive historical eruptions, but whether it had effects on global climate remains ambiguous. New petrologic analyses of the Cosigüina deposits reveal that the eruption released enough sulfur to explain a prominent ca. AD 1835 sulfate anomaly in ice cores from both the Arctic and Antarctic. A compilation of temperature-sensitive tree-ring chronologies indicates appreciable cooling of the Earth's surface in response to the eruption, consistent with instrumental temperature records. We conclude that this eruption represents one of the most important sulfur-producing events of the last few centuries and had a sizable climate impact rivaling that of the 1991 eruption of Mount Pinatubo.

Description: Due to the unprecedented rate at which our climate is changing, the ultimate consequence for many species is likely to be either extinction or migration to an alternate habitat. Certain species might, however, evolve at a rate that could make them resilient to the effects of a rapidly changing environment. This scenario is most likely to apply to species that have large population sizes and rapid generation times, such that the genetic variation required for adaptive evolution can be readily supplied. Emiliania huxleyi (Lohm.) Hay and Mohler (Prymnesiophyceae) is likely to be such a species, as it is the most conspicuous extant calcareous phytoplankton species in our oceans with growth rates of 1 day−1. Here we report on a validated set of microsatellites, in conjunction with the coccolithophore morphology motif genetic marker, to genotype 93 clonal isolates collected from across the world. Of these, 52 came from a single bloom event in the North Sea collected on the D366 United Kingdom Ocean Acidification cruise in June–July 2011. There were 26 multilocus genotypes (MLGs) encountered only once in the North Sea bloom and 8 MLGs encountered twice or up to six times. Each of these repeated MLGs exhibited Psex values of less than 0.05, indicating each repeated MLG was the product of asexual reproduction and not separate meiotic events. In addition, we show that the two most polymorphic microsatellite loci, EHMS37 and P01E05, are reporting on regions likely undergoing rapid genetic drift during asexual reproduction. Despite the small sample size, there were many more repeated genotypes than previously reported for other bloom-forming phytoplankton species, including a previously genotyped E. huxleyi bloom event. This study challenges the current assumption that sexual reproduction predominates during bloom events. Whilst genetic diversity is high amongst extant populations of E. huxleyi, the root cause for this diversity and ultimate fate of these populations still requires further examination. Nonetheless, we show that certain CMM genotypes are found everywhere, while others appear to have a regional bias.

Description: During the CINDY–DYNAMO field campaign of September 2011–January 2012, a Seaglider was deployed at 80°E and completed 10 north-south sections between 3 and 4°S, measuring temperature, salinity, dissolved oxygen concentration, and chlorophyll fluorescence. These high-resolution subsurface observations provide insight into equatorial ocean Rossby wave activity forced by three Madden-Julian Oscillation (MJO) events during this time period. These Rossby waves generate variability in temperature O(1°C), salinity O(0.2 g kg−1), density O(0.2 kg m−3), and oxygen concentration O(10 μmol kg−1), associated with 10 m vertical displacements of the thermocline. The variability extends down to 1000 m, the greatest depth of the Seaglider observations, highlighting the importance of surface forcing for the deep equatorial ocean. The temperature variability observed by the Seaglider is greater than that simulated in the ECCO-JPL reanalysis, especially at depth. There is also marked variability in chlorophyll fluorescence at the surface and at the depth of the chlorophyll maximum. Upwelling from Rossby waves and local wind stress curl leads to an enhanced shoaling of the chlorophyll maximum by 10–25 m in response to the increased availability of nutrients and light. This influence of the MJO on primary production via equatorial ocean Rossby waves has not previously been recognized.

Description: The atmospheric deposition of both macronutrients and micronutrients plays an important role in driving primary productivity, particularly in the low-latitude ocean. We report aerosol major ion measurements for five ship-based sampling campaigns in the western Pacific from similar to 25 degrees N to 20 degrees S and compare the results with those from Atlantic meridional transects (similar to 50 degrees N to 50 degrees S) with aerosols collected and analyzed in the same laboratory, allowing full incomparability. We discuss sources of the main nutrient species (nitrogen (N), phosphorus (P), and iron (Fe)) in the aerosols and their stoichiometry. Striking north-south gradients are evident over both basins with the Northern Hemisphere more impacted by terrestrial dust sources and anthropogenic emissions and the North Atlantic apparently more impacted than the North Pacific. We estimate the atmospheric supply rates of these nutrients and the potential impact of the atmospheric deposition on the tropical western Pacific. Our results suggest that the atmospheric deposition is P deficient relative to the needs of the resident phytoplankton. These findings suggest that atmospheric supply of N, Fe, and P increases primary productivity utilizing some of the residual excess phosphorus (P*) in the surface waters to compensate for aerosol P deficiency. Regional primary productivity is further enhanced via the stimulation of nitrogen fixation fuelled by the residual atmospheric iron and P*. Our stoichiometric calculations reveal that a P* of 0.1 mu mol L-1 can offset the P deficiency in atmospheric supply for many months. This study suggests that atmospheric deposition may sustain similar to 10% of primary production in both the western tropical Pacific.

Description: In this study we report diapycnal diffusive fluxes of dissolved iron (dFe), dissolved aluminium (dAl) and the major macronutrients to the surface waters of the North Atlantic subpolar gyre. Turbulent diffusivities at the base of the summer mixed layer ranged from 0.01 to 0.5 (median 0.07) cm2 s−1 and daily macronutrient fluxes into the surface mixed layer typically represented 〈 0.5% of integrated mixed layer inventories, although fluxes were highly variable. Elevated nutrient fluxes of up to 4% of mixed layer inventories were identified on the Greenland Shelf, where integrated nutrient pools were lowest due to localised shoaling of the mixed layer. Diffusive fluxes of dFe and dAl were typically 〈0.1% of mixed layer inventories but were also highly variable between stations. Approximations of daily phytoplankton nutrient and Fe uptake indicate that the diffusive flux may at best represent 〈10% of phytoplankton macronutrient uptake, and only 1% of daily phytoplankton Fe uptake. The daily turbulent diffusive flux of dFe was comparable in magnitude to coincident estimates of aeolian Fe supply but despite shallower than normal convective mixing in winter 2010 the diffusive supply was 22 and 59 times smaller than the annual convective supply of Fe to the Irminger and Iceland basins respectively. The general picture obtained from this study is one of small magnitude diffusive nutrient and Fe fluxes to the subpolar North Atlantic during the period of annual nutrient minima and indicates that the diffusive supply mechanism is unlikely to alleviate the recently identified presence of seasonal iron limitation within the North Atlantic subpolar gyre; a condition exacerbated by low dFe:NO3− ratios in subsurface source waters.

Description: The sea-surface microlayer (SML) is the ocean's uppermost boundary to the atmosphere and in control of climate relevant processes like gas exchange and emission of marine primary organic aerosols (POA). The SML represents a complex surface film including organic components like polysaccharides, proteins, and marine gel particles, and harbors diverse microbial communities. Despite the potential relevance of the SML in ocean-atmosphere interactions, still little is known about its structural characteristics and sensitivity to a changing environment such as increased oceanic uptake of anthropogenic CO2. Here we report results of a large-scale mesocosm study, indicating that ocean acidification can affect the abundance and activity of microorganisms during phytoplankton blooms, resulting in changes in composition and dynamics of organic matter in the SML. Our results reveal a potential coupling between anthropogenic CO2 emissions and the biogenic properties of the SML, pointing to a hitherto disregarded feedback process between ocean and atmosphere under climate change.

Description: Calcifying foraminifera are expected to be endangered by ocean acidification, However, the response of a complete community kept in natural sediment and over multiple generations under controlled laboratory conditions has not been constrained to date. During 5 six month incubation, foraminiferal assemblages were treated with pCO2 enriched seawater of 430, 907, 1865 and 3247 μatm pCO2. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Òcalc was much higher in the sedi10 ment than in the water column in all pCO2 treatments and remained close to saturation. As a result, the life cycle of living assemblages was largely unaffected by the tested pCO2 treatments. Growth rates, reproduction and mortality, and therefore population densities and size-frequency distribution of Ammonia aomoriensis varied markedly during the experimental period. Growth rates varied between 25 and 50 μm per month, 15 which corresponds to an addition of 1 or 2 new chambers per month. According to the size-frequency distribution, foraminifera start reproduction at a diameter of 250 μm. Mortality of large foraminifera was recognized, commencing at a test size of 285 μm at a pCO2 ranging from 430 to 1865 μatm, and of 258 μm at 3247 μatm. The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of dry 20 weight. Living individuals had a calcium carbonate production rate of 0.47 gm−2 yr−1, whereas dead empty tests accumulated at a rate of 0.27 gm−2a−1. Although Òcalc was close to 1, some empty tests of Ammonia aomoriensis showed dissolution features at the end of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. This species specific response could be explained by differences in 25 the elemental test composition, in particular the higher Mg-concentrations in Ammonia aomoriensis tests. Our results emphasize that the sensitivity to ocean acidification of endobenthic foraminifera in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment.

Description: A general pattern in water mass distribution and potential shelf–basin exchange is revealed at the Laptev Sea continental slope based on hydrochemical and stable oxygen isotope data from the summers 2005–2009. Despite considerable interannual variations, a frontal system can be inferred between shelf, continental slope and central Eurasian Basin waters in the upper 100 m of the water column along the continental slope. Net sea-ice melt is consistently found at the continental slope. However, the sea-ice meltwater signal is independent from the local retreat of the ice cover and appears to be advected from upwind locations. In addition to the along-slope frontal system at the continental shelf break, a strong gradient is identified on the Laptev Sea shelf between 122° E and 126° E with an eastward increase of riverine and sea-ice related brine water contents. These waters cross the shelf break at ~ 140° E and feed the low-salinity halocline water (LSHW, salinity S 〈 33) in the upper 50 m of the water column. High silicate concentrations in Laptev Sea bottom waters may lead to speculation about a link to the local silicate maximum found within the salinity range of ~ 33 to 34.5, typical for the Lower Halocline Water (LHW) at the continental slope. However brine signatures and nutrient ratios from the central Laptev Sea differ from those observed at the continental slope. Thus a significant contribution of Laptev Sea bottom waters to the LHW at the continental slope can be excluded. The silicate maximum within the LHW at the continental slope may be formed locally or at the outer Laptev Sea shelf. Similar to the advection of the sea-ice melt signal along the Laptev Sea continental slope, the nutrient signal at 50–70 m water depth within the LHW might also be fed by advection parallel to the slope. Thus, our analyses suggest that advective processes from upstream locations play a significant role in the halocline formation in the northern Laptev Sea.

Description: Ash layers from explosive volcanic eruptions (i.e. tephra) represent isochronous surfaces independent from the environment in which they are deposited and the distance from their source. In comparison to eastern Beringia (non-glaciated Yukon and Alaska), few Plio-Pleistocene distal tephra are known from western Beringia (non-glaciated arctic and subarctic eastern Russia), hindering the dating and correlation of sediments beyond the limit of radiocarbon and luminescence methods. The identification of eight visible tephra layers (T0–T7) in sediment cores extracted from Lake El'gygytgyn, in the Far East Russian Arctic, indicates the feasibility of developing a tephrostratigraphic framework for this region. These tephra range in age from ca. 45 ka to 2.2 Ma, and each is described and characterized by its major-, minor-, trace-element and Pb isotope composition. These data show that subduction zone related volcanism from the Kurile–Kamchatka–Aleutian–Arc and Alaska Peninsula is the most likely source, with Pb isotope data indicating a Kamchatkan volcanic source for tephra layers T0–T5 and T7, while a source in the Aleutian Arc is possible probable for Tephra T6. The location of Lake El'gygytgyn relative to potential source volcanoes (〉1000 km) suggests these tephra are distributed over a vast area. These deposits provide a unique opportunity to correlate the high-resolution paleoenvironmental records of Lake El'gygytgyn to other terrestrial paleoenvironmental archives from western Beringia and marine records from the northwest Pacific and Bering Sea. This is an important first step towards the development of a robust integrated framework between the continuous paleoclimatic records of Lake El'gygytgyn and other terrestrial and marine records in NE Eurasia.

Description: Under modern conditions only North Pacific Intermediate Water is formed in the Northwest Pacific Ocean. This situation might have changed in the past. Recent studies with General Circulation Models indicate a switch to deep-water formation in the Northwest Pacific during Heinrich Stadial 1 (17.5–15.0 kyr) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the Northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the mid-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in δ13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the δ13C intermediate water (700–1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (〉2100 m water depth) during the last glacial termination. Thus, intermediate and deep-water masses of the Northwest Pacific have a differing ventilation history during the last deglaciation.

Description: The global ocean–climate system has been highly sensitive to the formation and advection of deep overflow water from the Nordic Seas as integral part of the Atlantic Meridional Overturning Circulation (AMOC) but its evolution over the Pliocene–Pleistocene global cooling is not fully understood. In particular, changes in the sources and mixing of prevailing deep waters that were involved in driving overturning throughout the Pliocene–Pleistocene climate transitions are not well constrained. Here we investigate the evolution of a substantial deep southward return overflow of the AMOC over the last 4 million years. We present new records of the bottom-water radiogenic neodymium isotope (ϵNd) variability obtained from three sediment cores (DSDP site 610 and ODP sites 980/981 and 900) at water depths between 2170 and 5050 m in the northeast Atlantic. We find that prior to the onset of major Northern Hemisphere glaciation (NHG) ∼3 million years ago (Ma), ϵNd values primarily oscillated between −9 and −11 at all sites, consistent with enhanced vertical mixing and weak stratification of the water masses during the warmer-than-today Pliocene period. From 2.7 Ma to ∼2.0 Ma, the ϵNd signatures of the water masses gradually became more distinct, which documents a significant advection of Nordic Seas overflow deep water coincident with the intensification of NHG. Most markedly, however, at ∼1.6 Ma the interglacial ϵNd signatures at sites 610 (2420 m water depth (w.d.)) and 980/981 (2170 m w.d.) synchronously and permanently shifted by 2 to 3 ϵNd units to less radiogenic values, respectively. Since then the difference between glacial and interglacial ϵNd values has been similar to the Late Quaternary at each site. A decrease of ∼2ϵNd units at 1.6 Ma was also recorded for the deepest water masses by site 900 (∼5050 m w.d.), which thereafter, however, evolved to more radiogenic values again until the present. This major ϵNd change across the 1.6 Ma transition reflects a significant reorganization of the overturning circulation in the northeast Atlantic paving the way for the more stratified water column with distinct water masses prevailing thereafter.

Description: X-ray absorption near edge structure (XANES) studies on calcium phosphate species (Ca-P) deal with marginal differences among subtle spectral features despite a hitherto missing systematic breakdown of these differences. Related fingerprinting approaches depend therefore on spectral libraries that are not validated against each other, incomplete and scattered among publications. This study compiled a comprehensive spectral library from published reference compound libraries in order to establish more clear-cut criteria for Ca-P determination by distinctive phosphorus K-edge XANES features. A specifically developed normalization method identified diagnostic spectral features within the compiled library, e.g. by uniform calculation of ratios between white-line and secondary peak heights. Post-processing of the spectra (n = 81) verified distinguishability among most but not all phases, which included hydroxylapatite (HAP), poorly crystalline HAP, amorphous HAP, fluorapatite, carbonate fluorapatite (CFAP), carbonate hydroxylapatite, β-tricalcium phosphate, octacalcium phosphate (OCP), brushite, monetite, monocalcium phosphate, amorphous calcium phosphate (ACP), anapaite, herderite, scholzite, messelite, whiteite and P on CaCO3. Particularly, peak height ratios significantly improved analyte specificity, e.g. by supplementary breakdown into OCP and ACP. The spectral analysis also revealed Ca-P standards that were rarely investigated or inappropriately synthesized, and thus provides a basis for standard selection and synthesis. The developed method and resulting breakdown by species were subsequently tested on Ca-P spectra from studies on bone and sediment. The test indicated that bone material likely comprises only poorly crystalline apatite, which implies direct nucleation of apatite in bone. This biological apatite formation is likely opposed to that of sedimentary apatite, which apparently forms by successive crystallization. Application of the method to μXANES spectra of sediment particles indicated authigenic apatite formation by an OCP precursor.

Description: Subduction of the oceanic Cocos plate offshore Costa Rica causes strong advection of methane-charged fluids. Presented here are the first direct measurements of microbial anaerobic oxidation of methane (AOM) and sulfate reduction (SR) rates in sediments from the two mounds, applying radiotracer techniques in combination with numerical modeling. In addition, analysis of carbonate δ18O, δ13C, and 87Sr / 86Sr signatures constrain the origin of the carbonate-precipitating fluid. Average rates of microbial activities showed differences with a factor of 4.8 to 6.3 between Mound 11 [AOM 140.71 (±40.84 SD); SR 117.25 (±82.06 SD) mmol m−2 d−1, respectively] and Mound 12 [AOM 22.37 (±0.85 SD); SR 23.99 (±5.79 SD) mmol m−2 d−1, respectively]. Modeling results yielded flow velocities of 50 cm a−1 at Mound 11 and 8–15 cm a−1 at Mound 12. Analysis of oxygen and carbon isotope variations of authigenic carbonates from the two locations revealed higher values for Mound 11 (δ18O: 4.7 to 5.9‰, δ13C: −21.0 to −29.6‰), compared to Mound 12 (δ18O: 4.1 to 4.5‰, δ13C: −45.7 to −48.9‰). Analysis of carbonates 87Sr / 86Sr indicated temporal changes of deep-source fluid admixture at Mound 12. The present study is in accordance with previous work supporting considerable differences of methane flux between the two Mounds. It also strengthens the hypothesis of a predominantly deep fluid source for Mound 11 versus a rather shallow source of biogenic methane for Mound 12. The results demonstrate that methane-driven microbial activity is a valid ground truthing tool for geophysical measurements of fluid advection and constraining of recent methane fluxes in the study area. The study further shows that the combination of microbial rate measurements, numerical modeling, and authigenic carbonate analysis provide a suitable approach to constrain temporal and spatial variations of methane charged fluid flow at the Pacific Costa Rican margin.

Description: We use an eddying realistic primitive equation model of the Southern Ocean to examine the spatial and temporal distribution of near-inertial wind-power input (WPI) and near-inertial energy (NIE) in the Southern Ocean. We find that the modelled near-inertial WPI is almost proportional to inertial wind-stress variance (IWSV), while the modelled NIE is modulated by the inverse of the mixed-layer depth. We go on to assess recent decadal trends of near-inertial WPI from trends of IWSV based on reanalysis wind-stress. Averaged over the Southern Ocean, annual-mean IWSV is found to have increased by 16 percent over the years 1979 through 2011. Part of the increase of IWSV is found to be related to the positive trend of the Southern Annular Mode over the same period. Finally, we show that there are horizontal local maxima of NIE at depth that are almost exclusively associated with anticyclonic eddies.

Description: With an extension of 〉40 km2 the recently discovered Campeche cold-water coral province located at the northeastern rim of the Campeche Bank in the southern Gulf of Mexico belongs to the largest coherent cold-water coral areas discovered so far. The Campeche province consists of numerous 20 to 40 m high coral ridges that are developed in intermediate water depths of 500 to 600 m. The ridges are colonized by a vivid cold-water coral ecosystem that covers the upper flanks and summits. The rich coral community is dominated by the framework-building scleractinia Enallopsammia profunda and Lophelia pertusa while the associated benthic megafauna shows a rather scarce occurrence. The recent environmental setting is characterized by a high surface water production caused by a local upwelling center and a dynamic bottom water regime comprising vigorous bottom currents, internal waves and strong density contrasts, which all together provide optimal conditions for the growth of cold-water corals. The strong hydrodynamics – potentially supported by the diel vertical migration of zooplankton in the Campeche area – drive the delivering of food particles to the corals. The Campeche cold-water coral province is, thus, an excellent example highlighting the importance of the hydrographic setting in securing the food supply for the development of large and vivid cold-water coral ecosystems.

Description: Extension of the continental lithosphere leads to the formation of rift basins or rifted continental margins if breakup occurs. Seismic investigations have repeatedly shown that conjugate margins have asymmetric tectonic structures and different amount of extension and crustal thinning. Here we compare two coincident wide-angle and multichannel seismic profiles across the northern Tyrrhenian rift system sampling crust that underwent different stages of extension from north to south and from the flanks to the basin center. Tomographic inversion reveals that the crust has thinned homogeneously from ~24 km to ~17 km between the Corsica Margin and the Latium Margin implying a β factor of ~1.3–1.5. On the transect 80 km to the south, the crust thinned from ~24 km beneath Sardinia to a maximum of ~11 km in the eastern region near the Campania Margin (β factor of ~2.2). The increased crustal thinning is accompanied by a zone of reduced velocities in the upper crust that expands progressively toward the southeast. We interpret that the velocity reduction is related to rock fracturing caused by a higher degree of brittle faulting, as observed on multichannel seismic images. Locally, basalt flows are imaged intruding sediment in this zone, and heat flow values locally exceed 100 mW/m2. Velocities within the entire crust range 4.0–6.7 km/s, which are typical for continental rocks and indicate that significant rift-related magmatic underplating may not be present. The characteristics of the pre-tectonic, syn-tectonic and post-tectonic sedimentary units allow us to infer the spatial and temporal evolution of active rifting. In the western part of the southern transect, thick postrift sediments were deposited in half grabens that are bounded by large fault blocks. Fault spacing and block size diminish to the east as crustal thinning increases. Recent tectonic activity is expressed by faults cutting the seafloor in the east, near the mainland of Italy. The two transects show the evolution from the less extended rift in the north with a fairly symmetric conjugate structure to the asymmetric margins farther south. This structural evolution is consistent with W-E rift propagation and southward increasing extension rates.

Description: From October 2008 to November 2010, CH3I concentrations were measured in the Kiel Fjord together with potentially related biogeochemical and physical parameters. A repeating seasonal cycle of CH3I was observed with highest concentrations in summer (ca. 8.3 pmol L−1; June and July) and lowest concentrations in winter (ca. 1.5 pmol L−1; December to February). A strong positive correlation at zero lag between [CH3I] and solar radiation (R2 = 0.93) was observed, whereas correlations with other variables (SST, Chlorophyll a) were weaker, and they lagged CH3I by ca. 1 month. These results appear consistent with the hypothesis that SSR is the primary forcing of CH3I production in surface seawater, possibly through a photochemical pathway. A mass balance of the monthly averaged data was used to infer mean rates of daily net production (Pnet) and losses for CH3I over the year. The sea-to-air flux of CH3I in the Kiel Fjord averaged 3.1 nmol m−2 d−1, the mean chemical loss rate was 0.047 pmol L−1 d−1, and Pnet varied systematically from winter to summer (from 0 to 0.6 pmol L−1 d−1). Pnet was correlated at zero lag with SST, SSR, and Chla (R2 = 0.55, 0.67, and 0.73, respectively, p 〈〈 0.01). The lagged cross-correlation analysis indicated that SSR led Pnet by 1 month, whereas the strongest cross correlations with Chla were at lags of 0 to +1 month, and SST lagged Pnet by 1 month. The broad seasonal peak of Pnet makes it difficult to determine the key factor controlling CH3I net production using in situ concentration data alone.

Description: Ventilation is the primary pathway for atmosphere–ocean boundary perturbations, such as temperature anomalies, to be relayed to the ocean interior. It is also a conduit for gas exchange between the interface of atmosphere and ocean. Thus it is a mechanism whereby, for instance, the ocean interior is oxygenated and enriched in anthropogenic carbon. The ventilation of the Mediterranean Sea is fast in comparison to the world ocean and has large temporal variability. Here we present transient tracer data from a field campaign in April 2011 that sampled a unique suite of transient tracers (SF6, CFC-12, 3H and 3He) in all major basins of the Mediterranean. We apply the transit time distribution (TTD) model to the data in order to constrain the mean age, the ratio of the advective / diffusive transport and the number of water masses significant for ventilation. We found that the eastern part of the eastern Mediterranean can be reasonably described with a one-dimensional inverse Gaussian TTD (IG-TTD), and thus constrained with two independent tracers. The ventilation of the Ionian Sea and the western Mediterranean can only be constrained by a linear combination of IG-TTDs. We approximate the ventilation with a one-dimensional, two inverse Gaussian TTD (2IG-TTD) for these areas and demonstrate a possibility of constraining a 2IG-TTD from the available transient tracer data. The deep water in the Ionian Sea has a mean age between 120 and 160 years and is therefore substantially older than the mean age of the Levantine Basin deep water (60–80 years). These results are in contrast to those expected by the higher transient tracer concentrations in the Ionian Sea deep water. This is partly due to deep water of Adriatic origin having more diffusive properties in transport and formation (i.e., a high ratio of diffusion over advection), compared to the deep water of Aegean Sea origin that still dominates the deep Levantine Basin deep water after the Eastern Mediterranean Transient (EMT) in the early 1990s. The tracer minimum zone (TMZ) in the intermediate of the Levantine Basin is the oldest water mass with a mean age up to 290 years. We also show that the deep western Mediterranean has contributed approximately 40% of recently ventilated deep water from the Western Mediterranean Transition (WMT) event of the mid-2000s. The deep water has higher transient tracer concentrations than the mid-depth water, but the mean age is similar with values between 180 and 220 years.

Description: We investigate potential relations between variations in seafloor relief and age of the incoming plate and interplate seismicity. Westward from Osa Peninsula in Costa Rica, a major change in the character of the incoming Cocos Plate is displayed by abrupt lateral variations in seafloor depth and thermal structure. Here a Mw 6.4 thrust earthquake was followed by three aftershock clusters in June 2002. Initial relocations indicate that the main shock occurred fairly trenchward of most large earthquakes along the Middle America Trench off central Costa Rica. The earthquake sequence occurred while a temporary network of OBH and land stations ∼80 km to the northwest were deployed. By adding readings from permanent local stations, we obtain uncommon P wave coverage of a large subduction zone earthquake. We relocate this catalog using a nonlinear probabilistic approach within both, a 1-D and a 3-D P wave velocity models. The main shock occurred ∼25 km from the trench and probably along the plate interface at 5–10 km depth. We analyze teleseismic data to further constrain the rupture process of the main shock. The best depth estimates indicate that most of the seismic energy was radiated at shallow depth below the continental slope, supporting the nucleation of the Osa earthquake at ∼6 km depth. The location and depth coincide with the plate boundary imaged in prestack depth-migrated reflection lines shot near the nucleation area. Aftershocks propagated downdip to the area of a 1999 Mw 6.9 sequence and partially overlapped it. The results indicate that underthrusting of the young and buoyant Cocos Ridge has created conditions for interplate seismogenesis shallower and closer to the trench axis than elsewhere along the central Costa Rica margin.

Description: Coastal hypoxia and anoxia have become a global key stressor to marine ecosystems, with almost 500 dead zones recorded worldwide. By triggering cascading effects from the individual organism to the community-and ecosystem level, oxygen depletions threaten marine biodiversity and can alter ecosystem structure and function. By integrating both physiological function and ecological processes, animal behaviour is ideal for assessing the stress state of benthic macrofauna to low dissolved oxygen. The initial response of organisms can serve as an early warning signal, while the successive behavioural reactions of key species indicate hypoxia levels and help assess community degradation. Here we document the behavioural responses of a representative spectrum of benthic macrofauna in the natural setting in the Northern Adriatic Sea (Mediterranean). We experimentally induced small-scale anoxia with a benthic chamber in 24m depth to overcome the difficulties in predicting the onset of hypoxia, which often hinders full documentation in the field. The behavioural reactions were documented with a time-lapse camera. Oxygen depletion elicited significant and repeatable changes in general (visibility, locomotion, body movement and posture, location) and species-specific reactions in virtually all organisms (302 individuals from 32 species and 2 species groups). Most atypical (stress) behaviours were associated with specific oxygen thresholds: arm-tipping in the ophiuroid Ophiothrix quinquemaculata, for example, with the onset of mild hypoxia (〈 2mLO(2) L-1), the emergence of polychaetes on the sediment surface with moderate hypoxia (〈 1mLO(2) L-1), the emergence of the infaunal sea urchin Schizaster canaliferus on the sediment with severe hypoxia (〈 0.5mLO(2) L-1) and heavy body rotations in sea anemones with anoxia. Other species changed their activity patterns, for example the circadian rhythm in the hermit crab Paguristes eremita or the bioherm-associated crab Pisidia longimana. Intra-and interspecific reactions were weakened or changed: decapods ceased defensive and territorial behaviour, and predator-prey interactions and relationships shifted. This nuanced scale of resolution is a useful tool to interpret present benthic community status (behaviour) and past mortalities (community composition, e.g. survival of tolerant species). This information on the sensitivity (onset of stress response), tolerance (mortality, survival), and characteristics (i. e. life habit, functional role) of key species also helps predict potential future changes in benthic structure and ecosystem functioning. This integrated approach can transport complex ecological processes to the public and decision-makers and help define specific monitoring, assessment and conservation plans

Description: The Mediterranean Sea (MedSea) is considered a "laboratory basin" being an ocean in miniature, suffering dramatic changes in its oceanographic and biogeochemical conditions derived from natural and anthropogenic forces. Moreover, the MedSea is prone to absorb and store anthropogenic carbon due to the particular CO2 chemistry and the active overturning circulation. Despite this, water column CO2 measurements covering the whole basin are scarce. This work aims to be a base-line for future studies about the CO2 system space-time variability in the MedSea combining historic and modern CO2 cruises in the whole area. Here we provide an extensive vertical and longitudinal description of the CO2 system variables (total alkalinity – TA, dissolved inorganic carbon – DIC and pH) along an East-West transect and across the Sardinia-Sicily passage in the MedSea from two oceanographic cruises conducted in 2011 measuring CO2 variables in a coordinated fashion, the RV Meteor M84/3 and the RV Urania EuroFleets 11, respectively. In this sense, we provide full-depth and length CO2 distributions across the MedSea, and property-property plots showing in each sub-basin post-Eastern Mediterranean Transient (EMT) situation with regard to TA, DIC and pH. The over-determined CO2 system in 2011 allowed performing the first internal consistency analysis for the particularly warm, high salinity and alkalinity MedSea waters. The CO2 constants by Mehrbach et al. (1973) refitted by Dickson and Millero (1987) are recommended. The sensitivity of the CO2 system to the atmospheric CO2 increase, DIC and/or TA changes is evaluated by means of the Revelle and buffer factors.

Description: We study the erosive convergent margin of north-central Chile (at similar to 31 degrees S) by using high-resolution bathymetric, wide-angle refraction, and multichannel seismic reflection data to derive a detailed tomographic 2-D velocity-depth model. In the overriding plate, our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6.0-6.5km/s, which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the Coastal Cordillera. Beneath the lower and middle continental slope, however, the presence of a zone of reduced velocities (3.5-5.0km/s) is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion. At the landward edge of the outermost fore arc, the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp (similar to 1km offset), which overlies a strong lateral velocity contrast from similar to 5.0 to similar to 6.0km/s. This pronounced velocity contrast propagates deep into the continental crust, and it resembles a major normal listric fault. We interpret this seismic discontinuity as the volcanic-continental basement contact of the submerged Coastal Cordillera characterized by a gravitational collapse of the outermost fore arc. Subduction erosion has, most likely, caused large-scale crustal thinning and long-term subsidence of the outermost fore arc.

Description: Major stratospheric sudden warmings are prominent disturbances of the Northern Hemisphere polar winter stratosphere. Understanding the factors controlling major warmings is required, since the associated circulation changes can propagate down into the troposphere and affect the surface climate, suggesting enhanced prediction skill when these processes are accurately represented in models. In this study we investigate how different natural and anthropogenic factors, namely, the quasi-biennial oscillation (QBO), sea surface temperatures (SSTs), anthropogenic greenhouse gases, and ozone-depleting substances, influence the frequency, variability, and life cycle of major warmings. This is done using sensitivity experiments performed with the National Center for Atmospheric Research's Community Earth System Model (CESM). CESM is able to simulate the life cycle of major warmings realistically. The QBO strengthens the climatological stratospheric polar night jet (PNJ) and significantly reduces the frequency of major warmings through reduction of planetary wave propagation into the PNJ region. Variability in SSTs weakens the PNJ and significantly increases the major warming frequency due to enhanced wave forcing. Even extreme climate change conditions (RCP8.5 scenario) do not influence the total frequency but determine the prewarming phase of major warmings. The amplitude and duration of major warmings seem to be mainly determined by internal stratospheric variability. We also suggest that SST variability, two-way ocean/atmosphere coupling, and hence the memory of the ocean are needed to reproduce the observed tropospheric negative Northern Annular Mode pattern after major warmings.

Description: The Baltic Sea is a seasonally ice-covered marginal sea located in a densely populated area in northern Europe. Severe sea ice conditions have the potential to hinder the intense ship traffic considerably. Thus, sea ice fore- and nowcasts are regularly provided by the national weather services. Typically, the forecast comprises several ice properties that are distributed as prognostic variables, but their actual usefulness is difficult to measure, and the ship captains must determine their relative importance and relevance for optimal ship speed and safety ad hoc. The present study provides a more objective approach by comparing the ship speeds, obtained by the automatic identification system (AIS), with the respective forecasted ice conditions. We find that, despite an unavoidable random component, this information is useful to constrain and rate fore- and nowcasts. More precisely, 62–67% of ship speed variations can be explained by the forecasted ice properties when fitting a mixed-effect model. This statistical fit is based on a test region in the Bothnian Sea during the severe winter 2011 and employs 15 to 25 min averages of ship speed.

Description: The widely used concept of constant ”Redfield” phytoplankton stoichiometry is often applied for estimating which nutrient limits phytoplankton growth in the surface ocean. Culture experiments, in contrast, show strong relations between growth conditions and cellular stoichiometry with often substantial deviations from Redfield stoichiometry. Here we investigate to what extent both views agree by analyzing remote sensing and in situ data with an optimality-based model of nondiazotrophic phytoplankton growth in order to infer seasonally varying patterns of colimitation by light, nitrogen (N), and phosphorus (P) in the global ocean. Our combined model-data analysis suggests strong N and N-P colimitation in the tropical ocean, seasonal light, and N-P colimitation in the Northern Hemisphere, and strong light limitation only during winter in the Southern Ocean. The eastern equatorial Pacific appears as the only ocean area that is essentially not limited by N, P, or light. Even though our optimality-based approach specifically accounts for flexible stoichiometry, inferred patterns of N and P limitation are to some extent consistent with those obtained from an analysis of surface inorganic nutrients with respect to the Redfield N:P ratio. Iron is not part of our analysis, implying that we cannot accurately predict N cell quotas in high-nutrient, low-chlorophyll regions. Elsewhere, we do not expect a major effect of iron on the relative distribution of N, P, and light colimitation areas. The relative importance of N, P, and light in limiting phytoplankton growth diagnosed here by combining observations and an optimal growth model provides a useful constraint for models used to predict future marine biological production under changing environmental conditions.

Description: Continental shelves are predominately (~70%) covered with permeable, sandy sediments. While identified as critical sites for intense oxygen, carbon, and nutrient turnover, constituent exchange across permeable sediments remains poorly quantified. The central North Sea largely consists of permeable sediments and has been identified as increasingly at risk for developing hypoxia. Therefore, we investigate the benthic O2 exchange across the permeable North Sea sediments using a combination of in situ microprofiles, a benthic chamber, and aquatic eddy correlation. Tidal bottom currents drive the variable sediment O2 penetration depth (from ~3 to 8 mm) and the concurrent turbulence-driven 25-fold variation in the benthic sediment O2 uptake. The O2 flux and variability were reproduced using a simple 1-D model linking the benthic turbulence to the sediment pore water exchange. The high O2 flux variability results from deeper sediment O2 penetration depths and increased O2 storage during high velocities, which is then utilized during low-flow periods. The study reveals that the benthic hydrodynamics, sediment permeability, and pore water redox oscillations are all intimately linked and crucial parameters determining the oxygen availability. These parameters must all be considered when evaluating mineralization pathways of organic matter and nutrients in permeable sediments.

Description: Coccolithophores are a key functional group in terms of the pelagic production of calcium carbonate (calcite), although their contribution to shelf sea biogeochemistry, and how this relates to environmental conditions, is poorly constrained. Measurements of calcite production (CP) and coccolithophore abundance were made on the north-west European shelf to examine trends in coccolithophore calcification along natural gradients of carbonate chemistry, macronutrient availability and plankton composition. Similar measurements were also made in three bioassay experiments where nutrient (nitrate, phosphate) and pCO2 levels were manipulated. Nanoflagellates (〈 10 μm) dominated chlorophyll biomass and primary production (PP) at all but one sampling site, with CP ranging from 0.6 to 9.6 mmol C m−2 d−1. High CP and coccolithophore abundance occurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and Western English Channel. Coccolithophore abundance and CP showed no correlation with nutrient concentrations or ratios, while significant (p 〈 0.01) correlations between CP, cell-specific calcification (cell-CF) and irradiance in the water column highlighted how light availability exerts a strong control on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northern North Sea, Norwegian Trench) showed a strong response in terms of CP to combined nitrate and phosphate addition, mediated by changes in cell-CF and growth rates. In contrast, an offshore diverse coccolithophore community (Bay of Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling this community. Sharp decreases in pH and a rough halving of calcite saturation states in the bioassay experiments led to decreased CP in the Bay of Biscay and northern North Sea, but not the Norwegian Trench. These decreases in CP were related to slowed growth rates in the bioassays at elevated pCO2 (750 μatm) relative to those in the ambient treatments. The combined results from our study highlight the variable coccolithophore responses to irradiance, nutrients and carbonate chemistry in north-west European shelf waters, which are mediated by changes in growth rates, cell-CF and species composition.

Description: The ongoing oceanic uptake of anthropogenic carbon dioxide (CO2) is significantly altering the carbonate chemistry of seawater, a phenomenon referred to as ocean acidification. Experimental manipulations have been increasingly used to gauge how continued ocean acidification will potentially impact marine ecosystems and their associated biogeochemical cycles in the future; however, results amongst studies, particularly when performed on natural communities, are highly variable, which may reflect community/environment-specific responses or inconsistencies in experimental approach. To investigate the potential for identification of more generic responses and greater experimentally reproducibility, we devised and implemented a series (n = 8) of short-term (2–4 days) multi-level (≥4 conditions) carbonate chemistry/nutrient manipulation experiments on a range of natural microbial communities sampled in Northwest European shelf seas. Carbonate chemistry manipulations and resulting biological responses were found to be highly reproducible within individual experiments and to a lesser extent between geographically separated experiments. Statistically robust reproducible physiological responses of phytoplankton to increasing pCO2, characterised by a suppression of net growth for small-sized cells (〈10 μm), were observed in the majority of the experiments, irrespective of natural or manipulated nutrient status. Remaining between-experiment variability was potentially linked to initial community structure and/or other site-specific environmental factors. Analysis of carbon cycling within the experiments revealed the expected increased sensitivity of carbonate chemistry to biological processes at higher pCO2 and hence lower buffer capacity. The results thus emphasise how biogeochemical feedbacks may be altered in the future ocean.

Description: The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simulated realistic dust deposition events (10 gm(-2)) into large mesocosms (52m(3)). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3-) (net increases up to +9.8 mu M NO3- at 0.1m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3-. By transiently increasing DIP and NO3- concentrations in N-P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3- relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3-/DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust deposition events, from initially 15% to 60-70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3-/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity.

Description: The marine CaCO3 cycle is an important component of the oceanic carbon system and directly affects the cycling of natural and the uptake of anthropogenic carbon. In numerical models of the marine carbon cycle, the CaCO3 cycle component is often evaluated against the observed distribution of alkalinity. Alkalinity varies in response to the formation and remineralization of CaCO3 and organic matter. However, it also has a large conservative component, which may strongly be affected by a deficient representation of ocean physics (circulation, evaporation, and precipitation) in models. Here we apply a global ocean biogeochemical model run into preindustrial steady state featuring a number of idealized tracers, explicitly capturing the model's CaCO3 dissolution, organic matter remineralization, and various preformed properties (alkalinity, oxygen, phosphate). We compare the suitability of a variety of measures related to the CaCO3 cycle, including alkalinity (TA), potential alkalinity and TA*, the latter being a measure of the time-integrated imprint of CaCO3 dissolution in the ocean. TA* can be diagnosed from any data set of TA, temperature, salinity, oxygen and phosphate. We demonstrate the sensitivity of total and potential alkalinity to the differences in model and ocean physics, which disqualifies them as accurate measures of biogeochemical processes. We show that an explicit treatment of preformed alkalinity (TA0) is necessary and possible. In our model simulations we implement explicit model tracers of TA0 and TA*. We find that the difference between modelled true TA* and diagnosed TA* was below 10% (25%) in 73% (81%) of the ocean's volume. In the Pacific (and Indian) Oceans the RMSE of A* is below 3 (4) mmol TA m−3, even when using a global rather than regional algorithms to estimate preformed alkalinity. Errors in the Atlantic Ocean are significantly larger and potential improvements of TA0 estimation are discussed. Applying the TA* approach to the output of three state-of-the-art ocean carbon cycle models, we demonstrate the advantage of explicitly taking preformed alkalinity into account for separating the effects of biogeochemical processes and circulation on the distribution of alkalinity. In particular, we suggest to use the TA* approach for CaCO3 cycle model evaluation.

Description: Understanding the causes of the observed expansion of tropical ocean's oxygen minimum zones (OMZs) is hampered by large biases in the representation of oxygen distribution in climate models, pointing to incorrectly represented mechanisms. Here we assess the oxygen budget in a global biogeochemical circulation model, focusing on the Atlantic Ocean. While a coarse (0.5°) configuration displays the common bias of too large and too intense OMZs, the oxygen concentration in an eddying (0.1°) configuration is higher and closer to observations. This improvement is traced to a stronger oxygen supply by a more realistic representation of the equatorial and off-equatorial undercurrents, outweighing the concurrent increase in oxygen consumption associated with the stronger nutrient supply. The sensitivity of the eastern tropical Atlantic oxygen budget to the equatorial current intensity suggests that temporal changes in the eastward oxygen transport from the well-oxygenated western boundary region might partly explain variations in the OMZs.

Description: In this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back-arc basin combining refraction and wide-angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)-2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a ~450 km long 2-D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time-migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity-derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin, extension has thinned the crust from ~20 km under the coastline to ~13 km ~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back-arc oceanic settings. The velocities-depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10–20 km wide, relatively low-velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

Description: The Baltic Sea is a seasonally ice-covered, marginal sea in central northern Europe. It is an essential waterway connecting highly industrialised countries. Because ship traffic is intermittently hindered by sea ice, the local weather services have been monitoring sea ice conditions for decades. In the present study we revisit a historical monitoring data set, covering the winters 1960/1961 to 1978/1979. This data set, dubbed Data Bank for Baltic Sea Ice and Sea Surface Temperatures (BASIS) ice, is based on hand-drawn maps that were collected and then digitised in 1981 in a joint project of the Finnish Institute of Marine Research (today the Finnish Meteorological Institute (FMI)) and the Swedish Meteorological and Hydrological Institute (SMHI). BASIS ice was designed for storage on punch cards and all ice information is encoded by five digits. This makes the data hard to access. Here we present a post-processed product based on the original five-digit code. Specifically, we convert to standard ice quantities (including information on ice types), which we distribute in the current and free Network Common Data Format (NetCDF). Our post-processed data set will help to assess numerical ice models and provide easy-to-access unique historical reference material for sea ice in the Baltic Sea. In addition we provide statistics showcasing the data quality. The website http://www.baltic-ocean.org hosts the post-processed data and the conversion code. The data are also archived at the Data Publisher for Earth & Environmental Science, PANGAEA (doi:10.1594/PANGAEA.832353)

Description: Acoustic velocities were measured during triaxial deformation tests of silty clay and clayey silt core samples from the Nankai subduction zone (Integrated Ocean Drilling Program Expeditions 315, 316, and 333). We provide a new data set, continuously measured during pressure increase and subsequent axial deformation. A new data processing method was developed using seismic time series analysis. Compressional wave velocities (V-p) range between about 1450 and 2200 m/s, and shear wave velocities (V-s) range between about 150 and 800 m/s. V-p slightly increases with rising effective confining pressure and effective axial stress. Samples from the accretionary prism toe show the highest Vp, while fore-arc slope sediments show lower Vp. Samples from the incoming plate, slightly richer in clay minerals, have the lowest values for V-p. V-s increases with higher effective confining pressures and effective axial stress, irrespective of composition and tectonic setting. Shear and bulk moduli are between 0.2 and 1.3 GPa, and 3.85 and 8.41 GPa, respectively. Elastic moduli of samples from the accretionary prism toe and the footwall of the megasplay fault (1.50 and 3.98 GPa) are higher than those from the hanging wall and incoming plate (0.59 and 0.88 GPa). This allows differentiation between normal and overconsolidated sediments. The data show that in a tectonosedimentary environment of only subtle compositional differences, acoustic properties can be used to differentiate between stronger (accretionary prism toe) and weaker (fore-arc slope, incoming plate) sediments. Especially V-p/V-s ratios may be instrumental in detecting zones of low effective stress and thus high pore fluid pressure