ALBERT

Description: The analysis of high-resolution vector magnetic data acquired by deep-sea submersibles (DSSs) requires the development of specific approaches adapted to their uneven tracks. We present a method that takes advantage of (1) the varying altitude of the DSS above the seafloor and (2) high-resolution multibeam bathymetric data acquired separately, at higher altitude, by an Autonomous Underwater Vehicle, to estimate the absolute magnetization intensity and the magnetic polarity of the shallow subseafloor along the DSS path. We apply this method to data collected by DSS Nautile on a small active basalt-hosted hydrothermal site. The site is associated with a lack of magnetization, in agreement with previous findings at the same kind of sites: the contrast between nonmagnetic sulfide deposits/stockwork zone and strongly magnetized basalt is sufficient to explain the magnetic signal observed at such a low altitude. Both normal and reversed polarities are observed in the lava flows surrounding the site, suggesting complex history of accumulating volcanic flows.

Description: Mesoscale eddies are ubiquitous features in the Southern Ocean, yet their phenomenology is not well quantified. To tackle this task, we use satellite observations of sea level anomalies and sea surface temperature (SST) as well as in situ temperature and salinity measurements from profiling floats. Over the period 1997–2010, we identified over a million mesoscale eddy instances and were able to track about 105 of them over 1 month or more. The Antarctic Circumpolar Current (ACC), the boundary current systems, and the regions where they interact are hot spots of eddy presence, representing also the birth places and graveyards of most eddies. These hot spots contrast strongly to areas shallower than about 2000 m, where mesoscale eddies are essentially absent, likely due to topographical steering. Anticyclones tend to dominate the southern subtropical gyres, and cyclones the northern flank of the ACC. Major causes of regional polarity dominance are larger formation numbers and lifespans, with a contribution of differential propagation pathways of long-lived eddies. Areas of dominance of one polarity are generally congruent with the same polarity being longer-lived, bigger, of larger amplitude, and more intense. Eddies extend down to at least 2000 m. In the ACC, eddies show near surface temperature and salinity maxima, whereas eddies in the subtropical areas generally have deeper anomaly maxima, presumably inherited from their origin in the boundary currents. The temperature and salinity signatures of the average eddy suggest that their tracer anomalies are a result of both trapping in the eddy core and stirring.

Description: The Eastern Tropical South Pacific oxygen minimum zone (ETSP-OMZ) is a site of intense nitrous oxide (N2O) flux to the atmosphere. This flux results from production of N2O by nitrification and denitrification, but the contribution of the two processes is unknown. The rates of these pathways and their distributions were measured directly using 15N tracers. The highest N2O production rates occurred at the depth of peak N2O concentrations at the oxic-anoxic interface above the oxygen deficient zone (ODZ) because slightly oxygenated waters allowed (1) N2O production from both nitrification and denitrification and (2) higher nitrous oxide production yields from nitrification. Within the ODZ proper (i.e., anoxia), the only source of N2O was denitrification (i.e., nitrite and nitrate reduction), the rates of which were reflected in the abundance of nirS genes (encoding nitrite reductase). Overall, denitrification was the dominant pathway contributing the N2O production in the ETSP-OMZ.

Description: Vital processes relevant for exploited stocks, for example growth, predation and recruitment are closely related to the environmental conditions. Here, we present a generic method to include state of the art knowledge on environmental impacts and environmental forecasting into short-term predictions and the formulation of environment-based harvest control rules for exploited stocks. The method consists of three elements: First, the linkage between environmental parameters and stock dynamics, second the short-term prediction of both environment and stock dynamics, and third the scaling of otherwise constant reference values for fishing mortality in accordance with the environmental situation. The method is exemplified for Eastern Baltic cod. Recruitment is treated as dependent on oxygen conditions, and the formulation of the proposed fishing intensity is accounting for the actual oxygen conditions and predicted conditions for the year following the assessment year. Finally, the resulting advice is compared to advice that has been given not accounting for the oxygen conditions.

Description: The iron hypothesis suggests that in large areas of the ocean phytoplankton growth and thus photosynthetic CO2-uptake is limited by the micronutrient iron. Phytoplankton requires iron in particular for nitrate uptake, light harvesting and electron transport in photosynthesis, suggesting a tight coupling of iron and light limitation. One important source of iron to the open ocean is dust deposition. Previous global biogeochemical modeling studies have suggested a low sensitivity of oceanic CO2-uptake to changes in dust deposition. Here we show that this sensitivity is increased significantly when iron-light colimitation, i.e. the impact of iron bioavailability on light harvesting capabilities, is explicitly considered. Accounting for iron-light colimitation increases the shift of export production from tropical and subtropical regions to the higher latitudes of subpolar regions at high dust deposition and amplifies iron limitation at low dust deposition. Our results re-emphasize the role of iron as a key limiting nutrient for phytoplankton.

Description: A long-standing problem in climate modeling is the inaccurate simulation of tropical Atlantic (TA) sea surface temperature (SST), known as the TA SST bias. It has far-reaching consequences for climate prediction in that area as it goes along, among others, with erroneous precipitation patterns. We show that the TA SST bias can be largely reduced by increasing both the atmospheric horizontal and vertical resolution in a climate model. At high horizontal resolution, enhanced vertical resolution is indispensable to substantially improve the simulation of TA SST by enhancing surface wind stress. This also reduces biases in the upper ocean thermal structure and precipitation patterns. Although, enhanced horizontal resolution alone leads to some improvement in the mean climate, typical bias patterns characterized by a reversed zonal SST gradient at the equator and too warm SST in the Benguela upwelling region are mostly unchanged at a coarser vertical resolution.

Description: The Late Quaternary variability of the South Asian (or Indian) monsoon has been linked with glacial-interglacial and millennial scale climatic changes but past rainfall intensity in the river catchments draining into the Andaman Sea remains poorly constrained. Here we use radiogenic Sr, Nd, and Pb isotope compositions of the detrital clay-size fraction and clay mineral assemblages obtained from sediment core NGHP Site 17 in the Andaman Sea to reconstruct the variability of the South Asian monsoon during the past 60 kyr. Over this time interval eNd values changed little, generally oscillating between 27.3 and 25.3 and the Pb isotope signatures are essentially invariable, which is in contrast to a record located further northeast in the Andaman Sea. This indicates that the source of the detrital clays did not change significantly during the last glacial and deglaciation suggesting the monsoon was spatially stable. The most likely source region is the Irrawaddy river catchment including the Indo-Burman Ranges with a possible minor contribution from the Andaman Islands. High smectite/(illite1chlorite) ratios (up to 14), as well as low 87Sr/86Sr ratios (0.711) for the Holocene period indicate enhanced chemical weathering and a stronger South Asian monsoon compared to marine oxygen isotope stages 2 and 3. Short, smectite-poor intervals exhibit markedly radiogenic Sr isotope compositions and document weakening of the South Asian monsoon, which may have been linked to short-term northern Atlantic climate variability on millennial time scales.

Description: Denmark Strait Overflow Water (DSOW) supplies the densest contribution to North Atlantic Deep Water and is monitored at several locations in the subpolar North Atlantic. Hydrographic (temperature and salinity) and velocity time series from three multiple-mooring arrays at the Denmark Strait sill, at 180 km downstream (south of Dohrn Bank) and at a further 320 km downstream on the east Greenland continental slope near Tasiilaq (formerly Angmagssalik), were analyzed to quantify the variability and track anomalies in DSOW in the period 2007-2012. No long-term trends were detected in the time series, while variability on time scales from interannual to weekly was present at all moorings. The hydrographic time series from different moorings within each mooring array showed coherent signals, while the velocity fluctuations were only weakly correlated. Lagged correlations of anomalies between the arrays revealed a propagation from the sill of Denmark Strait to the Angmagssalik array in potential temperature with an average propagation time of 13 days, while the correlations in salinity were low. Entrainment of warm and saline Atlantic Water and fresher water from the East Greenland Current (via the East Greenland Spill Jet) can explain the whole range of hydrographic changes in the DSOW measured downstream of the sill. Changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of DSOW. Fresh anomalies found in downstream measurements of DSOW within the Deep Western Boundary Current can therefore not be attributed to Arctic climate variability in a straightforward way

Description: An empirical function is derived for predicting the rate-depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O2 and NO3− fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically-resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71 % of paired O2 and NO3− fluxes to within 50% of measured values. It further provides realistic geochemical concentration-depth profiles, NO3− penetration depths and apparent first-order POC mineralization rate constants. The model performs less well on the continental shelf due to the high heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr−1 of N and a POC burial rate of 107 ± 52 Tg yr−1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi-G type models and is suitable for non-steady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.

Description: We present measurements of pCO2, O2 concentration, biological oxygen saturation (ΔO2/Ar), and N2 saturation (ΔN2) in Southern Ocean surface waters during austral summer, 2010–2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chl a) concentrations in regions of frontal mixing and sea ice melt. pCO2 and ΔO2/Ar exhibited large spatial gradients (range 90 to 450 µatm and −10 to 60%, respectively) and covaried strongly with Chl a. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time scales of O2 and CO2 air-sea equilibration. We measured significant oceanic CO2 uptake, with a mean air-sea flux (~ −10 mmol m−2 d−1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean ΔN2 of +2.5%), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean ΔO2phys = 2.1%). Box model calculations were able to reproduce much of the spatial variability of ΔN2 and ΔO2phys along the cruise track, demonstrating significant effects of air-sea exchange processes (e.g., atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment-corrected surface ΔO2/Ar data, ranged from ~ −40 to 〉 300 mmol O2 m−2 d−1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and stratified regions of sea ice melt, reflecting physical controls on surface water light fields and nutrient availability.

Description: Siberian river water is a first-order contribution to the Arctic freshwater budget, with the Ob, Yenisey, and Lena supplying nearly half of the total surface freshwater flux. However, few details are known regarding where, when, and how the freshwater transverses the vast Siberian shelf seas. This paper investigates the mechanism, variability, and pathways of the fresh Kara Sea outflow through Vilkitsky Strait toward the Laptev Sea. We utilize a high-resolution ocean model and recent shipboard observations to characterize the freshwater-laden Vilkitsky Strait Current (VSC), and shed new light on the little-studied region between the Kara and Laptev Seas, characterized by harsh ice conditions, contrasting water masses, straits, and a large submarine canyon. The VSC is 10-20 km wide, surface intensified, and varies seasonally (maximum from August to March) and interannually. Average freshwater (volume) transport is 500 ± 120 km3 a-1 (0.53 ± 0.08 Sv), with a baroclinic flow contribution of 50-90%. Interannual transport variability is explained by a storage-release mechanism, where blocking-favorable summer winds hamper the outflow and cause accumulation of freshwater in the Kara Sea. The year following a blocking event is characterized by enhanced transports driven by a baroclinic flow along the coast that is set up by increased freshwater volumes. Eventually, the VSC merges with a slope current and provides a major pathway for Eurasian river water toward the western Arctic along the Eurasian continental slope. Kara (and Laptev) Sea freshwater transport is not correlated with the Arctic Oscillation, but rather driven by regional summer pressure patterns.

Description: Widespread seepage of methane from seafloor sediments offshore Svalbard close to the landward limit of the gas hydrate stability zone (GHSZ) may, in part, be driven by hydrate destabilization due to bottom water warming. To assess whether this methane reaches the atmosphere where it may contribute to further warming, we have undertaken comprehensive surveys of methane in seawater and air on the upper slope and shelf region. Near the GHSZ limit at ∼400 m water depth, methane concentrations are highest close to the seabed, reaching 825 nM. A simple box model of dissolved methane removal from bottom waters by horizontal and vertical mixing and microbially mediated oxidation indicates that ∼60% of methane released at the seafloor is oxidized at depth before it mixes with overlying surface waters. Deep waters are therefore not a significant source of methane to intermediate and surface waters; rather, relatively high methane concentrations in these waters (up to 50 nM) are attributed to isopycnal turbulent mixing with shelf waters. On the shelf, extensive seafloor seepage at 〈100 m water depth produces methane concentrations of up to 615 nM. The diffusive flux of methane from sea to air in the vicinity of the landward limit of the GHSZ is ∼4-20 μmol m-2 d-1, which is small relative to other Arctic sources. In support of this, analyses of mole fractions and the carbon isotope signature of atmospheric methane above the seeps do not indicate a significant local contribution from the seafloor source.

Description: We performed simulations with a global model of ocean biogeochemistry forced with orbitally driven anomalies of oceanic conditions for the mid-Holocene, known as Holocene climate optimum, to investigate natural variability in the eastern equatorial Pacific oxygen minimum zone (EEP OMZ). While the global mean temperature during the mid-Holocene was likely slightly higher than the 1961–1990 mean, the sea surface temperature in the EEP was slightly lower. Mid-Holocene oxygen concentrations in the EEP OMZ are generally increased, locally by up to 50%, and the EEP OMZ volume was, depending on definition of the OMZ threshold, at least 6% lower. These higher oxygen levels are the combined result of competing physical and biogeochemical processes. Our results imply that mechanisms for past changes in the EEP OMZ intensity and extension can differ from the global warming driven decline in oxygen levels observed for the recent decades and predicted for the future.

Description: Foulden Maar is a highly resolved maar lake deposit from the South Island of New Zealand comprising laminated diatomite punctuated by numerous diatomaceous turbidites. Basaltic clasts found in debris flow deposits near the base of the cored sedimentary sequence yielded two new ⁴⁰Ar/³⁹Ar dates of 24.51 ± 0.24 and 23.38 ± 0.24 Ma (2σ). The younger date agrees within error with a previously published ⁴⁰Ar/³⁹Ar date of 23.17 ± 0.19 Ma from a basaltic dyke adjacent to the maar crater. The diatomite is inferred to have been deposited over several tens of thousands of years in the latest Oligocene/earliest Miocene, and may have been coeval with the period of rapid glaciation and subsequent deglaciation of Antarctica known as the Mi-1 event. Sediment magnetic properties and SEM measurements indicate that the magnetic signal is dominated by pseudo-single domain pyrrhotite. The most likely source of detrital pyrrhotite is schist country rock fragments from the inferred tephra ring created by the phreatomagmatic eruption that formed the maar. Variations in magnetic mineral concentration indicate a decrease in erosional input throughout the depositional period, suggesting long-term (tens of thousands of years) environmental change in New Zealand in the latest Oligocene/earliest Miocene.

Description: A precise age for the collision of the Kohistan-Ladakh block with Eurasia along the Shyok suture zone (SSZ) is one key to understanding the accretionary history of Tibet and the tectonics of Eurasia during the India-Eurasia collision. Knowing the age of the SSZ also allows the suture to be used as a piercing line for calculating total offset along the Karakoram Fault, which effectively represents the SE border of the Tibetan Plateau and has played a major role in plateau evolution. We present a combined structural, geochemical, and geochronologic study of the SSZ as it is exposed in the Nubra region of India to test two competing hypotheses: that the SSZ is of Late Cretaceous or, alternatively, of Eocene age. Coarse-continental strata of the Saltoro Molasse, mapped in this area, contain detrital zircon populations suggestive of derivation from Eurasia despite the fact that the molasse itself is deposited unconformably onto Kohistan-Ladakh rocks, indicating that the molasse is postcollisional. The youngest population of detrital zircons in these rocks (approximately 92 Ma) and a U/Pb zircon date for a dike that cuts basal molasse outcrops (approximately 85 Ma) imply that deposition of the succession began in the Late Cretaceous. This establishes a minimum age for the SSZ and rules out the possibility of an Eocene collision between Kohistan-Ladakh and Eurasia. Our results support correlation of the SSZ with the Bangong suture zone in Tibet, which implies a total offset across the Karakoram Fault of approximately 130–190 km.

Description: Key Points: Daily snapshots of TIL strength; synoptic-Scale behavior of the TIL and shear/curl contributions to relative vorticity; TIL within ridges in midlatitude winter is stronger than polar summer TIL High-resolution GPS radio occultation temperature profiles from the COSMIC satellite mission (2007–2013) are used to obtain daily snapshots of the strength of the extratropical tropopause inversion layer (TIL). Its horizontal structure and day-to-day variability are linked to the synoptic situation at near-tropopause level. The strength of the TIL in cyclonic as well as anticyclonic conditions is investigated by separating relative vorticity into curl and shear terms. The analysis shows that the TIL has high zonal variability, and its strength is instantaneously adjusted to the synoptic situation at near-tropopause level. Our key finding is that the TIL within midlatitude ridges in winter is as strong as or stronger than the TIL in polar summer. The strongest TIL in anticyclonic conditions is related to the shear term, while the weaker TIL in cyclonic conditions is enhanced by the curl term.

Description: Seismicity and tectonic structure of the Alboran Sea were derived from a large amphibious seismological network deployed in the offshore basins and onshore in Spain and Morocco, an area where the convergence between the African and Eurasian plates causes distributed deformation. Crustal structure derived from local earthquake data suggests that the Alboran Sea is underlain by thinned continental crust with a mean thickness of about 20 km. During the 5 months of offshore network operation, a total of 229 local earthquakes were located within the Alboran Sea and neighboring areas. Earthquakes were generally crustal events, and in the offshore domain, most of them occurred at crustal levels of 2 to 15 km depth. Earthquakes in the Alboran Sea are poorly related to large-scale tectonic features and form a 20 to 40 km wide NNE-SSW trending belt of seismicity between Adra (Spain) and Al Hoceima (Morocco), supporting the case for a major left-lateral shear zone across the Alboran Sea. Such a shear zone is in accord with high-resolution bathymetric data and seismic reflection imaging, indicating a number of small active fault zones, some of which offset the seafloor, rather than supporting a well-defined discrete plate boundary fault. Moreover, a number of large faults known to be active as evidenced from bathymetry, seismic reflection, and paleoseismic data such as the Yusuf and Carboneras faults were seismically inactive. Earthquakes below the Western Alboran Basin occurred at 70 to 110 km depth and hence reflected intermediate depth seismicity related to subducted lithosphere.

Description: The scientific work during SO242/2 (28. August - 01. October 2015) was part of the JPIO Pilot Action ‘Ecological Aspects of Deep-Sea Mining‘. The main goal was to study the potential long-term ecological impact of anthropogenic disturbances on the deep-sea floor from mining polymetallic Mn-nodules. The expedition SO242 built on studies of the former German TUSCH projects (1989-1996) with four RV SONNE cruises to the DISCOL Experimental Area in the Peru Basin, South Pacific (7°S, 88.5° W; 4150 m water depth) between 1989 and 1996 (DISCOL and ATESEPP projects). The integrated ecological studies were carried out within and next to plough tracks of the original DISCOL experiment 1989, which mimicked seafloor disturbances similar to those occurring during nodule mining. Leg 242/2 extended the investigations started during leg 242/1 with a focus on biogeochemical and biological sampling and observations, including comparative studies of the composition of benthic communities (all size classes) as well as of ecosystem functions (remineralization rates, transfer of matter and energy in food webs, ecotoxicology). In addition, observations were continued of the physicochemical characteristics of the DEA, including the overlying benthic boundary layer. The nodule fields surrounding the DEA were used as references for undisturbed areas. A large proportion of the work was based on autonomous instruments and sensor modules that were deployed by means of ROV and lander systems. In addition, ROV-manipulated and telemetryguided instruments such as the Ocean Floor Observatory System were used for targeted sampling and surveys. Food-web experiments including some small-scale disturbances were carried out and sampled directly at the seafloor by the ROV.

Description: Observations show that the Equatorial Atlantic Zonal Mode (ZM) obeys similar physics to the El Niño Southern Oscillation (ENSO): positive Bjerknes and delayed negative feedbacks. This implies the ZM may be predictable on seasonal timescales, but models demonstrate little prediction skill in this region. In this study using different configurations of the Kiel Climate Model (KCM) exhibiting different levels of systematic error, we show that a reasonable simulation of the ZM depends on realistic representation of the mean state, i.e., surface easterlies along the equator, upward sloping thermocline to the east, with an equatorial SST cold tongue in the east. We further attribute the differences in interannual variability among the simulations to the individual components of the positive Bjerknes and delayed negative feedbacks. Differences in the seasonality of the variability are similarly related to the impact of seasonal biases on the Bjerknes feedback. Our results suggest that model physics must be enhanced to enable skillful seasonal predictions in the Tropical Atlantic Sector, although some improvement with regard to the simulation of Equatorial Atlantic interannual variability may be achieved by momentum flux correction. This pertains especially to the seasonal phase locking of interannual SST variability.

Description: We investigate how well a suite of regional climate models (RCMs) from the ENSEMBLES project represents the residual spatial dependence of daily precipitation. The study area we consider is a 200 km×200 km region in south central Norway, with RCMs driven by ERA-40 boundary conditions at a horizontal resolution of approximately 25 km×25 km. We model the residual spatial dependence with pair-copula constructions, which allows us to assess both the overall and tail dependence in precipitation, including uncertainty estimates. The selected RCMs reproduce the overall dependence rather well, though the discrepancies compared to observations are substantial. All models overestimate the overall dependence in the west-east direction. They also overestimate the upper tail dependence in the north-south direction during winter, and in the west-east direction during summer, whereas they tend to underestimate this dependence in the north-south direction in summer. Moreover, many of the climate models do not simulate the small-scale dependence patterns caused by the pronounced orography well. However, the misrepresented residual spatial dependence does not seem to affect estimates of high quantiles of extreme precipitation aggregated over a few grid boxes. The underestimation of the area-aggregated extreme precipitation is due mainly to the well-known underestimation of the univariate margins for individual grid boxes, suggesting that the correction of RCM biases in precipitation might be feasible

Description: Growing slowly, marine N2 fixers are generally expected to be competitive only where nitrogen (N) supply is low relative to that of phosphorus (P) with respect to the cellular N:P ratio (R) of non-fixing phytoplankton. This is at odds with observed high N2 fixation rates in the oligotrophic North Atlantic where the ratio of nutrients supplied to the surface is elevated in N relative to the average R (16:1). In this study, we investigate several mechanisms to solve this puzzle: iron limitation, phosphorus enhancement by preferential remineralization or stoichiometric diversity of phytoplankton, and dissolved organic phosphorus (DOP) utilization. Combining resource competition theory and a global coupled ecosystem-circulation model we find that the additional N and energy investments required for exo-enzymatic break-down of DOP gives N2 fixers a competitive advantage in oligotrophic P-starved regions. Accounting for this mechanism expands the ecological niche of N2-fixers also to regions where the nutrient supply is high in N relative to R, yielding, in our model, a pattern consistent with the observed high N2-fixation rates in the oligotrophic North Atlantic.

Description: Mesoscale eddies in Oxygen Minimum Zones (OMZ's) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent ‘natural tracer experiments’ with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus the heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO3−), nitrite (NO2−) and biogenic N2 associated with an anticyclonic eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO3− was nearly exhausted, we measured the highest δ15N values for both NO3− and NO2− (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ~40 µmol L−1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2− reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3− reduction (up to ~30‰ in the presence of NO2−). However, the overall ε for N-loss was calculated to be only ~13-14‰ (as compared to canonical values of ~20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO3− + NO2−) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε for NO3− reduction to NO2− and NO2− reduction to N2 of ~16-21‰ and ~12‰, respectively, when the effect of NO2− oxidation could be removed. These results, together with the relationship between N and O of NO3− isotopes and the difference in δ15N between NO3− and NO2-, confirm a role for NO2− oxidation in increasing the apparent ε associated with NO3− reduction. The lower ε for NO3− and NO2− reduction as well as N-loss calculated in this study could help reconcile the current imbalance in the global N budget if they are representative of OMZ N-loss.

Description: The observed geomorphology and calculated thermal histories of the Bhutan Himalaya provide an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a convergentmountain range. However, little consensus has been reached regarding the late Cenozoic history of the Bhutan Himalaya. Some researchers have argued that observed geologic relationships show slowing deformation rates, such that the range is decaying from a geomorphic perspective, while others see the range as growing and steepening. We suggest that a better understanding is possible through the integrated interpretation of geomorphic and thermochronometric data from the comparison of predictions from models of landscape evolution and thermal-kinematic models of orogenic systems. New thermochronometric data throughout Bhutan aremost consistent with a significant decrease in erosion rates, from2 to 3 km/Ma down to 0.1–0.3 km/Ma, around 6–4Ma. We interpret this pattern as a decrease in rock uplift rates due to the activation of contractional structures of the Shillong Plateau, an uplifted region approximately 100 km south of Bhutan. However, low-relief, fluvial landscapes throughout the Bhutanese hinterland record a late pulse of surface uplift likely due to a recent increase in rock uplift rates. Constraints from our youngest thermochronometers suggest that this increase in rock uplift and surface uplift occurred within the last 1.75Ma. These results imply that the dynamics of the Bhutan Himalaya and Shillong Plateau have been linked during the late Cenozoic, with structural elements of both regions active in variable ways and times over that interval.

Description: Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw ≥ 7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the Gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting data set shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink-type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater-sediment interface for sulfate, calcium, strontium, and magnesium result from rapid geochemical processes, AOM, and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane.

Description: Understanding of the evolution of fluid-fault interactions during earthquake cycles is a challenge that acoustic gas emission studies can contribute. A survey of the Sea of Marmara using a shipborne, multibeam echo sounder, with water column records, provided an accurate spatial distribution of offshore seeps. Gas emissions are spatially controlled by a combination of factors, including fault and fracture networks in connection to the Main Marmara Fault system and inherited faults, the nature and thickness of sediments (e.g., occurrence of impermeable or gas-bearing sediments and landslides), and the connectivity between the seafloor and gas sources, particularly in relation to the Eocene Thrace Basin. The relationship between seepage and fault activity is not linear, as active faults do not necessarily conduct gas, and scarps corresponding to deactivated fault strands may continue to channel fluids. Within sedimentary basins, gas is not expelled at the seafloor unless faulting, deformation, or erosional processes affect the sediments. On topographic highs, gas flares occur along the main fault scarps but are also associated with sediment deformation. The occurrence of gas emissions appears to be correlated with the distribution of microseismicity. The relative absence of earthquake-induced ground shaking along parts of the Istanbul-Silivri and Princes Islands segments is likely the primary factor responsible for the comparative lack of gas emissions along these fault segments. The spatiotemporal distribution of gas seeps may thus provide a complementary way to constrain earthquake geohazards by focusing the study on some key fault segments, e.g., the northern part of the locked Princes Islands segment.

Description: We examine the impact of sea surface temperature (SST) bias on inter-annual variability during boreal summer over the equatorial Atlantic using two suites of partially coupled model (PCM) experiments with and without surface heat flux correction. In the experiments, surface wind stress anomalies are specified from observations while the thermodynamic coupling between the atmospheric and oceanic components is still active as in the fully coupled model. The results show that the PCM can capture around 50% of the observed variability associated with the Atlantic Niño from 1958 to 2013, but only when the bias is substantially reduced using heat flux correction, with no skill otherwise. We further show that ocean dynamics explain a large part of the SST variability in the eastern equatorial Atlantic in both observations (50-60%) and the PCM experiments (50-70%) with heat flux correction, implying that the seasonal predictability potential may be higher than currently thought.

Description: Submarine landslides can cause damaging tsunamis, the height of which scales up with the volume of the displaced mass. The largest underwater landslides are far bigger than any landslides on land, and these submarine megaslides tend to occur on open continental slopes with remarkably low gradients of less than 2°. For geohazard assessments it is essential to understand what preconditions and triggers slope failure on such low gradients. Previous work has suggested that generation of high excess pore pressure due to rapid sediment deposition plays a key role in such failures. However, submarine slope failure also occurs where sedimentation rates are low (〈0.15 m/kyr), such as off northwest Africa. We use a fully coupled stress and fluid flow finite element model to test whether such low sedimentation rates can generate sufficient excess pore pressures to cause failure of a 2° slope. The sensitivity of overpressure generation and slope stability is assessed with respect to different sedimentation rates and patterns, sediment consolidation properties, and stratigraphic layer configurations. The simulations show that, in general, it is difficult to generate significant excess pore pressure if sediment accumulation is slow and the only pressure source. However, we identify a sediment compression behavior that can lead to submarine landslides in locations worldwide. Our results imply that compressibility is an important factor for the stability of low gradient continental slopes.

Description: The Baltic Sea is a marginal sea, located in a highly industrialized region in Central Northern Europe. Saltwater inflows from the North Sea and associated ventilation of the deep exert crucial control on the entire Baltic Sea ecosystem. This study explores the impact of anticipated sea level changes on the dynamics of those inflows. We use a numerical oceanic general circulation model covering both the Baltic and the North Sea. The model successfully retraces the essential ventilation dynamics throughout the period 1961–2007. A suite of idealized experiments suggests that rising sea level is associated with intensified ventilation as saltwater inflows become stronger, longer, and more frequent. Expressed quantitatively as a salinity increase in the deep central Baltic Sea, we find that a sea level rise of 1 m triggers a saltening of more than 1 PSU. This substantial increase in ventilation is the consequence of the increasing cross section in the Danish Straits amplified by a reduction of vertical mixing

Description: Current estimates suggest that more than 60% of the global seafloor are covered by millions of abyssal hills and mountains. These features introduce spatial fluid-dynamic granularity whose influence on deep-ocean sediment biogeochemistry is unknown. Here we compare biogeochemical surface-sediment properties from a fluid-dynamically well-characterized abyssal hill and upstream plain: (1) In hill sediments, organic-carbon and -nitrogen contents are only about half as high as on the plain while proteinaceous material displays less degradation; (2) on the hill, more coarse-grained sediments (reducing particle surface area) and very variable calcite contents (influencing particle surface charge) are proposed to reduce the extent, and influence compound-specificity, of sorptive organic-matter preservation. Further studies are needed to estimate the representativeness of the results in a global context. Given millions of abyssal hills and mountains, their integrative influence on formation and composition of deep-sea sediments warrants more attention.

Description: The cruise SO239 EcoResponse took place between 11th of March and 30th of April 2015. Aim of the cruise was to study the biodiversity, geological and geochemical settings across a productivity gradient in the CCZ. Also to study the genetic connectivity between distant deepsea populations, to compare the fauna from seamounts with the fauna living attached to the nodules and to sample an APEI for the first time. The AUV was used to test the usefulness of photographic and side-scan sonar survey for future monitoring of mining activities. We visited 6 working areas in 4 ISA contractor areas (from BGR, IOM, DEME, Ifremer) and the APEI number 3. On all sites sediment samples were taken with the Multicorer, Box-Corer and Gravity Corer. Additionally epibenthic fauna and scavengers were sampled with the Epibenthic Sledge and Amphipod Trap, respectively. CTD cast and water samples were taken on each of the areas. An AUV was used to perform detail bathymetric mapping in addition to side-scan sonar and photographic surveys. ROV was used to sample megafauna organisms, to sample sediments inside dredge tracks, and to perform video transect.

Description: The Southern Ocean (SO) is an important CO2 reservoir, some of which enters via the production, sinking, and remineralization of organic matter. Recent work suggests that the fraction of production that sinks is inversely related to production in the SO, a suggestion that we confirm from 20 stations in the Scotia Sea. The efficiency with which exported material is transferred to depth (transfer efficiency) is believed to be low in high-latitude systems. However, our estimates of transfer efficiency are bimodal, with stations in the seasonal ice zone showing intense losses and others displaying increases in flux with depth. Zooplankton fecal pellets dominated the organic carbon flux and at stations with transfer efficiency 〉100% fecal pellets were brown, indicative of fresh phytodetritus. We suggest that active flux mediated by zooplankton vertical migration and the presence of sea ice regulates the transfer of organic carbon into the oceans interior in the Southern Ocean.

Description: Information about oceanic nitrate is crucial for making inferences about marine biological production and the efficiency of the biological carbon pump. While there are no optical properties that allow direct estimation of inorganic nitrogen, its correlation with other biogeochemical variables may permit its inference from satellite data. Here we report a new method for estimating monthly mean surface nitrate concentrations employing local multiple linear regressions on a global 1° by 1° resolution grid, using satellite-derived sea surface temperature, chlorophyll, and modeled mixed layer depth. Our method is able to reproduce the interannual variability of independent in situ nitrate observations at the Bermuda Atlantic Time Series, the Hawaii Ocean Time series, the California coast, and the southern New Zealand region. Our new method is shown to be more accurate than previous algorithms and thus can provide improved information on temporal and spatial nutrient variations beyond the climatological mean at regional and global scales.

Description: Arctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234-thorium based estimates of the export flux of POC to onboard incubation-based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE-ratio) showed large variability (0.09 ± 0.19–1.3 ± 0.3). The highest ThE-ratio (1.3 ± 0.3) was recorded in a mono-specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE-ratios (0.1 ± 0.1–0.5 ± 0.2), while mixed and/or prebloom communities showed lower ThE-ratios (0.10 ± 0.03–0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralized in the upper mesopelagic (〈300 m). Our results suggest that blooming diatoms and P. pouchetii can export a significant fraction of their biomass below the surface layer (100 m) in the open Arctic Ocean. Also, we show evidence that the material sinking from a P. pouchetii bloom may be remineralized (〉100 m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings.

Description: Large amounts of methane hydrate locked up within marine sediments are vulnerable to climate change. Changes in bottom water temperatures may lead to their destabilization and the release of methane into the water column or even the atmosphere. In a multimodel approach, the possible impact of destabilizing methane hydrates onto global climate within the next century is evaluated. The focus is set on changing bottom water temperatures to infer the response of the global methane hydrate inventory to future climate change. Present and future bottom water temperatures are evaluated by the combined use of hindcast high-resolution ocean circulation simulations and climate modeling for the next century. The changing global hydrate inventory is computed using the parameterized transfer function recently proposed by Wallmann et al. (2012). We find that the present-day world's total marine methane hydrate inventory is estimated to be 1146Gt of methane carbon. Within the next 100years this global inventory may be reduced by ∼0.03% (releasing ∼473Mt methane from the seafloor). Compared to the present-day annual emissions of anthropogenic methane, the amount of methane released from melting hydrates by 2100 is small and will not have a major impact on the global climate. On a regional scale, ocean bottom warming over the next 100years will result in a relatively large decrease in the methane hydrate deposits, with the Arctic and Blake Ridge region, offshore South Carolina, being most affected.

Description: The cyanobacterium Trichodesmium is responsible for a significant proportion of the annual "new" nitrogen introduced into the global ocean. Despite being arguably the best studied marine diazotroph, the factors controlling the distribution and growth of Trichodesmium remain a subject of debate, with sea surface temperature, the partial pressure of CO2, and nutrients including iron (Fe) and phosphorus (P), all suggested to be important. Synthesizing data from seven cruises collectively spanning large temporal and spatial scales across the Atlantic Ocean, including two previously unreported studies crossing the largely undersampled South Atlantic gyre, we assessed the relationship between proposed environmental drivers and both community N2 fixation rates and the distribution of Trichodesmium. Simple linear regression analysis would suggest no relationship between any of the sampled environmental variables and N2 fixation rates. However, considering the concentrations of iron and phosphorus together within a simplified resource-ratio framework, illustrated using an idealized numerical model, indicates the combined effects these nutrients have on Trichodesmium and broader diazotroph biogeography, alongside the reciprocal maintenance of different biogeographic provinces of the (sub)tropical Atlantic in states of Fe or P oligotrophy by diazotrophy. The qualitative principles of the resource-ratio framework are argued to be consistent with both the previously described North-South Atlantic contrast in Trichodesmium abundance and the presence and consequence of a substantial non-Trichodesmium diazotrophic community in the western South Atlantic subtropical gyre. A comprehensive, observation-based explanation of the interactions between Trichodesmium and the wider diazotrophic community with iron and phosphorus in the Atlantic Ocean is thus revealed.

Description: Literature data on benthic dissolved iron (DFe) fluxes (µmol m−2 d−1), bottom water oxygen concentrations (O2BW, μM), and sedimentary carbon oxidation rates (COX, mmol m−2 d−1) from water depths ranging from 80 to 3700 m were assembled. The data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic DFe fluxes: inline image where γ (= 170 µmol m−2 d−1) is the maximum flux for sediments at steady state located away from river mouths. This simple function unifies previous observations that COX and O2BW are important controls on DFe fluxes. Upscaling predicts a global DFe flux from continental margin sediments of 109 ± 55 Gmol yr−1, of which 72 Gmol yr−1 is contributed by the shelf (〈200 m) and 37 Gmol yr−1 by slope sediments (200–2000 m). The predicted deep-sea flux (〉2000 m) of 41 ± 21 Gmol yr−1 is unsupported by empirical data. Previous estimates of benthic DFe fluxes derived using global iron models are far lower (approximately 10–30 Gmol yr−1). This can be attributed to (i) inadequate treatment of the role of oxygen on benthic DFe fluxes and (ii) improper consideration of continental shelf processes due to coarse spatial resolution. Globally averaged DFe concentrations in surface waters simulated with the intermediate-complexity University of Victoria Earth System Climate Model were a factor of 2 higher with the new function. We conclude that (i) the DFe flux from marginal sediments has been underestimated in the marine iron cycle and (ii) iron scavenging in the water column is more intense than currently presumed.

Description: Dissolved organic nitrogen (DON) and phosphorus (DOP) represent the most abundant form of their respective nutrient pool in the surface layer of the oligotrophic oceans and play an important role in nutrient cycling and productivity. Since DOP is generally more labile than DON, it provides additional P that may stimulate growth of nitrogen-fixing diazotrophs that supply fixed nitrogen to balance denitrification in the ocean. In this study, we introduce semirecalcitrant components of DON and DOP as state variables in an existing global ocean-atmosphere-sea ice-biogeochemistry model of intermediate complexity to assess their impact on the spatial distribution of nitrogen fixation and the size of the marine fixed nitrogen inventory. Large-scale surface data sets of global DON and Atlantic Ocean DOP are used to constrain the model. Our simulations suggest that both preferential DOP remineralization and phytoplankton DOP uptake are important "non-Redfield" processes (i.e., deviate from molar N:P=16) that need to be accounted for to explain the observed patterns of DOP. Additional non-Redfield DOP sensitivity experiments testing dissolved organic matter (DOM) production rate uncertainties that best reproduce the observed spatial patterns of DON and DOP stimulate additional nitrogen fixation that increases the size of the global marine fixed nitrogen inventory by 4.7±1.7% compared to the simulation assuming Redfield DOM stoichiometry that underestimates the observed nitrogen inventory. The extra 8Tgyr-1 of nitrogen fixation stimulated in the Atlantic Ocean is mainly responsible for this increase due to its large spatial separation from water column denitrification, which buffers any potential nitrogen surplus in the Pacific Ocean. Our study suggests that the marine fixed nitrogen budget is sensitive to non-Redfield DOP dynamics because access to the relatively labile DOP pool expands the ecological niche for nitrogen-fixing diazotrophs.

Description: Spaceborne sea surface salinity (SSS) measurements provided by the European Space Agency's (ESA) “Soil Moisture and Ocean Salinity” (SMOS) and the National Aeronautical Space Agency's (NASA) “Aquarius/SAC-D” missions, covering the period from May 2012 to April 2013, are compared against in situ salinity measurements obtained in the northern North Atlantic between 20°N and 80°N. In cold water, SMOS SSS fields show a temperature-dependent negative SSS bias of up to −2 g/kg for temperatures 〈5°C. Removing this bias significantly reduces the differences to independent ship-based thermosalinograph data but potentially corrects simultaneously also other effects not related to temperature, such as land contamination or radio frequency interference (RFI). The resulting time-mean bias, averaged over the study area, amounts to 0.1 g/kg. A respective correction applied previously by the Jet Propulsion Laboratory to the Aquarius data is shown here to have successfully removed an SST-related bias in our study area. For both missions, resulting spatial structures of SSS variability agree very well with those available from an eddy-resolving numerical simulation and from Argo data and, additionally they also show substantial salinity changes on monthly and seasonal time scales. Some fraction of the root-mean-square difference between in situ, and SMOS and Aquarius data (approximately 0.9 g/kg) can be attributed to short time scale ocean processes, notably at the Greenland shelf, and could represent associated sampling errors there.

Description: The western boundary current system off Brazil is a key region for diagnosing variations of the Atlantic meridional overturning circulation (AMOC) and the southern subtropical cell. In July 2013 a mooring array was installed off the coast at 11°S similar to an array installed between 2000 and 2004 at the same location. Here we present results from two research cruises and the first 10.5 months of moored observations in comparison to the observations a decade ago. Average transports of the North Brazil Undercurrent and the Deep Western Boundary Current (DWBC) have not changed between the observational periods. DWBC eddies that are predicted to disappear with a weakening AMOC are still present. Upper layer changes in salinity and oxygen within the last decade are consistent with an increased Agulhas leakage, while at depths water mass changes are likely related to changes in the North Atlantic as well as tropical circulation changes.

Description: As a result of the raising CO2-emissions and the resultant ocean acidification (decreasing pH and carbonate ion concentration), the impact on marine organism that build their skeletons and protective shells with calcium carbonate (e.g., mollusks, sea urchins, coccolithophorids, and stony corals) becomes more and more detrimental. In the last few years, many experiments with tropical reef building corals have shown, that a lowering of the carbonate ion concentration significantly reduces calcification rates and therefore growth (e.g., Gattuso et al. 1999; Langdon et al. 2000, 2003; Marubini et al. 2001, 2002). In the middle of this century, many tropical coral reefs may well erode faster than they can rebuild. Cold-water corals are living in an environment (high geographical latitude, cold and deep waters) already close to a critical carbonate ion concentration below calcium carbonate dissolves. Actual projections indicate that about 70% of the currently known Lophelia reef structures will be in serious danger until the end of the century (Guinotte et al. 2006). Therefore L. pertusa was cultured at GEOMAR to determine its long-term response to ocean acidification. Our work has revealed that – unexpectedly and controversially to the majority of warm-water corals – this species is potentially able to cope with elevated concentrations of CO2. Whereas short-term (1 week) high CO2 exposure resulted in a decline of calcification by 26-29 % for a pH decrease of 0.1 units and net dissolution of calcium carbonate, L. pertusa was capable to acclimate to acidified conditions in long-term (6 months) incubations, leading to slightly enhanced rates of calcification (Form & Riebesell, 2012). But all these studies were carried out in the laboratory under controlled conditions without considering natural variability and ecosystem interactions with the associated fauna. Moreover, only very little is known about the nutrition (food sources and quantity) of cold-water corals in their natural habitat. In a multifactorial laboratory study during BIOACID phase II we could show that food availability is one of the key drivers that promote the capability of these organisms to withstand environmental pressures such as alterations in the carbonate chemistry and temperature (Büscher, Form & Riebesell, in prep.). To take into account the influences of natural fluctuations and interactions (e.g. bioerosion), we aim to merge in-situ results from the two research cruises POS455 and POS473 with laboratory experimental studies for a comprehensive understanding of likely ecosystem responses under past, present and future environmental conditions.

Description: Variability of mid-latitude blocking in the boreal winter northern hemisphere is investigated for the period 1960/61 to 2001/02 by means of relaxation experiments with the model of the European Center for Medium-Range Weather Forecasts. It is shown that there is pronounced interannual and decadal variability in blocking, especially over the Eurasian continent, consistent with previous studies. The relaxation experiments show that realistic variability in the tropics can account for a significant part of observed interannual blocking variability, but also that about half of the observed variability can only be explained by extratropical tropospheric variability. On the quasi-decadal time scale, extratropical sea surface temperature and sea-ice, in addition to tropical variability, play a more important role. The stratosphere, which has been shown to influence interannual variability of the North Atlantic Oscillation in previous studies, has no significant influence on blocking according to our analysis.

Description: This manual represents a review of the potential sources and methods to be applied when providing prior information to Bayesian stock assessments and marine risk analysis. The manual is compiled as a product of the EC Framework 7 ECOKNOWS project (www.ecoknows.eu). The manual begins by introducing the basic concepts of Bayesian inference and the role of prior information in the inference. Bayesian analysis is a mathematical formalization of a sequential learning process in a probabilistic rationale. Prior information (also called ”prior knowledge”, ”prior belief”, or simply a ”prior”) refers to any existing relevant knowledge available before the analysis of the newest observations (data) and the information included in them. Prior information is input to a Bayesian statistical analysis in the form of a probability distribution (a prior distribution) that summarizes beliefs about the parameter concerned in terms of relative support for different values. Apart from specifying probable parameter values, prior information also defines how the data are related to the phenomenon being studied, i.e. the model structure. Prior information should reflect the different degrees of knowledge about different parameters and the interrelationships among them. Different sources of prior information are described as well as the particularities important for their successful utilization. The sources of prior information are classified into four main categories: (i) primary data, (ii) literature, (iii) online databases, and (iv) experts. This categorization is somewhat synthetic, but is useful for structuring the process of deriving a prior and for acknowledging different aspects of it. A hierarchy is proposed in which sources of prior information are ranked according to their proximity to the primary observations, so that use of raw data is preferred where possible. This hierarchy is reflected in the types of methods that might be suitable – for example, hierarchical analysis and meta-analysis approaches are powerful, but typically require larger numbers of observations than other methods. In establishing an informative prior distribution for a variable or parameter from ancillary raw data, several steps should be followed. These include the choice of the frequency distribution of observations which also determines the shape of prior distribution, the choice of the way in which a dataset is used to construct a prior, and the consideration related to whether one or several datasets are used. Explicitly modelling correlations between parameters in a hierarchical model can allow more effective use of the available information or more knowledge with the same data. Checking the literature is advised as the next approach. Stock assessment would gain much from the inclusion of prior information derived from the literature and from literature compilers such as FishBase (www.fishbase.org), especially in data-limited situations. The reader is guided through the process of obtaining priors for length–weight, growth, and mortality parameters from FishBase. Expert opinion lends itself to data-limited situations and can be used even in cases where observations are not available. Several expert elicitation tools are introduced for guiding experts through the process of expressing their beliefs and for extracting numerical priors about variables of interest, such as stock–recruitment dynamics, natural mortality, maturation, and the selectivity of fishing gears. Elicitation of parameter values is not the only task where experts play an important role; they also can describe the process to be modelled as a whole. Information sources and methods are not mutually exclusive, so some combination may be used in deriving a prior distribution. Whichever source(s) and method(s) are chosen, it is important to remember that the same data should not be used twice. If the 2 | ICES Cooperative Research Report No. 328 plan is to use the data in the analysis for which the prior distribution is needed, then the same data cannot be used in formulating the prior. The techniques studied and proposed in this manual can be further elaborated and fine-tuned. New developments in technology can potentially be explored to find novel ways of forming prior distributions from different sources of information. Future research efforts should also be targeted at the philosophy and practices of model building based on existing prior information. Stock assessments that explicitly account for model uncertainty are still rare, and improving the methodology in this direction is an important avenue for future research. More research is also needed to make Bayesian analysis of non-parametric models more accessible in practice. Since Bayesian stock assessment models (like all other assessment models) are made from existing knowledge held by human beings, prior distributions for parameters and model structures may play a key role in the processes of collectively building and reviewing those models with stakeholders. Research on the theory and practice of these processes will be needed in the future.

Description: We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ~150 km. At this depth, the mantle resistivity decreases to values of ~100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20-30km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.

Description: Climate model resolution can affect both the climate change signal and present-day representation of extreme precipitation. The need to parametrize convective processes raises questions about how well the response to warming of convective precipitation extremes is captured in such models. In particular, coastal precipitation extremes can be sensitive to sea surface temperature (SST) increase. Taking a recent coastal precipitation extreme as a showcase example, we explore the added value of convection-permitting models by comparing the response of the extreme precipitation to a wide range of SST forcings in an ensemble of regional climate model simulations using parametrized and explicit convection. Compared at the same spatial scale, we find that the increased local intensities of vertical motion and precipitation in the convection-permitting simulations play a crucial role in shaping a strongly nonlinear extreme precipitation response to SST increase, which is not evident when convection is parametrized. In the convection-permitting simulations, SST increase causes precipitation intensity to increase only until a threshold is reached, beyond which further SST increase does not enhance the precipitation. This flattened response results from an improved representation of convective downdrafts and near-surface cooling, which damp the further intensification of precipitation by stabilizing the lower troposphere locally and also create cold-pools that cause subsequent convection to be triggered at sea, rather than by the coastal orography. These features are not well represented in the parametrized convection simulations, resulting in precipitation intensity having a much more linear response to increasing SSTs

Description: The major geochemical characteristics of Red Sea brine are summarized for 11 brine-filled deeps located along the central graben axis between 19°N and 27°N. The major element composition of the different brine pools is mainly controlled by variable mixing situations of halite-saturated solution (evaporite dissolution) with Red Sea deep water. The brine chemistry is also influenced by hydrothermal water/rock interaction, whereas magmatic and sedimentary rock reactions can be distinguished by boron, lithium, and magnesium/calcium chemistry. Moreover, hydrocarbon chemistry (concentrations and δ 13 C data) of brine indicates variable injection of light hydrocarbons from organic source rocks and strong secondary (bacterial or thermogenic) degradation processes. A simple statistical cluster analysis approach was selected to look for similarities in brine chemistry and to classify the various brine pools, as the measured chemical brine compositions show remarkably strong concentration variations for some elements. The cluster analysis indicates two main classes of brine. Type I brine chemistry (Oceanographer and Kebrit Deeps) is controlled by evaporite dissolution and contributions from sediment alteration. The Type II brine (Suakin, Port Sudan, Erba, Albatross, Discovery, Atlantis II, Nereus, Shaban, and Conrad Deeps) is influenced by variable contributions from volcanic/ magmatic rock alteration. The chemical brine classification can be correlated with the sedimentary and tectonic setting of the related depressions. Type I brine-filled deeps are located slightly off-axis from the central Red Sea graben. A typical " collapse structure formation " which has been defined for the Kebrit Deep by evaluating seismic and geomorphological data probably corresponds to our Type I brine. Type II brine located in depressions in the northern Red Sea (i.e., Conrad and Shaban Deeps) could be correlated to " volcanic intrusion-/extrusion-related " deep formation. The chemical indications for hydrothermal influence on Conrad and Shaban Deep brine can be related to brines from the multi-deeps region in the central Red Sea, where volcanic/magmatic fluid/rock interaction is most obvious. The strongest hydrothermal influence is observed in Atlantis II brine (central multi-deeps region), which is also the hottest Red Sea brine body in 2011 (*68.2 °C).

Description: The Atlantis II Deep of the Red Sea hosts the largest known hydrothermal ore deposit on the ocean floor and the only modern analog of brine pool-type metal deposition. The deposit consists mainly of chemical-clastic sediments with input from basin-scale hydrothermal and detrital sources. A characteristic feature is the millimeter-scale layering of the sediments, which bears a strong resemblance to banded iron formation (BIF). Quantitative assessment of the mineralogy based on relogging of archived cores, detailed petrography, and sequential leaching experiments shows that Fe-(oxy)hydroxides, hydrothermal carbonates, sulfides, and authigenic clays are the main “ore” minerals. Mn-oxides were mainly deposited when the brine pool was more oxidized than it is today, but detailed logging shows that Fe-deposition and Mn-deposition also alternated at the scale of individual laminae, reflecting short-term fluctuations in the Lower Brine. Previous studies underestimated the importance of nonsulfide metal-bearing components, which formed by metal adsorption onto poorly crystalline Si-Fe-OOH particles. During diagenesis, the crystallinity of all phases increased, and the fine layering of the sediment was enhanced. Within a few meters of burial (corresponding to a few thousand years of deposition), biogenic (Ca)-carbonate was dissolved, manganosiderite formed, and metals originally in poorly crystalline phases or in pore water were incorporated into diagenetic sulfides, clays, and Fe-oxides. Permeable layers with abundant radiolarian tests were the focus for late-stage hydrothermal alteration and replacement, including deposition of amorphous silica and enrichment in elements such as Ba and Au.

Description: The age of the subducting Nazca Plate off Chile increases northward from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaíso (32°S). Age-related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental fore arc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under fore arc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water-releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust, and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the fore arc is only about one fourth of the latter. This high variability over a subduction zone of 〈1500 km length shows that it is insufficient to consider subduction zones as uniform entities in global estimates of subduction zone fluxes.

Description: High-resolution magnetic surveys have been acquired over the partially sedimented Palinuro massive sulfide deposits in the Aeolian volcanic arc, Tyrrhenian Sea. Surveys flown close to the seafloor using an autonomous underwater vehicle (AUV) show that the volcanic-arc-related basalt-hosted hydrothermal site is associated with zones of lower magnetization. This observation reflects the alteration of basalt affected by hydrothermal circulation and/or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and/or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids. To discriminate among these inferences, estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area—the stockwork—we use high-resolution vector magnetic data acquired by the AUV Abyss (GEOMAR) above a crater-shaped depression hosting a weakly active hydrothermal site. Our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones. Thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit, forming lobe-shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones.

Description: High-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites.

Description: The cause of massive blooms of Ethmodiscus rex laminated diatom mats (LDMs) in the eastern Philippine Sea (EPS) during the Last Glacial Maximum (LGM) remains uncertain. In order to better understand the mechanism of formation of E. rex LDMs from the perspective of dissolved silicon (DSi) utilization, we determined the silicon isotopic composition of single E. rex diatom frustules (δ30SiE. rex) from two sediment cores in the Parece Vela Basin of the EPS. In the study cores, δ30SiE. rex varies from −1.23‰ to −0.83‰ (average −1.04‰), a range that is atypical of marine diatom δ30Si and that corresponds to the lower limit of reported diatom δ30Si values of any age. A binary mixing model (upwelled silicon versus eolian silicon) accounting for silicon isotopic fractionation during DSi uptake by diatoms was constructed. The binary mixing model demonstrates that E. rex dominantly utilized DSi from eolian sources (i.e., Asian dust) with only minor contributions from upwelled seawater sources (i.e., advected from Subantarctic Mode Water, Antarctic Intermediate Water, or North Pacific Intermediate Water). E. rex utilized only ~24% of available DSi, indicating that surface waters of the EPS were eutrophic with respect to silicon during the LGM. Our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool, thus revising previously proposed models for the formation of E. rex LDMs.

Description: We use surface air temperature to evaluate the decadal forecast skill of the fully coupled Max Planck Institut Earth System Model (MPI-ESM) initialized using only surface wind stress applied to the ocean component of the model (Modini: Model initialization by partially coupled spin-up). Our analysis shows that the greenhouse gas forcing alone results in a significant forecast skill on the 2–5 and 6–9 year range even for uninitialized hindcasts. For the first forecast year, the forecast skill of Modini is generally comparable with previous initialization procedures applied to MPI-ESM. But only Modini is able to generate a significant skill (correlation) in the tropical Pacific for a 2–5 year (and to a lesser extent for a 6–9 year) hindcast. Modini is also better able to capture the observed hiatus in global warming in hindcast mode than the other methods. Finally, we present forecasts for 2015 and the average of years 2016–2019 and 2020–2024, predicting an end to the hiatus.

Description: Ocean–atmosphere interactions play a key role in climate variability on a wide range of time scales from seasonal to decadal and longer. The extratropical oceans are thought to exert noticeable feedbacks on the atmosphere especially on decadal and longer time scales, yet the large-scale atmospheric response to anomalous extratropical sea surface temperature (SST) is still under debate. Here we show, by means of dedicated high-resolution atmospheric model experiments, that sufficient daily variability in the extratropical background SST needs to be resolved to force a statistically significant large-scale atmospheric response to decadal North Pacific SST anomalies associated with the Pacific Decadal Oscillation (PDO), which is consistent with observations. The large-scale response is mediated by atmospheric eddies. This implies that daily extratropical SST fluctuations must be simulated by the ocean components and resolved by the atmospheric components of global climate models to enable realistic simulation of decadal North Pacific sector climate variability.

Description: The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous.

Description: A global ocean model with 1/12∘ horizontal resolution is used to assess the seasonal cycle of surface Eddy Kinetic Energy (EKE). The model reproduces the salient features of the observed mean surface EKE, including amplitude and phase of its seasonal cycle in most parts of the ocean. In all subtropical gyres of the Pacific and Atlantic, EKE peaks in summer down to a depth of ∼350 m, below which the seasonal cycle is weak. Investigation of the possible driving mechanisms reveals the seasonal changes in the thermal interactions with the atmosphere to be the most likely cause of the summer maximum of EKE. The development of the seasonal thermocline in spring and summer is accompanied by stronger mesoscale variations in the horizontal temperature gradients near the surface which corresponds, by thermal wind balance, to an intensification of mesoscale velocity anomalies towards the surface.

Description: The timing of initiation of human impacts on the global climate system is actively debated. Anthropogenic effects on the global climate system are evident since the Industrial Revolution, but humans may have altered biomass burning, and hence the climate system, for millennia. We use the specific biomarker levoglucosan to produce the first high-temporal resolution hemispheric reconstruction of Holocene fire emissions inferred from ice core analyses. Levoglucosan recorded in the Greenland North Greenland Eemian ice core significantly increases since the last glacial, resulting in a maximum around ~2.5 ka and then decreasing until the present. Here we demonstrate that global climate drivers fail to explain late Holocene biomass burning variations and that the levoglucosan maximum centered on ~2.5 ka may be due to anthropogenic land clearance.

Description: The West Spitsbergen Current, which flows northward along the western Svalbard continental slope, transports warm and saline Atlantic water (AW) into the Arctic Ocean. A combined analysis of highresolution seismic images and hydrographic sections across this current has uncovered the oceanographic processes involved in horizontal and vertical mixing of AW. At the shelf break, where a strong horizontal temperature gradient exists east of the warmest AW, isopycnal interleaving of warm AW and surrounding colder waters is observed. Strong seismic reflections characterize these interleaving features, with a negative polarity reflection arising from an interface of warm water overlying colder water. A seismic-derived sound speed image reveals the extent and lateral continuity of such interleaving layers. There is evidence of obliquely aligned internal waves emanating from the slope at 450–500 m. They follow the predicted trajectory of internal S2 tidal waves and can promote vertical mixing between Atlantic and Arctic-origin waters.

Description: International Workshop: Applications and Perspectives of Cavity Enhanced Optical Detection Techniques in Ocean Sciences; Kiel, Germany, 20–21 April 2015

Description: Oxygen minimum zones are increasing, yet the effects of these zones on the genetic composition of marine fish stocks has been neglected. We assessed the combined effects of stock size and structure, and the prevailing oxygen situation, on Eastern Baltic cod (ICES SD25) genetic diversity. For this purpose, we used an integrative long-term otolith sample and data series (1995-2013) to (1) calculate the approximate number of females with surviving eggs in a given year, i.e., contributing to reproduction (n F), and (2) the annually resolved cohort mean allelic richness as proxy of genetic diversity, based on 12 microsatellite markers. Cohort mean allelic richness showed strong year-to-year fluctuations though no permanent decline. Importantly, it was highly correlated with n F, but with an unexpected 1.5 year time lag that may be an artefact of Eastern Baltic cod ageing problems. Our findings indicated that environmental pressure can effect rapid alterations in exploited fish stock genetic composition, and pointed to the importance of large females for Eastern Baltic cod reproduction during stagnation periods. Considering the importance of standing genetic variation for the evolutionary potential of populations, this is relevant for projections of the future state of cod stocks under global change.

Description: Cruise SO242/1 ran from 28 July to 25 August 2015 starting and ending in Guayaquil, Ecuador. In total, 40 scientists from five European countries took part in this cruise of the JPIO project ‚Ecological Aspects of Deep‐Sea Mining‘ to study the ecological long‐term impact of deep sea disturbances. The working area, the DISCOL area in the Peru Basin, was ploughed in 1989 and thoroughly studied in the years thereafter, the last cruise, SO106, happened in 1996. SO242/2 aimed at mapping the DISCOL Experimental Area (DEA) in great detail using ship‐based and AUV‐based hydroacoustic and optical methods. To see changes between differently disturbed areas and study the possible recovery of the ecosystem, biological sampling occurred with TV‐guided multi‐coring (MUC), box coring (BC) and epi‐benthic sledge tracks (EBS). Additional biological sampling for scavenging animals occurred with baited Amphipod‐Traps within and further outside the DEA. For geochemical sampling, multi‐, box and gravity coring (BC) was used. Two lander systems equipped with physical sensors such as ADCPs and CTDs were used for current measurements and to monitor sediment plume dispersal created by the EBS. Additional visual studies of the fauna distribution occurred with camera tows (OFOS). Five main sampling areas were selected, two within the DEA targeting heavily disturbed (ploughed) locations and three reference areas 3 to 4 nmi outside the DEA. All five areas had been sampled in the past and can be directly compared concerning ecological changes. Despite a four‐day break due to medical reasons the work program could almost be completed. Four of the working areas were at least sampled with five BCs and MUCs each, and one GC. Box coring could not be performed in the western reference area. In total, 5 CTDs, 27 MUCs, 25 BCs, 7 GCs, 8 EBSs, 5 Amphipod‐Traps, 6 lander deployments and 6 OFOS tracks were successfully undertaken and one thermistor mooring was deployed. In addition, 16 AUV dives clearly showed that plough marks are still well visible after 26 years. There is a slight sediment cover next to the plough tracks, but first analyses of the faunal distribution show that the sessile fauna has not yet recolonised the tracks. Stalked sponges, corals and anemones can be found outside the tracks but still within the DEA. Their distribution patterns inside the DEA do not vary clearly from those on reference sites. The Mn‐nodule distribution is not homogenous; there are areas inside the DEA that do not have nodules at the seafloor surface; they are typically linked to depressions that show low backscatter intensity in the AUV side scan sonar data pointing towards less dense sediment infill. In gravity cores, nodules could be recovered even in 9m sediment depth, finding several more or less intact nodules throughout the entire sediment column was common. Water current measurements show slow currents (max. 6cm/s) and a strong tide‐influenced current direction, whereas no general direction could be observed. Two ‘disturbance experiments’ demonstrated that sediment plumes can be monitored using high frequency ADCPs (1200 kHz). The disturbance by the EBS created a sediment plume that stayed close to the seafloor. First analyses of current trajectories showed that the sediment resettled rather quickly. It became clear that plume behaviour during large‐scale mining cannot be extrapolated from these small‐scale and short‐term experiments. In resume, cruise SO242/1 was very successful and research should continue in the DEA area, which is undoubtedly the best studied long‐term deep sea disturbance site in the ocean.

Description: The subpolar North Atlantic (SPNA) is important in the global carbon cycle because of the deep water ventilation processes that lead to both high uptake of atmospheric CO2 and large inventories of anthropogenic CO2 (Cant). Thus, it is crucial to understand its response to increasing anthropogenic pressures. In this work, the budgets of dissolved inorganic carbon (DIC), Cant and natural DIC (DICnat) in the eastern SPNA in the 2000s, are jointly analyzed using in situ data. The DICnat budget is found to be in steady state, confirming a long-standing hypothesis from in situ data for the first time. The biological activity is driving the uptake of natural CO2 from the atmosphere. The Cant increase in the ocean is solely responsible of the DIC storage rate which is explained by advection of Cant from the subtropics (65%) and Cant air-sea flux (35%). These results demonstrate that the Cant is accumulating in the SPNA without affecting the natural carbon cycle.

Description: An analysis of seasonal variations in climatological surface chlorophyll points to distinct biogeographical zones in the North Atlantic subpolar gyre. In particular, the Labrador Sea appears well delineated into two regions on either side of the 60°N parallel, with very different climatological phytoplankton biomass cycles. Indeed, north of 60°N, an early and short spring bloom occurs in late April, while south of 60°N, the bloom gradually develops 1 month later and significant biomass persists all summer long. Nevertheless, at climatological scale, the first-order mechanism that controls the bloom is identical for both bioregions. The light-mixing regime can explain the bloom onset in both bioregions. In the Labrador Sea, the blooms seem to rely on a mean community compensation irradiance threshold value of 2.5 mol photon m−2 d−1 over the mixed layer.

Description: The East Asian winter monsoon (EAWM) is one of the most active components of the global climate system. Climate anomalies associated with the EAWM differ between extratropical and tropical regions due to the EAWM's meridional extent. Spatial differences in the EAWM variability on centennial and millennial time scales during the Holocene have not been well understood. This study describes Holocene spatiotemporal features of the EAWM based on comparisons of proxy records and climate simulations. The analysis specifically compared four proxy records located throughout China to assess the EAWM's spatial variability during the Holocene. These records indicate a stronger EAWM during the early Holocene than that during the late Holocene. The EAWM also shows a rapid, asynchronous decline from northwestern to southeastern China. The EAWM declined in northwestern China from 10 to 7.5 ka B.P., whereas the decline did not occur in southern China and the eastern Tibetan Plateau until 6–4.5 ka B.P. Coupled equilibrium and transient simulations of climate evolution during the Holocene indicate that the decline of the EAWM from 10 to 7.5 ka B.P. was probably caused by melting of Northern Hemisphere (NH) ice sheets and enhanced Atlantic meridional overturning circulation (AMOC). The decline of the EAWM from 6 to 4.5 ka B.P. over the eastern Tibetan Plateau and southern China is related to an abrupt increase in sea surface temperatures (SSTs) of the tropical western Indian Ocean. We therefore argue that the regional shift in EAWM intensity was probably related to a distinguishing response to high-latitude (NH ice sheets and AMOC) and low-latitude (tropical SSTs) forcings.

Description: Much uncertainty exists about the state of the oceanic and atmospheric circulation in the tropical Pacific over the last glacial cycle. Studies have been hampered by the fact that sediment cores suitable for study were concentrated in the western and eastern parts of the tropical Pacific, with little information from the central tropical Pacific. Here we present information from a suite of sediment cores collected from the Line Islands Ridge in the central tropical Pacific, which show sedimentation rates and stratigraphies suitable for paleoceanographic investigations. Based on the radiocarbon and oxygen isotope measurements on the planktonic foraminifera Globigerinoides ruber, we construct preliminary age models for selected cores and show that the gradient in the oxygen isotope ratio of G. ruber between the equator and 8°N is enhanced during glacial stages relative to interglacial stages. This stronger gradient could reflect enhanced equatorial cooling (perhaps reflecting a stronger Walker circulation) or an enhanced salinity gradient (perhaps reflecting increased rainfall in the central tropical Pacific).

Description: Dust aerosol is important in the Earth system, but the relative impact of meteorological mechanisms on North African dust emission remains unclear. This study presents the first climatology of dust emission amounts associated with Harmattan surges (HSs), characterized by postfrontal strengthening of near-surface winds. A new automated identification uses their strong isallobaric winds as an indicator for HSs in 32 years of ERA-Interim reanalysis. Their impact on dust aerosol emission is estimated by combining the identified events with derived dust emissions. The estimate highlights that about one third of the total emission mass is associated with HSs. Spring shows the largest associated emissions of 30–50% of the monthly totals consistent with the largest number and duration of HSs. Regional emission contributions of up to 80% in the north coincide with the overall largest emission maxima in spring. The importance of HSs for dust emission implies that aerosol-climate models need to accurately represent synoptic-scale storms. Key Points: - First climatology of North African dust emission mass linked with Harmattan surges (HSs) - One third of total emission linked to HSs annually and spatially averaged - Regionally up to 80% of springtime maximum in emission mass is associated with HSs

Description: Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

Description: The North Brazil Current (NBC) constitutes a bottleneck for the mean northward return flow of the Atlantic Meridional Overturning Circulation (AMOC) in the tropical South Atlantic. Previous studies suggested a link between interannual to multidecadal NBC and AMOC transport variability and proposed to use NBC observations as an index for the AMOC. Here we use a set of hindcast, sensitivity, and perturbation experiments performed within a hierarchy of ocean general circulation models to show that decadal to multidecadal buoyancy-forced changes in the basin-scale AMOC transport indeed manifest themselves in the NBC. The relation is, however, masked by a strong interannual to decadal wind-driven gyre variability of the NBC. While questioning the NBC transport as a direct index for the AMOC, the results support its potential merit for an AMOC monitoring system, provided that the wind-driven circulation variability is properly accounted for.

Description: The complex deglacial to Holocene oceanographic development in the Gulf of Guayaquil (Eastern Equatorial Pacific) is reconstructed for sea surface and subsurface ocean levels from (isotope) geochemical proxies based on marine sediment cores. At sea surface, southern sourced Cold Coastal Water and tropical Equatorial Surface Water/Tropical Surface Water are intimately related. In particular since ~10 ka, independent sea surface temperature proxies capturing different seasons emphasize the growing seasonal contrast in the Gulf of Guayaquil, which is in contrast to ocean areas further offshore. Cold Coastal Water became rapidly present in the Gulf of Guayaquil during the austral winter season in line with the strengthening of the Southeast Trades, while coastal upwelling off Peru gradually intensified and expanded northward in response to a seasonally changing atmospheric circulation pattern affecting the core locations intensively since 4 ka BP. Equatorial Surface Water, instead, was displaced and Tropical Surface Water moved northward together with the Equatorial Front. At subsurface, the presence of Equatorial Under Current-sourced Equatorial Subsurface Water was continuously growing, prominently since ~10–8 ka B.P. During Heinrich Stadial 1 and large parts of the Bølling/Allerød, and similarly during short Holocene time intervals at ~5.1–4 ka B.P. and ~1.5–0.5 ka B.P., the admixture of Equatorial Subsurface Water was reduced in response to both short-term weakening of Equatorial Under Current strength from the northwest and emplacement by tropical Equatorial Surface Water, considerably warming the uppermost ocean layers.