ALBERT

Description: Dense Nordic waters enter the North Atlantic through passages in the Greenland-Scotland Ridge at a mean rate of 6 Sv. Subsequent entrainment of ambient water into the sinking plumes downstream of the sills approximately double this flux. Decade-long observations show these fluxes to be stable with no discernible trends. Hydraulic control of the overflows and the buffering effect of the Nordic basins effectively filter out short-term variability of dense water production associated with white noise North Atlantic Oscillation forcing. Simulations with directly forced and coupled atmosphere-ocean models show, under present climate conditions, overflow variability on multi-decadal time scales but no longterm trends.

Description: Wave attenuation is an important physical property of hydrate-bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate-bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors estimated from zero-offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada, demonstrate significant wave attenuation for hydrate-bearing sediments. These results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals. The application of an inverse Q-filter to compensate attenuation effects of permafrost and hydrate-bearing sediments improved the resolution of surface 3D seismic data and its correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate-bearing strata and hence, the volume of hydrates in place.

Description: High interannual variability of summer surface salinity over the Laptev and East Siberian Sea shelves derived from historical records of the 1950s–2000s is attributed to atmospheric vorticity variations. In the cyclonic regime (positive vorticity) the eastward diversion of the Laptev Sea riverine water results in a negative salinity anomaly to the east of the Lena Delta and farther to the East Siberian Sea, and a positive anomaly to the north of the Lena Delta. Anticyclonic (negative) vorticity results in negative salinity anomalies northward from the Lena Delta due to freshwater advection toward the north, and a corresponding salinity increase eastward.

Description: Identifying the source of atmospheric dust is crucial to better understand the global marine biogeochemical cycle as phytoplankton growth relies on dissolved micronutrient elements available in the open ocean. Mineralogical, geochemical and Sr isotope analyses of a one year-time series (April 2003–April 2004) of deep ocean particle flux at the Madeira Abyssal Plain in the subtropical northeast Atlantic are presented. The lithogenic fraction has a high occurrence of palygorskite and smectite and an absence of kaolinite together with Sr isotopic compositions similar to the Northeast Atlantic aerosols. This indicates the Anti-Atlas Moroccan chain of Paleozoïc age as the source region. The lithogenic fraction supplies 4 times more Fe during a dust event than during the spring-summer aeolian input. A continuous input of only 1% of the lithogenic iron made available over a year period, could lead to an increase in primary productivity of 40% relative to today's value.

Description: CTD measurements carried out in the southern Adriatic Sea and in the western Ionian basin (Eurafrican Mediterranean Sea) during May 2003 by the German research vessel Poseidon (Poseidon cruise 298) and numerical simulations are used to elucidate aspects of the abyssal circulation of this oceanic region. The observations reveal that dense waters of Adriatic origin were strongly diluted along their way on the Italian continental slope, whilst their characteristics remained better preserved in a region located further east. Numerical simulations carried out by means of a nonlinear, reduced-gravity plume model confirm the observations and contribute to explain their cause: The very steep topographic slope along the Italian shelf in the region of the Gulf of Taranto induces strong entrainment of intermediate waters in the bottom layers. Instead, the bottom waters of Adriatic origin which, along their path further east, encounter gentler topographic variations, are weakly diluted by turbulent mixing and, therefore, better preserve their original characteristics. The remarkable differences in the simulated turbulent mixing along these two different paths are accentuated by the presence of a noticeable zonal gradient of potential density existing in the near-bottom layers of the northern Ionian basin.

Description: Gas hydrate samples recovered from a cold vent field offshore Vancouver Island were studied in detail both by macroscopic observations and instrumental methods (powder X-ray diffraction method (PXRD), nuclear magnetic resonance (NMR), and Raman spectroscopy). It was found that gas hydrates were massive from 2.64 to 2.94 m below seafloor (mbsf), elongated, nodular and tabular from 4.60 to 4.81 mbsf, and vein-like from 5.48 to 5.68 mbsf, showing a trend of decreasing hydrate content with increasing depth. All samples were determined to be structure I hydrate from PXRD, NMR, and Raman spectroscopies. The hydration numbers were estimated to be 6.1 ± 0.2 on average as determined from the methane distribution over the cage sites from NMR and Raman analytical results. Estimates of conversion levels indicated that ∼78% of the water in the massive samples was hydrate, down to a low value of ∼0.4% for the pore hydrate samples. The results are compared with measurements on synthetic hydrates and samples recovered from below the permafrost on the Mallik site. Differences in methane content and lattice parameters for synthetic and natural samples are relatively minor. Additional work is needed to address the presence of minor gas components and the heterogeneity of natural hydrate samples.

Description: Large-scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing 〉20–25 km3 of the volcano's north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large-scale landslides and subsequent volcanism/magmatism during Teno's evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Teno's most evolved, low-density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low-Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long-term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano-tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near-surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse-induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short-term compositional variations may also directly relate to major flank collapse events.

Description: The extent of the Yermak Slide has been revised on the basis of new acoustic and detailed bathymetric data. The true geometry, with an affected area of at least 10,000 km2 and more than 2400 km3 of involved sedimentary material, puts the Yermak Slide among the largest exposed submarine slides worldwide, comparable to the Storegga Slide off central Norway. Details from the side's internal structure give evidence for one main slide event during MIS 3 followed by repeated minor events. The timing coincides with the transition of the Kapp Ekholm Interstadial into Glaciation G of Svalbard (Mangerud et al., 1998) and the buildup phase of the Svalbard-Barents Sea Ice Sheet. Thus the slide occurred during a period of falling sea level, increasing ice volume, and, presumably, increasing glaciotectonic activity. The side's geometry and internal physical appearance point to a tectonically induced partial shelf collapse.

Description: The study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off Spitsbergen and direct seabed observations using a Remotely Operated Vehicle (ROV). In addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the Barents Sea with short sediment cores from a selected fjord environment (Storfjord). While sedimentation in the fjords is mainly controlled by river/meltwater discharge and coastal erosion by sea ice/glaciers resulting in high supply of terrigenous organic matter, Atlantic water inflow, and thus enhanced marine organic matter supply, characterizes the environment on the outer shelf and slope. Local deviations from this pattern, particularly on the shelf, are due to erosion and out washing of fine-grained material by bottom currents. Spots dominated by marine productivity close to the island have been found at the outer Isfjord and west off Prins Karls Forland as well as off the Kongsfjord/Krossfjord area and probably reflect local upwelling of nutrient-rich Atlantic water–derived water masses. Accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century. Negative correlation of the Isfjord temperature record with reconstructed productivities in the Storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming. Extremely high accumulation rates of marine organic carbon between 5.4 and 17.2 g m−2 yr−1 mark the Storfjord area, and probably high-latitude fjord environments in general, as a sink for carbon dioxide.

Description: This study presents a comparison of sea surface temperature (SST) estimates based on Mg/Ca ratios of Globigerinoides ruber and alkenone unsaturation index (U37 K′) in core sediment recovered from the Gulf of Guinea, eastern equatorial Atlantic. Mg/Ca- and U37 K′-based SST estimates yield fairly comparable results for the time interval 21,000–14,500 years and for the late Holocene. The early and middle Holocene, however, are largely characterized by a discrepant trend, with warm Mg/Ca and cold U37 K′ based SST estimates. This discrepant SST trend is accompanied by low sea surface salinity estimates (high riverine runoff) and biogenic sediment, which is characterized by high biogenic opal content, low carbonate content, and relatively low alkenone concentration. We hypothesize that the discrepancy in the reconstructed SSTs during the middle and early Holocene presumably suggests a period of elevated riverine input of dissolved silica and dominantly siliceous phytoplankton bloom in a low saline and warm surface water, while alkenone producers were likely prevalent in a season of cold SST and low riverine silica input. This study suggests that changes in the local hydrography and nutrient input have strong influence on the U37 K′-based SST estimate that may be unraveled by combining different SST proxies.

Description: We present a modern method used in nonlinear time series analysis to investigate the relation of two oscillating systems with respect to their phases, independently of their amplitudes. We study the difference of the phase dynamics between El Niño/Southern Oscillation (ENSO) and the Indian Monsoon on inter-annual time scales. We identify distinct epochs, especially two intervals of phase coherence, 1886–1908 and 1964–1980, corroborating earlier findings from a new point of view. A significance test shows that the coherence is very unlikely to be the result of stochastic fluctuations. We also detect so far unknown periods of coupling which are invisible to linear methods. These findings suggest that the decreasing correlation during the last decades might be a typical epoch of the ENSO/Monsoon system having occurred repeatedly. The high time resolution of the method enables us to present an interpretation of how volcanic radiative forcing could cause the coupling.

Description: An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752-1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

Description: Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (〉60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate 〈 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Description: We present the first complete budget of the interannual variability in Arctic springtime ozone taking into account anthropogenic chemical and natural dynamical processes. For the winters 1991/1992 to 2003/2004 the Arctic chemical ozone loss is available from observations. This work investigates the dynamical supply of ozone to the Arctic polar vortex due to mean transport processes for the same winters. The ozone supply is quantified in a vortex-averaged framework using estimates of diabatic descent over winter. We find that the interannual variability of both dynamical ozone supply and chemical ozone loss contribute, in equal shares, to the variability of the total ozone change. Moreover, together they explain nearly all of the interannual variability of Arctic springtime column ozone. Variability in planetary wave activity, characterized by the Eliassen-Palm flux at 100 hPa, contributes significantly to the variability of ozone supply, chemical ozone loss and total springtime ozone.

Description: Observed multidecadal variability (30 yr running means, trends, and moving standard deviations) of the North Atlantic Oscillation (NAO) during the instrumental record is compared to that simulated by two different coupled general circulation models in extended-range control experiments. Simulated NAO exhibits strong low frequency fluctuations, even on multi-centennial time scale. Observed multi-decadal NAO variations agree well with the model variability. Trend probability distribution functions, observed and simulated, were not found to be different with statistical significance. Thus, multi-decadal NAO changes similar to those observed during the instrumental record, including the recent increase in 1965–1995, may be internally generated within the coupled atmosphere-ocean system without considering external forcing.

Description: Lake Tahoe is an ultra-oligotrophic subalpine lake that is renowned for its clarity. The region experiences little cloud cover and is one of the most UV transparent lakes in the world. As such, it is an ideal environment to study the role of UV radiation in aquatic ecosystems. Long-term trends in Secchi depths showed that water transparency to visible light has decreased in recent decades, but limited data are available on the UV transparency of the lake. Here we examine how ultraviolet radiation varies relative to longer-wavelength photosynthetically active radiation (PAR, 400-700 nm, visible wavelengths) horizontally along inshore-offshore transects in the lake and vertically within the water column as well as temporally throughout 2007. UV transparency was more variable than PAR transparency horizontally across the lake and throughout the year. Seasonal patterns of Secchi transparency differed from both UV and PAR, indicating that different substances may be responsible for controlling transparency to UV, PAR, and Secchi. In surface waters, UVA (380 nm) often attenuated more slowly than PAR, a pattern visible in only exceptionally transparent waters with very low dissolved organic carbon. On many sampling dates, UV transparency decreased progressively with depth suggesting surface photobleaching, reductions in particulate matter, increasing chlorophyll a, or some combination of these increased during summer months. Combining these patterns of UV transparency with data on visible light provides a more comprehensive understanding of ecosystem structure, function, and effects of environmental change in highly transparent alpine and subalpine lakes such as Tahoe.

Description: The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Description: A new 46-year hindcast dataset for seasonal-to-annual ensemble predictions has been created using a multi-model ensemble of 5 state-of-the-art coupled atmosphere-ocean circulation models. The multi-model outperforms any of the single-models in forecasting tropical Pacific SSTs because of reduced RMS errors and enhanced ensemble dispersion at all lead-times. Systematic errors are considerably reduced over the previous generation (DEMETER). Probabilistic skill scores show higher skill for the new multi-model ensemble than for DEMETER in the 4–6 month forecast range. However, substantially improved models would be required to achieve strongly statistical significant skill increases. The combination of ENSEMBLES and DEMETER into a grand multi-model ensemble does not improve the forecast skill further. Annual-range hindcasts show anomaly correlation skill of ∼0.5 up to 14 months ahead. A wide range of output from the multi-model simulations is becoming publicly available and the international community is invited to explore the full scientific potential of these data.

Description: [1] The present geological setting west of Svalbard closely parallels the situation off mid-Norway after the last glaciation, when crustal unloading by melting of ice induced very large earthquakes. Today, on the modern Svalbard margin, increasing bottom water temperatures are destabilizing marine gas hydrates, which are held in continental margin sediments consisting of interlayered contourite deposits and glacigenic debris flows. Both unloading earthquakes and hydrate failure have been identified as key factors causing several megalandslides off Norway during early Holocene deglaciation. The most prominent event was the Storegga Slide 8200 years B.P. which caused a tsunami up to 23 m high on the Faroe and Shetland islands. Here we show by numerical tsunami modeling that a smaller submarine landslide west of Svalbard, 100 m high and 130 km wide, would cause a tsunami capable of reaching northwest Europe and threatening coastal areas. A tsunami warning system based on tiltmeters would give a warning time of 1–4 h.

Description: This study proposes a mechanism that explains the marked shift in the correlation between the El Niño/Southern Oscillation (ENSO) and the isotopic composition (δ18Oc) of a Porites coral from the Chagos Archipelago (71°E/5°S). Only after the mid‐1970s a strong ENSO signal emerges in the δ18Oc during the analyzed period 1950–1994. In the 1970s, the increasing sea surface temperature (SST) shifted the mean SST closer to the deep convection threshold at about 28.5°C. ENSO‐related SST variability largely controls the deep convection and precipitation in the central equatorial Indian Ocean (CEIO) when the SST is at this critical level. The anomalies in the precipitation induce changes in the isotopic composition of the surface ocean waters. The precipitation signal amplifies the SST signal in the coral δ18Oc and raises the correlation to ENSO. The presented results have important implications for the reconstruction of ENSO indices from corals within the Indian Ocean.

Description: The Arctic Ocean is the missing piece for any global model. Records of processes at both long and short timescales will be necessary to predict the future evolution of the Arctic Ocean through what appears to be a period of rapid climate change. Ocean monitoring is impoverished without the long-timescale records available from paleoceanography and the boundary conditions that can be obtained from marine geology and geophysics. The past and the present are the key to our ability to predict the future.

Description: Surface seawater pCO2 and related parameters were measured at high frequency onboard the volunteer observing ship M/V Falstaff in the North Atlantic Ocean between 36° and 52°N. Over 90,000 data points were used to produce monthly CO2 fluxes for 2002/2003. The air-sea CO2 fluxes calculated by two different averaging schemes were compared. The first approach used gas transfer velocity determined from wind speed retrieved at the location of the ship and called colocated winds, while for the second approach a monthly averaged gas transfer velocity was calculated from the wind for each grid pixel including the variability in wind. The colocated wind speeds determined during the time of passage do not capture the monthly wind speed variability of the grid resulting in fluxes that were 47% lower than fluxes using the monthly averaged wind products. The Falstaff CO2 fluxes were in good agreement with a climatology using averaged winds. Over the entire region they differed by 2–5%, depending on the time-dependent correction scheme to account for the atmospheric in increase in pCO2. However, locally the flux differences between the ship measurements and the climatology were greater, especially in regions north of 45°N, like the eastern sector. A comparison of two wind speed products showed that the annual CO2 sink is 4% less when using 6 hourly NCEP/NCAR wind speeds compared to the QuikSCAT wind speed data.

Description: Hydroxylamine (NH(2)OH) is an intermediate of the marine nitrogen cycle and in marine environments dissolved NH(2)OH is short-lived. In order to investigate the distribution of NH(2)OH under varying oxygen conditions, its seasonal variability was investigated on a monthly basis from July 2005 to May 2006 at the time series station Boknis Eck located in the Eckernforde Bay (southwestern Baltic Sea). NH(2)OH concentrations were generally low and close to the detection limit. However, a pronounced increase was observed after the seasonal thermohaline stratification period with low oxygen/anoxic conditions in the deep layers was terminated in November 2005. The increase of NH(2)OH was associated with the re-oxygenation of the water column. We conclude that NH(2)OH was produced in-situ during nitrification. We suggest that the detection of significant amounts of NH(2)OH can be used as an indicator for a "fresh" nitrifying system.

Description: Bubbles evolving from active gas seeps can be traced by hydroacoustic imaging up to 1000 m high in the Black Sea water column. Although methane concentrations are not distinguishable between the water column above the deep seep and reference sites, atmospheric noble gas measurements clearly show the constant input of gases (mainly methane) via seepage into the Black Sea. Archaea (ANME-1, ANME-2) and methanotrophic bacteria detected with specific 16S rRNA-targeted oligonucleotide probes are related to active gas seeps in the oxic and anoxic water column. It is suggested that methane seeps have a much greater influence on the Black Sea methane budget than previously acknowledged and that ANME-1 and ANME-2 are injected via gas bubbles from the sediment into the anoxic water column mediating methane oxidation. Our results show further that only minor amounts of methane evolving from Black Sea gas seeps reach the atmosphere due to the very effective microbial barrier. Hence only major thermodynamically and/or tectonically triggered gas hydrate dissociation has the potential to induce rapid climate changes as suggested by the “clathrate gun hypothesis.”

Description: The zonal equatorial circulation of the upper 700 m in the central tropical Atlantic is studied based on 11 cross-equatorial ship sections taken at 23–29°W during 1999 to 2005 and on data from a pair of moored Acoustic Doppler current profilers deployed on the equator at 23°W during February 2004 to May 2005. The observations on the equator reveal the existence of two mean westward cores of the Equatorial Intermediate Current below the Equatorial Undercurrent. In contrast to the 2002 moored observations at the same position the intraseasonal variability during the mooring period is dominated by zonal instead of meridional velocity fluctuations.

Description: The dynamics and predictability of the decadal to multidecadal Atlantic merid­ional overturning circulation (MOC) variability are described from observations and models. The investigation focuses on two modes that involve the MOC: One mode exhibits a quasi-decadal period, while the other is multidecadal. The two modes have completely different underlying dynamics, which is reflected in their rather different spatial characteristics. While the quasi-decadal mode represents a damped mode of the coupled ocean-atmosphere system, the multidecadal mode can be basically understood as the MOC response to the multidecadal forcing by the North Atlantic Oscillation (NAO). "Perfect model" predictability studies indicate a rather high predictability potential of the MOC variability on decadal timescales. Variations of the MOC are associated with variations in the meridional heat trans­port that drive sea surface temperature (SST) anomalies. SST anomalies in the North Atlantic thus exhibit a similar decadal predictability potential as the MOC. The decadal predictability carries over to the atmosphere. The probability density function of European surface air temperature anomalies, for instance, changes sig­nificantly with the state of the MOC. A reconstruction of the MOC for the 20th cen­tury from observed SSTs shows considerable variability on decadal timescales, but no strong sustained long-term trend. Furthermore, an assessment of the observed hydrographical changes in the Nordic Seas, with the aid of ocean general circula­tion model experiments and the analysis of recent scenario integrations with global climate models, indicates that the expected anthropogenic weakening of the MOC may not exceed the level of the internal variability within the next decades.

Description: The investigations carried out between 2002 and 2004 during six field experiments within the Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA) project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Center, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X band (9.42 GHz) vertical (VV) polarized river radar and was mounted on board the research vessel Ludwig Prandtl during the experiments in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The important progress realized in this investigation is the availability of calibrated X band radar data. Another central point of the study is to demonstrate the applicability of the quasi-specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed. Radar data have been taken at very low grazing angles ≤2.6° of flood and ebb tide–oriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ≤0.6 m s−1 have been measured at wind speeds ≤4.5 m s−1 and water depths ≤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tide–oriented sand wave is 27%. For the flood tide–oriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tide–oriented sand wave is 10%, and for the flood tide–oriented sand wave, a value of 43% has been derived. Phases of measured and simulated NRCS modulations correspond to asymmetric sand wave slopes. The results of the simulated NRCS modulation show the qualitative trend but do not always quantitatively match the measured NRCS modulation profiles because the quasi-specular scattering theory at very low grazing angle is a first-order theory.

Description: There is growing concern about the transfer of methane originating from water bodies to the atmosphere. Methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport. This work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere. The bubble model predicts the evolving bubble size, gas composition, and rise distance and is suitable for almost all aquatic environments. The model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep, oxic, saline water with excellent results. Methane bubbles from within the hydrate stability zone (typically below ∼500 m water depth in the ocean) are believed to form an outer hydrate rim. To explain the subsequent slow dissolution, a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone. The calibrated model explains the impressively tall flares (〉1300 m) observed in the hydrate stability zone of the Black Sea. This study suggests that only a small amount of methane reaches the surface at active seep sites in the Black Sea, and this only from very shallow water areas (〈100 m). Clearly, the Black Sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere, although substantial amounts of methane may reach the surface in shallow lakes and reservoirs.

Description: The majority of known high-temperature hydrothermal vents occur at mid-ocean ridges and back-arc spreading centers, typically at water depths from 2000 to 4000 meters. Compared with 30 years of hydrothermal research along spreading centers in the deep parts of the ocean, exploration of the approximately 700 submarine arc volcanoes is relatively recent [de Ronde et al., 2003]. At these submarine arc volcanoes, active hydrothermal vents are located at unexpectedly shallow water depth (95% at 〈1600-meter depth), which has important consequences for the style of venting, the nature of associated mineral deposits, and the local biological communities. As part of an ongoing multinational research effort to study shallow submarine volcanic arcs, two hydrothermal systems in the submerged part of the Aeolian arc have been investigated in detail during research cruises by R/V Poseidon (July 2006) and R/V Meteor (August 2007). Comprehensive seafloor video surveys were conducted using a remotely operated vehicle, and drilling to a depth of 5 meters was carried out using a lander-type submersible drill. This research has resulted in the first detailed, three-dimensional documentation of shallow submarine hydrothermal systems on arc volcanoes

Description: We reconstruct the hydrologic history of the tropical western Indian Ocean by calculating the δ18Oseawater from coupled coral Sr/Ca and δ18O measurements in a massive Porites coral from Mayotte (Comoros) between 1881 and 1994. We found that the precipitation-evaporation balance varies naturally on time scales of 5–6 years and 18–25 years. High (low) SSTs are associated with positive (negative) δ18Oseawater implying that atmospheric variability is linked with remote climate modes in the Indian Ocean and the tropical/extratropical Pacific Ocean. Warm El Niño-Southern Oscillation events are associated with a negative freshwater balance at Mayotte. This case study demonstrates that a much denser network of δ18Oseawater reconstructions is crucial for understanding the spatial patterns of hydrological conditions.

Description: Observations and model results both indicate increasing oxygen minimum zones (OMZ) in the tropical oceans. Here we report on record low dissolved oxygen minimum concentrations in the eastern tropical North Atlantic in fall of 2008, with less than 40 mu mol kg(-1) in the core of the OMZ. There we find a deoxygenation rate of similar to 0.5 mu mol kg(-1) a(-1) during the last decades on two repeat sections at 7.5 and 11 degrees N. The potential temperature and salinity in the surface and central water layers increased on both sections compared to previous observations. However, in contrast to the oxygen decrease in the core of the OMZ, increasing oxygen concentrations were observed in the central water layer above the OMZ. The observed deoxygenation was thus restricted to the core of the oxygen minimum layer. It remains unclear whether the vertical expansion of the oxygen minimum represents a long time trend or decadal variations

Description: Wide–angle reflection seismic experiments were performed at the Storegga slide offshore Norway in 2002 with the goal to quantify the amount of gas hydrate and free gas in the sediment. Twenty‐two stations with Ocean Bottom Hydrophones (OBH) and Seismometers (OBS) were deployed for a 2D and a 3D experiment. Kirchhoff depth migration is used to transform the seismic wide–angle data into images of the sediment layers and to obtain P wave velocity–depth functions. The gas hydrate and free gas saturations are estimated from the elastic properties of the sediment on the basis of the Frenkel–Gassmann equations. There is 5–15% gas hydrate in the pore space of the sediment in the gas hydrate stability zone (GHSZ). The free gas saturation takes the value of 0.8% for a homogeneous distribution of gas in the pore water and 7% for the model of a patchy gas distribution.

Description: Primary production and calcification in response to different partial pressures of CO2 (PCO2) (“glacial,” “present,” and “year 2100” atmospheric CO2 concentrations) were investigated during a mesocosm bloom dominated by the coccolithophorid Emiliania huxleyi. The day-to-day dynamics of net community production (NCP) and net community calcification (NCC) were assessed during the bloom development and decline by monitoring dissolved inorganic carbon (DIC) and total alkalinity (TA), together with oxygen production and 14C incorporation. When comparing year 2100 with glacial PCO2 conditions we observed: (1) no conspicuous change of net community productivity (NCPy); (2) a delay in the onset of calcification by 24 to 48 hours, reducing the duration of the calcifying phase in the course of the bloom; (3) a 40% decrease of NCC; and (4) enhanced loss of organic carbon from the water column. These results suggest a shift in the ratio of organic carbon to calcium carbonate production and vertical flux with rising atmospheric PCO2.

Description: The growing world population increases the demand for water, energy, and land. This demand for natural resources impacts the transport of material and the supply of nutrients in the coastal ocean by rivers. We assess the potential impact of river N, Si, Fe, and organic carbon (OC) fluxes on the global and coastal ocean biogeochemistry, using an ocean biogeochemistry model and observations, in eight different scenarios. We assess two extreme scenarios, one with no river nutrients, corresponding to a complete stop of nutrient input by rivers, and one with high nutrient fluxes, corresponding to a world population of 12 billion people. Compared to today's scenario values, primary production (PP) changes from −5% to +5% for the open ocean, and from −16% to +5% for the coastal ocean. In the coastal ocean the impact of river nutrients on PP depends on regional nutrient limitation. River inputs have a larger impact on PP in areas where upwelling and high runoff are combined. The coastal ocean is typically N‐ or Si‐limited. River Fe not assimilated by the phytoplankton is exported to open ocean areas, and its fertilizing effect depletes coastal and open ocean surface waters from N and Si. The impact on PP is reflected on global ocean low‐O2 areas whose extent changes from −16% to +23% across the range of scenarios. River nutrients have a modest impact on the global ocean CO2 sink of up to 0.4 Pg C a−1, depending on the amount of inorganic and organic carbon transported by the rivers.

Description: Submarine high‐intensity methane seeps have been surveyed in the Sorokin Trough and Paleo Dnepr Area in the Black Sea from May to June, 2003 to estimate the sea‐air methane flux. The Sorokin Trough mud volcano area in around 2080 m water depth shows no direct effects on the methane concentration in the surface water and the atmosphere (average methane saturation ratios (SR) of 143%). The average sea‐air methane flux can be determined as 0.2–0.57 nmol m−2 s−1, using two different sea‐air gas exchange models; mean wind speed were extraordinary low throughout the cruise (1.16 m s−1). The investigations in the Paleo Dnepr Area (60 to 800 m water depth) reflects a more diverse pattern. Spots of high methane concentrations in the surface water have been recorded above a seep location in around 90 m water depth (SR up to 294%). The air‐sea methane flux above this seep site (0.96–2.32 nmol m−2 s−1) is 3 times higher than calculated for the surrounding shelf (0.32–0.77 nmol m−2 s−1) and 5 times higher than assessed for open Black Sea waters (water depth 〉 200 m, 0.19–0.47 nmol m−2 s−1).

Description: Eddy length scales are calculated from satellite altimeter products and in an eddy-resolving model of the North Atlantic Ocean. Four different measures for eddy length scales are derived from kinetic energy densities in wave number space and spatial decorrelation scales. Observational estimates and model simulation agree well in all these measures near the surface. As found in previous studies, all length scales are, in general, decreasing with latitude. They are isotropic and proportional to the local first baroclinic Rossby radius (L r) north of about 30°N, while south of 30°N (or for L r 〉 30 km), zonal length scales tend to be larger than meridional ones, and (scalar) length scales show no clear relation to L r anymore. Instead, they appear to be related to the local Rhines scale. In agreement with a recent theoretical prediction by Theiss [2004], the observed and simulated pattern of eddy length scales appears to be indicative of two different dynamical regimes in the North Atlantic: anisotropic turbulence in the subtropics and isotropic turbulence in the subpolar North Atlantic. Both regions can be roughly characterized by the ration between L r and the Rhines scales (L R), with L R 〉 L r in the isotropic region and L R 〈 L r in the anisotropic region. The critical latitude that separates both regions, i.e., where L R = L r, is about 30°N.

Description: Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated décollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Description: Recent current measurements in the tropical eastern North Atlantic reproduce the components of the large scale flow field. However, the observations as well as the 1/12°-FLAME model computations indicate that a lot of eddy scale variability is superimposed on the mean flow field. Despite of the disturbance by variability the signature of the Guinea Dome is well present. In November 2002 the Guinea Dome transport from direct observations was about 2.8 Sv above σ θ = 25.8 kg/m3 and 4 Sv between σ θ = 25.8 and 27.1 kg/m3. The oxygen minimum in the shadow zone comprises the central water and the Antarctic Intermediate Water (AAIW) layers and is located between the equatorial current system and the North Equatorial Current. The North Equatorial Counter- and Undercurrents at 3° to 6°N are major oxygen sources for the central water layer of the low-oxygen regions in the northeastern tropical Atlantic. A second, northern North Equatorial Countercurrent (nNECC) band exists at 8° to 10°N. The nNECC carries oxygen rich water from the southern hemisphere eastward but with an admixture of water from the northern hemisphere. A float at 200 m depth was spreading eastward in the North Equatorial Undercurrent (NEUC), at 28°W it shifted northward into the nNECC, and then was trapped in the Guinea Dome region for more than 3 years. The model indicates the region 22° to 32°W as the area of exchange between the NECC/NEUC and the nNECC bands. In the AAIW layer the northern Intermediate Countercurrent acts as oxygen source for the oxygen minimum zone.

Description: The process of plate accretion at mid-ocean ridges, once thought to occur in a relatively simple, magmatic system, has been shown in recent years to possess unexpected layers of complexity [e.g., Cannat, 1996; Escartín and Lin, 1998; Jokat et al., 2003; Michael et al., 2003]. Particularly at lower spreading rates, the magma supply to some or all of the ridge decreases, with the plate spreading motion being taken up instead on faults. The balance between these magmatic and tectonic processes governs such features as the topography, seismic activity location of hydrothermal vents, and degree of chemical exchange between crust and ocean at spreading axes. It therefore has important implications for the hydrothermal marine biosphere and global chemical budgets.

Description: A coupled ecosystem-circulation model of the North Atlantic is used to examine the individual contributions by wind stress and surface heat fluxes to naturally driven interannual-to-decadal variability of air-sea fluxes of CO2 and O2 during 1948–2002. The model results indicate that variations in O2 fluxes are mainly driven by variations in surface heat fluxes in the extratropics (15°N to 70°N), and by wind stress in the tropics (10°S to 15°N). Conversely, variations in simulated CO2 fluxes are predominantly wind-stress driven over the entire model domain (18°S to 70°N); while variability in piston velocity and surface heat fluxes is less important. The simulated uptake of O2 by the North Atlantic amounts to 70 ± 11 Tmol yr−1 to which the subpolar region (45°N to 70°N) contributes by 62 ± 10 Tmol yr−1. Whereas the subpolar North Atlantic takes up more than 2/3 of the total carbon absorbed by the North Atlantic in our model (about 0.3 Pg C yr−1), interannual variability of air-sea CO2 fluxes reaches similar values (about 0.01 Pg C yr−1 each) in the subpolar (45°N to 70°N), the subtropical (15°N to 45°N) and the equatorial (10°S to 15°N) Atlantic.

Description: This is a study about the general circulation of the southwestern Mediterranean Sea based on observations of currents carried out in the southwestern Mediterranean Sea in the framework of the Mass Transfer and Ecosystem Response (MATER) program (EEC/MAST3 program). From July 1997 to August 2002, profiling floats (MEDPROF experiment), isobaric floats (LIWEX experiment), and moored current meters (ELISA experiment) give evidence of two large-scale barotropic cyclonic circulations, the here-called Western and Eastern Algerian Gyres, centered around [3730′N, 230′E] and [3830′N, 600′E], respectively. These gyres have typical horizontal scales of 100–300 km and are characterized by orbital velocities of about 5 cm/s corresponding to rotational periods of about 4 months. They are strongly related to the bottom topography of the basin and to the planetary vorticity gradient: closed f/H isocontours (f is the planetary vorticity, H the water depth) correspond to the locations of the gyres and favor such circulations as free geostrophic modes. A linear and barotropic model is used to investigate the possibility of wind driving, but the results suggest that the wind stress is not responsible for establishing such circulations. The boundary currents flowing along the continental slope of Africa, Sardinia, and the Balearic Islands are proposed to be the main drivers of these gyres.

Description: Over the past several decades, the Arctic Ocean has undergone substantial change. Enhanced transport of warmer air from lower latitudes has led to increased Arctic surface air temperature. Concurrent reductions in Arctic ice extent and thickness have been documented. The first evidence of warming in the intermediate Atlantic Water (AW, water depth between 150 and 900 meters) of the Arctic Ocean was found in 1990. Another anomaly, found in 2004, suggests that the Arctic Ocean is in transition toward a new, warmer state [Polyakov et al., 2005, and references therein].

Description: As methane is consumed in the deep sea, its 13C/12C ratio progressively increases because of kinetic isotope fractionation. Many submarine hydrothermal vents emit methane with carbon isotope ratios that are higher than those of background methane in the surrounding ocean. Since the latter exists at low concentrations, mixing of background methane with vent fluid tends to decrease the 13C/12C ratio as concentration decreases, opposite to the trend produced by consumption. We investigated CH4 concentration and δ13C together with δ3He in plumes from the Logatchev hydrothermal field (LHF) located at 14°45′N, 45°W, which generates relatively heavy methane (δ13C ≈ −13‰) by serpentinization of ultramafic rock. The measured methane and δ3He were well correlated at high concentrations, indicating a CH4/3He ratio of 1 × 108 in the vent fluids. These tracer distributions were also simulated with an advection-diffusion model in which methane consumption only occurs above a certain threshold concentration. We utilized δ3He to calculate the methane remaining in solution after oxidation, f, and the deviation of δ13C from the value expected from mixing alone, Δδ13C. Both in the model and in the data, the entire set of Δδ13C values are not correlated with log f, which is due to continuous oxidation within the plume while mixing with background seawater. A linear relationship, however, is found in the model for methane at concentrations sufficiently above background, and many of the samples with elevated CH4 north of LHF exhibit a linear trend of Δδ13C versus log f as well. From this trend, the kinetic isotope fractionation factor in the LHF plumes appears to be about 1.015. This value is somewhat higher than found in some other deep-sea studies, but it is lower than found in laboratory incubation experiments.

Description: Global satellite observations of temperature and geopotential height (GPH) from the Microwave Limb Sounder (MLS) on the EOS Aura spacecraft are discussed. The precision, resolution, and accuracy of the data produced by the MLS version 2.2 processing algorithms are quantified, and recommendations for data screening are made. Temperature precision is 1 K or better from 316 hPa to 3.16 hPa, degrading to ∼3 K at 0.001 hPa. The vertical resolution is 3 km at 31.6 hPa, degrading to 6 km at 316 hPa and to ∼13 km at 0.001 hPa. Comparisons with analyses (Goddard Earth Observing System version 5.0.1 (GEOS-5), European Centre for Medium-range Weather Forecasts (ECMWF), Met Office (MetO)) and other observations (CHAllenging Minisatellite Payload (CHAMP), Atmospheric Infrared Sounder/Advanced Microwave Sounder Unit (AIRS/AMSU), Sounding of the Atmosphere using Broadband Radiometry (SABER), Halogen Occultation Experiment (HALOE), Atmospheric Chemistry Experiment (ACE), radiosondes) indicate that MLS temperature has persistent, pressure-dependent biases which are between −2.5 K and +1 K between 316 hPa and 10 hPa. The 100-hPa MLS v2.2 GPH surface has a bias of ∼150 m relative to the GEOS-5 values. These biases are compared to modeled systematic uncertainties. GPH biases relative to correlative measurements generally increase with height owing to an overall cold bias in MLS temperature relative to correlative temperature measurements in the upper stratosphere and mesosphere.

Description: The collapse of the Laurentide Ice Sheet over Hudson Bay ∼8.47 ka allowed the rapid drainage of glacial Lake Agassiz into the Labrador Sea, an event identified as causing a reduction in Atlantic meridional overturning circulation (AMOC) and the 8.2 ka cold event. Atmosphere-ocean models simulations based on this forcing, however, fail to reproduce several characteristics of this event, particularly its duration. Here we use planktonic foraminifera U/Ca records to document the routing of western Canadian Plains runoff that accompanied ice-sheet collapse. Geochemical modeling of the ∼7 nmol/mol increase in U/Ca at the opening of Hudson Bay indicates an increase in freshwater discharge of 0.13 ± 0.03 Sverdrups (106 m3 s−1) from routing, a sufficient magnitude to cause an AMOC reduction. We suggest that this routing event suppressed AMOC strength for several centuries after the drainage of Lake Agassiz, explaining multi-centennial climate anomalies associated with the 8.2 ka cold event.

Description: Periods of enhanced terrigenous input to the ocean's basins of the North Atlantic have been reported for the last glacial period. We present a set of new sediment cores recovered from the Sophia Basin north of Svalbard which exhibit widespread ice-rafted debris layers reflecting enhanced terrigenous input throughout the last ∼200 ka B.P. Their consistent stratigraphic position, sedimentological character, high sedimentation rate, and geochemical characteristic point to synchronously deposited layers which we name terrigenous input events (TIEs). Owing to their higher densities, they generate excellent reflectors for sediment-penetrating acoustic devices and prominent acoustic layers in the imagery of sedimentary structures. Therefore TIEs can be used for regional acoustic stratigraphy. Each of the events can be linked to major glacial activity on Svalbard. However, the Early Weichselian glaciation is not recorded as a TIE and, in agreement with other work, might not have occurred on Svalbard as a major glacial advance to the shelf break. Nonsynchronous timing of western and northern sources on Svalbard points against sea level–induced iceberg discharge events.

Description: [1] Although most Central American magmas have a typical arc geochemical signature, magmas in southern Central America (central Costa Rica and Panama) have isotopic and trace element compositions with an ocean island basalt (OIB) affinity, similar to the Galapagos-OIB lavas (e.g., Ba/La 〈 40, La/Yb 〉 10, 206Pb/204Pb 〉 18.8). Our new data for Costa Rica suggest that this signature, unusual for a convergent margin, has a relatively recent origin (Late Miocene ∼6 Ma). We also show that there was a transition from typical arc magmas (analogous to the modern Nicaraguan volcanic front) to OIB-like magmas similar to the Galapagos hot spot. The geographic distribution of the Galapagos signature in recent lavas from southern Central America is present landward from the subduction of the Galapagos hot spot tracks (the Seamount Province and the Cocos/Coiba Ridge) at the Middle American Trench. The higher Pb isotopic ratios, relatively lower Sr and Nd isotopic ratios, and enriched incompatible-element signature of central Costa Rican magmas can be explained by arc–hot spot interaction. The isotopic ratios of central Costa Rican lavas require the subducting Seamount Province (Northern Galapagos Domain) component, whereas the isotopic ratios of the adakites and alkaline basalts from southern Costa Rica and Panama are in the geochemical range of the subducting Cocos/Coiba Ridge (Central Galapagos Domain). Geological and geochemical evidence collectively indicate that the relatively recent Galapagos-OIB signature in southern Central America represents a geochemical signal from subducting Galapagos hot spot tracks, which started to collide with the margin ∼8 Ma ago. The Galapagos hot spot contribution decreases systematically along the volcanic front from central Costa Rica to NW Nicaragua.

Description: Fifteen-minute Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) infrared dust index images are used to identify dust source areas. The observations of dust source activation (DSA) are compiled in a 1° × 1° map for the Sahara and Sahel, including temporal information at 3-hourly resolution. Here we use this data set to identify the most active dust source areas and the time of day when dust source activation occurs most frequently. In the Sahara desert 65% of DSA (March 2006 to February 2008) occurs during 0600–0900 UTC, pointing toward an important role of the breakdown of the nocturnal low-level jet (LLJ) for dust mobilization. Other meteorological mechanisms may lead to dust mobilization including density currents initiated by deep convective systems which mobilize dust fronts (haboobs) occurring preferentially in the afternoon hours and cyclonic activities. The role of the nocturnal LLJ for dust mobilization in the Sahara is corroborated by regional model studies and analysis of meteorological station data.

Description: For modeled sediment cores of the open ocean, a method for predicting simultaneously the ages of four different solid sediment compounds with respect to their depositional year onto the sediment surface is presented. The simulation of time-dependent age distribution in the sediment mixed layer and the eventually accumulating sediment is a prerequisite of a proper data assimilation of marine sediment core data into predictive climate models. Through such a data assimilation, marine paleoclimate data could then be efficiently used in order to optimally determine adjustable model parameters. The age simulation is based on a passive tracer transport method taking into account varying vertical advection rates within the sediment top layers, chemical pore water reactions, and bioturbation. It turns out that different weight fractions of the modeled sediment have different ages in one horizontal geometric depth-in-core level depending on the particle rain onto the sediment and the reactivity of the material within the sediment pore waters. For simultaneous consideration of paleoclimatic tracers associated within one and the same weight fraction, e.g., for calcium carbonate, tracers such as foraminiferal δ13C, and calcium carbonate weight percentages, this may not be critical. However, for simultaneous consideration of calcium carbonate and opal weight percentages, the age difference in the observed weight fractions may have to be corrected. The age offset between CaCO3 and opal depends critically on the sediment accumulation rate. Low-accumulation sites are more strongly affected than high-accumulation sites.

Description: Oxygen and stable carbon isotope records along the growth direction on shells of the bivalve species Astarte borealis and Serripes groenlandicus reliably record all important aspects of the bottom water hydrography in the shallow southeastern Kara Sea, despite uncertainties about the isotopic range due to sparse sampling and the possibility of growth rate changes. Changing freshwater supply from the rivers Ob and Yenisei is the main cause for seasonal temperature and salinity variations near the three sampling locations in 20 to 70 m water depth as suggested by CTD measurements and modeling. Peak winter salinity of the simulated hydrographic data series and peak winter values in the isotope records follow negative trends, which indicate a freshening of the bottom water due to an increasing fraction of river water during the 1990s. This freshening affected the whole Kara Sea, and coincided with a lowering of regional air pressure gradients, as indicated by the declining Arctic oscillation index. The resulting weakening of the prevailing southwesterly winds diminished the inflow of saline Atlantic-derived water from the Barents Sea through the Kara Strait in the southwest, and, additionally, reduced the export of river water toward the north and northeast into the Arctic basin. Saline Atlantic-derived water thus was replaced by freshwater, which was successively accumulated in the Kara Sea and accordingly imprinted on the stable isotope composition of the bivalve shells. The 1990s freshening in the Kara Sea thus may be caused by natural variations rather than being a signal for global change.

Description: [1] A systematic search for methane-rich fluid seeps at the seafloor was conducted at the Pacific continental margin offshore southern Nicaragua and northern central Costa Rica, a convergent margin characterized by subduction erosion. More than 100 fluid seeps were discovered using a combination of multibeam bathymetry, side-scan sonar imagery, TV-sled observations, and sampling. This corresponds, on average, to a seep site every 4 km along the continental slope. In the northwestern part of the study area, subduction of oceanic crust formed at the East Pacific Rise is characterized by pervasive bending-induced faulting of the oceanic plate and a relatively uniform morphology of the overriding continental margin. Seepage at this part of the margin typically occurs at approximately cone-shaped mounds 50 - 100 m high and up to 1 km wide at the base. Over 60 such mounds were identified on the 240 km long margin segment. Some normal faults also host localized seepage. In contrast, in the southeast, the 220 km long margin segment overriding the oceanic crust formed at the Cocos-Nazca Spreading Centre has a comparatively more irregular morphology caused mainly by the subduction of ridges and seamounts sitting on the oceanic plate. Over 40 seeps were located on this part of the margin. This margin segment with irregular morphology exhibits diverse seep structures. Seeps are related to landslide scars, seamount-subduction related fractures, mounds, and faults. Several backscatter anomalies in side-scan images are without apparent relief and are probably related to carbonate precipitation. Detected fluid seeps are not evenly distributed across the margin but occur in a roughly margin parallel band centered 28 ± 7 km landward of the trench. This distribution suggests that seeps are possibly fed to fluids rising from the plate boundary along deep-penetrating faults through the upper plate.

Description: A generalized heat function is defined for diagnosing the pathways by which heat is carried by the ocean. In contrast to previous work, our generalized heat function varies along an isentrope only in the presence of mixing. The generalized heat function is diagnosed using the Levitus global ocean data set, net northward heat transport based on the data set of Grist and Josey, and different specifications for mixing in the ocean. The separation between the heat flux carried by the shallow wind driven cells and the deep overturning circulation is clearly revealed, with up to 0.4 PW being associated with the spreading of North Atlantic Deep Water. The importance of eddy-induced mixing near the surface of the Southern Ocean is evident.

Description: On the basis of integrations of an eddy-permitting coupled physical-biological model of the tropical Pacific we explore changes in the simulated mean circulation as well as its intraseasonal to interannual variability driven by the biologically modulated vertical absorption profiles of solar radiation. Three sensitivity ocean hind-cast experiments, covering the period from 1948 to 2003, are performed. In the first one, simulated chlorophyll affects the attenuation of light in the water column, while in the second experiment, the chlorophyll concentration is kept constant in time by prescribing an empirically derived spatial pattern. The third experiment uses a spatially and temporally constant value for the attenuation depth. The biotically induced differential heating is generated by increased absorption of light in the surface layers, leading to a surface warming and subsurface cooling. The effect is largest in the eastern equatorial Pacific. However, the initial vertical redistribution of heat leads to considerable changes of the near-surface ocean circulation subsequently influencing the near-surface temperature structure. In general, including biophysical coupling improves the model performance in terms of temperature and ocean circulation patterns. In particular, the upwelling in the eastern equatorial Pacific is enhanced, the mixed layer becomes shallower, the warm bias in the eastern Pacific is reduced, and the zonal temperature gradient increases. This leads to stronger La Niña events and an associated increase in the variability of the Niño3 SSTA time series. Furthermore, the eddy kinetic energy (EKE) associated with mesoscale eddies in the eastern equatorial Pacific increases by almost 100% because of enhanced EKE production due to enhanced horizontal and vertical shear of the mean currents.

Description: Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.

Description: A radiocarbon-calibrated box model for today's ocean suggests that a lag of about 1750 years should exist between the arrival of the midpoint of the deglaciation 18O signal in the deep Atlantic Ocean and its arrival in the deep Pacific Ocean. In order to assess the actual lag, we have carried out accelerator radiocarbon measurements on two cores from the Atlantic Ocean and one core from the Pacific Ocean. Although the results are not definitive, there is a suggestion that the actual time lag was about 1000 years.

Description: In an attempt to create a scenario for the cause of the glacial to interglacial CO2 change recorded in air trapped in polar ice, we call on an increase in the alkalinity of polar surface waters. In this way we circumvent a major deficiency of the polar nutrient scenarios of Sarmiento and Toggweiler (1984), Siegenthaler and Wenk (1984) and Knox and McElroy (1984). Namely, our scenario does not require a drop in the nutrient content of polar surface waters in conformity with the demonstration by Boyle (1988a, b) that the cadmium content of planktonic foraminifera from polar regions did not decrease from late glacial to Holocene time. The rise in alkalinity required by our model is a natural consequence of the demise, during glacial time, of North Atlantic Deep Water as a major force in ocean circulation and of the nutrient maximum deepening of Boyle (1988b). Rather than being original, our hypothesis builds on the concept basic to the polar nutrient hypotheses, namely that the CO2 partial pressure in polar waters controls that for both the atmosphere and warm surface ocean. It also requires the alkalinity increase in surface waters produced by Boyle's nutrient deepening.

Description: The basement morphology and sediment thickness of the Hess Rise, an oceanic plateau in the central North Pacific, have been mapped on the basis of seismic reflection profiles. The acoustic stratigraphy on and around the rise is correlated with the lithostratigraphy at Deep Sea Drilling Project sites 464, 310, 465, and 466. A total sediment isopach chart of the rise reveals small-scale departures from the expected sedimentary pattern (thick sediment in shallow areas; thin sediment in deep areas). Sediment-filled basement depressions result from mass transport; thin sediment (〈50 m) occurs on steep scarps, basement ridges, and areas affected by bottom currents. A pre-Senonian sediment isopach chart shows a thickening from less than 50 m to more than 250 m of sediment from the northeast to the southwest. This trend seems explainable only in terms of the time-transgressive nature of seafloor formed at a mid-ocean ridge. The axial trend of the rise (N30°W) parallels nearby Mesozoic magnetic lineations and seems to be isochronous as deduced from the Deep Sea Drilling Project data. The Hess Rise began developing in late Aptian time along a segment of the Pacific-Farallon Ridge. Important events in the history of the rise are late-stage volcanism on the southern margin of the rise along the Mendocino Fracture Zone, tectonism and volcanism about 85 Ma that resulted in a major regional unconformity (reflector C), and another period of tectonism and volcanism between 65 and 43 Ma that coincided with the formation of the Emperor Seamounts and created structural benches on the western side of the rise. A significant change in the paleoenvironment that apparently occurred around the Paleogene-Neogene boundary (∼25–20 Ma) caused pronounced changes in the depositional environment and resulted in another major regional unconformity (reflector A).

Description: A suit of sediment cores close to and south of the Strait of Gibraltar (12°-36°N, 500–2800 m water depth) were analyzed for stable isotopes in epibenthic foraminifers Cibicidoides wuellerstorfi and Planulina ariminensis. During peak glacial times, the data exhibit higher δ13C values of up to 1.6‰ at intermediate depths near the Strait of Gibraltar (36°N). The values decrease to the south as evidenced by our data, but also to the north as revealed by data of intermediate depth cores north of 38°N (in Duplessy et al. [1987]). Thus, the distribution pattern of δ13C provides crucial evidence for an increased influence of nutrient depleted Mediterranean Outflow Water (MOW) on the glacial northeast Atlantic hydrography. During oxygen isotope Terminations I and II, the meridional carbon isotope gradient indicates a significantly decreased but still active MOW. As deduced from the δ18O fluctuations, the temperatures of the MOW in the Atlantic were lower during glacial times by as much as 5°C. During glacial times and during Termination I the maximum δ13C values of the MOW correlate with minimum values of the North Atlantic Deep Water (NADW) and vice versa. This inverse response to climatic change of the carbon isotope signals of both water masses indicates, that the supply of saline MOW to the north Atlantic may be less important for the formation of NADW than previously assumed.

Description: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low- and mid-latitude upwelling cells started to decline by more than 2-4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO2 as recorded in ice cores and, with a slight delay of more than 1000 years, to a large-scale gradual CO2 depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ13C by 0.4–0.5‰. The decrease in new production also matches a clear 13C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high-latitude insolation, i.e., to orbital forcing as primary cause.

Description: The cold upwelling waters of the eastern tropical oceans not only interact with the atmospheric circulation via changing the sea surface temperatures but also influence the biological activity via affecting the nutrient and oxygen contents of the upwelling waters. While the sources of the equatorial upwelling associated with the Equatorial Undercurrent (EUC) have been studied extensively, the relevance of the northern and southern off-equatorial undercurrents (NEUC, SEUC) for the off-equatorial upwelling regions has remained unclear. In this study we use output from a high-resolution, 1/12° model (FLAME) to investigate the mean pathways and variability of the off-equatorial undercurrents (OEUCs) in the Atlantic. In particular, a calculation of Lagrangian trajectories helps to gain insight into the source waters of the OEUCs and their connection to the upwelling regions in the eastern tropical Atlantic. In the model solution the sources of both OEUCs belong almost exclusively to the Southern Hemisphere. The pathways of the source waters are found to be governed by strong recirculations between the different eastward and westward zonal currents because of intense eddy motions associated with the tropical instability wave activity. Whereas the SEUC is predominantly fed through the recirculation in the ocean interior, the NEUC is also fed by a weak inflow from the western boundary current. Investigation of the fate of the NEUC shows only a weak direct supply to the upwelling in the Guinea Dome and along the African coast but a significant contribution to the equatorial upwelling.

Description: δ18O profiles in drifting Arctic sea ice are coupled with back trajectories of ice drift and an ice growth model to reconstruct the surface hydrography of the Arctic Ocean interior. The results compare well with δ18O values obtained by traditional oceanographic methods and known water mass distributions. Analysis of the stable isotopic composition of sea ice floes sampled at strategic and relatively accessible locations, e.g., Fram Strait, could aid in mapping spatial and temporal variations in Arctic Ocean surface waters.

Description: The anthropogenic CO2 accumulation rate for the North Atlantic Ocean was estimated on the basis of the decrease in the δ13C of the dissolved inorganic carbon measured between cruises in 1981 (Transient Tracers in the North Atlantic), 1993 (OACES) and 2003 (Repeat Hydrography). A mean depth‐integrated δ13C change of −15.0 ± 3.8‰ m yr−1 was estimated by applying a multiple linear regression approach to determine the anthropogenic δ13C decrease at 22 stations where δ13C depth profiles were compared. The largest and deepest anthropogenic δ13C decreases occurred in the subpolar ocean and, in contrast, the smallest and shallowest decreases occurred in the tropical ocean. A mean anthropogenic CO2 accumulation rate of 0.63 ± 0.16 mol C m−2 yr−1 (0.32 ± 0.08 Pg C yr−1) in the North Atlantic Ocean over the last 20 years was determined from the mean depth‐integrated δ13C change and a ratio of anthropogenic δ13C to DIC change of −0.024‰ (μmol kg−1)−1. Only half of the accumulated anthropogenic CO2 in the North Atlantic during the last 20 years was the result of air‐sea CO2 uptake, based on a comparison of the air‐sea 13CO2 flux to the DIC13 inventory change, with the other half likely a result of northward advective transport.

Description: Measurements of chlorofluorocarbon inventories during 1997–2003 allow the detection and quantification of significant changes in the formation rates of two modes of Labrador Sea Water (LSW): Upper (ULSW) and deep LSW, both here defined in fixed density intervals. Both modes contribute to the cold limb of the Meridional Overturning Circulation. Results reveal that the lighter ULSW formed since the mid-1990s has started to replace the large pool of the deep LSW stored in the western North Atlantic. Formation of deep LSW was absent in 1997–2003. Formation of ULSW compensated for this absence during 1998/99 (7.9 Sv), but afterwards significantly declined to 2.5 Sv. The decrease of the overall LSW formation throughout 1997–2003 correlates with a declining eastward baroclinic mass transport between the centers of the subpolar and subtropical gyres since 1997, a warming of LSW, and a gradually decreasing North Atlantic Oscillation index after 1999.

Description: We present full 2004–2005 seasonal cycles of CO2 partial pressure (pCO2) and dissolved oxygen (O2) in surface waters at a time series site in the central Labrador Sea (56.5°N, 52.6°W) and use these data to calculate annual net air-sea fluxes of CO2 and O2 as well as atmospheric potential oxygen (APO). The region is characterized by a net CO2 sink (2.7 ± 0.8 mol CO2 m−2 yr−1) that is mediated to a major extent by biological carbon drawdown during spring/summer. During wintertime, surface waters approach equilibrium with atmospheric CO2. Oxygen changes from marked undersaturation of about 6% during wintertime to strong supersaturation by up to 10% during the spring/summer bloom. Overall, the central Labrador Sea acts as an O2 sink of 10.0 ± 3.1 mol m−2 yr−1. The combined CO2 and O2 sink functions give rise to a sizable APO flux of 13.0 ± 4.0 mol m−2 yr−1 into surface waters of the central Labrador Sea. A mixed layer carbon budget yields a net community production of 4.0 ± 0.8 mol C m−2 during the 2005 productive season about one third of which appears to undergo subsurface respiration in a depth range that is reventilated during the following winter. The timing of the spring bloom is discussed and eddies from the West Greenland Current are thought to be associated with the triggering of the bloom. Finally, we use CO2 and O2 mixed layer dynamics during the 2005 spring bloom to evaluate a suite of prominent wind speed-dependent parameterizations for the gas transfer coefficient. We find very good agreement with those parameterizations which yield higher transfer coefficients at wind speeds above 10 m s−1.

Description: We use clinopyroxene-liquid thermobarometry, aided by petrography and mineral major element chemistry, to reconstruct the magma plumbing system of the late Miocene, largely mafic Teno shield-volcano on the island of Tenerife. Outer rims of clinopyroxene and olivine phenocrysts show patterns best explained by decompression-induced crystallization upon rapid ascent of magmas from depth. The last equilibrium crystallization of clinopyroxene occurred in the uppermost mantle, from ∼20 to 45 km depth. We propose that flexural stresses or, alternatively, thermomechanical contrasts create a magma trap that largely confines magma storage to an interval roughly coinciding with the Moho at ∼15 km and the base of the long-term elastic lithosphere at ∼40 km below sea level. Evidence for shallow magma storage is restricted to the occurrence of a thick vitric tuff of trachytic composition emplaced before the Teno shield-volcano suffered large-scale flank collapses. The scenario developed in this study may help shed light on some unresolved issues of magma supply to intraplate oceanic volcanoes characterized by relatively low magma fluxes, such as those of the Canary, Madeira and Cape Verde archipelagoes, as well as Hawaiian volcanoes in their postshield stage. The data presented also support the importance of progressive magmatic underplating in the Canary Islands.

Description: During the SAMUM field campaign in southern Morocco in May and June 2006 density currents generated by evaporative cooling after convective precipitation were frequently observed at the Sahara side of the Atlas Mountain chain. The associated strong surface cold-air outflow during such events has been observed to lead to dust mobilization in the foothills. Here a regional model system is used to simulate a density current case on 3 June 2006 and the subsequent dust emission. The model studies are performed with different parameterization schemes for convection, and with different horizontal model grid resolutions to examine to which extent the model system can be used for reproducing dust emissions in this region. The effect of increasing the horizontal model grid resolution from 14 km to 2.8 km on the strength on the density currents and thus on dust emission is smaller than the differences due to different convection parameterization schemes in this case study. While the results in reproducing the observed density current at the Atlas Mountain varied with different convection parameterizations, the most realistic representation of the density current is obtained at 2.8 km grid resolution at which no parameterization of deep convection is needed.

Description: We collected 56 marine gravity cores from the Pacific seafloor offshore Central America which contain a total of 213 volcanic ash beds. Ash-layer correlations between cores and with their parental tephras on land use stratigraphic, lithologic, and compositional criteria. In particular, we make use of our newly built database of bulk-rock, mineral, and glass major and trace element compositions of plinian and similarly widespread tephras erupted since the Pleistocene along the Central American Volcanic Arc. We thus identify the distal ashes of 11 Nicaraguan, 8 El Salvadorian, 6 Guatemalan, and 1 Costa Rican eruptions. Relatively uniform pelagic sedimentation rates allow us to determine ages of 10 previously undated tephras by their relative position between ash layers of known age. Linking the marine and terrestrial records yields a tephrostratigraphic framework for the Central American volcanic arc from Costa Rica to Guatemala. This is a useful tool and prerequisite to understand the evolution of volcanism at a whole-arc scale.

Description: In this study we present the first combined investigation into the composition of the major matrices involved in calcification processes (surrounding water, extrapallial fluid, aragonite, and calcite) of Mytilus edulis with respect to their calcium isotope (d44/40Ca) and elemental compositions (Sr/Ca and Mg/Ca). Our aim was to examine the suitability of Mytilus edulis as a proxy archive and to contribute to the understanding of the process of biomineralization. Mytilus edulis specimens were live collected from the Schwentine Estuary, Kiel Fjord, and North Sea (Sylt). d44/40Ca was determined by thermal ionization mass spectrometry (TIMS) accompanied by measurements of Mg/Ca and Sr/Ca using inductively coupled plasma–optical emission spectroscopy (ICP-OES). The elemental and isotopic compositions of the investigated matrices showed systematic offsets. The carbonates are strongly depleted in their magnesium and strontium concentrations and fractionated toward lighter calcium isotope compositions relative to the surrounding Schwentine Estuary water. The opposite is observed for the extrapallial fluid (EPF). Our findings extend the results of previous studies reporting a strong biological control and the interaction of different environmental conditions influencing biomineralization. Future studies should focus on the temporal development of the interrelation between the different matrices.

Description: The oceans absorb and store a significant portion of anthropogenic CO2 emissions, but large uncertainties remain in the quantification of this sink. An improved assessment of the present and future oceanic carbon sink is therefore necessary to provide recommendations for long‐term global carbon cycle and climate policies. The formation of North Atlantic Deep Water (NADW) is a unique fast track for transporting anthropogenic CO2 into the ocean's interior, making the deep waters rich in anthropogenic carbon. Thus the Atlantic is presently estimated to hold 38% of the oceanic anthropogenic CO2 inventory, although its volume makes up only 25% of the world ocean. Here we analyze the inventory change of anthropogenic CO2 in the Atlantic between 1997 and 2003 and its relationship to NADW formation. For the whole region between 20°S and 65°N the inventory amounts to 32.5 ± 9.5 Petagram carbon (Pg C) in 1997 and increases up to 36.0 ± 10.5 Pg C in 2003. This result is quite similar to earlier studies. Moreover, the overall increase of anthropogenic carbon is in close agreement with the expected change due to rising atmospheric CO2 levels of 1.69% a−1. On the other hand, when considering the subpolar region only, the results demonstrate that the recent weakening in the formation of Labrador Sea Water, a component of NADW, has already led to a decrease of the anthropogenic carbon inventory in this water mass. As a consequence, the overall inventory for the total water column in the western subpolar North Atlantic increased only by 2% between 1997 and 2003, much less than the 11% that would be expected from the increase in atmospheric CO2 levels.

Description: Combined delta O18/salinity data reveal a distinctive water mass generated during winter sea ice formation which is found predominantly in the coastal polynya region of the southern Laptev Sea. Export of the brine-enriched bottom water shows interannual variability in correlation with atmospheric conditions. Summer anticyclonic circulation is favoring an offshore transport of river water at the surface as well as a pronounced signal of brine-enriched waters at about 50 m water depth at the shelf break. Summer cyclonic atmospheric circulation favors onshore or an eastward, alongshore water transport, and at the shelf break the river water fraction is reduced and the pronounced brine signal is missing, while on the middle Laptev Sea shelf, brine-enriched waters are found in high proportions. Residence times of bottom and subsurface waters on the shelf may thereby vary considerably: an export of shelf waters to the Arctic Ocean halocline might be shut down or strongly reduced during "onshore'' cyclonic atmospheric circulation, while with "offshore'' anticyclonic atmospheric circulation, brine waters are exported and residence times may be as short as 1 year only.

Description: The influence of the Atlantic and Indo-Pacific oceans on Atlantic-European climate is investigated by analyzing ensemble integrations with the atmospheric general circulation model ECHAM4 forced by anomalous sea surface temperature and sea ice conditions restricted to the Atlantic (AOGA) and Indo-Pacific (I+POGA) oceans. The forcing from both the Indo-Pacific and Atlantic oceans are important for the generation of the sea level pressure (SLP) variability in the Atlantic region in the boreal winter season. Over the North Atlantic the SLP response in the I+POGA experiment projects on the North Atlantic Oscillation, while it projects on the East Atlantic Pattern in the AOGA experiment. In both experiments (I+POGA and AOGA) a quadrupole-type 500 hPa height anomaly pattern is simulated which emerges from the tropical Pacific and Atlantic oceans, respectively. In boreal summer the influence of the Atlantic Ocean dominates the SLP response in the Atlantic region. The tropical North Atlantic is a key region in forcing the SLP response over the Caribbean Sea in this season.

Description: Direct observations at the Grand Banks have raised a quandary concerning the pathways of the lower branch of the meridional overturning circulation: In contrast to moored current meters that depict an intense, narrow Deep Western Boundary Current (DWBC), observations using different float types failed to show this continuous export path. Here, this issue is addressed by a Lagrangian analysis of synthetic particles in an eddy-resolving circulation model. Due to intense eddy activity around the Grand Banks, about 40% of the deep water in the DWBC is diverted into the interior, spreading southward along the western flank of the Mid-Atlantic Ridge or with the eddying flow field in the basin interior. Imposing constraints on the vertical displacements of particles similar to those experienced by observational floats further reduces their adherence to the DWBC, particularly near the southern tip of the Grand Banks.

Description: Atmospheric radiative transfer plays a central role in understanding global climate change and anthropogenic climate forcing, and in the remote sensing of surface and atmospheric properties. Because of their opacity and highly scattering nature, clouds (covering more than half the planet at any time) pose unique challenges in atmospheric radiative transfer calculations. Some widely-used assumptions regarding clouds—such as having a flat top and base, horizontal uniformity, and infinite extent—are amenable to simple one-dimensional (1-D) radiative transfer and are therefore attractive from a computational point of view. However, these assumptions are completely unrealistic and yield errors. The ever-increasing need to realistically simulate cloud radiative processes in remote sensing and energy budget applications has contributed to the recent rapid growth of the three-dimensional (3-D) radiative transfer (RT) community [e.g., Marshak and Davis, 2005].

Description: We measured halogen concentrations and I-129/I ratios in five drilling sites of Integrated Ocean Drilling Program Expedition 311 (offshore Vancouver Island, Canada) in order to identify potential sources of fluids and methane in gas hydrate fields. Iodine is dominated by organic decomposition and transports with fluids in reducing environments and the presence of the cosmogenic radioisotope I-129 (T-1/2 = 15.7 Ma) allows the age determination of organic sources for iodine. Here we report halogen concentrations in 135 pore water samples, I concentrations in 48 sediment samples, and I-129/I ratios measured in a subset of 20 pore water samples. Most I-129/I ratios fall into a range around 500 x 10(-15), corresponding to a minimum age of 25 Ma and the lowest ratio of 188 x 10(-15) (T-min = 47 Ma) was observed at 208 m below sea floor (mbsf) in Site 1326. These ages are considerably older than that of the local sediments in the gas hydrate fields and that of the subducting sediments on the Juan de Fuca plate, indicating that old, accreted sediments in the accretionary wedge contribute a significant amount of iodide and, by association, of methane to the gas hydrate occurrences. A geochemical transport-reaction model was applied to simulate the advection of deeply sourced fluids and the release of iodide, bromide, and ammonia in the host sediments due to organic matter degradation. The model was first tested with data from two well studied areas, Ocean Drilling Program Site 1230 (Peru margin) and Site 1245 (Hydrate Ridge). The model results for the Expedition 311 sites indicate that the in situ release of young iodine is relatively minor in comparison to the contribution of migrating fluids, carrying large amounts of old iodine from deep sources. The comparison between the sites demonstrates that the total organic content has a strong effect on the rate of in situ iodine release and that lateral flows along fractures can have a significant influence on pore water chemistry, especially at the Cascadia margin. The iodine results indicate that mobilization and transport of methane from sources in the upper plate of active margins is an important process which can also play a substantial role in the formation of gas hydrate fields.

Description: Thickness diffusivity (κ) according to the Gent and McWilliams parameterisation which accounts for eddy-driven advection in the ocean, is estimated using output from an eddy-resolving model of the Southern Ocean. A physically meaningful definition of rotational eddy fluxes leads almost everywhere to positive κ. Zonally averaged near surface values of κ remain smaller than 200 m2/s poleward of the polar front, increases between 60–45°S to about 600 m2/s and peak between 45–35° S at almost 3000 m2/s. κ stays high in the upper 500 m but decreases with depth and is essentially zero below 2500 m. In addition to the thickness diffusion (κ) there is eddy-induced eastward (westward) advection of isopycnal thickness at the poleward (equatorward) flank of the ACC pointing toward strong anisotropic lateral mixing.

Description: A seismic wide‐angle and refraction experiment was conducted offshore of Nicaragua in the Middle American Trench to investigate the impact of bending‐related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. On the basis of the reflectivity pattern of multichannel seismic reflection (MCS) data it has been suggested that bending‐related faulting facilitates hydration and serpentinization of the incoming oceanic lithosphere. Seismic wide‐angle and refraction data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent normal faults on the seaward trench slope. A tomographic joint inversion of seismic refraction and wide‐angle reflection data yield anomalously low seismic P wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2–0.5 km s−1 compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6–7.8 km s−1 and hence 5–7% lower than the typical velocity of mantle peridotite. These systematic changes in P wave velocity from the outer rise toward the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. If hydration is indeed affecting the seismic properties of the mantle, serpentinization might be reaching 12–17% in the uppermost 3–4 km of the mantle, depending on the unknown degree of fracturing and its impact on the elastic properties of the subducting lithosphere.

Description: A simple geochemical box model for the global cycle of methane (CH4) has been developed and applied to reconstruct the evolution of atmospheric CH4 over the entire Phanerozoic. According to the model, the partial pressure of atmospheric CH4 (pCH4) increased up to approximately 10 ppmv during the Carboniferous coal swamp era. This implies a maximum radiative forcing of about 3.5 W m−2 via CH4. Through its radiative forcing, CH4 heated the average global surface temperature by up to 1°C. The elevated pCH4 values during the Permian-Carboniferous cold period may have moderated the temperature decline caused by the coeval drawdown of atmospheric CO2. Additional runs with a global carbon model indicate that the heating induced by elevated pCH4 favored the drawdown of atmospheric pCO2 via enhanced rates of silicate weathering. Simulations with a state-of-the-art climate model reveal that the effects of atmospheric CH4 on average global surface temperature also depend on the partial pressures of CO2. The CH4 climate effect is amplified by high background levels of atmospheric CO2 such that a coeval increase in the partial pressure of both greenhouse gases has a much stronger climate effect than previously anticipated.

Description: Middepth current measurements in the equatorial Atlantic are characterized by elevated levels of energy contained in zonal flows of high baroclinic mode number. These alternating zonal flows, often called equatorial stacked jets, have amplitudes up to 20 cm s−1 and vertical wavelengths of 600 m. The jets are most pronounced in the depth range between 500 and 2500 m. Repeated direct velocity observations at 35°W indicate that the jets are coherent within ±1° of the equator. Individual jets can persist for 1–2 years, but they appear and decay rather irregularly. The equatorial stacked jets are also found in realistic general circulation model simulations. The features grow in amplitude with increasing horizontal and vertical model resolution. However, even at very high model resolutions, their amplitudes are still underestimated. In all model simulations, high levels of energy related to the stacked jets are found in the vicinity of the western boundary currents (WBCs). Depth range and strength of the WBCs in different experiments are related to depth range and strength of the jets. In the interior, stacked jets are characterized by eastward wave propagation suggesting that high baroclinic mode Kelvin waves radiate energy generated in the WBC into the interior and form the stacked jets.

Description: The extratropical large-scale atmospheric circulation is often described in terms of a few preferred and recurrent patterns referred to as weather regimes. Here, we investigate the influence of the observed Indian and western Pacific Ocean (IP) warming over the last decades, on the frequency of occurrence of North Atlantic weather regimes. A multi-model approach is adopted in which five different atmospheric general circulation models are forced with idealized sea surface temperature patterns mimicking the IP warming. Despite some discrepancies, three models suggest a stronger occurrence of the Zonal regime when IP is warm, compensated by less frequent Greenland Anticyclone regimes, consistently with the observed positive trend of the North Atlantic Oscillation. The other two models simulate instead an increase in the frequency of occurrence of the Atlantic Ridge regime. Variance decomposition into stationary and transient waves suggest two mechanisms at work in the individual models. The AR regime favoured occurrence is associated with a strong transient wave activity along the wave guide in the North Pacific and downstream in the North Atlantic. The Zonal favoured excitation is interpreted as an indirect response to changes in the Tropical Atlantic associated with a global alteration of the Walker cell.

Description: Eddy diffusivities in the Labrador Sea (LS) are estimated from deep eddy resolving float trajectories, moored current meter records, and satellite altimetry. A mean residence time of 248 days in the central LS is observed with several floats staying for more than 2 years. By applying a simple random walk diffusion model, the observed distribution of float residence times in the central LS could be explained by a mean eddy diffusivity of about 300 m2 s−1. Estimates from float trajectories themselves and from moored current meter records yield significantly higher eddy diffusivities in the central LS of 950–1100 m2 s−1. This discrepancy can be explained by an inhomogeneity of the eddy diffusivity at middepth with high/low values in the central LS/region between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE. between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE.

Description: [1] The Terceira Rift formed relatively recently (∼1 Ma ago) by rifting of the old oceanic lithosphere of the Azores Plateau and is currently spreading at a rate of 2–4mm/a. Together with the Mid-Atlantic Ridge, the Terceira Rift forms a triple junction that separates the Eurasian, African, and American Plates. Four volcanic systems (São Miguel, João de Castro, Terceira, Graciosa), three of which are islands, are distinguished along the axis and are separated by deep avolcanic basins similar to other ultraslow spreading centers. The major element, trace element and Sr-Nd-Pb isotope geochemistry of submarine and subaerial lavas display large along-axis variations. Major and trace element modeling suggests melting in the garnet stability field at smaller degrees of partial melting at the easternmost volcanic system (São Miguel) compared to the central and western volcanoes, which appear to be characterized by slightly higher melting degrees in the spinel/garnet transition zone. The degrees of partial melting at the Terceira Rift are slightly lower than at other ultraslow mid-ocean ridge spreading axes (Southwest Indian Ridge, Gakkel Ridge) and occur at greater depths as a result of the melting anomaly beneath the Azores. The combined interaction of a high obliquity, very slow spreading rates, and a thick preexisting lithosphere along the axis probably prevents the formation and eruption of larger amounts of melt along the Terceira Rift. However, the presence of ocean islands requires a relatively stable melting anomaly over relatively long periods of time. The trace element and Sr-Nd-Pb isotopes display individual binary mixing arrays for each volcanic system and thus provide additional evidence for focused magmatism with no (or very limited) melt or source interaction between the volcanic systems. The westernmost mantle sources beneath Graciosa and the most radiogenic lavas from the neighboring Mid-Atlantic Ridge suggest a mantle flow from Graciosa toward the Mid-Atlantic Ridge and hence a flux of mantle material from one spreading axis into the other. The Terceira Rift represents a unique oceanic rift system situated within the thickened, relatively old oceanic lithosphere and thus exhibits both oceanic and continental features.

Description: [1] The global-scale quantification of organic carbon (Corg) degradation pathways in marine sediments is difficult to achieve experimentally due to the limited availability of field data. In the present study, a numerical modeling approach is used as an alternative to quantify the major metabolic pathways of Corg oxidation (Cox) and associated fluxes of redox-sensitive species fluxes along a global ocean hypsometry, using the seafloor depth (SFD) as the master variable. The SFD dependency of the model parameters and forcing functions is extracted from existing empirical relationships or from the NOAA World Ocean Atlas. Results are in general agreement with estimates from the literature showing that the relative contribution of aerobic respiration to Cox increases from 〈10% at shallow SFD to 〉80% in deep-sea sediments. Sulfate reduction essentially follows an inversed SFD dependency, the other metabolic pathways (denitrification, Mn and Fe reduction) only adding minor contributions to the global-scale mineralization of Corg. The hypsometric analysis allows the establishment of relationships between the individual terminal electron acceptor (TEA) fluxes across the sediment-water interface and their respective contributions to the Corg decomposition process. On a global average, simulation results indicate that sulfate reduction is the dominant metabolic pathway and accounts for approximately 76% of the total Cox, which is higher than reported so far by other authors. The results also demonstrate the importance of bioirrigation for the assessment of global species fluxes. Especially at shallow SFD most of the TEAs enter the sediments via bioirrigation, which complicates the use of concentration profiles for the determination of total TEA fluxes by molecular diffusion. Furthermore, bioirrigation accounts for major losses of reduced species from the sediment to the water column prohibiting their reoxidation inside the sediment. As a result, the total carbon mineralization rate exceeds the total flux of oxygen into the sediment by a factor of 2 globally.

Description: We document through the analysis of 2002–2005 observational data the recent Atlantic Water (AW) warming along the Siberian continental margin due to several AW warm impulses that penetrated into the Arctic Ocean through Fram Strait in 1999–2000. The AW temperature record from our long-term monitoring site in the northern Laptev Sea shows several events of rapid AW temperature increase totaling 0.8°C in February–August 2004. We hypothesize the along-margin spreading of this warmer anomaly has disrupted the downstream thermal equilibrium of the late 1990s to earlier 2000s. The anomaly mean velocity of 2.4–2.5 ± 0.2 cm/s was obtained on the basis of travel time required between the northern Laptev Sea and two anomaly fronts delineated over the Eurasian flank of the Lomonosov Ridge by comparing the 2005 snapshot along-margin data with the AW pre-1990 mean. The magnitude of delineated anomalies exceeds the level of pre-1990 mean along-margin cooling and rises above the level of noise attributed to shifting of the AW jet across the basin margins. The anomaly mean velocity estimation is confirmed by comparing mooring-derived AW temperature time series from 2002 to 2005 with the downstream along-margin AW temperature distribution from 2005. Our mooring current meter data corroborate these estimations.

Description: Focused fluid expulsion at cold vents is a common feature of subduction zones, serving as an important backflux of water and volatile elements to the oceanic reservoir. The strong enrichment of iodine in fluids collected from mounds along the Central American convergent Margin allowed the determination of 129I/I ratios for age calculations in order to determine potential source formations in this active, erosional margin. The majority of the determined iodine ages are between 40 and 20 Ma. Because these ages are older than the age of host sediments and underthrust sediments on the oceanic plate (〈18 Ma), a major contribution of iodine must come from old, organic rich sources in the upper plate. Both the iodine concentrations and ages determined for the mounds in this study are similar to reported values for hydrate fields at accretionary margins, indicating that iodine and associated organic carbon cycling at both erosional and accretionary margins may occur on similar time scales.

Description: Cenozoic biostratigraphic, cosmogenic isotope, magnetostratigraphic, and cyclostratigraphic data derived from Integrated Ocean Drilling Program Expedition 302, the Arctic Coring Expedition (ACEX), are merged into a coherent age model. This age model has low resolution because of poor core recovery, limited availability of biostratigraphic information, and the complex nature of the magnetostratigraphic record. One 2.2 Ma long hiatus occurs in the late Miocene; another spans 26 Ma (18.2–44.4 Ma). The average sedimentation rate in the recovered Cenozoic sediments is about 15 m/Ma. Core-seismic correlation links the ACEX sediments to the reflection seismic stratigraphy of line AWI-91090, on which the ACEX sites were drilled. This seismostratigraphy can be correlated over wide geographic areas in the central Arctic Ocean, implying that the ACEX age model can be extended well beyond the drill sites.

Description: Energy levels of internal waves are estimated from seismic reflection data. Three legacy seismic sections from 1993 and 1997 obtained off the Iberian Peninsula have been analyzed for acoustic reflections within the water column. The reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m. Depth variations of these reflections are thought to be caused by the background internal wave field. From the variations we derive horizontal wave number spectra of normalized internal wave displacement. The general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves. Significant differences within the sections can be found when sufficiently large subsections are averaged. The spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy.

Description: The upper branch of the meridional overturning circulation in the North Atlantic is fed by cross‐equatorial transport of various water masses from the Southern Hemisphere. Here, we study the large‐scale spreading of South Atlantic Water (SAW) into the western tropical North Atlantic from the equator to 25°N. The fractions of SAW in the upper ocean water masses are quantified using a water mass analysis applied on a data set of conductivity‐temperature‐depth data from the Hydrobase project and the Argo float program. To fill gaps in the data coverage and to gain insight into the mechanisms involved, the observations are complemented with results from the high‐resolution Family of Linked Atlantic Model Experiments model (equation image°), which has been shown to realistically simulate the inflow of SAW into the Caribbean. The analysis reveals the mean SAW propagation pathways in the North Atlantic and identifies the regions of largest variability. High SAW fractions in the thermocline and central water layers are limited to the region south of 10°N, where the water body consists of 80%–90% SAW. Thus, the zonal currents in the equatorial gyre are mainly formed of SAW. The weaker currents in the intermediate layer combined with a northward excursion of the North Equatorial Current allow the SAW in this layer to intrude farther north compared to the layers above. The transition into North Atlantic Water occurs gradually from 12°N to 20°N in the intermediate layer.

Description: Strontium and neodymium radiogenic isotope ratios in early to middle Eocene fossil fish debris (ichthyoliths) from Lomonosov Ridge (Integrated Ocean Drilling Program Expedition 302) help constrain water mass compositions in the Eocene Arctic Ocean between ∼55 and ∼45 Ma. The inferred paleodepositional setting was a shallow, offshore marine to marginal marine environment with limited connections to surrounding ocean basins. The new data demonstrate that sources of Nd and Sr in fish debris were distinct from each other, consistent with a salinity-stratified water column above Lomonosov Ridge in the Eocene. The 87Sr/86Sr values of ichthyoliths (0.7079–0.7087) are more radiogenic than Eocene seawater, requiring brackish to fresh water conditions in the environment where fish metabolized Sr. The 87Sr/86Sr variations probably record changes in the overall balance of river Sr flux to the Eocene Arctic Ocean between ∼55 and ∼45 Ma and are used here to reconstruct surface water salinity values. The ɛNd values of ichthyoliths vary between −5.7 and −7.8, compatible with periodic (or intermittent) supply of Nd to Eocene Arctic intermediate water (AIW) from adjacent seas. Although the Norwegian-Greenland Sea and North Atlantic Ocean were the most likely sources of Eocene AIW Nd, input from the Tethys Sea (via the Turgay Strait in early Eocene time) and the North Pacific Ocean (via a proto-Bering Strait) also contributed.

Description: As one of the few places in the ocean where winter cooling and mixing creates conditions where water from the surface can penetrate into the deep ocean the Labrador Sea is an area of interest to people studying climate change in the ocean. Persistent cloud cover over this area makes it impossible to use infrared satellite imagery to relate space/time changes in sea surface temperature (SST) to changes in surface currents and air-sea interaction. Using passive microwave SSTs from the Advanced Microwave Scanning Radiometer (AMSR-E), we plot space/time changes in SST in the Labrador Sea and relate these changes to both simultaneous in situ measurements of temperature and numerical model SSTs. A direct comparison between the microwave SSTs, infrared SSTs, and in situ temperatures measured from profiling floats reveals that the microwave SSTs are a good representation of space/time changes in infrared SST and in ocean temperatures down to 10 m below the sea surface. Comparisons between the microwave SSTs and time series of temperatures at depths below 50 m reveal that winter/spring surface cooling makes the SST similar to temperatures at these deeper depths in the convection region of the central Labrador Sea. Detailed comparison of the annual cycle between the microwave SSTs and the model SST and 10 m currents reveals overall good agreement and some interesting differences.

Description: Seismic reflection profiles across the Hikurangi Plateau Large Igneous Province and adjacent margins reveal the faulted volcanic basement and overlying Mesozoic-Cenozoic sedimentary units as well as the structure of the paleoconvergent Gondwana margin at the southern plateau limit. The Hikurangi Plateau crust can be traced 50–100 km southward beneath the Chatham Rise where subduction cessation timing and geometry are interpreted to be variable along the margin. A model fit of the Hikurangi Plateau back against the Manihiki Plateau aligns the Manihiki Scarp with the eastern margin of the Rekohu Embayment. Extensional and rotated block faults which formed during the breakup of the combined Manihiki-Hikurangi plateau are interpreted in seismic sections of the Hikurangi Plateau basement. Guyots and ridge-like seamounts which are widely scattered across the Hikurangi Plateau are interpreted to have formed at 99–89 Ma immediately following Hikurangi Plateau jamming of the Gondwana convergent margin at ∼100 Ma. Volcanism from this period cannot be separately resolved in the seismic reflection data from basement volcanism; hence seamount formation during Manihiki-Hikurangi Plateau emplacement and breakup (125–120 Ma) cannot be ruled out. Seismic reflection data and gravity modeling suggest the 20-Ma-old Hikurangi Plateau choked the Cretaceous Gondwana convergent margin within 5 Ma of entry. Subsequent uplift of the Chatham Rise and slab detachment has led to the deposition of a Mesozoic sedimentary unit that thins from ∼1 km thickness northward across the plateau. The contrast with the present Hikurangi Plateau subduction beneath North Island, New Zealand, suggests a possible buoyancy cutoff range for LIP subduction consistent with earlier modeling.

Description: The South Equatorial Undercurrent (SEUC) in the western to central tropical Atlantic is investigated by a combination of shallow floats, with a few acoustically tracked, shipboard current measurements and hydrography. Float trajectories show a well confined SEUC revealing large standing meanders near its western origin. Transports determined from 31 sections across the SEUC increase from 5.6 Sv at 35°W near the western boundary to 10.2 Sv 800 km farther east. Internal recirculations north and south of the SEUC were indicated by the float trajectories and a weak transport reduction farther along its eastward progression is observed. The deep part of the South Equatorial Current carries on both sides of the SEUC interior water masses westward, and supplies almost 5 Sv to the SEUC between 35°W and 28°W, or about half of the SEUC transport in the interior tropical Atlantic.

Description: The anomalously strong hurricane activity in the Atlantic sector during the recent years led to a controversy about the impact of global warming on hurricane activity in the Atlantic sector. Here we show that the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans (Indo-Pacific) is a key parameter in controlling the vertical wind shear over the Atlantic, an important quantity for hurricane activity. The stronger warming of the tropical North Atlantic relative to that of the Indo-Pacific during the most recent years drove reduced vertical wind shear over the Atlantic and is thus responsible for the strong hurricane activity observed. In 2006, however, the temperature difference between the tropical North Atlantic and the tropical Indian and Pacific Oceans is much reduced, which explains the relatively weak hurricane season.