ALBERT

Description: Changes of the meridional overturning circulation (MOC) due to surface heat flux variability related to the North Atlantic Oscillation (NAO) are analyzed in various ocean models, i.e., eddying and non‐eddying cases. A prime signature of the forcing is variability of the winter‐time convection in the Labrador Sea. The associated changes in the strength of the MOC near the subpolar front (45°N) are closely related to the NAO‐index, leading MOC anomalies by about 2–3 years in both the eddying and non‐eddying simulation. Further south the speed of the meridional signal propagation depends on model resolution. With lower resolution (non‐eddying case, 4/3° resolution) the MOC signal propagates equatorward with a mean speed of about 0.6 cm/s, similar as spreading rates of passive tracer anomalies. Eddy‐permitting experiments (1/3°) show a significantly faster propagation, with speeds corresponding to boundary waves, thus leading to an almost in‐phase variation of the MOC transport over the subtropical to subpolar North Atlantic.

Description: The long-term data sets of total alkalinity (TA) (1929–2002 A.D.) and δ18O (1966–2002 A.D.) are used to investigate freshwater and brine distributions in the Arctic Ocean. Fractions of sea ice meltwater and other freshwaters (OF) (precipitation, river runoff, and freshwater carried by Pacific water implied as salinity deficit) are calculated on the basis of salinity-TA and salinity-δ18O relationships. Rejected brine during sea ice growth resides in surface water in the central Arctic Ocean, while net melting is found along the surface flow of water from the Pacific and Atlantic oceans. Distribution of OF at 10 m water depth suggests that Russian runoff leaves the shelf mainly west of the Mendeleyev Ridge, enters into the deep basin, and exits from the ocean through the western part of Fram Strait. The influence of Mackenzie River water is limited in the region and in depth. Accumulation of freshwater in the Canadian Basin is caused by deep penetration of OF with brine, indicating the transport of freshwater by shelf-derived water. The major origin of shelf-derived water entering into the upper halocline layer in the Canadian Basin should be the Chukchi and East Siberian Sea shelves, and the main freshwater sources are the salinity deficit of Pacific water and/or Russian runoff. An increase in OF inventory accompanied by an increase in brine content may suggest an increase of the shelf-derived water supply into the western Canadian Basin in anticyclonic years.

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: In this study we present a global distribution pattern and budget of the minimum flux of particulate organic carbon to the sea floor (JPOCα). The estimations are based on regionally specific correlations between the diffusive oxygen flux across the sediment-water interface, the total organic carbon content in surface sediments, and the oxygen concentration in bottom waters. For this, we modified the principal equation of Cai and Reimers [1995] as a basic monod reaction rate, applied within 11 regions where in situ measurements of diffusive oxygen uptake exist. By application of the resulting transfer functions to other regions with similar sedimentary conditions and areal interpolation, we calculated a minimum global budget of particulate organic carbon that actually reaches the sea floor of ∼0.5 GtC yr−1 (〉1000 m water depth (wd)), whereas approximately 0.002–0.12 GtC yr−1 is buried in the sediments (0.01–0.4% of surface primary production). Despite the fact that our global budget is in good agreement with previous studies, we found conspicuous differences among the distribution patterns of primary production, calculations based on particle trap collections of the POC flux, and JPOCα of this study. These deviations, especially located at the southeastern and southwestern Atlantic Ocean, the Greenland and Norwegian Sea and the entire equatorial Pacific Ocean, strongly indicate a considerable influence of lateral particle transport on the vertical link between surface waters and underlying sediments. This observation is supported by sediment trap data. Furthermore, local differences in the availability and quality of the organic matter as well as different transport mechanisms through the water column are discussed.

Description: Time series of hydrographic and transient tracer measurements were used to study the variability of Greenland Sea water mass transformation between 1991 and 2000. Increases in tracer inventories indicate active renewal of Greenland Sea Intermediate Water (GSIW) at a rate of 0.1 to 0.2 Sv (1 Sv = 1 × 106 m3 s−1) (10-year average). A temperature maximum (Tmax) was established at the base of the upper layer (500 m) as a consequence of anomalously strong freshwater input into the near-surface layer at the beginning of the 1990s. Tmax rapidly descended to 1500 m by 1995 followed by a much slower rate of descent. GSIW became warmer and less saline compared to the 1980s. During the deepening phase of Tmax, atmospheric data revealed above-average wind stress curl and oceanic heat loss. In addition, high Arctic Ocean sea-ice export and lack of local sea-ice formation have been documented for that period. A combination of all these factors may have evoked the renewal of GSIW with anomalously freshwater from the upper layers. The Tmax layer established a stability maximum that inhibits vertical exchange between intermediate and deeper waters. Temperature and salinity of deep waters continued to increase at rates of 0.01°C yr−1 and 0.001 yr−1, respectively. However, since 1993, decrease in and homogenization of deep water transient tracer concentrations indicate that renewal occurred predominantly by addition of Arctic Ocean waters. In 2000 the water column (500 m to 3400 m) required an additional 60 W m−2 (110 W m−2) over the annual mean heat loss to restore its heat content to 1989 (1971) values.

Description: This study was motivated by a strong warming signal seen in mooring-based and oceanographic survey data collected in 2004 in the Eurasian Basin of the Arctic Ocean. The source of this and earlier Arctic Ocean changes lies in interactions between polar and sub-polar basins. Evidence suggests such changes are abrupt, or pulse-like, taking the form of propagating anomalies that can be traced to higher-latitudes. For example, an anomaly found in 2004 in the eastern Eurasian Basin took ∼1.5 years to propagate from the Norwegian Sea to the Fram Strait region, and additional ∼4.5–5 years to reach the Laptev Sea slope. While the causes of the observed changes will require further investigation, our conclusions are consistent with prevailing ideas suggesting the Arctic Ocean is in transition towards a new, warmer state.

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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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 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: An improved knowledge of iron biogeochemistry is needed to better understand key controls on the functioning of high-nitrate low-chlorophyll (HNLC) oceanic regions. Iron budgets for HNLC waters have been constructed using data from disparate sources ranging from laboratory algal cultures to ocean physics. In summer 2003 we conducted FeCycle, a 10-day mesoscale tracer release in HNLC waters SE of New Zealand, and measured concurrently all sources (with the exception of aerosol deposition) to, sinks of iron from, and rates of iron recycling within, the surface mixed layer. A pelagic iron budget (timescale of days) indicated that oceanic supply terms (lateral advection and vertical diffusion) were relatively small compared to the main sink (downward particulate export). Remote sensing and terrestrial monitoring reveal 13 dust or wildfire events in Australia, prior to and during FeCycle, one of which may have deposited iron at the study location. However, iron deposition rates cannot be derived from such observations, illustrating the difficulties in closing iron budgets without quantification of episodic atmospheric supply. Despite the threefold uncertainties reported for rates of aerosol deposition (Duce et al., 1991), published atmospheric iron supply for the New Zealand region is ∼50-fold (i.e., 7- to 150-fold) greater than the oceanic iron supply measured in our budget, and thus was comparable (i.e., a third to threefold) to our estimates of downward export of particulate iron. During FeCycle, the fluxes due to short term (hours) biological iron uptake and regeneration were indicative of rapid recycling and were tenfold greater than for new iron (i.e. estimated atmospheric and measured oceanic supply), giving an “fe” ratio (uptake of new iron/uptake of new + regenerated iron) of 0.17 (i.e., a range of 0.06 to 0.51 due to uncertainties on aerosol iron supply), and an “Fe” ratio (biogenic Fe export/uptake of new + regenerated iron) of 0.09 (i.e., 0.03 to 0.24).

Description: We measured the vertical water column distribution of nitrous oxide (N2O) during the European Iron Fertilization Experiment (EIFEX) in the subpolar South Atlantic Ocean during February/March 2004 (R/V Polarstern cruise ANT XXI/3). Despite a huge build‐up and sedimentation of a phytoplankton bloom, a comparison of the N2O concentrations within the fertilized patch with concentrations measured outside the fertilized patch revealed no N2O accumulation within 33 days. This is in contrast to a previous study in the Southern Ocean, where enhanced N2O accumulation occurred in the pycnocline. Thus, we conclude that Fe fertilization does not necessarily trigger additional N2O formation and we caution that a predicted radiative offset due to a Fe‐induced additional release of oceanic N2O might be overestimated. Rapid sedimentation events during EIFEX might have hindered the build‐up of N2O and suggest, that not only the production of phytoplankton biomass but also its pathway in the water column needs to be considered if N2O radiative offset is modeled.

Description: During the MATER experiment, an eddy characterized by marked Levantine Intermediate Water (LIW) in its core at intermediate depth, and called Sardinian Eddy (SE), has been observed in the Algerian Basin located in the western Mediterranean Sea. Here, results from a numerical simulation allow to investigate thoroughly such structures. The formation of SEs and their life cycle in the model are detailed and compared to in-situ observations. The formation of SEs is associated with the detachment from the continental slope of Sardinia of the large-scale cyclonic gyre found in the Algerian Basin. Once formed, SEs leave the slope westwards. They are strongly barotropic and about 3 to 4 SEs are created each year. At a later stage, SEs develop a baroclinic component as revealed by an emerging surface signature about 1 month after their formation. Coalescence processes and merging of these eddies are documented between Minorca and Sardinia where these eddies accumulate.

Description: Here we present a high-resolution marine sediment record from the El Niño region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Niño flood events on the continent are used as paleo–El Niño–Southern Oscillation proxy data. The onset of stronger El Niño activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Niño activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Niño events is 60–80 years. El Niño events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Niño flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions.

Description: To constrain the fluxes of methane (CH4) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (−63 to −66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L−1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (−56 to −28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (−25 to −6‰ PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of “downcurrent” plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h−1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × 104 mol h−1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally.

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. us to present an interpretation of how volcanic radiative forcing could cause the coupling.

Description: A new model of the marine ecosystem coupled into a global Earth System Climate Model suitable for long-term (multimillennial timescale) simulations is presented. The model is based on nitrate as the sole limiting nutrient. Prognostic equations for nutrients, phytoplankton, zooplankton, and detritus are solved online in the three-dimensional ocean circulation model component. Experiments with different parameterizations of vertical mixing, including a scheme of tidally driven mixing, changes in buoyancy forcing in the Southern Ocean, different particle sinking velocities, and the inclusion of dissolved organic matter are performed, and the results are compared with observations. The results reemphasize the roles of Southern Ocean freshwater forcing and diapycnal mixing in the low-latitude pycnocline in setting the global deep water circulation and properties. The influence of high mixing in the Southern Ocean as inferred from observations is much more limited. The deep water circulation also has a strong influence on the marine ecosystem and nutrient distributions. We demonstrate that larger values of vertical diffusion lead to a shallower nutricline due to increased upwelling. Export production and nutrient distributions respond sensitively to changes in mixing and to the ratio of particle sinking to remineralization in the upper ocean. The best fits to global measurements of temperature, salinity, deep ocean radiocarbon, mixed layer depth, nutrients, and chlorophyll are obtained for values of vertical mixing in the pycnocline of around 0.2–0.3 × 10−4 m2/s and for e-folding depth for particle remineralization of 100–200 m. A simple parameterization of dissolved organic matter dynamics increases primary production and nutrient concentrations in the upper ocean and improves chlorophyll distributions in the subtropical gyres but has no discernible influence on particulate export fluxes. Remaining model deficiencies are identified, and strategies for future model improvement are outlined.

Description: Four indirect methods to determine carbon and nutrient regeneration ratios in the ocean are applied to results from a physical-biogeochemical model with prescribed element ratios for organic matter (de-)composition. The aim is to test whether these methods are suitable to reproduce C org :N:P element ratios of organic matter remineralization, which in contrast to the real ocean are exactly known in the model framework. The model experiment is carried out using the classical C:N:P Redfield ratio of 106:16:1 for production and decomposition of organic material under preindustrial atmospheric pCO2. Two methods rely on predefined end member values, while the others do not. The first method is a simple linear regression of two parameters, neglecting mixing effects, and yields remineralization signals biased by isopycnal tracer gradients induced by contributions of different water masses. The second method is based on multiple linear regression of three parameters, includes mixing of three, but not-prescribed end members. It can, in part, reproduce the prescribed remineralization ratios. However, considerable bias appears as a result of water mass mixing. The third method considers isopycnal mixing of three prescribed end member water masses by using temperature/salinity as conservative tracers on the two density surfaces σΘ = 26.8 and σΘ = 27.2. On the basis of a mixing triangle approach, the method is able to reproduce the regeneration rates best in the low latitudes, where the integrated signal of remineralization is high. The fourth method uses the full set of available parameters to derive mixing fractions and remineralization and is applied to the density range from σΘ = 26.8 to σΘ = 27.2, yielding the best reproduction of prescribed remineralization ratios. As expected, results from the last two methods are sensitive to the choice of end member concentrations. In general, best agreement between modeled and reconstructed ratios is found between 20°N and 20°S and deviations occur toward the outcrop regions, which we account to the low amount of remineralized material together with uncertainties in prescribed end member values. Our investigation shows how apparent variability of remineralization ratios can be generated through methodological shortcomings only.

Description: Recent studies comparing shipboard data between the 1950's and the 1990's have shown significant, heterogeneous adjustments of the temperature-salinity structure of the N. Atlantic Ocean. Here, we present proxy records of temperature and salinity from aragonite sclerosponge skeletons, extending existing records of the Salinity Maximum Waters (SMW) of the N. Atlantic back to 1890. These proxy records show secular temperature increases of 1.6–2.0°C, higher than published global averages, and salinity increases of 0.35–0.5 psu, smaller than short-term secular trends recently measured. Salinity reconstructions vary more significantly on the decadal scale, showing changes that are related to low-frequency variations of the North Atlantic Oscillation (NAO). On both secular and decadal time scales, the records indicate significant thermohaline changes in the SMW, either via forcing at the surface or increasing depths of density surfaces in the Bahamas.

Description: The global thermohaline circulation is an important part of Earth's climate system. Cold, dense water formed in the Nordic Seas enters the Atlantic Ocean as overflows across the sills of the Greenland-Scotland Ridge. The Denmark Strait Overflow (DSO) is one of the main sources of North Atlantic Deep Water. Until now the DSO has been believed to be stable on interannual timescales. Here, for the first time, evidence is presented from a 4-year program of observations showing that overflow transports in 1999/2000 were approximately 30% higher than previous estimates. Later, transports decreased remarkably during the observation period, coincident with a temporary temperature increase of about 0.5°C.

Description: The impact of 11-year solar cycle variations on stratospheric ozone (O3) is studied with the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB-CMAM-CHEM). To consider the effect of variations in charged particle precipitation we included an idealized NO x source in the upper mesosphere representing relativistic electron precipitation (REP). Our results suggest that the NO x source by particles and its transport from the mesosphere to the stratosphere in the polar vortex are important for the solar signal in stratospheric O3. We find a positive dipole O3 signal in the annual mean, peaking at 40–45 km at high latitudes and a negative O3 signal in the tropical lower stratosphere. This is similar to observations, but enhanced due to the idealized NO x source and at a lower altitude compared to the observed minimum. Our results imply that this negative O3 signal arises partly via chemical effects.

Description: In this study, direct measurements of the air/ sea flux of acetone were made over the North Pacific Ocean. The results demonstrate that the net flux of acetone is into, rather than out of the oceans. The extrapolated global ocean uptake of 48 Tg yr(-1) requires a major revision of the atmospheric acetone budget. This result is consistent with a recent reevaluation of acetone photodissociation quantum yields.

Description: Global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the North Atlantic that disrupted deepwater production. These massive freshwater pulses known as Heinrich events arose, in part, from instabilities within the Laurentide ice sheet. Paleoevidence from the North Atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the Northern Hemisphere. In the tropical western Pacific sea, surface temperatures and salinity varied together with ocean and climate changes at high latitudes. Here we present results from coupled modeling experiments that shed light on a possible dynamical link between the North Atlantic Ocean and the western tropical Pacific. This link involves a global oceanic standing wave pattern brought about by millennial-scale glacial density variations in the North Atlantic, atmospheric teleconnections triggered by meridional sea surface temperature gradients, and local air-sea interactions. Furthermore, our modeling results are compared with hydrological records from the Cariaco basin, the Indian Ocean, the Sulu Sea, and northern Australia.

Description: We reconstruct Caribbean seawater temperatures from sclerosponge Sr/Ca ratios using a specimen of Ceratoporella nicholsoni that grew at 20 m below sea level in a reef cave at Jamaica. We sample the time interval from 1620 to 1745 A.D. with almost monthly resolution. This interval includes the Maunder sunspot minimum, one of the coldest periods of the Little Ice Age. Reconstructed annual temperature amplitudes are on the order of about 1°C. The mean growth rate calculated from the annual Sr/Ca variations corresponds perfectly with U-Th-based growth rates. We find that the interannual climate variability is determined by El Niño–Southern Oscillation and by a decadal signal, most likely originating from the tropical North Atlantic. On a multidecadal timescale the Maunder Minimum is characterized by a 1°–2°C cooling and reduced amplitudes of the interannual and decadal temperature variations.

Description: The assumption that abiotic air-sea gas exchange is, via the temperature dependence of the gas' solubility, proportional to the surface heat flux is often used to distinguish between physically and biotically inferred oxygen fluxes across the sea surface. We quantitatively investigate its validity in the context of an eddy-permitting circulation model that contains an abiotic oxygen compartment. In the model, the “true” abiotic oxygen air-sea fluxes are systematically lower than those predicted by the air-sea heat flux relation. This discrepancy is caused by the nonlinear relationship between temperature and solubility that results in the saturation of a mixed water parcel being higher than the arithmetic mean saturation of the mixed components. This effect results in a simulated additional sea-to-air oxygen flux of about 0.5 mol O2 m−2 a−1 north of 40°N, which is not accounted for by the heat-flux relation and which is of similar magnitude as, though at the lower end of, biotically induced oxygen fluxes. Simulated outgassing of the model's abiotic oxygen is also higher than that predicted by the heat-flux relation at the equator (by ≈0.25 mol O2 m−2 a−1), where numerical artifacts endemic to state-of-the-art z level ocean models are found to affect simulated air-sea gas exchange. In addition to discrepancies in the annual mean fluxes, model results also indicate that the subtropical seasonal cycle in abiotic air-sea oxygen exchange is smaller by approximately 20% than the estimate based on air-sea heat fluxes, a result consistent with admittedly sparse observations of argon saturation.

Description: [1] As the eastern Galápagos Spreading Center (GSC) shallows westward toward the Galápagos Archipelago, axial morphology evolves from a low-relief, valley-and-ridge terrain to an increasingly prominent axial ridge, closely mirroring the western GSC. Between the Inca Transform (∼85.5°W) and its western termination near 91°W, the eastern GSC comprises seven morphological segments, separated by five morphological discontinuities and the eastward propagating 87°W overlapping spreading center. Combined morphologic and geochemical data divide the eastern GSC into two domains independent of the fine-scale morphologic segmentation. The western domain is defined by its axial ridge morphology and highly variable lava population. Elemental data define steep along-axis gradients, reflecting a complex source that includes one or more hot spot–related components in addition to a highly depleted component. The eastern domain is defined by transitional, valley-and-ridge morphologies and a surprisingly invariant lava population. This population is dominated by shallow crystal fractionation processes and displays significantly less variability attributable to multiple source components. The Galápagos hot spot has long been known to have a symmetrical, long-wavelength influence on crustal accretion along the GSC. Existing isotopic and new elemental data define twin “geochemical peaks” that we interpret as loci for transfer of distinct source components from the Galápagos plume to the GSC. Although Na8 and Fe8 values lie within the negatively correlated global array, Na8 increases with decreasing axial depth, contrary to global trends and consistent with emerging deep, hydrous melting models that predict decreasing overall extent of melting despite increasing melt production. Support for hydrous melting comes from decreasing heavy REE, increasing La/Sm and La/Yb, and the systematics of decreasing FeO and increasing CaO and Al2O3 with decreasing distance to the hot spot. Overall, an enriched, deep melt component appears to coexist in the shallow mantle with a ubiquitous, depleted primitive melt component, consistent with new models for channelized melt flow connecting a deep hydrous melt regime with the dry shallow mantle. Nevertheless, an absence of low-Fe lavas suggests that hydrous melting is strictly limited beneath the eastern GSC, becoming dominant only near the western geochemical peak where input from a hydrous “Northern” or “Wolf-Darwin” plume component is inferred.

Description: In 1996, about 320 kg of SF6 were introduced in the center of the Greenland Sea gyre. We use this signal together with the CFC distribution to follow the spreading of Greenland gyre water from the Denmark Strait through the Irminger Basin and the Labrador Sea to the Grand Banks. In the summer of 2003 Denmark Strait Overflow Water tagged with deliberately released SF6 could be traced throughout the Irminger Basin to the central Labrador Sea, confirming that water with potential density of 28.045 contributes to the Denmark Strait Overflow. The upper limit of the transfer time from the central Greenland Sea to the Labrador Sea was found to be 7 years. This study suggests that roughly 4 kg of excess SF6 has been transported over the Denmark Strait and confirm earlier reported transport through the Faroe Bank Channel. These results should be considered when using SF6 as a transient tracer.

Description: The Gravity Recovery and Climate Experiment (GRACE) processing centers at the GeoForschungsZentrum Potsdam (GFZ) and the University of Texas Center for Space Research (UTCSR) provide time series of monthly gravity field solutions covering the period since mission launch in March 2002. Although the achieved accuracy still remains an order of magnitude below the mission's baseline goal, these time series have successfully been used to study terrestrial phenomena such as water storage variations. Over the oceans, the monthly gravity field solutions can be converted into estimates of the fluctuating ocean bottom pressure (OBP), which is the sum of atmospheric and oceanic mass variations. The GRACE products may be validated against in situ OBP observations which are available from a ground truth site in the tropical northwest Atlantic Ocean. Large differences are observed between the in situ and GRACE-derived OBP which are investigated by comparing the tidal and nontidal ocean models used at GFZ and UTCSR for dealiasing short-term (〈2 months) mass variations from satellite measurements. Results show that the barotropic nontidal and tide models need improvement at periods shorter than 1 day and longer than 2 weeks. On a global scale the monthly OBP fields from GRACE generally overestimate the variability compared to ocean general circulation models, especially in tropical regions. This may be attributed to continuing deficiencies in GRACE data processing. Nevertheless, there is some initial evidence that GRACE possesses the potential to observe large-scale averages of bottom pressure fluctuations.

Description: Emissions of methane gas from cold seeps on the seafloor have a strong impact on a number of biogeochemical processes. These processes include the development of deepsea benthic ecosystems via the process of anaerobic oxidation of methane [Boetius et al., 2000] or the precipitation of carbonates [Ritger et al., 1987]. The fluxes of other chemical species associated with methane emissions may even influence the chemical composition of seawater [Aloisi et al., 2004]. Such gas emissions may have been much more intensive in the past with a strong impact on global climate [Dickens, 1999], as suggested by carbon isotope data.

Description: Most climate models predict a weakening of the North Atlantic thermohaline circulation for the 21st century when forced by increasing levels of greenhouse gas concentrations. The model spread, however, is rather large, even when the forcing scenario is identical, indicating a large uncertainty in the response to forcing. In order to reduce the model uncertainties a weighting procedure is applied considering the skill of each model in simulating hydrographic properties and observation-based circulation estimates. This procedure yields a “best estimate” for the evolution of the North Atlantic THC during the 21st century by taking into account a measure of model quality. Using 28 projections from 9 different coupled global climate models of a scenario of future CO2 increase (SRESA1B) performed for the upcoming fourth assessment report of the Intergovernmental Panel on Climate Change, the analysis predicts a gradual weakening of the North Atlantic THC by 25(±25)% until 2100.

Description: Subduction erosion has dominated the evolution of the north Chile convergent continental margin since at least the Mesozoic. We investigate the structure of the Antofagasta (23°S) sector of this margin along a transect using coincident wide-angle and near-vertical seismic profiling and gravity data. A 2-D velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography. A velocity-derived density distribution was used to model marine gravity data and substantiate the velocity model. The gravity and velocity models imply that the overriding plate is mainly made of arc-type igneous basement. The upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion, the top coincident with a reflection in near-vertical seismic images. The seismic boundary is interpreted as a detachment separating an upper extended domain with large-scale normal faulting from a lower domain apparently undergoing a different type of deformation. Velocity-derived porosity indicates that the front of the margin is probably fluid-saturated and disaggregated by fracturation as a consequence of frontal subduction erosion. Fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate. At depths greater than ∼20 km, porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured. The change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 Antofagasta earthquake (M w = 8.0) defining the seismogenic zone.

Description: [1] The effects of low-temperature alteration on the Rb-Sr and Sm-Nd isotope systems were investigated in 14–28 Ma mid-ocean ridge basalts recovered during Ocean Drilling Program (ODP) Leg 187 from the Australian Antarctic Discordance through comparison of pristine glass and associated variably altered basalts. Both Nd and Sm are immobile during low-temperature alteration, and 143Nd/144Nd displays mantle values even in heavily altered samples. In contrast, 87Sr/86Sr and Rb concentrations increase during seawater-rock interaction, which is especially apparent in single samples with macroscopically zoned alteration domains. The increase in 87Sr/86Sr roughly correlates with the visible degree of alteration, indicating a higher seawater/rock ratio in the more altered samples. Sr concentrations, however, do not systematically increase with increasing degree of alteration, most likely reflecting exchange of Sr in smectite interlayer sites. The degree of alteration in the uppermost oceanic crust of the Australian Antarctic Discordance is independent of crustal age. A comparison with literature data for young and old altered oceanic crust suggests that most low-temperature alteration occurs within a few million years after formation of the oceanic crust, probably reflecting greater fluid flux through the crust during its early history as a result of higher permeability and increased fluid circulation near the ridge.

Description: We compare alkalinity and total dissolved inorganic carbon (DIC) measurements made during the Transient Tracers in the Ocean, North Atlantic Study (TTO‐NAS) in 1981 with modern measurements from a TTO reoccupation cruise in 2004 (M60/5). We find that the TTO‐NAS alkalinity values are 3.6 ± 2.3 μmol kg−1 higher than modern alkalinity data tied to Certified Reference Materials. The TTO‐NAS DIC values re‐calculated from original alkalinity and discrete‐pCO2 data using currently accepted constants are 3.8 μmol kg−1 higher than those reported in the revised TTO data set. This difference is reduced to 0.7 μmol kg−1 when our suggested correction to the TTO‐NAS alkalinity is applied. These re‐calculated DIC values are 2.4 μmol kg−1 too low relative to contemporaneous measurements made by the vacuum extraction/manometric Certified method. Application of this correction brings the TTO data into almost perfect agreement with modern measurements for slowly‐ventilated deep water of the eastern Atlantic.

Description: Conducted in the northeast Atlantic Ocean (15°20′–21°20′W, 38°N–45°N), the Programme Océan Multidisciplinaire Méso Echelle (POMME) is a research project aimed at a better understanding of the biological production and the carbon budget of the region in relation to the formation mechanisms of the 11°–12°C mode water of the northeast Atlantic. With the help of two research vessels, several tens of floats and drifters, and nine moorings, the field experiment was carried out between autumn 2000 and autumn 2001, with a more intensive phase in the winter and early spring of 2001. The field experiment resolved small (several kilometers) to regional (several hundred kilometers) scales and daily to seasonal variability. A first analysis of the rich data set focused on the large-scale and the mesoscale variability. It shows that the distribution of water mass characteristics and biological activity is strongly influenced by the mesoscales in this supposedly quiet transition zone between the subtropical and subpolar gyres. The seasonal variability, however, presents an imprint of the large-scale structures with a clear north-south gradient in properties and budgets. This region is found on an annual average to be a sink of atmospheric CO2. Smaller scales, associated with fronts and filaments, were clearly observed in many fields (temperature, but also chlorophyll, oxygen, biogenic particles, etc.), with modeling studies suggesting that they play a significant role in subduction, ventilation, and transport of biogeochemical tracers in the POMME region.

Description: A new 0.5° resolution Mediterranean climatology of the mixed layer depth based on individual profiles of temperature and salinity has been constructed. The criterion selected is a threshold value of temperature from a near‐surface value at 10 m depth, mainly derived by a method applied on the global (de Boyer Montégut et al., 2004 dBM04). With respect to dBM04, the main differences reside in the absence of spatial interpolation of the final fields and in the improved spatial resolution. These changes to the method are necessary to reproduce the Mediterranean mixed layer's behavior. In the derived climatological maps, the most relevant features of the basin surface circulation are reproduced, as well as the areas prone of the deep water formation are clearly identified. Finally, the role of density in the definition of the mixed layer's differing behaviors between the oriental and the occidental regions of the basin is presented.

Description: By means of numerical modeling, we analyze the cycling of iron between its various physical (dissolved, colloidal, particulate) and chemical (redox state and organic complexation) forms in the upper mixed layer. With our proposed model it is possible to obtain a first quantitative assessment of how this cycling influences iron uptake by phytoplankton and its loss via particle export. The model is forced with observed dust deposition rates, mixed layer depths, and solar radiation at the site of the Bermuda Atlantic Time-series Study (BATS). It contains an objectively optimized ecosystem model which yields results close to the observational data from BATS that has been used for the data-assimilation procedure. It is shown that the mixed layer cycle strongly influences the cycling of iron between its various forms. This is mainly due to the light dependency of photoreductive processes, and to the seasonality of primary production. The daily photochemical cycle is driven mainly by the production of superoxide, and its amplitude depends on the concentration and speciation of dissolved copper. Model results are almost insensitive to the dominant form of dissolved iron within dust deposition, and also to the form of iron that is taken up directly during algal growth. In our model solutions, the role of the colloidal pumping mechanism depends strongly on assumptions on the colloid aggregation and photoreduction rate.

Description: Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.

Description: The S reflection west of Iberia has been interpreted as a low‐angle detachment fault separating crustal fault blocks from partially serpentinized mantle. We apply full waveform inversion to investigate the fine structure of S. Our results confirm that S is largely a step increase in velocity (and density), probably from crustal rocks to partially serpentinized mantle peridotites. A ∼50 m thick low velocity zone above S might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone. Both interpretations imply focused fluid flow along S, raising the possibility that low‐angle movement along S was aided by the development of local, transient high fluid pressures.

Description: We have studied faulting associated with bending of the incoming oceanic plate along segments of Middle America and Chile subduction zones and its relationship to intermediate-depth intraslab seismicity and slab geometry. Multibeam bathymetry shows that bending-related faulting forms patterns made of sets of faults with orientations ranging from parallel to almost perpendicular to the trench axis. These fault patterns may change along a single subduction zone within along-strike distances of several hundred kilometers or less. Where available, near-trench intraplate earthquakes show normal-fault focal mechanisms consistent with mapped bending-related normal faults. The strike of bending-related faults in the incoming oceanic plate is remarkably similar to the strike of the nodal planes of intermediate-depth earthquakes for each segment of the study areas. This similarity in strike is observed even for faults oriented very oblique to the trench and slab strikes. Thus, in the studied subduction zones, results strongly support that many intraslab earthquakes do not occur along the planes of maximum shear within the slab and that much intermediate-depth seismicity occurs by reactivation of faults formed by plate bending near the trench. Furthermore, a qualitative relationship between trench faulting and intraslab seismicity is indicated by segments of the incoming plate with pervasive bend-faulting that correspond to segments of the slabs with higher intermediate-depth seismicity.