Deep Southern Ocean Research Articles

The role of ocean circulation and regolith removal in triggering the Mid-Pleistocene Transition: Insights from authigenic Nd isotopes

Approximately 1,250,000 to 700,000 years ago, the pacing of glacial-interglacial cycles changed from 41,000 years to ∼100,000 years, a shift known as the ‘Mid-Pleistocene Transition’ (MPT). The cause – or causes – of this shift remain uncertain. However, changes in ocean circulation and removal of northern hemisphere regolith have both been proposed as potential triggers. Here, we present continuous, orbitally resolved reconstructions of deep ocean neodymium isotopes from three locations in the equatorial Atlantic, Indian and southwest Pacific oceans, spanning 1.7 million years from the Holocene to before the MPT, to test these two hypotheses. We find that global seawater neodymium isotope variability over glacial-interglacial cycles is controlled by changes in both neodymium input to the North Atlantic and deep ocean mixing. Using this neodymium isotope data, we show that enhanced northern hemisphere regolith removal began approximately 1.45 million years ago, ∼200,000 years prior to the onset of the MPT and ∼500,000 years prior to a major expansion in northern hemisphere ice sheets between ∼900,000 and 870,000 years ago. This ice sheet expansion was immediately preceded by an interval of reduced mixing of Atlantic-sourced waters into the deep southwest Pacific Ocean. We hypothesize that this circulation reorganization reflected increased stratification of the deep Southern Ocean interior, possibly in response to cooling and Antarctic sea ice expansion at this time. Taken together, these data suggest an expansion and/or thickening of northern hemisphere ice sheets during the MPT was facilitated by a combination of reduced northern hemisphere regolith cover alongside Southern Ocean circulation changes. Together, these shifts allowed the build up of larger northern hemisphere ice sheets that were more resistant to deglaciation, facilitating the longer glacial cycles of the post-MPT world.

Millennial‐Scale Carbon Flux Variability in the Subantarctic Pacific During Marine Isotope Stage 3 (57–29 ka)

AbstractAntarctic ice cores reveal a glacial climate state during Marine Isotope Stage 3 (MIS‐3; 57–29 ka) punctuated by millennial‐scale warming events and pulses of CO2. This study further explores how changes in Southern Ocean carbon cycling contributed to these millennial‐scale fluctuations in climate. Evidence from South Atlantic sediment cores suggests that warming events were associated with decreased dust‐borne iron flux, reduced export production, and increased upwelling from the deep Southern Ocean (SO). These processes are considered to have contributed to rising atmospheric CO2 during periods of rapid warming. Here we investigate whether the same processes occurred in the southwest Pacific sector of the SO at TAN1106‐28. We show that reduced New Zealand glaciation and localized iron limitation in the southwest Pacific led to reduced export production during millennial‐scale warming events. Decreases in foraminifera‐bound δ15N during all MIS‐3 warming events may reflect increased nutrient supply by upwelling. Increased calcium carbonate flux during MIS‐3 warming events likely reflects coccolithophore production in response to sea surface temperatures, which, would increase carbonate counter pump strength and reduce CO2 sequestration. Concomitant decreases in bottom water oxygen, inferred from redox‐sensitive U and Mn sediment concentrations, and increases in the 14C age of deep waters, suggest that old, nutrient‐rich waters influenced southwest Pacific middepth waters during warming events. This signature may reflect an expansion of Pacific Deep Water into the SO during warming. Taken together, our multi‐proxy data set reveals that the southwest subantarctic Pacific acted as a source of CO2 during millennial‐scale warming events of MIS‐3.

Disentangling Biological Transformations and Photodegradation Processes from Marine Dissolved Organic Matter Composition in the Global Ocean.

Dissolved organic matter (DOM) holds the largest amount of organic carbon in the ocean, with most of it residing in the deep for millennia. Specific mechanisms and environmental conditions responsible for its longevity are still unknown. Microbial transformations and photochemical degradation of DOM in the surface layers are two processes that shape its molecular composition. We used molecular data (via Fourier transform ion cyclotron resonance mass spectrometry) from two laboratory experiments that focused on (1) microbial processing of fresh DOM and (2) photodegradation of deep-sea DOM to derive independent process-related molecular indices for biological formation and transformation (Ibio) and photodegradation (Iphoto). Both indices were applied to a global ocean data set of DOM composition. The distributions of Iphoto and Ibio were consistent with increased photodegradation and biological reworking of DOM in sunlit surface waters, and traces of these surface processes were evident at depth. Increased Ibio values in the deep Southern Ocean and South Atlantic implied export of microbially reworked DOM. Photodegraded DOM (increased Iphoto) in the deep subtropical gyres of Atlantic and Pacific oceans suggested advective transport in warm-core eddies. The simultaneous application of Iphoto and Ibio disentangled and assessed two processes that left unique molecular signatures in the global ocean.

Transport of Anthropogenic Carbon From the Antarctic Shelf to Deep Southern Ocean Triggers Acidification

AbstractFlow of dense shelf water provide an efficient mechanism for pumping CO2 to the deep ocean along the continental shelf slope, particularly around the Antarctic bottom water (AABW) formation areas where much of the global bottom water is formed. However, the contribution of the formation of AABW to sequestering anthropogenic carbon (Cant) and its consequences remain unclear. Here, we show prominent transport of Cant (25.0 ± 4.7 Tg C yr−1) into the deep ocean (>2,000 m) in four AABW formation regions around Antarctica based on an integrated observational data set (1974–2018). This maintains a lower Cant in the upper waters than that of other open oceans to sustain a stronger CO2 uptake capacity (16.9 ± 3.8 Tg C yr−1). Nevertheless, the accumulation of Cant can further trigger acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr−1. Our findings elucidate the prominent role of AABW in controlling the Southern Ocean carbon uptake and storage to mitigate climate change, whereas its side effects (e.g., acidification) could also spread to other ocean basins via the global ocean conveyor belt.

Top-down control over dissolved organic carbon in the bottom water of the Weddell Sea and its implication for the continental shelf pump

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation’s capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 μM-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 ± 3.7 μM-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 ± 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Paleoproductivity and deep-sea oxygenation in Cosmonaut Sea since the last glacial maximum: impact on atmospheric CO2

The paleoproductivity in the Southern Ocean plays a crucial role in controlling the atmospheric CO2 concentration. Here, we present the sediment record of gravity core ANT37-C5/6-07, which was retrieved from the Cosmonaut Sea (CS), Indian Ocean sector of the Southern Ocean. We found that the change in the oxygen concentration in the CS bottom water is strongly correlated with the atmospheric CO2 fluctuations since the Last Glacial Maximum (LGM). Based on the change in the export production, we reconstructed the evolution history of the deep-water ventilation/upwelling in the study area. During the LGM, a large amount of respiratory carbon was stored in the deep Southern Ocean due to the effect of the low export productivity and restricted ventilation. The oxygen concentration was also low at this time. Despite the increase in paleoproductivity, the biological pump efficiency remained at a low level during the Last Deglaciation. Vast quantities of CO2 were released into the atmosphere through enhanced upwelling. The recovery of ventilation during this period facilitated the supply of oxygen-rich surface water to the deep ocean. Moreover, signals were identified during the transitions between the Heinrich Stage 1 (HS1), Antarctic Cold Reverse (ACR), and Younger Drays (YD) periods. During the Holocene, the productivity increased overall, and the oxygen in the bottom water was consumed but still remained at a high level. This may have been caused by the enhanced ventilation and/or the prevalence of East Cosmonaut Polynya (ECP) near Cape Ann.

Atlantic circulation changes across a stadial–interstadial transition

Abstract. We combine consistently dated benthic carbon isotopic records distributed over the entire Atlantic Ocean with numerical simulations performed by a glacial configuration of the Norwegian Earth System Model with active ocean biogeochemistry in order to interpret the observed Cibicides δ13C changes at the stadial–interstadial transition corresponding to the end of Heinrich Stadial 4 (HS4) in terms of ocean circulation and remineralization changes. We show that the marked increase in Cibicides δ13C observed at the end of HS4 between ∼2000 and 4200 m in the Atlantic can be explained by changes in nutrient concentrations as simulated by the model in response to the halting of freshwater input in the high-latitude glacial North Atlantic. Our model results show that this Cibicides δ13C signal is associated with changes in the ratio of southern-sourced (SSW) versus northern-sourced (NSW) water masses at the core sites, whereby SSW is replaced by NSW as a consequence of the resumption of deep-water formation in the northern North Atlantic and Nordic Seas after the freshwater input is halted. Our results further suggest that the contribution of ocean circulation changes to this signal increases from ∼40 % at 2000 m to ∼80 % at 4000 m. Below ∼4200 m, the model shows little ocean circulation change but an increase in remineralization across the transition marking the end of HS4. The simulated lower remineralization during stadials compared to during interstadials is particularly pronounced in deep subantarctic sites, in agreement with the decrease in the export production of carbon to the deep Southern Ocean during stadials found in previous studies.

Anthropogenic and internal drivers of wind changes over the Amundsen Sea, West Antarctica, during the 20th and 21st centuries

Abstract. Ocean-driven ice loss from the West Antarctic Ice Sheet is a significant contributor to sea-level rise. Recent ocean variability in the Amundsen Sea is controlled by near-surface winds. We combine palaeoclimate reconstructions and climate model simulations to understand past and future influences on Amundsen Sea winds from anthropogenic forcing and internal climate variability. The reconstructions show strong historical wind trends. External forcing from greenhouse gases and stratospheric ozone depletion drove zonally uniform westerly wind trends centred over the deep Southern Ocean. Internally generated trends resemble a South Pacific Rossby wave train and were highly influential over the Amundsen Sea continental shelf. There was strong interannual and interdecadal variability over the Amundsen Sea, with periods of anticyclonic wind anomalies in the 1940s and 1990s, when rapid ice-sheet loss was initiated. Similar anticyclonic anomalies probably occurred prior to the 20th century but without causing the present ice loss. This suggests that ice loss may have been triggered naturally in the 1940s but failed to recover subsequently due to the increasing importance of anthropogenic forcing from greenhouse gases (since the 1960s) and ozone depletion (since the 1980s). Future projections also feature strong wind trends. Emissions mitigation influences wind trends over the deep Southern Ocean but has less influence on winds over the Amundsen Sea shelf, where internal variability creates a large and irreducible uncertainty. This suggests that strong emissions mitigation is needed to minimise ice loss this century but that the uncontrollable future influence of internal climate variability could be equally important.

Multi-omics analysis of the specialised metabolism of two novel Pseudonocardia spp. isolated from the deep Southern Ocean

Multidrug-resistant pathogens have become a global threat. In this context, filamentous Actinobacteria has been proven to be an exceptional source of antimicrobial metabolites. In particular, rare actinomycetes isolated from marine environments have been proposed as a potential source of yet untapped specialised metabolites. In this study, two novel species, Pseudonocardia abyssalis sp. nov. and Pseudonocardia oceani sp. nov, isolated from deep Southern Ocean sediments are described, both in terms of their phenotypic and genomic characterization. Furthermore, the genomic architecture, with a focus on Biosynthetic Gene Cluster (BGC), across eight strains belonging to the two novel species were investigated. A total of 14 Gene Cluster Families (GCF) were identified, of which five GCFs comprise BGCs from both species, and nine were specific to each species. Moreover, a correlation of GCFs to phylogeny was observed. Following genome analysis, a comparative mass-spectrometry based metabolomics analysis was carried out with one strain from each new species, as well as Pseudonocardia ammonioxydans and Pseudonocardia sediminis, also of marine origin. The metabolomics profiles agreed with the GCF distribution, where a group of ubiquitous metabolites were produced by both new Pseudonocardia spp., while groups of species-specific metabolites were also detected. This metabolic-repertoire was found to be elicitated through the addition of N-acetyl glucosamine (GlcNAc), revealing chemically-inducible bioactivity against the fungi Candida albicas and multidrug-resistant Candida auris. These results showcase the power of a combined genomic-metabolomics approach to investigate rare-actinomycetes from understudied locations and have uncovered a wealth of both biosynthetic and chemical diversity for further investigation.

Covariation of Deep Antarctic Pacific Oxygenation and Atmospheric CO2 during the Last 770 kyr

Abstract We present new geochemical evidence of changes in oxygenation of the deep Antarctic Pacific over the last 770 kyr. Our data are derived from redox-sensitive metals and export production proxies extracted from gravity core ANT34/A2-10 at 4217 m water depth. Our results show that oxygen levels in the deep Antarctic Zone (AZ) varied in line with the release of deeply sequestered remineralized carbon to the atmosphere during glacial–interglacial (G–IG) cycles, with lower oxygen concentrations and more carbon storage during glacial periods. Subsequent reductions in the amount of carbon stored at depth were closely associated with improved ventilation during glacial terminations. The systematic and repeated glacial-to-interglacial increases in export production in the AZ region indicate a robust pattern of enhanced Southern Ocean (SO) ventilation during interglacial periods. In addition to the decline in atmospheric CO2 caused by iron fertilization in the Subantarctic AZ (SAZ) during the latter half of the glacial progression, decreases in productivity in the central AZ suggest that the weakening of SO ventilation induced deep AZ carbon sequestration and that this might have made a continuous additional contribution to the CO2 decline from each interglacial peak to glacial maximum. Observed variations in the degree of deep oxygenation and “organic carbon pump” efficiency in the central AZ might be driven primarily by physical “ventilation” processes (i.e., overturning circulation, mixing, and/or air–sea gas exchange). Our records of abyssal oxygenation in the central AZ, which vary in concert with atmospheric CO2 levels over the last several G–IG cycles, provide strong evidence that SO ventilation plays a significant role in controlling variations in both the amount of respired carbon sequestered in the deep ocean and atmospheric CO2 concentrations on G–IG timescales. Specifically, we suggest that the “organic carbon pump” (OCP) in the SAZ and the physical ventilation processes in the AZ (the “carbon venting valve”) acted together synergistically, but dominated at different intervals over G–IG cycles, to repeatedly switch the SO between carbon sink and carbon source, thereby modulating the atmospheric CO2 over the last 770 kyr. These findings provide new insights into the role of the AZ in controlling deep SO carbon sequestration and atmospheric CO2 levels in G–IG cycles.

Southern Ocean contribution to both steps in deglacial atmospheric CO2 rise

The transfer of vast amounts of carbon from a deep oceanic reservoir to the atmosphere is considered to be a dominant driver of the deglacial rise in atmospheric CO2. Paleoceanographic reconstructions reveal evidence for the existence of CO2-rich waters in the mid to deep Southern Ocean. These water masses ventilate to the atmosphere south of the Polar Front, releasing CO2 prior to the formation and subduction of intermediate-waters. Changes in the amount of CO2 in the sea water directly affect the oceanic carbon chemistry system. Here we present B/Ca ratios, a proxy for delta carbonate ion concentrations Δ[CO32−], and stable isotopes (δ13C) from benthic foraminifera from a sediment core bathed in Antarctic Intermediate Water (AAIW), offshore New Zealand in the Southwest Pacific. We find two transient intervals of rising [CO32−] and δ13C that that are consistent with the release of CO2 via the Southern Ocean. These intervals coincide with the two pulses in rising atmospheric CO2 at ~ 17.5–14.3 ka and 12.9–11.1 ka. Our results lend support for the release of sequestered CO2 from the deep ocean to surface and atmospheric reservoirs during the last deglaciation, although further work is required to pin down the detailed carbon transfer pathways.

Pseudonocardia abyssalis sp. nov. and Pseudonocardia oceani sp. nov., two novel actinomycetes isolated from the deep Southern Ocean.

The actinomycetes strains KRD168T and KRD185T were isolated from sediments collected from the deep Southern Ocean and, in this work, they are described as representing two novel species of the genus Pseudonocardia through a polyphasic approach. Despite sharing >99 % 16S rRNA gene sequence similarity with other members of the genus, comparative genomic analysis allowed species delimitation based on average nucleotide identity and digital DNA–DNA hybridization. The KRD168T genome is characterized by a size of 6.31 Mbp and a G+C content of 73.44 mol%, while the KRD185T genome has a size of 6.82 Mbp and a G+C content of 73.98 mol%. Both strains contain meso-diaminopimelic acid as the diagnostic diamino acid, glucose as the major whole-cell sugar, MK-8(H4) as a major menaquinone and iso-branched hexadecanoic acid as a major fatty acid. Biochemical and fatty acid analyses also revealed differences between these strains and their phylogenetic neighbours, supporting their status as distinct species. The names Pseudonocardia abyssalis sp. nov. (type strain KRD168T=DSM 111918T=NCIMB 15270T) and Pseudonocardia oceani (type strain KRD185T=DSM 111919T=NCIMB 15269T) are proposed.

Neodymium isotope evidence for coupled Southern Ocean circulation and Antarctic climate throughout the last 118,000 years

The chain of events surrounding the initiation and intensification of the last glacial cycle remain relatively poorly understood. In particular, the role of Southern Ocean paleocirculation changes is poorly constrained, in part, owing to a paucity of sedimentary records from this region. In this study we present multiproxy data – including neodymium isotope and sortable silt measurements – for paleocirculation changes within the deep (3167 m water depth) Indian sector of the Southern Ocean from a new sediment core, TT1811-34GGC (41.718°S, 80.163°E). We find a tight coupling between circulation changes, Antarctic climate, and atmospheric CO2 concentrations throughout the last 118,000 years, even during the initial stages of glacial inception of Marine Isotope Stage (MIS) 5.4 to 5.1. We find that periods of cooling correspond to reductions in the entrainment of North Atlantic-sourced waters within the deep Southern Ocean, as evidenced by more radiogenic neodymium isotope values of deep water bathing our core site. Cooling also corresponds to generally slower bottom water flow speeds, as indicated by finer sortable silt size fractions. A reduction in entrainment of North-Atlantic sourced waters occurred during MIS 5.4–5.1, when Atlantic circulation was strong, suggesting a Southern hemisphere control on paleocirculation changes at that time. We hypothesise that expanded Southern Ocean sea-ice during MIS 5.4 increased the density of the deep Southern Ocean, reducing the ability of Atlantic-sourced waters to mix into Lower Circumpolar Deep Water. This led to an expanded contribution of Pacific Deep Water within the lower circulation cell and increased stratification within the deep Southern Ocean. These paleocirculation changes can help account for the reduction in atmospheric CO2 across the MIS 5.5 to 5.4 transition, and in doing so help explain the chain of events surrounding the decent into the last glacial period.

On the control of subantarctic stratification by the ocean circulation

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.

Changes in Antarctic Bottom Water Formation During Interglacial Periods

AbstractIn the modern Southern Ocean and during the last interglacial period, Marine Isotope Stage 5e, there are observations that point to reduced Antarctic Bottom Water (AABW) formation. These reductions are believed to be driven by an increase in the strength of the Southern Ocean density stratification due to Antarctic ice melt‐induced surface water freshening. Any reduction in AABW formation has important implications for global climate as AABW plays a vital role in the cycling of carbon in the world's ocean. The primary question this study seeks to answer is do these AABW reductions occur during any of the other interglacials of the past 470,000 years? To study AABW changes in the paleoceanographic record, we look at changes in the redox record. Newly formed AABW is oxygen‐rich, so any reduction should lead to a decrease in oxygen concentrations in the deep Southern Ocean. The trace element uranium is useful for studying these redox changes as it is enriched in marine sediments under low‐oxygen conditions. When accounting for other factors, such as paleoproductivity, that can also decrease the oxygen concentrations in sedimentary porewater, it is possible to identify changes in AABW using authigenic uranium. The survey conducted by this study found a possible AABW reduction during late Marine Isotope Stage 11 (~397 ka). The cause of this event is less clear than others studied, and we explore the possibilities of ice melt‐induced freshening or a change in the position or strength of the Southern Hemisphere westerly winds.

Sea-ice control on deglacial lower cell circulation changes recorded by Drake Passage deep-sea corals

The sequence of deep ocean circulation changes between the Last Glacial Maximum and the Holocene provides important insights for understanding deglacial climate change and the role of the deep ocean in the global carbon cycle. Although it is known that significant amounts of carbon were sequestered in a deep overturning cell during glacial periods and released during deglaciation, the driving mechanisms for these changes remain unresolved. Southern Ocean sea-ice has recently been proposed to play a critical role in setting the global deep ocean stratification and circulation, and hence carbon storage, but testing such conceptual and modelling studies requires data constraining past circulation changes. To this end, we present the first deglacial dataset of neodymium (Nd) isotopes measured on absolute-dated deep-sea corals from modern Lower Circumpolar Deep Water depths in the Drake Passage. Our record demonstrates deglacial variability of 2.5 εNd units, with radiogenic values of up to εNd=−5.9 during the Last Glacial Maximum providing evidence for a stratified glacial circulation mode with restricted incorporation of Nd from North Atlantic Deep Water in the lower cell. During the deglaciation, a renewed Atlantic influence in the deep Southern Ocean is recorded early in Heinrich Stadial 1, coincident with Antarctic sea-ice retreat, and is followed by a brief return to more Pacific-like values during the Antarctic Cold Reversal. These changes demonstrate a strong influence of Southern Ocean processes in setting deep ocean circulation and support the proposed sea-ice control on deep ocean structure. Furthermore, by constraining the Nd isotopic composition of Lower Circumpolar Deep Water in the Southern Ocean, our new data are important for interpreting deglacial circulation changes in other ocean basins and support a spatially asynchronous return of North Atlantic Deep Water to the deep southeast and southwest Atlantic Ocean.

Global biogeography of marine amphipod crustaceans: latitude, regionalization, and beta diversity

Studying the biogeography of amphipod crustaceans is of interest because they play an important role at lower trophic levels in ecosystems. Because they lack a planktonic larval stage, it has been hypothesized that marine benthic amphipod crustaceans may have short dispersal distances, high endemicity, and spatial turnover in species composition, and consequently high global species richness. In this study, we examined over 400000 distribution records of 4876 amphipod species, and identified 12 regions of endemicity. The number and percent of endemic species peaked at 30°-35°S and coincided with 3 of these regions of high endemicity: Australia, New Zealand, and southern Africa. Pelagic species of marine amphipod crustaceans were more cosmopolitan than benthic species. The latitudinal patterns of richness (alpha, gamma, and ES50) and species turnover were at least bimodal. Most occurrence records and greater alpha and gamma richness were in mid-latitudes, reflecting sampling bias. Both ES50 and beta diversity had similar richness in the tropics, mid-latitudes, and on the Antarctic shelf around 70°S. These 2 indices exhibited a sharp dip in the deep Southern Ocean at 55°S. ES50 peaked at 30°-35°S and a small dip was apparent near the equator at 5°-10°N. Beta diversity was driven mostly by turnover rather than nestedness. These findings support the need for conservation in each realm of species endemicity—and for amphipods, particularly in Antarctica and the coastal mid-latitudes (30°-35°S) of the Southern Hemisphere.

No detectable Weddell Sea Antarctic Bottom Water export during the Last and Penultimate Glacial Maximum

Weddell Sea-derived Antarctic Bottom Water (AABW) is one of the most important deep water masses in the Southern Hemisphere occupying large portions of the deep Southern Ocean (SO) today. While substantial changes in SO-overturning circulation were previously suggested, the state of Weddell Sea AABW export during glacial climates remains poorly understood. Here we report seawater-derived Nd and Pb isotope records that provide evidence for the absence of Weddell Sea-derived AABW in the Atlantic sector of the SO during the last two glacial maxima. Increasing delivery of Antarctic Pb to regions outside the Weddell Sea traced SO frontal displacements during both glacial terminations. The export of Weddell Sea-derived AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO2 rise in the transition to the Holocene and the Eemian. Our new records lend strong support for a previously inferred AABW overturning stagnation event during the peak Eemian interglacial.

Radiogenic isotopic and clay mineralogical signatures of terrigenous particles as water-mass tracers: New insights into South Atlantic deep circulation during the last termination

The past evolution of the Southern Ocean, one of the major components of the climatic system, is still a matter of debate. This study provides new insights into the deep Southern Ocean circulation based on the radiogenic isotopes and clay mineralogical signature of the terrigenous fractions transported by the main deep water masses to sediments recovered in core MD07-3076Q from the central South Atlantic. This approach successfully permits: (1) provenance identification of the various grain-size fractions (clay, cohesive silt and sortable silt); (2) assignment of each grain-size fraction to a specific water-mass; (3) reconstruction of past changes in the main deep water-mass pathways. These data document the evolution of deep-water masses in the South Atlantic Ocean during the last deglaciation. The Antarctic Bottom Water (AABW) speed and northward extension were maximum at the end of the Last Glacial Maximum (LGM), associated with strong bottom water production in the Weddell Sea, together with a vigorous Lower Circumpolar Deep Water (LCDW). In contrast the North Atlantic Deep Water (NADW) circulation was weaker than today. The onset of the deglaciation (from 17.5 ka to 15 ka, ∼Heinrich Stadial 1, HS 1) was marked by weakening and southerly retreat of the AABW and by an increase of mixing between AABW and LCDW. The speed of the AABW remained at its lowest during the Bølling Allerød (B/A) and the Younger Dryas (YD), and the LCDW slowed and retreated to the south, while the NADW progressively migrated southward, deepened, and strengthened between the beginning of the Bølling Allerød and the Holocene (from ∼15 to 10 ka).

Projected slow down of South Indian Ocean circulation

Using an ensemble of 28 climate models, we examine hindcasts and ‘business as usual’ future changes to large-scale South Indian Ocean dynamics. We compare model ensemble seasonal-to-annual volume transports to observations and explore drivers of past and future circulation variability and change. Off the west coast of Australia, models consistently project a weakening of the Leeuwin Current and Undercurrent due to reduced onshore flow and downwelling. The reduced onshore flow is related to changes in the alongshore pressure gradient. While the alongshore pressure gradient change is consistent with the Indonesian Throughflow projected weakening, we found no inter-model relationship between these changes. In the south-western Indian Ocean, the models project a robust weakening of the North East and South East Madagascar Currents, Agulhas Current and transport through the Mozambique Channel. This reduced Indian Ocean western boundary flow is partly associated with a weaker Indonesian Throughflow and overturning circulation, where the latter is related to a decrease in the convergence of deep Southern Ocean waters into the Indian Ocean. In contrast to the weakening of other features, the westward flowing Agulhas Current extension south of Africa is projected to strengthen, which is consistent with an intensification of the Antarctic Circumpolar Current.