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In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom‐bound 15N/14N (δ15Ndb) and reduced export production during the ice ages indicates more complete nitrate (NO3−) consumption. This evidence points to an ice age decline in gross NO3− supply from the deep ocean to the surface wind‐mixed layer, which may help to explain the reduced CO2 levels of the ice age atmosphere. We use a seasonally resolved, two‐layer model of the N isotopes in the Antarctic Zone upper ocean to quantify the ice age decline in gross NO3− supply implied by the data. When model parameters are varied to reflect reduced gross NO3− supply, the concentration of wintertime upper ocean NO3− is lowered, but with a much weaker increase in NO3− δ15N than predicted by analytical models such as the Rayleigh and steady state models. Physical mixing is the dominant cause, with a modest contribution from foodweb dynamics. As a result, the observed δ15Ndb rise of ~3‰–4‰ must be explained mostly by a greater summertime increase in NO3− δ15N during the ice ages. The high degree of NO3− consumption required to generate this summertime δ15N rise indicates a >80% reduction in gross NO3− supply. Half or more of the modern gross NO3− supply is from wind‐forced Antarctic upwelling that drives the upper cell of Southern Ocean overturning. Thus, the decrease in NO3− supply cannot be achieved solely by a decline in vertical mixing or wintertime convection; rather, it requires an ice age weakening of the upper cell. Key Points: A two‐layer model of Antarctic Zone seasonality that includes upper ocean N cycling captures key N isotopic patternsThe model indicates that an ice age rise of ~3‰‐4‰ in diatom‐bound δ15N requires a >80% reduction in gross NO3− supply from deep watersThis large change implies a weakening of the upper cell of global ocean overturning that passes through the Antarctic surface [ABSTRACT FROM AUTHOR]

The Antarctic Peninsula (AP) region is one of the areas under faster warming rates worldwide, and where food web changes have been observed in the last decades. Among these changes are the development of cryptophytes under warmer conditions in detriment of diatoms, and the reduced krill availability in the environment. An isotopic approach was used to investigate whether the temporal and spatial patterns of energy transfer from phytoplankton (using particulate organic matter – POM – as a proxy of primary producers) to baleen whales (humpback – Megaptera novaeanglieae, fin – Balaenoptera physalus, Antarctic minke – Balaenoptera bonaerensis) , and killer whales – Orcinus orca – is similar in areas under different effects of warming around the northern Antarctic Peninsula (NAP). Samples of POM (n = 65), krill (n = 29) and skin of baleen (n = 106) and, opportunistically, killer whales (n = 5) were collected in Gerlache and Bransfield Straits (western AP) and the Powell Basin (northeastern AP) during the austral summers of 2013–2016. Mean isotope values for δ 13 C and δ 15 N values were, respectively, ‐26.3‰ (± 2.9) and 0.9‰ (± 1.7) for POM, ‐25.6‰ (± 0.9) and 5.3‰ (± 1.1) for krill, ‐24.1‰ (± 2) and 8.9‰ (± 1.5) for humpback, ‐24.6‰ (± 1.2) and 8.2‰ (± 0.7) for fin, ‐24.4‰ (± 1.6) and 8.7‰ (± 1) for Antarctic minke whales, and −23.6‰ (± 1.2) and 8.9‰ (± 1.7) for killer whales. Interannual significant differences were found for δ 13 C values of POM and fin whales’ samples, while spatial differences were found for δ 13 C values of POM samples and humpback whales and for δ 15 N values of POM, humpback and Antarctic minke whales. Lower δ 13 C and δ 15 N values for the base of the food web tended to be observed towards open sea regions (Powell Basin and an area under the influence of the Bellingshausen Sea waters). The isoscapes generated for the baseline of the NAP ecosystem provided unprecedented information, to the best of our knowledge, of how the δ 13 C and δ 15 N values of POM varied spatially and temporally in the region. HPLC-CHEMTAX pigment analysis indicated that two of the main phytoplankton groups in the study region were diatoms and cryptophytes. The contribution of these groups to the total phytoplankton biomass was positively and negatively correlated with the POM δ 13 C values, respectively. Despite the spatial and temporal limited interpretation of our results due to our reduced sampling effort to the east of the AP and to the relatively short temporal range investigated, the differences observed in the isotopic composition are considered representative of contrasting environmental conditions. The present study provides new insights on stable isotope values in the Antarctic ecosystem and may help to foresee the consequences of physico-chemical changes in water properties to the biota due to global warming. [ABSTRACT FROM AUTHOR]

We report wintertime nitrogen and oxygen isotope ratios (δ15N and δ18O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ15N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6-3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5-8‰). The δ18O data from the same depth profiles and lateral δ15N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ15N for nitrite in the winter mixed layer (−40‰ to −20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ15N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ15N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ15N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer. [ABSTRACT FROM AUTHOR]

Abstract: A new fishing gear was used to sample the macrozooplankton and micronekton community in the surface layer (0–2m) under ice and in open water, the Surface and Under Ice Trawl (SUIT). In total, 57 quantitative hauls were conducted in the Lazarev Sea (Southern Ocean) during 3 different seasons (autumn 2004, winter 2006, summer 2007/2008). At least 46 species from eight phyla were caught in all 3 seasons combined. Biomass density was dominated by Antarctic krill Euphausia superba. The average biomass density was highest under the winter sea ice and lowest under the young ice in autumn. In summer, macrozooplankton biomass was dominated by ctenophores in open water and by Antarctic krill under ice. The community composition varied significantly among seasons, and according to the presence of sea ice. The response of the community composition to the presence of sea ice was influenced by species that were significantly more abundant in open water than under ice (Cyllopus lucasii, Hyperiella dilatata), only seasonally abundant under ice (Clione antarctica), or significantly associated with sea ice (Eusirus laticarpus). A number of abundant species showed distinct diel patterns in the surface occurrence both under ice and in open water, indicating that the surface layer serves as a foraging ground predominantly at night. Our results emphasize the potential of a number of non-euphausiid macrozooplankton and micronekton species to act as energy transmitters between the production of sea ice biota and the pelagic food web. By providing a regional-scale quantitative record of macrofauna under Antarctic sea ice covering 3 seasons, this study adds new and direct evidence that the ice-water interface layer is a major functional node in the ecosystem of the Antarctic seasonal sea ice zone. [Copyright &y& Elsevier]

Abstract: The Sea Ice Physics and Ecosystem eXperiment (SIPEX) was conducted in the East Antarctic pack ice zone between 115-130°E from 9 September – 11 October, 2007. In situ measurements of sea-ice and snow properties were conducted at 15 ice stations, together with ship-based ASPeCt observations. The ice and snow thickness varied considerably in different regions of the pack ice, with particularly thick ice associated with deformation and a strong slope jet in the southwest of the study region. The mean ice thickness was 0.99m (1.57m excluding the northern marginal ice zones), but varied from 0.61m along the southern leg to 1.80m along the western leg, with pockets of considerably thicker ice in some regions. Swell was observed on two occasions penetrating more than 330km south of the ice edge into regions with 80-100% ice concentration. Ice thicknesses calculated from near coincident ICESat laser altimetry (1.74m) are similar to the in-situ observations in the central pack (1.57m). [Copyright &y& Elsevier]

Abstract: Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)4 and biogenic silica (bSiO2) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (<500m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (>1000m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0±0.7molSim−2 yr−1. This supply must be equivalent to the net annual bSiO2 production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5±0.2molSim−2 yr−1). This discrepancy reveals that some 1.5±0.7molSim−2 yr−1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (∆30Si) probably reflects (1) a significant impact of bSiO2 dissolution, enriching the bSiO2 pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)4 from below when the surface Si(OH)4 pool is significantly depleted. We estimated a net integrated bSiO2 production of 10.5±1.4molSim−2 yr−1 in the AASW above the plateau, which includes a significant contribution of bSiO2 production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to −1.2±0.2‰ for the Antarctic Circumpolar Current. [Copyright &y& Elsevier]

Abstract: In the winter 2007–2008, the CAML-CEAMARC cruises prospected in the Eastern part of the Antarctic continental shelf (Dumont d''Urville Sea, off Terre Adélie). The Australian R/V “Aurora Australis” and the Japanese R/V “Umitaka Maru” sampled in locations and at depths previously uninvestigated in this region. In total, 538 teleost specimens collected during these cruises were sequenced for the mitochondrial cytochrome oxidase I gene (COI), with the goal of barcoding a representative sampling from the campaign. The efficiency of barcoding for identification has been questioned for some taxonomic groups, thus we compared the COI results for a few of the families and genera included here (genus Trematomus, Artedidraconidae, Liparidae) to results for other markers for the same specimens. To better explore intra- and interspecific variability, sequences from previous campaigns and public databases were added to the analysis for these groups. The congruence among the results for different genes (COI, cytochrome b, D-loop and the nuclear rhodopsin retrogene) and morphological identification was used to assess the efficiency of the COI dataset at recovering species delimited using other data. Where discrepancies were present among the different data sources, a morphological re-identification was performed. The partial COI sequence yields reliable identification in most Antarctic teleost families when using their position in the clusters on a NJ tree. However, for several groups of species neither COI nor the other molecular markers investigated nor morphology recover unambiguously the currently accepted species. The taxonomy of these groups needs to be reconsidered. Identification through sequence similarity using the Barcode of Life Data System (BOLD) works for some groups, but is hampered by the incompleteness of the taxonomic coverage for antarctic teleosts. For four families (Artedidraconidae, Zoarcidae, Liparidae and Channichthyidae), several interspecific divergences were very small, and of the same magnitude as intraspecific divergences for other antarctic species. Despite these small divergences, almost all the species investigated in artedidraconids have molecular synapomorphies in the COI sequences, and a barcoding gap from the closest species. In the genus Trematomus, almost all species are well separated except for two pairs of closely related species that could not be distinguished by the other molecular markers either. For the typically hard to identify zoarcids and liparids, the results of barcoding are in agreement with in-depth morphological study. Once a reasonably complete reference dataset is available, barcoding will be invaluable to discriminate species from one another in these families. A careful comparison of the morphological and molecular results for our specimens allowed us to add numerous well-identified specimens (including some rare species) and sequences to BOLD. It helped to pinpoint the specimens that needed to be re-identified morphologically, and highlighted groups where barcoding is most helpful for specimen identification (Chionodraco species). This large-scale project underlines the need for further taxonomic work in antarctic actinopterygians. [Copyright &y& Elsevier]

Abstract: Many members of the benthic fauna of the Antarctic continental shelf share close phylogenetic relationships to the deep-sea fauna adjacent to Antarctica and in other ocean basins. It has been suggested that connections between the Southern Ocean and the deep sea have been facilitated by the presence of a deep Antarctic continental shelf coupled with submerging Antarctic bottom water and emerging circumpolar deep water. These conditions may have allowed ‘polar submergence’, whereby shallow Southern Ocean fauna have colonised the deep sea and ‘polar emergence’, whereby deep-sea fauna colonised the shallow Southern Ocean. A recent molecular study showed that a lineage of deep-sea and Southern Ocean octopuses with a uniserial sucker arrangement on their arms appear to have arisen via polar submergence. A distantly related clade of octopuses with a biserial sucker arrangement on their arms (historically placed in the genus Benthoctopus) is also present in the deep-sea basins of the world and the Southern Ocean. To date their evolutionary history has not been examined. The present study investigated the origins of this group using 3133 base pairs (bp) of nucleotide data from five mitochondrial genes (12S rRNA, 16S rRNA, cytochrome c oxidase subunit I, cytochrome c oxidase subunit III, cytochrome b) and the nuclear gene rhodopsin from at least 18 species (and 7 outgroup taxa). Bayesian relaxed clock analyses showed that Benthoctopus species with a high-latitude distribution in the Southern Hemisphere represent a paraphyletic group comprised of three independent clades. The results suggest that the Benthoctopus clade originated in relatively shallow Northern Hemisphere waters. Benthoctopus species distributed in the Southern Ocean are representative of polar emergence and occur at shallower depths than non-polar Benthoctopus species. [Copyright &y& Elsevier]

Abstract: The partial pressure of CO2 (pCO2) and dissolved inorganic carbon (DIC) were monitored in shallow coastal waters located inside and outside giant kelp beds (Macrocystis pyrifera) located in the Kerguelen Archipelago (Southern Ocean). Photosynthesis and respiration by microplankton and kelp lead to marked pCO2 and DIC diel cycles. Daily variations of pCO2 and DIC are significant in the spring and summer, but absent in the winter, reflecting the seasonal cycle of biological activity in the kelp beds. If the kelp beds seem to favour the onset of phytoplankton blooms, most of the primary production inside the kelp beds is due to the kelp itself. The primary production of Macrocystis kelp beds in the Sub-Antarctic high-nutrient, low-chlorophyll (HNLC) waters off the Kerguelen Archipelago is elevated and closely linked to light availability. This production is significant from October to March and reaches its climax in December at the solar radiation maximum. [Copyright &y& Elsevier]

Abstract: The Antarctic Peninsula region is undergoing rapid change: a warming in winter of almost 6°C since 1950, the loss of six ice shelves, the retreat of 87% of the marine glaciers, and decreases in winter sea-ice duration. Concurrently, there is evidence of ecosystem change along the western Antarctic Peninsula (wAP). Since the life histories of most polar marine species are synchronized with the seasonal cycle of sea ice, we assess how the seasonal sea-ice cycle is changing in the wAP region. Four new metrics of seasonal sea-ice variability were extracted from spatial maps of satellite derived daily sea-ice concentration: (a) day of advance, (b) day of retreat, (c) the total number of sea-ice days (between day of advance and retreat), and (d) the percent time sea-ice was present (or sea-ice persistence). The spatio-temporal variability describes distinct on-to-offshore and alongshore differences in ice–ocean marine habitats, characterized overall by a longer sea-ice season in coastal regions (6.8–7.9 months) versus a shorter sea-ice season over the shelf (4.1–5.3 months), with on-to-offshore differences increasing south-to-north. Large perturbations in the seasonality of the marine habitat occur in association with ENSO and Southern Annular Mode (SAM) variability. The local atmospheric response to these climate modes is largely a strengthening of the meridional winds during spring-to-autumn, which in turn affect the timing of the sea-ice retreat and subsequent advance. These perturbations are embedded in overall trends towards a later sea-ice advance, earlier retreat and consequently shorter sea-ice season, the impacts of which are expected to affect ecosystem functionality in the wAP region. A suite of ocean–atmosphere–ice interactions are described that are consistent with the amplified warming in late autumn, early winter. [Copyright &y& Elsevier]

Abstract: The Palmer Long-Term Ecological Research (LTER) program seeks to obtain a comprehensive understanding of various components of the Antarctic marine ecosystem—the assemblage of plants, microbes, animals, ocean, and sea-ice south of the Antarctic Polar Front. A central hypothesis of the Palmer LTER is that the seasonal and interannual variability of sea ice affects all levels of the Antarctic marine ecosystem, from the timing and magnitude of seasonal primary production to the breeding success and survival of apex predators. In the context of this high-latitude ecosystem we use satellite imagery to examine physical forcing and possible mechanisms influencing the distribution of phytoplankton biomass in the region to the west of the Antarctic Peninsula. We evaluate the spatial and temporal variability of pigment biomass (estimated as chlorophyll-a concentrations using SeaWiFS data) in response to the spatial and temporal variability of sea-ice extent (estimated from passive microwave satellite data). While the ocean-color data record is relatively short (7 years) and contains high interannual variability, there are persistent spatial patterns of phytoplankton biomass that indicate important regional-scale physical mechanisms including: the marginal ice zone and its impact on the mixed-layer depth, the timing of spring sea-ice retreat, the importance of the southern Antarctic Circumpolar front, and teleconnections with sub-polar regions. The SeaWIFS imagery presented here provides the most complete synoptic space/time views of phytoplankton biomass within this region to date. These observations suggest that the southern Antarctic Circumpolar front may have a more profound influence on the western Antarctic Peninsula ecosystem than previously thought. [Copyright &y& Elsevier]

Abstract: We report measurements of dissolved iron (dFe, <0.4μm) in seawater collected from the upper 300m of the water column along the CLIVAR SR3 section south of Tasmania in March 1998 (between 42°S and 54°S) and November–December 2001 (between 47°S and 66°S). Results from both cruises indicate a general north-to-south decrease in mixed-layer dFe concentrations, from values as high as 0.76nM in the Subtropical Front to uniformly low concentrations (<0.1nM) between the Polar Front and the Antarctic continental shelf. Samples collected from the seasonal sea-ice zone in November–December 2001 provide no evidence of significant dFe inputs from the melting pack ice, which may explain the absence of pronounced ice-edge algal blooms in this sector of the Southern Ocean, as implied by satellite ocean-color images. Our data also allow us to infer changes in the dFe concentration of surface waters during the growing season. South of the Polar Front, a comparison of near-surface with subsurface (150m depth) dFe concentrations in November–December 2001 suggests a net seasonal biological uptake of at least ∼0.14–0.18nM dFe, of which ∼0.05–0.12nM is depleted early in the growing season (before mid December). A comparison of our spring 2001 and fall 1998 data indicates a barely discernible seasonal depletion of dFe (∼0.03nM) within the Polar Frontal Zone. Further north, most of our iron profiles do not exhibit near-surface depletions, and mixed-layer dFe concentrations are sometimes higher in samples from fall 1998 compared to spring 2001; here, the near-surface dFe distributions appear to be dominated by time-varying inputs of aerosol iron or advection of iron-rich subtropical waters from the north. [Copyright &y& Elsevier]

Abstract: A survey of the spatial and temporal behavior of the atmospheric general circulation as it relates to both polar regions is presented. The review is based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40), updated using ECMWF operational analyses. The analysis spans 1960–2005 in the Northern Hemisphere, but is restricted to 1979–2005 in the Southern Hemisphere because of difficulties experienced by ERA-40 prior to the modern satellite era. The seasonal cycle of atmospheric circulation is illustrated by focusing on winter and summer. The huge circulation contrasts between the land-dominated Northern Hemisphere and the ocean-dominated Southern Hemisphere stand out. The intensification of the North Atlantic Oscillation/Northern Annular Mode and the Southern Annular Mode in DJF is highlighted and likely due to warming of the tropical Indian Ocean. The Arctic frontal zone during northern summer and the semi-annual oscillation throughout the year in the Southern Hemisphere are prominent features of the high latitude circulation in the respective hemispheres. Rotated principal component analysis (RPCA) is used to describe the primary modes of temporal variability affecting both polar regions, especially the links with the tropical forcing. The North Atlantic Oscillation is a key modulator of the atmospheric circulation in the North Atlantic sector, especially in winter, and is the dominant control on the moisture transport into the Arctic Basin. The Pacific-South American teleconnection patterns are primary factors in the high southern latitude circulation variability throughout the year, especially in the Pacific sector of Antarctica where the majority of moisture transport into the continent occurs. [Copyright &y& Elsevier]

Abstract: For 1992–2002, the paper investigates the heat loss and area of three polynyas in the western and central Ross Sea. These are the Ross Sea Polynya (RSP) and the much smaller Terra Nova Bay and McMurdo Sound polynyas. The importance of these polynyas is that their associated salt rejection contributes to the formation of the High Salinity Shelf Water (HSSW) that is crucial to the Antarctic Bottom Water formation. The study divides into two parts, 1992–1999, when there was negligible iceberg activity, and 2000–2002, when major icebergs calved and interacted with the polynyas. To retrieve the ice thicknesses and heat fluxes within the polynyas, the paper uses an algorithm based on the ratio of the vertically and horizontally polarized Special Sensor Microwave/Imager (SSM/I) 25-km resolution 37-GHz channels, combined with meteorological data. Because of sidelobe contamination, the ice shelf and icebergs are masked. Our results show that for the polynyas, and consistent with other observations, their mean winter area is about 30,000 km2 and their combined ice production is about 500 km3y−1. We also find that the polynya ice production approximately equals the ice export. This is in contrast to the Weddell Sea, where the polynya ice production equals about 6% of the ice export. For the years 2000 and 2002, the calving of large icebergs directly affect the ice production by inhibiting the ice production off the shelf due to piling up of first year ice upwind of the bergs and by generating new polynyas downwind of the bergs. The period 1992–2001 exhibits an upward trend in polynya productivity. The decadal increase in the ice production suggests that the observed HSSW salinity decrease in the western Ross Sea is not due to the polynyas, but is rather due to a change in the properties of the water flowing into the Ross. [Copyright &y& Elsevier]

Abstract: Several studies have provided evidence of a reduction of the Antarctic sea ice extent. However, these studies were conducted either at a global scale or at a regional scale, and possible inter-regional differences were not analysed. Using the long-term whaling database we investigated circum-Antarctic changes in summer sea ice extent from 1931 to 1987. Accounting for bias inherent in the whaling method, this analysis provides new insight into the historical ice edge reconstruction and inter-regional differences. We highlight a reduction of the sea ice extent occurring in the 1960s, mainly in the Weddell sector where the change ranged from 3° to 7.9° latitude through summer. Although the whaling method may not be appropriate for detecting fine-scale change, these results provide evidence for a heterogeneous circumpolar change of the sea ice extent. The shift is temporally and spatially consistent with other environmental changes detected in the Weddell sector and also with a shift in the Southern Hemisphere annular mode. The large reduction of the sea ice extent has probably influenced the ecosystem of the Weddell Sea, particularly the krill biomass. [Copyright &y& Elsevier]

Abstract: Seasonal and interannual variations of the mixed layer properties in the Antarctic Zone (AZ) south of Tasmania are described using 7 WOCE/SR3 CTD sections and 8 years of summertime SURVOSTRAL XBT and thermosalinograph measurements between Tasmania and Antarctica. The AZ, which extends from the Polar Front (PF) to the Southern Antarctic Circumpolar Current Front (SACCF), is characterized by a 150m deep layer of cold Winter Water (WW) overlayed in summer by warmer, fresher water mass known as Antarctic Surface Water (AASW). South of Tasmania, two branches of the PF divide the AZ into northern and southern zones with distinct water properties and variability. In the northern AZ (between the northern and southern branches of the PF), the mixed layer depth (MLD) is fairly constant in latitude, being 150m deep in winter and around 40–60m in summer. In the southern AZ, the winter MLD decreases from 150m at the S-PF to 80m at the SACCF and from 60 to 35m in summer. Shallower mixed layers in the AZ-S are due to the decrease in the wind speed and stronger upwelling near the Antarctic Divergence. The WW MLD oscillates by ±15m around its mean value and modest interannual changes are driven by winter wind stress anomalies. The mixed layer is on annual average 1.7°C warmer, 0.06 fresher and 0.2kgm-3 lighter in the northern AZ than in the southern AZ. The Levitus (1998) climatology is in agreement with the observed mean summer mixed layer temperature and salinity along the SURVOSTRAL line but underestimates the MLD by 10–20m. The winter MLD in the climatology is also closed to that observed, but is 0.15 saltier than the observations along the AZ-N of the SR3 line. MLD, temperature and density show a strong seasonal cycle through the AZ while the mixed layer salinity is nearly constant throughout the year. During winter, the AZ MLD is associated with a halocline while during summer it coincides with a thermocline. Interannual variability of the AZ summer mixed layer is partly influenced by large scale processes such as the circumpolar wave which produces a warm anomaly during the summer 1996–1997, and partly by local mechanisms such as the retroflection of the S-PF which introduces cold water across the AZ-N. [Copyright &y& Elsevier]

Time-series sediment traps were deployed between December 1998 and January 2000 and from March 2000 to February 2001 at two offshore Prydz Bay sites within the seasonal ice zone (SIZ) of the Southern Indian Ocean located between 62–63°S and 73–76°E to quantify seasonal biogeochemical particle fluxes. Samples were obtained from traps placed at 1400, 2400, and 3400 m during the first deployment year (PZB-1) and from 3300 m in the second deployment year (PZB-2). All geochemical export fluxes were highly seasonal with primary peaks occurring during the austral summer and relatively low fluxes prevailing through the winter months. Secondary flux peaks in mid-winter and in early spring were suggestive of small-scale, sea-ice break-up events and the spring retreat of seasonal ice, respectively. Biogenic silica represented over 70% (by weight) of the collected trap material and provided an annual opal export of 18 g m−2 to 1 km and 3–10 g m−2 to 3 km. POC fluxes supplied an annual export of approximately 1 g m−2, equal to the estimated ocean-wide average. Elevated particulate Corg/Cinorg and Sibio/Cinorg molar ratios indicate a productive, diatom-dominated system, although consistently small fluxes of planktonic foraminifera and pteropod shells document a heterotrophic source of carbonate to deeper waters in the SIZ. The observation of high Sibio/Corg ratios and the δ15N time-series data suggest enhanced rates of diatom-POC remineralization in the upper 1000 m relative to bioSiO2. The occurrence in this region of a pronounced temperature minimum, associated with a strong pycnocline and subsurface particle maximum at 50–100 m, may represent a zone where sinking, diatom-rich particulates temporarily accumulate and POC is remineralized. [Copyright &y& Elsevier]

The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica, during two field seasons, one in austral spring 1994 and one in early summer, 1995/96. Aggregate abundance, size and potential sinking rates were determined by photographic techniques. Measurements of water column parameters, including particulate organic carbon concentrations, were assessed simultaneously, as was the flux of organic matter with floating sediment traps. The numbers of aggregates (and to a lesser extent their size) increased with time, although there was substantial spatial variability in these variables at any point in time. Some aggregates appeared to sink extremely rapidly, and for these, our photographic measurements were able to estimate only a minimum sinking rate, which equaled 288 m d−1. Estimates of aggregate organic carbon flux were compared to those determined by floating sediment traps. From these results, aggregate fluxes appear to have dominated the vertical export of organic matter from the euphotic zone. The genesis and flux of aggregates in the Ross Sea are the critical processes controlling the export of biogenic material from the surface layer. [Copyright &y& Elsevier]

We present 230Th-normalized dust and export production fluxes for two contrasted marine sediment cores spanning the Antarctic Polar Front, close to the Kerguelen Plateau in the Southern Indian Ocean, covering the last glacial cycle. We report glacial lithogenic fluxes comparable to the South Atlantic and higher than in the South Pacific sectors of the Southern Ocean. Structural and temporal discrepancies with dust reconstructions from Antarctic ice cores and the Atlantic and Pacific sectors of the Subantarctic Zone (SAZ) point towards Southern Africa and/or the Kerguelen Plateau as an additional source of lithogenic material to the Southern Indian Ocean during the last ice age. In the SAZ, export production proxies respond to iron (Fe) fertilization with total organic carbon (TOC) fluxes as high as those previously reported from the Atlantic sector of the Southern Ocean. However, the correlation between export production and dust proxies is weaker than in the other sectors, and shows a muted response of export production during peak glacials. We hypothesize that this muted response may be related to macronutrient (co-)limitation imposed on phytoplankton growth possibly induced by a northward displacement of wind-driven upwelling and/or the polar frontal system during peak glacials. The Antarctic Zone (AZ) record depicts the typical pattern of enhanced export production during interglacials and comparatively low productivity during glacials suggesting a decrease in the supply of macronutrients to the AZ surface during ice ages compared to warm periods, as previously proposed. However, a muted response of opal fluxes during marine isotope stage (MIS) 5e argues for a southward migration of the frontal system during warmer MIS 5e, possibly causing silicic acid (co-)limitation at this site. These results illustrate the important role of Fe fertilization and changes in wind-driven upwelling intensities in modulating the strength and efficiency of the biological carbon pump in the Southern Ocean, but also highlight the potential role of frontal movements in explaining export production variability at sites located close to the Antarctic Circumpolar Current (ACC) fronts. Overall, our new datasets provide new insights on the regional differences in dust and export production patterns that need to be considered in order to evaluate the efficiency of carbon sequestration in the different sectors of the Southern Ocean during ice ages. • Multiple sources of dust supplied to the Southern Indian Ocean during ice ages. • Iron fertilization may have been impeded by nutrient (co-)limitation. • Nutrient (co-)limitation may be related to frontal migration. • Iron fertilization enhanced carbon export in all three basins of the Southern Ocean. [ABSTRACT FROM AUTHOR]