Subantarctic Zone Research Articles

The Contribution of the Vendée Globe Race to Improved Ocean Surface Information: A Validation of the Remotely Sensed Salinity in the Sub-Antarctic Zone

The Vendée Globe is the world’s most famous solo, non-stop, unassisted sailing race. The Institute of Marine Sciences and the Barcelona Ocean Sailing Foundation installed a MicroCAT on the One Ocean One Planet boat. The skipper, Dídac Costa, completed the round trip in 97 days, from 8 November 2020 to 13 February 2021, providing one measurement of temperature and conductivity every 30 s during navigation. More than half of the ship’s route was in the sub-Antarctic zone, between the tropical and polar fronts, and it passed through areas of oceanographic interest such as Southern Patagonia (affected by glacier melting), the Brazil–Malvinas confluence, the Southern Pacific Ocean, and the entire Southern Indian Ocean. This sailing race gave a rare opportunity to measure in-situ sea surface salinity in a region where satellite salinity measurements are not reliable. Due to the decreased sensitivity of brightness temperature to salinity in cold seas, retrieving sea surface salinity at high latitudes remains a major challenge. This paper describes how the data are processed and uses the data to validate satellite salinity products in the sub-Antarctic zone. The sailing race measurements represent surface information (60 cm depth) not available from drifters or Argo floats. Acquiring measurements using round-the-world sailing races would allow us to analyse the evolution of ocean salinity and the impact of changes in the ice extent around Antarctica.

Distinctive zonation of planktonic ostracods assemblages in the dynamic Southeastern Atlantic

Species diversity, abundance and spatial distribution of planktonic Ostracoda in the frontal waters of the South Atlantic were studied in summer 2009. Mesozooplankton materials were collected using WP-2 net in the top 300 m layer, from 32 stations located along the profile designated from the southern coast of Africa towards the Antarctic. Twenty-one species of ostracods were recorded. Ostracoda species diversity generally decreased with increasing latitude. Their total abundance showed a parabolic pattern of distribution, with lower numbers at the edges of the transect and higher numbers in the middle. We identified three distinctive areas occupied by different assemblages of Ostracoda. The first area, the Benguela Current (BENG), was inhabited mainly by subtropical ostracods. The second area - Southern Subtropical Convergence, Subantarctic Front, Subantarctic Zone and Polar Front (SSTC, SAF, SANT and PF) - was characterized by the presence of a mixture of subtropical, subantarctic, and Antarctic species. The third area, the Antarctic Zone (ANTA), was inhabited mostly by subantarctic and Antarctic ostracods. Moreover, we confirmed that the SSTC was an almost impassable hydrographical barrier for subtropical-tropical Ostracoda species. This study demonstrated the distribution of some species extending further north (e.g., Metaconchoecia australis) than previously recorded. Morphometric analysis of the most common ostracod species, Discoconchoecia elegans, showed an increasing trend in its mean individual size from the BENG zone to the ANTA zone.

Physical control on the inter-annual variability of summer dissolved nutrient concentration and phytoplankton biomass in the Indian sector of the Southern Ocean

To understand the role of physical processes and their interannual variability on the dissolved nutrient concentration and phytoplankton biomass distribution, field data collected in the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2009–2011 are used. In the subtropical zone, macronutrients were limited (N:P < 1, N:Si < 1, Si:P ≈ 1) and the phytoplankton biomass variability was mainly governed by the mesoscale eddy activity associated with the Agulhas Return Current. High nutrient low chlorophyll condition prevailed in the sub-Antarctic zone and further south. A South-North gradient of the upper layer dissolved SiO2 was higher than that of NO3. The sub-Antarctic zone was characterized by the highest N:Si ratio (>4) and it was associated with the enhanced draw down of silicate due to the winter/spring diatom blooms in the region. The chlorophyll-a (CHL) concentration in the Polar Frontal Zone was low (∼0.2 mg m–3) in 2009 and 2010 but it was high (0.5 mg m–3) in 2011. This increase in CHL in 2011 was due to the supply of dissolved iron from the strong winds and subsequent mixing during the winter of 2010. Further, the increased CHL values in the Antarctic zone (0.5 mg m–3) in 2011 compared to 2009 and 2010 could be due to the increased sea ice melting associated with positive Southern Annular Mode. The increased phytoplankton biomass in the summer of 2011 coincides with an increase in nitrate utilization (N:P≈13) compared to 2009 and 2010. Observations showed that ISSO frontal zones are characterized by inter-annual variability in terms of nutrient utilization and phytoplankton biomass production.

Dust correlation and oxygen isotope stratigraphy in the Southern Ocean over the last 450 kyrs: An Indian sector perspective

The chronology of Southern Ocean (SO) marine sediment cores forms the basis of understanding the SO paleoceanography, with significant implications for global climate. Because tuning of the oxygen isotope (δ18O) record of a marine sediment core to a δ18O stack (δ18O stratigraphy) is difficult in the SO because of a general paucity of calcareous foraminifera, tuning of the dust proxy signal of a marine sediment core to the dust record of an ice core (dust correlation) is a promising way to construct an age–depth model. However, the reliability of dust correlation has not been established, especially beyond the last ∼300 kyrs, and such work has been performed more in the Atlantic and Pacific sectors of the SO than in the Indian sector. Here we present a new dust correlation using the rock magnetic record, together with continuous δ18O stratigraphy, for a marine sediment core in the Indian sector of the SO over the last ∼410 kyrs. The δ18O stratigraphy is consistent with the dust correlation within their chronological uncertainties, supporting the reliability of the latter chronology for glacial–interglacial timescales. However, the dust correlation often produces older ages than the δ18O stratigraphy by up to 3 kyrs (410–126 ka and 84–50 ka). We additionally compiled, based on dust correlation, available dust proxy records in the circum-Antarctic SO for the last 450 kyrs to discuss variations in the dust proxy record within the Indian sector and between all three sectors. For the Indian sector, there is a marked difference in dust proxy signals for Marine Isotope Stage (MIS) 6 between two cores that have markedly different latitudinal positions relative to the oceanic fronts. Increased biogenic magnetite production by iron fertilization in the Subantarctic Zone during MIS 6 may partly explain the difference between the two cores. For the circum-Antarctic SO, moving correlation coefficients were computed between the marine sediment-core dust proxy signals and the dust flux of an Antarctic ice core. Strong correlation was recognized for intervals of high dust flux in the ice core, except for MIS 6. Although minor, the slightly lower correlation in the Indian sector than in the other sectors indicates a contribution from local dust source and volcanic materials and thus suggests the necessity for caution when performing dust correlation in the Indian sector of the SO.

A silicon isotopic perspective on the contribution of diagenesis to the sedimentary silicon budget in the Southern Ocean

Diatoms are known to fractionate silicon isotopes during the formation of their frustules causing the silicon isotopic composition of biogenic silica to track the degree of silicic acid consumption in surface waters. Despite a growing body of work that uses this proxy to reconstruct past changes in silicic acid utilization, the understanding of the benthic silicon cycle, particularly the identification and quantification of the processes that potentially alter the silicon isotopic composition of biogenic silica during early diagenesis is still lacking. We investigated these processes by comparing the silicon isotopic composition of pore water silicic acid, biogenic silica and, for the first time, lithogenic silica from five sediment cores collected in the deep basin of the Southern Ocean representing a diversity of sedimentation regimes. Silicic acid concentrations and the isotopic composition of Southern Ocean pore waters were the result of a dynamic balance between the dissolution of biogenic silica, reactive lithogenic silica phases and Si re-precipitation with the relative importance of each processes differing significantly between regions. The results are consistent with the formation of authigenic alumino-silicates derived from dissolved biogenic silica in the Sub-Antarctic Zone and in the Antarctic Zone (on average 12 ± 5% and 17 ± 13%, respectively). Since this latter process can fractionate silicon isotopes, this implies that, even if the silicon isotopic composition of diatoms preserved in the sediments is a reliable proxy for silicic acid utilization in the past ocean, care must be taken to extract a clean biogenic silica phase free of authigenic clays and lithogenic phases from sediments to eliminate this potential bias when interpreting isotopic records.

The Daily‐Resolved Southern Ocean Mixed Layer: Regional Contrasts Assessed Using Glider Observations

AbstractWater mass transformation in the Southern Ocean is vital for driving the large‐scale overturning circulation, which transports heat from the surface to the ocean interior. Using profiling gliders, this study investigates the role of summertime buoyancy forcing and wind‐driven processes on the intraseasonal (1–10 days) mixed layer thermohaline variability in three Southern Ocean regions southwest of Africa important for water mass transformation—the Subantarctic Zone (SAZ), Polar Frontal Zone (PFZ), and Marginal Ice Zone (MIZ). At intraseasonal time scales, heat flux was shown as the main driver of buoyancy gain in all regions. In the SAZ and MIZ, shallow mixed layers and strong stratification enhanced mixed layer buoyancy gain by trapping incoming heat, while buoyancy loss resulted primarily from the entrainment of cold, salty water from below. In the PFZ, rapid mixing linked to Southern Ocean storms set persistently deep mixed layers and suppressed mixed layer intraseasonal thermohaline variability. In the polar regions, lateral stirring of meltwater from seasonal sea‐ice melt dominated daily mixed layer salinity variability. We propose that these meltwater fronts are advected to the PFZ during late summer, indicating the potential for seasonal sea‐ice freshwater to impact a region where the upwelling limb of overturning circulation reaches the surface. This study reveals a regional dependence of how the mixed layer thermohaline properties respond to small spatiotemporal processes, emphasizing the importance of surface forcing occurring between 1 and 10 days on the mixed layer water mass transformation in the Southern Ocean.

Biogeochemical controls on ammonium accumulation in the surface layer of the Southern Ocean

Abstract. The production and removal of ammonium (NH4+) are essential upper-ocean nitrogen cycle pathways, yet in the Southern Ocean where NH4+ has been observed to accumulate in surface waters, its mixed-layer cycling remains poorly understood. For surface seawater samples collected between Cape Town and the Marginal Ice Zone in winter 2017, we found that NH4+ concentrations were 5-fold higher than is typical for summer and lower north than south of the Subantarctic Front (0.01–0.26 µM versus 0.19–0.70 µM). Our observations confirm that NH4+ accumulates in the Southern Ocean's winter mixed layer, particularly in polar waters. NH4+ assimilation rates were highest near the Polar Front (12.9 ± 0.4 nM d−1) and in the Subantarctic Zone (10.0 ± 1.5 nM d−1), decreasing towards the Marginal Ice Zone (3.0 ± 0.8 nM d−1) despite the high ambient NH4+ concentrations in these southernmost waters, likely due to the low temperatures and limited light availability. By contrast, rates of NH4+ oxidation were higher south than north of the Polar Front (16.0 ± 0.8 versus 11.1 ± 0.5 nM d−1), perhaps due to the lower-light and higher-iron conditions characteristic of polar waters. NH4+ concentrations were also measured along five transects of the Southern Ocean (Subtropical Zone to Marginal Ice Zone) spanning the 2018/19 annual cycle. These measurements reveal that mixed-layer NH4+ accumulation south of the Subantarctic Front derives from sustained heterotrophic NH4+ production in late summer through winter that, in net, outpaces NH4+ removal by temperature-, light-, and iron-limited microorganisms. Our observations thus imply that the Southern Ocean becomes a biological source of CO2 to the atmosphere in autumn and winter not only because nitrate drawdown is weak but also because the ambient conditions favour net heterotrophy and NH4+ accumulation.

Emiliania huxleyi biometry and calcification response to the Indian sector of the Southern Ocean environmental gradients

An increase in the atmospheric pCO2 and temperature is expected to reduce ocean CO32− concentration, make oceans warmer and alter ocean circulation patterns. This will also affect the production and biogeographic distribution of marine calcifying organisms including coccolithophores. The lowering of oceanic CO32− is expected to interfere with the coccolithophore calcification process and cause malformation of coccoliths, whereas changes in the oceanic temperature and circulation patterns may shift their biogeographic boundaries. In this study, we have investigated Emiliania huxleyi coccolith and coccosphere size response to the wide-ranging physico-chemical conditions of the Indian sector of the Southern Ocean between latitudes 38oS and 58oS during the austral summer of 2010 (January–February). This study helps to understand the response of E. huxleyi coccolith/coccosphere morphometry and mass changes to the fluctuating temperature, salinity, CO32−, pCO2, and nutrient values. Our results show that in the Indian sector of the Southern Ocean, E. huxleyi coccoliths are larger and coccospheres are smaller in the Subtropical Zone (STZ). In contrast, coccoliths size is smaller and coccospheres size is larger in the Subantarctic Zone (SAZ), which is due to the decrease in Sea Surface Temperature, Sea Surface Salinity and increase in nutrient concentrations. In the Indian sector of the Southern Ocean, E. huxleyi shows a north-to-south morphotype shift from the heavily calcified ‘Group A’ (E. huxleyi morphotype A) to the weakly calcified ‘Group B’ (E. huxleyi morphotypes B/C, C) forms. We demonstrate that although weakly calcified E. huxleyi morphotypes (morphotypes B/C and C) comprise less mass than that of the E. huxleyi morphotype A, due to the large-sized coccospheres and numerous coccoliths per coccosphere, ‘Group B’ coccospheres precipitate large amount of CaCO3 in the SAZ compared to ‘Group A’ coccospheres located in the STZ. We have documented the presence of large E. huxleyi overcalcified coccospheres with large-sized coccoliths in the southernmost cold, high pCO2, and nutrient-rich waters which show extracellular calcite precipitation. The energy dispersive spectrometry analysis indicates the presence of a large amount of Mg in the overcalcified E. huxleyi specimens. We suspect that E. huxleyi in the colder nutrient-rich waters, with future projected changes in the carbonate chemistry, may adapt to low pH, high pCO2 conditions through extracellular Ca and Mg mineralization.

The Late Neogene to Quaternary Surface Water Changes as Responses from Planktonic Foraminifera at the Transitional Subantarctic Zone

The Late Neogene to Quaternary Surface Water Changes as Responses from Planktonic Foraminifera at the Transitional Subantarctic Zone

The Late Neogene Surface Water Changes as Responses from Planktonic Foraminifera at the Transitional Subantarctic Zone

The Late Neogene Surface Water Changes as Responses from Planktonic Foraminifera at the Transitional Subantarctic Zone

Influence of environmental variability and Emiliania huxleyi ecotypes on alkenone-derived temperature reconstructions in the subantarctic Southern Ocean

Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dominance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correlations were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach

Abstract. Improvements in our capability to reconstruct ancient surface-ocean conditions based on organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean provide an opportunity to better establish past position, strength and oceanography of the subtropical front (STF). Here, we aim to reconstruct the late Eocene to early Miocene (37–20 Ma) depositional and palaeoceanographic history of the STF in the context of the evolving Tasmanian Gateway as well as the potential influence of Antarctic circumpolar flow and intense waxing and waning of ice. We approach this by combining information from seismic lines (revisiting existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A) in the western Tasmanian continental slope. We apply improved taxonomic insights and palaeoecological models to reconstruct the sea surface palaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma) assemblages show a progressive transition from dominant terrestrial palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by heterotrophic dinoflagellates to predominantly outer-neritic/oceanic autotrophic taxa. This transition reflects the progressive deepening of the western Tasmanian continental margin, an interpretation supported by our new seismic investigations. The dominance of autotrophic species like Spiniferites spp. and Operculodinium spp. reflects relatively oligotrophic conditions, like those of regions north of the modern-day STF. The increased abundance in the earliest Miocene of Nematosphaeropsis labyrinthus, typical for modern subantarctic zone (frontal) conditions, indicates a cooling and/or closer proximity of the STF to the site . The absence of major shifts in dinocyst assemblages contrasts with other records in the region and suggests that small changes in surface oceanographic conditions occurred during the Oligocene. Despite the relatively southerly (63–55∘ S) location of Site 1168, the rather stable oceanographic conditions reflect the continued influence of the proto-Leeuwin Current along the southern Australian coast as Australia continued to drift northward. The relatively “warm” dinocyst assemblages at ODP Site 1168, compared with the cold assemblages at Antarctic Integrated Ocean Drilling Program (IODP) Site U1356, testify to the establishment of a pronounced latitudinal temperature gradient in the Oligocene Southern Ocean.

Pleistocene surface-ocean changes across the Southern subtropical front recorded by cryptic species of Orbulina universa

The largest ocean temperature gradients occur in the Southern Ocean, where the Antarctic circumpolar current frontal systems serve as hydrographic barriers with distinctive flora and fauna. During Pleistocene glacial-interglacial cycles, the position and intensity of these frontal systems are thought to have changed. This study examines the occurrence of distinct cryptic species of Orbulina universa during the Mid-Pleistocene Transition at two sites in the South Atlantic, Ocean Drilling Program Holes 1088B and 1090B that are separated by 2° latitude. We discuss abundance records of the Caribbean and Mediterranean species and their relationship to Total Organic Carbon and stable isotope records to further understand the sea-surface changes of this region during the Pleistocene. Data from 1090B, the more southerly Hole, shows the presence of few Mediterranean species together with aberrant and Biorbulina bilobata forms. At ODP Hole1088B, on the other hand, the presence of both cryptic species are observed throughout the Pleistocene, indicating distinct surface water sources and properties. The abnormal forms indicate the possible disappearance of O. universa under stressful environmental conditions in the sub-antarctic zone with reseeding of the species at glacial-interglacial transitions (Marine isotopic stages 23/22, 14/13, 8/7 and 2/1). Although nutrient-rich surface waters and shallow nutricline favours high productivity in this region during glacial intervals, it is possible that Mediterranean species at 1090B would have been restricted by cold sub-polar surface waters. Comparison with published data suggests that the presence of subtropical Caribbean species at 1088B was driven by the prominence of northern sourced waters and interocean exchange south of Africa. Our data point to the presence of sub-tropical waters at 1088B during each glacial-interglacial transition with maxima occurring at three intervals (1200 ka, 700–600 ka and 250–100 ka) that coincide with the inception of the Mid-Pleistocene Transition and sea surface temperatures changes at the subtropical front. The interocean transfer and surface water changes at the subtropical front in the late Pleistocene are consistent with modulation by eccentricity-induced changes from a 41-ky world to a 100-ky world.

Evidence for the Impact of Climate Change on Primary Producers in the Southern Ocean

Within the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO), this paper brings together analyses of recent trends in phytoplankton biomass, primary production and irradiance at the base of the mixed layer in the Southern Ocean and summarises future projections. Satellite observations suggest that phytoplankton biomass in the mixed-layer has increased over the last 20 years in most (but not all) parts of the Southern Ocean, whereas primary production at the base of the mixed-layer has likely decreased over the same period. Different satellite models of primary production (Vertically Generalised versus Carbon Based Production Models) give different patterns and directions of recent change in net primary production (NPP). At present, the satellite record is not long enough to distinguish between trends and climate-related cycles in primary production. Over the next 100 years, Earth system models project increasing NPP in the water column in the MEASO northern and Antarctic zones but decreases in the Subantarctic zone. Low confidence in these projections arises from: (1) the difficulty in mapping supply mechanisms for key nutrients (silicate, iron); and (2) understanding the effects of multiple stressors (including irradiance, nutrients, temperature, pCO2, pH, grazing) on different species of Antarctic phytoplankton. Notwithstanding these uncertainties, there are likely to be changes to the seasonal patterns of production and the microbial community present over the next 50–100 years and these changes will have ecological consequences across Southern Ocean food-webs, especially on key species such as Antarctic krill and silverfish.

Avian orthoavulavirus 1 (Newcastle Disease virus) antibodies in five penguin species, Antarctic peninsula and Southern Patagonia.

Avian orthoavulavirus 1 (AOaV-1) causes Newcastle disease, one of the most important and contagious infections in poultry, where migratory birds can play a key role as a reservoir. Seven hundred and seven serum samples were collected from five penguin species (King, Magellanic, Gentoo, Chinstrap and Adelie penguins) in the Antarctic and Sub-Antarctic zones. Using a competitive ELISA to detect antibodies against AOaV-1, we identified positive individuals in all penguin species. The Magellanic penguin showed the highest seropositivity rate (30.3%), suggesting it could be a natural reservoir of this virus. At the Antarctic zones, Chinstrap penguin showed the highest occurrence (7.5%). Interesting, positive sera was only obtained in Sub-Antarctic and Northern zones at the Antarctic peninsula, no seroreactivity was observed in Southern locations. Further studies are needed to establish the role of these penguin species in the epidemiology of the AOaV-1 and determine the effects of this virus in these populations.

Effects of Ocean Acidification on Calcification of the Sub-Antarctic Pteropod Limacina retroversa

Ocean acidification is expected to impact the high latitude oceans first, as CO2 dissolves more easily in colder waters. At the current rate of anthropogenic CO2 emissions, the sub-Antarctic Zone will start to experience undersaturated conditions with respect to aragonite within the next few decades, which will affect marine calcifying organisms. Shelled pteropods, a group of calcifying zooplankton, are considered to be especially sensitive to changes in carbonate chemistry because of their thin aragonite shells. Limacina retroversa is the most abundant pteropod in sub-Antarctic waters, and plays an important role in the carbonate pump. However, not much is known about its response to ocean acidification. In this study, we investigated differences in calcification between L. retroversa individuals exposed to ocean carbonate chemistry conditions of the past (pH 8.19; mid-1880s), present (pH 8.06), and near-future (pH 7.93; predicted for 2050) in the sub-Antarctic. After 3 days of exposure, calcification responses were quantified by calcein staining, shell weighing, and Micro-CT scanning. In pteropods exposed to past conditions, calcification occurred over the entire shell and the leading edge of the last whorl, whilst individuals incubated under present and near-future conditions mostly invested in extending their shells, rather than calcifying over their entire shell. Moreover, individuals exposed to past conditions formed larger shell volumes compared to present and future conditions, suggesting that calcification is already decreased in today’s sub-Antarctic waters. Shells of individuals incubated under near-future conditions did not increase in shell weight during the incubation, and had a lower density compared to past and present conditions, suggesting that calcification will be further compromised in the future. This demonstrates the high sensitivity of L. retroversa to relatively small and short-term changes in carbonate chemistry. A reduction in calcification of L. retroversa in the rapidly acidifying waters of the sub-Antarctic will have a major impact on aragonite-CaCO3 export from oceanic surface waters to the deep sea.

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

AbstractAtmospheric dust is a primary source of iron (Fe) to the open ocean, and its flux is particularly important in the high nutrient, low chlorophyll (HNLC) Southern Ocean where Fe currently limits productivity. Alleviation of this Fe limitation in the Subantarctic Zone of the Atlantic by increased dust‐borne Fe supply during glacial periods has been shown to increase primary productivity. However, previous work has found no such increase in productivity in the Pacific sector. In order to constrain the relative importance of Southern Ocean Fe fertilization on glacial‐interglacial carbon cycles, records of dust fluxes outside of the Atlantic sector of the Southern Ocean at the Last Glacial Maximum (LGM) are required. Here we use grain size and U‐series analyses to reconstruct lithogenic and CaCO3 fluxes and Nd, Sr, and Pb isotopes to ascertain the provenance of terrigenous material delivered to four deep water cores in the SW Pacific Ocean over the last ~30 kyr. We find evidence for an increase in the relative proportion of fine‐grained (0.5–12 μm) terrigenous sediment and higher detrital fluxes during the LGM compared to the Holocene. The provenance of the LGM dust varied spatially, with an older, more “continental” signature (low εNd, high 87Sr/86Sr) sourced from Australia in the northern cores, and a younger, more volcanogenic source in the southern cores (high εNd, low 87Sr/86Sr), likely sourced locally from New Zealand. Given this increase in lithogenic flux to the HNLC subantarctic Pacific Southern Ocean during the LGM, factors besides Fe supply must have regulated the biological productivity here.

Impact of the Agulhas Return Current on the oceanography of the Kerguelen Plateau region, Southern Ocean, over the last 40 kyrs

The oceanography of the western Indian sector of the Southern Ocean is extremely complex due to the presence of several subantartic islands and plateaus that alter the zonal flow of the Antarctic Circumpolar Current. The circulation is even more complex around the Kerguelen Islands (KI) as the hydrological fronts merge with the Agulhas Return Current, the latter transporting warm surface waters from the low latitudes to the Subantarctic Zone (SAZ) east of KI. Here we present new sea-surface and sub-surface temperatures, based on diatom and radiolarian census counts respectively, in two cores covering the last 40,000 years (40 kyrs) to infer past oceanographic changes in the KI region. In core MD11-3353, located in the Polar Front Zone (PFZ) west of KI, both sea-surface and sub-surface temperatures follow the expected glacial-interglacial pattern, with a constant difference of ∼3 °C, suggesting that the structure of the top 500 m of the water column did not change drastically over this period. In core MD12-3396CQ, located in the SAZ east of KI, the sub-surface temperature record again follows the expected glacial-interglacial pattern while the diatom-based reconstruction shows a strong warming during the 40-24 kyrs period, with a mean difference between the two of ∼9 °C. The temperature difference subsequently reverts to ∼3 °C during the glacial termination and Holocene. We suggest that the large difference between surface and sub-surface temperatures during the 40-24 kyrs period resulted from the injection of warm surface waters from the Indian low latitudes. This signal was transported to the core site by a strengthened Agulhas Return Current during the glacial, when the Agulhas leakage to the Atlantic was reduced. Our study thus proposes that the surface conditions in the SAZ of the KI region are shaped by the interplay between the migration of the Southern Ocean hydrological fronts and the Agulhas (Return) Current, both being climatically modulated.

A circumpolar dust conveyor in the glacial Southern Ocean

The increased flux of soluble iron (Fe) to the Fe-deficient Southern Ocean by atmospheric dust is considered to have stimulated the net primary production and carbon export, thus promoting atmospheric CO2 drawdown during glacial periods. Yet, little is known about the sources and transport pathways of Southern Hemisphere dust during the Last Glacial Maximum (LGM). Here we show that Central South America (~24‒32°S) contributed up to ~80% of the dust deposition in the South Pacific Subantarctic Zone via efficient circum-Antarctic dust transport during the LGM, whereas the Antarctic Zone was dominated by dust from Australia. This pattern is in contrast to the modern/Holocene pattern, when South Pacific dust fluxes are thought to be primarily supported by Australian sources. Our findings reveal that in the glacial Southern Ocean, Fe fertilization critically relies on the dynamic interaction of changes in dust-Fe sources in Central South America with the circumpolar westerly wind system.

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.