Abstract. The primary production and zooplankton life cycles in the Southern Ocean synchronize with seasonal sea ice melting, during which particulate organic carbon (POC) and nitrogen (PN), including ice algae, are supplied from sea ice into the water column. To understand carbon flow through food webs in this productive marginal ice zones, it is necessary to determine the quantity and quality of organic carbon and nitrogen in sea ice. However, knowledge regarding brash sea ice is scarce, even though it is the predominant form at the Antarctic ice edge. In this study, 102 sea-ice samples were collected from the Indian sector (20–160°E) during summer to clarify variations in isotopic baselines and the potential contribution of sea ice-derived carbon and nitrogen to the water column. The δ13C and δ15N values of sea-ice particulate matter were −24.5±3.2‰ and +1.6±2.2‰ (mean±standard deviation), respectively, showing greater variability compared to those in seawater (−28.8±1.0‰ for δ13C and −0.5±1.4‰ for δ15N). We identified correlations of δ13C and δ15N with organic matter and nutrient concentrations and their ratios (e.g., silicate:nitrate), indicating that nutrient consumption and organic matter decomposition influenced those isotopic characteristics in sea ice. Using freshwater flux from sea ice melt and the measured sea-ice POC concentrations, this supply was estimated to be 4.6±0.3 Tg C year−1, equivalent to >14% of primary production in the marginal ice zone of the studied region. These results suggest that the carbon supply from sea ice is a crucial factor supporting ecosystems in the seasonal sea ice zone.

The Ross Sea, a critical region for global carbon cycling, receives increasing glacial meltwater from West Antarctica, yet the impact on regional carbon cycling remains poorly understood. A key uncertainty is how this meltwater influences dissolved organic matter (DOM), particularly in the historically inaccessible eastern Ross Sea. This study provides the first comprehensive analysis of meltwater-driven DOM dynamics during exceptional 2023 low sea ice conditions. By analyzing dissolved organic carbon (DOC) concentrations, DOM optical properties, and biogeochemical parameters across 42 stations, we examined associations between glacial meltwater, DOM, and regional carbon cycling. DOC concentrations varied regionally, with the eastern Ross Sea exhibiting significantly higher levels (75 +/- 20 mu M C) than the central (65 +/- 16 mu M C) and western regions (52 +/- 10 mu M C). In the eastern Ross Sea, high meltwater fractions were associated with elevated iron (Fe) concentrations (0.54 +/- 0.18 nM) and enhanced biological production, evidenced by nutrient depletion and elevated particulate organic carbon, which subsequently promoted DOC accumulation. Modified Circumpolar Deep Water (mCDW) intrusion through the Hayes Bank creates a hydrographic boundary confining DOC-rich waters. Accumulated DOM undergoes microbial processing, as indicated by optical properties showing low-molecular-weight characteristics and biological breakdown. These processes contribute to the deep water microbial loop and suggest that accelerating West Antarctic ice loss may alter Southern Ocean carbon sequestration by creating regional carbon hotspots that enhance the biological carbon pump. The 2023 observations provide critical insights into how glacial meltwater inputs during low sea ice conditions could influence regional carbon cycling patterns.

Because the partial pressure of carbon dioxide (pCO2) at the ocean surface is the main determinant of the air-sea CO2 flux, understanding what causes pCO2 to vary is important to the issue of carbon sequestration. The strength of the CO2 flux is strongly influenced by wind speed and, in polar regions, the presence of sea ice which affects gas exchange efficiency. In this study, oceanographic measurements were made near the Mertz Glacier at 140-150 degrees E during the 2018-2019 austral summer to clarify the effects of melting sea ice and glacial ice and biological production on surface ocean pCO2. Water column samples were collected and analyzed for stable oxygen isotopic ratios and concentrations of dissolved inorganic carbon, total alkalinity, and nutrients. We evaluated the direct effects of meteoric water and sea ice meltwater on pCO2, finding that, at 10 dbar, they reduced pCO2 by 8 +/- 1 & micro;atm and 8 +/- 2 & micro;atm, respectively, relative to pCO2 of Circumpolar Deep Water, 447 +/- 21 & micro;atm. We then evaluated the indirect effects of biological production, finding that the reduction of pCO2 due to biological production, calculated from nutrient uptake during winter and summer, was 107 +/- 31 & micro;atm. The reduction of pCO2 at 10 dbar (113 +/- 29 & micro;atm) relative to pCO2 of Circumpolar Deep Water in the coastal region of the Mertz Glacier was influenced significantly by biological production. Iron limitation was assessed based on the ratio of nutrient consumption (ASi/AN) of phytoplankton from winter to summer. Net community production was higher and ASi/AN was lower closest to the continent. As low ASi/AN indicates iron-replete conditions, the implication is that iron had enhanced biological production. These results support the hypothesis that iron supplied by glacial meltwater and coastal sediments controls biological production and pCO2 variability in this area.

A hydrographic section along 55° E, south of 30° S, was visited from December 2019 to January 2020 as the first occupation under the Global Ocean Ship-based Hydrographic Investigations Program. The water column was measured from the sea surface to 10 dbar above the bottom with eddy-resolving station spacings and state-of-the-art accuracy. The upper profile was characterised by a conspicuous front between 42.5 and 43° S and a cold-core eddy at 39° S. The front was identified as the confluence of Subtropical and Subantarctic fronts. The Agulhas Return Current front was found at 41.6° S. When combined with the section north of 30° S observed in 2018, another subsurface front was found in dissolved oxygen around 28° S at depths of 1500 to 3000 dbar. In the eastern Weddell–Enderby Abyssal Plain, no obvious large-scale flow was observed at depths greater than 3000 dbar. We used transient tracers to estimate isopycnal diffusivity there to be 72±16 m2 s−1. Antarctic Bottom Water in the basin consisted of water masses originating from the Cape Darnley region (0 %–35 %) and Weddell Sea Deep Water (5 %–75 %) diluted by Lower Circumpolar Deep Water above. These snapshot observations not only confirm hydrographic features reported earlier in the Madagascar and Crozet basins, but also describe the diffusive nature of the deep to bottom circulation in the Weddell–Enderby Abyssal Plain.

Antarctic Bottom Water (AABW) is derived from dense water that sinks from the Antarctic continental shelf to the deep ocean. The sinking of AABW is balanced by a return flow of lighter water, and the resulting overturning circulation determines the density stratification of the deep ocean, regulates ocean storage of heat and carbon, and supplies oxygen to the deep sea. In this Review, we highlight progress in understanding how and why AABW is changing and the consequences for the deep overturning circulation. Since the mid-1980s, ocean heat content below 4,000 dbar has increased at a rate of 12.9 (±1.8) trillion watts, and the AABW has thinned by more than 50 dbar decade−1, with more rapid thinning observed closer to the sources of AABW. The abyssal overturning circulation has slowed in response to freshening of shelf waters by glacial melt and changes in sea ice formation. Numerical model simulations indicate that these trends will accelerate under projected increases in meltwater input. Future research priorities include sustained observations in the deep ocean and on the Antarctic continental shelf; exploration of feedbacks between ocean circulation, sea ice, dense water formation and ice shelf melt; and improved representation of AABW in ocean and climate models. Antarctic Bottom Water (AABW) sinks near Antarctica and fills the deep ocean. This Review discusses how AABW is formed, past changes to its properties and transport, and projects future changes in AABW and the deep overturning circulation.

Newly formed sea ice can incorporate considerable amounts of organic matter including phytoplankton during its freezing and growth. This initial incorporation influences the standing stocks of ice algae, a fundamental component of high-latitude marine ecosystems. Physical incorporation via the accretion of frazil ice, instead of congelation growth, is considered to incorporate particles efficiently. However, for thin ice (<0.20 m thickness), information on the relationship between ice texture and particle incorporation is scarce. Here, we assessed sea-ice texture (proportion of frazil ice-origin layer), macronutrient concentrations, fraction of snow, and algal composition in Antarctic sea ice and their effects on the enrichment of algae into thin ice in terms of chlorophyll a (Chl a) and biogenic silica (BSi). Chl a and BSi concentrations of thin ice ranged over 0.2–11.6 µg L−1 and 0.5–11.1 µmol L−1, respectively, being significantly concentrated compared to seawater. BSi enrichment in thin ice correlated negatively with the proportion of frazil ice-origin layer, while no relationship was observed with Chl a enrichment. These results suggest that frazil ice could play a minor role in entraining algal cells under certain circumstances, possibly due to the low turbulence and wind speeds of the late summer–early autumn season dampening the interaction between frazil ice and phytoplankton. Algal composition data could suggest that ice algae from the previous winter’s sea ice were released and re-incorporated into the surrounding thin ice on its formation, given the high similarity in species composition observed between older ice and several thin ice samples. This mechanism potentially promotes algal enrichment without the accretion of frazil ice, as indicated by the maximum enrichment in thin ice which developed via congelation growth. Our results show that algal concentration in thin ice could be affected by the presence of old sea ice and hydrographic and meteorological conditions.

Circumpolar Deep Water inflow onto Antarctica’s continental shelves is a key driver of accelerated Antarctic Ice Sheet mass loss, both presently and during the last deglaciation. However, the mechanisms driving enhanced inflow and the resultant impact on large-scale ice-sheet retreat events are still not fully understood. Here we address this topic using marine sediment cores from Lützow–Holm Bay, East Antarctica, through analyses of sedimentary beryllium isotopes and complementary proxies. These records, when compared to inland mountain outcrop records, show that ice-shelf collapse and simultaneous inland ice-sheet thinning 9,000 years ago were associated with enhanced Circumpolar Deep Water inflow and sea-level rise. A hierarchical modelling approach that combines climate and high-resolution ocean simulations suggests that freshwater discharge from adjacent Antarctic sectors into the Southern Ocean probably enhanced the regional inflow into submarine troughs in Lützow–Holm Bay between 10,000 and 9,000 years ago. We propose a feedback loop whereby meltwater from rapidly retreating Antarctic sectors since the Last Glacial Maximum enhanced stratification and Circumpolar Deep Water incursions onto adjacent shelves. Alongside relative sea-level rise, this meltwater feedback triggers further ice-shelf instability and enhances dynamic inland ice discharge, highlighting a mechanism relevant to future Antarctic Ice Sheet changes. Early Holocene retreat of an ice shelf in East Antarctica was linked to ocean-driven forcing enhanced by ice-sheet meltwater from adjoining regions, as unveiled through the integration of proxy records with ocean and climate modelling.

Ocean circulation around the Antarctic coastal margins plays a critical role in heat delivery to Antarctica, melting ice shelves. However, until recently, satellite‐based sea‐level observations have been limited by the presence of sea ice. With improved algorithms, it is now possible to monitor sea‐level fluctuations over most of the Southern Ocean, including sea ice‐covered areas. We identified several clockwise gyres along East Antarctic coastal margins in satellite‐derived dynamic ocean topography (DOT). Singular value decomposition analyses revealed that the coastal DOT deepening and anomalous clockwise circulation consistently occur during the positive phase of the southern annular mode (SAM), which is associated with negative wind stress curl anomalies. Shifting of the SAM to a more positive phase since the 20th century and its expected continuation into the coming century could lead to enhanced clockwise gyres along East Antarctica, contributing to increased poleward ocean heat transport.

The coastal polynya formed off Cape Darnley, in the Southern Ocean, is a favorable site for a secondary phytoplankton bloom in the late summer and autumn. In late February of 2018, we conducted in‐situ observations onboard an icebreaker and measured surface water chlorophyll a concentrations reaching 5.5 μg/L. Concurrently, in turbulent conditions associated with wind speeds exceeding 20 m/s, ocher‐colored newly‐formed sea ice, in the form of grease and pancake ice, spread across the polynya. Chlorophyll a concentrations measured in the grease and pancake ice were 47 times (260.0 μg/L) and 11 times (61.8 μg/L) higher than in the surrounding seawater, respectively. The corresponding sea ice algal concentration was sufficiently high to discolor the ice. Moreover, water temperatures were at or below the freezing point at depths shallower than 30 m, suggesting that suspended frazil ice came into contact with phytoplankton, which were particularly abundant in the water column during the bloom, and scavenged them. The diatom Fragilariopsis curta represented more than 83% of both total diatom cell abundance and biovolume in sea ice. Combining known algal growth rates with our results of chlorophyll a concentration in newly‐formed ice, most of the ice algae originated from phytoplankton incorporated and accumulated by frazil ice. The considerable algal concentrations measured in new ice suggest that this accumulation process could contribute to the algal standing stock in Antarctic sea ice. The abundant bloom‐forming phytoplankton incorporated into sea ice suggests that they could be a seed population for subsequent ice‐algal or ice‐edge bloom formation.

Continuous moored time series of temperature, salinity, pressure and current speed and direction are of great importance for understanding the continental shelf and under-ice-shelf dynamics and thermodynamics that govern water mass transformations and ice melting in and around Antarctic marginal seas. In these regions, icebergs and sea ice make ship-based mooring deployment and recovery challenging. Nevertheless, over decades, expeditions around the fringe of Antarctica sporadically deployed and recovered hundreds of moored instruments, including those facilitated through ice shelves boreholes. These datasets tend to be archived in a wide range of data centres, with, to our knowledge, no clear format standardisation. As a result, systematic analysis of historical mooring time series in the marginal seas is often challenging. Here we present the first version of a standardised pan-Antarctic moored hydrography and current time series compilation, with broad international contributions from data centres, research institutes and individual data owners. The mooring records in this compilation span over five decades, from the 1970s to the 2020s, providing an opportunity for a systematic study of the pan-Antarctic water mass transport and shelf connectivity. As a demonstration of the utility of this compilation, we present spectral analysis of the compiled current velocity time series, which unsurprisingly shows the dominating presence of tidal variability within most records. This component of the variability is fitted using multi-linear regression to tidal frequencies, and the tidal fit is removed from the original time series to leave de-tided variability. Given the limited record durations to months to years, de-tided variability is dominated by synoptic (3-10 d period), intraseasonal (10-80 d) and seasonal (similar to 6 months-1 year) signals. The spatial distribution of the kinetic energy integrated within frequency bands is presented and discussed within respective regional contexts, and future avenues of research are proposed. This data compilation is assembled under the endorsement of Ocean-Cryosphere Exchanges in ANtarctica: Impacts on Climate and the Earth System (OCEAN ICE) project (https://ocean-ice.eu/, last access: 23 October 2025) funded by the European Commission and UK Research and Innovation. It is available and regularly updated in NetCDF format with the SEANOE database at 10.17882/99922 (Zhou et al., 2024a).

The first systematic sediment survey by the icebreaker Shirase (AGB-5003) was conducted during the 61st Japanese Antarctic Research Expedition (JARE-61). Sediment samples were obtained from a total of 28 sites in 3 areas of the Totten Glacier front–Dalton polynya, Lützow-Holm Bay and Cape Darnley polynya using a large-bore gravity corer and GSJ-type K-grab sampler. In this paper, details of the survey operation are described as a reference for the future experience. In addition, preliminary results of the obtained sediments are reported as a fundamental information for future analysis using these samples.

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150 degrees E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150 degrees E), the poleward shift of the southern boundary of the ACC (50-120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1-1.6 degrees C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (similar to 0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10-20 mu mol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112-140 degrees E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 degrees C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m(3)) was found to the east of Mertz Polynya (142-148 degrees E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.

The Southern Ocean plays a central role in Earth's climate, ecology, and biogeochemical cycles. Therefore, understanding long-term changes in Southern Ocean water masses in the geologic past is essential for assessing the role of the Southern Ocean in the climate system. Radiolarian fossils are a useful tool to reconstruct the water masses of the Southern Ocean. However, the radiolarian assemblages in the high latitudes of the Southern Ocean (south of the polar front (PF)) are still poorly understood. In this paper, we report the radiolarian assemblages in surface marine sediment and plankton tow samples collected from the high latitudes south of the PF. In the surface sediments, four factors (named F1–F4) of the radiolarian assemblages were identified using Q-mode factor analysis, which are related to different water masses and hydrological conditions. F1 is related to the surface waters south of the southern boundary (SB) of the Antarctic Circumpolar Current (ACC), which are cooled by melting sea ice and ice sheets. F2 is associated with water masses north of the SB. A comparison with the vertical distribution of the radiolarian assemblages in plankton tow samples indicates that characteristic species are associated with the Circumpolar Deep Water (CDW) and surface waters north of the SB. F3 is associated with modified Circumpolar Deep Water (mCDW). The radiolarian assemblage of F4 does not seem specifically related to any of the water mass here analyzed. However, the species in this assemblage are typically dwells within ice shelf and/or sea ice edge environments. Radiolarian assemblages here identified and associated with water masses, and ice edge environments are useful to reconstruct the environment south of the PF in the geologic past.

In East Antarctica, warm ocean water travels toward the Totten Ice Shelf. This water melts and thins the ice shelf, and speeds up the rate at which ice moves into the sea, leading to sea-level rise. Scientists often get on board ships called icebreakers to study the ice and water in these regions. However, sea ice and icebergs are major obstacles to navigation and scientific operations. For example, American, Australian, and Japanese icebreakers tried but could only observe a small area where sea ice was more broken up. So, we used a helicopter to measure the ocean during one of our research expeditions. Helicopters can travel faster than icebreakers. They can fly over sea ice and icebergs, and trained workers can drop sensors into small gaps in the ice. In 6 days, we observed ocean temperatures at 67 sites, covering a large area that could not be studied before. We identified wide pathways of warm water flowing toward the Totten Ice Shelf.

To investigate the spatial distributions and determinants of nutrient concentrations, we measured NO3- +NO2-, PO43- , and Si(OH)4 concentrations in the eastern Indian Ocean sector of the Antarctic Ocean (80 - 150 degrees E, south of 60 degrees S) between December 2018 and February 2019. In the region influenced by the Antarctic Circumpolar Current, nutrient concentrations were increased by nutrients supplied from the deep layer and by organic matter decomposition and remineralization within the seasonal pycnocline after the development of strong stratification. Strong stratification also enhanced phytoplankton growth and nutrient consumption by photosynthesis. In contrast, in the subpolar region, nutrient concentrations were increased by nutrients supplied by brine discharged during sea ice formation and decreased by dilution with sea ice meltwater. Although high salinity in the surface and subsurface layers corresponded well to upwelling areas around subpolar subgyres, high salinity was not necessarily correlated with nutrient concentrations. We estimated primary production both from in situ nutrient data and from satellite-acquired chlorophyll-a data. According to both estimation methods, primary production was high in the subpolar region, especially around 120 - 130 degrees E. However, nutrient-based estimation also showed high production in coastal areas where, because of sea ice and cloud cover, estimation based on satellite data was not possible. To understand primary production in seasonal ice areas, the best estimation method should be selected for the research goals or multiple methods should be used in combination.

The Totten Ice Shelf (TIS) and Moscow University Ice Shelf (MUIS), along the Sabrina Coast of Wilkes Land, are the floating seaward terminuses of the second-largest freshwater reservoir in the East Antarctic Ice Sheet. Being a marine ice sheet, it is vulnerable to the surrounding ocean conditions. Recent comprehensive oceanographic observations, including bathymetric measurements off the Sabrina Coast, have shed light on the widespread intrusion of warm modified Circumpolar Deep Water (mCDW) onto the continental shelf and the intense ice–ocean interaction beneath the TIS. However, the spatiotemporal coverage of the observation is very limited. Here, we use an ocean–sea ice–ice shelf model with updated bathymetry to better understand the regional ocean circulations and ocean–cryosphere interactions. The model successfully captured the widespread intrusions of mCDW, local sea ice production and the ocean heat and volume transports into the TIS cavity, facilitating an examination of the overturning ocean circulation within the ice shelf cavities and the resultant basal melting. We found notable differences in the temporal variability in ice shelf basal melting across the two adjacent ice shelves of the TIS and the western part of the MUIS. Ocean heat transport by mCDW controls the low-frequency interannual-to-decadal variability in ice–ocean interactions, but the sea ice production in the Dalton Polynya strongly modifies the signals, explaining the regional difference between the two ice shelves. The formation of a summertime eastward-flowing undercurrent beneath the westward-flowing Antarctic Slope Current is found to play an important role in the seasonal delivery of ocean heat to the continental shelf.

Warm, salty Circumpolar Deep Water (CDW) has long been regarded as the climatological driver for Antarctica, but the mechanism of how it can reach the continental shelf remains unsettled. Motivated by the absence of observational eddy flux estimation in the Antarctic margin, we quantify isopycnal diffusivity of CDW by hydrographic records and satellite altimetry under the mixing length framework. For comparison, spiciness and thickness are used as the isopycnal tracer, and two yield similar results. Over the extent of Antarctic Circumpolar Current (ACC), we find a general agreement with the mixing suppression theory and its exception in the lee of the topography as previously reported. In contrast, no mixing length’s dependency on mean flow is obtained to the pole, reflecting a stagnant flow regime in the Antarctic margin. Isopycnal diffusivity ranges 100–500 m2 s-1 to the south of the ACC. Eddy diffusion is likely enhanced where the CDW intrusion is localized by the recirculating gyres, mostly attributable to the small gradient of isopycnal thickness. Volume transport is then estimated by the layer thickness gradient. Thickness-diffusive onshore heat flux across the continental slope (~3.6/1.2 TW in the eastern/western Indian sectors) is quantitatively consistent with cryospheric heat sinks by sea ice formation and ice shelf basal melt, suggesting that the isopycnal eddy diffusion is the main cause of the onshore CDW intrusion. We emphasize that the thickness field is essential for determining the eddy fluxes in the Antarctic margin.