This review compiles studies on paleoceanographic and paleoenvironmental changes in the high-latitude Southern Ocean during the late Quaternary based on Chinese Antarctic Expeditions. Four major findings are summarized as follows. (1) Diatom compositions in sediment record sea surface condition changes in the Bransfield Strait and Cosmonaut Sea, indicating an increase in sea ice cover since the Mid-Holocene for both areas, possibly linked to cold water mass injection from the Weddell Sea ice shelf. (2) Export productivity records from Antarctic marginal seas show a glacial-interglacial variation pattern, with higher and lower productivity during the interglacial and glacial periods, respectively, controlled by both the intensity of Circumpolar Deep Water upwelling and sea ice cover. An overall increase of export productivity from Marine Isotope Stage (MIS) 12 to MIS 6 is noticed. (3) Ventilation state of the abyssal Southern Ocean is closely related to global climate change and atmospheric CO₂ concentrations, with elevated ventilation efficiency during deglacial periods responsible for elevated atmospheric CO₂ concentrations. (4) Antarctic marine-based ice sheets/glaciers wax and wane in line with global and regional ice volume changes, predominantly subject to oceanic forcing, while land-based ice sheets might only reorganize in specific glacial intervals. Despite significant progress in understanding the role of the Southern Ocean and the Antarctic Ice Sheet within the global climate system, challenges remain in sample collection and research methods. Future studies should aim to collect samples from a broader range of latitudes, ensure higher quality and longer temporal span, resolve dating issues, reconstruct seawater temperature, and focus on reconstructions of Southern Ocean carbon cycling and changes of intermediate to deep water masses.

Research on changes in the redox conditions of bottom waters is essential for understanding deep water circulation, global ocean currents, climate change, and ecosystem health. Through sedimentary geological methods, a deeper understanding of the complex relationships between various environmental changes can be achieved, providing detailed evidence and theoretical support for global climate change research. The Ross Sea in Antarctica plays a key role in the formation of Antarctic bottom water (AABW), and the complex climate changes since the last glacial maximum (LGM) make it particularly significant for study. This research analyzes core ANT32-RB16C from the Ross Sea using geochemical proxies such as major and trace elements, grain size, and redox-sensitive indicators like Mn/Ti, Co/Ti, Mo/Ti, Cd/Ti, U/Th, and Ni/Co molar concentration ratios. Combining this data with a previously established chronological framework, the study explores the evolution of redox conditions in the Ross Sea’s deep waters since the LGM. The results show that the deep waters have remained oxygen-rich since the LGM, with significant changes in four stages. Stage 1 (24.7–15.7 cal ka BP): Strong oxidizing conditions, likely due to enhanced formation of Ross Sea bottom water (RSBW), increasing oxygen levels. Stage 2 (15.7–4.5 cal ka BP): Weakened oxidizing conditions as temperatures rose and ice shelves retreated, increasing primary productivity and depleting oxygen. Stage 3 (4.5–1.5 cal ka BP): Continued decline in oxidizing conditions, possibly linked to high primary productivity and oxygen consumption. Stage 4 (1.5 cal ka BP to present): A rapid recovery of oxidizing conditions, likely driven by temperature drops, increased RSBW formation, and decreased productivity.

The melting of seasonal sea ice in Antarctica plays a pivotal role in the region’s carbon cycle, influencing global carbon storage and the exchange of carbon between the atmosphere and the ocean. However, the impact of variability in the timing of seasonal sea ice retreat on the flux and composition of sinking particulate matter remains to be elucidated. In this study, we deployed sediment traps in Prydz Bay during the austral summers of 2019/2020 and 2020/2021, noting that sea ice melting occurred approximately one and a half months earlier in the former summer compared to the latter. We analyzed sediment trap data, which included total mass flux (TMF), particulate organic carbon (POC), biogenic silica (BSi), particulate inorganic carbon, and lithogenic particle (Litho) fluxes, as well as the stable isotopes δ¹³C and δ¹⁵N of particulate organic matter (POM). Additionally, we incorporated remote sensing data on sea ice concentration and chlorophyll a. This dramatic delay in sea ice melting timing could result in a significant increase in TMF, BSi and POC fluxes in the summer of 2020/2021 compared to 2019/2020. Elevated BSi fluxes and more ¹³C-depleted POC in the austral summer of 2020/2021 suggest that the delayed melting of sea ice may have stimulated the productivity of centric diatoms. Furthermore, the higher BSi/POC ratio and more negative δ¹⁵N values of POM, along with a reduced presence of krill in the traps, indicate a diminished grazing pressure from zooplankton, which collectively enhanced the sedimentation efficiency of POC during the austral summer of 2020/2021. These findings highlight the critical role of sea ice melting timing in regulating productivity, flux and composition of sinking particulate matter in the Prydz Bay ecosystem, with significant implications for carbon cycling in polar oceans.

Dissolved organic carbon (DOC) constitutes the largest organic carbon reservoir in the ocean and plays a vital role in the oceanic carbon cycle. However, research on DOC in Antarctica has been limited, largely due to difficulties in sample collection. In this study, we investigate chromophoric dissolved organic matter (CDOM) in the Cosmonaut Sea and Cooperation Sea, Antarctica. The relative abundances of CDOM, as indicated by the absorption coefficient at 254 nm (a₂₅₄), exhibit significant variability, ranging from 1.29 m⁻¹ to 8.37 m⁻¹. The diatom species B is the primary contributor to CDOM in the region influenced by the Antarctic slope current. Using excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis (EEM-PARAFAC), we identify three fluorescent components: one protein-like component (C1) and two humic-like components (C2 and C3). Our findings indicate that strong microbial activity in the euphotic zones of the Cosmonaut Sea and Cooperation Sea serves as a primary pathway for the removal of protein-like substances while also acting as a significant source of humic-like substances. Microbial degradation likely accounts for the decoupling of the vertical distribution of DOC and phytoplankton. Furthermore, the intrusion of modified circumpolar deep water (mCDW) into the euphotic zones of open ocean areas influences the distribution of CDOM at depths shallower than 200 m. These results have important implications for enhancing our understanding of the dynamics of CDOM and DOC in east Antarctica, as well as for improving assessments of the carbon cycle.

Arctic climate changes have profoundly influenced the polar environmental changes in recent years. The Arctic Oscillation (AO), as a key component of the Arctic climate system’s internal variability, affects the source to sink processes and interactions across the multilayer Arctic system by regulating the land, ocean, sea ice, and atmospheric processes. The East Siberian Arctic Shelf (ESAS) has experienced significant changes in the input, transport, and burial of sedimentary organic carbon (OC) due to climate warming and shifts in the AO phase in recent decades. This study analyzes grain size, total organic carbon (TOC), total nitrogen (TN), and stable carbon isotope (δ¹³C) in two sediment cores from the ESAS to reconstruct the burial record of OC over the past few decades and examine the response mechanism of sedimentary OC records to regional-scale climate forcing. The results show that the OC in the two sediment cores originates from mixed sources with a dominant terrestrial contribution. In the LV83-28 core from the Laptev Sea, the TOC and TN contents have increased at an accelerated rate since the 1990s, with a noticeable rise in the contribution of terrestrial OC. This trend is linked to an increase in terrigenous input caused by the positive AO phase. Core LV83-39 in the East Siberian Sea could have accumulated more terrestrial OC transported along the continental shelf during the positive AO. This implies that, under the interannual regulation of the AO regime, the input and cross-shelf transport of terrigenous OC in the ESAS showed consistent sedimentary responses. This finding could enhance the understanding of the burial mechanism of sedimentary OC and its environmental response to regional climate change.

Nutrient uptake status dominates phytoplankton biomass and community structure in the Southern Ocean during austral summer, yet how nutrient utilization variability responds to phytoplankton community succession is still unclear, partly due to lack of data spanning the entire summer. In this study, nitrate, phosphate, and silicate combined with temperature, salinity, and apparent oxygen utilization (AOU) were analyzed along 45°E in the Cosmonaut Sea during December 2019, January 2021, and February 2022. The variations in nutrient utilization in the euphotic layer were studied using biogeochemical tracers, and seasonal nutrient depletion was also estimated. The results showed that nutrient distribution varied significantly from December to February. Significant positive correlations were observed for nitrate and silicate concentrations with salinity and AOU, indicating that nutrient distributions were mainly influenced by water mass and phytoplankton production. Increasing ∆[N^*] and decreasing ∆[Si^*] in the upper 50 m were observed south of 63.5°S from December to February, which possibly contributed to a progressive shift in dominant phytoplankton population from Phaeocystis antarctica to diatoms. The seasonal nutrient depletion generally increased from December to February. Moreover, the consumption of silicate substantially increased compared to nitrate, indicating that the abundance of diatoms was increasing with time during the austral summer. Our observations suggest that nutrient utilization status is closely related to phytoplankton community structure in the euphotic layer of the Cosmonaut Sea.

The global ocean is a major source of the climate-relevant atmospheric trace gas nitrous oxide (N₂O). However, an accurate assessment of the global oceanic emissions of N₂O is hampered by missing data on dissolved N₂O from large regions such as the Southern Ocean. To address this deficit, N₂O was measured in the Prydz Bay in February 2015 during the 31st Chinese National Antarctic Research Expedition. N₂O concentrations (saturation) in the surface layer were generally low (undersaturation with respect to atmospheric equilibrium) and ranged from 13.3 nmol/L to 16.1 nmol/L (83%–102%) at the time of sampling. A comparison of our observations with archived data revealed that no discernible trend in N₂O concentrations in the surface waters of Prydz Bay could be detected for the period between 2006 and 2015. Temperature and salinity changes driven by meltwater input were the predominant controls on N₂O concentrations in surface waters. At depth, the distribution of N₂O concentrations was dominated by production via nitrification in offshore deep waters and vertical convection in the shelf waters, where concentrations were lower and gradients were less steep. Our results suggest a rather unusual pattern of N₂O distribution in the Prydz Bay (low N₂O in shelf waters compared with the open ocean), providing important insights into the coastal dynamics of N₂O in high-latitude polar regions.

Polynyas and their adjacent seasonal ice zones (SIZs) represent the most productive regions in the Southern Ocean, supporting unique food webs that are highly sensitive to climate change. Understanding the dynamics of phytoplankton and the carbon pool in these areas is crucial for assessing the role of the Southern Ocean in global carbon cycling. During the late stage of an algal bloom, seawater samples at 14 stations were collected in the Amundsen Sea Polynya (ASP) and adjacent SIZ. Using nutrients, phytoplankton pigments, organic carbon (OC), remote sensing data, and physicochemical measurements, as well as CHEMTAX model simulations, we investigated the response of the phytoplankton crops, taxonomic composition, and OC pool to environmental factors. Our analyses revealed that hydrodynamic regimes of the polynya, adjacent SIZs and open sea were regulated by the regionally varying intrusion of Circumpolar Deep Water, photosynthetically active radiation and sea ice melt water. The ASP exhibited the highest seasonal nutrient utilization rates [ΔN = (1059 ± 386) mmol/m², ΔP = (50 ± 17) mmol/m² and ΔSi = (956 ± 904) mmol/m²], while the open sea had lower rates. The integrated chlorophyll a (Chl a) concentration at depths of 0–200 m ranged from 20.4 mg/m² to 1420.0 mg/m² and peaked in the polynya. In the study area, Haptophytes Phaeocystis antarctica was the dominant functional group (34% ± 27%), and diatoms acted as a secondary contributor (23% ± 14%). The major functional group and particulate OC (POC) contributor varied from diatoms (36% ± 12%) in the open sea to haptophytes (48% ± 31%) in the polynya waters. Strong light conditions and microelement limitations promoted the dominance of P. antarctica (low Fe forms) dominance in the ASP. The strong correlations between the POC and Chl a depth-integrated concentration suggest that the POC was primarily derived from phytoplankton, while dissolved OC (DOC) was influenced by consumer activity and water mass transport. In addition, the transport of OC in the upper 200 m of the water column within the ASP was quantified, revealing the predominantly westward fluxes for both DOC [9.0 mg/(m²·s)] and POC [7.2 mg/(m²·s)]. The latitudinal transport exhibited the northward transport of DOC [8.1 mg/(m²·s)] and southward transport of POC [4.3 mg/(m²·s)] movement. These findings have significant implications for enhancing our understanding of how hydrodynamics influence OC cycling in polynya regions.

Seasonal variation in phytoplankton composition influences the pathways and efficiency of energy flow, reshaping the structure of the trophic pyramid in the Ross Sea. However, field investigation of grazing processes presents challenges that hinder our understanding of energy pathways. This study aims to provide insights into energy flow using a three-dimensional ecosystem model applied to the Ross Sea. By analyzing the simulation results, the role of the seasonal phytoplankton succession, specifically the shift from dominance by Phaeocystis antarctica to diatoms, in energy allocation is explored. The short-lived spring bloom of P. antarctica mainly fuels microzooplankton, creating a brief food chain where energy transfers primarily among smaller plankton. In contrast, the subsequent summer bloom of diatoms, which persists longer, provides nearly half of the total phytoplankton energy loss (via ingestion and mortality) to larger mesozooplankton. Our findings indicate that phytoplankton succession in the Ross Sea extends the bloom duration, particularly for diatoms, thereby facilitating energy transfer to higher trophic levels and improving overall energy utilization. This suggests that phytoplankton succession, an ecological strategy adapted to iron-deficient environments in the Ross Sea, explains why the colder region in front of the Ross Ice Shelf is significantly more productive than the northern areas, ultimately favored by top predators.