Millennial-scale variability in biological productivity south of the Antarctic Polar Front during the last glacial period

The Southern Ocean plays a key role in regulating atmospheric CO₂ through ocean circulation, sea-ice dynamics, and biological carbon sequestration. However, the response of biological productivity south of the Antarctic Polar Front (APF) to millennial-scale climate variability during the Last Glacial Period remains incompletely understood, largely due to the scarcity of high-resolution records from this region. Here we present a multiproxy reconstruction of biological productivity and paleoceanographic conditions over the past ~26 kyr based on sediment core KH-19-6-PC07 recovered from the eastern South Sandwich Islands region.

We combine geochemical proxies (total organic carbon, Br/Ti, and Si/Ti ratios), stable nitrogen and carbon isotopes, diatom assemblage data, and grain-size analyses with a dust-correlated age model. The age model is supported by radiocarbon dating of acid-insoluble organic carbon and geomagnetic relative paleointensity. The results indicate persistently low biological productivity and seasonally extensive sea-ice cover between 26 and 24 ka. During this interval, diatom assemblages are dominated by taxa associated with cold, sea-ice-influenced conditions, consistent with reduced nutrient availability and limited light conditions.

In contrast, Antarctic Isotope Maximum 2 (AIM2; ~24–23 ka) is characterized by a marked increase in diatom productivity, a decline in δ¹⁵N values, and major shifts in diatom community composition. These changes are accompanied by an abrupt retreat of both summer and winter sea-ice margins, suggesting enhanced nutrient supply and reduced nutrient utilization efficiency under more open-water conditions. A transient dominance of Thalassiothrix antarctica further points to the development of strong surface-water stratification following rapid sea-ice melt.

Comparison with nearby sedimentary records from the Scotia Sea and the eastern Weddell Sea shows that, despite comparable or higher diatom concentrations, KH-19-6-PC07 consistently records lower abundances of sea-ice-associated diatom taxa. This pattern suggests relatively warmer surface conditions in the eastern South Sandwich Islands region during the investigated interval. We propose that these regional characteristics reflect intensified heat and nutrient supply linked to changes in ocean circulation, potentially associated with poleward shifts or enhanced meandering of the southern boundary of the Antarctic Circumpolar Current along the South Sandwich Trench. Overall, our results highlight pronounced spatial heterogeneity in Southern Ocean surface conditions during millennial-scale climate events and emphasize the importance of ocean circulation and bathymetric influences in modulating biological productivity south of the APF during the Last Glacial Period.