Chemical Fingerprints in Antarctic Mud Reveal 200 Years of Growing Sea Ice in the Ross Sea

Beneath the frigid waters of the southwestern Ross Sea, the seafloor has been quietly keeping a diary. A team of researchers led by Emma M. de Jong of the Antarctic Research Centre at Victoria University of Wellington has now learned to read it, decoding chemical and microscopic traces left by tiny algae to reconstruct how sea ice in one of Earth’s most productive marine regions has changed over the past two centuries. Their findings, published in the journal Biogeosciences, reveal a striking and somewhat counterintuitive story: sea ice duration in the southwestern Ross Sea has increased over the last 200 years, even as the planet has warmed.

The Ross Sea is no ordinary stretch of ocean. It is among the most seasonally productive regions of the global ocean, where massive blooms of microscopic plants fuel the entire Southern Ocean food web and drive the uptake and storage of carbon dioxide. Three biological hotspots dominate the region: Terra Nova Bay, McMurdo Sound, and the central Ross Sea. Each hosts a different cast of phytoplankton, including diatoms, the glass-shelled algae that form the bulk of the biomass; haptophytes, most notably the colony-forming species Phaeocystis antarctica; dinoflagellates; and bacteria. Which group wins out in any given season depends on a delicate interplay of sea ice cover, water stratification, light, temperature, and the supply of iron, a nutrient that limits growth across much of the Southern Ocean.

Understanding how these communities have responded to past climate shifts has been hampered by a fundamental data problem. Satellite measurements of chlorophyll, the green pigment that betrays the presence of phytoplankton, only extend back a couple of decades. To place recent changes in context, scientists need longer baselines, and the last 200 years offer a natural one: this window captures the tail end of a prolonged Antarctic cooling that lasted from roughly 1200 to 1900 CE, followed by the onset of industrial-era warming and rising atmospheric carbon dioxide. Sediment cores, which accumulate layer by layer on the seafloor, provide exactly the archive needed to look back through this pivotal transition.

De Jong and her colleagues, including Xavier Crosta of the University of Bordeaux, Sebastian Naeher of Lincoln University, and corresponding author V. Holly L. Winton of Victoria University of Wellington, collected six surface sediment samples and three short sediment cores along a north-south transect stretching from the McMurdo Sound coastal polynya to south of Terra Nova Bay. The samples were gathered in 2014 and 2015 by the Korean Polar Research Institute vessel RV Araon. Polynyas, areas of open water surrounded by sea ice, are engines of Antarctic productivity, and the transect deliberately crossed a gradient of sea ice conditions, from sites covered by ice only 20 to 40 percent of the year near McMurdo Sound to sites north of the Drygalski Ice Tongue that are ice-covered roughly 80 percent of the year.

The analytical approach combined two complementary lines of evidence. The first was diatom assemblages: the researchers identified and counted the microscopic silica shells of 58 diatom species or species groups, using the ecological preferences of each taxon as a window into past conditions. Species such as Fragilariopsis curta, which thrives under persistent fast ice or pack ice that melts late in the season, signal heavy sea ice influence, while Chaetoceros resting spores and open-ocean species indicate stratified, ice-free waters and high productivity. The second line of evidence was lipid biomarkers, specific fat molecules produced by different organisms. Among these, the highly branched isoprenoid IPSO25, short for Ice Proxy Southern Ocean, is widely used as a marker for sea ice algae, while particular fatty acids and sterols can be traced to diatoms, Phaeocystis, dinoflagellates, or bacteria.

The spatial patterns were unambiguous. At the southern end of the transect near McMurdo Sound, where summer sea ice is lower and shorter-lived, the sediments held increased proportions of open-ocean diatom species and bacterial fatty acids, along with signs of a diatom-dominated phytoplankton community and higher summer biomass. At the northern end near Terra Nova Bay, under heavier ice, the sediments were richer in sea ice-associated diatoms, sea ice diatom-derived fatty acids, and IPSO25. Fatty acids linked to Phaeocystis antarctica also increased toward the north, suggesting that this haptophyte may flourish under heavier ice conditions. A statistical technique called Principal Component Analysis confirmed the structure, resolving three distinct groupings: diatoms and fatty acids associated with heavier sea ice, an opposing group of open-water and fast-ice diatom assemblages, and an independent cluster of fatty acids and HBIs shaped by post-depositional degradation.

One of the study’s most valuable technical contributions is the proposal of a new sea ice biomarker. The saturated fatty acid C24:0 correlated strongly and positively with the Fragilariopsis curta group, with a correlation coefficient of 0.80, and strongly and negatively with Chaetoceros resting spores. Because the unsaturated sea ice diatom fatty acids C24:1ω9 and C24:1ω11 degrade over time into C24:0, inflated concentrations of this compound appear precisely where sea ice diatoms once thrived. The authors propose C24:0 as a candidate sedimentary biomarker of sea ice change in the southwestern Ross Sea, though they note that in East Antarctica the same compound has been attributed to open-water diatoms, a reminder that biomarker sources can differ from region to region.

The downcore records from the three short cores, dated using the radioactive decay of lead-210, tell the temporal story. The longest core reaches back to 1794 CE, with each centimetre of sediment representing roughly three to four years. Within these archives, sea ice-associated diatoms such as Fragilariopsis curta and the sea ice proxy PIPSO25, an index that combines IPSO25 with open-water sterols, all indicate an increase in sea ice extent over the last 200 years, accompanied by a decline in open-ocean diatom species. Notably, the phytoplankton-derived fatty acids alone show little change in community composition over the same period, a trend the authors attribute partly to degradation: fatty acid concentrations drop by about 50 percent in the top 20 centimetres of sediment, roughly the first 76 to 95 years, before stabilising. The sterols, which are less reactive, remain comparatively stable and preserve the signal.

The 200-year increasing sea ice trend aligns closely with independent reconstructions from ice cores and data assimilation, which show relatively stable conditions before 1900 followed by rising sea ice extent from around 1950 onward. The convergence of evidence from sediments, ice cores, and satellites underscores a robust regional pattern, even as the authors caution that coastal sea ice duration, which their cores record, is not identical to regional sea ice extent, and that local processes such as fast ice formation, polynya activity, and storm-driven winds shape conditions at their sites. Beyond settling a regional question, the work delivers a practical tool: by independently distinguishing pelagic diatoms, Phaeocystis antarctica, and sea ice-associated diatoms in the sediment record, these biomarkers open the door to longer, decadal-resolution reconstructions of sea ice and phytoplankton change, exactly the kind of baseline needed to anticipate how the Southern Ocean’s engines of carbon drawdown will respond as warming reshapes the ice around Antarctica.

Subject of Research: Biomarker and diatom records of sea ice and phytoplankton variability in Antarctic marine sediments

Article Title: Biomarkers and diatoms as tracers of phytoplankton communities and past sea ice conditions in the southwestern Ross Sea, Antarctica: drivers and variability over the last 200 years

Article References: Biomarkers and diatoms as tracers of phytoplankton communities and past sea ice conditions in the southwestern Ross Sea, Antarctica: drivers and variability over the last 200 years. (n.d.). https://doi.org/10.5194/bg-23-6979-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-6979-2026

Keywords: Ross Sea, Antarctica, sea ice, biomarkers, diatoms, phytoplankton, IPSO25, Phaeocystis antarctica, sediment cores, paleoclimate, Southern Ocean, Biogeosciences