Sea ice proxies are used to reconstruct the climate and environmental history in both polar regions. In the Southern Ocean, the biomarker IPSO25 ‒a highly branched isoprenoid (HBI) diene‒ is produced by sea ice diatoms (Belt et al., 2016). We evaluated the abundance and distribution of IPSO25 in recent ocean surface sediments through comparisons with satellite data and diatom assemblages for sea ice studies in the Western Antarctic Peninsula area. Further, analyses of bulk data and several biomarkers (HBIs, phytosterols, GDGTs) as well as XRF scans were conducted on a sediment core from the Bransfield Basin (add lat&long and core name?) and an age model was developed based on acid-insoluble organic 14C dating. The piston core provides insights on the development of spring sea ice, primary production and sea surface temperature (SST) over the past 20.000 years. The rapid decrease of IPSO25 and slight increase of open ocean indicators from HBI trienes and phytosterols as well as the SST reflect the warming after the Last Glacial Maximum. There is clear evidence for the Antarctic Cold Reversal and a mid-Holocene cooling event about 5.000 years BP. Seasonal sea ice cover remains high during the early Holocene and high variability occurs since the Holocene Climatic Optimum with an overall decrease of sea ice towards the present. Belt et al., 2016. Nature Communications, v. 7, p. 12655.