Abstract. In the last decades, changing climate conditions have had a severe
impact on sea ice at the western Antarctic Peninsula (WAP), an area rapidly
transforming under global warming. To study the development of spring sea
ice and environmental conditions in the pre-satellite era we investigated
three short marine sediment cores for their biomarker inventory with a
particular focus on the sea ice proxy IPSO25 and micropaleontological
proxies. The core sites are located in the Bransfield Strait in shelf to
deep basin areas characterized by a complex oceanographic frontal system,
coastal influence and sensitivity to large-scale atmospheric circulation
patterns. We analyzed geochemical bulk parameters, biomarkers (highly
branched isoprenoids, glycerol dialkyl glycerol tetraethers, sterols), and diatom abundances and diversity over the past 240 years and
compared them to observational data, sedimentary and ice core climate
archives, and results from numerical models. Based on biomarker
results we identified four different environmental units characterized by
(A) low sea ice cover and high ocean temperatures, (B) moderate sea ice
cover with decreasing ocean temperatures, (C) high but variable sea ice
cover during intervals of lower ocean temperatures, and (D) extended sea ice
cover coincident with a rapid ocean warming. While IPSO25
concentrations correspond quite well to satellite sea ice observations for
the past 40 years, we note discrepancies between the biomarker-based sea ice
estimates, the long-term model output for the past 240 years, ice core
records, and reconstructed atmospheric circulation patterns such as the El
Niño–Southern Oscillation (ENSO) and Southern Annular Mode (SAM). We
propose that the sea ice biomarker proxies IPSO25 and PIPSO25 are
not linearly related to sea ice cover, and, additionally, each core site
reflects specific local environmental conditions. High IPSO25 and
PIPSO25 values may not be directly interpreted as referring to high
spring sea ice cover because variable sea ice conditions and enhanced
nutrient supply may affect the production of both the sea-ice-associated and
phytoplankton-derived (open marine, pelagic) biomarker lipids. For future
interpretations we recommend carefully considering individual biomarker
records to distinguish between cold sea-ice-favoring and warm sea-ice-diminishing environmental conditions.
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