Abstract
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.
Highlights
Observations of global mean surface temperatures show a warming of approximately 1.0 ± 0.2 ◦C (IPCC, 2018) above the 1850–1900 baseline as a result of progressive industrialization since the mid-19th century
Core PS97/0682 from Orleans Trough mainly consists of diatom-bearing silty clay (Lamy, 2016) and spans from 1780 to 2007 CE with sedimentation rates from 0.1 to 0.5 cm a−1
We note that the interpretation of the biomarker data for past sea ice estimates in Antarctica is strongly impacted by the origin of water masses, mixing, nutrient input and dynamics of seaice-related primary production
Summary
Observations of global mean surface temperatures show a warming of approximately 1.0 ± 0.2 ◦C (IPCC, 2018) above the 1850–1900 baseline as a result of progressive industrialization since the mid-19th century. An acceleration of this trend due to anthropogenic forcing has been projected (IPCC, 2019). The changes in sea ice cover are related to warm water intrusion and higher sea surface temperatures (SSTs) along the WAP (Martinson and McKee, 2012; Meredith and King, 2005), and to largescale modes of atmospheric circulation such as the Southern Annular Mode (SAM) (e.g., Barbara et al, 2013) and the El Niño–Southern Oscillation (ENSO) (e.g., Liu et al, 2004) or a combination of both (Etourneau et al, 2013; Stammerjohn et al, 2008b, a)
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