Demystifying the anomalous high-latitude marine alkenones: novel paleo-sea ice and paleo-SST proxies

Abstract:

Alkenones have been widely and successfully used for paleo-SST reconstruction for nearly 40 years. However, alkenones in northern high latitude oceans often display anomalous distributions with high %C_37:4 and warm-biased UK37’-inferred SSTs. The apparent correlation between alkenone %C_37:4 and sea surface salinity (SSS) has enabled use of %C_37:4 as a paleo-SSS proxy, rationalized as some mysterious source from low salinity, polar water masses. We have now used combined genomic and lab-culture approaches to identify a new lineage of Isochrysidales capable of producing exceptionally high C_37:4 alkenone in the northern high latitude oceans. This new Isochrysidales lineage co-occurs widely with sea ice in marine environments and belongs to Group II Isochrysidales (Group II-i), and is phylogenetically distinct from the common marine alkenone producer, Emiliania huxleyi. We show that %C_37:4 in particulate organic matter and surface sediments is strongly correlated with annual mean sea ice concentrations. High resolution records of %C_37:4 in sediment cores from the Svalbard region provide robust and quantitative records of sea ice in the past 14 kyrs, consistent with multi-proxy sea ice and temperature records. Because Group II-i does not produce C₃₈ methyl ketones, we demonstrate through surface sediment calibrations and downcore records that UK38me provides more robust paleo-SST reconstructions than the traditional UK37’ index. In addition, we report new and specific biomarkers from Group II Isochrysidales that can be used to trace non-E. huxleyi-produced alkenones in ocean sediments.