Highly branched isoprenoids as proxies for variable sea ice conditions in the Southern Ocean

International audienceConcentrations of a highly branched isoprenoid (HBI) diene determined in over 200 sediment samples from the Arctic co-vary with those of an HBI monoene (IP 25) shown previously to be a sedimentary sea ice proxy for the Arctic. The same diene, but not monoene IP 25 , occurred in nine sea ice samples collected from various locations around Antarctica. The diene has been reported previously in Antarctic sea ice diatoms and the 13 C isotopic compositions of the diene determined in two Antarctic sea ice samples were also consistent with an origin from sea ice diatoms (d 13 C-5.7 to-8.5%). In contrast, HBIs found in two Antarctic phytoplankton samples did not include the diene but comprised a number of tri-to pentaenes. In sediment samples collected near Adélie Land, East Antarctica, both the diene and the tri-to pentaenes often co-occurred. 13 C isotopic compositions of the tri-to pentaenes in three sediment samples ranged from-35 to-42% whereas that of the diene in a sediment sample was-18%. We propose the presence of this isotopically 13 C enriched HBI diene in Antarctic sediments to be a useful proxy indicator for contributions of organic matter derived from sea ice diatoms. A ratio of the concentrations of diene/ trienes might reflect the relative contributions of sea ice to phytoplanktonic inputs of organic matter to Antarctic sediments