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

Concentrations of a highly branched isoprenoid (HBI) diene determined in over 200 sediment samples from the Arctic co-vary with those of an HBI monoene (IP25) shown previously to be a sedimentary sea ice proxy for the Arctic. The same diene, but not monoene IP25, 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 C-13 isotopic compositions of the diene determined in two Antarctic sea ice samples were also consistent with an origin from sea ice diatoms (delta C-13-5.7 to -8.5 parts per thousand). 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 Adelie Land, East Antarctica, both the diene and the tri-to pentaenes often co-occurred. C-13 isotopic compositions of the tri-to pentaenes in three sediment samples ranged from -35 to -42 parts per thousand whereas that of the diene in a sediment sample was -18 parts per thousand. We propose the presence of this isotopically C-13 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.