O28 NOVEL HIGHLY BRANCHED ISOPRENOID BIOMARKERS AS INDICATORS OF SEA-ICE DIATOMS: IMPLICATIONS FOR HISTORICAL SEA-ICE RECORDS AND FUTURE PREDICTIONS Guillaume MASSÉ 1 , Simon BELT 1 , Steve ROWLAND 1 , Lindsay VARE 1 , Michel POULIN 2 , Marie-Alexandrine SICRE 3 and Christine MICHEL 4 1. School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, Devon, United Kingdom 2. Research Division, Canadian Museum of Nature, PO Box 3443, Station D, Ottawa, Ontario, K1P 6P4, Canada 3. Laboratoire des Sciences du Climat et de l'Environnement (LSCE) Bât 12, Domaine du CNRS Avenue de la Terrasse F-91198 Gif-sur-Yvette Cedex France 4. Freshwater Institute, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada Polar oceans are important contributors to the Earth’s climate systems. In particular, sea-ice influences the exchanges of heat and moisture between polar oceans and atmosphere, and its high albedo means that it reflects much of incoming solar radiation. In addition, when melting, the outflow of low-salinity surface water impacts on the global deep oceanic circulation, which can influence the climate to a large extent. Therefore, increasing our knowledge about how changes in past sea-ice extent contributes to our understanding of the actual changes in climate is critical if we aim to succeed in predicting future changes (Thomas and Dieckmann, 2003). Direct global estimates of sea-ice cover derived from remote sensing observations are now routine but have only been possible since the 1970’s (Stroeve et al., 2005). Previously, analysis of data derived from early ship records have been carried out, providing an observed sea ice record for the 20 th century and earlier. Such records have been based on archive materials including lighthouse diaries, ships logs, travellers journals and newspaper reports. However, only the most recent direct estimations of sea-ice cover are believed to be reliable and therefore, longer time scales studies are only possible using proxy data sources. In the current project, we are investigating the potential to use chemical biomarkers of sea-ice associated diatoms to serve as a proxy of sea-ice cover in the Arctic (Belt et al ., 2007) To date, our investigations have revealed that a restricted number of diatoms biosynthesise a class of secondary metabolite chemicals termed highly branched isoprenoids (HBIs). These chemicals are ubiquitous to marine sediments, but only one structural form of the HBIs exists in Arctic sea-ice. In turn, this chemical (a C 25 HBI mono-unsaturated alkene – IP 25 ) can almost certainly be associated with some Haslea spp. which are known to occur in Arctic sea- ice. Indeed, we have identified three such species in our work and now have them in culture.