UPWELLING

Species from the diatom genus Proboscia are weakly silicified. They can adjust their buoyancy and migrate to deeper levels below the euphotic zone to obtain nutrients that are not available to other algae. In seasonal upwelling regions, this migration often enables them to reach nutrient-rich water layers before mixing in the photic zone takes place, resulting in a high contribution to the primary production by these diatoms. Therefore, remnants of Proboscia species may serve as biomarkers for upwelling conditions. Diatom silica tests are commonly used for palaeoenvironmental reconstruction but are prone to dissolution. Recently, we have identified a unique suite of lipid biomarkers in Proboscia diatoms: saturated and unsaturated C28 and C30 1,14-diols and C27 and C29 12-hydroxy methyl alkanoates (Sinninghe Damsté et al., 2003). Eustigmatophyceae algae are the only other marine organisms known to produce similar compounds, but they mainly produce 1,13- and 1,15-diols with C28 to C34 carbon chain lengths and long-chain mid-chain hydroxy methyl alkanoates with different homologues than those found in Proboscia diatoms. Thus, long-chain 1,14-diols and 12-hydroxy methyl alkanoates (Proboscia lipids) can be used as indicators for seasonal upwelling. Indeed, sediment trap samples from the Arabian Sea showed high fluxes o