A 90 kyr upwelling record from the northwestern Indian Ocean using a novel long-chain diol index

Presently, upwelling is of major importance for driving primary productivity in the Arabian Sea but its intensity in the past is not well constrained. Here we used long-chain 1,14-alkane diols, specific lipids of diatoms of the genus Proboscia, as new proxies to reconstruct upwelling conditions in the Arabian Sea. Variations in the seasonal lipid fluxes were determined using sediment traps in the Somalia upwelling system deployed 80 km off the coast on the Somali continental slope (NIOP 905, 10 degrees 45.444'N / 51 degrees 56.655'E) at 1265 m water depth, 268 m above the sea floor and 270 kM off the coast in the deep Somali Basin south of Socotra (NIOP 915, 10 degrees 43.068'N / 53 degrees 34.422'E), at 3047 m depth, 1000 m above the sea floor. Highest fluxes of C-28 and C-30 1,14-diols (up to almost 600 mu g M-2 day(-1)) were only observed during nutriciine shoaling at the onset of the Southwest monsoon (SWM), prior to massive upwelling. By contrast, fluxes of C-30 1,15-diols, derived from as yet undefined biological sources, only increased marginally during the SWM and also during the Northeast monsoon (NEM), when, instead of upwelling, enhanced vertical mixing led to a second productivity pulse. Sediment core NIOP 905 taken at the continental slope site showed strong fluctuations in relative concentrations of long-chain 1,14- and 1,15-diols with time, which we quantified as the summed concentrations of C-28 and C-30 1,14-diols divided by the summed concentrations of C-28 and C-30 1,14-diols and C-30 1,15-diols. This diol index follows the same trend as other upwelling intensity records from the Arabian Sea that are based on sea surface temperature reconstructions, organic carbon content, barium/aluminium ratios, and abundance and stable isotope composition of specific foraminiferal species. The diol index was relatively high during the Holocene (ca. 0.7) but much lower during the Late Glacial Maximum (ca. 0.2). It was generally low during the last Glacial but elevated values were found during the first half of marine isotope stage 3 (between 60 and similar to 45 ka) and at the end of marine isotope stage 5.1 (approximately 80 ka), suggesting intensified glacial upwelling. Our data shows that long chain diols are suitable proxies to reconstruct past upwelling intensities in the Arabian Sea. (C) 2008 Elsevier B.V. All rights reserved.