Lack of organic matter accumulation on the upwelling-influenced Somalia margin in a glacial-intelglacial transition

Monsoon-induced upwelling has been operating off Somalia for at least the last 160,000 years, making the NW Indian Ocean one of the most productive oceanic areas in the world. We have studied the sediments (recovered during the Indusom cruise of the R/V Marion-Dufresne) from three different environments off this margin; a core from the highly productive surface waters (core MD85-674); a core where the oxygen minimum intersects the slope (core MD85-664); and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production (core MD85-668). Though the first two environments are a priori favourable to organic matter (OM) accumulation, no marine OM enrichment can be observed in the cores. To understand this anomaly, emphasis is placed upon the transition from isotopic Stage 6 to Stage 5, because the end of the glacial Stage 6 is characterised by intensified upwelling and primary productivity in this region, whereas Stage 5 experienced a marked decrease in productivity. Various palaeoenvironmental markers, such as OM, V, Ni, Cu, Zn, Ba and P, have been used to identify periods of high productivity and/or bottom water oxygen-depletion. The results suggest that (1) the lack of marine OM in two settings favourable to organic carbon storage may be due to the nature of primary productivity. Coccolithophorid production would not be prone to OM accumulation, in contrast to the contribution of OM by diatoms or, particularly, naked phytoplankton (dinoflagellates or other peridinians); (2) Ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of Ba as a proxy indicator; (3) the period of intensified upwelling in Stage 6 as well as the Stage 6-5 transition left no geochemical imprint on the sediments. This means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments. In other words, the Somalia upwelling does not participate to the so-called biological pump and has no positive effect upon long-term carbon storage.