Holocene evolution of summer winds and marine productivity in the tropical Indian Ocean in response to insolation forcing: data-model comparison
1LSCE/IPSL (laboratoire CEA/CNRS/UVSQ), Domaine du CNRS, UMR 8212, bâtiment 12, 91198 Gif-sur-Yvette, France
2Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB, UK
3LSCE/IPSL (laboratoire CEA/CNRS/UVSQ), Centre CEA-Saclay, Orme-les-Merisiers, 91190 Gif-sur-Yvette, France
4Université de la Méditerranée, CNRS, Laboratoire d'Océanographie Physique et Biogéochimique, LOPB – UMR 6535 Campus de Luminy, Case 901, 13288 Marseille Cedex 9, France
Abstract. The relative abundance of Globigerinoides bulloides was used to infer Holocene paleo-productivity changes at ODP Site 723 (19°03' N, 57°37' E; Oman Margin) and core MD77-191 (07°30' N, 76°43' E; Southern tip of India). Today, the primary productivity at both sites peaks during the summer season, when monsoon winds result in local Eckman pumping, which brings more nutrients to the surface. On a millennium time-scale, however, the % G.~bulloides records indicate an opposite evolution of paleo-productivity at these sites through the Holocene. The Oman Margin productivity was maximal at ~9 ka (boreal summer insolation maximum) and decreased since then, suggesting a direct response to insolation forcing. On the opposite, the productivity at the southern tip of India was minimum at ~9 ka, and strengthened towards the present.
Paleo-reconstructions of wind patterns, marine productivity and foraminifera assemblages were obtained using the IPSL-CM4 climate model coupled to the PISCES marine biogeochemical model and the FORAMCLIM ecophysiological model. These reconstructions are fully coherent with the marine core data. They confirm that the evolution of particulate export production and foraminifera assemblages at our two sites have been directly linked with the strength of the upwelling. Model simulations at 9 ka and 6 ka BP show that the relative evolution between the two sites since the early Holocene can be explained by the weakening but also the southward shift of monsoon winds over the Arabian sea during boreal summer.