A multi-proxy analysis of late Quaternary Indian monsoon dynamics for the Maldives, Inner Sea

Abstract

Abstract. We present a detailed multi-proxy data record to reveal the late Quaternary changes in marine sedimentation and biogeochemical processes of the upper bathyal Maldives (equatorial Indian Ocean) and how they are related to the benthic ecosystem dynamics. We investigated the sediment core SO-236-052-4 from the central part of the Inner Sea, Maldives, focusing on Fe / Ca and Si / Ca ratios as proxies for terrigenous sediment delivery, as well as Total Organic Carbon (TOC) and Ba / Ca ratios as proxies for marine productivity. Benthic foraminiferal fauna distributions, sortable silt records and stable oxygen and carbon isotope analyses were used for reconstructing the past ecosystem, as well as changes in the intermediate water circulation, bottom water current velocity and oxygenation. This multi-proxy data record shows an enhanced dust supply during the glacial intervals, represented by increased Fe / Ca and Si / Ca ratios, an overall coarsening of the sediment and increasing amount of agglutinated benthic foraminifera. The enhanced dust fluxes can be attributed to higher dust availability in the Asian desert and loess areas and its transport by intensified winter monsoon winds during glacial conditions. These combined effects of wind-induced mixing of surface waters and dust fertilisation during the cold phases resulted in increased surface water productivity and related organic carbon fluxes. Thus, the development of highly diverse benthic foraminiferal faunas and the distribution of certain detritus and suspension feeders were fostered. The difference in the stable carbon isotope signal between epifaunal and deep infaunal benthic foraminifera reveals intermediate water oxygen concentrations between approximately 40 and 100 μmol kg−1. The pattern of oxygen changes resembles that from the deep Arabian Sea suggesting an expansion of the Oxygen Minimum Zone (OMZ) from the Arabian Sea into the tropical Indian Ocean, further controlled by the inflow of the Antarctic Intermediate Water (AAIW). The precessional circulation pattern of the bottom water oxygenation is overprinted by glacial-/interglacial changes resulting in a long phase of reduced ventilation during the last glacial period. The latter process is likely linked to the combined effects of generally enhanced oxygen consumption rates during high-productivity phases, reduced AAIW production and restriction of bathyal environments of the Inner Sea of the Maldives during sea-level lowstands. Thus, this multi-proxy record provide a close linkage between the Indian monsoon oscillation, intermediate water circulation, productivity and sea-level changes on orbital time-scale.