Late Quaternary productivity changes in the equatorial Indian Ocean (ODP Hole 716A)

Abstract

The equatorial Indian Ocean is swept by the Indian Ocean equatorial westerlies (IEW) which intensify in monsoon transitions during April–May and October–November, driving Eastward Equatorial Current (EEC) to the upper ocean. The EEC brings significant changes in the surface productivity in the equatorial region influencing the life style of the surface and deep-sea fauna. This study is aimed at understanding ~300Kyr record (~450–150Kyrs ago) of productivity changes linked to variations in IEW–EEC in the equatorial Indian Ocean using high resolution microfaunal record from Integrated Ocean Drilling Program (IODP) Hole 716A, Maldives Ridge. In total, 451 core samples of 10cm3 volume from the ~300Kyr long sequence (ranging ~450–150Kyrs ago) were analyzed at every 1cm interval to generate census data of the benthic foraminiferal fauna. Factor and cluster analyses using percentages of 53 highest-ranked benthic foraminiferal species were run that enabled us to identify five biofacies, indicating the varied nature of deep-sea environments during the late Quaternary. The results show a major shift roughly 300Kyr ago, across Marine Isotope Stages 9 and 8 (MIS 9/8), recorded in this study as mid-Brunhes transition at Hole 716A. The present study also combines published planktic foraminiferal fauna and pteropod records from Hole 716A to understand if the surface to shallow and intermediate water paleoenvironments were coupled with deep-water changes during the late Quaternary in the Maldivian Archipelago. Biofacies Robulus nicobarensis–Trifarina reussi (Rn–Tr), Uvigerina porrecta–Reussella simplex (Upo–Rs) and Cymbaloporetta squammosa–Bolivinita sp. (Cs–Bsp) dominate before the mid-Brunhes transition and document high organic flux with relatively low oxygen paleoenvironment, similar to the planktic foraminifer Globigerina bulloides population, while benthic foraminiferal biofacies Hoeglundina elegans–Miliolinella subrotunda (He–Ms) and Uvigerina peregrina–Quinqueloculina seminulum (Upe–Qs) record high seasonality in food supply with well-oxygenated deep water following the mid-Brunhes climatic transition (~300Kyr ago at Hole 716A). These changes are also recorded in the population of planktic foraminiferal species Globigerinoides ruber and Neogloboquadrina dutertrei as well as pteropods. Organic carbon flux linked to changes in the IEW–EEC strength appears to be the most plausible factor controlling faunal assemblages and hence paleoproductivity dynamics in the Maldivian region. Our results suggest that the strength of the IEW is inversely related with the Indian Ocean Dipole (IOD). The IOD strengthened across MIS 9/8 during which time the IEW weakened, leading to major changes at Hole 716A.