Monsoonal variability in the northeastern Arabian Sea during the past 5000 years:: geochemical evidence from laminated sediments

A high-resolution laminated sediment record from the oxygen-minimum zone in the northeastern Arabian Sea off Pakistan was studied to reconstruct climatic variations. To infer the monsoon-driven "moisture history" in the northeastern Arabian Sea during the past 5000 years, we have used three types of independant proxies: (1) varve thickness, (2) stable isotope ratios of planktic foraminifera; and (3) inorganic geochemical composition. On the basis of these proxies we have subdivided the sedimentary record into five intervals. The first time interval from 5000 to 3900 yr BP (Period I) is dominated by summer-monsoon activity. Deduced from Ti/Al, Zr/Al, K/Al and Sr/Ca ratios and oxygen-isotope data of planktonic foraminifera slightly enhanced winter-monsoon conditions can be inferred for the period from 3900 to about 3000 yr BP (Period II). Maximum precipitation during enhanced winter-monsoon activity at about 3100 yr BP was followed by an onset of aridification at 3000 yr BP which continued until about 2000-2200 yr BP (Period III). The textural, mineralogical and geochemical data suggest that this aridification was a gradual, century-scale process rather than an abrupt event. Comparison to late Holocene records in NW- and SW-India, the Near East and tropical Africa supports our interpretation that Period III from 3000 to 2000 yr BP is characterized by a gradual change in the atmospherical moisture budget over much of this area. Reduced monsoonal rainfall may be linked to variations of Eurasian snow cover. As deduced from varve thickness and element to Al-ratios, Period IV from 2000 to about 1500 yr BP is characterized by a summer-monsoon dominated sequence with higher precipitation than during Period III. The delta O-18 record shows an abrupt shift at 2000 yr BP coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern Arabian Sea. Present-day conditions in the Makran area were established at 1500 yr BP (Period V). Based on inorganic geochemical data and meteorological records at several stations in the Makran we conclude that Period V is dominated by precipitation during the winter season. Here, declined rainfall associated with the summer-monsoon may be related to a less northward position of the intertropical convergence zone (ITCZ) during summer and a stronger influence of winter-monsoon linked phenomenona. (C) 2001 Elsevier Science B.V. All rights reserved.