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Abstract

We use the flux of bulk sediment (CaCO3, biogenic opal, organic carbon, lithogenic material), and of planktic foraminifera (PF) and other shell-bearing plankton from sediment trap EPT-2 off Pakistan to (1) constrain the seasonal pattern of regional productivity and (2) search for indications of the NE monsoon winter situation that may serve as a modern analogue to better reveal the seasonal climatic signals preserved in the sedimentary record of the Arabian Sea. Our trap data show a clear seasonality of fluxes that can also be traced in the composition of non-bioturbated (varved) summer and winter sediment laminae preserved within the oxygen minimum zone. In EPT-2, the flux of PF is low during summer, but during winter and late spring it is higher, as at trap station WAST, in the upwelling area of the western Arabian Sea. Globigerina bulloides, a PF species linked to summer upwelling and high productivity, is of minor importance off Pakistan. In contrast, Globigerina falconensis dominates in flux and relative abundance, and is indicative of winter mixing, when NE monsoonal winds cool the highly saline surface waters and break up stratification. An enhanced horizontal flux of suspended sediments stirred up on the shelf and upper slope is clearly shown by the peak in occurrence of small benthic foraminifera during winter. Altogether, our data suggest that the particle flux in the northeastern Arabian Sea is determined by local sediment resuspension and winter productivity rather than by summer monsoonal upwelling, representing a ‘non-upwelling’ environment, in contrast to the ‘summer upwelling’ regime off Oman, Somalia and southern India. We used this evidence to reconstruct the seasonal intensity of both monsoons for the past 25 ka: the SW and NE monsoon both were weak during the last glacial period. The NE monsoon peaked during the cool phases of the glacial to interglacial climatic transition (i.e. during the Younger Dryas (YD) and Heinrich Event HI). The SW monsoon was reinforced after the YD. Both monsoons were enhanced during early Holocene time, when summer insolation and hence atmospheric forcing was at a maximum.

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