2010. Reconstruction of the variability of the southwest monsoon during the past 3 ka, from the continental margin of the southeastern Arabian Sea.ABSTRACT: From temporal variation in d 18 O in Globigerinoides ruber and G. sacculifer and geochemical indices of weathering/erosion (chemical index of alteration, Al and Ti), we infer rapid southwest monsoon (SWM) deterioration with dwindling fluvial and detrital fluxes at ca. 450-650, 1000 and 1800-2200 cal. a BP during the late Holocene. We have evaluated the role of solar influx (reconstructed) and high-latitude climate variability (archived in GRIP and GISP-2 cores) on SWM precipitation. Broadly, our d 18 O climate reconstruction is concordant with GRIP and GISP-2, and supports a teleconnection through atmospheric connection between the SWM and the North Atlantic climate -albeit temporal extents of the Little Ice Age and Medieval Warm Period from high latitude are not entirely coeval. Moreover, there is a humid climate and enhanced precipitation during the terminal stages of the Little Ice Age. The medieval warming (ca. AD 800-1300) is not synchronous either, and is punctuated by an arid event centred at 1000 a BP. Although the delineation of the specific influence of solar influx on SWM precipitation is elusive, we surmise that SWM precipitation is a complex phenomenon and local orography along southwestern India may have a role on the entrapment of moisture from the southwest trade winds, when these hit land.

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[1] Reconstruction of century-scale Indian monsoon and winter hydrography is made from an AMS-dated core located in the unique region of the southeast Arabian Sea which lies in the pathways of the lowsalinity Bay of Bengal Waters, advecting during winter northeast monsoon (NEM). Based upon clay mineral analyses in seawaters, we identify chlorite and kaolinite as specific clays supplied by the Bay of Bengal Waters and local fluvial flux (by the southwest monsoon (SWM) precipitation from the Peninsular India), respectively, along the southwest continental margin of India. An evaluation of clay flux and d18 O in G. ruber portrays century-scale weaker SWM precipitation events during ∼450-650 yr, ∼1000 yr, and 1800-2200 cal yr BP. Kaolinite wt % and flux were found to be low during all these events, though chlorite had a persistent or enhanced flux. From the enhanced flux of chlorite and reduced kaolinite/chlorite ratio, during weaker phases of SWM, we deduce a stronger NEM (winter hydrography), implying an inverse coupling between the summer and the winter monsoon.

The Gulf of Kachchh is a funnel shaped, macrotidal water body located in the arid region of northwestern India with ~ 50 cm annual rainfall and insignificant fluvial input. The Gulf waters, however, have high-suspended matter. Time series measurements of total suspended matter (TSM) and synchronous, validated hydrodynamic modeling have been used to decipher the dispersal pathways and the sources of the high turbidity. Contrary to the prevalent offshore reducing trend for most of the Indian Coast, the Gulf is anomalous for having an enhanced turbidity at the mouth with lower concentrations in the inland areas. The hydrography of the Gulf is dominated by strong, alongshore currents at the mouth which move in (out) during flood (ebb), and undergo cyclic, dynamic changes with tidal phases. The flood tidal currents amplify inland with propagating tides, pulling in the offshore waters and acting as a feeder of high saline, turbid offshore waters into the Gulf, most of which are trapped inland due to time lag of ebb and flood between the outer and the inner Gulf. Based upon the distribution maps of TSM and clays in the water column, it is deduced that a large segment of the Gulf is nourished by contributions from the Indus River.

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From a transact along 15 • N latitude in the middle Bengal Fan, temporal and spatial variations in the granulometric parameters and clay minerals in 14 C dated box cores from the eastern, the central and the western regions were studied to determine climate induced changes in the hydrography. Clay assemblages have spatial and temporal changes and are markedly different in the eastern and the western bay. From a high abundance of the clay smectite, which has its major source in the Deccan Basalt in peninsular India, it is inferred that the western bay is predominantly a depocenter of 'peninsular sources'. The eastern and the central regions of the bay, however, mostly receive sediments of the 'Himalayan source'. Related to unstable climate, the reported dominant illite-chlorite (I + C) assemblage in the eastern region of the bay (I + C > 60% smectite < 15%), between 18 and 12.6 ka BP, points to a predominant supply from the Himalayan sources through equatorwards dispersal by the winter hydrography. Higher smectite, and reduced clays of the Himalayan sources (smectite > 25%; I + C > 45%) are reported also after 12.5 ka BP from the eastern bay. These are interpreted as evidences of an intensified SW monsoon and associated change in the dispersal pattern by stronger summer monsoon hydrography which supports across bay dispersal by anticyclonic gyre. The influence of climate on hydrographic changes is consistent during the short events of arid climate (weak NE monsoon) in Holocene in core 31/1(western bay), in which the enhanced contents of the clays of the Himalayan sources are observed (smectite < 40% I + C > 50%). These findings have implications for climate regulated influence of fluvial processes over the areas, hitherto, considered unaffected by the Indian peninsular fluvial sources.

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