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40549-B2
Testing the Climate-Weathering Feedback on the Carbon Cycle Over Quaternary Monsoon Cycles in the Bay of Bengal
We investigated how marine productivity responds to changes in weathering regime and riverine input in the northern Bay of Bengal and the more southerly Andaman Sea. We focused on the interval from 200 to 90 ka which included the glacial/interglacial transition from marine isotope stages (MIS) 6-4, and several high amplitude precessional changes in summer insolation. Study focused in increasing the resolution of the preliminary Andaman Sea record we previously produced from core MD77-169 and generating a new record from the northern Bay of Bengal RC12-343. We applied a new productivity indicator, Sr/Ca ratios in coccolith shells. This indicator is set in the photic zone and tracks primary production. In the RC12-343 we also measured Ba/Ti ratios, an indicator of export production. Finally, we examined foraminiferal assemblages to assess the role of changes in eddy pumping and upwelling which may also affect productivity.
In the Northern Bay of Bengal, we find high productivity during MIS 5 interglacial and low during the glacial (MIS 6 and MIS 4). Productivity shows little sensitivity to precessional changes. Sr/Ca ratios of coccolith C. leptoporus increase by 40% during the early stage 5 interglacial relative to the glacial. Ba/Ti ratios show a concomitant large increase during the early interglacial, from 0.06 to 0.2 g/g. Coupled together, higher production and higher export indicates a net addition of nutrients to the photic zone. Two possible nutrient sources include eddy pumping of deep nutrients or riverine addition of nutrients. Our prior studies in Bay of Bengal sediment traps reveal that distinct planktic foraminiferal assemblages characterize intervals of sea surface height anomalies conducive to eddy pumping. Foraminiferal assemblages indicate stronger eddy pumping during glacial period, perhaps because of decreased stratification and altered wind regimes. Consequently higher interglacial productivity cannot be attributed to stronger eddy pumping and likely reflects increased nutrient delivery from riverine sources. The Ganges-Brahmaputra system delivers 10% of the worlds annual river flux of dissolved inorganic phosphorus, so changes in river flux may contribute significantly to nutrient budgets and production in this area. The inferred decrease in river nutrient delivery is consistent with a significant decrease in discharge from the Ganges-Brahmaputra system during glacial periods which we previously identified from Nd isotopic measurements on planktic foraminifera in this core.
In the Andaman Sea, Sr/Ca records indicate high productivity during precessional maxima in summer insolation. Sr/Ca is typically 50% higher during insolation maxima. Peak productivity is the same during precessional summer insolation maxima regardless of whether they fall during glacials or interglacial periods. The Andaman Sea productivity record closely tracks variations in the smectite/(illite+chlorite) in the same core which has been interpreted as an indicator of chemical weathering intensity. Both chemical weathering intensity and river nutrient flux are likely responding to fluctuations in summer monsoon precipitation which closely follows precessional forcing in this more southerly location.
Williams undergraduate Andrea Burke, who conducted much of the work in this phase of the study, presented her results at the 2007 EGU meeting in Vienna, Austria and is preparing a manuscript for journal submission.
