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 northernmost Bay of Bengal in a high accumulation rate core 200 km south of the Ganges-Brahmaputra Delta (SO93-126KL, water depth 1253m). This core includes the last 50 ky of sedimentation. We focused on the last 20 ky of deglaciation, and also a high resolution sampling of millennial events between 34 and 38 ka. We sought to apply a new productivity indicator, Sr/Ca ratios in coccolith shells. This indicator is set in the photic zone and tracks primary production. Typically we use species Calcidiscus leptoporus which exhibits the highest amplitude productivity response. However, the delta sediments are extremely rich in detrital materials with sparse microfossils. The dominant coccolith is Gephyrocapsa oceanica and C. leptoporus are scarce, so we selected G. oceanica individuals and prepared them for Sr/Ca measurement using the Cameca IMS-3f Ion microprobe at Woods Hole Oceanographic Institute.
In this record we find a much lower amplitude Sr/Ca response in G. oceanica than in other genera, consistent with work at other sites. Nonetheless, a detectable signal of increased productivity is observed following glaciation and coincident with the Early Holocene maximum in the summer monsoon. A major transient peak in productivity occurs at 12 ka as river flux was increasing to early Holocene maximum levels. This may reflect a transient pulse in nutrients during the first few thousand years of release of sediment-laden meltwater and higher river discharge from enhanced monsoon. Productivity during millennial monsoon peaks between 34-38 ka is higher than glacial maximum values and comparable to the Early Holocene levels.
We conduced more in depth work on foraminiferal assemblages in RC12-343 core. We resampled the core to obtain larger sample sizes for more accurate statistics, and sieved to >150 um to include smaller specimens than in our previous initial examination. We conducted complete assemblage counts of samples from glacial and interglacial endmembers to evaluate change in upwelling or eddy pumping intensity. Previous productivity data indicate enhanced interglacial productivity but the mechanism (river runoff or more wind-driven eddy pumping) is unclear. Analysis of the assemblages indicates that classic upwelling indicators (G. bulloides) have comparably low abundance (4%) in glacial and interglacial samples. Significant enrichment of glacial samples in G. menardii implies that preservational differences may contribute to assemblages and must be considered in interpretation.
