Articles written in Journal of Earth System Science

    • Coupling of thermocline depth and strength of the Indian, summer monsoon during deglaciation

      Sushant S Naik Divakar Naidu P

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      We investigated the variations in thermocline depth using the difference in $\delta^{18}$Oc values between the two species of planktonic foraminifera, surface dwelling Globigerinoides ruber (s.s.) and thermocline dwelling Neogloboqudrina dutertrei, covering a time span of 9–23 kyr from the sediment core SK218/1 from the western Bay of Bengal (BoB). Here we show that during the strong phase of the Indian summer monsoon (12–9 kyr), a strong stratification leads to a shallow mixed layer and thermocline depth in the BoB as evident from higher $\Delta\delta^{18}$O between the mixed layer and thermocline dwelling planktonic foraminifera species. Thus, a strong coupling between the Indian summer monsoon and thermocline depth in the BoB prevailed at a millennial time scale.

    • Shell weights of foraminifera trace atmospheric $\rm{CO}_{2}$ from the Miocene to Pleistocene in the central Equatorial Indian Ocean


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      The Maldives Sea is a region dominated by the South Asian monsoon (SAM) and at present, a $\rm{CO}_{2}$ source to the atmosphere. Ti/Al elemental ratios from Site U1467 and U1468 recovered from the Maldives Sea show a gradual increase from $\sim 12 \rm{Ma}$ and indicate terrigenous inputs to this region associated with increasing wind intensity associated with initiation of the SAM. Shell weights of planktonic foraminifera, Globigerinoides trilobus have been used to understand variations in surface water carbonate ion concentration for the last 20 Ma. Shell weights show a good correspondence with global $\rm{CO}_{2}$ records and show heavier shell weights during the colder periods than compared to warmer intervals which reveals that the Maldives Sea behaved similar to other tropical oceanic regions in terms of its surface water carbonate chemistry. A significant decrease in $\rm{CaCO}_{3}$ wt.%, decrease in foraminifera shell weights and dissolution of spines along with an increase in organic carbon (OC%) towards 10.5 Ma is linked to the reduced carbonate deposition and increased productivity during monsoon which is a feature in all tropical sediment cores. Lower shell weights and dissolution features on foraminiferal shells were observed during periods of intense Oxygen Minimum Zone (OMZ) suggesting calcite dissolution due to an increase in bottom water $\rm{CO}_{2}$.

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