Delta ¹³C depleted oceans before the termination 2: More nutrient-rich deep-water formation or light-carbon transfer?
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Carbon-isotopes (delta ¹³C) composition of benthic foraminifera has been extensively used to understand the link between deep-water circulation and climate. Equatorial Indian Ocean delta ¹³C records of planktic- and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-delta ¹³C around the penultimate glacial termination (T2: 132 ka). The time-series planktic- and benthic- species delta ¹³C records exhibit two distinct mean- delta ¹³C levels. The low mean delta ¹³C characterises the pre-T2 period (250 ka–132 ka), while the post-T2 (approx. 95 ka – Present) period records high mean delta ¹³C, generating a one-time shift of approx. 0.4 ppt within the last approx. 250 kyr time-period. This shift is a result of consistently higher delta ¹³C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 (approx.132 ka), lasts until approx. 95 ka, and sustained through the T1. The normally observed glacial-interglacial delta ¹³C variations of approx. 0.3 ppt occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2 delta ¹³C shift appears to be an inherent feature of the world oceans although with certain timing offsets. Therefore, it should represent a fundamental change in deep-ocean circulation (nutrient) dynamics. But, the leading hypotheses of circulation-driven oceanic distribution of delta ¹³C fail to explain the observed mean-delta ¹³C shift. Therefore it is proposed that, in addition to changes in deep-water circulation, the oceans before T2 were characterised by significantly lower-delta ¹³C than after. Such low-delta ¹³C mean-ocean during the pre-T2 period might have been the result of significantly increased transfer of terrestrial light-carbon to the ocean reservoir due to changes in global wind patterns.
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