δ13C depleted oceans before the Termination 2: More nutrient-rich deep-water formation or light-carbon transfer? (original) (raw)
Carbon-isotopes (δ 13 C) composition of benthic foraminifera has been extensively used to understand the link between deep-water circulation and climate. Equatorial Indian Ocean δ 13 C records of planktic-and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-δ 13 C around the penultimate glacial termination (T2: 132 ka). The time-series planktic-and benthic-species δ 13 C records exhibit two distinct mean-δ 13 C levels. The low mean-δ 13 C characterises the pre-T2 period (250 ka-132 ka), while the post-T2 (~95 ka-Present) period records high mean-δ 13 C, generating a one-time shift of ~0.4 ‰ within the last ~250 kyr time-period. This shift is a result of consistently higher-δ 13 C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 (~132 ka), lasts until ~95 ka, and sustained through the T1. The normally observed glacial-interglacial δ 13 C variations of ~0.3 ‰ occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2-δ 13 C 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 δ 13 C fail to explain the observed mean-δ 13 C shift. Therefore it is proposed that, in addition to changes in deep-water circulation, the oceans before T2 were characterised by significantly lower-δ 13 C than after. Such low-δ 13 C 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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