Abstract:
Carbon-isotopes (delta (super 13) C) composition of benthic foraminifera has been extensively used to understand the link between deep water circulation and climate. Equatorial Indian Ocean delta (super 13) C records of planktic- and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-delta (super 13) C around the penultimate glacial termination (T2: 132 ka). The time-series planktic- and benthic- species delta (super 13) C records exhibit two distinct mean-delta (super 13) C levels. The low mean-delta (super 13) C characterises the pre-T2 period (250 ka-132 ka), while the post-T2 ( approximately 95 ka - Present) period records high mean-delta (super 13) C, generating a one-time shift of approximately 0.4 per mil within the last approximately 250 kyr time-period. This shift is a result of consistently higher-delta (super 13) C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 ( approximately 132 ka), lasts until approximately 95 ka, and sustained through the T1. The normally observed glacial-interglacial delta (super 13) C variations of approximately 0.3 per mil occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2-delta (super 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 delta (super 13) C fail to explain the observed mean-delta (super 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-delta (super 13) C than after. Such low-delta (super 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.