Cartoons illustrating the mechanisms for δ13C-lead-δ18O at the 405-ka cycle during the MCO. (IMAGE)
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(a) Eccentricity maxima can cause a shrinkage of the Antarctic ice volume and an increased 16O (isotopically light oxygen) transfer to the ocean. Simultaneously, enhanced monsoons and continental weathering can transport more alkalinity and nutrients to the ocean, releasing more 12C-enriched carbon into the deep sea. (b) During eccentricity minima, the opposite processes occur. Therefore, benthic δ18O-δ13C interactions are nearly in phase at eccentricity cycles. (c) Cross-spectral coherence and phase angles between parallel δ18O and δ13C records from IODP/ODP sites 1146, U1337, U1338 and U1505 for the MCO interval, and they show that variations of benthic δ13C lead those of δ18O in the 405-ka bands. In general, the relatively long residence time of carbon in the deep ocean facilitates a lead of benthic δ18O relative to δ13C. The MCO greenhouse effect is likely to have accelerated the response of marine carbon cycle to eccentricity forcing, generating the δ13C-lead-δ18O scenario.
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