Abstract

Changes in the vertical water mass structure of the Vema Channel during the Pliocene have been inferred from benthic foraminiferal assemblages and stable isotopic analyses from three sites of DSDP Leg 72 (South Atlantic). Faunal and isotopic results from Sites 516A and 518 suggest that a major change occurred in deep-water circulation patterns in the late Pliocene near 3.2 Ma.

Benthic oxygen isotopic records from Sites 516A and 518 show a characteristic increase in δ18O values near 3.2 Ma. This has been documented in numerous Pliocene isotopic records. The magnitude of the oxygen isotopic enrichment near 3.2 Ma appears to increase with water depth from an average enrichment of 0.34‰ in Site 516A (1,313 m) to an average enrichment of 0.58‰ in Site 518 (3,944 m). We suggest that this enrichment resulted partly from a change in deep-water circulation patterns which included a decrease in bottom-water temperatures. Planktonic δ18O values near 3.2 Ma show no evidence of an enrichment which would be indicative of an increase in global ice volume. On the contrary, δ18O values in Sites 517 and 518 become more depleted near 3.2 Ma, indicating a surface-water warming perhaps due to a change in the strength and/or position of the Brazil Current.

An increase in the relative abundance of the benthic foraminifer Nuttalides umbonifera, which is associated with Antarctic Bottom Water (AABW) in the modern ocean, coincides with the benthic 18O enrichment in Site 518. At 3.2 Ma, oxygen and carbon isotopic gradients between Sites 518 (3,944 m) and 516A (1,313 m) show a marked increase such that Site 518 becomes enriched in 18O and depleted in 13C relative to Site 516A. This enrichment in δ18O is interpreted as partly representing a temperature decrease at Site 518; the depletion in δ13C indicates a corrosive water mass which is high in metabolic CO2. We suggest that benthic foraminiferal and stable isotopic changes in Site 518 resulted from a pulse-like increase in the formation of AABW near 3.2 Ma. The cause of this circulation event may have been linked to global cooling and/or the final closure of the Central American Seaway.

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