Mid-Pliocene deep-sea bottom-water temperatures based on ostracode Mg/Ca ratios

Abstract

We studied magnesium:calcium (Mg/Ca) ratios in shells of the deep-sea ostracode genus Krithe from a short interval in the middle Pliocene between 3.29 and 2.97 Ma using deep-sea drilling sites in the North and South Atlantic in order to estimate bottom water temperatures (BWT) during a period of climatic warmth. Results from DSDP and ODP Sites 552A, 610A, 607, 658A, 659A, 661A and 704 for the period Ma reveal both depth and latitudinal gradients of mean Mg/Ca values. Shallower sites (552A, 610A and 607) have higher mean Mg/Ca ratios (10.3, 9.7, 10.1 mmol/mol) than deeper sites (661A, 6.3 mmol/ mol), and high latitude North Atlantic sites (552A, 610 and 607) have higher Mg/Ca ratios than low latitude (658A: 9.8 mmol/ mol, 659A: 7.7 mmol/mol, 661A: 6.3 mmol/mol) and Southern Ocean (704: 8.0 mmol/mol) sites. Converting Mg/Ca ratios into estimated temperatures using the calibration of Dwyer et al. (1995) [Dwyer, G.S., Cronin, T.M., Baker, P.A., Raymo, M.E., Buzas, J.S., Corrège, T., 1995. North Atlantic deepwater temperature change during late Pliocene and late Quaternary climatic cycles. Science 270, 1347Science 270, -1351 suggests that mean middle Pliocene bottom water temperatures at the study sites in the deep Atlantic were about the same as modern temperatures. However, brief pulses of elevated BWT occurred several times between 3.29 and 2.97 Ma in both the North and South Atlantic Ocean suggesting short-term changes in deep ocean circulation. Published by Elsevier B.V.

Milankovitch Forcing and Meridional Moisture Flux in the Atmosphere: Insight from a Zonally Averaged Ocean–Atmosphere Model

“…These results are consistent with deep ocean temperature changes derived from paleoclimate records for the warm climate of the middle Pliocene (Dwyer et al 1995;Cronin et al 2005; see AMM10). On the other hand, as discussed in section 5d, the model underestimates air temperature changes inferred from ice-core data for the glacialinterglacial cycles of the late Pleistocene.…”

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“…The accurate chronology of sedimentary boulders needs to be corroborated by further pollen assemblage studies and the offshore sediment record in Prydz Bay. Even now, our chronological estimation of large scale ice margin retreat of the EAIS adds to evidence obtained otherwise, for example, the ages of volcanic ash bed in diatom-bearing glaciomarine strata cored in Ferrar Fiord by the CIROS drill -hole, the fossil plants from the Pliocene Sirius Group (Barrett et al, 1992;Francis and Hill, 1996), the modeling results of ice sheet variation during the middle Pliocene (Whitehead and McKelvey, 2002), and the deep-sea bottom-water temperatures base on ostracod Mg/Ca ratios (Cronin et al, 2005).…”

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