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2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 6
Presentation Time: 2:45 PM

PALEOGENE SEAWATER CHEMISTRY INFERRED FROM CORAL STABLE ISOTOPIC AND ELEMENTAL CHEMISTRY


IVANY, Linda C., Department of Earth Sciences, Syracuse Univ, Syracuse, NY 13244, PETERS, Stephen C., Department of Geological Sciences, Univ of Michigan, 2534 CC Little Building, Ann Arbor, MI 48109-1063, WILKINSON, Bruce H., Department of Geological Sciences, Univ of Michigan, Ann Arbor, MI 48109, LOHMANN, Kyger C., Department of Geological Sciences, Univ of Michigan, 2534 C.C.Little, 425 E. University, Ann Arbor, MI 48109-1063 and REIMER, Beth A., Department of Geology and Geological Engineering, Univ of Mississippi, University, MS 38677, lcivany@syr.edu

A sectioned and polished specimen of the colonial coral Arcohelia from the early Oligocene Byram Formation (~30Ma) near Vicksburg, MS, reveals 12 prominent annual growth bands from center to margin. Microsampling of original aragonite across the growth axis produced carbonate powders from 78 successive growth-band-parallel milling paths. Stable oxygen isotopic compositions show clear, well-constrained seasonal fluctuations ranging between about -2.2 and -4.2 per mil, which, when converted to temperature using the equation of Leder et al. (1996) and assuming a seawater composition of -0.5 per mil, yield temperature estimates ranging between about 12 and 23 degrees C. These values are consistent with temperatures derived from isotopic data from other taxa found in the same unit. Dark growth bands occur during the transitions from warm to cool temperatures. Powders from five of the twelve annual bands used for isotopic analysis were also run for Mg/Ca and Sr/Ca ratios in order to evaluate the utility of these proxies for paleoclimate reconstruction. Paleotemperatures calculated using the equations of Mitsuguchi et al. (1996) and DeVilliers et al. (1994) developed from modern corals are consistently lower than those derived from oxygen isotope values. If Oligocene oxygen isotope-derived temperatures are correct, we can calculate the expected Mg/Ca and Sr/Ca ratios of Paleogene seawater. Observed Oligocene Mg/Ca ratios are about 0.8 times that of the expected value, and Sr/Ca ratios about 1.08 times expected. These preliminary results suggest that Oligocene seawater was depleted in Mg and enriched in Sr relative to calcium in comparison to today's ocean. This is consistent with a Paleogene transition from "calcite" to "aragonite" seas, perhaps in response to a decrease in sea-floor spreading rates with a concomitant reduction in scavenging of magnesium from seawater, and the increased sequestration of strontium in sedimentary carbonate.

Session No. 219

Paleoclimatology/Paleoceanography II
Wednesday, 30 October 2002: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 34, No. 6, September 2002, p.500
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