HR: 15:10h
AN: B13E-07    [Abstracts]
TI: The Interpretation of Mg/Ca in Ostracode Valves: Biokinetic vs. Thermodynamic Controls
AU: * Dettman, D L
EM: dettman@geo.arizona.edu
AF: Dept. Geosciences, University of Arizona, Tucson, AZ 85721 United States
AU: Palacios-Fest, M R
EM: terra_nostra_mx@yahoo.com.mx
AF: Terra Nostra Earth Sci. Research, 3220 W. Ina Rd. \#8105, Tucson, AZ 85741 United States
AU: Cohen, A S
EM: acohen@geo.arizona.edu
AF: Dept. Geosciences, University of Arizona, Tucson, AZ 85721 United States
AB: The geochemistry of the calcite valves of ostracodes (a group of micro-crustacean) is often used to reconstruct the history of aqueous environments in both marine and fresh-water settings. These benthic animals can be very abundant in lakes and ponds and their low-Mg calcite valves are easily recovered from sediment cores. Many studies have used minor-element ratios (Mg/Ca and Sr/Ca) as indicators of temperature and/or salinity change through time and numerous calibration studies have been undertaken. There is considerable disagreement on the interpretation of both historical data and calibration studies because of differing views on what controls elemental ratios in ostracode valves. Here we focus on Mg/Ca ratios and critique the dominant assumption that Mg/Ca ratios in ostracode calcite are interpretable as a temperature-dependant distribution (or partition) coefficient. The use of a distribution coefficient, usually defined as a ratio of shell-to-water Mg/Ca ratios, assumes that the ratio in the water plays a significant role in the resultant ratio in the shell. Ostracode biomineralization is most commonly viewed as equivalent to inorganic precipitation of low-Mg calcite from solution, a system in which distribution coefficients are probably valid models. However, a re-examination of published studies shows that in many cases Mg/Ca(water) has no statistically demonstrable affect on the Mg/Ca ratio of ostracode valve calcite. The valve Mg/Ca ratio is most often a function of ambient temperature. In a number of studies the importance of the water's Mg/Ca ratio cannot be determined due to auto-correlation with other environmental factors. This implies that there is considerable biological control on the minor element chemistry of the ostracode valve. This is supported by a number of observations: valve calcification is rapid and initiated by the animal; Mg/Ca ratios within the valve vary greatly on a microscopic scale; the earliest carbonate formed during calcification approaches magnesite; other crustaceans accumulate Mg prior to molting. Finally, we note that ostracode valves typically fall into the 0.003 to 0.04 range of molar Mg/Ca ratios, and that ratios greater than 0.05 are extremely rare in the valves of fully-calcified adult ostracodes of any species. Yet these animals thrive in waters where the range of Mg/Ca ratio covers more than 3 orders of magnitude. A distribution coefficient model incorrectly predicts a similar 3 order of magnitude range in shell chemistry. Alternatively a bio-kinetic model for valve chemistry yields very promising results for the use of Mg/Ca ratios in ostracode valves as a paleothermometer.
DE: 3670 Minor and trace element composition
DE: 4239 Limnology
DE: 4267 Paleoceanography
DE: 1045 Low-temperature geochemistry
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting