Trace element shell chemistry of continental ostracodes and the applicability of experimentally-derived multiple regression models to paleoenvironmental reconstructions in southwestern North America.

Abstract

Lacustrine ostracodes have been studied during the last three decades because of their apparent utility in reconstructing aquatic paleoenvironments. These microcrustaceans are ideal for paleoenvironmental analysis because: (1) they are cosmopolitan and abundant lacustrine inhabitants; (2) they show high diversity; (3) they are sensitive to climate-related limnological variations; (4) their valves incorporate some trace elements from host waters that may provide information on paleolimnologic conditions; and (5) their valve calcification appears to be in oxygen stable isotopic equilibrium with the host water. I designed a two-part research project to understand the significance of ostracode shell chemistry in response to the environment. First, an experiment to determine the Mg²⁺and Sr²⁺ partition coefficients (K(p)) for Limnocythere staplini, a widely distributed species in North America was executed to establish its applicability to the geologic record. Three hundred specimens of this species were individually cultured in 20 ml capped vials at discrete temperatures (15, 20 and 25 °C) and conductivities (10, 15, 20, 25, 30 μMohs/cm). After the ostracodes molted, the carapaces were removed for Ca²⁺, Mg²⁺, and Sr²⁺ analysis by inductively coupled plasma mass spectrometry (ICP-MS). Ostracode shell chemistry shows that ᵐ(Mg/Ca)(valve} is temperature dependent but not related to ᵐ(Mg²⁺/Ca²⁺)(water}. ᵐ(Sr/Ca)(valve} is dependent on TDS and possibly to ᵐ(Sr²⁺/Ca²⁺)(water}. Experimental results are applicable to a limited number of water bodies within the ranges of these analyses. Second, the equations generated in these experiments were used in a case study from the geologic record to test the applicability of this technique. Las Acequias Hohokam irrigation system provided the material for the first attempt to use ostracode shell chemistry to reconstruct the history of canal operation during Hohokam occupation (8th-15th Century A.D.). One hundred and twenty nine valves of Limnocythere staplini were analyzed by ICP-MS. I studied these valve data and associated information to determine the human water usage patterns and impact on the environment by ancient Native Americans. Current results suggest that experimental ᵐ(Mg/Ca)(valve} ratios may be used as a paleothermometer to reconstruct aquatic paleoenvironments. ᵐ(Sr/Ca)(valve} may prove to be a reliable source for reconstructing paleosalinity. The combined trace element analyses provides a powerful tool for understanding both paleoclimatic and human impact changes. However, paleoenvironmental reconstructions through partition coefficients is not warranted because thermodynamic equilibrium may not exist between water and Limnocythere staplini.