OCR Text |
Show 8/ 14/ 97 Smith and Forester 1 Pandemic vs Endemic Ostracode Biogeography in the Great Basin: Physical vs Biological Selection Pressure Alison J. Smith Dept. Geology, Kent State University, Kent, OH 44242 asmith@ geology. kent. edu Richard M. Forester U. S. Geological Survey, Denver Federal Center, Denver, CO 80225 forester@ usgs. gov Abstract For many continental ecosystems, the configuration and duration of air masses control temperature ranges and effective moisture, setting the stage for the development of regional ecotones. Continental ecosystems are also strongly affected by the sources and supply of groundwater recharge and discharge, as in the development of wetlands and lakes, and the sustainment of deep aquifers. Groundwater flow and atmospheric circulation are linked through the hydrologic cycle. Climate- induced changes in the hydrologic cycle have had pronounced effects on hydrologic distribution and in biological systems. One well- known example includes the development of pluvial lakes in the Great Basin during the last glacial cycle in response to ice sheet growth and jet stream deflection. Within the framework of ecology, we can define hydrologic ecosystems consisting of communities of organisms that interact with different components of the hydrologic cycle. We include here aquifers, wetlands, springs, and surface water, all of which characterize and affect much of the terrestrial landscape. These ecosystems include rheocrene communities that are linked to 1) confined aquifers, 2) the phreatic conditions of shallow unconfined aquifers, and 3) the wetlands, streams, and littoral zones of lakes. The different components of the hydrologic cycle include, but are not limited to, atmospheric precipitation and evaporation, water- rock interaction, and groundwater recharge and discharge. Together, they mark hydrologic ecotones that have both spatial and temporal definitions. The epigean organisms of springs, seeps, wetlands, streams and lakes offer information about the effects of climate on the landscape, whereas hypogean organisms in deep aquifers provide hydrochemical information. Certain organisms, such as ostracodes, also provide a temporal record in the form of fossil remains. Ostracodes are microscopic ( typically < 1mm in length) crustaceans that produce bivalved calcite shells. Their shell composition, sensitivity to environmental variables, and wide distribution make them valuable sources of paleoclimate information. Their low- Mg calcite shells are abundant microfossils in many sedimentary environments and may provide geochemical data on a variety of past aquatic conditions including temperature, salinity, and isotopic composition of water and dissolved inorganic carbon. Ostracodes play a role similar to that of foraminifera in the marine world by providing vital information about the interactive nature of climate and the hydrologic cycle through their ecology, shell geochemistry, and biogeographical distribution. |