HOLOCENE-SCALE GROUNDWATER-SURFACE WATER INTERACTION IN PRAIRIE LAKES: ASSESSING THE INDIVIDUAL RESPONSE OF LAKES TO HYDROLOGIC AND CLIMATIC CHANGE

The prairie lakes of the North American Great Plains are part of local or regional hydrologic systems, and their response to climate forcing is mediated by hydrology. The challenge is to identify the link between lakes and the regional shallow and deep ground water, and to identify a lake's individual response to change within the context of its hydrologic setting. Ostracodes (microscopic crustaceans) are common microfossils of the Holocene sediment records from prairie lakes, and can be used to track ground water and surface water interactions through their ecology, isotopic composition and trace metal record, including changing source water, solute evolution, and cation exchange. The modern North American biogeography of ostracodes indicates that solute composition, temperature and total concentration (TDS) are key factors in species distribution. For example, some species occupy similar salinity ranges but different solute compositional ranges, and thus track solute change. Although ground water can mediate the climate signal recorded in prairie lakes, significant hydrologic changes occur over Holocene time that may not have been induced by climate change. An example of solute changes related to changes in ground water hydrology is the Holocene record from Rice Lake on the Missouri Coteau of central North Dakota. Today, Rice Lake has a total concentration of about 1.8 grams/liter, and is a bicarbonate enriched saline lake with an ionic composition dominated by Mg-Ca-HCO3-Cl. Rice Lake's Holocene record begins as a freshwater lake in early Holocene time, and evolves along a bicarbonate depleted solute path (dominated by Limnocythere staplini) to a saline lake during mid-Holocene time. However, in late Holocene time (approximately 2500 cal yrs ago) L. staplini is replaced by L. ceriotuberosa, which in turn gives way to L. sappaensis, a species associated with bicarbonate enriched saline water. This change in the solute path of the lake is likely groundwater controlled, and may be attributed to a change in source water (from bicarbonate depleted to bicarbonate enriched). Today, only wells south of the lake have Alk/Ca ratios less than 1, whereas most of the wells around the lake have Alk/Ca ratios greater than 1. Changes in the solute history of this lake shed light on the changing flow paths rather than informing us directly about climate change.