MODIFICATION OF CLIMATE SIGNALS BY HYDROLOGIC, HYDROCHEMICAL, AND BIOLOGICAL FILTERS: THE POWER OF MULTIPROXY APPROACHES TO HELP SEE THROUGH THE FILTERS

Proxy archives of past climate preserved in lacustrine sediments are indispensable tools of paleoclimate reconstruction. Most of these proxies, however, are not direct recorders of climate (e.g., mean annual temperature and P-ET), but are affected by physiographic, geomorphologic, and hydrologic setting, aquifer (sensu lato) and regolith lithology, and seasonality, to name a few. Hence, there is no simple correlation between, for example, salinity and aridity, or d¹⁸O and mean annual temperature. Hydrochemistry and its evolution are predetermined by aquifer lithology and mineralogy and driven by the hydrologic balance. Variability of temperature and hydrochemistry, in turn, affects the species of algae, diatoms, chironomids, ostracodes, and other biogenic proxies that are present in the lakes, and in the case of ostracodes, also their shell chemistry. Thus every proxy archive is an indirect record of past climate, and the filters acting upon these proxies are non-linear. Multiproxy reconstruction is a powerful and perhaps the only way to see though these filters and limit possible interpretations, even though they may or may not offer insights into the cause of change.

In the study of a core from Elk Lake (Grant Co.), MN, increases in the Mg/Ca, d¹⁸O and d¹³C were accompanied by a decrease in the Sr/Ca of ostracode shells at 5385 cal yrBP (Smith et al., 1997, 2002). The Mg/Ca and d¹⁸O changes suggest increased aridity, and d¹³C increased degassing of DIC from a shallow highly productive lake, but the Sr/Ca suggests freshening. The XRD analyses of bulk sediments reveal that endogenic carbonates changed from calcite to aragonite as a result of increased Mg/Ca of water: resulting in the uptake of Sr by aragonite. The extreme lowering of the lake level and the changeover of the coring site to a spring-fed wetland are further verified by the presence of the ostracode Heterocypris fretensis. A later period of prolonged drought at 5220 cal yrBP is characterized by increased Mg/Ca but decreased d¹⁸O and the presence of ostracode Limnocythere staplini. The latter two proxies indicate the change in hydrologic balance from surface run-off dominated to groundwater-fed shallow lake of much diminished volume. Any one proxy such as Mg/Ca or d¹⁸O would not have been able to even hint at these prolonged drought events or their magnitude.