, Volume 17, Issue 1, pp 33-49

Major-ion chemistry of compositionally diverse lakes, Western Nebraska, U.S.A.: implications for paleoclimatic interpretations

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Abstract

The western Sand Hills, Nebraska, contains hundreds of compositionally diverse lakes with total dissolved solids ranging from 200 mg l-1 to over 100000 mg l-1 on occasion. This region has a semi-arid, mid-continental climate that is likely to be sensitive to future changes in climate. To interpret the record of Holocene climate in the lake sediments using available paleolimnological techniques, especially proxies for geochemical variations such as diatoms. ostracodes, and authigenic carbonates, requires an understanding of the modern spatial and temporal variability in lake chemistry. This paper provides: (1) a general description of the region and its many lakes; (2) a review of previous investigations related to the origin of these lakes; (3) major-ion data collected in 1992 and 1993 for 27 lakes in Sheridan County, NE; and (4) a discussion of these data in relation to a qualitative conceptual model, implications for lake history, and interpretations of paleoclimatic indicators that employ Mg/Ca ratios. Implications of the data include: (1) compositional differences between lakes are a function of local hydrologic variability, largely the magnitude of the input and output components and the lakes lifetime; (2) the lifetime of a lake is likely related to the complex geomorphic and hydrologic development of the ancestral Blue Creek basin; and (3) solutes in these lakes were not acquired from a high TDS groundwater source. The observed concentrations can be produced by evaporatively concentrating local groundwater. The available Mg and Ca data indicate that for low salinity lakes (e.g., Dillings and Krause lakes) there is a systematic relationship between seasonal increases in Mg/Ca ratios and salinity. Analyses of single ostracodes of the same species could provide a detailed record of seasonal salinity variations, as well as year-to-year hydrologic variability. In the more saline lakes (e.g., Potash, Shriner, Homestead, UNNJ, Wilkinson lakes), the Mg/Ca ratios in authigenic carbonate or ostracodes would be more difficult to interpret. In general, the Mg/Ca ratio decreases with increasing salinity. The dynamics of Ca- and Mg-bearing mineral phases need to be considered for a detailed interpretation of the Mg/Ca ratios in these lakes.