Water isotopic and hydrochemical evolution of a lake chain in the northern Great Plains and its paleoclimatic implications

Oxygen isotopes and geochemistry from lake sediments are commonly used as proxies of past hydrologic and climatic conditions, but the importance of present-day hydrologic processes in controlling these proxies are sometimes not well established and understood. Here we use present-day hydrochemical data from 13 lakes in a hydrologically connected lake chain in the northern Great Plains (NGP) to investigate isotopic and solute evolution along a hydrologic gradient. The δ18O and δ2H of water from the chain of lakes, when plotted in δ2H - δ18O space, form a line with a slope of 5.9, indicating that these waters fall on an evaporation trend. However, 10 of the 13 lakes are isotopically similar (δ18O = –6 ± 1‰ VSMOW) and show no correlation with salinity (which ranges from 1 to 65‰). The lack of correlation implies that the isotopic composition of various source waters rather than in-lake evaporation is the main control of the δ18O of the lakes. Groundwater, an important input in the water budget of this chain of lakes, has a lower δ18O value (−16.7‰ in 1998) than that of mean annual precipitation (−11‰) owing to selective recharge from snow melt. For the lakes in this chain with salinity < 15‰, the water Mg/Ca ratios are strongly correlated with salinity, whereas Sr/Ca is not. The poor correlation between Sr/Ca and salinity results from uptake of Sr by endogenic aragonite. These new results indicate that δ18O records may not be interpreted simply in term of climate in the NGP, and that local hydrology needs to be adequately investigated before a meaningful interpretation of sedimentary records can be reached.