Mg/Ca, Sr/Ca, δ¹⁸O and δ¹³C Chemistry of quaternary lacustrine ostracode shells from the north american continental interior

The application of fossil ostracode shell chemistry to the reconstruction of past continental climate must take into consideration both short-term and long-term changes in the hydrologic budget of the lakes. This is particularly true when working in areas of strong seasonality in temperature and precipitation and negative effective moisture where many lakes owe their existence to inflow from groundwater. Moreover the chemistry (especially Mg/Ca and Sr/Ca) and δ¹⁸O of lake water may respond differently to hydrologic changes depending on whether they are caused by climate or other factors. These complications make clear how important it is to have information on ostracode ecology, to conduct a time-series monitoring study of the modern system, and to employ other proxy records. Examples of such studies are available from many semi-arid and arid regions of the world. Here I use Rice and Coldwater Lakes, North Dakota, as examples from the northern Great Plains of North America to illustrate these points. Decadally resolved Mg/Ca ratio variations for Rice Lake for the last 2000 years show excellent correlation with the GISP2 ice core δ¹⁸O record sampled at the same temporal resolution. Mg/Ca data for Rice and Coldwater Lakes also show good agreement with each other and with diatom-inferred salinity records for Coldwater as well as for Moon Lake located 100 km N N E of Coldwater Lake. However, δ¹⁸O records for neither lake are correlated with Mg/Ca records and highlight the importance of knowing the hydrology of each system.