HOLOCENE HYDROLOGIC AND HYDROCHEMICAL CHANGES OF LAKE MANITOBA, CANADA, THE OSTRACODE SHELL CHEMISTRY AND STABLE ISOTOPE RECORD

Mid to late Holocene ostracode shell chemical data from the South Basin of the Lake Manitoba are consistent with isostatically and climatically induced changes in the basin’s fluvial and groundwater sourced hydrologic budget. Interpretation of the shell data is complicated by wind-induced mixing of sediment in the large shallow basin as well as the existence of (semi-) isolated groundwater seeps with solute compositions that differ from those of the lake. Hence, shells of Fabaerformiscandona rawsoni in a given 1-cm sample may reflect different times and or solutes. Despite these complications, F. rawsoni shells from the time when the north and south basins were separated nonetheless show hydrologically closed basin behavior with notable exceptions such as high δ¹⁸O and low Mg/Ca and vice versa. High δ¹⁸O, low Mg/Ca samples may identify times of significant inflow from Assiniboine River, whereas low δ¹⁸O, high Mg/Ca samples may reflect shell transport from isolated saline groundwater seeps. The full stratigraphic δ¹⁸O values show an upward increase. However, many replicates show a wide spread, indicating a highly variable hydrochemical environment (over one or multiple summers) likely coupled with the site complicating factors, belying the trend. High-Mg calcite isotopes (Last et al, 1994) are in broad agreement with F. rawsoni except in the upper 1.5m (modern hydrology established) where the trends diverge. The difference might be explained by F. rawsoni living in shallow littoral area of the lake subject to more evaporation (higher δ¹⁸O) and higher productivity (higher DIC δ¹³C) and transported by wind-mixing to the center of the main basin, whereas high-Mg calcite is precipitated in the main basin as evidenced by sediment trap data (Last, pers. comm.). The shell isotope and metal data are a product of the system’s primary and secondary (reworking, multiple solute sources) variability throughout the history of the record. The importance of secondary sources of data variability in the South Basin record offers a cautionary note about interpretation of shell isotope and metal data from any record (Ito and Forester, 2009). Accordingly, the high variability of the system is real while any reconstruction of causes of that variability remains speculative.