THE USE OF MULTIPLE STABLE ISOTOPES TO DISCERN SOLUTE SOURCES AND GEOCHEMICAL PROCESSES IN ANTARCTIC LAKES

We have analyzed surface and hypolimnion waters from four saline and one freshwater closed-basin lakes in the McMurdo Dry Valleys, Antarctica, for their δ¹¹B and δ⁴⁴Ca signatures. These lakes (Bonney, Fryxell, Hoare, Joyce, and Vanda) have been studied since the early 1960s, yet debate still exists over their geochemical origins and evolution. These new data have been coupled with previous published ⁸⁷Sr/⁸⁶Sr and δ⁷Li values in order to better assess the source of solutes to these lakes as well as to understand if internal lake processes might effect isotopic distributions. All isotopic signatures indicate that the weathering of aluminosilicates either as dust on glacier surfaces or in the hyporheic zones of the stream channels is a major source of these elements to the surface waters of these lakes. For two of the saline lakes the δ⁴⁴Ca of the bottom waters are greater than that of the surface waters, suggesting the precipitation of Ca salts at depth. The δ¹¹B value increases with salinity of the lakes up to +51‰ relative to NIST 951, which suggests that the original seawater B isotope composition of the lakes (+39‰) was modified through preferential ¹⁰B removal in secondary minerals. ⁸⁷Sr/⁸⁶Sr ratios vary greatly from lake to lake suggesting different sources of Sr to each lake reflecting the variable mineralogy/petrology of the watershed tills. In general, the δ⁷Li and δ¹¹B are greater in the hypolimnetic waters, suggesting preferential removal of ⁶Li and ¹⁰B and/or the input of seawater-derived Li and B. Groundwater-derived lacustrine waters, on the other hand, have lower δ⁷Li and δ¹¹B than the hypolimnetic waters. When combined, this suite of isotopic signatures reveals that all of these isotopes can be used to identify solute sources, but in-lake processes can later modify Ca, Li, and B isotopic signatures.