AN ELEMENTAL AND ISOTOPIC INVESTIGATION IN WATER COLUMN OF LAKE TEXOMA

Lake Texoma, a large impoundment on the Texas-Oklahoma border formed by the confluence of the Red River (Total Dissolved Solids, TDS > 1000 mg/l) and the Washita River (TDS < 500 mg/l), provides an opportunity to study saline and fresh water mixing processes, element budgets and metal cycling. Water column samples, collected at 20 locations throughout the lake in summer and fall 2003 and analyzed for elemental and stable isotope (sulfur and carbon) compositions show distinct spatial and seasonal variations related to inflow mixing, biogenic consumption and summer anoxia. Ca, K, Mg, Li, Na and Sr exhibit conservative behavior in both the main lake and two river arms with lake concentrations in good agreement with inflow cation budgets. Main lake Zn concentration is higher than both the Red and the Washita river arms, which indicates an extra source. In contrast, lower Si concentration in the main lake compared with two river arms suggests biogenic consumption. During summer, dissolved oxygen decreases with depth (from 7 to 0.1 mg/l) while Fe, Mn and HS^- show complementary increases. Thus, summer anoxia apparently leads to reduction of Fe and Mn oxyhydroxides in sulfide-rich bottom waters.

Lake Texoma and two river arms show relatively small variations in d³⁴S (from 11.5 to 13.4 ‰ CTD), similar to Permian/Cretaceous marine gypsum and anhydrite (d³⁴S 10 — 15‰) in headwater regions. Increasing d³⁴S with depth (>16m) in the main lake suggests fractionation associated with sulfate reduction in anoxic bottom waters. d¹³C_DIC values become more negative (from –2.5 to –8.2 ‰ PDB) with depth in summer, suggesting bacterial oxidation of organic matter linked to sulfate reduction.