A STABLE ISOTOPE RECORD FROM LAKE CAHUILLA SHORELINE TUFA, SOUTHWESTERN CALIFORNIA

Tufa from Pleistocene Lake Cahuilla provides a detailed isotopic record of processes occurring in Lake Cahuilla and the Colorado River drainage basin. The tufa may also provide a detailed record of the process of switching on the Colorado River delta between Salton Trough and the Gulf of California. A 60 cm-thick slice of tufa from the ancient northwestern shoreline was subsampled at 4 mm intervals and analyzed for stable isotopes. The result is a near-symmetrical plot of covarying changes in d¹⁸O and d¹³C, with values descending from, respectively, -3.4 and 3.6 to -7.8 and 1 per mil (PDB), and back again. Over 20 second-order variations of greater than 1 per mil d¹⁸O are tracked by corresponding changes in d¹³C.

Several hypotheses are being considered as possible explanations for these trends, including, 1) periodic mixing of fresh, Colorado River-derived lake waters with marine waters from the Gulf, 2) paleotemperature changes, and 3) hydrologic fluctuations associated with precipitation changes in the Colorado River drainage basin. The first of these is unlikely due to the absence of marine fauna in Lake Cahuilla sediments (Arnal, 1961), and temperature fluctuations are probably not able to account for the magnitude nor frequency of the changes observed. This leaves variation in input as the most likely hypothesis. Whether the lake in which these hydrologic changes occurred was open or closed is also still being investigated. A generally hydrologically open lake is suggested by the coincidence in elevation of the uppermost part of the tufa and its associated shoreline with that of the outflow sill. This is reinforced by the relatively low covariance between oxygen and carbon isotopes of 0.52--high enough to suggest a lake with a relatively long residence time, but probably too low for complete closure.