GEOCHEMICAL CLIMATE PROXY DEVELOPMENT IN TUFA FROM THE MONO BASIN, CALIFORNIA

We are interested in the climatic evolution of Mono Lake reflected in changing lake conditions, for instance lake level and pH. There are a restricted number of tools to interpret these changes, in particular in the ancient lake (> 10 ka). This research is part of ongoing studies aimed at evaluating the viability of new geochemical proxies in terrestrial systems. This study had two primary goals: to evaluate geochemical retentivity of different textural varieties of carbonate precipitates (tufa) through the analysis of young, well-characterized samples; and to apply those proxies to dated ancient tufas to evaluate paleolake conditions.

A collection of tufa samples reflecting distinct textural attributes was examined; these broadly break down into dense forms, commonly laminated, and those which are more texturally heterogeneous, generally spongy. Total lanthanoid rare Earth element (REE) concentrations of densely laminated forms of tufa are higher (> 20 ppm) and more homogeneous than spongy morphologies (mainly < 10 ppm). Carbonate-associated sulfate from a preliminary subset of samples behave as predicted, with elevated CAS in samples with low total REE. The densely laminated samples also have restricted stable (C, O) isotopic variability compared to the spongy forms. The data from the former textural variety are consistent with retention of undisturbed elemental systematics.

The use of texture-specific samples was applied to dated layers from an ancient tufa mound ranging over 25 kyr. The samples show a factor of 1.6 variation in total REE concentration. The initial group of samples show a modest negative correlation between REE total and the magnitude of Ce anomaly, from samples with high REE and negative anomalies (Ce/Ce* = 0.6) to those with lower total REE and slightly positive anomalies (Ce/Ce* = 1.1). Decrease in the total REE (as well as Ba content) follows the c. 40 ka rise in lake level.