ORIGIN OF LAMINATED LACUSTRINE CARBONATES: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTION

Our ability to accurately predict how the Earth will respond to rapid global climate change depends on insights gained from ancient high resolution paleoclimate records. Recent advances in paleoenvironmental and paleoclimatic research demonstrate the ability to create such climate records based on the structure and composition of laminations present in ambient temperature fresh water carbonates. We expanded upon this prior research through construction of a high resolution stable isotopic stratigraphy for a middle Miocene tufa from the Barstow formation exposure in the central Mojave Desert.

The major, trace, rare earth element and stable isotopic composition of the Barstow Formation tufa support the interpretation that this laminated carbonate formed in a low-temperature surficial environment. Major and trace element concentrations indicate that the Barstow tufa is ~94% calcite with low Mg/Ca, Mn/Ca, and Sr/Ca ratios (all <0.01) Chondrite normalized rare earth element compositions for the Barstow Tufa range between 3.1 and 33.0 ppm, comparable to post Pleistocene tufas from Mono Lake and Trona, California. In contrast, travertines from Hot Creek and Travertine Springs, California exhibit much lower REE concentrations with chondrite normalized values ranging between .08 and 6.6 ppm.

A high-resolution stable isotopic record for the middle Miocene Barstow Formation Tufa suggests this precipitation of calcite occurred via mixing between cold ground-water fed springs and closed-basin lake waters. No clear seasonal changes in the aqueous system are recognizable from this record, however, episodic, possibly decadal scale, evaporative events are clearly recognized in both tufa structure and stable isotopic composition.