RECONSRUCTION OF THE PALEOECOLOGY OF PLEISTOCENE LAKE LAHONTAN, HUMBOLDT COUNTY, NEVADA

The Bilk Creek Mountains of Humboldt County in northwestern Nevada contain sporadic outcrops of Late Pleistocene shoreline sediments deposited during the high stand period for Lake Lahontan. As Lake Lahontan is now almost completely evaporated away, these sediments and the fossils they contain are the only reliable record of the ancient environmental conditions and paleoecology of the lake. Previous sampling of these deposits revealed the presence of a Molluscan-dominated fauna composed of the gastropods Gyraulus parvus, Valvata virens, and Amnicola longinqua, and the bivalve Pisidium ultramontanum. The 12.5 kg sample yielded 2,962 individual fossils for a fossil density of 4.22 fossils/gram. The sample was dominated by the planispiral, cosmopolitan gastropod, G. parvus, which comprised over 67% of all individuals. Pleistocene lake faunas with the low-density and high dominance signature differ greatly from coeval lake deposits from the upper Midwest, such as Lake Scuppernong of southeastern Wisconsin. Such differences are normally interpreted to reflect paleoclimatic variations, but without accurate age determination, temporal variation cannot be falsified. In addition, the study of Lake Lahontan’s paleoecology establishes a pre-human base-line with which to compare the modern ecologies of extant lakes. The Lake Lahontan fossils are assumed to be between twelve and eighteen-thousand years old, but have never been dated by any radiometric means. Reconnaissance work in Nevada revealed the presence of strandline concentrations of the common fossils fauna – dating these fossils can determine the genuine highstand age of the lake. Preliminary sampling of these fossils from 5.8 kg of sediment has yielded 1594 individuals (for a fossil density of 0.2 fossils/gram), but these results will be greatly enhanced once a definitive date has been determined for the fossils via radiometric dating to be performed by Beta Analytic labs.