Late Quaternary Chronology of Tufa Deposits, Walker Lake, Nevada

Abstract

Walker Lake, a terminal desertic lake in Mineral County, Nevada, has fluctuated greatly in lake level throughout its long history due to changes in climate and drainage. In its Late Pleistocene high stand, Walker Lake formed the southernmost embayment of pluvial Lake Lahontan. Calcareous tufa and other shoreline deposits of Pleistocene and Holocene ages provide a valuable record of past lake levels. Studies of tufa morphology and stable isotope character, closely integrated with sedimentologic data, show great promise for elucidating the post-glacial climatic history of the Great Basin. Three prominent shoreline carbonate deposits below 1250 m have been studied in the field and with stable isotope and radiocarbon analyses. The oldest deposit (18,000 radiocarbon yrs B.P.) consists of massive, bowl-shaped tufa heads outcropping from conspicuous wavecut and fan-deltaic terraces. Three "bigheads" are covered by alluvial fan deposits, which in turn are coated by an "encrusting tufa" of 3 cm average thickness. This encrusting tufa, dated at 2150 yrs B.P. and exhumed during the historic drop in lake level, coats virtually every undisturbed boulder around the entire lake. The third deposit, a 2 mm "outer rind," coats the first two deposits and has modern pre A-bomb or A-bomb radiocarbon levels depending on sample elevation. The $$\delta^{13}C$$ values for the bighead deposits average $$3.41 \pm 0.10\%0$$, and the $$\delta^{18}O$$ values average $$-1.47 \pm 0.61\%0$$. The $$\delta^{13}C$$ and $$\delta^{18}O$$ values for the encrusting tufas average $$3.21 \pm 0.12\%0$$ and $$-0.68 \pm 0.25\%0$$, respectively, and the $$\delta^{13}C$$ and $$\delta^{18}O$$ values for the outer rinds average $$3.09 \pm 0.17\%0$$ and $$-0.10 \pm 0.34\%0$$, respectively. Isotopic paleotemperatures for the two Holocene deposits range from 27 to 35°C, the same as shoreline values measured in July and August. Our results indicate late Pleistene low stands below 1222 m circa 18,000 yrs B.P., a post-Pleistocene period of desiccation during which alluvial fans formed, another rise to 1250 m, another period of desiccation circa 2000 yrs B.P., a rise to the historic high stand of 1244 m and, since 1880, a drop to 1206 m.