Kiel device connected to a Finnigan MAT 251 mass spectrometer

Prior to TIC and d18O analyses, each sample was combined withdeionize d water, shaken and centrifuged for 15 min at 20,000 rpm using a Sorval Superspeed RC2B.1 After centrifugation, the conductivity of the supernatant was measured and the supernatant discarded. This procedure was repeated until the conductivity of the supernatant was o3 times the conductivity of Boulder, Colorado, tap water. The sample was then freeze dried and homogenized with a mortar and pestle.

Continuous, high-resolution d18O records from cored sediments of Pyramid Lake, Nevada, indicate that oscillations in the hydrologic balance occurred. When drought occurs in the Sierra Nevada, the flow of the Truckee River is reduced and Pyramid Lake falls or the amount of overflow to Winnemucca Lake basin decreases. This results in increasing d18O values of lake water. For the purposes of this paper, we define ??hydrologic droughts?? as periods during which Dd18O=Dt tends to be positive; that is, times when lake volume (or spill rate) decreases.

There are two principal sources of d18Oin to Pyramid Lake: Sierran snowmelt and Lake Tahoe overflow.Evaporation from Lake Tahoe has resulted in a d18O value (5.6m) much larger than that of snowmelt (14.6m). Thus, during droughts when Lake Fig. 19. Plot of TOC, d13C and d18O data from PLB98-2 (solid lines) and the top of PLC97-1 (dashed lines) indicating overlap of cores. 670 L. Benson et al. / Quaternary Science Reviews 21 (2002) 659?682 Tahoe ceases spilling, the d18O value of Truckee River discharge decreases from B11m to B14.6m. Th e input of 14.6m water to Pyramid lake causes its d18O value to first increase as its volume decreases; however, its steady-state value (1.6m) is much smaller than when the lake receives spill from Lake Tahoe.

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