CARBONATE DEPOSITION, PYRAMID LAKE SUBBASIN, NEVADA .4. COMPARISON OFTHE STABLE-ISOTOPE VALUES OF CARBONATE DEPOSITS (TUFAS) AND THE LAHONTAN LAKE-LEVEL RECORD

In this paper, the fundamental importance of changes in hydrologic bal ance and hydrologic state on the delta(18)O and delta(13)C values of w ater and dissolved inorganic carbon (DIC) in lakes of the Lahontan bas in is illustrated. Abrupt changes in delta(18)O and delta(13)C values of carbonate deposits (tufas) from the Pyramid Lake subbasin, Nevada, coincide with abrupt changes in lake-level and hydrologic state. Minim a in lake-level at similar to 26,000, similar to 15,500 and similar to 12,000 yr B.P. are associated with relatively heavy delta(18)O and de lta(13)C values; maxima in the lake-level record at similar to 14,000 and similar to 10,500 yr B.P. are associated with relatively light del ta(18)O and delta(13)C values. We believe that the correlation between maxima and minima in the lake-level and delta(18)O records reflect th e fundamental effect of lake-level dynamics on the delta(18)O value of lake water. Evaporation increases the delta(18)O value of lake water, whereas, streamflow discharge and on-lake precipitation decrease the delta(18)O value. Variation in the delta(18)O value of lake water, the refore, indicates change in the hydrologic balance; increases in delta (18)O accompany decreases in lake volume and decreases in delta(18)O a ccompany increases in lake volume. Covariance of delta(13)C and delta( 18)O indicates that change in delta(13)C values of DIC also accompany change in lake volume. We offer the hypothesis (first put forward by J .A. McKenzie) that change in the productivity (photosynthesis) respira tion balance is responsible for much of the observed variation in delt a(13)C. Most Great Basin lakes, including Lake Lahontan, experienced c hanges in hydrologic state during the late Wisconsin. When a lake beco mes hydrologically open, the residence time of water decreases. The gr eater the rate of spill, the greater the volume of evaporated (O-18-en riched) water removed from the spilling lake and the more negative the delta(18)O value of water remaining in the spilling lake. The concent ration of DIC, as well as the concentrations of photosynthesis limitin g nutrients (e.g., phosphorus, nitrogen, silica, molybdenum) decrease as spill increases. Increasing rates of spill, therefore, lead to over all decreases in photosynthetic rates relative to respiration rates an d, as a consequence, the delta(13)C values of DIC become more negative .