Carbonate deposition, Pyramid Lake Subbasin, Nevada .4. Comparison of the stable isotope values of carbonate deposits (tufas) and the Lahontan lake-level record

In this paper, the fundamental importance of changes in hydrologic balance and hydrologic state on the delta(18)O and delta(13)C values of water and dissolved inorganic carbon (DIC) in lakes of the Lahontan basin 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. Minima 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 delta(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 delta(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 the 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, therefore, indicates change in the hydrologic balance; increases in delta(18)O accompany decreases in lake volume and decreases in delta(18)O accompany 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) respiration balance is responsible for much of the observed variation in delta(13)C. Most Great Basin lakes, including Lake Lahontan, experienced changes in hydrologic state during the late Wisconsin. When a lake becomes hydrologically open, the residence time of water decreases. The greater the rate of spill, the greater the volume of evaporated (O-18-enriched) water removed from the spilling lake and the more negative the delta(18)O value of water remaining in the spilling lake. The concentration of DIC, as well as the concentrations of photosynthesis limiting nutrients (e.g., phosphorus, nitrogen, silica, molybdenum) decrease as spill increases. Increasing rates of spill, therefore, lead to overall decreases in photosynthetic rates relative to respiration rates and, as a consequence, the delta(13)C values of DIC become more negative.