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 δ18O and δ13C values of water and dissolved inorganic carbon (DIC) in lakes of the Lahontan basin is illustrated. Abrupt changes in δ18O and δ13C 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 ∼26,000, ∼15,500 and ∼12,000 yr B.P. are associated with relatively heavy δ18O and δ13C values; maxima in the lake-level record at ∼14,000 and ∼10,500 yr B.P. are associated with relatively light δ18O and δ13C values. We believe that the correlation between maxima and minima in the lake-level and δ18O records reflect the fundamental effect of lake-level dynamics on the δ18O value of lake water. Evaporation increases the δ18O value of lake water, whereas, streamflow discharge and on-lake precipitation decrease the δ18O value. Variation in the δ18O value of lake water, therefore, indicates change in the hydrologic balance; increases in δ18O accompany decreases in lake volume and decreases in δ18O accompany increases in lake volume. Covariance of δ13C and δ18O indicates that change in δ13C 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 δ13C. reat 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 (18O-enriched) water removed from the spilling lake and the more negative the δ18O 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 δ13C values of DIC become more negative.