A NEW MODEL FOR THE INTERPRETATION OF CARBON ISOTOPE RECORDS OF LACUSTRINE CALCITE: INFLUENCE OF SEASONAL PRECIPITATION AND HUMIDITY

Changes in d¹³C_(CaCO3) values are generally attributed to variation in d¹³C_(DIC) whereby increased aquatic productivity partitions a greater proportion of ¹²C_(DIC) into organic matter that in turn increases d¹³C_(DIC) values. Productivity within the water column is in turn proportional to the availability of light and nutrients. A comparison of orbitally-forced variation in mid latitude insolation with d¹³C_(CaCO3) values through the Holocene reveals a negative correlation between summer insolation and d¹³C_(CaCO3) values. Additionally, d¹³C_(CaCO3) values do not display any relationship with % organic matter. We propose a new intermediate mechanism in which d¹³C_(CaCO3) values are ultimately a function of d¹³C_(DIC) that is in turn driven by respiration, productivity, and input of terrestrial organic matter. We propose that variation in d¹³C_(Organic) values of terrestrial organic matter supplied to soil in the Midwestern US and the lake carbon budget control d¹³C_(DIC) values, ultimately to be recorded as d¹³C_(CaCO3) values in lake sediment. A strong negative relationship between soil moisture/precipitation and d¹³C_(Cellulose) values has been reported for trees (trunk wood, twigs, leaves, and sap). When trees exposed to low soil moisture lose twigs, branches and leaves, the decay of that organic matter should result in an increase of the soil carbon d¹³C_(Organic) value which will ultimately enter the lake with groundwater and surface runoff. Likewise a shift in flora to species resistant to low and/or seasonally variable soil moisture (C₄ pathway) would result in an increase in d¹³C_(Organic) values of soil organic matter because C₄ plants have much higher d¹³C_(Organic) values. The resolution of our observed variability in d¹³C_(CaCO3) values is too high to suggest a shift to C₄ vegetation (which may take several hundred years). We therefore propose that a decrease in summer moisture forces an increase in terrestrial organic matter d¹³C_(Organic) values and ultimately d¹³C_(CaCO3) values. To evaluate the relationship between summer precipitation and the d¹³C_(CaCO3) value of lake sediment we present 20th century carbonate and tree cellulose data from several central New York lakes that demonstrate a strong relationship between summer moisture and lacustrine d¹³C_(CaCO3) values.