2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 2
Presentation Time: 1:45 PM


PATTERSON, William P., Dept. of Geological Sciences, Univ. of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2, Canada, KIRBY, Matthew E., Department of Geological Sciences, California State University, Fullerton, Fullerton, CA 92834, ANDERSON, William T., Earth Sciences Department, Florida Int'l Univ, 11200 S.W. 8th Street, Miami, FL 33199 and MULLINS, Henry T., Earth Sciences, Syracuse Univ, Syracuse, NY 13244, bill.patterson@usask.ca

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.