HR: 1330h
AN: PP22A-1199    [PDF]
TI: A model for carbon and oxygen isotope co-variation in closed lakes based on study of Lake Junin, Peru
AU: * Burns, S J
EM: sburns@geo.umass.edu
AF: Department of Geosciences University of Massachusetts, Morrill Science Center, Amherst, MA 01003 United States
AU: Seltzer, G
EM: goseltze@syr.edu
AF: Dept. of Earth Sciences, Syracuse University, Syracuse, NY 13244 United States
AU: Rodbell, D
EM: rodbelld@union.edu
AF: Geology Department, Union College, Schenectady, NY 12308 United States
AB: In closed lake systems C and O stable isotope ratios often co-vary, although it is seldom clear why. Based on results from Lake Junin, Peru ($11\deg$ S, 4100 masl), we have developed a simple C mass balance model that treats the lake as C-limited by low influx of dissolved bicarbonate. The model can explain both the highly enriched C isotope values of the sediments and the strong correlation between O and C isotopes, and is a good general explanation for co-variation in C and O other closed lakes. The sediment record from Lake Junin, Peru, provides an unusual opportunity to reconstruct tropical climatic variability through the analysis of authigenic lacustrine carbonates. Carbonate-rich sediments have been accumulating rapidly in the lake since shortly after deglaciation of the region (Seltzer et al., 2000). We have made a several hundred measurements of O and C stable isotopes of both authigenic calcite and ostracods from the lake. During the Late Glacial, C isotope values are around +8 to + 9 $\permil$, and at around 12 ka increase to between + 13 to + 15 $\permil$ until around 8 ka. From 8 ka to about 3 ky BP, C isotope values decreases to a low of +3 $\permil$, then increases again to around + 8 $\permil$. Throughout the core, the C isotope values very closely track oxygen isotope values (r$^{2}$ = 0.9). Carbon isotope ratios of lacustrine carbonates are often interpreted in terms of changes in lake productivity, yet organic C concentrations and accumulation rates in lake Junin are relatively constant and are not correlated to carbonate C isotope values. A C isotope mass balance model for the lake suggests that the C isotope values are instead controlled by the flux of bicarbonate to the lake from surface inflow, which controls the degree of drawdown of dissolved CO$_{2}$. The model treats the dissolved CO$_{2}$ as susceptible to isotopic enrichment through a Rayleigh fractionation process as the light isotope is lost to organic matter. Variation in the isotopic ratio of dissolved carbon is driven not by changes in organic C burial, however, but by variation in the rate of resupply of bicarbonate. Wetter climate conditions result in both lower O isotope ratios because a smaller fraction of water lost is to evaporation, and lower C isotope ratios because of an increased flux of dissolved inorganic C.
DE: 1620 Climate dynamics (3309)
DE: 1699 General or miscellaneous
DE: 1845 Limnology
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting