Use of [delta]13C values of soil organic matter found in speleothems as a new proxy for paleovegetation and interpreting paleoclimate
Elkins, Joe Travis
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Isotopic analysis of soil organic matter and of DOC in cave drip water indicates that ä13C of these substances reflect overlying vegetation. Soil and drip water samples were collected at three southern U.S. caves from regions with different plant cover. Thirteen soil samples were also collected along a transect across Texas and southeastern New Mexico in a transition from a C3-dominated to a C4 dominated landscape. The ä13C of DOC in drip waters are more negative than one would expect from regional trends in soil ä13C. They nonetheless preserve the trend toward greater ä13C in more C4-dominated regions. Thirty-two samples of 13 speleothems from a variety of environments yield ä13Cof organic matter that range from -25.3 to -7.3‰ vs. PDB and TOC contents ranging from 40 to 197 ppm. About half of the samples have ä13Corg greater than -19‰, which is compatible with an origin in dominantly C4 vegetation, but have ä13CCO3 less than -6‰, which would conventionally be interpreted as reflecting C3 vegetation. The most likely interpretations of the data are that either the speleothem organic matter and spelean calcite are of different ages and thus not derived from plants with the same photosynthetic pathway, or that previous geochemical models attributing certain spelean ä13CCO3 to certain types of vegetation did not take into account the large variability in spelean ä13CCO3 possible in nature. These results suggest that ä13C of organic matter in speleothems may be a better indicator of ancient vegetation than ä13C of spelean carbonate. Treatments to destroy organic matter traditionally used on carbonates prior to geochemical analysis have no statistical effect on the ä13C value of speleothem carbonate. The treatments examined in this study were oven roasting at 340° C, soaking in bleach, soaking in 30% H2O2, and soaking in deionized distilled water (DDW). Bleach seems to be effective in destroying organic matter in carbonate, but the other treatments concentrate organics, presumably as the result of dissolution of CaCO3. The fact that organic matter remains in speleothem calcite after treatment may be a result of low permeability of void spaces in which organic matter resides.