d13C VALUE OF SOIL ORGANIC MATTER IN SPELEOTHEMS: A NEW PROXY TO DETERMINE PALEOVEGETATION AND INTERPRET PALEOCLIMATE

Factors such as kinetic fractionation, insufficient residence time of meteoric water with soil CO₂, and the contribution of past dead carbon from aging soil organic matter and its effect on increased bedrock dissolution cause the d¹³C values of calcite to be suspect when using those values to interpret paleovegetation. Recent data by Elkins and Railsback (2001) from karts terrains in the southern U.S. show that the d¹³C value of humic substances in present day soils and drip water fall within the values expected of the vegetation in modern landscapes and that the d¹³C values of organic matter in groundwater (drip water) better reflects modern vegetation than does the organic matter in soil. Our recent research shows that d¹³C values of soil organic matter in speleothems may be a more reliable indicator of paleovegetation than the d¹³C of spelean carbonate and has resulted in the development of a new technique to obtain the d¹³C values of humic substances in speleothems. We are pleased to be the first to share such values. In 50% of the 33 speleothem samples analyzed, the d¹³C values of the calcite did not indicate the same proportion of C3 v. C4 and CAM photosynthetic pathway plants growing above the cave at the time of crystallization as the d¹³C values of the humic substances within the same carbonates. Furthermore, in 30% of the speleothem samples analyzed, the d¹³C values of the calcite indicate the opposite type of vegetation indicated by the d¹³C values of the humic substances. Organic substances have also previously been shown to have an effect on the d¹³C values of carbonates. This research also shows that treatments conventionally used to remove organic matter from other types of carbonates have variable effects on the removal of organic material from speleothems and that after treatment, there was no demonstrable effect on the d¹³C values of spelean carbonate.