Geological Society of America

T199 High-Resolution Terrestrial and Marine Geochemical Proxies of Global Change: Progress, Problems, and Utility II

Tuesday, 20 October 2009
1:30 PM - 5:30 PM
Portland Ballrooms 251/258

UNDERSTANDING SPELEOTHEM δ13C VARIABILITY USING 14C AT MOANING CAVE, CALIFORNIA

Paper number: T199-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Jessica Oster Presenting Author
University of California, Davis, Davis, CA
Isabel Montanez
University of California, Davis, CA
Warren D. Sharp
Berkeley, CA
Thomas P. Guilderson
Lawrence Livermore National Laboratory, LIvermore, CA

Abstract

The majority of speleothem paleoclimate studies focus on δ¹⁸O records as proxies of variability in the isotopic composition of rainfall. However, many other proxies (trace elements, Sr isotopes, growth rates) can shed light on climate variability. While multi-proxy studies allow the investigation of a range of environmental processes, including water-soil-rock interactions and shifts in vegetation amount and type, they can also highlight the complexity and variability of cave systems. For example, speleothem δ¹³C can respond to a number of complex processes that are both directly and indirectly related to climate.

Here we develop use of the dead carbon proportion (dcp), estimated from coupled ¹⁴C and ²³⁰Th/U measurements of a stalagmite taken from Moaning Cave, central Sierra Nevada foothills, CA and independently calibrated records of atmospheric radiocarbon content. The dead carbon proportion provides an additional constraint on how water-rock interaction, degassing and prior calcite precipitation have affected the stalagmite δ¹³C record, and by extension, the trace element and ⁸⁷Sr/⁸⁶Sr records. This, in turn, allows us to constrain the degree to which shifts in Moaning Cave stalagmite δ¹³C compositions record external environmental changes such as variation in climate-driven vegetation density and soil productivity above the cave. Our results demonstrate that degassing and prior calcite precipitation can exert a significant control on stalagmite δ¹³C values, as up to 90% of the HCO₃^- reservoir can be lost before precipitation of stalagmite calcite. This work has implications for the interpretation of speleothem proxy records, especially from caves in arid and semi-arid environments.