SGER - Carbon cycle processes affected by ENSO and transitional SOI conditions: Development of a new geologic tool for using speleothems as a proxy for carbon cycle dynamics

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ABSTRACTSGER - Carbon cycle processes affected by ENSO and transitional SOI conditions:Development of a new geologic tool for using speleothems as a proxy for carbon cycledynamics.Using high-resolution microsampling techniques for stable isotope analysis of speleothems from Belize, we found large fluctuations in stalagmite d13C values at inter-annual to sub-seasonal time scales. These results reveal a remarkably strong correlation between d13C values and recent El NiNo events. Discovery of this relation is puzzling in light of the absence of significant weather anomalies during El NiNo and La NiNa intervals for this area. d13C values of this stalagmite likely reflect changes in the overlying forest ecosystem carbon budget that are sensitive to subtle modifications of local weather related to ENSO. Preliminary work suggests that the stalagmite d13C variations reflect changes in important ecosystem processes such as soil pCO2 and/or soil-atmospheric CO2 flux. As such, speleothems may be used as a tool for exploring carbon cycle dynamics and their response to environmental conditions.Our discovery of the correlation of SOI and the geologic record of C isotopes raises anumber of questions. Does this record reveal a new source of interannual variability in the global carbon cycle due to ENSO through its effects on terrestrial biogeochemical systems? Does El NiNo produce a stress on the ecosystem that is reflected in isotopic fractionation? Is the source of the fractionation difference in the vegetation, in the soil, or in the groundwater? What process(es) within the ecosystem serve to amplify the magnitude of ENSO-related environmental changes and leads to their being transmitted by groundwater and recorded in stalagmites? Consequently, the specific objectives of this SGER project are to capture carbon isotopic composition of vegetation, litter, soil gas/moisture, ground water, and cave drip water during a neutral phase of SOI, to complement the samples already collected during the recent El NiNo. Compare the carbon isotopic analyses of El NiNo and transitional samples to constrain location of differences in isotopic fractionation (e.g. vegetation, soil respiration, etc.). This SGER project will serve to capture the brief transition between El NiNo and La NiNa conditions that is expected to occur during the summer of 2003. As such it is time-sensitive, as the next transition through neutral conditions will not occur for several years. We will collect samples from ambient air, vegetation, leaf litter, soil, soil gas and water, groundwater, rainwater, cave air, and drip water. U.S. import permits have been obtained as necessary for soil and vegetation samples. Results from this project will provide a neutral baseline that will enable us to explore how the cave carbon biogeochemical system responds to ENSO events and identify the biogeochemical mechanisms responsible for the ENSO signal preserved in the geologic record. Ultimately this proposed project will provide the basis for new insights into the relationships between terrestrial ecosystems, weather, the carbon cycle, and the geological record.