EFFECTS OF DIFFERENT ORGANIC-MATTER SOURCES ON ESTIMATES OF ATMOSPHERIC AND SOIL pCO(2) USING PEDOGENIC CARBONATE

摘要

Carbon isotope analysis of paleosol carbonates is one of the most widely used methods for producing quantitative estimates of CO2 levels in the ancient atmosphere, and is increasingly used to estimate soil pCO(2) as a proxy for primary productivity. Recent efforts to improve the carbonate CO2 paleobarometer by refining input parameters (e.g., soil temperature, soil CO2 production function) have yielded more accurate estimates of ancient atmospheric pCO(2). The carbonate CO2 paleobarometer is especially sensitive to input values for the carbon-isotope composition of soil organic matter (OM), which should ideally reflect the delta C-13 value of OM present when pedogenic carbonates were forming. Published soil pCO(2) estimates derived from pedogenic carbonate in Upper Jurassic paleosols are re-evaluated here using OM occluded within pedogenic carbonates rather than average values of fossilized plant material collected throughout the sampled stratigraphic sections. The new soil pCO(2) estimates calculated using occluded OM range from 4,600 to 20,000 ppmV and are much lower than the previously published estimates, which were often in excess of 60,000 ppmV. In order to determine which OM source provides more accurate results, estimates of atmospheric pCO(2) obtained using plant material and occluded OM from a carbonate-bearing modern soil are compared with measured, pre-industrial atmospheric CO2 levels. In the modern soil profile, plant OM delta C-13 is highly variable and slightly more negative than the delta C-13 of occluded OM. The observed similar to 1% offset between the average delta C-13 values of plant material and occluded OM is much less than the overall range of similar to 6% in plant OM. Estimates of atmospheric pCO(2) from the modern soil that are calculated using occluded OM differ by less than 100 ppmV, on average, from estimates based on plant OM. These results suggest that occluded OM produces reasonably accurate pCO(2) estimates when used with the carbonate CO2 paleobarometer. Applying a similar to 1% correction to the delta C-13 of occluded OM also produces accurate pCO(2) estimates, but the extreme variability in delta C-13 of plant matter leads to inaccurate pCO(2) estimates, even when samples are averaged.