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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research
Vol. 86 (2016), No. 7. (July), Pages 800-812
Research Articles

Effects of Different Organic-Matter Sources On Estimates of Atmospheric and Previous HitSoilNext Hit pCO2 Using Pedogenic Carbonate

Timothy S. Myers, Neil J. Tabor, Louis L. Jacobs, Robert Bussert

Abstract

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 Previous HitsoilNext Hit pCO2 as a proxy for primary productivity. Recent efforts to improve the carbonate CO2 paleobarometer by refining input parameters (e.g., Previous HitsoilNext Hit temperature, Previous HitsoilNext Hit CO2 production function) have yielded more accurate estimates of ancient atmospheric pCO2. The carbonate CO2 paleobarometer is especially sensitive to input values for the carbon-isotope composition of Previous HitsoilNext Hit organic matter (OM), which should ideally reflect the δ13C value of OM present when pedogenic carbonates were forming. Published Previous HitsoilNext Hit pCO2 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 Previous HitsoilNext Hit pCO2 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 pCO2 obtained using plant material and occluded OM from a carbonate-bearing modern Previous HitsoilNext Hit are compared with measured, pre-industrial atmospheric CO2 levels. In the modern Previous HitsoilNext Hit profile, plant OM δ13C is highly variable and slightly more negative than the δ13C of occluded OM. The observed ∼ 1‰ offset between the average δ13C values of plant material and occluded OM is much less than the overall range of ∼ 6‰ in plant OM. Estimates of atmospheric pCO2 from the modern Previous HitsoilTop 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 pCO2 estimates when used with the carbonate CO2 paleobarometer. Applying a −1‰ correction to the δ13C of occluded OM also produces accurate pCO2 estimates, but the extreme variability in δ13C of plant matter leads to inaccurate pCO2 estimates, even when samples are averaged.


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