Paper No. 7
Presentation Time: 9:50 AM
INGERSON LECTURE: MODERN SOIL SYSTEM CONSTRAINTS ON RECONSTRUCTING DEEP-TIME ATMOSPHERIC CO2 : A NEW VIEW OF PHANEROZOIC PCO2
Paleosol carbonate-based estimates of paleo-atmospheric CO2 play a prominent role in constraining radiative-forcing and climate sensitivity in the deep-time. Large uncertainty in mineral-based paleo-CO2 estimates reflects their sensitivity to soil-respired CO2 (Sz), a poorly constrained parameter due to a paucity of soil CO2 measurements during carbonate formation in modern soils and a lack of widely applicable proxies of paleo-soil CO2. In this study, the δ13C values of carbonate and soil organic matter pairs from a literature compilation of 230 Holocene soil horizons are applied to a two-component CO2-mixing equation in order to define soil order-specific ranges of soil CO2 applicable for constraining Sz in their corresponding paleosol analogues. Equilibrium carbonate-soil organic matter (SOM) pairs, characterized by Δ13Ccarb-SOM values of 12 to 16‰ and most likely to record open system two-component CO2 mixing, indicate total soil CO2 contents during calcite formation of ~500 to 10,000 ppmv with lower values (<500 to 2500 ppmv) for soils characterized by a net-moisture deficit. Overall higher values (2000 to 5000 ppmv) are indicated for soils characterized by higher moisture content and productivity. Vertisols define the largest range in total soil CO2 (<1000 to >25,000 ppmv) reflecting their seasonally driven dynamic hydrochemistry. A large subset of non-equilibrium pairs with Δ13C values exceeding 16‰ support the recently proposed hypothesis that pedogenic carbonate precipitation occurs during periods of low productivity in a soil atmosphere with a large component of atmospheric CO2. The results further indicate that measured δ13Ccarb and δ13CSOM in paleosols can be used to refine the proposed ranges of S(z) for study-specific reconstruction of paleo-atmospheric pCO2.The resulting soil CO2 estimates are compared with other recently proposed proxies of soil CO2 (mean annual precipitation and depth to the calcic nodular zone), and are utilized to recalibrate the Phanerozoic record of paleo-CO2 developed by previous studies using the pedogenic carbonate CO2-paleobarometer.