INGERSON LECTURE: MODERN SOIL SYSTEM CONSTRAINTS ON RECONSTRUCTING DEEP-TIME ATMOSPHERIC CO2 : A NEW VIEW OF PHANEROZOIC PCO2 (Invited Presentation)

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 Δ¹³C_carb-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 δ¹³C_carb and δ¹³C_SOM 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.