T315 Deciphering Climate Change and Climate Variability in the Geologic Record Using Palaeosols, Loess, Palaeoweathering Profiles, Pedosedimentary Sequences, and Polygenetic Soils: Their Applications and Limitations as Terrestrial Proxies in Understanding Impacts of Climatic Changes

Wednesday, 25 October 2017
8:00 AM - 12:00 PM
Skagit 3

REVISION OF THE PALEOSOL PCO2 PROXY THROUGH DIRECT MEASUREMENT OF SOIL CARBONATE-HOSTED FLUID INCLUSIONS

Paper number: T315-11
Program: Technical Sessions
Time: 10:35 AM – 10:50 AM

Authors

Abstract

The paleosol diffusion model is a commonly used atmospheric pCO2 proxy due to the abundance and high frequency of paleosols in the terrestrial rock record. The model is based on equilibrium mixing between atmospheric CO2 and soil-respired CO2, which is tracked by means of carbon isotope mass balance on paleosol carbon phases. However, the concentration of CO2 in soil is a crucial parameter that cannot be directly measured on paleosols, and represents the single largest source of inaccuracy in the model. Using a modern vertisol from Dallas, TX, we show that soil carbonate-hosted fluid inclusions retain geochemical signatures that are most likely indicative of soil conditions at the time of carbonate precipitation. Analysis of CO2 from fluid inclusions permits the direct measurement of two commonly assumed components of the paleosol diffusion model: 1) the carbon isotopic composition of soil CO2; and 2) total soil CO2 concentration, both of which have a strong soil depth dependency. We develop a new methodology to make high precision isotope ratio and concentration measurements on small amounts of CO2 released from pedogenic carbonate inclusions by online crushing into both an isotope ratio and a quadrupole mass spectrometer. When the full paleosol gas diffusion model is applied to these new soil-CO2 constraints from a detailed depth transect in our test soil, it predicts near modern atmospheric CO2 concentrations. While a more rigorous modern validation of the proxy using contemporaneous soil gas measurements is still in development, we apply the technique to Late Triassic age soil carbonates as a proof of concept. The good agreement between our new method and modern atmospheric conditions along with promising results from paleosol carbonates demonstrates that our fluid-inclusion approach results in a substantial revision of both the accuracy and precision of the paleosol pCO2 proxy.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306657

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