GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 209-4
Presentation Time: 8:55 AM


BREECKER, Daniel O.1, FRICKE, Henry2, MCNEECE, Colin J.1, MILLER, Nathan R.1, SCHALLER, Morgan F.3, CLYDE, William C.4 and JORDAN, Jacob S.1, (1)Department of Geological Sciences, the University of Texas at Austin, Austin, TX 78712, (2)Department of Geology, Colorado College, 14 East Cache La Poudre St, Colorado Springs, CO 80903, (3)Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, (4)Dept. of Earth Sciences, University of New Hampshire, Durham, NH 03824,

Carbonate nodules in paleosol successions offer an important terrestrial proxy of atmospheric pCO2. However, accuracy and precision of this proxy are limited by poorly quantified soil-respired CO2 concentrations (S). Differences in S among stacked paleosols should result in differences in pH of (paleo) soil solutions, which in turn should result in predictable differences in [B] and δ11B values of paleosol carbonates. We work backwards from the B chemistry of soil carbonates to constrain secular change in S, which can then be used in the soil carbonate CO2 proxy to calculate corresponding changes in atmospheric CO2. B sorption onto particle surfaces must be considered to determine pH from B in soil carbonates. Therefore, we speciate B with PHREEQC using the constant capacitance model and surface complexation constants from the literature. We find that the inclusion of complexation contracts the B speciation shift into a narrower pH range, suggesting that B in soil carbonates is a very sensitive pH proxy between 7.5 to 9. In order to apply the hybrid proxy, we assume that 1) observed secular shifts in paleosol carbonate [B] and δ11B values are pH-controlled, 2) calcite precipitates in thermodynamic equilibrium with the soil solution in the CO2- H2O-CaCO3-surface system, and 3) there is no B isotope fractionation between aqueous tetrahedral B and soil carbonate B. With these assumptions, observed secular changes in both [B] and δ11B values of soil carbonates can be related to possible ranges of secular changes in S. The intersection of constraints from [B] and δ11B gives a single estimate of secular change in S and thereby, using the soil carbonate CO2 proxy, corresponding change in atmospheric CO2.

We applied this hybrid proxy to a suite of samples from the Bighorn Basin of Wyoming that record an early Eocene hyperthermal event (ETM2). To measure the concentrations and δ11B values of B in the calcite component of soil carbonate nodules, we treated drilled powders with a volume of 0.33M acetic acid sufficient to dissolve 80% of the calcite and analyzed leachates by MC-ICP-MS and ICP-MS. Preliminary results suggest a 10x increase in S, which corresponds to a 6x increase in atmospheric CO2 during this time.