BORON IN SOIL CARBONATES: DEVELOPMENT AND INITIAL APPLICATION OF A HYBRID PALEO-CO2 PROXY (Invited Presentation)

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.