ASSESSING THE VIABILITY OF PEDOGENIC GIBBSITE AS A PROXY FOR PALEO-PCO2

Depth trends in the stable carbon and oxygen isotope ratios of carbonate occluded in gibbsite from the Georgia Piedmont indicate that the simple one-dimensional Fickian diffusion model used in calculating PCO₂ may not be appropriate for this mineral. Paleo-PCO₂ proxies used to determine Earth’s atmospheric history are key to constraining Phanerozoic geochemical models such as GEOCARBSULF (Berner, 2006). δ¹³C trends preserved in paleosol carbonates and oxy-hydroxides have been the most exploited proxies for estimates of past atmospheric CO₂ concentrations, which often return values that trend positively with model results. In an effort to further test the assumptions that are implicit in the determination of paleo-PCO₂ from oxy-hydroxides we have explored the systematics of a modern/relict soil environment where mineralization and capture of the δ¹³C trend is being recorded in pedogenic gibbsite. The trend shows that the δ¹³C value of CO₂ deep in the profile (>100 cm) is consistant with that expected for the C3 vegetation that is known to have persevered at the site for centuries (i.e., -24 ‰). Near the surface (<40cm), where active biological and hydrological processes serve to mix the soil, there is no discernible trend with depth. The δ¹³C values maximize to -14 ‰ at 40cm but do not follow the expected Fickian mixing line between the surface and 50cm. These results are contrary to previously published data and necessitate that care must be used when attempting to use pedogenic gibbsite as a proxy for paleo-PCO₂. We recommend that the use of this proxy be limited to samples that can be identified to be deep in the soil profile to avoid the complications that are introduced in the near surface.

Berner, R.A., 2006. GEOCARBSULF: A combined model for Phanerozoic atmospheric O₂ and CO₂ over Phanerozoic time. Geochemica et Cosmochimica Acta, 70, 5653-5664.