TY - CHAP
T1 - Relating Weathering Fronts for Acid Neutralization and Oxidation to pCO2 and pO2
AU - Brantley, S. L.
AU - Lebedeva, M.
AU - Bazilevskaya, E.
N1 - Funding Information:
EB and ML acknowledge support from the Department of Energy Office of Basic Sciences grant DE-FG02-05ER15675 to SLB. SLB also acknowledges support from the NASA Astrobiology Institute Cooperative Agreement NCC2-1057 to C. House. Conversations with M. Pavich, J.J. Braun, L. Kump, A. Maloof, V. Balashov, H. Buss, and Y. Godderis influenced this manuscript. Nathan Sheldon and Steve Driese are acknowledged for helpful reviews. Dick Holland also reviewed an early version of this chapter before his death. Holland's contributions to our understanding of Earth history, and especially to the topics in this chapter, were immense.
PY - 2013/11
Y1 - 2013/11
N2 - As mineral assemblages formed at depth reequilibrate at Earth's surface, reactions with O2 and CO2 are recorded as depletion profiles in regolith. We use models to explore reaction fronts in regolith developed on two lithologies exposed on ridgetops in the Virginia Piedmont (United States). The ratios, R0, of capacities to consume O2:CO2 differ between the Virginia diabase (R0=0.04) and granite (0.02) protoliths. However, the ratios, R, of actual consumption of O2 (by FeO oxidation) versus CO2 (by silicate weathering) recorded by regolith are identical (~0.02). Although soil gases were not measured, we propose that R=~0.02 because the ratio of soil pO2 to pCO2 (=R') equals ~0.02 for both sites. For Fe-rich diabase, however, R'0 and CO2 is depleted deeper than O2, allowing ferrous iron to be lost from regolith. In contrast, R'>R0 for Fe-poor granite and O2 is consumed deeper than CO2, causing iron to be retained. In the granite, the volume constraints during oxidation promote fracturing. In turn, fracturing promotes advection and development of thick regolith. Indeed, modern regolith is generally thicker on granites than diabase, as expected if the condition, 0.022/pCO2<~0.02, thick regolith was also not likely.
AB - As mineral assemblages formed at depth reequilibrate at Earth's surface, reactions with O2 and CO2 are recorded as depletion profiles in regolith. We use models to explore reaction fronts in regolith developed on two lithologies exposed on ridgetops in the Virginia Piedmont (United States). The ratios, R0, of capacities to consume O2:CO2 differ between the Virginia diabase (R0=0.04) and granite (0.02) protoliths. However, the ratios, R, of actual consumption of O2 (by FeO oxidation) versus CO2 (by silicate weathering) recorded by regolith are identical (~0.02). Although soil gases were not measured, we propose that R=~0.02 because the ratio of soil pO2 to pCO2 (=R') equals ~0.02 for both sites. For Fe-rich diabase, however, R'0 and CO2 is depleted deeper than O2, allowing ferrous iron to be lost from regolith. In contrast, R'>R0 for Fe-poor granite and O2 is consumed deeper than CO2, causing iron to be retained. In the granite, the volume constraints during oxidation promote fracturing. In turn, fracturing promotes advection and development of thick regolith. Indeed, modern regolith is generally thicker on granites than diabase, as expected if the condition, 0.022/pCO2<~0.02, thick regolith was also not likely.
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U2 - 10.1016/B978-0-08-095975-7.01317-6
DO - 10.1016/B978-0-08-095975-7.01317-6
M3 - Chapter
AN - SCOPUS:84903808486
SN - 9780080983004
VL - 6
SP - 327
EP - 352
BT - The Atmosphere - History
PB - Elsevier Inc.
ER -