TY - JOUR
T1 - Feedbacks among O2 and CO2 in deep soil gas, oxidation of ferrous minerals, and fractures
T2 - A hypothesis for steady-state regolith thickness
AU - Kim, Hyojin
AU - Stinchcomb, Gary
AU - Brantley, Susan L.
N1 - Funding Information:
We would like to thank two anonymous reviewers and Dr. George Hilley for constructive comments. This study is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Award Number DE-FG02-05ER15675. We would like to thank the Fairfax County Park Authority of Virginia for all the help conducting our study in the county's forests. We are also grateful to Jacques R. Courtillet for providing field support. We thank Xin Gu and Laura Liermann for assisting us to analyze the soil carbon content. SLB acknowledges conversations over many years with M. Pavich.
Publisher Copyright:
© 2016 Elsevier B.V.
PY - 2017/2/15
Y1 - 2017/2/15
N2 - O2 and CO2, the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops—one on granite and two on diabase—in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO2 and pCO2 measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO2 versus pCO2 on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O2 to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO2 and weathering-induced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO2, a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.
AB - O2 and CO2, the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops—one on granite and two on diabase—in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO2 and pCO2 measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO2 versus pCO2 on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O2 to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO2 and weathering-induced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO2, a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.
KW - Fe redox cycling
KW - mafic vs. felsic bedrocks
KW - regolith development
KW - soil pO and pCO
KW - weathering-induced fracturing
UR - http://www.scopus.com/inward/record.url?scp=85006377757&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2016.12.003
DO - 10.1016/j.epsl.2016.12.003
M3 - Article
AN - SCOPUS:85006377757
SN - 0012-821X
VL - 460
SP - 29
EP - 40
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
ER -