Title: Feedbacks among O₂ and CO₂ in deep soil gas, oxidation of ferrous minerals, and fractures: A hypothesis for steady-state regolith thickness

Abstract

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 partly 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 causemore » 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.« less

Authors:

Kim, Hyojin ^[1]; Stinchcomb, Gary  ^[2];

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• Search DOE PAGES for ORCID "0000-0003-3220-7762"

Brantley, Susan L.  ^[1]

• Search DOE PAGES for author "Brantley, Susan L."
• Search DOE PAGES for ORCID "0000-0002-9574-2693"

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1. Pennsylvania State Univ., University Park, PA (United States)
2. Murray State Univ., Murray, KY (United States)

Publication Date:

Mon Dec 19 00:00:00 EST 2016

Research Org.:

Pennsylvania State Univ., University Park, PA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences, and Biosciences Division

OSTI Identifier:

1607344

Alternate Identifier(s):

OSTI ID: 1396578

Grant/Contract Number:  

FG02-05ER15675

Resource Type:

Accepted Manuscript

Journal Name:

Earth and Planetary Science Letters

Additional Journal Information:

Journal Volume: 460; Journal Issue: C; Journal ID: ISSN 0012-821X

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; 54 ENVIRONMENTAL SCIENCES