A comparison of closed- and open-system models for porewater pH and calcite-saturation state

Journal Article · Fri Jan 01 04:00:00 EST 1993 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(93)90434-X· OSTI ID:6708601

Boudreau, B P ^[1]; Canfield, D E ^[2]

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1. Dalhousie Univ., Halifax, Nova Scotia (Canada)
2. Georgia Inst. of Technology, Atlanta (United States)

The authors compare the theoretical predictions of a closed- and an open-system model for the evolution of pH and calcite-saturation state during the sequential oxidation of organic matter by O[sub 2], NO[sub 3][sup [minus]], and SO[sub 4][sup =]. The model calculations reveal that closed and open systems have qualitatively similar behavior with respect to pH and carbonate saturation, [Omega][sub c]. However, a quantitative comparison establishes that the closed-system model represents usually an upper limit on pH in the oxic zone of sediments, while it always sets a lower bound on pH in the zone of SO[sub 4][sup =] reduction. The changes in [Omega][sub c] in closed and open systems during oxic decay are more complex than those of pH. The closed system will present an upper limit on [Omega][sub c] when the initial pH is high (i.e., 8.0), but does not exhibit limiting behavior if the initial pH is low (i.e., 7.0). The closed-system model always places a lower limit on [Omega][sub c] during sulfate reduction. Both models predict that CaCO[sub 3] dissolution can buffer the potential pH fall due to oxic CO[sub 2] production; however, quantitative evaluation of this effect requires a diagenetic model with realistic dissolution kinetics. The authors' investigations also demonstrate that vigorous mono-sulfide precipitation from an FeOOH-iron source can mitigate the fall in pH normally associated with organic matter decay as this mineral-forming reaction consumes an important protolytic spcies, H[sub 2]S. On the other hand, the precipitation of CaCO[sub 3] has only a modest lowering effect on pH. 82 refs., 11 figs., 7 tabs.

• OSTI ID: 6708601
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 57:2; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
540310* -- Environment
Aquatic-- Basic Studies-- (1990-)
CALCITE
CARBON CYCLE
CARBONATE MINERALS
CHEMICAL REACTIONS
DECAY
DISSOLUTION
ELEMENTS
EVALUATION
KINETICS
MATHEMATICAL MODELS
MATTER
MINERALS
NITRATES
NITROGEN COMPOUNDS
NITROGEN CYCLE
NONMETALS
ORGANIC MATTER
OXIDATION
OXYGEN
OXYGEN COMPOUNDS
PH VALUE
PRECIPITATION
REDUCTION
SATURATION
SEDIMENTS
SEPARATION PROCESSES
SULFATES
SULFUR COMPOUNDS
SULFUR CYCLE