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AAPG Bulletin

Abstract


Volume: 30 (1946)

Issue: 4. (April)

First Page: 477

Last Page: 513

Title: Studies on Redox Potential of Marine Sediments

Author(s): Claude E. ZoBell (2)

Abstract:

The redox potential of sediments may be used advantageously in the study and interpretation of the morphology, general nature, and chemical processes in unconsolidated sediments. The redox potential is believed to have a pronounced effect on the diagenesis of sedimentary materials, including the conversion of organic matter into petroleum.

The modern concepts of oxidation, reduction, and redox potentials are discussed. The redox potential is measured in volts and is expressed as an Eh value, Eh being the e.m.f. of an oxidation-reduction system referred to a standard hydrogen half-cell. The Eh of a system is a quantitative expression of its oxidizing or reducing intensity. Redox potential may be defined as the electron-escaping tendency of a reversible oxidation-reduction system, and thus is an intensity factor. The reducing or oxidizing capacity of sediments is independent of the redox potential, although there is a relation between the intensity and capacity factors. The pH and temperature at which measurements are made influence the redox potential of sediment samples.

The Eh of sediments, like the pH, can be measured either colorimetrically or electrometrically. A description of the methods of measuring the Eh of sedimentary materials helps to clarify the concepts of redox potentials. Methods of measuring the capacity as well as the intensity factor are given.

Data on more than 1,000 samples of bottom deposits indicate that each type of sediment has its own characteristic Eh and pH. Eh values ranging from +0.350 to -0.500 volt have been observed in samples of recent sediments in which the pH ranged from 6.4 to 9.5. As a very general rule, the pH of sediments increases with core depth and the Eh decreases, or conditions become more alkaline and more reducing with core depth. The reducing capacity decreases with core depth.

Positive Eh values are generally characteristic of bottom deposits which are well oxygenated, those which consist of coarse sediments, or those which are poor in organic matter. Negative Eh values are characteristic of bottom deposits rich in organic matter and which consist largely of fine sediments. An abundance of readily decomposable organic matter appears to promote reducing conditions. In the presence of organic matter, bacteria and allied microorganisms create reducing conditions. Such conditions are maintained by certain organic compounds, ferrous iron, reduced manganese, hydrogen sulphide, and other inorganic constituents of sediments.

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