Near-surface rates of microbiological sulfate reduction in most sediments often exceed rates of iron release, which consequently limit the precipitation of early diagenetic iron sulfides. Excess hydrogen sulfide produced by sulfate reduction generally escapes from the sediment before it can be fixed as iron sulfide; however, the efficiency of hydrogen sulfide fixation may be significantly increased by the presence of metastable, easily dissolved iron-bearing phases, such as volcanic glass. Sediments that efficiently trap hydrogen sulfide may have lower ratios of carbon to sulfur than those typically found in normal marine environments. The abundance of volcanic ash in sediments related to the closing of the proto-Atlantic may account for the low ratios of carbon to pyritic sulfur in lower Paleozoic sediments from Europe and the United States compared to equivalent sediments from Australia.
The increase in the efficiency of trapping of hydrogen sulfide does not affect the amount of organic matter oxidized or sulfate reduced. It does, however, decrease the amount of sulfide that escapes the sediment and is reoxidized to sulfate. The net effect is a decrease in oceanic sulfate and an increase in atmospheric oxygen.