264-1 Unsteady sediment deposition plays a major role in the weakening of the anaerobic methane oxidation filter in coastal sediments

Abstract:

While it can be tempting to assume that stratigraphic geochemical variations record secular changes in seawater conditions, it is well known that changes in depositional environments can also drive variations in geochemical parameters without any major change in the marine realm. This does not mean that these geochemical records are not useful, just that they are telling a different story.  We present geomicrobiological data from a modern setting of enhanced organic matter burial – The Chesapeake Bay – where eutrophication and land-use change have led to increased anoxia and sedimentation rates. Deposition of ³⁴S-enriched pyrite at the sulfate methane transition zone occurs at two sites close to the predominant source of terrestrial material. Methane is effectively oxidized aerobically with sulfate at the site with more terrestrial organic matter and less sulfate. At the site with more sulfate and planktonic organic matter, demand for sulfate for organoclastic sulfate reduction reduces the efficiency of anaerobic methane oxidation coupled to sulfate reduction.  Other, less common pathways of anaerobic methane oxidation occur at the site, but they cannot keep up with methane production. The ingrowth of ³⁴S-enriched pyrite at the sulfate methane transition zone is similar between sites and occurs tens of centimeters below where sediment is being deposited today. While it may not be possible to discern if methane oxidation was efficient at ancient sulfate methane transition zones, this work documents that elevated anoxia and sedimentation rates can result in pyrite sulfur isotope values similar to (or perhaps exceeding) seawater sulfate sulfur isotope values.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18880

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