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How could a freshwater swamp produce a chemical signature characteristic of a saltmarsh?

February 1, 2018

Reduction–oxidation (redox) reaction conditions, which are of great importance for the soil chemistry of coastal marshes, can be temporally dynamic. We present a transect of cores from northwest Florida wherein radical postdepositional changes in the redox regime has created atypical geochemical profiles at the bottom of the sedimentary column. The stratigraphy is consistent along the transect, consisting of, from the bottom upward, carbonate bedrock, a gray clay, an organic mud section, a dense clay layer, and an upper organic mud unit representing the current saltwater marsh. However, the geochemical signature of the lower organic mud unit suggests pervasive redox reactions, although the interval has been identified as representing a freshwater marsh, an unlikely environment for such conditions. Analyses indicate that this discrepancy results from postdepositional diagenesis driven by millennial-scale environmental parameters. Rising sea level that led to the deposition of the capping clay layer, created anaerobic conditions in the freshwater swamp interval, and isolated it hydrologically from the rest of the sediment column. The subsequent infiltration of marine water into this organic material led to sulfate reduction, the buildup of H2S and FeS, and anoxic conditions. Continued sulfidation eventually resulted in euxinic conditions, as evidenced by elevated levels of Fe, S, and especially Mo, the diagnostic marker of euxinia. Because this chemical transformation occurred long after the original deposition the geochemical signature does not reflect soil chemistry at the time of deposition and cannot be used to infer syn-depositional environmental conditions, emphasizing the importance of recognizing diagenetic processes in paleoenvironmental studies.

Citation Information

Publication Year 2018
Title How could a freshwater swamp produce a chemical signature characteristic of a saltmarsh?
DOI 10.1021/acsearthspacechem.7b00098
Authors Terrence A. McCloskey, Christopher G. Smith, Kam-biu Liu, Marci E. Marot, Christian Haller
Publication Type Article
Publication Subtype Journal Article
Series Title ACS Earth and Space Chemistry
Index ID 70194959
Record Source USGS Publications Warehouse
USGS Organization St. Petersburg Coastal and Marine Science Center