Diagenesis and remobilization of carbon and sulfur in mid-Pleistocene organic-rich freshwater sediment

Abstract

Analyses of carbon and sulfur concentrations, speciation of sulfur, sulfur isotopic data, and magnetic remanence properties are used to investigate the diagenetic history of carbon and sulfur in a Pleistocene organic-rich freshwater sediment, the West Runton Freshwater Bed (Norfolk, UK). Although this unit is of freshwater origin on paleontological grounds, bulk C/S ratios for the sediment are low (2 to 7), indicative of diagenesis in a marine environment. This apparent contradiction arises from two factors. Firstly, much of the S is organically bound and bulk C/S does not reflect an environment where pyrite is the dominant sink for diagenetic sulfide (the basis for discrimination of marine and freshwater environments by the C/S method). Secondly, post-diagenetic remobilization of S has occurred in the bed during a phase of oxic groundwater weathering. Sulfur has been leached from the upper part of the unit and is fixed as: (1) ferrimagnetic greigite; (2) pyritic-S; and (3) organic-S lower in the section where reducing conditions have been maintained. The lowest parts of the unit appear relatively unaffected by this process; C/pyritic-S here is indicative of the original freshwater depositional environment, and early diagenetic greigite is preserved. We conclude that bulk C/S ratios are not a reliable diagnostic tool for organic-rich freshwater sediments, since significant organic-S concentrations can be present; pyritic-S must be determined specifically in organic-rich sediment to give adequate distinction of depositional environment. More worrying is the apparent ease with which S may be redistributed within a bed, obscuring original depositional signatures. Whilst this is apparent when the sequence is examined in detail, samples collected with less care or where less control is feasible (e.g., drill chips or sidewall cores) may yield erroneous data.