DIAGENESIS AND REMOBILIZATION OF CARBON AND SULFUR IN MIDPLEISTOCENE ORGANIC-RICH FRESH-WATER SEDIMENT

Analyses of carbon and sulfur concentrations, speciation of sulfur, su lfur isotopic data, and magnetic remanence properties are used to inve stigate the diagenetic history of carbon and sulfur in a Pleistocene o rganic-rich freshwater sediment, the West Runton Freshwater Bed (Norfo lk, UK), Although this unit is of freshwater origin on paleontological grounds, bulk C/S ratios for the sediment are low (2 to 7), indicativ e of diagenesis in a marine environment, This apparent contradiction a rises from two factors, Firstly, much of the S is organically bound an d bulk C/S does not reflect an environment where pyrite is the dominan t sink for diagenetic sulfide (the basis for discrimination of marine and freshwater environments by the C/S method), Secondly, post-diagene tic remobilization of S has occurred in the bed during a phase of oxic groundwater weathering, Sulfur has been leached from the upper part o f 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 unaff ected 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 CIS ratios are not a reliable diagnos tic tool for organic-rich freshwater sediments, since significant orga nic-S concentrations can be present; pyritic-S must be determined spec ifically in organic-rich sediment to give adequate distinction of depo sitional environment, More worrying is the apparent ease with which S may be redistributed within a bed, obscuring original depositional sig natures, Whilst this is apparent when the sequence is examined in deta il, samples collected with less care or where less control is feasible (e.g., drill chips or sidewall cores) may yield erroneous data.