Sedimentary carbon, sulfur, and iron relationships in modern and ancient diagenetic environments of the Eel River Basin (USA)

Depositional and diagenetic controls on the distributions of carbon, sulfur , and iron (C-S-Fe) in modern sediments and upper Pleistocene mudrocks of t he Eel River Basin (ERB), northern California continental margin, mere inve stigated using a combination of geochemical, radioisotopic, and sedimentolo gical methods. A mass balance based on down-core profiles of porewater and solid-phase constituents and diagenetic modeling suggests that only 12-30% of the pyrite-S produced via SO4-2 reduction during burial is retained in m odern shelf and upper slope deposits of the ERB, Bioturbational reoxidation of initially reduced S is inferred to be the major control on S preservati on, on the basis of an observed inverse relationship between pyrite-S reten tion and biological mixing intensity, estimated from profiles of excess Th- 234. Importantly, these findings argue that massive depositional episodes o n the shelf following hoods of the Eel River have a negligible long-term im pact on bioturbating macrofauna and the potential to affect geochemical pro perties of the sediments. Down-core profiles of reactive Fe3+ and Py-Fe(II) for the modern deposits suggest that highly reactive Fe phases are sulfidi zed well within similar to 500-2000 years of burial, thereby limiting later pyritization, which could occur through sulfidation of less reactive phase s. This result explains the low (less than or equal to 0.4) degree of pyrit ization (DOP) values exhibited by both modern and ancient deposits of the E RB and lends support to the notion that pyritization in aerobic continental -margin sediments is largely associated with highly reactive detrital Fe ox ides. Comparable mean C/S weight ratios for modern sediments (5.4 +/- 3.3, 1 sigma) and mudrocks (6.9 +/- 4.5) of the ERB suggest that the upper Pleis tocene strata reflect a geochemical environment analogous to that of the mo dern margin. Specifically, the CS-Pe signatures shared be the modern and an cient deposits are a consequence of similar detrital Fe mineralogies, initi al organic matter content (C-org less than or equal to 1%) and composition (C/N = 13 to 17, delta C-13(org) = -22 to -25 parts per thousand), burial r ate, and importantly, bioturbation intensity, The findings of this study ha ve important implications for the use of C-S-Fe signatures as indicators of diagenetic processes in dynamic, continental-margin environments.