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Coastal upwelling and a history of organic-rich mudstone deposition off Peru

Summary The present-day upwelling circulation off Peru, the regional pattern of organic matter in surface sediments and the stable carbon isotope characteristics of Neogene and Quaternary carbonate lithologies suggest a unique feedback mechanism in continental margin deposition and subsequent alteration after burial. In such a scenario, the high bioproductivity, the position of a poleward flowing undercurrent and the rate of subsidence of margin basins appear to be the principal variables controlling this mechanism. Transfer of organic matter from the sea surface to the sea floor is particularly efficient in the upwelling ecosystem off Peru. Preservation and burial are enhanced by high bulk sedimentation rates along the upper continental slope (between 11°–15°S) at depths where the subsurface current velocities decrease below those normally associated with the poleward flow. Burial and preservation are diminished, however, where shallow water depths promote continuous reworking of the bottom sediments by onshore flows and alongshore water movement (between 6°–10°S). The resulting sedimentary facies are distinctly different from each other in that the former process yields an organic-rich (> 5 wt % Corg) and the latter process yields a calcareous (> 15 wt % CaCo3) mud facies. The bulk sediment accumulation and individual component fluxes are estimated for both portions of the margin situated between 6° and 15°S latitude and lying in < 500 m of water depth. Furthermore, the chemical environment of organic-matter decomposition in the rapidly accumulating carbonate-poor facies is dominated by microbial fermentation and methanogenesis, whereas, the muds containing lesser amounts of organic matter are dominated by microbial sulphate reduction. These differences in facies composition persist throughout the subsequent stages of compaction and diagenesis. Most prominent among these is the formation of ‘organic’ dolomites with distinctly different isotopic signatures and mineral assemblages. The original upwelling facies (i.e., organic-rich muds or calcareous muds), the extent of reworking by subsurface currents, and the subsidence history of the margin basins may be inferred from these sedimentary signatures.