Geochemistry of pelagic and hemipelagic carbonates: criteria for identifying systems tracts and sea-level change

The elemental (Si, Ti, Al, Mn, Ca, Zr) and carbon stable-isotope (delta C-13) geochemistry of a biostratigraphically well-constrained Cenomanian-Turonian (Upper Cretaceous) Chalk succession on the Isle of Wight, southern England, shows systematic variation that corresponds closely to a published sequence stratigraphic model for the Cenomanian. Six sequences and their constituent systems tracts, defined elsewhere using sedimentological criteria, are clearly distinguishable from bulk-sediment elemental profiles, and an additional Upper Cenomanian sequence previously identified in Spain is recognized in England from these geochemical data. The manganese curve is particularly instructive, exhibiting minima around sequence boundaries and through lowstands, rising values from the transgressive surfaces through transgressive systems tracts, maxima around maximum flooding surfaces, and declining values through highstands. Silica and trace-element (Ti, Zr) aluminium ratios peak around transgressive surfaces and maximum flooding surfaces, indicating pulses of increased siliciclastic input. Positive delta C-13 excursions are confirmed at the base of the Middle Cenomanian and spanning the Cenomanian-Turonian boundary but are not evident in other sequences. Variation in Mn is related to bulk sedimentation rate and detrital versus biogenic supply, which control the Mn flux and the efficiency of the diagenetic Mn 'pump' that leads to elevated Mn contents in sediments. Manganese peaks do not generally correlate with positive delta C-13 excursions, and although near-coincident Mn and delta C-13 peaks occur around the Cenomanian-Turonian boundary, the former is not necessarily linked to the oceanic anoxic event occurring at that lime. The global oceanic Mn flux may have been enhanced during the Cenomanian as a result of hydrothermal activity during rapid sea-floor spreading and oceanic plateau Formation. Elemental chemostratigraphy provides a new tool for developing sequence stratigraphic models in pelagic and hemipelagic carbonate successions.