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-1 3) geochemistry of a biostratigraphically well-constrained Cenomanian-Turon ian (Upper Cretaceous) Chalk succession on the Isle of Wight, southern Engl and, shows systematic variation that corresponds closely to a published seq uence stratigraphic model for the Cenomanian. Six sequences and their const ituent systems tracts, defined elsewhere using sedimentological criteria, a re clearly distinguishable from bulk-sediment elemental profiles, and an ad ditional Upper Cenomanian sequence previously identified in Spain is recogn ized in England from these geochemical data. The manganese curve is particu larly instructive, exhibiting minima around sequence boundaries and through lowstands, rising values from the transgressive surfaces through transgres sive systems tracts, maxima around maximum flooding surfaces, and declining values through highstands. Silica and trace-element (Ti, Zr) aluminium rat ios peak around transgressive surfaces and maximum flooding surfaces, indic ating pulses of increased siliciclastic input. Positive delta C-13 excursio ns are confirmed at the base of the Middle Cenomanian and spanning the Ceno manian-Turonian boundary but are not evident in other sequences. Variation in Mn is related to bulk sedimentation rate and detrital versus biogenic su pply, which control the Mn flux and the efficiency of the diagenetic Mn 'pu mp' 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 bo undary, the former is not necessarily linked to the oceanic anoxic event oc curring at that lime. The global oceanic Mn flux may have been enhanced dur ing the Cenomanian as a result of hydrothermal activity during rapid sea-fl oor spreading and oceanic plateau Formation. Elemental chemostratigraphy pr ovides a new tool for developing sequence stratigraphic models in pelagic a nd hemipelagic carbonate successions.