Interpretation of the paleoceanography during the mid-Cretaceous from geochemical data and simple box model

The deep ocean conditions and circulation during the Cretaceous have been reconstructed mainly from data from the Tethys Sea and the proto-Atlantic Ocean. However, sedimentological data from the Pacific during that period have been quite limited. Here, we present geochemical and filed data from the northwestern Pacific during the mid-Cretaceous. The objectives of this study are to evaluate the origin of organic matter in the sediments, to reconstruct the redox condition and dissolution of carbonate, and to evaluate the deep ocean circulation and ocean environments in the North Pacific during the Albian (~112 Ma) by comparison for the distribution of carbonate and organic matter between the Tethys and the Pacific. Especially in order to consider this dissolution effect, we conducted experiments by using the simple 3-box model. On the basis of Corg/N atomic ratios, terrestrial organic carbon was an important contributor. The low organic carbon and carbonate contents combined with concentrations of some elements suggest that the mid-depth northwest Pacific during the time of deposition of these sediments was oxic but corrosive to sedimentary carbonate, while in the Tethys, dissolved oxygen was largely consumed and carbonate was well-preserved. Our model results indicate that under high atmospheric CO2 content during the mid-Cretaceous, an increase in the oceanic inventory of alkalinity is likely necessary to preserve any carbonate on the seafloor, and suggest that the northwest Pacific hydrography was quite distinct from the Tethys Sea and the proto-Atlantic Ocean.