The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation

Abstract

Summary Correlation of the δ 13 C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger et al. (this volume), indicates that a global episode of intense organic carbon ( org C) burial took place during the latest Cenomanian-earliest Turonian ‘Oceanic Anoxic Event’ (OAE) ( A. plenus through I. labiatus macrofossil zones and upper R. cushmani TRZ through W. archecretacea PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the δ 13 C curve indicates that rates of org C burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of org C burial. The δ 13 C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in δ 13 C reflects decreasing rates of org C burial following the Cenomanian-Turonian ‘oceanic anoxic event’ as well as the probable oxidation and return of significant amounts of org C to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor. The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for org C deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced org C preservation. The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities.