Late Cretaceous (Campanian) carbon isotope events, sea-level change and correlation of the Tethyan and Boreal realms

摘要

Carbon stable-isotope stratigraphy provides unique insights into environmental change during the Campanian, the longest (83.5-71.3 Ma), but least well-understood stage of the Late Cretaceous. A new carbon isotopes δ~(13)C) profile for a 500-m-thick Campanian-basal Maastrichtian Tethyan pelagic-hemipelagic section near El Kef, northern Tunisia, is calibrated using data from a biostratigraphically well-constrained succession at Kalaat Senan. The general shapes of the Tunisian δ~(13)C reference curve and published Tethyan δ~(13)C profiles from Elles (Tunisia) and Bidart (SW France), and a Boreal curve for the Trunch borehole (eastern England), are remarkably similar in all three areas. A positive carbon isotope event of +0.2‰ δ~(13)C in the mid-Campanian dated at 78.7 Ma and a negative excursion of -0.4‰ in the upper Campanian at 74.8% Ma can be correlated between Tunisia and England. A positive excursion of +0.3% at 83.7 Ma spans Santonian-Campanian boundary. These isotope events enable precise inter-regional correlations that are consistent with published nannofossil data. Review of Campanian sea-level data from North Africa, the Middle East and northern Europe indicates that major shifts in δ~(13)C profiles coincide with changes in eustatic sea-level. Relatively stable δ~(13)C values in the lower Campanian and their long-term fall through the upper Campanian reflect high and then falling eustatic sea-levels, and increased carbonate production. Short-term (～600 kyr) positive excursions record greater organic productivity and/or organic matter preservation, and decreased carbonate fluxes during periods of rapid sea-level rise and the drowning of carbonate platforms. Excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand. Negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea-level fall. Carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of Campanian global biostratigraphic frameworks, and improve our understanding of the nature and timing of Late Cretaceous sea-level change.