CALIBRATING THE LATE CRETACEOUS CARBON CYCLE USING ASTRONOMICALLY TUNED CARBON ISOTOPE RECORDS AT DEMERARA RISE (TROPICAL NORTH ATLANTIC)

Floating astronomical time scales (ATS), based on periodic variation in Earth’s orbital and rotational parameters (Milankovitch Cycles), provide precise durations for climatic, geochemical, and biotic events in Mesozoic strata, potentially with smaller uncertainty than radioisotopic geochronology. However, cyclostratigraphic analyses are limited to depositional environments where sediment geochemistry and/or lithological properties fluctuate in response to Milankovitch forcing. Many sequences of interest are therefore precluded from cyclostratigraphic analysis, and other quantitative stratigraphic techniques must be employed to develop chronostratigraphic frameworks. This research couples an ATS derived from cyclostratigraphic analysis with a high-resolution carbon isotope chemostratigraphy (δ¹³C) of the rhythmically bedded Turonian Stage at Demerara Rise (Tropical N. Atlantic ODP Leg 207 - Sites 1259 & 1261). Bulk organic carbon isotope (δ¹³C_org) and carbonate carbon isotope (δ¹³C_carb) chemostratigraphies are presented for the Turonian, and durations between excursions are quantified. Combining both δ¹³C_carb and δ¹³C_org chemostratigraphies aides in the construction of a δ¹³C reference curve that more accurately tracks δ¹³C of marine dissolved inorganic carbon, as divergence between the two records can indicate diagenesis or local overprinting. Additionally, the floating astronomical time scale is anchored to the refined Cenomanian-Turonian Boundary age (93.9±0.15 Ma) from the GSSP, and numerical ages are calculated for named Turonian δ¹³C excursions. Excursions in δ¹³C records can serve as correlative time horizons to develop robust chronostratigraphic frameworks in basins distant from the tropical reference site.

By coupling δ¹³C with cyclostratigraphy at suitable sites, durations between δ¹³C excursions can be assigned and time control can be exported globally. This approach provides geologists with the ability to confidently compare geographically separated sites that are coeval, and also yields better constraint on rates of change during important events in Earth history. Astronomically tuned δ¹³C stratigraphy is particularly suited for application in the Late Cretaceous due to δ¹³C fluctuations driven by intervals of accelerated organic carbon burial.