BRIDGING THE TURONIAN GAP IN THE MESOZOIC ASTRONOMICAL TIME SCALE VIA THE INTEGRATION OF CHEMOSTRATIGRAPHIC AND CYCLOSTRATIGRAPHIC DATA FROM DEMERARA RISE

The Cenomanian-Turonian (Late Cretaceous) section, including Oceanic Anoxic Event II, is a global source rock interval commonly characterized by rhythmically bedded limestone and organic carbon-rich mudstone lithologies. This stratigraphic interval was recovered in 2003 from multiple sites at Demerara Rise in the Tropical North Atlantic (Ocean Drilling Program Leg 207) yielding a near-continuous record of cyclical hemipelagic sedimentation through the Turonian Stage. This record provides the potential to develop a continuous astrochronology that bridges the “Turonian gap”, an interval that separates published Turonian and Coniacian astronomical time scales that have been previously integrated with radioisotopic data (from the Western Interior Basin, USA). Astrochronologic testing of geophysical log data and geochemical data at sites Site 1261 (upper Cenomanian-mid Turonian) and Site 1259 (mid-Turonian-lower Coniacian) using the Average Spectral Misfit algorithm reveals strong Milankovitch periodicities, and provides an objective calibration of the astronomical time scale. Lithostratigraphic and carbon isotope (δ¹³C_org) chemostratigraphic correlations between the two sites facilitated compilation of a composite section spanning the entire Turonian Stage. The new astronomically-tuned duration for the Turonian is statistically indistinguishable from recently revised radioisotope ages (⁴⁰Ar-³⁹Ar) from bentonite horizons in the Western Interior Basin (U.S.A.), signaling agreement among independent chronometers. The base of the floating orbital timescale has been tied to the Cenomanian-Turonian Boundary providing an anchored age model for perturbations in the global carbon cycle recorded by δ¹³C_org measurements. The astronomically-tuned Turonian-Coniacian δ¹³C_org reference curve serves as a chronostratigraphic framework for comparing records among geographically separated sections and for evaluating the timing and duration of major geologic events in the Turonian including: an Early Turonian climatic optimum, Earth system responses to the aftermath of a major oceanic anoxic event, and a significant Middle-Late Turonian fall in relative sea level documented globally.