Chronology And Eccentricity Phasing For The Early Turonian Greenhouse (Similar To 93-94 Ma): Constraints On Astronomical Control Of The Carbon Cycle

The Early Turonian interval represents a unique confluence of climatic and oceanographic conditions including peak surface temperatures, high greenhouse-gas concentrations, and maximum Phanerozoic sea level. The susceptibility of this climate mode to astronomical insolation forcing remains poorly understood partly due to a limited time control and unknown phasing of astronomical cycles in this interval. Here, we offer a refined astrochronology of the Early Turonian based on laterally consistent precession signals preserved in offshore strata of the Bohemian Cretaceous Basin (central Europe). Pristine amplitude modulation verified through interference patterns in depth-frequency plots provides a robust indication of similar to 100 and 405-kyr eccentricity phases (maxima and minima) that are pinned to ammonite biozones and new carbon-isotope data from two cores. The Early Turonian is estimated as 885 +/- 41 (2 sigma) thousand years (kyr) in duration, with the Cenomanian/Turonian boundary predating the first Turonian 405-kyr maximum (no. 232 in the Geological Time Scale 2020) by 82 +/- 70 (2 sigma) kyr. The results support a possible link of the recovery from Oceanic Anoxic Event II to increasing magnitude of seasonal insolation extremes due to rising eccentricity on 405-kyr and million-year (Myr) time scales. Superimposed upon this trend are small-scale carbon-isotope anomalies the pacing of which passes from similar to 110 kyr, resembling short eccentricity, to similar to 170 kyr, possibly related to obliquity modulation. This eccentricity-to-obliquity transition, paralleling the rising phase of Myr-scale eccentricity, suggests decoupling of the carbon-cycle perturbations from low-latitude seasonal insolation. Mid-latitude to high-latitude carbon reservoirs became involved toward the Middle Turonian.