Synchronizing Rock Clocks in the Cambrian

Home , Jiangshanian

EGU21-5082 https://doi.org/10.5194/egusphere-egu21-5082 EGU General Assembly 2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License.

Zhengfu Zhao1, Nicolas Thibault1, Tais W. Dahl2, Niels H. Schovsbo3, Aske L. Sørensen2, Christian M.Ø. Rasmussen2, and Arne T. Nielsen1 1Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark 2GLOBE institute, University of Copenhagen, Denmark 3Geological Survey of Denmark and Greenland, Denmark

Profound environmental and biological changes took place during the Cambrian, yet, compared to other Phanerozoic intervals, the Cambrian time framework remains poorly constrained, which severely hinders a detailed understanding of the timing and progression of these major geological events. In this study, we report a radiometrically anchored astrochronologic framework across the late Cambrian interval, using high-resolution aluminum (Al) series (1 mm resolution) through the Alum Shale Formation in Scania, southernmost Sweden, based on the fully cored Albjära-1 well. Significant cycles with periods of 405 kyr (long eccentricity), 108 kyr (short eccentricity), 30.4 kyr (obliquity) and 18.8 kyr (precession), associated with long-term amplitude modulation of obliquity and precession, confirmed the orbital imprint on late Cambrian climate. Using the U-Pb dating at 486.78±0.53Ma for the Cambro-Ordovician boundary as anchor point, our timescale spans from ~483.9 to ~500.0 Ma, covering 7 trilobite superzones and 3 graptolite zones. The calibration indicates ages of 491.2±0.54 Ma, 493.9±0.67 Ma, 497.3±0.67 Ma and 500.4±0.67 Ma for the lower boundaries of provisional Stage10, Jiangshanian, Paibian and Guzhangian stages, respectively. This radiometrically anchored astrochronology also provides precise age constrains on regional superzones or even biozones within Scandinavia, and hopefully pave the way for better understanding the late Cambrian major geological events globally.