Determining the Duration of the Ediacaran Shuram C-isotope Excursion using Rock Magnetic Cyclostratigraphy

点击翻译按钮获取中文摘要

Determining the Duration of the Ediacaran Shuram C-isotope Excursion using Rock Magnetic CyclostratigraphyPI: Kenneth P. Kodama, Lehigh University, EAR-1322002ABSTRACTThis award is designated as an OIIA/ISE Global Venture Fund Award and is being co-funded by NSF's OIIA International Science and Engineering Section. This project will develop high-resolution time sequences, or chronostratigraphies, for the sedimentary rocks that carry a record of the Ediacaran (~555 Ma) Shuram negative carbon isotope excursion at two distant localities, one in South China and the other in South Australia. The chronostratigraphies will be determined by a new technique, rock magnetic cyclostratigraphy, which can detect global climate cycles that are driven by variations in the magnitude and direction of Earth's axial tilt and the ellipticity of Earth's orbit around the Sun. The astronomically-forced global climate cycles have been very regular throughout Earth history, so they provide a high-resolution metronome to develop time sequences for sedimentary rocks. The rock magnetic chronostratigraphies should provide up to 15,000 year resolution for the duration of the Shuram carbon isotope excursion recorded by the Doushantuo Formation in South China and by the Wonoka Formation in South Australia, both marine, silt-carbonate sedimentary rocks. Magnetizations applied in the laboratory to samples collected at close stratigraphic spacing (10-30 cm) will measure variations in the concentration of depositional magnetic minerals in the rocks. PIs previous work shows that the magnetic mineral concentration variations can record astronomically-forced global climate cycles. Oriented samples will also be collected from the Wonoka and Doushantuo rocks for standard paleomagnetic analysis to detect polarity reversals of the Earth's magnetic field. The reversal stratigraphy developed will help constrain the duration of the rock magnetically detected cycles to ensure they are astronomically-forced climate cycles. Comparing the rock magnetic cyclostratigraphies from two widely separated localities will provide a strong test of the primary nature of the Shuram carbon isotope excursion. Post-depositional alteration of the rocks is one explanation for the Shuram excursion. Identical high-resolution durations for the excursion in Australia and China would support a primary origin and would bolster the interpretation that the Shuram excursion is a singular event in Earth history and resulted from the oxidation of the world ocean just before life exploded into multi-cellular diversity. The development of the rock magnetic cyclostratigraphy technique for Ediacaran rocks that will result from this project will afford higher resolution relative time assignment for these ancient rocks than afforded by current geochronological techniques.