Astrochronology Of The Early-Middle Triassic In South China And Astronomical Forced Paleoenvironment Change

The Milankovitch theory that quasi-periodic oscillations in the Earth-Sun position have induced prominent 104-106 year variations in the stratigraphic record of climate is widely accepted. Analysis and interpretation of fossil astronomical signals derived from climate proxies in sedimentary rocks can help us calibrate geological time, understand the past climate of the Earth and predict future climate change. This thesis studied multiple paleoclimate proxies from well-studied marine Lower-Middle Triassic sections in South China. In combination of biostratigraphic results and radioisotopic dating, study of astrochronology and magnetostratigraphy provided a high-resolution continous astronomical time scale for the Early-Middle Triassic sections in South China, which can serve as a reference for international chronostratigraphic chart. The new time scale provided unique opportunities to decipher mechanisms of recurrent Earth system upheavals and paleoenvironment changes that occurred during the Early-Middle Triassic.Multiple paleoclimate proxies developed from sedimentary sequences enhance confidence in the characterization of paleoclimate and paleoenvironmental change. However, the relationship among different proxies and their sensitivities to climate change are rarely well understood. This thesis reviews current status and outstanding problems of physical climate proxies. To decipher the relationship among proxies and to estimate their individual sensitivities, this thesis propose two criteria for multiple proxy datasets and introduce two new methods to test these criteria. I have studied a large suite of paleoclimate proxies derived from the sedimentary record and proposed the first ever described cluster tree of 16 physical climate proxies from the Early Triassic marine section in South China.The timing of the end-Permian mass extinction and subsequent prolonged recovery during the Early Triassic Epoch can be established from astronomically controlled climate cycles recorded in continuous marine sedimentary sections. Astronomical-cycle tuning of spectral gamma-ray logs from biostratigraphically-constrained cyclic stratigraphy through marine sections at Meishan, Chaohu, Daxiakou and Guandao in South China yields an integrated time scale for the Early Triassic, which is consistent with scaling of magnetostratigraphy from climatic cycles in continental deposits of the Germanic Basin. The main marine mass extinction interval at Meishan is constrained to less than 40% of a 100-kyr (kilo-year) cycle (i.e.,< 40 kyr) and the sharp negative excursion in δ13C is estimated to have lasted < 6 kyr. The sharp positive shift in δ13C from -2%o to 4%o across the Smithian-Spathian boundary at Chaohu was completed in 50 kyr. The earliest marine reptiles at Chaohu that are considered to represent a significant recovery of marine ecosystems did not appear until 4.7 myr (million years) after the end-Permian extinction. The durations of the Griesbachian, Dienerian, Smithian and Spathian substages, including the uncertainty in placement of widely used conodont biostratigraphic datums for their boundaries, are 1.4±0.1,0.6±0.1,1.7±0.1 and 1.4±0.1 myr, implying a total span for the Early Triassic of 5.1±0.1 myr. Therefore, relative to an assigned 251.902±0.024 Ma for the Permian-Triassic boundary from the Meishan GSSP, the ages for these substage boundaries are 250.5±0.1 Ma for base Dienerian,249.9±0.1 Ma for base Smithian (base of Olenekian stage),248.2±0.1 Ma for base Spathian, and 246.7±0.1 Ma for the base of the Anisian Stage. This astronomical-calibrated timescale provides rates for the recurrent carbon isotope excursions and for trends in sedimentation accumulation through the Early Triassic of studied sections in South China.The timeline of the Anisian Stage was understood today only in an uncertain framework. Here we present high-resolution (3-5 kyr) spectral gamma-ray (GR) and magnetic susceptibility (MS) measurements from the well-defined marine sections at Guandao section in South China. The spectral GR and MS variations document a series of Milankovitch frequencies (eccentricity, obliquity and precession) that support the prediction of astronomical solutions. Astronomical tuning of spectral GR and MS series to 405-kyr cycles yields an astronomical time scale for the Anisian Stage. The durations of the Anisian stage are 5.6±0.2 myr, implying a 241.2±0.2 Ma age for the Anisian-Ladinian boundary. The result refines the presently ～242 Ma age for the base Anisian stage. The astronomical-calibrated Anisian timescale provides constraints for geological events, i.e. global sea-level change, carbon-isotope curves, magnetic polarity timescale and biostratigraphic zonations.The start of the Mesozoic Era is marked by roughly five million years (myr) of Earth system upheavals, including unstable biotic recovery, repeated global warming, ocean anoxia, and perturbations in the global carbon cycle. Intervals between crises were comparably hospitable to life. The causes of these upheavals are unknown, but are thought to be linked to recurrent Siberian volcanism. Here, two marine sedimentary successions at Chaohu and Daxiakou, South China are evaluated for paleoclimate change from astronomical forcing. In these sections, gamma-ray variations indicative of terrestrial weathering reveal enhanced obliquity cycles over prolonged intervals, characterized by a 32.8 kiloyear periodicity with strong 1.2-myr modulations. These suggest a 22-hour length-of-day and a 1.2-myr interaction between the orbital inclinations of Earth and Mars. Comparing the 1.2-myr modulation of obliquity cycles in these sections with Early Triassic records of global sea-level, temperature, redox and biotic evolution suggests that long-term astronomical forcing was involved in the repeated climatic and biotic upheavals that took place throughout the Early Triassic.Records of past sea level change provide insight into the understanding of current sea level change. High amplitude (up to 200 m) and short term (<106 year) eustatic changes are associated with the waxing and waning of continental ice sheets. The Triassic Period represents the warmest geologic period of the past 600 million years without ice sheets; however, Haq and his colleagues have noted numerous third-order scale (0.5-3 myr) sea level oscillations with amplitudes of tens of meters within these intervals, which, if confirmed to be global in scale, challenges known mechanisms of sea level change. Here, a new approach called "dynamic noise in sea-level" or "dynamic non-Milankovitch signal in sea-level" (DNS) provides a means for estimating the time scale of the Triassic sea level variations. Application of this approach in Early Triassic sections of South China provides a high-resolution time scale for the recorded sea level variations. The results indicate that abrupt and high amplitude sea-level falls in European sections are synchronous with those in South China. Without ice, recharge and discharge of continental aquifers is a proposed mechanism for these high amplitude sea-level variations in the Early Triassic hothouse. The continental aquifers may have an impact on global sea level changes across the whole Phanerozoic. The DNS metric is expected to apply in the whole geologic past for water-level of marine and even lakes and likely makes a fresh look for high precision global sea-level variations.