Cyclostratigraphic Study Of The Middle Triassic Series In South China

Earth's history reveals that over time the Earth has gone through some dramatic evolutionary events that changed the evolutionary trajectory of life driven by paleoenvironmental changes.Understanding the timing and mechanism of these geological events has implications to projected global warming and other associated events.The Mesozoic Era immediately follows the end-Permian mass extinction event which is the largest bio-crisis in Earth history,which had significant implications for the evolution of all life.Study of the palaeoclimate and palaeoenvironment can provide key clues for us to understand ecosystem restructuring following the mass extinction.Over the last decade,the role of Malankovitch cycles,which results in changes in the hydrological cycle and palaeoclimate,in the restructuring of ecosystems has gained more interest.Here,cyclostratigraphic studies were applied for the three Middle Triassic sections,i.e.,the Yongning,Yongyue and Pohong sections in Guizhou,South China,for the first time.The main aims are to build high-resolution astronomical time scales by 405-kyr eccentricity tuning for the Middle Triassic and building high-resolution palaeoclimatic and palaeoenvironmental proxies(magnetic susceptibility,Fe/Al and V/Al)data series to reconstruct palaeoclimate and paleoenvironment dynamics.These timescales were then correlated with key sections in South China that record major events in the evolution of marine ecosystems,and put into a palaeoclimatic and palaeoceanographic context.This study reveals that:?Spectral analysis of proxies for continental weathering(terrestrial flux)through studied sections indicate orbital cycles that reflect 405-kyr long-eccentricity,100-kyr short-eccentricity,?33-kyr obliquity,and?20-kyr precession forcing,respectively,in the Middle Triassic.Spectral analysis of proxies for terrestrial flux(e.g.,Fe/Al)of three(Middle Triassic)marine platform succession(Yongning section,Pohong and Yongyue,South China)indicate significant cycles.Based on available age constraints and sedimentation rate modeling,the recognized cycles include 405-kyr long eccentricity,100-kyr short eccentricity and?20-kyr precession cycles.Spectral analyses of magnetic susceptibility,Fe/Al and P through the Yongyue and Pohong sections show significant cycles of?85-m,?7-m and?4-m.Based on independent age constraints and sedimentation rate modelling,these cycles are interpreted as orbital cycles that reflect 405-kyr long-eccentricity,?33-kyr obliquity,and?20-kyr precession forcing,respectively.?Basing on the time series analysis an astronomical time scale spanning?9.6Myr is produced,which almost covers the whole Middle Triassic.Anchoring these signals to a dated and widespread“green bean”volcanic ash layer(246.6 Ma)and tuning to stable 405-kyr long-eccentricity cycles yields an astronomical time scale spanning?4.2±0.3 Myr(246.7 to 242.5+0.3/-1.4 Ma)in the Anisian.This time scale constrains the ages of the Luoping biota(which marks the earliest complex Mesozoic marine ecosystem in South China)and the Panxian marine vertebrate biota to243.8+0.3/-1.4 and 243.4+0.3/-1.4 Ma,respectively.Tuning of the records to the stable405-kyr long-eccentricity parameter in the Yongyue and Pohong section yields a tuned astronomical time scale spanning?7.4±0.2 Myr,which covers from ca.244.4 to 237.0Ma and includes the entire Ladinian Stage.?Reconstruction of continental weathering changes in the study area using high-resolution proxies of magnetic susceptibility and Fe/Al.The early Anisian is characterized by intercalated claystone and marls with high values in terrestrial flux proxies,indicating an intensified hydrological cycle due to a humid climate in the study area.This was followed by a long-term cooling trend into the late Anisian,as indicated by declining terrestrial flux proxy values.Magnetic susceptibility and Fe/Al data from the studied sections show intervals with relatively high values in the middle-late Anisian(ca.244.0 to 242.6 Ma),and the latest Anisian to early Ladinian(ca.241.8 to 240.9 Ma).These geochemical changes,combined with lithological changes,indicate enhanced terrigenous flux caused by the acceleration of hydrological circulation under a warmer and more humid climate.In addition,P data records two sharp falls that appear broadly coeval with the previously postulated late Ladinian ecological crisis.?Reconstruction of seawater redox change in the Middle Triassic using V/Al and Mo/Al.Seawater redox conditions were assessed using V/Al,Mo/Al and lithofacies analyses,and these data reveal possibly two putative deoxygenation events during the Anisian.Our new high-resolution redox data can also be correlated with results on Anisian ocean redox changes elsewhere.The deoxygenation events also happened in the late Ladinian,at ca.238.4-238 Ma and 237.2-237 Ma.This phenomenon was the first documentation,its implications on the“late Ladinian warming events”and the“Ladinian ecological crisis”need further investigation.?Reconstruction of 3^rd order relative sea level change using both classic sequence stratigraphic methods and sedimentary dynamic noise modelling.Our new astrochronology also reveals that?1.2-Myr obliquity amplitude modulation cycles are expressed in the Middle Triassic.The regional sea level changes inferred from sequence stratigraphy and sedimentary noise modelling can be correlated with Tethyan and global sea level change.This work demonstrates the utility and power of sedimentary noise modeling in shallow marine environments for inferring past sea level dynamics.It also furthers constrains the timing of putative Middle Triassic climatic and ecological events.Our new astronomical timescale and high-resolution geochemical data provide key constraints on the timing of marine ecosystem recovery after the end-Permian mass extinction.Orbital forcing seems to have played an important role in modulating continental weathering and seawater chemistry dynamics that impacted the timing of ecosystem recovery.