Marine Redox Evolution And Environmental Change From The Late Pliensbachian To Early Toarcian In The Early Jurassic

Extreme paleoclimate change event from Earth history can be used to better understand the Earth’s potential responses to anthropogenic climate change.The Toarcian Oceanic Anoxic Event（T-OAE;～183 Ma）was one of the most significant climatic changes and environmental perturbations of the Phanerozoic,characterized by global warming associated with substantial carbon release,enhanced hydrological cycling and continental chemical weathering,widespread seawater deoxygenation,and extinction.Stratigraphically the event is recorded as a negativecarbon isotope excursion（Toarcian NCIE）,which has been identified in all major biospheric reservoirs of carbon.A smaller magnitude carbon cycle perturbation occurred at the preceding Pliensbachian–Toarcian boundary（Pl/To）,and this event has been similarly linked to carbon release with increased temperature and continental weathering.These events provide a valuable opportunity to dissect a period of extreme paleoclimate change that has been linked to voluminous carbon emissions and global warming.The recognition of the T-OAE was originally based on the identification of widespread organic-rich sediments,interpreted to have been deposited as a result of expanded oxygen minimum zones（OMZ）in the global ocean.OMZ expansion was linked intimately to global warming because warmer water leads to sluggish ocean circulation,lower dissolved atmospheric O₂ content,and increased oxygen demand in the water column through respiration associated with enhanced primary productivity.Evidence for pervasive oceanic anoxia has mainly been described from European sections deposited in hydrographically restricted basins.These sites are thus not representative of the global ocean,and the uncertainty with regard to the redox of the global ocean is exacerbated due to the paucity of records from outside of Europe,especially from open-ocean environments.In addition,the redox conditions of marine environments across the Pl/To are largely uncertain.Moreover,evidence for environmental perturbations during the T-OAE is derived mainly from low-latitude tropical regions.The lack of geological records from relatively high-latitude regions prohibits the further analysis of the extent to which global warming could affect seawater redox,hydrological cycling and continental weathering at different latitudes.In this thesis,I present high-resolution pyrite S-isotopes,organic C-isotopes and Fe-speciation data,combined with elemental concentration data,across three Pliensbachian–Toarcian sedimentary sections.Together,these data reveal changes in marine redox conditions and the hydrological cycling.The sedimentary successions studied were deposited in the deep and shallow Panthalassic Ocean（Sakahogi and Sakuraguchi-dani sections,respectively）,and which are now exposed in Japan,and in the Hebrides basin（paleolatitude～40°N）,Scotland（Raasay section）,which connected the polar regions through the Viking corridor to the north,and close to west Tethys northwest European shelf,where detailed analyses on T-OAE have been carried out,to the south.These records provide a better understanding of the evolution of open-ocean redox conditions outside of Europe,and of the effects of climate changes in sub-tropical regions from the late Pliensbachian to early Toarcian.The findings are outlined as follows:1.The T-OAE is been identified for the first time in the Hebrides basin based on the identification of T-NCIE in the Raasay section,representing the T-OAE interval that has been widely identified especially in north Europe.Majority of analyses on climatic and environmental perturbations during the T-OAE come from low-latitude tropical regions.The thesis is focused on Raasay section,located at a higher-latitude sub-tropical area in the Hebrides basin.The existing detailed biostratigraphy in the Hebrides basin has assigned the study Raasay successionto lower Toarcian.On this basis,I have carried out high-resolution organic carbon isotope work throughout the succession.The results show a stepwise negative carbon isotope excursion with a magnitude of 2.7‰.This variation in carbon isotopes recorded in the Raasay section matchs the widely identified Torcian NCIE from other coeval sections,thus for the first chemostratigraphically constraining the stratigraphic age of the Raasay section.Given that the Toarcian NCIE is a typical marker of the T-OAE,my work for the first time identifies the stratigraphic position of the T-OAE in the Raasay section,Hebrides basin,which paves the way for further studies of redox condition and enrvironmental change at a higher-latitude sub-tropical area during the T-OAE.2.Revealing the local seawater redox conditions in the Hebrides basin（Raasay section）,indicating the redox conditions were highly varible between different basins during the T-OAE,and highlighting the role of local factors in affecting seawater redox coditions.After the identificationof the T-OAE in the Raasay section based on organic C-isotopes,I have carried out the elemental analysis of Raasay bulk rocks.Low enrichment of redox-sensitive trace elements（e.g.,Mo,U）,combined with paleoecological information（the occurrenceof benthic fossils）indicate local bottom water in the Raasay section was oxic/suboxic during the T-OAE,in contrast with strong euxinic conditions（anoxic waters with free H₂S）in the nearby Cleveland basin.This reflects that redox conditions were spatially variable during the T-OAE.Furthur analysis suggests that local factors may have played an important role in causing such redox variability.In detail,for example,i.Differences in basin hydrography.The water depth of Raasay section in the Hebrides basin was shallower than that of Yorkshire section in the Cleveland basin,which has also been demonstrated highly restricted during the T-OAE.In this case,chemocline of the Hebrides basin might be more unstable because shallower waters be more susceptible to local water disturbance associated with currents and prevailing winds.This could have resulted in prevailing oxic/suboxic conditions in the Raasay section.By contrast,deeper water depth and highly restricted hydrography in the Yorshire section（Cleveland basin）would have limited the water exchange between the basin and open oceans,leading to strongly reduing conditions.ii.Differences in basin paleogeography.Modelling results suggest polar cold and oxygenated flows might have moved southward and entered the northwest European shelf through the Viking corridor during the T-OAE.Because the Raasay section was paleogeographically closer to the Viking corridor,local waters might be more susceptible to the southward flows,thus being more oxygenated.By contrast,the flows could have not significantly influenced the waters in the Cleveland basin due to the topographic barriers and highly restricted hydrography.iii.Local runoff.Salinity analysis of the Yorkshire section,Cleveland basin indicates bracklish to even fresh waters prevailed during the T-OAE.This suggests that substantial runoff might have entered the basin,which would have caused strong stratification,thus exacerbating reducing conditions.3.Revealing enhanced local hydrological cycling and continental chemical weathering in the Hebrides basin（Raasay section）during the T-OAE,and highlighting higher sub-tropical latitudes were also significantly influenced by T-OAE global warming.In this thesis,sediment elemental grain-size proxies（Zr/Rb,Ti/K,Ti/Al）and chemical weathering proxies（K/Al,Rb/Al）are analyzed trough the Raasay section.The results show sediment coarsening in the Raasay section from the onset of the T-OAE,consistent with enhanced chemical weathering inferred from the chemical weathering proxies.This suggests active hydrological cycling even at higher-latitude sub-tropical area because more coarse-grained terrestrial materials could have been transported to oceans under stronger hydrodynamic conditions.Furthermore,the occurrenceofcross beddings in the Raasay section indicates storm activity,supporting active hydrological cycling in the Raasay section.Taken together,my work in this thesis demonstrates enhanced hydrological cycling and chemical weathering at higher-latitudes,emphasizing the significant impact of climate warming on the local environments at different latitudes during the T-OAE.4.Revealing a long-term evolution of Panthalassic Ocean redox conditions at widely differing water depths from the late Pliensbachian to early Toarcian,and clarifying for the first time at least local/regional euxinia in the Panthalassic deep waters during the T-OAE.The paucity of geological records directly from the global open ocean has led to many uncertainties regading the open ocean redox conditions during the T-OAE.In this thesis,two sections,one from the Panthalassic central deep waters（Sakahogi section）and the other from the Panthalassic western margin shallow waters（Sakuraguchi-dani section）are analyzed to reconstruct the evolution of redox conditions from the late Pliensbachian to early Toarcian.Fe-speciation data,combined with redox-sensitive trace element data from the Sakahogi section indicated that Panthalassic deep waters were at least locally/regionally anoxic/ferruginous(anoxic waters with Fe²⁺,but no free H₂S)from the late Pliensbachian to the end of the T-OAE.More intense development of sulfidic pore waters at the sediment-water interface may have occurred around the Pl/To boundary.At least intermittent bottom water euxinia characterized the T-OAE,followed by a subsequent transition toward more oxygenated conditions.By contrast,low enrichment of redox-sensitive elements（e.g.,Mo,U）during the T-OAE in the study Panthalassic shallow-water section（Sakuraguchi-dani）,coupled with the frequent occurrence of bioturbation signals in the coeval stratigraphy,suggests that shallow waters remained largely oxic/suboxic.5.Proposing a distinct Panthalassic deep-water upwelling model to account for the high enrichment of highly reactive iron on the shelf at the Panthalassic western margin.Fe-speciation data from the Panthalassic shallow-water section show anomalous enrichment of highly reactive iron,which suggests persistent anoxic/ferruginous conditions,at odds with oxic/suboxic conditions inferred from redox-sensitive elements and ecological information.Where should have the excessive iron deposited on the shelf come from?Addressing this issue may be the key to resolve this contradiction related to the interpretation of redox conditions on the shelf.Bacause majority of iron involved in marine iron cycling derives from terrestrial input,my work first analyzes the relative contributions of terrestrial iron to the sediments on the shelf.The chemical alteration index（CIA）was used to reflect the continental chemical weatering,supplemented by proxies reflecting changes in grain size and input of terrestrial plant debris.The results show neglible/weak correlations between these proxies and highly reactive iron,suggesting an insignificant contribution of terrestrial iron to the sediments deposited on the shelf.On this basis,this thesis has proposed an upwelling model in the Panthalassic Ocean during the T-OAE to account for the high enrichment of iron on the shelf.The deeper waters brought anoxic/ferruginous waters onto the shelf through upwelling,where Fe²⁺oxidation was initiated in oxic shallow waters.This model is supported by a global eustatic sea-level rise,active hydrological cycling and prevailing winds during the T-OAE,which could have partly facilitated upwelling and water mixing.6.Revealing the marine sulfur cycling across the Panthalassic transect from the late Pliensbachian to early Toarcian,and highlighting the role of local/regional sedimentary environment on pyrite S-isotopes.A positive shift（>10‰）in pyrite sulfur isotopes across Pl/To and the T-OAE intervals in the deep-water Sakahogi section and a larger positive shift（>20‰）during the T-OAE in the shallow-water Sakuraguchi-dani section indicate significant marine sulfur cycle perturbations across the Panthalassic transect.Local sedimentary environments rather than global redox conditions might have played a more important role in driving these sulfur cycle perturbations.On the shallow shelf generally low and highly variable pyrite sulfur concentrations and the larger positive shift in pyrite S-isotopes during the T-OAE were likely attributable mainly to elevated sedimentation rates associated with enhanced hydrological cycling and chemical weathering.In the deep-water Sakahogi section,positive shifts in pyrite S-isotopes at the Pl/To and aross the T-OAE were most likely controlled by increased organic matter supply to the seafloor linked to elevated export prduction and/or favorable preservation conditions.