Mineralogical Record Of The Paleocene-Eocene Thermal Maximum(PETM) In Terrestrial Environments

Continental weathering is thought to have a negative feedback effect on greenhouse gases,as global warming enhances the chemical weathering of continental silicate minerals then consuming the CO₂.Clay minerals,which are important products of silicates chemical weathering,can be used to indirectly indicate the intensity of chemical weathering of continental silicate minerals.The Paleocene-Eocene Thermal Maximum（PETM,at～56 Ma）was the most significant extreme warming event since the Cenozoic era,and maily due to a rapid increase in atmospheric CO₂ concentration causing global warming.The widespread increases in kaolinite in marine and coastal marine sediments across the Paleocene-Eocene boundary have been considered the key evidence of the enhanced intensity of continental chemical weathering because of global warming.However,in recent years evidence such as microfossils,isotopes and other indicators in marine and coastal marine sediments suggested that the increases of kaolinite in sediment are likely related to increased catchment erosin.Therefore,there is still controversy over using marine sediments as carriers to study the types of continental weathering.This study focuses on paleosols which are in-situ records of continental weathering and applies sedimentary facies analysis,isotope geochemistry,sediment grain size,clay minerals,and iron oxide minerals to analyze the sedimentary sections of the PETM recorded in the Bighorn Basin,United States and the Nanxiong Basin,Guangdong,China,to explore the mineralogical response of different regional weathering types during the PETM.It’s of great significance to understand the impact of greenhouse effects on climate and the ecological environment,as well as its self-healing mechanism.The main findings are as follow.The Polecat Bench section in the Bighorn Basin of the USA was a braided river facies deposit,dominated by mudstone,siltstone,and conglomerate.Paleosol belonged to an aggradational paleosol,mainly developed on sandstone or conglomerate.The clay mineral assebmages in the paleosol were mainly smectite,mixed-layer illite-smectite（I-S）,kaolinite,and small amounts of illite and chlorite.The paleosols contained two major iron oxide minerals,hematite and goethite,whoes content changes corresponded to the observed color changes of the section in the field.Comparing the two iron oxide minerals,the content of hematite showed a significant increase during the hyperthermal event,indicating that the Bighorn Basin was under an enhanced seasonal arid climatic conditions during the PETM.Analyses of the clay minerals,sediment morphology and structure,grain size,and level of soil development of the Polecat Bench section showed that the smectite content increased significantly after the onset of the PETM and returned to the level of the pre-PETM after the recovery of the PETM.The contents of I-S,kaolinite,and illite changed in opposite to that of smectite.There were obvious differences in morphology of clay minerals between during and outside of the PETM.However,the increase in smectite content during the PETM in the Bighorn Basin was not related to in-situ soil formation,but was mainly due to the intensification of continental physical weathering,which led to an increase in detrital smectite.During the PETM,the Bighorn Basin was mainly characterized by enhance seasonal climate,which led to a decrease in the degree of consolidation of riverbank and upstream soils.Meanwhile,the more frequent heavy rain during this period exacerbated the erosion of smectite-rich bentonite from the Cretaceous strata in the source area,leading to an increase in smectite content in the Bighorn Basin during the PETM.At the same time,by comparing the changes in clay mineral content along the whole section,it is found that the clay mineral content change in the phases of onset and recovery stages lagged behind the onset of the PETM and the recovery of the PETM,which may be related to the delayed signal of sedimentary input from the source area.Field observation revealed that the strata of the Maogouwan section in the Nanxiong Basin was mainly composed of fluvial facies and interrupted by lacustrine facies deposits.The lithology was mainly mudstone,siltstone,medium-coarse-grained sandstone,and conglomerate.Red paleosols were developed in the floodplain facies.The paleosols showed brownish-red to purplish-red in color and developed typical soil-forming features（such as slickensides,mottles,burrows,and carbonate nodules）.The clay minerals in the paleosol of Nanxiong Basin are mainly I-S,illite,mixed-layer kaolinite-smectite,kaolinite,and a small amount of vermiculite and smectite.The isotopic records of pedogenesis carbonate nodules in the Maogouwan section revealed a clear negative carbon isotope excursion（CIE）,with a decreasing magnitude by～-5‰,consistent with the CIE of pedogenesis carbonate nodules recorded at different regions around the world by is～-4 to-6‰within the PETM.By comparing the stratigraphic age in the Nanxiong Basin,it is likely that the PETM event was recorded in the Maogouwan section,including the body of the PETM（section location:165-225 m）,the recovery of the PETM（section location:135-165 m）,and the post-PETM（section location:0-135 m）.Oxygen isotope calculations show that the mean annual temperature（MAT）during the body of the PETM was～14.2℃,and the MAT of the post-PETM was～13.4℃.By comparing the MAT bwtween the Bighorn Basin and Nanxiong Basin during the PETM,it is found that the two basins had similar MAT during the body of the PETM.Clay minerals,sediment morphology and structure,grain size analysis,and level of soil development analysis were conducted to exlopre the responses of the Paleocene-Eocene transition in the Nanxiong Basin.The results showed that during the PETM,the content of I-S in the basin increased significantly,while the content of illite decreased.Morphological analysis showed that I-S mainly formed on the surface or edge of illite minerals.The in-situ soil formation during PETM and post-PETM were basically consistent with the formation of clay minerals.The higher content of I-S during PETM mainly came from detrital sedimentation.Vermiculite was a product of soil formation under warm and humid seasonal climate.Combined with the characteristics of peloclimate in different regions of China during the same period,it indicates that the paleoclimate of the Nanxiong Basin during the PETM was mainly characterized by seasonally warm and humid climate.The increase in rainfall could have increased the kinetic energy and transportation capacity of upstream rivers,and eroded I-S and illite from the source area,leading to an increase in I-S content in the Nanxiong Basin during the PETM.The abundant I-S in the Cretaceous and Paleocene strata could mainly come from the transformation of illite.Results of the diffuse reflectance spectroscopy（DRS）analysis showed the paleosol contains two types of iron oxide minerals,hematite and goethite insediments from Maogouwan section in Nanxiong Basin,whose variations in content corresponded to the color changes observed in the field.By comparing the iron oxide content during the body of the PETM and the post-PETM,both hematite and goethite showed a significant increase and fluctuation during the PETM,suggesting that the Nanxiong Basin was under a seasonal climatic condition with obviously alternated dry and wet periods during the PETM.Both the Bighorn Basin and the Nanxiong Basin were terrestrial fluvial basins,located at the same latitude and under similar temperature conditions during the PETM.However,due to differences in the distribution of land and sea,the two basins had different climatic and environmental characteristics,resulting in different types of clay minerals formed during pedogenesis.The increase in clay minerals during the PETM was mainly derived from detrital clay minerals,which were related to the enhanced physical weathering and erosion,rather than in situ pedogenesis after deposition.This suggests that chemical weathering changs were likely overruled by enhanced physical weathering and erosion at the continental interior during the PETM.