Volcanism And Hydroclimate During The Paleocene-eocene Thermal Maximum

Global warming is unequivocal,with retreating glaciers,frequent wildfires,and extreme precipitation events occurring more frequently,leaving humans facing a hotter,drier,and in places wetter,Earth.As of 2023,atmospheric CO₂levels continue to rise steadily.Anthropogenic greenhouse gas emissions have been rapidly altering Earth’s climate,and pushing the Earth system toward a warmer state with no historical precedent.Hyperthermal events from Earth history can be used to better understand the response of the Earth’s hydroclimate to anthropogenic carbon emissions and global warming.In this thesis,we focus on the Paleocene-Eocene Thermal Maximum（PETM,～56 Ma）,the most abrupt extreme warming event of the Cenozoic and one that has historically been treated as a potentially useful analogue for understanding regional and global responses to future climate change.The PETM was characterised by a 5-8℃rapid global warming and a>2‰negative carbon isotope excursion（CIE）that affected the major biospheric reservoirs of carbon and which has been attributed to the release of ¹²C-enriched carbon.The PETM was also associated with significant ecological changes,including ocean acidification,ocean deoxygenation,extinction of deep-sea benthic foraminifera increased chemical weathering and enhanced hydrological cycling.Research on hydroclimate change across the PETM,especially the change of extreme precipitation frequency,has important significance for the prediction of changes and trends in hydroclimate in the future.Emerging evidence indicates that the hydrological cycle was overall intensified during the PETM,but with likely latitudinal differences and heterogeneous distribution.There is a paucity of data on the hydroclimate changes across the PETM in the tropical Tethyan zone,which hinders our understanding of global hydroclimate change during the PETM.There have been a number of studies on the distribution and the change in frequency of extreme precipitation across the PETM on a global scale published in recent years.However,these studies are based mainly on models,and direct geological evidence is lacking.During the PETM,sedimentary records in some regions（e.g.,Spain,Tibet,and the North Sea Basin）show an increase in grain size,indicating possible extreme precipitation events.However,the response of turbidites to changing hydroclimate in the PETM is unclear.Increase in extreme precipitation in tropics may be expected because this is where tropical cyclones are generated and there is intense seasonal rainfall.However,the response of temperate areas remains ambiguous.The ultimate cause of the PETM has been the subject of intense study,and has focused on a handful of likely causes that all involve large-scale release of ¹²C-enriched carbon.These include biogenic methane release from the destabilization of methane hydrates,CO₂release from volcanic activity,or greenhouse gas release from intrusion of mantle-derived melts into organic-rich sediments.Recent work has suggested that the¹²C-enriched carbon was probably volcanogenic and sourced from the North Atlantic Igneous Province（NAIP）.Mercury（Hg）,which is primarily sourced from volcanism,in sedimentary rocks is an emerging tool suitable for tracing ancient large-scale volcanism.However,sedimentary Hg anomalies shows strong heterogeneity during the PETM and Hg is readily influenced by diagenesis,alteration,and variations in the ability of sediments to host Hg.Mass-independent fractionation of Hg-isotopes may more effectively indicate likely sources of Hg,but there is currently no published record of Hg-isotope variations through the PETM.In this thesis,geochemical and sedimentological proxies are used to reveal and better understand hydroclimate change in tropical and temperate regions during the PETM.In addition,new records of Hg and Hg-isotopes are used to robustly examine the hypothesized link between volcanic activity and the PETM.The findings are outlined as follows:1.Time series analysis of turbidite recurrence from a North Sea well indicates a～21 kyr astronomical precession-paced climate control on turbidite frequency.The section studied is the thickest PETM sedimentary archive yet recorded,and the work uncovers a clear link between astronomical forcing of climate change,intensification of the hydrological cycle,and large-scale changes in sediment supply in the PETM over long,multi-millennial timescales.In this thesis,we present the thickest sedimentary archive yet recorded of the PETM from a cored well in the North Sea Basin,offshore UK.The negative CIE marking the PETM in this succession spans 140.2 m and is coeval with the occurrence of>200 turbidite sandstone beds.The CIE and these turbidites occur in an otherwise mud-dominated succession and suggest an order of magnitude increase in sedimentation rates coeval with the PETM.Time series analysis of turbidite sandstone recurrence reveals a pervasive～5 m cyclicity.Based on available age constraints,this cyclicity is attributed to 21 kyr astronomical precession cycles,indicating how precession scale changes in climate drove regional changes in hydrology,sediment supply and turbidity currents.A time lag between the onset of the CIE and a marked increase in turbidite deposition is also recognized,potentially reflecting a lagged response of the sedimentary system to environmental change,at least in the studied area.This work uncovers a clear link between extreme climate warming,intensification of the hydrological cycle,and large-scale changes in sediment supply at the PETM.Moreover,this findings reveal the importance of astronomical climate forcing in mediating these factors over long,multi-millennial timescales.2.New isoprenoid glycerol dialkyl glycerol tetraethers（iso GDGT）data from two shallow marine sections in present-day Southern Tibet reveal an increase in seawater temperature of～7℃,supporting a reduced meridional temperature gradient during the PETM.At the same time,elemental proxies and mineralogical data suggest complex controls on variations in terrigenous sediment supply in this region,including sea level fluctuations,hinterland weathering,erosion and sediment delivery.To fill a key gap in our knowledge of hydroclimate response of shallow-water carbonate platforms during the PETM in the tropical eastern Tethys,we reconstruct PETM sea-surface temperatures（SST）from the distribution of isoprenoid glycerol dialkyl glycerol tetraethers（iso GDGT）in two new shallow marine carbonate sections exposed in present-day Southern Tibet.Our new temperature dataset demonstrates that the absolute peak PETM mean SST values reached up to 38°C and likely increased～7°C during the PETM.This is consistent with previous studies and provide a good supporting evidence for a reduced meridional temperature gradient during the PETM.Elemental data and quartz content suggest an increase in terrigenous flux prior to the CIE onset,consistent with pre-onset warming.However,the changes in terrigenous flux during the PETM from our studied sections and other published sections in eastern Tethys are inconsistent.It highlights the multiple controls on variations in terrigenous sediment supply in this region,including sea level fluctuations,hinterland weathering,erosion and sediment delivery.3.The first Hg-isotope record across the PETM was obtained from well 23/16b-9,located close to active NAIP volcanism.The data support a temporal link between CO₂release from volcanic activity and the initiation of the PETM.Emerging geochemical and modeling evidence has suggested that the ¹²C-enriched carbon released at the PETM was largely volcanogenic and sourced from the NAIP.Hg abundance and Hg-isotope data are present from well 23/16b-9,located close to the active NAIP volcanism,to investigate the link between NAIP-related volcanic activity and the PETM.It provides the first Hg-isotopic evidence for a causal link between NAIP and the PETM.A marked transient shift ofΔ¹⁹⁹Hg towards 0‰with increased Hg abundance occurs within the PETM onset,indicating a causal link to extrusive volcanic activity from the NAIP.IncreasingΔ¹⁹⁹Hg values through the body of the PETM indicate a protracted interval of magmatism.Towards the end of,and after,the PETM the data suggest an overall waning influence of direct volcanogenic Hg outgassing.This work can explain both the triggering mechanism and long duration of the PETM,supporting a temporal link between CO₂release from volcanic activity and the initiation of the event.4.A compilation of new and existing mercury and other geochemical data across the PETM from 19 globally distributed sites reveals the temporal and spatial distribution characteristics of mercury abundance through the PETM.This new compilation indicates at least intercontinental Hg enrichments related to volcanism～27kyr prior to and within the CIE onset.To gain a global overview of the temporal and spatial distribution of Hg enrichment during the PETM,a compilation of new and existing Hg and associated geochemical data across this event from 19 sites has been analyzed.Our findings suggest that there are extensive and at least intercontinental Hg enrichments～27 kyr prior to and within the CIE onset.These data are consistent with Hg-isotope and osmium-isotope evidence which together indicate that volcanism was the trigger of the PETM.Evidence for volcanism from the Hg data during the CIE body is much more complex and equivocal,and we note in particular a general lack of Hg enrichment in the latter part of the CIE body.Reduced Hg content in our compilation toward and after the end of the CIE,despite local evidence near the NAIP for increased ash deposition,reveal complexity between volcanism and the expression of volcanism in Hg enrichment data.This work also emphasizes how Hg can be influenced by multiple factors,and suggests that distance from the NAIP had a greater impact on Hg content than,for example,water depth.Thus,transport distance of Hg may be a clear cause of the spatial heterogeneity of the PETM Hg records we observe.