Barium Isotope Compositions Of Barite:Analytical Methods,Digenesis And The Application For The PETM Study

Marine export production is a key parameter controlling global carbon cycle and climate change.As the bio-barite accumulation rate（BAR）in marine sediments is well correlated with the organic carbon export flux in overlying column,the BAR has been commonly used to reconstruct paleo-ocean export production.However,because the BAR may be affected by the dilution of carbonates minerals and the variable preservation rate of bio-barite,these effects will generate great uncertainty in reconstruction of the paleo-ocean export production.Recently,Ba isotope composition of bio-barite（excess Ba）has been proposed as a novel tool to reconstruct marine export production based on the positive correlation between Ba isotope composition of biobarite and BAR.Compared with BAR,the Ba isotope composition of bio-barite is insensitive to dilution and preservation rate of bio-barite,which can reliably indicate the change of paleo-ocean export production.In the context of previous study,this dissertation aims to trace the export production in equatorial Pacific during the Paleocene-Eocene thermal maximum（PETM）using Ba isotope composition of bio-barite.For this purpose,it is necessary to establish the reliable barite Ba isotope analysis methods and understand whether the Ba isotope composition of bio-barite will be affected by sedimentary diagenesis.Therefore,we established reliable and rapid barite Ba isotope analysis methods,then studied the diagenetic effect on the Ba isotope of barite,and finally used the bio-barite Ba isotope data to trace the paleo-ocean export production during the PETM period.Firstly,we develop a practical method of high precision Ba isotope measurement for barite using Na2CO3 exchange reaction,one column cation resin（0.5 mL,AG50WX12）,and 135Ba-136Ba double-spike method by MC-ICP-MS.The precision and accuracy of this method were tested by measurement of a synthetic barite standard made by reacting Ba（NO3）2 with Na2SO4.The δ^138/134Ba value of the synthetic barite is 0.07± 0.03‰,which is well consistent with the recommended value（0.09 ± 0.05‰）.The robustness of this method was further assessed by replicated analyses of six barite reference materials,which cover a large range of BaSO4 contents from 18. ⁸⁷ wt.%to nearly 100 wt.%.The δ^138/134Ba value of pure barite international standard NBS127 and natural standards GBW07811,07812,07814,07815 and 07816 are is-0.28 ± 0.03‰,0.08± 0.07‰,0.32± 0.07‰,0.03± 0.03‰,-0.06± 0.06‰,-0.02± 0.06‰,respectively.The efficient analytical methods are very important for geological application.Based on Na2CO3 exchange reaction,we establish a rapid and simple method for high precision Ba isotope measurement of barite using H2O extraction.H2O was used as a dispersant to rapidly extract～20-75 μg Ba as barite micro-particles（invisible to the naked eye）into suspension,and then these micro-particles were easily dissolved in 3 mol/L HCl.The precision and accuracy of this method were tested by repeatedly analyzed of five barite standards（NBS127,GBW07811,GBW07812,GBW07814,and GBW07816）.The Ba isotope compositions of these barites using the H2O extraction method are well consistent with the data previously measured by the Na2CO3 exchange method.Compared with the Na2CO3 exchange method,the H2O-extraction method reduces the time of the chemical process from five days to two hours and significantly simplifies the digestion procedure and improves the efficiency of barite Ba isotope analyses.Secondly,this thesis studies the Ba isotope compositions of diagenetic barites in the Early Cambrian Yurtus formation in the Tarim Basin,and proposes to use ⁸⁷Sr/ ⁸⁶Sr to monitor the effect of diagenesis on the Ba isotope composition of barite.Primary barite will dissolve and reprecipitate if the sulfate concentration in the porewater drops to zero at reducing environments.The impacts of barite dissolution and reprecipitation on Ba isotope of barite are unclear.Therefore,it is necessary to understand the influence of diagenesis on barite Ba isotopes before using barite Ba isotopes to reconstruct paleoocean production.The deviation of ⁸⁷Sr/ ⁸⁶Sr ratio of diagenetic barite with respect to Early Cambrian seawater suggests that the ⁸⁷Sr/ ⁸⁶Sr values of barite were modified by siliciclastic sediments.The δ^138/134Ba values of diagenetic barite is positively correlated with ⁸⁷Sr/ ⁸⁶Sr,indicating that the barium of diagenetic barite may also be modified by surrounding sediments.Therefore,we propose that the barites with low ⁸⁷Sr/ ⁸⁶Sr（consistent with coeval seawater）and low δ^138/134Ba can represent the primary barite,while the barites with ⁸⁷Sr/ ⁸⁶Sr ratio higher than coeval seawater and high δ^138/134Ba may be modified by diagenesis.We suggest to use the combination of ⁸⁷Sr/ ⁸⁶Sr andδ^138/134Ba of barite to exclude the influence of diagenesis when using barite Ba isotope data to trace paleo-ocean production.Thirdly,we use Ba isotopes of bio-barite to trace the marine export production in the equatorial Pacific during the PETM period.The PETM（～56 Ma）was a transient global warming event associated with a huge perturbation to the global carbon cycle.Changes in marine export production may contribute to the rapid recovery from this climate change event,by driving the burial of inorganic and organic carbon.This study investigates the Ba isotopic composition of bio-barite from the sediments of Site 1221 in the equatorial Pacific.The δ^138/134Ba values of bio-barites are 0.07±0.06‰ before PETM,significantly decreased to-0.06 ± 0.05‰ following the PETM onset,and increased to a maximum of 0.18± 0.02‰ during the recovery period.After we exclude the influence of diagenesis and exogenous input on the Ba isotope of bio-barite,the variation of Ba isotopes of bio-barites during PETM period indicates the trend of decrease and then increase of ocean export production in the equatorial Pacific.The similar pattern was observed for the δ^138/134Ba value of excess Ba in the sediments at Southern Ocean（Site 690）and South Atlantic（Site 1263）.Combining the δ^138/134Ba of bio-barite and excess Ba in these three sites,it is clear that the magnitude of variations in ocean export production during the PETM period decreased with increasing latitude.This may indicate that the abrupt changes in climate have a pronounced impact on the export production of low-latitude oceans,and subtle impact on the export production of high-latitude oceans.