Barium Isotopic Geochemi Stry Of Loess And Latosols

The oceanic cycle of barium(Ba)is closely associated with ocean productivity.Accumulations of Ba and barite are robust indicators of ocean productivity in marine sediments.However,those indicators can be compromised due to changing in barite preservation.With the development of analytical methods in recent years,it is now possible to acquire the high-precision data of Ba isotope compositions.Recent studies have revealed heterogeneous Ba isotope compositions in the ocean,suggesting that Ba isotopes are potential indicators for Ba cycling and paleao-productivity in the ocean.River input is the major source of oceanic Ba.Studies on river systems show that river water is enriched in heavy Ba isotopes with δ137/134 Ba of 0.14±0.10‰(1SD,n=8)relative to the upper continental crust with a δ137/134 Ba of 0.00±0.03‰,implying that heavy Ba isotopes may be preferentially leached out from crustal rocks during continental weathering.Because continental weathering is critical in forming soils and affecting the Ba isotope compositions of river and ocean waters,it is necessary to examine how Ba isotopes are fractionated during the weathering process.To study Ba isotope fractionation during weathering,we analyzed Ba isotope compositions of loess from the China Loess Plateau for illuminating Ba isotope fractionation during weathering under arid-semiarid climate.We also measured Ba isotope compositions of latosols from South China and performed sequential extraction experiments on latosol for deciphering Ba isotope fractionation during weathering under wet monsoon climate.Furthermore,we conducted basaltic dissolution experiments to investigate Ba isotope fractionation during basaltic dissolution.During loess formation in the China Loess Plateau,chemical weathering is dominated by dissolution and precipitation of carbonate minerals,accompanied with the dissolution and transformation of easily weathered silicates and the formation and transformation of Fe(oxyhydr)oxides.Results of sequential extraction experiments on loess show the homogeneity of Ba isotope composition in carbonate and silicates minerals(δ137/134 Ba=～0‰).Therefore,leaching of carbonate minerals cannot lead to the variation of Ba isotope composition in loess.Because loess originate from a wide range and is formed by weak chemical weathering,the Ba isotope composition in loess can be used for estimating Ba isotope composition of the upper continental crust.Data of δ56Fe in loess show that samples have extremely homogeneous Fe isotope composition with δ56Fe ranging from 0.06‰ to 0.12‰,regardless of variations in the major element composition and weathering intensity.Our study indicates that since Fe isotopes are not significantly fractionated during loess formation,the loess can be regarded as representative of upper continental crust.It follows that the average δ56 Fe of upper continental crust is 0.09±0.03‰2SD),consistent with previous estimates based on igneous rock data.To decipher Ba isotope fractionation during extreme weathering,we analyzed a latosol profile developed from basalts from Zhanjiang,Guangdong Province,South China.The profile can be grouped into Ba-depleted layers(D-layers)and Ba-enriched layers(E-Ⅰ,E-Ⅱ,and E-Ⅲ)according to the values of τTn,Ba which evaluates the mass fraction of Ba lost or gained from the weathered profile relative to the bedrock.Theδ137/134 Ba of the soil profile ranges from-0.22‰ to 0.02‰,lower than that of the parent basaltic rocks(0.03 ± 0.03‰,2SD n = 2).A mass balance model suggests that the average δ137/134 Ba of the materials leached from the weathered profile is～0.08‰,indicating the loss of heavy Ba isotopes into the hydrosphere during extreme weathering of basalt.In the D-layers,Ba isotope fractionation can be modelled by Rayleigh fractionation,implying that heavy Ba isotopes are preferentially leached during minerals dissolution.On the contrary,the Ba-enriched layers are not consistent with the Rayleigh fractionation model.They are enriched in isotopically light Ba isotopes.Ba is preserved in different species in soils.Distinguishing Ba isotope composition and partition in diverse species can be helpfut to decipher Ba isotopic behaviors during weathering.Sequential extraction experiments have been widely used to separate metals into different species.We performed sequentially extraction experiments on latosol to separate Ba in exchangeable forms,easily reducible Fe-Mn(oxyhydr)oxides(such as ferrihydrite and lepidocrocite),and the residue.Results of sequential extraction experiments show that most of Ba is preserved in exchangeable fraction or easily reducible Fe-Mn(oxyhydr)oxides in Ba enriched layers.The exchangeable Ba is the dominant fraction below E-I.After being released from minerals,Ba can be adsorbed on secondary minerals or precipitated on easily reducible Fe-Mn(oxyhydr)oxides in weak eluviation condition.Those processes account for the preserving of Ba in latosols.δ137/134 Ba in exchangeable fraction(-0.28‰ to-0.15‰)and easily reducible Fe-Mn(oxyhydr)oxides(-0.32‰ to-0.16‰)is lower than that in bedrocks(0.03±0.03‰)and bulk soil(-0.22‰ to 0.02‰)implying that light Ba isotopes are preferential to be adsorbed on secondary minerals and associated in easily reducible Fe-Mn(oxyhydr)oxides.In summary,those processes play important roles in the preservation of lighter Ba isotopes in the latosol.The residual Ba is preserved in crystalline phases.The iRe-Ba is defined to evaluate the mass fraction of Ba loss from the crystalline relative to the bedrock.The correlation between δ137/134BaRe and τRe-Ba shows that δ137/134BaRe decreases with the loss of Ba underneath the residue above E-Ⅲ;but δ137/134BaRe increases with the loss of Ba in the E-Ⅲ.In the E-Ⅲ,samples contain a large amount of alkali and alkaline earth metals(MgO>2.6 wt.%,CaO>1.7 wt.%,Na2O>0.5 wt.%,K2O>0.2 wt.%)suggesting the presence of alkali and alkaline earth metals enriched minerals such as smectites.Therefore,the crystal Ba may be mainly preserved in these minerals.On the contrary,the depletion of alkali and alkaline earth metals(MgO<0.2 wt.%,CaO<0.1 wt.%,Na2O<0.1 wt.%,K2O<0.1 wt.%)above E-Ⅲ implies the decomposition of minerals enriched in alkali and alkaline earth metals.The crystal Ba is mainly preserved in minerals with depleted alkali and alkaline earth metals such as kaolinites.In the E-Ⅲ,the δ137/134Ba in the residue increases with the τRe-Ba decreasing implying that light Ba isotopes prefer to be released during dissolution of minerals enriched in alkali and alkaline earth metals enriched(e.g.smectites).In samples above E-Ⅲ,the δ137/134Ba in the residue decreases with decreasing τRe-Ba suggesting that heavy Ba isotopes prefer to be released into fluid during dissolution of minerals depleted in alkali and alkaline earth metals(e.g.kaolinites).Results of basaltic dissolution experiments suggest that although the oxalate anion can accelerate the release of Ba from basalts,it does not result in significant Ba isotope fractionation.During the basaltic dissolution,light Ba isotopes prefer to be released into the solution,with Δ137/134Bafluid-solid=-0.55‰ in the solution of ammonium chloride,and Δ137/134Bafluid-solid=-0.61‰ in ammonium oxalate.It is in concordance with the results of sequential extraction experiments.Our studies illuminate the mechanism of Ba isotopes fractionation during the dissolution of minerals,adsorption on secondary minerals and formation of easily reducible Fe-Mn(oxyhydr)oxides.Those processes play important roles in Ba cycles of earth’s surface.