Variation And Their Mechanisms Of Elemental And Sr-Nd Isotopic Systematics During Chemical Weathering Of Limestone:a Case Study In The Northern Guangdong,South China

Limestones are broadly outcropped on earth surface.They are very easily weathered,which contributes the main of the dissolved alkaline earth elements,such as Ca and Sr.Therefore,limestones weathering not only plays an important role in regulating the budgets of many elements to rivers and hence to the oceans,but also likely influence Sr isotopic composition in river water,and then regulate the global Sr isotope budgets.However,the variations and their mechanisms of elemental and Sr isotopic systematics during chemical weathering of limestone,is still a matter of debate.Besides,consecutive variation of elemental and strontium isotopic compositions in weathering profile have not been investigated.In this work,we present a detailed study of major and trace elements,as well as Sr and Nd isotopic compositions of the saprolites sampled from a weathering profile developed on limestone in the northern Guangdong province,South China,in order to?i?get an insightful understanding of the mobilization and re-distribution of major and trace elements during weathering of limestones,?ii?reveal the mechanism for the variations of Sr isotopic compositions during limestones weathering.The main findings of this study are as follows.1.The results indicate that almost all of Sr and Ca was removed from the profile by aqueous solution during moderate chemical weathering of limestones,due to high solubility of calcite.while,the other alkaline earth metals,such as Mg,and Ba and alkali metals,as well as the transition metals,are relatively enriched in the upper profile,mainly owing to adsorption of these elements by clay minerals.2.It is worth noting that the UCC-normalized rare earth element?REE?pattern of the saprolites in the profile is characterized by a stronger enrichment in HREE?heavy REE?relative to LREE?light REE?.The mobilization and re-distribution of the REEs in this profile was controlled by the variation of p H values.In the nearly neutral aqueous solution,the REEs tend to form complexes with carbonate ion.As a result,their capacity of transferring is significantly improved.At the sections in basic conditions,the carbonate complexes decompose,and the REEs are deposited and enriched.Given that HREE were preferentially combined with carbonate ion compared to LREE,HREEs are likely easier to be transported.As a result,more HREEs will be enriched at the lower sections than LREEs,showing a significant HREE-enriched fractionation patterns of this profile.3.Interestingly,the results also show that the Sr isotope exhibited a large variation range,with ⁸⁷Sr/⁸⁶Sr ratios varying from 0.70979 at the bottom of the profile to 0.72216 at the top.Well,the potential extraneous inputs,such as river water and rain water,as well as dust deposits,appear not contribute much extraneous Sr to the saprolites of the profile.It is worth noting that a significant negative correlation between ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations happens on the profile,implying that the potential scenario for the variations of Sr isotope composition largely attributed to the incongruent weathering of minerals during limestone weathering,as there are dramatic differences in resistance to weather and ⁸⁷Sr/⁸⁶Sr ratios between silicate and carbonate minerals.As a result,dissolved Sr sourced from the weathering of calcite,were removed from the weathering profile obviously prior to Sr occurred in silicate minerals.Finally,⁸⁷Sr/⁸⁶Sr ratios of the saprolites move towards the values that are similar to the silicate minerals with the strengthening of the chemical weathering.