Meteoric Diagenesis In Pleistocene Carbonate Rocks In The Xisha Islands And Its Implications For Paleoclimate

Carbonate platform is mainly developed in the near surface Marine environment,which is susceptible to the transformation of meteoric water diagenesis caused by sea level drop.The response of carbonate platforms to meteoric water modification shows different mineralogical,petrological and geochemical characteristics due to the different climate and rainfall between regions.The Xisha Islands are located in the tropical sea area in the northern part of the South China Sea.Carbonate platform deposits have been developed extensively since the Early Miocene（～20 Ma）.Previous studies have shown that the East Asian monsoon（EAM）had an important influence on sedimentary and diagenetic processes in most parts of East Asia and South Asia during the Cenozoic.However,the connection between EAM and the widespread development of Marine carbonate platforms in the region remains unclear.Based on the above background,the well CK-2 is a deep drilling（928.75 m）,drilled on Chenhang Island,Xisha Islands,the core from the upper237 m section,the petrographic characteristics,carbon and oxygen isotopes(δ¹³C,δ¹⁸O),clumped isotope(Δ₄₇),trace elements（Sr,Fe,Mn）and rare earth elements（REE+Y）are analyzed.To explore the diagenesis of meteoric water in the Pleistocene carbonate rocks and its relationship with the East Asian Summer Monsoon（EASM）,the results show that:（1）The Pleistocene study interval of CK-2（237-16.7 m）has been subjected to extensive meteoric water diagenesis,forming four obvious exposed surfaces（denoted as A,B,C,D）,which can be divided into four stratigraphic units:Unit 1（185-237 m）is mainly composed of LMC（low magnesium calcite）and content<10%of dolomite,unit 2-4（16.7-185 m）are mainly LMC.The development time of these four exposed surfaces was consistent with the global sea-level,which shows that the four sedimentary discontinuities of CK-2 in Pleistocene should be generated by sea-level decline.（2）The Pleistocene carbonate succession of well CK-2 has undergone neomorphism,meniscus,fibrous,prismatic,and blocky calcite,etc.cementation,and dissolution.The upper part（16.7-150 m）is dominated by meteoric diagenetic environment,supplemented by a small amount of seawater diagenetic environment,and the two diagenetic environments sometimes alternate.The lower part（150-237 m）is dominated by seawater diagenetic environment.（3）Geochemical parameters have a good correspondence to the characteristics of diagenetic transformation in meteoric water.The sensitivity ofδ¹⁸O to meteoric water is higher than that of trace elements and cluster isotopes(Sr、Mn、Fe、REE+Y、δ¹³C、Δ₄₇).The comprehensive analysis ofΔ₄₇andδ¹⁸O indicates that the limestones of Ledong Formation of Xisha Islands in the South China Sea were formed in two stages,from meteoric water-seawater to meteoric water,with the fluid temperature ranging from 22.1to 32.1℃.（4）The diagenetic characteristics of meteoric water during the exposure period are related to the enhancement of summer monsoon.The B exposed surface（0.9±0.5 Ma）at the top of Unit 2 in CK-2 has the most negativeδ¹³C value（-6.2‰）and the thickest calc red soil（16.5m red-brown iron oxide）.Fe,Mn andΔ₄₇show significant increases near the retention surface,which is basically consistent with the time of summer wind intensification reported in other studies.The exposed surfaces A and C which are not obviously modified by meteoric water indicate that the degree of diagenesis of meteoric water is not strong,and the strengthening degree of summer monsoon is weakening.Based on the petrology and geochemistry data from the Pleistocene section of CK-2,this paper reveals the response and record of meteoric water diagenesis on the carbonate platform in Xisha area to the East Asian monsoon.Furthermore,this study provides insight to the diagenesis on other carbonate platforms in monsoon regions around the world.