Paleosalinity reconstruction in offshore lacustrine basins based on elemental geochemistry:a case study of Middle Upper Eocene Shahejie Formation,Zhanhua Sag,Bohai Bay Basin  

作　　者：Long SUN Shenghe WU Dali YUE Wenfu CUI 

机构地区：[1]National Key Laboratory of Petroleum Resources and Engineering,China University of Petroleum(Beijing),Beijing 102249,China [2]College of Geosciences,China University of Petroleum(Beijing),Beijing 102249,China [3]Shengli Oil Production Plant,Shengli Oilfield Company,SINOPEC,Dongying 257400,China

出　　处：《Journal of Oceanology and Limnology》2024年第4期1087-1105,共19页海洋湖沼学报（英文）

基　　金：Supported by the National Natural Science Foundation of China(No.42272110)。

摘　　要：Salinity is a crucial property of water body and is essential for the restoration of paleoecology and paleoenvironment.However,the theoretical method of application of elemental geochemical proxies to paleosalinity reconstruction is still underdeveloped.Moreover,accurate determination and reconstruction of paleosalinity and its variation in an offshore lacustrine basin have been extremely challenging thus far.This study presents detailed elemental geochemical investigations from the Zhanhua Sag in the southern Bohai Bay Basin to reconstruct the salinity variation in the Paleogene Eocene Shahejie Formation(50.8-33.9 Ma).Based on the variation of strontium barium ratio(Sr/Ba)and boron gallium ratio(B/Ga),we determined three typical salinity types of water body:salty water(Sr/Ba>0.5,B/Ga>6),brackish water(0.2<Sr/Ba<0.5,3<B/Ga<6),and fresh water(Sr/Ba<0.2,B/Ga<3),after eliminating carbonate-sourced strontium(Sr).The salinity values following Couch’s paleosalinometer r anged from 3.1 to 11.9,reflecting the overall characteristics of oligohaline(0.5<salinity value<5)to mesohaline(5<salinity value<18)brackish water.All proxies yielded similar trends in paleosalinity variation,demonstrating a clear trend of rising and then declining from 50.8 Ma to 33.9 Ma.We considered that the B/Ga ratio had the highest reliability and resolution in determining the salinity types of water body in the study area.The environmental factors causing paleosalinity variation were also thoroughly analysed based on the temporal relationship among the salinity types of watermasses,paleoclimate characteristics from pollen records,and marine transgression events from marine fossils.Our research established a model of paleoclimatic and eustatic mechanisms to explain paleosalinity variation,providing reasonable and integral driving forces for the salinity variation of all offshore lacustrine basins.

关 键 词：PALEOSALINITY PALEOCLIMATE Zhanhua Sag Middle-Upper Eocene Shahejie Formation 

分 类 号：P618.13[天文地球—矿床学]