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