SALINITY IS AN ESSENTIAL ELEMENT FOR LACUSTRINE SYSTEMS, PALEOSALINITY IS CRITICAL IN PALEOENVIRONMENTAL AND PALEOECOLOGICAL STUDIES

Reconstructing the paleosalinity changes in the lacustrine or marine deposits is a key to depict the paleoclimatic and paleoecological evolutions in the past world. However, mechanisms of the paleosalinity changes and its correlation with astronomically forcing in the past remain poorly understood. In this study, Rb/Sr data from the Luo-69 core in the Bohai Bay Basin, East China, was used to cyclostratigraphic analysis. A high-resolution astronomical time scale (ATS) for the lower third member of Shahejie Formation (Es3l) is established using the stable 405 kyr eccentricity tuning. Paleosalinity proxies of the B/Ga, Sr/Ba and S/TOC show a good match with 405 kyr eccentricity cycle, with peak values corresponding with low salintity and trough values corresponding with high salinity. During the peak value of 405 kyr eccentricity with relatively high power of the obliquity signal, stronger East Asian summer monsoon leads to higher riverine flux, and thus low water salinity. Increase in terrestrial organic matter input during high riverine flux period triggered higher primary production and associated with higher nitrogen fixation. These result in higher preservation of organic matter, and higher clay mineral and lower carbonate mineral contents within the bulk sediments. On the contrary, during the minima value of long orbital eccentricity accompanied by low the obliquity singal, weaker summer monsoon leads to lower riverine influx. The higher evaporation and lower precipitation lead to the accumulation of evaporite sediments, and thus high paleosalinity. Therefore, our study indicates the sensitivity of paleosalinity proxies to the astronomically forcing in lacustrine strata. Increase in obliquity power in the upper part of Es3l member (~40.5 Ma) corresponds to the middle Eocene climate Optimium (MECO), in response to high temperature at this interval as recorded in low value δ¹⁸O of benthic marine fossils.