Journal
SEDIMENTARY GEOLOGY
Volume 421, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.sedgeo.2021.105963
Keywords
Nitrogen fixation; Denitrification; Eocene; Paleosalinity; Shahejie Formation
Categories
Funding
- Chinese Postdoctoral Innovative Talent Support Program [BX20200312]
- National Science Foundation of China [41690131]
- National Key Basic Research Program of China [2017ZX05049004-001]
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Nitrogen isotopes have been studied in the Bohai Bay Basin to understand the paleoenvironmental factors affecting sedimentary conditions. The results suggest a correlation between nitrogen isotopic signals and ancient salinity changes, potentially related to marine transgression events and stratification of the water column.
Nitrogen isotopes have been widely used as indicators of depositional conditions in studies of ancient sediments. Bohai Bay Basin is a major continental hydrocarbon province in NE China, but nitrogen isotopic signatures of this lacustrine system(connected with the open ocean occasionally) have barely been investigated. We report a study of paleoenvironmental information recorded by N isotopic variations in the lower third member (Es3L) and upper fourth member (Es4U) of the Eocene Shahejie Formation in the Jiyang Sub-basin, southern Bohai Bay Basin. Paleosalinity (B/Ga, Sr/Ba, and S/TOC) and paleoredox (C-org/P, DOP, U-EF, and Fe/Al) proxies and delta C-13(org) values were used to elucidate paleoenvironmental factors that affect N isotopic signals in the study core. Results indicate that delta N-15 values are positively correlated with paleosalinity proxies in the Luo-69 and Niu-1 drill-cores. Two possibilities may account for this: (1) a paleosalinity surge caused by marine transgression events led to stratification of the water column, creating an anoxic-suboxic deep-water layer and promoting denitrification in sediments; or (2) strong stratification of the water column impeded the upwelling process that brought nutrients to the upper water layer, weakening nitrogen fixation there. In marginal-marine intervals such as the Eocene Shahejie Formation, water-column salinity may play a critical role in regulating the delta N-15 signal. This study indicates that nitrogen isotopic compositions may aid paleoenvironmental and paleoclimatic interpretations of marginal-marine shale, particularly when applied together with multiple paleoenvironmental proxies to improve the understanding of ancient sedimentary conditions in marginal-environment. (C) 2021 Elsevier B.V. All rights reserved.
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