The effect of lake level on sulfur isotope systematics in the Lockatong Formation of the Newark Basin, New Jersey.

摘要

The Lockatong Formation is composed of lacustrine sediments that were deposited in the Newark Basin during the Late Triassic (∼222-218 Ma). This formation is characterized by black, organic rich layers interspersed with iron-oxide rich, red layers at various intervals, both of which contain pyrite, indicating that the lake supported a community of sulfate reducing bacteria. The stratigraphic patterns of this formation reflect cyclic depositional environments of the ancient lake from a deep, productive freshwater lake to a playa or completely dried out lake bed. Wet-dry cycles forced by climatic changes are proposed as the primary influence on the sulfate concentration in the paleo-lakes of the Newark Basin, which influences the delta 34S of sulfide, produced from bacterial sulfate reduction and is preserved in pyrite.;In modern closed systems, stratification influences sulfate concentration. Deep lakes with euxinic bottom waters have sulfate concentrations that limit bacterial sulfate reduction producing sulfide with high delta34S values, which are preserved in sedimentary pyrite. Conversely, low lake levels have a higher sulfate concentration, resulting in more fractionation and lower delta 34S values. This study tests this hypothesis using the varying lake levels in the Triassic Newark Basin. Comparison of delta34S values from pyrite and sedimentary depth rankings show that deep lake pyrite samples have relatively high delta34S values (+5 +/- 5 ‰) while shallow lake deposits have lower values (-6‰ +/- 5 ‰). These findings indicate a relationship between climatically controlled lake level and delta34S values that are preserved in sedimentary pyrite.