TESTING WATER COLUMN REDOX CONTROL ON THE OFFSET OF PAIRED KEROGEN-PYRITE SULFUR ISOTOPES

Analyzing marine sediments can provide insights into the ocean’s redox state through time, helping to develop a framework for understanding Earth’s history. Sulfur isotopes (δ34S) measured in ancient marine sediments from around the world have been analyzed to reconstruct past global marine redox and estimate past atmospheric oxygen content. Sulfur is commonly buried in its reduced form as the mineral pyrite (FeS2), however the δ34S values of diagenetic pyrite convolve many local biological, chemical, and physical processes that make distinguishing local from global isotope signals difficult. Here, we investigate the utility of adding coeval δ34S measurements from sulfur bound into kerogen, to track environmental conditions recorded in δ34S.

To explore the relationship between water column redox and sulfur isotopes, we measured the kerogen and pyrite δ34S, total organic carbon (TOC), and Fe from International Ocean Discovery Program (IODP) ocean cores representing different depositional environments. For this study we focused on comparing two localities with varying water column redox, samples from the Agulhas Current and Baltic Sea. The samples included surface samples (within the top 10% of depths), ones below the surface but before the SMTZ (sulfur methane transition zone), and five within the deep portion of the core. We did a sequential sulfur extraction on a single sediment aliquot to separate the different sulfur phases. The δ34S values for the pyrite and kerogen fractions were then measured using an Elemental Analyzer and Isotope Ratio Mass Spectrometer. We predict sulfurization of organic matter in an anoxic water column leads to the formation of δ34S-depleted kerogen and a smaller kerogen-pyrite offset. Our results will help us link sulfur cycling to water column redox and organic and iron input, allowing for better predictions to be made about key aspects of the marine system we seek to reconstruct. Understanding the environmental information recorded in the δ34S offset for coeval pyrite and kerogen in the modern era will give us new insights on using sulfur records to reconstruct the biogeochemical response of the ocean due to past climatic events and varying oxygen levels of the water column.