Volume-dependent magnetic susceptibility (κ), the magnetic response of bulk sediment to an applied magnetic field, is indicative of relative abundances of paramagnetic, diamagnetic, and ferrimagnetic minerals, and thus is used extensively to reconstruct paleoenvironments, decipher depositional processes, examine changes in sediment provenance, and understand diagenetic processes in terrestrial and marine settings Abstract Volume-dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post-depositional alteration by H 2 S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane-bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X-ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high-resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H 2 S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ 13 C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleopositions of the sulfate-methane transition zone, past enhancements of OSR or paleo-methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps.
Plain Language SummaryIn continental margin environments, variation in the magnetic susceptibility (κ) of marine sediments is influenced primarily by the delivery of detrital magnetic minerals from continental erosion and secondarily by diagenetically driven dissolution and/or growth of new magnetic minerals within the sediments. Bulk sediment measurements of κ often record a mixed signal from these processes, making it difficult to distinguish the original depositional detrital signal from in situ diagenetic effects. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration in methane-bearing marine sediments in the offshore Krishna-Godavari basin, eastern peninsular India. We utilize measurements of κ, magnetic properties, a heavy mineral proxy (Zr/Rb ...