Isolating Detrital And Diagenetic Signals In Magnetic Susceptibility Records From Methane-Bearing Marine Sediments

Volume-dependent magnetic susceptibility (kappa) 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, kappa is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post-depositional alteration by H2S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases kappa. Here, we provide a new approach for isolating the detrital signal in kappa and identifying intervals of diagenetic alteration of kappa 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 kappa in unaltered sediments, we predict the primary detrital kappa signal and identify intervals of decreased kappa, which correspond to increased total sulfur content. Our approach is a rapid, high-resolution method that can identify overprinted kappa resulting from pyritization of titanomagnetite due to H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate delta C-13 measurements indicate that both OSR and AOM can drive the observed kappa loss, but AOM drives the greatest decreases in kappa. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo-positions 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.