Isolating Detrital and Diagenetic Signals in Magnetic Susceptibility Records From Methane‐Bearing Marine Sediments.

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 H2S, 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 H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ13C 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 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. Plain Language Summary: In 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 from X‐ray fluorescence), total organic carbon (TOC) and total sulfur (TS), and authigenic carbonate carbon isotopes to determine which portions of the records reflect primary deposition and which reflect diagenesis associated with abundant TOC or abundant methane. Overall, by decoupling the mixed signal of κ, our approach can allow for improved interpretation of marine sediment sequences globally and has implications for reconstructing the role of detrital magnetic minerals on the marine sediment sulfur cycle and the development of chemosynthetic biological communities at methane seeps. Key Points: Zr/Rb as a heavy mineral proxy tracks well with magnetic susceptibility (κ) in stratigraphy unaltered by diagenesisPyritization driven by both organoclastic sulfate reduction and the anaerobic oxidation of methane results in a relative loss of (κ)At methane seeps pyritization is regulated by detrital Fe‐oxides, limiting or enhancing the role of pyrite as a global sulfur sink [ABSTRACT FROM AUTHOR]