Paleomagnetic Imprints of Sulfate Reduction Pathways in Continental Shelf Sediments: Organoclastic Versus Anaerobic Oxidation of Methane

Abstract

Marine continental shelf sediments with high deposition rates may provide useful archives of rapid geomagnetic secular variation as long as the primary magnetization is not altered substantially by diagenesis. To quantify the effects of sulfate (SO42‐) reduction, which is a dominant early diagenetic process in such sediments, on paleomagnetic recording, we analyzed four 6‐m long sediment cores from the eastern Mediterranean shelf. Two cores did not reach the methanogenic zone and are characterized by continuous organoclastic sulfate reduction (OSR), while the other two have a distinctive shallow sulfate‐methane transition zone (SMTZ). Age models based on 28 radiocarbon ages indicate steady deposition rates with spatially varying age spans, which suggest that different parts of the shelf stopped accumulating sediments at different times during the Holocene. The upper sediment column in all cores is dominated by detrital titanomagnetite and biogenic magnetite. OSR‐affected sediments record continuous (titano) magnetite dissolution, which resulted in steady magnetic susceptibility and remanence decreases. For cores that reach the methanogenic zone, similar behavior is observed at or above the STMZ, but magnetic properties stabilize at greater depths. Paleomagnetic directions in these sediments are more coherent, with better agreement with geomagnetic models than sediments affected by OSR. We suggest that methane‐rich sediments with a shallow SMTZ and high sedimentation rates can better preserve primary paleomagnetic signals than OSR‐dominated sediments due to a lack of dissolved sulfide in the main methanogenic zone, and that a susceptibility decline with depth should be a warning sign for paleomagnetic studies.