Diagenetic degradation of paleoenvironmental signals in magnetic susceptibility in sediments from the northern Bay of Bengal (IODP Expedition 353)

Magnetic susceptibility (κ) is a useful paleoenvironmental proxy that can track changes in terrigenous input and depositional conditions in marine sediments. However, detrital ferrimagnetic iron oxides that often drive these signals can be dissolved by hydrogen sulfide during sulfate reduction and anaerobic oxidation of methane (AOM). This diagenetic overprint of κ can be difficult to identify. We use rock magnetic and solid phase geochemistry measurements from the upper 50 m below seafloor (mbsf) in marine sediment cores from the Mahanadi Basin, northern Bay of Bengal (International Ocean Discovery Program Sites U1445 and U1446) to highlight the degradation of a primary detrital κ signal by diagenetic dissolution of magnetite and precipitation of pyrite.

Rock magnetic measurements indicate that the primary κ signal is driven by concentration of magnetite with minor contribution of goethite. κ decreases below the sulfate-methane transition (SMT) at both sites, but this decrease is most pronounced at U1445. Fine-grained magnetite is mostly absent below the SMT at both sites, shown by a lack of anhysteretic remanent magnetization. This decrease in κ corresponds to an increase in total sulfur, bulk δ³⁴S, and magnetic coercivity, which is most prominent at the modern SMT (18 mbsf at U1445 and 20 mbsf at U1446) and a second peak below the SMT at each site (26 mbsf at U1445 and 44 mbsf at U1446) representing likely stalled past positons of the SMT.

The amplitude of cyclic variation in κ has been reduced by pyritization of magnetite at both sites, but κ is more severely drawn down at U1445 compared to U1446, likely due to a lower sedimentation rate at U1445 resulting in increased exposure time of magnetite to hydrogen sulfide at the SMT. Site U1445 also contains numerous methane hydrate-bearing turbidites between 30 and 289 mbsf, suggesting the potential for periods of enhanced upward methane flux to the SMT, increasing AOM rates and magnetite dissolution. Due to the potential of diagenetic overprint, we suggest using caution when interpreting changes in paleoenvironmental conditions from variation in κ within methane-bearing continental margin sediments, such as these from the northern Bay of Bengal. Decreases in κ due to diagenesis may be subtle and episodic, requiring geochemical and rock magnetic measurements to identify.