Authigenic Ferrimagnetic Iron Sulfide Preservation Due to Nonsteady State Diagenesis: A Perspective From Perseverance Drift, Northwestern Weddell Sea

Abstract

• Magnetic mineral diagenesis is studied in Antarctic sediments using high-resolution porewater and rock magnetic data • Magnetic mineral dissolution and authigenesis is tightly coupled to dissolved sulfide and ferrous iron concentrations • Remanence-bearing iron sulfides form below the sulfate-methane transition and can be preserved following nonsteady state diagenesis

A Geochemical Mechanism for >10 m Apparent Downward Offsets of Magnetic Reversals Inferred From Comparison of Two Scotia Sea Drill Sites

“…This may not be surprising if the concentrations reflect the degree of dissolution during reductive diagenesis, as it is often observed that hematite is more resistant to dissolution than magnetite (Dillon & Bleil, 2006; Garming et al., 2005; J. Liu et al., 2004; Poulton et al., 2004; Reilly et al., 2020; Yamazaki et al., 2003). However, we also note a few anomalously high HIRM values in the U1538 stratigraphy without corresponding increases in other concentration dependent parameters, which are reminiscent of horizons with increased authigenic iron sulfide preservation that saturate above 300 mT, such as 3C pyrrhotite (Reilly et al., 2020).…”

“…Magnetic mineral authigenesis is largely controlled by porewater geochemistry, particularly through the production of dissolved Fe 2+ during Fe 3+ reduction and the production of H 2 S during SO 4 2− reduction (Amiel et al., 2020; Berner, 1984; Canfield & Berner, 1987; Karlin & Levi, 1983; Reilly et al., 2020). The order in which electron acceptors are used in microbial respiration is governed by their energetic favorability (Froelich et al., 1979), with Fe 3+ being more energetically favorable than SO 4 2− .…”

“…As a result, in situ dissolved Fe 2+ concentrations before porewater extraction may have even been higher than shown in Figure 2. While several mechanisms may be responsible for the Fe 2+ profile shown in Figure 2, the presence of dissolved Fe 2+ below SO 4 2− indicates that its production and/or diffusion rates are greater than its consumption rate by H 2 S. This type of geochemical environment, where H 2 S is limited with respect to Fe 2+ , is favorable for authigenic ferrimagnetic iron sulfide formation (Amiel et al., 2020; Kao et al., 2004; Reilly et al., 2020), with the sulfide formation depth most likely at the interface between upward diffusing Fe 2+ and downward diffusing H 2 S near the base of SO 4 2− reduction. These conditions occur at approximately 46 mbsf at Site U1538 today, but this sulfide formation depth likely varied through time as environmental conditions changed.…”

“…However, in nature, there are mechanisms by which Fe 3+ can also be used as an electron acceptor at greater depths, such as the proposed anaerobic oxidation of methane coupled with iron reduction (Beal et al., 2009; Chuang et al., 2021; Riedinger et al., 2014), in which Fe 2+ can accumulate below the SO 4 2− reduction zone. These mechanism and others can result in a geochemical environment where Fe 2+ accumulates more rapidly than it can be consumed by H 2 S, creating conditions that favor precipitation and preservation of sulfide‐limited iron sulfides, including the ferrimagnetic phases greigite and/or hexagonal 3C pyrrhotite, over pyrite (Blanchet et al., 2009; Horng, 2018; Horng & Roberts, 2018; Hunger & Benning, 2007; Kao et al., 2004; Reilly et al., 2020; Roberts et al., 2011; Rowan et al., 2009). Direct comparison of high‐resolution porewater geochemistry and magnetic measurements has clearly demonstrated ferrimagnetic enrichment where Fe 2+ concentrations are enhanced below the SO 4 2− reduction zone (Amiel et al., 2020; Reilly et al., 2020).…”

Control of Source‐To‐Sink Processes on the Dispersal, Fractionation, and Deposition of Magnetic Minerals in a Tropical Mesotidal Estuarine System

“…In contrast, relatively lower values of SIRM/χ lf (Figure 3e) followed by a consistent trend in the majority of the samples (except in Khandepar River) suggest that early reductive diagenesis has minimal impact on the Mandovi estuarine sediments. This can be explained by the fact that an enormous supply of detrital iron oxides from fluvial sources to the Mandovi estuary might have hindered the diagenetic reactions and hence the detrital magnetic particles were less affected by dissolution and remained preserved (Amiel et al, 2020;Gaikwad et al, 2021;Kao et al, 2004;Reilly et al, 2020). Biogenic magnetic particles synthesized by magnetotactic bacteria are ubiquitously found in estuarine environments (Hilgenfeldt, 2000;Hounslow & Maher, 1996;Kirschvink & Chang, 1984;Kopp & Kirschvink, 2008;Petersen et al, 1986;Roberts, 2015;Stolz et al, 1986).…”