Anaerobic oxidation of methane and greigite formation: Evidence of isotopically heavy pyrite in Pleistocene coastal sediments from the South Yellow Sea

Abstract

Diagenetic alteration of magnetic minerals, driven by closely linked C-S-Fe cycles, is highly likely to complicate the paleomagnetic record. In addition to the anomalous diagenetic paleomagnetic signatures caused by ferromagnetic greigite growth, pyrite sulfur isotope compositions are often “heavy” (i.e., δ³⁴S_pyr > 0). However, the dependencies and mechanistic origins of these signatures remain controversial. This study presents a high-resolution δ³⁴S_pyr record of a long sediment core collected from the South Yellow Sea, China. Ferromagnetic greigite is prominently identified in two coastal deposits within this core. The δ³⁴S_pyr values of these coastal deposits are isotopically (super) heavy, ranging from −10.6 to 22.8‰ and from −14.5 to 26.5‰, with mean and 1σ values of 5.9 ± 10.3‰ (n = 15) and 12.2 ± 9.8‰ (n = 33), respectively. Additionally, magnetic parameters show positive trends with δ³⁴S_pyr values throughout the sediment core. These positive trends, along with the enrichment of ferrous iron and sedimentary microtextural evidence of the authigenic growth sequence of framboidal pyrite, siderite, euhedral pyrite, and greigite, indicate that anaerobic oxidation of methane (AOM) is a fundamental factor for ferromagnetic greigite formation in coastal sediments with sulfate limitation. We estimate the delay time of greigite formation relative to the depositional age of surrounding sediments to be a few hundred years due to the rapid sedimentation rates and shallow burial depths of the sulfate-methane transition zone (SMTZ) in coastal deposits. Conversely, the deep burial of SMTZ likely suggests that a longer delay time is prevailing for greigite formation in hemipelagic sediments. This study highlights the role of AOM in controlling the formations of greigite and coeval (super) heavy pyrite.