Diagenetic barite-calcite-pyrite nodules in the Silurian Longmaxi Formation of the Yangtze Block, South China: A plausible record of sulfate-methane transition zone movements in ancient marine sediments

Abstract

The sulfate-methane transition zone (SMTZ) serves as an important geochemical transition especially during early diagenesis. Organoclastic sulfate reduction (OSR), sulfate-driven anaerobic oxidation of methane (SD-AOM), fermentation, and methanogenesis, the dominant diagenetic processes near the SMTZ, play key roles in marine sulfur and carbon cycling. However, the paleo-SMTZ has rarely been identified in the geologic record because of equivocal evidence. The presence of barite-calcite-pyrite assemblages (abundant nodules and pyritic laminae) concentrated over a 1-m thick interval of the Silurian Longmaxi Formation, Yangtze Block, South China, provide a unique opportunity to investigate possible vertical movements of the paleo-SMTZ in ancient sediments. The inferred paleo-SMTZ comprises nodules made up of barite (BaSO4), calcium carbonate (CaCO3), and pyrite (FeS2), and pyritic laminae. Strong 34S enrichment of barite (50 to 62.1‰ VCDT) and modestly 13C-depleted calcite (−11.4‰ to −5.3 VPDB) of the nodules associated with positive δ34S values of pyritic laminae (4 to 6.2‰ VCDT) suggest precipitation of these minerals sourced by mixed carbon and sulfur sources close to and/or within the paleo-SMTZ. Nodules from center to edge display mineral zonation that may reflect the effects of depth fluctuations of the paleo-SMTZ. Initial (stage 1) barite mineralization in nodule centers appears to have occurred within the base of the sulfate reduction zone (SRZ). By the time if stage 2 mineralization, the stratigraphic locus of precipitation had migrated closer to highly alkaline SMTZ resulting in barite dissolution and calcium carbonate precipitation. Higher proportions of calcium carbonate exhibiting some enrichment of 12C associated with cubic pyrite crystals in nodule centers are consistent with the occurrence of SD-AOM within the SMTZ. Stage 3 acicular barite mineralization along nodule edges likely took place near at the base of SRZ immediately above the SMTZ, whereas the replacement of barite by calcite during stage 4 mineralization occurred again within the SMTZ. Vertical movement of paleo-SMTZs suggested by the occurrences of barite-calcite-pyrite assemblages in the Longmaxi Formation across Yangtze Block, are likely the consequence of the complex interplay of varying sedimentation rate, organic matter quality, methane flux, and perhaps seawater sulfate concentration.