Description: The occurrence of microbially induced smectite-to-illite (S-I) reaction has challenged both the notions of solely inorganic chemical control for this reaction and the conventional concept of a semiquantitative illite geothermometer for the reconstruction of the thermal and tectonic histories of sedimentary basins. Here, we present evidence for a naturally occurring microbially induced S-I transition, via biotic reduction of phyllosilicate structural Fe(III), in mudstones buried at the Nankai Trough, offshore Japan (International Ocean Discovery Program Site C0023). Biotic S-I reaction is a consequence of a bacterial survival and growth strategy at diagenetic temperatures up to 80 °C within the Nankai Trough mudstones. These results have considerable implications for petroleum exploration, modification of fault behavior, and the understanding of microbial communities in the deep biosphere.

Description: The Cretaceous period (~145–65 m.y. ago) was characterized by intervals of enhanced organic carbon burial associated with increased primary production under greenhouse conditions. The global consequences of these perturbations, oceanic anoxic events (OAEs), lasted up to 1 m.y., but short-term nutrient and climatic controls on widespread anoxia are poorly understood. Here, we present a high-resolution reconstruction of oceanic redox and nutrient cycling as recorded in subtropical shelf sediments from Tarfaya, Morocco, spanning the initiation of OAE2. Iron-sulfur systematics and biomarker evidence demonstrate previously undescribed redox cyclicity on orbital time scales, from sulfidic to anoxic ferruginous (Fe-rich) water-column conditions. Bulk geochemical data and sulfur isotope modeling suggest that ferruginous conditions were not a consequence of nutrient or sulfate limitation, despite overall low sulfate concentrations in the proto–North Atlantic. Instead, fluctuations in the weathering influxes of sulfur and reactive iron, linked to a dynamic hydrological cycle, likely drove the redox cyclicity. Despite the potential for elevated phosphorus burial in association with Fe oxides under ferruginous conditions on the Tarfaya shelf, porewater sulfide generation drove extensive phosphorus recycling back to the water column, thus maintaining widespread open-ocean anoxia.

Description: Permeable sandy sediments cover 50-60% of the global continental shelf and are important bioreactors that regulate organic matter (OM) turnover and nutrient cycling in the coastal ocean. In sands, the dynamic porewater advection can cause rapid mass transfer and variable redox conditions, thus affecting OM remineralization pathways as well as the recycling of iron and phosphorus. In this study, North Sea sands were incubated in flow-through reactors (FTRs) to investigate biogeochemical processes under porewater advection and changing redox conditions. We found that the average rate of anaerobic OM remineralization was 12 times lower than the aerobic pathway, and Fe(III) oxyhydroxides were found as the major electron acceptors during 34 days of anoxic incubation. Abundant reduced Fe in the solid phase (expressed as Fe(II)) was measured before extensive Fe2+ release into porewater, and most of the reduced Fe (~96%) remained in the solid phase throughout the anoxic incubation. Fe(II) retained in the solid phase, either through the formation of authigenic Fe(II)-bearing minerals or adsorption, was easily re-oxidized upon exposure to O2 . Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe2+ with a constant P/Fe2+ ratio of 0.26. After 34 days of anoxic incubation, porewater was re-oxygenated and 〉99% of released P was coprecipitated through Fe2+ oxidation (so-called “Fe2+ curtain”). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as relatively immobile and rechargeable “redox battery” under dynamic porewater advection. Due to frequent oscillation of redox conditions, the Fe “redox battery” is characteristic for permeable sediments and plays an important role in coastal OM turnover. We also suggest that P liberated before Fe2+ release can escape the “Fe2+ curtain” in porewater advection, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions.