Evolution of (Bio‐)Geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

Biogeochemical processes in subseafloor sediments are closely coupled to global element cycles. To improve the understanding of changes in biogeochemical conditions on geological timescales, we investigate sediment cores from a 1,180 m deep hole in the Nankai Trough offshore Japan (Site C0023) drilled during International Ocean Discovery Program Expedition 370. During its tectonic migration from the Shikoku Basin to the Nankai Trough over the past 15 Ma, Site C0023 has experienced significant changes in depositional, thermal, and geochemical conditions. By combining pore‐water, solid‐phase, and rock magnetic data, we demonstrate that a transition from organic carbon‐starved conditions with predominantly aerobic respiration to an elevated carbon burial environment with increased sedimentation occurred at ∼2.5 Ma. Higher rates of organic carbon burial in consequence of increased nutrient supply and productivity likely stimulated the onset of anaerobic electron‐accepting processes during organic carbon degradation. A significant temperature increase by ∼50°C across the sediment column associated with trench‐style sedimentation since ∼0.5 Ma could increase the bioavailability of organic matter and enhance biogenic methanogenesis. The resulting shifts in reaction fronts led to diagenetic transformation of iron (oxyhydr)oxides into pyrite in the organic carbon‐starved sediments several millions of years after burial. We also show that high amounts of reducible iron(III) which can serve as electron acceptor for microbial iron(III) reduction are preserved and still available as phyllosilicate‐bound iron. This is the first study that shows the evolution of long‐term variations of (bio‐)geochemical processes along tectonic migration of ocean floor, thereby altering the primary sediment composition long after deposition.

Plain Language Summary: During the tectonic migration of ocean floor, deep sediments move over vast distances, thereby passing through different depositional and geochemical environments. We studied subseafloor sediments recovered at Site C0023 in the Nankai Trough offshore Japan during International Ocean Discovery Program Expedition 370, which aimed at exploring the prerequisites and limits of microbial life in marine sediments. Over the past 15 Ma, the sediments at Site C0023 migrated ∼750 km from its initial position to the Nankai Trough due to tectonic motion of the Philippine Sea plate. As a result, sedimentation rates and sediment temperature increased significantly. We use different geochemical and rock magnetic analyses to (1) reconstruct the evolution of (bio‐)geochemical processes, especially iron cycling, along the migration and to (2) investigate if iron(III) minerals are available to serve as energy substrates for microbial respiration in the deep sediments. Our results indicate that high amounts of phyllosilicate‐bound iron(III), which can be used by microbes to gain energy, are still available throughout the core. The changing depositional history, and consequently the organic carbon availability and temperature, ultimately determine the geochemical patterns we observe today. Such studies are needed to gain a better understanding of changes in (bio‐)geochemical cycling on geological timescales.

Key Points:

The tectonic migration of ocean floor led to a transition from an organic carbon‐starved to an elevated organic carbon burial environment.

Diagenetic transformation of iron oxides into pyrite within the carbon‐lean sediments occurred several millions of years after deposition.

Reducible iron(III), which is mostly present in phyllosilicates, can potentially fuel microbially mediated mineral alteration.