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

Abstract

Abstract 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.

Keywords

• IODP Expedition 370
• Site C0023
• Nankai Trough
• Deep biosphere
• Iron (oxyhydr)oxides
• Non‐steady state diagenesis