Slow slip events on subduction megathrusts are linked to subduction of rough crust and heterogeneous fault rock properties.

Knowledge of sedimentary organic carbon (OC) in hadal trenches, the deepest ocean realm, is rudimentary. Here, we conducted a comprehensive analysis of total OC (TOC), stable and radio‐carbon isotopes (δ¹³C and Δ¹⁴C), and biomarkers (e.g., n‐alkanes, n‐alkanols and n‐fatty acids) in 12 sediment cores collected from hadal (trench axis) and non‐hadal (abyssal plains and slopes) settings of the Kermadec Trench (Southwestern Pacific) and Atacama Trench (Southeastern Pacific) regions. Our results show that the TOC in the Atacama Trench region (0.86% ± 0.69%) is significantly higher than that of the Kermadec Trench region (0.29% ± 0.08%), likely related to different surface primary productivity. In both trench regions, the hadal sites are generally characterized by more negative δ¹³C, higher TOC/TN ratio, and similar or higher abundance ratio of terrigenous/marine biomarkers as compared to the non‐hadal sites, suggesting the selective preservation of terrigenous biogenic/fossil OC at the hadal trench axis. The linear increase in ¹⁴C age with sediment depth in non‐hadal cores reflects steady depositional conditions, whereas seven out of eight hadal sediment cores show ¹⁴C age reversals presumably due to occasional mass‐transport deposits. Our results suggest (1) a strong heterogeneity in sedimentary OC characteristics between trenches and within each trench and (2) relatively high accumulation rates of terrigenous OC in both the Kermadec Trench (0.35 ± 0.04 g m⁻² yr⁻¹) and the Atacama Trench (1.4 ± 0.5 g m⁻² yr⁻¹). Thus, hadal trenches appear to represent an important environment for terrigenous carbon deposition in the deep ocean.

Hadal trenches are considered as depocenters for organic matter and hotspots for microbial diagenetic activity. Here, we explore the sources, degradation, and transport of organic matter in the shelf‐trench continuum using seven short sediment cores collected along two transects with water depths ranging between 1,553 and 8,901 m in the New Britain Trench area, Papua New Guinea. Carbon isotopic compositions (δ¹³C) and radiocarbon contents (Δ¹⁴C) of sedimentary organic matter accompanied by total organic carbon/total nitrogen ratios suggest an important contribution from the preaged soil organic matter mixed by the marine algae and terrigenous C₃ vascular plants. In addition, the trench axis sites are characterized by elevated accumulation of terrigenous organic materials. Rates of organic matter mineralization approximated by dissolved inorganic carbon fluxes at the sediment‐water interface reveal an approximately threefold higher rate at the trench axis sites than the abyssal sites. ²¹⁰Pb_xs profiles and burial of carbonate (up to 50%) at both trench axis sites reflect recent occurrence of mass‐wasting events possibly induced by earthquakes, which is responsible for the transport of preaged, terrigenous organic matter to the trench bottom. This tectonically triggered deposition event, which was also shown to occur in the Japan Trench, the Tonga Trench, and probably in many other trenches, is likely to efficiently transport terrigenous organic matter to hadal trenches, thereby underpinning the importance of terrigenous organic matter burial in hadal trenches for the ocean organic carbon budget. Furthermore, we hypothesize that hadal trenches may host a distinct microbial community that is capable of feeding on old, refractory terrigenous organic matter.