Sources, Degradation, and Transport of Organic Matter in the New Britain Shelf-Trench Continuum, Papua New Guinea

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 (delta C-13) and radiocarbon contents (Delta C-14) 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 C-3 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. Pb-210(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. Plain Language Summary Hadal trenches that refer to seafloor areas covered by a water column with depths >6,000 m have been proposed as depocenters of organic material and hotspots for organic matter mineralization. In this study, we show that the sediments in the New Britain Shelf-Trench continuum with water depths ranging between 1,553 and 8,901 m receive a significant input of preaged soil organic matter mixed by marine algae and terrigenous C-3 vascular plants. Recent occurrence of mass-wasting events possibly induced by earthquakes accounts for the efficient burial of terrigenous organic matter and carbonate into the trench bottom. We highlight 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, which provides an additional possible explanation for the elevated diagenetic activity at the trench axis apart from the extant postulation of preferential focusing of relatively labile organic matter in the trench axis.