Sediment Accumulation and Carbon Burial in Four Hadal Trench Systems

Abstract

Hadal trenches are considered to act as depocenters for organic material, although pathways for the material transport and deposition rates are poorly constrained. Here we assess focusing, deposition and accumulation of material and organic carbon in four hadal trench systems underlying different surface ocean productivities; the eutrophic Atacama and Kuril-Kamchatka trenches, the mesotrophic Kermadec trench and the oligotrophic Mariana Trench. The study is based on the distributions of naturally occurring 210 Pb ex , 137 Cs and total organic carbon from recovered sediment cores and by applying previously quantified benthic mineralization rates. Periods of steady deposition and discreet mass-wasting deposits were identified from the profiles and the latter were associated with historic recorded seismic events in the respective regions. During periods without mass wasting, the estimated focusing factors along trench axes were elevated, suggesting more or less continuous downslope focusing of material toward the interior of the trenches. The estimated organic carbon deposition rates during these periods exhibited extensive site-specific variability, but were generally similar to values encountered at much shallower settings such as continental slopes and margins. Organic carbon deposition rates during periods of steady deposition were not mirrored by surface ocean productivity, but appeared confounded by local bathymetry. The inclusion of deposition mediated by mass-wasting events enhanced the sediment and organic carbon accumulations for the past ∼150 years by up to a factor of ∼4. Thus, due to intensified downslope material focusing and infrequent mass-wasting events, hadal trenches are important sites for deposition and sequestration of organic carbon in the deep sea.Plain Language Summary Hadal trenches (>6,000 m water depth) occupy ∼1% of the world's ocean floor yet are largely underexplored, but recent studies have shown that these environments are depocenters for organic material and microbial activity is intensified when compared to shallower abyssal plains. However, transport and accumulation of sediment material to these hadal trenches is poorly understood. This study investigates sedimentation and accumulation dynamics of organic carbon in trenches using results of radionuclide analysis (in sediment from the Atacama, Kuril-Kamchatka, Kermadec, and Mariana trenches). The analysis shows that trench sediments consist of discreet layers representing both periods of continuous deposition and sudden mass-wasting events often triggered by historic earthquakes. Down slope focusing of material, intensified the deposition along the trench axes. However, the deposition rates exhibited extensive site-specific variations that were partly related to mass-wasting events which greatly enhanced not only mass accumulation but also organic carbon accumulation at the trench axes. Our results illustrate that mass-wasting events play an important role on supplying organic carbon to hadal communities and suggest that hadal tr...

Organic matter degradation causes enrichment of organic pollutants in hadal sediments

“…The trench is located close to an intense upwelling region leading to high surface ocean productivity. Consequently, the trench is characterized by relatively high sediment accumulation rates with values along the trench axis ranging from 0.29 to 0.79 cm y −1 and mass accumulation rates of organic carbon from 1.1 to 5.3 g C m −2 y −1 15 . The intense deposition of material is facilitated by down-slope focusing and seismic driven mass wasting events.…”

“…97′ S 70° 58.70′ W 4050 0.96 80.5 1717 Sampling was done in 2018 15 , PCBs were analyzed in five sediment layers of each 20 mm from each site. The earliest approximate year of deposition is based on 210 Pb ex a TOC differs slightly from TOC data reported in Oguri et al 15 . due to analyses performed by different procedures, in two different laboratories and on parallel samples from different sediment cores.…”

“…It represents the relative fraction of the highly refractory organic carbon that can only break down during intense oxidation processes, normally not available in the subsurface sediments or oxygen restricted water. c Calculated based on sedimentation rates from Oguri et al 15 .…”

“…The trench systems act as collectors for organic material. This process is facilitated by tidal induced internal seiche 14 and downslope gravitational driven sediment displacement 15 . Apart from the more continuous downslope material transport, distinct sudden translocation of large amounts of previously deposited material, typically induced by earthquakes, may occur along the trench axis.…”

“…Apart from the more continuous downslope material transport, distinct sudden translocation of large amounts of previously deposited material, typically induced by earthquakes, may occur along the trench axis. Detailed investigation of 210 Pb ex (gamma-ray spectrometry of excess 210 Pb) profiles have shown that such “mass wasting” events contribute significantly to material deposition in the Atacama trench 15 . Hadal sediments harbor high abundance of microbial life that thrive at the extreme hydrostatic pressure 16 – 18 , and have been found to support higher degradation rates of organic carbon as compared to adjacent abyssal sediment and rates often scale with values encountered at continental slopes and margins 19 , 20 .…”