Hadal trenches act as depocenters for organic material, although pathways for hadal 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 provinces of different net primary productivity in the surface ocean – from the eutrophic Atacama and Kuril-Kamchatka trenches, and the mesotrophic Kermadec Trench, to the oligotrophic Mariana Trench. The study is based on the distributions of 210Pbex, 137Cs 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 continuous downslope focusing of material towards the interior of the trenches. The estimated accumulation rates of organic carbon 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 and accumulation rates during periods of steady deposition was not mirrored by surface ocean productivity, but appeared confounded by local bathymetry. Seismic driven mass wasting events markedly enhanced the accumulation of sediment and organic carbon by factors from 20 to 400. Thus, hadal trenches are important sites for deposition and sequestration of organic carbon in the deep-sea partly due to intensified downslope focusing of material but mainly due to mass-wasting events.

Polynyas represent regions of enhanced primary production due to the low, or absent, sea-ice cover coupled with the proximity of nutrient sources. However, studies throughout the Southern Ocean suggest elevated primary production does not necessarily result in increased carbon export. Three coastal polynyas in East Antarctica and an off-shelf region were visited during the austral summer of 2016/2017 to examine the vertical distribution of particulate organic carbon (POC). Carbon export was also examined using thorium-234 (234Th) as a proxy at two of the polynyas. Our results show that concentrations and integrated POC stocks were higher within the polynyas compared to the off-shelf sites. Within the polynyas, vertical POC concentrations were higher in the Mertz and Ninnis polynyas compared to the Dalton polynya. Similarly, higher carbon export was measured in the diatom-dominated Mertz polynya, where large particles (53 μm) represented a significant fraction of the particulate 234Th and POC, compared to the small flagellate-dominated Dalton polynya, where almost all the particulate 234Th and POC were found in the smaller size fraction (1 - 53 μm). The POC to Chlorophyll-a ratios suggests that organic matter below the mixed layer in the polynyas consisted largely of fresh phytoplankton at this time of the year. In combination with a parallel study on phytoplankton production at these sites, we find that increased primary production at these polynyas does lead to greater concentrations and export of POC and a higher POC export efficiency.