Abstract

Oceanic trenches are an important sink for organic matter (OM). However, little is known about how much of the OM reaching the hadal region derives from the sunlit surface ocean and other sources. We provide new insight into the OM sources in the Atacama Trench by examining the elemental and stable isotope composition of carbon and nitrogen in bulk OM throughout the entire water column and down to bathyal and hadal sediments. Moreover, we estimated the particulate organic carbon (POC) concentration and downward carbon flux. Our results, based on two-way variance analysis, showed statistical differences in delta N-15(PON) between the epipelagic zone and the deep zones. However, no statistical differences in delta C-13(POC) and C:N ratio between hadalpelagic and shallower pelagic zones were found, except for delta C-13(POC) in the oxygen-deficient zone. On the contrary, whereas the isotopic signatures of hadal sediments were distinct from those over the entire water column, they were similar to the values in bathyal sediments. Thus, our results suggest that bathyal sediments could contribute more OM to hadal sediments than the different zones of the water column. Indeed, whereas POC flux estimates derived from remote sensing data indicate that similar to 16%-27% of POC could evade surface remineralization within the top 200 m and potentially be exported to depths beyond the mesopelagic region, model estimates suggest that similar to 3.3% of it could reach hadal depths. Our results provide a quantitative baseline of pelagic-benthic coupling which can aid in assessment of carbon cycling changes in future climate scenarios.