The oceanic biological pump drives sequestration of carbon dioxide in the deep sea via sinking particles. Rapid biological consumption and remineralization of carbon in the “twilight zone” (depths between the euphotic zone and 1000 meters) reduce the efficiency of sequestration. By using neutrally buoyant sediment traps to sample this chronically understudied realm, we measured a transfer efficiency of sinking particulate organic carbon between 150 and 500 meters of 20 and 50% at two contrasting sites. This large variability in transfer efficiency is poorly represented in biogeochemical models. If applied globally, this is equivalent to a difference in carbon sequestration of more than 3 petagrams of carbon per year.

The ocean is an important global source of nitrous oxide (N₂O), a greenhouse gas that contributes to stratospheric ozone destruction. Bacterial nitrification and denitrification are thought to be the primary sources of marine N₂O, but the isotopic signatures of N₂O produced by these processes are not consistent with the marine contribution to the global N₂O budget. Based on enrichment cultures, we report that archaeal ammonia oxidation also produces N₂O. Natural-abundance stable isotope measurements indicate that the produced N₂O had bulk δ¹⁵N and δ¹⁸O values higher than observed for ammonia-oxidizing bacteria but similar to the δ¹⁵N and δ¹⁸O values attributed to the oceanic N₂O source to the atmosphere. Our results suggest that ammonia-oxidizing archaea may be largely responsible for the oceanic N₂O source.