The biological carbon pump, which transports particulate organic carbon (POC) from the surface to the deep ocean, plays an important role in regulating atmospheric carbon dioxide (CO 2 ) concentrations. We know very little about geographical variability in the remineralization depth of this sinking material and less about what controls such variability. Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization length scale for each site. Combining these results with corresponding data from the North Pacific, we show that the observed variability in attenuation of vertical POC flux can largely be explained by temperature, with shallower remineralization occurring in warmer waters. This is seemingly inconsistent with conclusions drawn from earlier analyses of deep-sea sediment trap and export flux data, which suggest lowest transfer efficiency at high latitudes. However, the two patterns can be reconciled by considering relatively intense remineralization of a labile fraction of material in warm waters, followed by efficient downward transfer of the remaining refractory fraction, while in cold environments, a larger labile fraction undergoes slower remineralization that continues over a longer length scale. Based on the observed relationship, future increases in ocean temperature will likely lead to shallower remineralization of POC and hence reduced storage of CO 2 by the ocean.biological carbon pump | particulate organic carbon | remineralization | mesopelagic A tmospheric carbon dioxide (CO 2 ) levels are strongly influenced by the production, sinking, and subsequent remineralization of particulate organic carbon (POC) in the ocean (1), with the atmospheric concentration partially set by the depth at which regeneration occurs (2). Numerous studies have endeavored to describe the complex interactions that produce the typically observed depth profile of sinking POC flux attenuation as relatively simple mathematical forms (3-6), with perhaps the most commonly used being a power law equation:where f z is the flux at depth z, normalized to flux at some reference depth, z 0 , and b is the coefficient of flux attenuation (7). This relationship was originally derived from POC flux measurements from several eastern North Pacific locations, and an open ocean composite b value of 0.86 was calculated (7), a value which has since been used extensively in biogeochemical models (8) and to normalize fluxes measured in different regions and at different depths (9, 10). Regional variations in b, from 0.6 to 2.0, have since been demonstrated by deep-sea (>2,000 m) sediment trap studies (11,12). More recently, mesopelagic POC flux attenuation between 150 m and 500 m depth was measured in the Vertical Transport in the Global Ocean (VERTIGO) project at two contrasting sites in the North Pacific (13), using neutrally buoyant sediment traps (NBSTs) developed to improve the reliability of upper ocean ...

The global carbon cycle is affected by biological processes in the oceans, which export carbon from surface waters in form of organic matter and store it at depth; a process called the 'biological carbon pump'. Most of the exported organic carbon is processed by the water column biota, which ultimately converts it into CO2 via respiration (remineralization). Variations in the resulting decrease in organic flux with depth 9 can, according to models, lead to changes in atmospheric CO 2 of up to 200 ppm 3 , indicating a strong coupling between biological activity in the ocean interior and oceanic storage of CO 2 .A key constraint in the analysis of carbon fluxes in the twilight zone is that, at steady state, the attenuation of particulate organic carbon (POC) flux with depth should be balanced by community metabolism. Published estimates of POC flux attenuation with depth are, however, up to 2 orders of magnitude lower than corresponding estimates of heterotrophic metabolism [4][5][6][7] . This discrepancy indicates that either estimates of POC flux and/or community metabolism are unreliable, or that additional, unaccounted for, sources of organic carbon to the twilight zone exist 8 .We compiled a comprehensive carbon budget of the twilight zone based on an based on the ratio between DOC concentrations and apparent oxygen utilization 15 , and on DOC gradients coupled to turbulent diffusivity measured from previous work at the study site 16 (Methods; Extended Data Fig. 2). DOC was estimated to supply 17% of total export in agreement with previous estimates of 9-20% across the North Atlantic basin 17 . Organic matter input via lateral advection was assumed to be negligible based on analyses of back-trajectories (derived from satellite-derived near-surface velocities over 3 months) of the water masses arriving at the PAP site during the study period, which suggested that the water had not passed over the continental slope (Extended Data Fig. 1b). The final source of DOC, excretion at depth by active flux, was estimated using net samples of zooplankton biomass and allometric equations 6,18 , giving a supply of 3 mg C m -2 d -1 . Defecation and mortality at depth present further sources of organic carbon to the twilight zone, but these were excluded from the budget due to large uncertainties associated with their estimation. Finally, chemolithoautotrophy has been suggested to be a significant source of organic matter in the deep ocean 19 , but without strong evidence that this poorly understood process could provide a major contribution at our study site, we chose to exclude it from our carbon budget.The remineralization of organic carbon by zooplankton and prokaryotes was estimated from zooplankton biomass and prokaryotic activity. It is crucial to note that in a steady state system, such as we assume this to be, organic carbon is lost from the system only by export or by remineralization. We focus entirely on community respiration as a measure of remineralization, a fundamental advance over previous methods to derive...

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north-south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol(-1) was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom(6). The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.