The "transfer efficiency" of sinking organic particles through the mesopelagic zone and into the deep ocean is a critical determinant of the atmosphere−ocean partition of carbon dioxide (CO 2 ). Our ability to detect large-scale spatial variations in transfer efficiency is limited by the scarcity and uncertainties of particle flux data. Here we reconstruct deep ocean particle fluxes by diagnosing the rate of nutrient accumulation along transport pathways in a data-constrained ocean circulation model. Combined with estimates of organic matter export from the surface, these diagnosed fluxes reveal a global pattern of transfer efficiency to 1,000 m that is high (∼25%) at high latitudes and low (∼5%) in subtropical gyres, with intermediate values in the tropics. This pattern is well correlated with spatial variations in phytoplankton community structure and the export of ballast minerals, which control the size and density of sinking particles. These findings accentuate the importance of high-latitude oceans in sequestering carbon over long timescales, and highlight potential impacts on remineralization depth as phytoplankton communities respond to a warming climate.biological pump | organic particles | remineralization | transfer efficiency | ocean carbon storage S inking organic particles deliver carbon from the surface euphotic zone (upper ∼100 m) of the ocean into deeper layers that do not exchange with the atmosphere (1). The longevity of oceanic carbon storage by this "biological pump" depends on the depth at which particulate organic matter (POM) decays and releases CO 2 into seawater (2). Most POM is consumed within the mesopelagic zone (100−1,000 m) of the water column, which recirculates rapidly to the surface, leaving only a small fraction to remineralize in the deep ocean where carbon can be sequestered on centennial and longer timescales (3). The "transfer efficiency" of particulate carbon from the euphotic zone to depth is therefore a critical determinant of atmospheric pCO 2 (4), but its underlying controls are poorly understood and crudely represented in Earth system models used to project global carbon cycling and climate.The depth scale over which POM fluxes attenuate hinges on both the sinking speed of particles and their rate of decomposition (5), each governed by a range of factors. Decomposition rates are thought to depend on the abundance of heterotrophic microbes (6) and the temperature sensitivity of their metabolism (7,8), as well as the palatability of the organic matter itself (9, 10). Particle sinking speeds depend on their size and density, which may be ultimately dictated by the plankton community structure and trophic web of the euphotic zone where they are produced (11,12). Because these factors exhibit distinct regional variations, their relative importance might be discerned by detecting large-scale patterns of transfer efficiency in the ocean.Arrays of neutrally buoyant sediment traps deployed at multiple depths provide the most direct estimate of particle fluxes from the euphotic...

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The “transfer efficiency” of organic particles from the surface to depth is a critical determinant of ocean carbon sequestration. Recently, direct observations and geochemical analyses have revealed a systematic geographical pattern of transfer efficiency, which is highest in high latitude regions and lowest in the subtropical gyres. We evaluate the possible causes of this pattern using a mechanistic model of sinking particle dynamics. The model represents the size distribution of particles, the effects of mineral ballast, seawater temperature (which influences both particle settling velocity and microbial metabolic rates), and O2. Parameters are optimized within reasonable ranges to best match the observational constraints. Our model shows that no single factor can explain the observed pattern of transfer efficiency, but the biological effect of temperature on remineralization rate and particle size effects together can reproduce most of the regional variability with both factors contributing to low transfer efficiency in the subtropical gyres and high transfer efficiency in high latitudes. Particle density from mineral ballast has a similar directional effect to temperature and size but plays a substantially smaller role in our optimum solution, due to the opposing patterns of silicate and calcium carbonate ballasting. Oxygen effects modestly improved model fit by depressing remineralization rates and thus increasing transfer efficiency in the Eastern Tropical Pacific. Our model implies that climate‐driven changes to upper ocean temperature and associated changes in surface plankton size distribution would reduce the carbon sequestration efficiency in a warmer ocean.