Attenuation of sinking particulate organic carbon flux through the mesopelagic ocean

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 (CO₂) 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 Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization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 CO₂ by the ocean.Atmospheric carbon dioxide (CO₂) 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:f_z = f_z0(z/z₀)^?b[1]where f_z is the flux at depth z, normalized to flux at some reference depth, z₀, 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 flux measurements (14). During VERTIGO, two deployments of multiple NBSTs at station ALOHA [“A Long-term Oligotrophic Habitat Assessment”; a tropical oligotrophic site characterized by low surface chlorophyll and warm temperatures (15)] and two at station K2 [a seasonally variable mesotrophic site in the northwest Pacific subarctic gyre with relatively cold waters (15)] yielded b values of 1.25 and 1.36 and of 0.57 and 0.49, respectively (16). Community structure (17), mineral ballasting (4, 12, 18), temperature (19), and oxygen concentration (20) have all been proposed as factors important in explaining these variations in the vertical profile of organic carbon remineralization, through their influence on particle sinking speed, POC degradation rate, or both.An alternative approach proposed to describe POC flux attenuation in the upper ocean is the use of an exponential equation (Eq. 2) that relates the flux at any depth to the flux measured at a reference depth by the remineralization length scale, z*, defined as the depth interval over which the flux decreases by a factor of 1/e (5, 6, 21),f_z = f_z0exp(?(z ? z₀)/z^?).[2]This approach is advocated by some because, unlike the power law equation (Eq. 1), which is sensitive to the reference depth used, the resulting length scale is not affected by use of either the absolute depth or the depth relative to the base of the mixed layer or euphotic zone (21).In this study, multiple neutrally buoyant PELAGRA (particle export measurement using a Lagrangian trap) sediment traps (22) (SI Text) were deployed at each of four locations in the North Atlantic: the Porcupine Abyssal Plain (PAP) time-series site, the Irminger and Iceland Basins, and within the North Atlantic subtropical gyre (NASG; Fig. 1A and Table S1). Fluxes of POC, lithogenic material, and the biominerals opal and calcium carbonate (CaCO₃) were measured at each site (Fig. 1 B and C), thereby increasing the range of locations for which these measurements have been made using NBSTs at multiple (≥3) depths.Open in a separate windowFig. 1.(A) Bathymetry map of the North Atlantic showing the locations of four multiple PELAGRA trap deployments; (B) measured fluxes of POM, opal, CaCO₃, and lithogenic material at each site; (C) corresponding measured POC fluxes at each site, with profiles fitted using Eq. 1 and extrapolated between 50 m and 1000 m depth. Error bars represent relative SD from replicate measurements and are generally smaller than symbols. POC flux plots show the calculated b value along with SE, r² value, and P value.