Attenuation of sinking particulate organic carbon flux through the mesopelagic ocean

Abstract

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 Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization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 ...

Deep ocean nutrients imply large latitudinal variation in particle transfer efficiency

“…S1) and/or analytical errors associated with the Th-based export observations used (29). On the other hand, our results are highly consistent with the few available POC flux profiles to 500 m compiled from neutrally buoyant sediment traps (8,14) (Fig. 3C), and extend the observed regional difference between subtropics and high latitudes over a much larger depth range.…”

“…S2). Red and blue dots are the T eff estimates derived from neutrally buoyant sediment trap observations (8,14). Thick dashed line in B and C is the prediction of the Martin curve relationship:…”

“…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).…”

“…This approach has recently revealed a pattern of rapid flux attenuation to ∼500 m in the subtropics, and slower attenuation in subarctic regions, that is consistent between the Atlantic and Pacific Oceans (8,14). Transfer efficiency to greater depth has been estimated by combining particle fluxes captured in bottom-moored sediment traps and thorium (Th)-based measures of surface export production, yielding an opposite pattern with highest values in the low latitudes (15).…”

Prediction of the Export and Fate of Global Ocean Net Primary Production: The EXPORTS Science Plan

“…It has been shown recently that the performance of the Siegel et al (2014) export flux model improves substantially if the parameters are regionally tuned supporting the importance of food web structure (Stukel et al, 2015). Further, recent field data summaries show strong relationships between the vertical scales of sinking flux attenuation in the TZ and both phytoplankton community structure in the EZ (Guidi et al, 2015;Puigcorbé et al, 2015) and environmental conditions in the TZ (Marsay et al, 2015). The explicit testing of the hypothesis and development of modeling tools to diagnose carbon cycling processes will require an extensive data set of a wide range of ECC states.…”