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 Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization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

Keywords: biological carbon pump; mesopelagic; particulate organic carbon; remineralization.

Fig. 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.

Fig. 2.

Scatter plots of temperature versus (A) the b coefficient of POC flux attenuation from Eq. 1 and (B) the remineralization length scale, z*, from Eq. 2. Values of b or z* calculated for POC flux measurements made with neutrally buoyant sediment traps are plotted against median temperature of the upper 500 m of the water column at each site, along with the resulting regression lines. Four North Atlantic values (this study) are shown by circles; four North Pacific values (16) are shown by inverted triangles. Error bars represent SE of b and z* values.

Fig. 3.

Global maps of b values from (Upper) the relationship between mesopelagic POC flux attenuation and upper water column (500 m) median temperature applied to World Ocean Atlas climatological data (29) and from (Lower) application of algorithms to satellite data, based on studies of thorium-derived POC export and POC flux to deep-sea sediment traps (adapted from ref. 28).

Fig. 4.

Conceptual comparison of POC flux profiles in areas with warm (red line) or cool (blue line) mesopelagic waters. In cooler waters, a smaller fraction of exported POC is remineralized in the upper water column (long-dashed lines from 500 m depth), but the nature of the material results in a greater fraction being remineralized before reaching the deep sea (short-dashed lines at 2,000 m), and vice versa in warm waters.