Global evaluation of particulate organic carbon flux parameterizations and implications for atmospheric pCO₂

Abstract

The shunt of photosynthetically derived particulate organic carbon (POC) from the euphotic zone and deep remineralization comprises the basic mechanism of the “biological carbon pump.” POC raining through the “twilight zone” (euphotic depth to 1 km) and “midnight zone” (1 km to 4 km) is remineralized back to inorganic form through respiration. Accurately modeling POC flux is critical for understanding the “biological pump” and its impacts on air‐sea CO2 exchange and, ultimately, long‐term ocean carbon sequestration. Yet commonly used parameterizations have not been tested quantitatively against global data sets using identical modeling frameworks. Here we use a single one‐dimensional physical‐biogeochemical modeling framework to assess three common POC flux parameterizations in capturing POC flux observations from moored sediment traps and thorium‐234 depletion. The exponential decay, Martin curve, and ballast model are compared to data from 11 biogeochemical provinces distributed across the globe. In each province, the model captures satellite‐based estimates of surface primary production within uncertainties. Goodness of fit is measured by how well the simulation captures the observations, quantified by bias and the root‐mean‐square error and displayed using “target diagrams.” Comparisons are presented separately for the twilight zone and midnight zone. We find that the ballast hypothesis shows no improvement over a globally or regionally parameterized Martin curve. For all provinces taken together, Martin's b that best fits the data is [0.70, 0.98]; this finding reduces by at least a factor of 3 previous estimates of potential impacts on atmospheric pCO2 of uncertainty in POC export to a more modest range [−16 ppm, +12 ppm].

The Role of Particle Size, Ballast, Temperature, and Oxygen in the Sinking Flux to the Deep Sea

“…Ballast also makes a comparatively minor contribution to global T EFF variability because it does not generate significant regional differences in particle density, at least given the simulated opal and carbonate export fluxes applied here. This finding conforms to Gloege et al's () observation that ballasting does not explain particulate organic carbon flux variability, even though those authors used thorium and sediment trap data that show the opposite TEFF pattern to the one we examined (Henson et al, ; Weber et al, ).…”

Vertical eddy iron fluxes support primary production in the open Southern Ocean

“…Ocean ecosystem. We use the Masachusetts Institute of Technology general circulation model 23 (MITgcm) with an embedded full ecosystem model 24,25 . The model configuration is identical to a companion paper 26 where we quantify the relative contribution of submesoscale and mesoscale dynamics on the total vertical iron transport.…”

The Contribution of Submesoscale over Mesoscale Eddy Iron Transport in the Open Southern Ocean

“…We couple the seasonally resolving physical simulation to the ecosystem model of Gloege et al (), which simplifies the Darwin biogeochemical model (Follows et al, ) to the complexity of the two species ecosystem described in Dutkiewicz et al (); there are two phytoplankton (diatoms and small phytoplankton) and two zooplankton functional groups respectively. The diatoms have a faster maximum growth rate but favor conditions with high nutrient concentrations, while the small phytoplankton are more resilient in low nutrient and light environments but have a slower maximum growth rate (Figure ).…”