Regina Usbeck

Particle fluxes in the ocean: comparison of sediment trap data with results from inverse modeling

Biological production lowers the CO2 concentrations in the surface layer of the ocean, and sinking detritus ‘‘pumps’’ nutrients and CO2 into the deep ocean. Quantifying the efficiency of the biological pump is a prerequisite for global CO2 budgets. Sediment traps are commonly used to directly measure the vertical particle flux; however, for logistical and financial reasons, traps cannot provide area-wide data sets. Moreover, it has been shown that sediment traps can under- or overestimate particle fluxes considerably. In this paper, we present a new technique to estimate the downward flux of particulate matter with an adjoint model. Hydrographic and nutrient data are used to calculate the mean ocean circulation together with parameters for particle fluxes using the AWI Adjoint Model for Oceanic Carbon Cycling (AAMOCC). The model is fitted to the property concentrations by systematically varying circulation, air–sea fluxes, export production and remineralization rates of particulate biogenic matter simultaneously. The deviations of model fluxes based on nutrient budgets from direct measurements with sediment traps yield an independent estimate of apparent trapping efficiencies. While consistent with hydrographic and nutrient data, model particle fluxes rarely agree with sediment trap data: (1) At shallow water depth (V1000 m), sediment trap fluxes are at the average 50% lower than model fluxes, which confirms flux calibrations using radionuclides; (2) in the very deep traps, model fluxes tend to be lower compared to data, which might be explained by lateral inputs into the traps. According to these model results, particle fluxes from the euphotic zone (EP) into mid water depth are considerably higher and the shallow loop of nutrient is more vigorous than would be derived from sediment trap data. Our results imply that fluxes as collected with sediment traps are inconsistent with model derived long-term mean particle fluxes based on nutrient budgets in the water column. In agreement with recent radionuclide studies, we conclude that reliable export flux estimates can only be obtained from sediment trap data if appropriate corrections are applied. D 2003 Elsevier Science B.V. All rights reserved.

Inverse Modeling of Particulate Organic Carbon Fluxes in the South Atlantic

The biological production of particulate material near the ocean surface and its subsequent remineralization during sinking and after deposition on the seafloor strongly affect the distribution of oxygen, dissolved nutrients and carbon in the ocean. Dissolved nutrient distributions therefore reveal the underlying biogeochemical processes, and these data can be used to determine rates of production, remineralization and accumulation with the aid of inverse techniques. Here, an ocean circulation, biogeochemical model that exploits the existing large sets of hydrographic, oxygen, nutrient and carbon data is presented and results for the export production of particulate organic matter, vertical fluxes in the water column, and sedimentation rates are presented. In the model, the integrated export flux of particulate organic carbon (POC) in the South Atlantic amounts to about 1300 Tg C yr1 (equivalent to 1.3 Gt C yr(1), most of which occurring in the Benguela/Namibia upwelling region and in a zonal band following the course of the Antarctic Circumpolar Current (ACC). Remineralization of POC in the upper water column is intense, and only about 7% of the export reaches a depth of 2000 m. Comparison of modeled particle fluxes with sediment trap data suggests that shallow traps tend to underestimate the downward flux, whereas the deep traps seem to be affected by the lateral input of material and apparently overestimate the vertical flux. These findings are consistent with recent radionuclide studies. The rapid degradation of POC with depth produces geographical patterns of POC fluxes to the seafloor and POC accumulation in the sediment that are very different from the pattern of surface productivity, because of the modulation with varying bottom depth. Whereas there is significant surface production in deep-water, open-ocean regions, the benthic fluxes occur predominantly in coastal and shelf areas.