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

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Usbeck, R. , Schlitzer, R. , Fischer, G. and Wefer, G. (2003): Particle fluxes in the ocean: Comparison of sediment trap data with results from inverse modeling , Journal of marine systems, 39 , pp. 167-183 .
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Biological production lowers the CO_2 concentrations in the surfacelayer of the ocean, and sinking detritus ``pumps'' nutrients andCO_2 into the deep ocean. Quantifying the efficiency of thebiological pump is a prerequisite for global CO_2 budgets. Sedimenttraps are commonly used to directly measure the vertical particleflux, however, for logistical and financial reasons traps cannotprovide area-wide data sets. Moreover, it has been shownthat sediment traps can under- or overestimate particle fluxes considerably.In this paper we present a new technique to estimate the downward fluxof particulate matter with an adjoint model. Hydrographic and nutrientdata are used to calculate the mean ocean circulation together withparameters for particle fluxes using the AWI Adjoint Model for OceanicCarbon Cycling (AAMOCC). The model is fitted to the propertyconcentrations by systematically varying circulation, air-sea fluxes,export production and remineralization rates of particulate biogenicmatter simultaneously.The deviations of model fluxes based on nutrient budgets from directmeasurements with sediment traps yield an independent estimate ofapparent trapping efficiencies. While consistent with hydrographicand nutrient data, model particle fluxes rarely agree with sedimenttrap data: (1) At shallow water depth (< 1000m), sediment trapfluxes are at the average 50% lower than model fluxes, which confirmsflux calibrations using radionuclides; (2) in the very deep traps,model fluxes tend to be lower compared to data which might beexplained by lateral inputs into the traps. According to these modelresults, particle fluxes from the euphotic zone into mid water depthare considerably higher and the shallow loop of nutrient is morevigorous than would be derived fromsediment trap data.Our results imply that fluxes as collected with sediment traps areinconsistent with model derived long-term mean particle fluxes basedon nutrient budgets in the water column. In agreement with recentradionuclide studies we conclude that reliable export flux estimatescan only be obtained from sediment trap data if appropriatecorrections are applied.

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