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

Abstract

Biological production lowers the CO 2 concentrations in the surface layer of the ocean, and sinking detritus ''pumps'' nutrients and CO 2 into the deep ocean. Quantifying the efficiency of the biological pump is a prerequisite for global CO 2 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 ( V 1000 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

Reconciling surface ocean productivity, export fluxes and sediment composition in a global biogeochemical ocean model

“…An evaluation of trapping efficiency of sediment traps (Yu et al, 2001) suggests a low and erratic efficiency in the mesopelagic. This is also confirmed by a comparison between sediment trap data and inverse modeling results (Usbeck et al, 2003). The low trapping efficiency of sediment traps at mid-depth might explain the systematic off-set between modeled fluxes and trap results.…”

On the variability of the exponent in the power law depth dependence of POC flux estimated from sediment traps

“…Global models might be able to provide additional constraints on the power law exponent by optimizing the exponent so as to best reproduce global dissolved inorganic carbon, phosphate or alkalinity fields (e.g. Usbeck et al, 2003). Ultimately a better understanding of the processes that affect POC fluxes in the water column should produce a mechanistically based model to replace the empirical power law parameterization (e.g.…”

Inverse Modeling of Particulate Organic Carbon Fluxes in the South Atlantic

“…Another experiment was conducted to enforce the trap data in the model over larger horizontal distances by applying smoothness constraints. This run showed that the smaller trap fluxes in the upper water column were inconsistent with the measured distributions of oxygen, nutrients and carbon in the ocean, because enforcing these fluxes over great areas led to un- realistic simulations for oxygen, nutrients and carbon (Usbeck et al 2003). …”