Description: A one-dimensional model "ADAM" is presented that allows the prognostic computation of the interactions between mineral particles (dust) and biologically formed aggregates. The model couples a 7-compartment biogeochemical component (NO(3), NH(4), phytoplankton aggregates, zooplankton, detritus, carbon, and chlorophyll) and a 4-compartment component for the tracing of mineral particles: single free particles in the water, particles aggregated with phytoplankton, incorporated in zooplankton, and attached to detritus. It resolves both annual and daily cycles of plankton and the fate of dust from eolian import into the ocean via biological activity, aggregation and disaggregation to sedimentation at the sea floor. The model results suggest that particle scavenging is essentially occurring in the mixed layer, where biological activity and shear aggregation regulate the formation of the aggregates. The aggregates interact intensively with the suspended pool of dust particles, and sink through the upper main thermocline with increasing speed. Particle break up and organic matter degradation are important mechanisms for particle cycling in the intermediate and deeper layers. The model predicts an 80% decrease of the annual carbon flux between 100 m and 3000 m depth. The vertical profile of Al-contents in suspended particulates and the annual average vertical flux of particulate organic matter are fairly well reproduced by the model, as well as the seasonal cycles of carbon and dust fluxes in the ocean interior. (C) 2010 Elsevier B.V. All rights reserved.

Description: Ocean biogeochemical models are routinely applied to assess the net global impact of ocean acidification and warming on pelagic CaCO3 cycling. As with respect to the net change of global air-sea carbon fluxes affected by the reduced calcification under future CO2 conditions, these models diverge by a factor of four. The standard method to evaluate modeled CaCO3 cycles is to compare alkalinity and CaCO3 saturation states with observations. In general, state-of-the-art models do feature strong deviations and it is unclear if, or to what extent, these are driven by a deficient representation of physics (ocean circulation) or a deficient representation of biogeochemistry. This points to a strong need for improvement of the data-based evaluation of the base state of global biogeochemical models used for ocean acidification research. Here we apply the TA* method to output from a variety of model experiments and observations (GLODAP). This method was originally developed to separate the signals of CaCO3 production and dissolution from the large, conservative alkalinity background in observations and is also a critical part of approaches used to quantify the inventory of anthropogenic CO2 in the ocean. The aim is our study is twofold. First, to assess the TA* method using additional explicit representations of preformed alkalinity, accumulated CaCO3 dissolution, and organic matter remineralisation in our models. And second, we aim to disentangle deficiencies in the physical and biogeochemical CaCO3-cyle module in a series of ocean biogeochemical models of increasing complexity. Finally, our modeling study provides a critical assessment of the ‘mystery of shallow CaCO3 dissolution’, i.e. apparent dissolution of major CaCO3 minerals well above their saturation horizon.