Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean

Abstract

Abstract. Marine aggregates are the vector for biogenically bound carbon and nutrients from the euphotic zone to the interior of the oceans. To improve the representation of this biological carbon pump in the global biogeochemical HAMburg Ocean Carbon Cycle (HAMOCC) model, we implemented a novel Microstructure, Multiscale, Mechanistic, Marine Aggregates in the Global Ocean (M4AGO) sinking scheme. M4AGO explicitly represents the size, microstructure, heterogeneous composition, density and porosity of aggregates and ties ballasting mineral and particulate organic carbon (POC) fluxes together. Additionally, we incorporated temperature-dependent remineralization of POC. We compare M4AGO with the standard HAMOCC version, where POC fluxes follow a Martin curve approach with (i) linearly increasing sinking velocity with depth and (ii) temperature-independent remineralization. Minerals descend separately with a constant speed. In contrast to the standard HAMOCC, M4AGO reproduces the latitudinal pattern of POC transfer efficiency, as recently constrained by Weber et al. (2016). High latitudes show transfer efficiencies of ≈0.25±0.04, and the subtropical gyres show lower values of about 0.10±0.03. In addition to temperature as a driving factor for remineralization, diatom frustule size co-determines POC fluxes in silicifier-dominated ocean regions, while calcium carbonate enhances the aggregate excess density and thus sinking velocity in subtropical gyres. Prescribing rising carbon dioxide (CO2) concentrations in stand-alone runs (without climate feedback), M4AGO alters the regional ocean atmosphere CO2 fluxes compared to the standard model. M4AGO exhibits higher CO2 uptake in the Southern Ocean compared to the standard run, while in subtropical gyres, less CO2 is taken up. Overall, the global oceanic CO2 uptake remains the same. With the explicit representation of measurable aggregate properties, M4AGO can serve as a test bed for evaluating the impact of aggregate-associated processes on global biogeochemical cycles and, in particular, on the biological carbon pump.

Effective Vertical Transport of Particulate Organic Carbon in the Western North Pacific Subarctic Region

“…This higher export flux and higher transfer efficiency in the deep sea in the subarctic region (station K2) than in the subtropical region (station S1) are inconsistent with previous reports of higher transport efficiencies in subtropical regions (e.g., Francois et al, 2002;Henson et al, 2012). Instead, the here reported higher transfer efficiency for the subarctic region (K2) compared to the subtropical region (S1) supports former observational and modeling studies featuring the found pattern (Marsay et al, 2015;Weber et al, 2016;DeVries and Weber, 2017;Cram et al, 2018;Maerz et al, 2020). In the next section, mechanisms related to the preservation of the POC flux and settling particles in the water column are discussed.…”

Particulate Trace Element Export in the North Atlantic (GEOTRACES GA01 Transect, GEOVIDE Cruise)

“…Here the particle sinking rate is estimated by dividing the particulate residence time by the surface-Eq depth layer. At Station 1, the sinking rate reaches 63 m/d for pFe, which is in the same order of magnitude as estimated using Stokes law (88 m/d for a particle of 53µm diameter and of density of 1860 kg/m 3 , representing an average density for a mixed aggregate composed of lithogenic, transparent exopolymer particles (TEP), detritus, opal and CaCO3; 116 ). Similarly, high particulate and dissolved inventories, high export fluxes and short τtotal are observed at Station #1 for other TEs, suggesting a fast supply to and removal from the upper ocean by lithogenic particles (Table 5).…”

New insights into large-scale trends of apparent organic matter reactivity in marine sediments and patterns of benthic carbon transformation

“…At estuarine sites, steady-state conditions may be impacted by the highly dynamic nature of such settings. The Severn estuary exhibits strong tidal dynamics that result in continuous cycles of sediment resuspension (Manning et al, 2010). The Rhone delta experiences strong pulses of fresh water and sediments associated with flood events, as well as variable sedimentation rates (Antonelli et al, 2008;Cathalot et al, 2010;Zebracki et al, 2015).…”

“…OM of terrestrial, anthropogenic, and marine sources mixes along the gradient and in- teracts with high loads of fine, suspended particular material. Additionally, intense tidally driven deposition-resuspension cycles (Dyer, 1984;Jonas and Millward, 2010;Manning et al, 2010;Uncles, 2010) result in a continuous re-exposure of benthic OM, which might cause the lower apparent reactivity of the buried OM. The Arabian Sea is another example where the k-h relationship deviates from the general global trends.…”