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

• Main Authors: Maerz, Joeran, Six, Katharina D., Stemmler, Irene, Ahmerkamp, Soeren, Ilyina, Tatiana
• Format: Text
• Language: English
• Published: 2019
• Subjects: Southern Ocean
• Online Access: https://doi.org/10.5194/bg-2019-378; https://www.biogeosciences-discuss.net/bg-2019-378/

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 (M 4 AGO) sinking scheme. M 4 AGO 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 M 4 AGO with the standard HAMOCC version, where POC fluxes follow a Martin curve approach with linearly increasing sinking velocity with depth, and temperature-independent remineralization. Minerals descend separately with a constant speed. In contrast to the standard HAMOCC, M 4 AGO reproduces the latitudinal pattern of POC transfer efficiency which has been 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, diatom frustule size co-determines POC fluxes in silicifiers-dominated ocean regions while calcium carbonate enhances the aggregate excess density, and thus sinking velocity in subtropical gyres. In ocean standalone runs and rising carbon dioxide (CO 2 ) without CO 2 climate feedback, M 4 AGO alters the regional ocean-atmosphere CO 2 fluxes compared to the standard model. M 4 AGO exhibits higher CO 2 uptake in the Southern Ocean compared to the standard run while in subtropical gyres, less CO 2 is taken up. Overall, the global oceanic CO 2 uptake remains the same. With the explicit representation of measurable aggregate properties, M 4 AGO can serve as a testbed for evaluating the impact of aggregate-associated processes on global biogeochemical cycles, and, in particular, on the biological carbon pump.