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.
Highlights
Marine aggregates transfer biologically bound carbon and nutrients from the sunlit surface waters, the euphotic zone, to the interior of the oceans
Developing M4AGO, we followed a process-oriented approach and explicitly incorporated ballasting and microstructure of aggregates of heterogeneous composition to calculate the mean sinking velocity. Introducing such complexity in Earth system models (ESMs) typically comes at the cost of high computational efforts
This is a non-negligible factor for model development which we reduce to a minimum with M4AGO
Summary
Marine aggregates transfer biologically bound carbon and nutrients from the sunlit surface waters, the euphotic zone, to the interior of the oceans. While uncertainty with respect to primary production estimates exists, about 4.0 to 11.2 Gt C yr−1 of biologically bound carbon is annually exported out of the euphotic zone of the global ocean (Laws et al, 2000; Najjar et al, 2007; Henson et al, 2012). The net withdrawal of carbon dioxide (CO2) from the ocean surface through export of carbon bound in particulate organic matter (POM) and biogenic minerals and subsequent release through microbial remineralization and dissolution during aggregate descent determine the strength of the so-called biological carbon pump. The biological carbon pump critically depends on phytoplankton growth, the replenishment of the euphotic zone with nutrients through mixing and upwelling processes, and the efficiency of biologically bound carbon transfer from surface waters to the interior of the oceans. We aim to advance the representation of marine aggregates in an ESM framework
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