Abstract
The element carbon plays a central role in climate and life on Earth. It is capable of moving among the geosphere, cryosphere, atmosphere, biosphere and hydrosphere. This flow of carbon is referred to as the Earth's carbon cycle. It is also intimately linked to the cycling of other elements and compounds. The ocean plays a fundamental role in Earth's carbon cycle, helping to regulate atmospheric CO2 concentration. The ocean biological carbon pump (OBCP), defined as a set of processes that transfer organic carbon from the surface to the deep ocean, is at the heart of the ocean carbon cycle. Monitoring the OBCP is critical to understanding how the Earth's carbon cycle is changing. At present, satellite remote sensing is the only tool available for viewing the entire surface ocean at high temporal and spatial scales. In this paper, we review methods for monitoring the OBCP with a focus on satellites. We begin by providing an overview of the OBCP, defining and describing the pools of carbon in the ocean, and the processes controlling fluxes of carbon between the pools, from the surface to the deep ocean, and among ocean, land and atmosphere. We then examine how field measurements, from ship and autonomous platforms, complement satellite observations, provide validation points for satellite products and lead to a more complete view of the OBCP than would be possible from satellite observations alone. A thorough analysis is then provided on methods used for monitoring the OBCP from satellite platforms, covering current capabilities, concepts and gaps, and the requirement for uncertainties in satellite products. We finish by discussing the potential for producing a satellite-based carbon budget for the oceans, the advantages of integrating satellite-based observations with ecosystem models and field measurements, and future opportunities in space, all with a view towards bringing satellite observations into the limelight of ocean carbon research.
Highlights
The ocean biological carbon pump (OBCP) can be defined as a suite of biological, physical, and chemical processes that contributes to, and controls, the transfer of organic carbon, and calcium carbonate, from the surface layer to the deep ocean (Volk and Hoffert, 1985)
Particulate Organic Carbon (POC) Particulate organic carbon is typically defined as all the organic carbon that is retained by GF/F filters
Recent advancements in satellite-based Particulate Inorganic Carbon (PIC) methods include: the development of band-difference PIC approaches (Mitchell et al, 2017), which have been seen to perform with better accuracy than standard methods (Balch et al, 2005) and found to be more resilient to errors in atmospheric-correction; methods that relate surface PIC to water-column integrated PIC, allowing quantification of PIC below the depths seen by the satellite (Hopkins et al, 2019); and quantification of the influence of coccolithophores on surface CO2 concentrations and gas fluxes (Shutler et al, 2013; Kondrik et al, 2019)
Summary
The ocean biological carbon pump (OBCP) can be defined as a suite of biological, physical, and chemical processes that contributes to, and controls, the transfer of organic carbon (in dissolved and particulate forms), and calcium carbonate, from the surface layer to the deep ocean (Volk and Hoffert, 1985). The magnitude of this pump has been esti mated to be between 4 and 12 Pg C y− 1 (Laws et al, 2000; Henson et al, 2011; DeVries and Weber, 2017). We discuss the future of satellite-based OBCP monitoring, and how satel lites, monitoring electromagnetic radiation at different frequencies and resolutions, can be used in synergy with other monitoring tools (e.g. ships-based and autonomous-based) and ecosystem models, to advance our understanding of the OBCP and the global carbon cycle
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