Carbon on CUSTARD: Episode 1

cover pic by Sofia Alexiou, NOC

Dr Maribel García-Ibáñez & Mr Gareth Lee, University of East Anglia, UK

28/12/2019

EPISODE 1: Why we measure carbon dissolved in the ocean

Atmospheric carbon dioxide (CO₂) enters the ocean through three “pumps”: the solubility pump (or physical pump), the biological pump, and the carbonate pump. Continuous contact and interaction between the atmosphere and the ocean allows CO₂ to be readily absorbed into the surface ocean. This is what is called the solubility pump or physical pump. The biological pump consists of the transformation in the ocean surface of dissolved CO₂ into organic matter, whose deposition creates a flow of organic carbon to the deep ocean. Phytoplankton, the base of the oceanic food webs, absorb dissolved CO₂ to synthesize organic matter. As it passes through the food web, the organic matter is transported to deeper layers of the oceans, being oxidized and decomposed. Part of this organic material reaches the seafloor, joining seabed sediments. The carbonate pump, on the contrary, releases CO₂ in the ocean surface layer through the creation of calcium carbonate external structures by some marine organisms such as coccolithophores and foraminifera. This calcium carbonate precipitates during photosynthesis and sinks.

Schematic of the global carbon cycle. Numbers represent mass reservoirs in PgC (1 PgC = 1015 gC) and annual carbon exchange fluxes (in PgC·yr–1). Black numbers and arrows indicate mass reservoirs and exchange fluxes estimated for the time prior to the Industrial Era (~1750). Red arrows and numbers indicate annual anthropogenic fluxes averaged over the 2000–2009 time period. These fluxes are a perturbation of the carbon cycle during Industrial Era (post 1750). The red arrows parts of Net land flux and Net ocean flux are the uptake of anthropogenic CO2 by the ocean and by terrestrial ecosystems (carbon sinks). Red numbers in the reservoirs denote cumulative changes of anthropogenic carbon over the Industrial Period 1750–2011. By convention, a positive cumulative change means that a reservoir has gained carbon since 1750. Uncertainties are reported as 90% confidence intervals. Source: Ciais et al. (2013).

Understanding how CO₂ behaves in the ocean, therefore, gives us information about how the ocean uptakes atmospheric CO₂ and how it is the redistributed in the ocean. Human activities have increased atmospheric CO₂ concentrations since the industrial revolution. These anthropogenic CO₂ emissions occur on top of an active natural carbon cycle that circulates carbon between the atmosphere, ocean and land reservoirs. The ocean dominates the storage of CO₂ due to its high solubility in seawater and its sequestration through water sinking away from the surface. In fact, the oceans have absorbed about 30% of the anthropogenic CO₂ emitted to the atmosphere since the industrial revolution. But this anthropogenic CO₂ is not evenly distributed throughout the oceans. While CO₂ concentration in the surface layers of the ocean increases as CO₂ increases in the atmosphere, its penetration into the deep ocean depends on the slow vertical mixing of the water column and the circulation of water. About half of the anthropogenic CO₂ is found in the first 400 m of the water column. However, in some regions where vertical movements of water are relatively fast, such as the Southern Ocean, the time scale necessary for deep penetration of anthropogenic CO₂ is of the order of decades instead of centuries.

During the CUSTARD cruise, we, the UEA CO₂ team, are quantifying two variables of the carbon system in the water of the Southern Ocean to help in answering the question of how deep in the ocean the CO₂ is stored and, therefore, for how long it is kept out of the atmosphere. Understanding how the ocean uptakes the atmospheric CO₂, which processes are responsible for this uptake, and where in the water column the CO₂ is stored will allow us to understand how the ocean will continue this task, favourable to us all, in the future.

Please see our next blog post ‘Carbon on CUSTARD- EPISODE 2’ to learn more about how we go about measuring dissolved carbon in the ocean.