The impact of remineralization depth on the air–sea carbon balance

Abstract

As particulate organic carbon rains down from the surface ocean it is respired back to carbon dioxide and released into the ocean's interior. The depth at which this sinking carbon is converted back to carbon dioxide-known as the remineralization depth-depends on the balance between particle sinking speeds and their rate of decay. A host of climate-sensitive factors can affect this balance, including temperature 1 , oxygen concentration 2 , stratification, community composition 3,4 and the mineral content of the sinking particles 5 . Here we use a three-dimensional global ocean biogeochemistry model to show that a modest change in remineralization depth can have a substantial impact on atmospheric carbon dioxide concentrations. For example, when the depth at which 63% of sinking carbon is respired increases by 24 m globally, atmospheric carbon dioxide concentrations fall by 10-27 ppm. This reduction in atmospheric carbon dioxide concentration results from the redistribution of remineralized carbon from intermediate waters to bottom waters. As a consequence of the reduced concentration of respired carbon in upper ocean waters, atmospheric carbon dioxide is preferentially stored in newly formed North Atlantic Deep Water. We suggest that atmospheric carbon dioxide concentrations are highly sensitive to the potential changes in remineralization depth that may be caused by climate change.The downward flux of particulate organic carbon, F , at a depth z below the base of the euphotic zone z c is often represented by the power law function F (z) = F (z c ) × (z/z c ) −b (ref. 6) where the exponent b controls the efficiency of transfer of particulate organic carbon to depth. The global mean value of the exponent b estimated using ocean biogeochemical tracer data is 0.9-1.0 (refs 7-9). Sediment trap observations indicate that the downward transfer efficiency may vary regionally and temporally 10 with the exponent b ranging from 0.5 to 2.0 (refs 4, 11-14). Previous studies using three-dimensional global ocean biogeochemistry models showed that the remineralization depth has an important role in controlling the global distributions of nutrients and carbon 7,8 . In ref. 7, a large change in atmospheric pCO 2 (partial pressure of carbon dioxide) of ∼100 ppm was obtained when the exponent b was changed from 0.9 to 2.0. We show here that even a small change in remineralization depth results in a substantial change in atmospheric pCO 2 and we explore the underlying mechanisms of this large sensitivity.We performed a systematic sensitivity analysis of the distributions of nutrients and carbon to changes in remineralization depth using a 3D global ocean biogeochemistry model 8 (see the Methods section) coupled with a one-box model of the atmosphere. Because the response of the new production of organic matter to nutrient supply is uncertain, we bracketed the ocean's response to changes in the remineralization profile by considering two extreme cases: a

Reconciliation of the carbon budget in the ocean’s twilight zone

“…Most of the exported organic carbon is processed by the water column biota, which ultimately converts it into CO2 via respiration (remineralization). Variations in the resulting decrease in organic flux with depth 9 can, according to models, lead to changes in atmospheric CO 2 of up to 200 ppm 3 , indicating a strong coupling between biological activity in the ocean interior and oceanic storage of CO 2 .…”

“…Prokaryotic heterotrophic production (PHP) was determined using bioassay-isotopedilution techniques using 3 Table 1). …”

Warm ocean processes and carbon cycling in the Eocene

“…The effect of a warmer ocean on the efficiency of the marine biological pump has also been emphasized, due to the fact that respiration rates in remineralizing microbes are temperature-dependent [3][4][5][6]. This may be important for understanding the response of the carbon cycle to current warming trends on short and geological time scales.…”

Controls on mesopelagic particle fluxes in the Sub-Antarctic and Polar Frontal Zones in the Southern Ocean south of Australia in summer—Perspectives from free-drifting sediment traps

“…The overall strength of the biological pump can be thought of as consisting of three steps: (i) primary production of organic matter by photosynthesis, (ii) export of a fraction of this production from the euphotic zone as sinking particles, and (iii) attenuation of this sinking flux by mesopelagic processes that limit the depth to which it is transferred and thus the duration over which it remains isolated from the atmosphere. Each of these steps exhibits similar variance and thus has similar potential importance in the control of the pump efficiency (Boyd and Trull, 2007;Kwon et al, 2009), but the factors controlling these steps are still under debate (Boyd and Trull, 2007;Buesseler and Boyd, 2009).…”