Impact of enhanced vertical mixing on marine biogeochemistry: lessons for geo-engineering and natural variability

S. Dutreuil,L. Bopp,A. Tagliabue

doi:10.5194/bg-6-901-2009

Abstract

Abstract. Artificially enhanced vertical mixing has been suggested as a means by which to fertilize the biological pump with subsurface nutrients and thus increase the oceanic CO2 sink. We use an ocean general circulation and biogeochemistry model (OGCBM) to examine the impact of artificially enhanced vertical mixing on biological productivity and atmospheric CO2, as well as the climatically significant gases nitrous oxide (N2O) and dimethyl sulphide (DMS) during simulations between 2000 and 2020. Overall, we find a large increase in the amount of organic carbon exported from surface waters, but an overall increase in atmospheric CO2 concentrations by 2020. We quantified the individual effect of changes in dissolved inorganic carbon (DIC), alkalinity and biological production on the change in pCO2 at characteristic sites and found the increased vertical supply of carbon rich subsurface water to be primarily responsible for the enhanced CO2 outgassing, although increased alkalinity and, to a lesser degree, biological production can compensate in some regions. While ocean-atmosphere fluxes of DMS do increase slightly, which might reduce radiative forcing, the oceanic N2O source also expands. Our study has implications for understanding how natural variability in vertical mixing in different ocean regions (such as that observed recently in the Southern Ocean) can impact the ocean CO2 sink via changes in DIC, alkalinity and carbon export.

Highlights

In the context of rising anthropogenic emissions of carbon dioxide (CO2) and increasing atmospheric CO2 concentrations, the ocean is a significant CO2 sink (on the order of 2 Pg yr−1 over the 1990s (Denman, et al, 2008))
We quantified the individual effect of changes in dissolved inorganic carbon (DIC), alkalinity and biological production on the change in pressure of CO2 (pCO2) at characteristic sites and found the increased vertical supply of carbon rich subsurface water to be primarily responsible for the enhanced CO2 outgassing, increased alkalinity and, to a lesser degree, biological production can compensate in some regions
Our analyses show that the impact of enhanced mixing on Southern Ocean FCO2 is dominated by the increased surface DIC concentrations associated with greater vertical mixing and that compensatory feedbacks by alkalinity and biology are relatively weak (Table 1)

Summary

Introduction

In the context of rising anthropogenic emissions of carbon dioxide (CO2) and increasing atmospheric CO2 concentrations, the ocean is a significant CO2 sink (on the order of 2 Pg yr−1 over the 1990s (Denman, et al, 2008)). Some geoengineering proposals seek mitigate for rising atmospheric CO2 by increasing the efficiency of the biological pump and the oceanic sink for atmospheric CO2. In the past, such proposals tended to focus on the artificial fertilization of ocean productivity by the micronutrient iron (Fe) (e.g., Markels and Barber, 2000; Leinen, 2008; Lampitt et al, 2008), which limits phytoplankton productivity in certain oceanic regions (such as the Southern Ocean) (Boyd et al, 2000). Experiments with numerical models have shown that the links between Fe, phytoplankton productivity, carbon export and atmospheric CO2 are by no means straightforward (e.g., Arrigo and Tagliabue, 2005; Aumont and Bopp, 2006), and might have unintended adverse consequences for ocean ecosystems and climate (Jin and Gruber, 2005; Cullen and Boyd, 2008)

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Conclusion