Abstract
A global ocean biogeochemical model is used to quantify the sensitivity of marine biogeochemistry and air‐sea CO2 exchange to variations in dust deposition over decadal timescales. Estimates of dust deposition generated under four climate states provide a large range in total deposition with spatially realistic patterns; transient ocean model experiments are conducted by applying a step-function change in deposition from a current climate control. Relative to current conditions, higher dust deposition increases diatom and export production, nitrogen fixation and oceanic net CO2 uptake from the atmosphere, while reduced dust deposition has the opposite effects. Over timescales less than a decade, dust modulation of marine productivity and export is dominated by direct effects in high-nutrient, low-chlorophyll regions, where iron is the primary limiting nutrient. On longer timescales, an indirect nitrogen fixation pathway has increased importance, significantly amplifying the ocean biogeochemical response. Because dust iron input decouples carbon cycling from subsurface macronutrient supply, the ratio of the change in net ocean CO2 uptake to change in export flux is large, 0.45‐0.6. Decreasing dust deposition and reduced oceanic CO2 uptake over the next century could provide a positive feedback to global warming, distinct from feedbacks associated with changes in stratification and circulation.
Highlights
Climate-induced variations in atmospheric dust deposition can significantly impact the marine carbon cycle because biological production and export are iron limited in many regions
Terrestrial dust sources, entrainment, atmospheric transport and ocean deposition are calculated within equilibrium climate simulations using the National Center for Atmospheric Research (NCAR) Community Climate System Model (CCSM3) using a slab ocean model; a complete model description, and comparison to observations for these cases is included in Mahowald et al (2006), so that we include only a brief description here
The most dramatic differences in biogeochemical fluxes were seen in the HNLC areas in response to the direct iron pathway; significant differences in export production and air–sea CO2 flux occurred within the subtropical gyres in response to the indirect nitrogen fixation pathway (Figs. 4–6)
Summary
Climate-induced variations in atmospheric dust deposition can significantly impact the marine carbon cycle because biological production and export are iron limited in many regions. A number of previous studies have discussed the potential climate feedbacks involving dust deposition, nitrogen fixation, the strength of the biological pump and air–sea exchange of CO2 (Falkowski, 1997; Michaels et al, 2001; Karl et al, 2002; Gruber, 2004; Moore et al, 2004; Jickells et al, 2005). We refer to this dust-nitrogen fixation-carbon linkage as the indirect pathway for dust deposition to influence air–sea CO2 exchange
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