Variable role of dust deposition on upper ocean nutrient distribution

Atmospheric transport and deposition of dust aerosol is very efficient in supplying iron to large part of global oceans. In this study, an Earth system model with ocean biogeochemistry component is used to explore how dust deposition can impact vertical distribution of dissolved iron (DFe) and phosphate in the upper 1000 m of the global oceans by impacting phytoplankton growth.  Although large areas of the global oceans show positive chlorophyll response following dust deposition, some regions (those having high levels of background DFe from continental shelf sediments and high atmospheric DFe input) experience net scavenging losses of DFe following dust depositions. Such regions experience a reduction in chlorophyll concentrations along with reduction in particulate organic carbon (POC) production and fluxes following dust deposition. While positive chlorophyll response is associated with low levels of background DFe and low atmospheric DFe input compared to regions experiencing negative chlorophyll response, the magnitude of chlorophyll increase depends on the background nitrate-to-iron ratio. With increase in the magnitude of positive chlorophyll response to atmospheric DFe deposition, an increase in POC production and resulting fluxes are encountered. Such an increase in POC flux can play the dual role of increase in scavenging removal of DFe as well as increase in PFe remineralization. The net result is that variation in NREG (Net REGeneration, taken as difference between PFe remineralization and DFe scavenging) in the upper 1000 m of the ocean has significant positive correlation with variation in POC fluxes, indicating that sinking organic matter following positive chlorophyll response to atmospheric iron deposition is the main driver of net DFe regeneration. Furthermore, a depth-wise difference between the impact of sinking POC and PFe fluxes on NREG is also evident. In the upper 150 m, high POC fluxes drive NREG while at deeper depths, PFe fluxes become important in driving NREG due to slow desorption release of iron from sinking PFe mass. As a result, with increase in POC fluxes, the depth of maximum NREG becomes shallower due to the shorter remineralization length scale of POC compared to lithogenic particles. On the contrary, with increases in the magnitude of atmospheric DFe , the depth of maximum NREG increases due to high dust deposition driving increased scavenging. Furthermore, increase in POC fluxes also leads to regeneration of phosphate at shallower depths. In this manner, the magnitude of chlorophyll response to atmospheric iron can significantly control the patterns of nutrient limitations.