The impact of different external sources of iron on the global carbon cycle

Abstract

Variable supply of iron to the ocean is often invoked to explain part of past changes in atmospheric CO 2 (CO 2atm ). Using model simulations, we find that CO 2atm is sensitive on the order of 15, 2, and 1 ppm to sedimentary, dust, and hydrothermal iron input. CO 2atm is insensitive to dust because it is not the major iron input to the Southern Ocean. Modifications to the relative export of Si(OH) 4 to low latitudes are opposite to those predicted previously. Although hydrothermalism is the major control on the iron inventory in~25% of the ocean, it remains restricted to the deep ocean, with minor effects on CO 2atm . Nevertheless, uncertainties regarding the iron-binding ligand pool can have significant impacts on CO 2atm . Ongoing expansion of iron observations as part of GEOTRACES will be invaluable in refining these results.

Overview of the mechanisms that could explain the ‘Boundary Exchange’ at the land–ocean contact

“…In addition, dust inputs are discrete in time and space, while sediments are more continuously discharged. Yet, atmospheric inputs could still bring relatively important amounts of elements at some specific locations, as recently demonstrated off the Saharan desert [27].…”

Mass Deposition Fluxes of Asian Dust to the Bohai Sea and Yellow Sea from Geostationary Satellite MTSAT: A Case Study

“…πr n(r)dr r πr n(r)dr =   (3) where n(r) is the particle size distribution, and Qext is the light extinction efficiency decided by Mie scattering. For the large size parameter of dust, the Qext is oscillating near 2. t and R are functions of re and τdu.…”

“…Dust aerosol in the atmosphere play an important role in the radiative forcing of the atmosphere via scattering and absorbing shortwave and longwave radiation [2]. Once deposited and dissolved in the ocean, it modifies biogeochemistry and carbon export [3]. Especially in the high nutrient low chlorophyll (HNLC) open ocean regions, the availability of iron is the limiting factor for phytoplankton growth [4].…”

More to hydrothermal iron input than meets the eye

“…As an illustration, modeling suggests changes to the residence time of Fe within observed uncertainties may have a larger impact on the global carbon cycle than perturbations to dust supply (2). In addition, new basin scale datasets from the GEOTRACES program highlight widespread variability in the Fe distributions in the ocean interior (Fig.…”

“…The results of this study equally imply that recent modeling results, suggesting hydrothermal sources contribute around one-third to the global Fe inventory (2), are likely underestimates. Better understanding the processes that regulate the long-range transport of hydrothermal Fe will enable global biogeochemical models to more accurately represent this Fe source and reduce uncertainty in how hydrothermalism impacts the cycling of both Fe and carbon in the ocean (2).…”