Oceanic Dust Deposition

Oceanic Dust Deposition

In open-ocean regions far from rivers and continental shelves, the deposition of aerosol dust is a primary source of micronutrients like Fe to the sea surface. Dust-borne trace metals are limiting nutrients for primary productivity, N2-fixation, and other important biogeochemical processes at the sea surface.

Schematic of 230Th and 232Th as tools for studying dust flux

Thorium isotopes are excellent tracers of dust input. Radiogenic 230Th is produced in the water column by uranium decay, while primordial 232Th is supplied to the ocean via the dissolution of lithogenic materials, like dust. Applying the in-situ residence derived computed from 230Th to 232Th allows for the determination of 232Th input rates, which can be converted to dust fluxes using the 232Th solubility and the Th concentration of dust. Similarly, particulate 230Th-normalization can be used to calculate 232Th fluxes, which can similarly be converted to dust fluxes. I am using these methods to determine dust input rates to the South Pacific Subtropical Gyre, an oceanic region modeled to have the lowest dust fluxes in the world. The iron fluxes from dust deposition are much lower than the theoretical lower limit needed to support observed rates of N2-fixation, instead requiring highly efficient microbial Fe recycling at the surface of the gyre (Pavia et al. in prep)

Oceanic dust deposition also varies on glacial-interglacial timescales, typically a factor of ~2-3 higher during glacial periods, which may have contributed to greater productivity and storage of respired CO2 in the deep ocean during glacials. I was recently involved in a study led by Allison Jacobel re-examining the role of iron in driving glacial-interglacial changes in export production in the Eastern Equatorial Pacific (Jacobel et al. 2019). We found that iron fertilization did not affect export productivity in the EEP, supporting a growing body of evidence suggesting that changes in ocean dynamics dictate the supply of subsurface nutrients, and thus productivity, in the EEP.