Reconstruction of Modern Dust Deposition in the South Pacific from Water Column and Sedimentary Methods

Aerosol dust deposition provides a source of limiting micronutrients such as iron to the surface ocean. Accurately modeling the deposition rate of dust to the ocean is critical for understanding the magnitude and regional variability of biogeochemical processes like export productivity and nitrogen fixation, both in present and future climates as well as in the geologic past. However, there is a paucity of dust flux data available for validating dust deposition models in remote ocean locales. Typical methods based on shipboard dust collectors only provide a snapshot on daily to weekly timescales. Additionally, paleoceanographic methods for reconstructing dust flux typically lack intercomparison with modern flux data. Geochemical methods for determining dust fluxes from upper ocean water column measurements thus provide a critical link between our understanding of paleo- dust deposition, modern micronutrient inputs, and models of future changes to the dust cycle.

We present results of a systematic intercomparison of four different methods for determining oceanic dust fluxes in the South Pacific Ocean. Two of the methods utilize water column measurements of the long-lived thorium isotopes ²³⁰Th and ²³²Th. The other two methods use surface sedimentary measurements of constant flux proxies commonly used to reconstruct paleo- dust inputs: ²³⁰Th-normalization and ³He-normalization. We find convergent estimates of dust flux across the South Pacific Gyre of 0.05-0.1 g m^-2 yr^-1 between the two water column methods and sedimentary ³He-normalization, but an order of magnitude higher estimates from sedimentary ²³⁰Th-normalization. We compare these results to output from the CAM4 dust model, and discuss implications for the links between aerosol iron inputs with the geographic distribution of nitrogen fixation in the modern South Pacific, as well as for paleoceanographic dust flux reconstructions using ²³⁰Th-normalization and ³He-normalization.