Constraining oceanic dust deposition using surface ocean dissolved Al

Abstract

[1] We use measurements of ocean surface dissolved Al, a global Biogeochemical Elemental Cycling (BEC) ocean model, and the global Dust Entrainment and Deposition (DEAD) model to constrain dust deposition to the oceans. Our Al database contains all available measurements with best coverage in the Atlantic. Vertical profiles and seasonal data exist in limited regions. Observations show that surface dissolved Al is distributed similarly to the dust deposition predicted by DEAD and other models. There is an equatorial Atlantic Al maximum that decreases toward higher latitudes. There are high Al concentrations in the Mediterranean Sea and the Arabian Sea and low concentrations in the Pacific and the Southern Ocean. The ocean basins maintain more distinct Al profiles than Fe profiles in the upper ocean, consistent with a weaker biological influence on Al than Fe. The BEC-predicted surface dissolved Al compares relatively well with observations. The Al distribution reflects the combined effects of Al input from dust and Al removal by particle scavenging and biological uptake by diatoms. Model-observed biases suggest a southward shift of maximum dust deposition compared to current dust model predictions. DEAD appears to overestimate deposition north of 30°N in the Pacific and to underestimate deposition south of 30°N. Observed Al concentrations and the ocean model-predicted surface Al lifetime provide a semi-independent method to estimate oceanic dust deposition. This technique indicates that DEAD may overestimate dust deposition to the north equatorial Atlantic but underestimate in other Atlantic regions, the Southern Ocean, and the Arabian Sea. However, spatial variations in aerosol Al solubility may also contribute to the model-observation mismatch. Our results have implications for all dust-borne ocean nutrients including Fe and demonstrate the potential of marine geochemical data to constrain atmospheric aerosol deposition fields.

Toxicity of atmospheric aerosols on marine phytoplankton

“…Data for day 5 for Synechococcus is not shown in the figure because the levels crashed to zero (i.e., all cells were dead) by day 5, and it is not possible to show a value of zero on the log scale we have used here. or other metal removal time scales ranging from 1 to 5 years (29)(30)(31). Under preindustrial conditions, potential Cu toxicity effects are concentrated in the northern hemisphere downwind of natural dust sources in low-chlorophyll subtropics in the Atlantic, Mediterranean and Indian Ocean basins ( Fig.…”

How well can we quantify dust deposition to the ocean?

“…A global analysis by Han et al [89] found that the distribution of surface dissolved Al concentrations is similar to that predicted by dust deposition models. Furthermore, their global compilation of dissolved Al data showed that surface levels are consistent with Al concentrations throughout the water column ( fig.…”

“…Another major unknown that limits model-derived estimates of dissolved Al residence time is whether dissolved Al is scavenged primarily by diatoms [88] or if it is removed more uniformly by sorption to all particulate phases [89]. Additional study of the scavenging behaviour of Al is warranted to explore its sensitivity to the abundance and composition of particles and thereby improve the algorithms used to derive dust fluxes from measured dissolved Al inventories.…”

“…A critical factor limiting the use of dissolved Al inventories to quantify the supply of dust is the uncertainty of the dissolved Al residence time, including its sensitivity to spatial and temporal variability of biological productivity as well as the abundance and composition of particles that scavenge Al from the water column [45,53,88,89]. To illustrate this point, we calculate the residence time of dissolved Al in surface waters that would be implied by applying an assumed aerosol settling rate of 1000 m day −1 to the surface aerosol Al concentration measured along section GA03 ( §3a) and multiplying by each set of measured aerosol Al solubility results ( §3c).…”

Iron biogeochemistry across marine systems – progress from the past decade

“…The work of Measures and colleagues (e.g. Han et al, 2008) has demonstrated the validity of a novel indirect approach of using surface water Al as a tracer of atmospheric deposition which provides data averaged over long time scales (months to years) in remote regions. Again this approach has significant uncertainties, but the broad agreement between this, long term field data, campaign data and models provides reassurance that the estimates of total dust deposition to the oceans and the regional patterns are realistic.…”