Trace element and isotope deposition across the air–sea interface: progress and research needs

Abstract

The importance of the atmospheric deposition of biologically essential trace elements, especially iron, is widely recognized, as are the difficulties of accurately quantifying the rates of trace element wet and dry deposition and their fractional solubility. This paper summarizes some of the recent progress in this field, particularly that driven by the GEOTRACES, and other, international research programmes. The utility and limitations of models used to estimate atmospheric deposition flux, for example, from the surface ocean distribution of tracers such as dissolved aluminium, are discussed and a relatively new technique for quantifying atmospheric deposition using the short-lived radionuclide beryllium-7 is highlighted. It is proposed that this field will advance more rapidly by using a multi-tracer approach, and that aerosol deposition models should be ground-truthed against observed aerosol concentration data. It is also important to improve our understanding of the mechanisms and rates that control the fractional solubility of these tracers. Aerosol provenance and chemistry (humidity, acidity and organic ligand characteristics) play important roles in governing tracer solubility. Many of these factors are likely to be influenced by changes in atmospheric composition in the future. Intercalibration exercises for aerosol chemistry and fractional solubility are an essential component of the GEOTRACES programme.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

“…Manganese enters the open ocean through lithogenic dust deposition (Baker et al, 2016) and lateral advection from reducing sediments . Sediments along the relatively shallow (<1000 m depth) ocean margins tend to receive more organic deposition, hence by enhanced microbial decomposition are more anoxic and a stronger source of reduced Mn than deep sea sediments.…”

How well can we quantify dust deposition to the ocean?

“…In regions where removal of dissolved metals by scavenging occurs at a much faster rate than transport by advection, ocean circulation is expected to contribute little to the overall uncertainty in dust deposition calculated from dissolved metal inventories [45,88]. In highly advective regimes, such as the eastern half of our study area under the influence of the North Equatorial Current [75], the dissolved metal distribution will be spatially smeared to a degree that may be quantifiable as biogeochemical modelling of dissolved trace element cycles improves.…”

“…Baker et al [45] emphasize the need to use the full equation (not shown here) for bulk (wet plus dry) aerosol deposition. They also point out that 7 Be is preferentially incorporated into fine aerosols and removed by wet deposition, which may require adjustments to the bulk deposition equation in areas where wet deposition is significant.…”

“…Baker et al [45] discuss the factors that cause aerosol settling rates to vary with environmental conditions (presented in §3a), and recommend strategies to reduce uncertainties, such as intercomparison of results obtained using different approaches. For example, here we show that dust deposition estimated using an average aerosol bulk settling velocity of 1000 m day −1 is consistent with values derived by normalizing surface aerosol Al concentrations to 7 Be (figure 2).…”

“…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).…”