Atmospheric deposition of soluble trace elements along the Atlantic Meridional Transect (AMT)

Alex R Baker,Tim D Jickells

doi:10.1016/j.pocean.2016.10.002

Abstract

We briefly review the role of atmospheric deposition measurements within the Atlantic Meridional Transect (AMT) programme and then go on to present new data on the soluble concentrations of a range of trace metals (Fe, Al, Mn, Ti, Zn, V, Ni and Cu) and major ions in aerosols collected along the AMT transect. The results allow us to identify emission sources of the trace metals particularly in terms of the relative importance of anthropogenic versus crustal sources. We identify strong gradients in concentrations and deposition for both crustal and anthropogenically sourced metals with much higher inputs to the North Atlantic compared to the South Atlantic, reflecting stronger land based emission sources in the Northern Hemisphere. We suggest anthropogenic sources of Ni and V may include an important component from shipping.We consider the extent to which these gradients are reflected in surface water concentrations of these metals based on the GEOTRACES water column trace metal data. We find there is a clear difference in the concentrations of surface water dissolved Al and Fe between the north and south Atlantic gyres reflecting atmospheric inputs. However for Mn, V or Ni, higher inputs to the North Atlantic compared to the South Atlantic are not clearly reflected in their water column concentrations.

Highlights

There is a long history of studies of the biogeochemical interactions between the atmosphere and the ocean over the North Atlantic dating at least from the Beagle expedition (Darwin, 1846) through classic studies in the early 1970s (Lovelock et al, 1972; Chester et al, 1972; Duce et al, 1975) and more recent studies (e.g. Church et al, 1991; Losno et al, 1992; Rädlein and Heumann, 1995; Norman and Leck, 2005)
We characterised atmospheric transport over the Atlantic using 5-day air parcel back trajectories. Using this approach (Baker et al, 2010, 2013) we identified seven air transport types relevant to the samples collected for the present study: Air transported from the surrounding continental land masses of Europe (EUR), North Africa, Southern Africa (SAF), including air impacted by biomass burning emissions (SAB) and South America (SAM), and air that had circulated over the remote North or South Atlantic for at least 5 days prior to collection (RNA and RSA respectively)
Apparent are other traits, such as the presence of relatively high levels of anthropogenic-origin species (NO3À, nssSO42À, s-V and s-Ni) in Saharan air, the generally higher concentrations of most species in aerosols over the remote North Atlantic compared to the remote South Atlantic, and the low median concentrations for most species in samples of the SAF type relative to the other continentallyinfluenced transport types

Summary

Introduction

There is a long history of studies of the biogeochemical interactions between the atmosphere and the ocean over the North Atlantic dating at least from the Beagle expedition (Darwin, 1846) through classic studies in the early 1970s (Lovelock et al, 1972; Chester et al, 1972; Duce et al, 1975) and more recent studies (e.g. Church et al, 1991; Losno et al, 1992; Rädlein and Heumann, 1995; Norman and Leck, 2005). Much of the AMT research on the biogeochemical impacts on the ocean of deposition from the atmosphere has focussed on the nutrient elements nitrogen, phosphorus and iron (e.g. Baker et al, 2006b). Silicon is well recognised as an important nutrient for marine phytoplankton (Moore et al, 2013), it is clear that the atmospheric supply of soluble silicon along AMT is small compared to those of other nutrients, and compared to the supply of silicon to the euphotic zone from below (Baker et al, 2006b; Krishnamurthy et al, 2010). The atmospheric deposition of phosphorus is small in comparison to the magnitude of atmospheric deposition of nitrogen and iron, and in comparison to phytoplankton requirements for these elements (Moore et al, 2013; Baker et al, 2003, 2007).

Methods

Results

Conclusion