High‐resolution Al and Fe data from the Atlantic Ocean CLIVAR‐CO₂ Repeat Hydrography A16N transect: Extensive linkages between atmospheric dust and upper ocean geochemistry

Abstract

Trace element sampling and shipboard flow injection analysis during the June–August 2003 Climate Variability and Predictability (CLIVAR)‐CO2 Repeat Hydrography A16N transect has produced a high‐resolution section of dissolved Fe and Al in the upper 1000 m of the Atlantic Ocean between 62°N and 5°S. Using the surface water dissolved Al and the Model of Aluminum for Dust Calculation in Oceanic Waters (MADCOW) model we have calculated the deposition of mineral dust to the surface ocean along this transect and compare that to dissolved Fe concentrations. The lowest mean mineral dust depositions of ≤0.2 g m−2 a−1 are found to the north of 51°N; a region which also exhibits characteristics of biological Fe limitation through its low dissolved surface water Fe (∼0.1 nM) and residual macronutrients, e.g., nitrate >2 μM. To the south of this region, mean dust deposition increases by an order of magnitude reaching ∼3 g m−2 a−1 at 10°N, underneath the Saharan dust outflow. Surface water Fe values also increase along this section to >1 nM. Distinct minima in Fe concentrations at the depth of the chlorophyll maximum in the vertical profiles between 18 and 4°N illuminate the role that active biological uptake plays in Fe cycling. An extensive subsurface zone of enhanced dissolved Fe concentrations (>1.5 nM) underlying this region is a result of the biological vertical transport and remineralization of the surface water Fe and is coincident with the intermediate nutrient maximum and oxygen minimum of this region. Elevated concentrations of dissolved Al in subsurface waters seen between 30 and 20°N coincide with the domain of the subtropical mode waters (STMW) which result from the sinking of surface waters in late winter in regions imprinted by dust deposition. The magnitude of the Al enrichment observed in this water mass implies that the predominant source to the STMW is from the more dust‐impacted western Atlantic, with only limited contributions from the STMW formation region near Madeira. A deeper subsurface Al enrichment (30–45°N) is associated with the outflow from the Mediterranean, another heavily dust‐impacted basin. These two regions of Al enrichment show the widespread geochemical connection between atmospheric transport processes and the North Atlantic and underscore its susceptibility to imprinting by atmospherically borne materials, natural as well as anthropogenic.

How well can we quantify dust deposition to the ocean?

“…Although models are not yet capable of quantifying the errors introduced by physical redistribution of dissolved metals supplied by dust, as noted above, dust fluxes derived from dissolved Al distributions are generally consistent with estimates based on other methods [13,[46][47][48][49]. 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.…”

“…Al, Th) in the ocean would offer an advantage when investigating the supply of dissolved iron. Although efforts to exploit this approach show promise [49,61], significant uncertainties associated with these methods require attention.…”

GEOTRACES IC1 (BATS) contamination‐prone trace element isotopes Cd, Fe, Pb, Zn, Cu, and Mo intercalibration

“…Two different sampling methods were used to collect samples for the CP-TEI intercalibration: an underway towed-fish near-surface pumped seawater system (provided by Ken Bruland's group and operated by Geoff Smith of UCSC) and the US GEOTRACES 24-sample 12-L trace element General Oceanics GO-Flo Rosette system deployed on a 8000 m-capable Kevlar cable and dedicated winch operated by Greg Cutter's ODU group, which is constructed and operated similarly to the CLIVAR system developed by Measures et al (2008). Fish samples were pressure-filtered through 0.45 µm Osmonics cartridge filters and GO-Flo samples were filtered through 0.2 µm Pall Acropak capsule filters after brief acid-leaching and flushing with ~500 mL each sample.…”

Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

“…Aluminium (Al) in the oceans has been used as a tracer for mineral dust deposition (Han et al, 2008;Measures and Vink, 2000;Measures and Brown, 1996) and water masses (Measures and Edmond, 1990).…”