Regional trends in the fractional solubility of Fe and other metals from North Atlantic aerosols (GEOTRACES cruises GA01 and GA03) following a two-stage leach

Abstract

Abstract. The fractional solubility of aerosol-derived trace elements deposited to the ocean surface is a key parameter of many marine biogeochemical models. Despite this, it is currently poorly constrained, in part due to the complex interplay between the various processes that govern the solubilisation of aerosol trace elements. In this study, we used a sequential two-stage leach to investigate the regional variability in fractional solubility of a suite of aerosol trace elements (Al, Ti, Fe, Mn, Co, Ni, Cu, Zn, Cd, and Pb) from samples collected during three GEOTRACES cruises to the North Atlantic Ocean (GA01, GA03-2010, and GA03-2011). We present aerosol trace element solubility data from two sequential leaches that provide a solubility window, covering a conservative lower limit to an upper limit, the maximum potentially soluble fraction, and discuss why this upper limit of solubility could be used as a proxy for the bioavailable fraction in some regions. Regardless of the leaching solution used in this study (mild versus strong leach), the most heavily loaded samples generally had the lowest solubility. However, there were exceptions. Manganese fractional solubility was relatively uniform across the full range of atmospheric loading (32 ± 13 and 49 ± 13 % for ultra high-purity water and 25 % acetic acid leaches, respectively). This is consistent with other marine aerosol studies. Zinc and Cd fractional solubility also appeared to be independent of atmospheric loading. Although the average fractional solubilities of Zn and Cd (37 ± 28 and 55 ± 30 % for Zn and 39 ± 23 and 58 ± 26 % for Cd, for ultra high-purity water and 25 % acetic acid leaches, respectively) were similar to Mn, the range was greater, with several samples being 100 % soluble after the second leach. Finally, as the objective of this study was to investigate the regional variability in TE solubility, the samples were grouped according to air mass back trajectories (AMBTs). However, we conclude that AMBTs are not sufficiently discriminating to identify the aerosol sources or the potential effects of atmospheric processing on the physicochemical composition and solubility of the aerosols.

Dissolved Pb and Pb isotopes in the North Atlantic from the GEOVIDE transect (GEOTRACES GA-01) and their decadal evolution

“…Like Bridgestock et al (2016) found in the tropical Atlantic, we see evidence of a natural Pb source to the northern North Atlantic which was previously obscured in the 1980s and 1990s by enormous anthropogenic inputs, and which is still obscured in magnitude by the relatively greater solubility of anthropogenic-derived atmospheric Pb. Aerosol samples collected concurrently with our seawater samples support our determination that Pb in the surface waters is partially of natural origin (Shelley et al, 2017(Shelley et al, , 2018, and work by Prospero et al (2012) introduces the possibility that much of the dust in the Irminger Sea and Iceland Basin is actually from high-latitude sources such as Icelandic dust. Future work to better constrain end-members could validate this hypothesis.…”

“…Although dust deposition to the North Atlantic Ocean is typically associated with northern African dust from the Sahara, Prospero et al (2012) and Bullard et al (2016) found that high-latitude dust emissions, specifically volcanic-based soils from Iceland, could be substantial enough to impact oceanic Fe cycling; therefore we suggest that the elevated Pb in the nearsurface waters of the Iceland Basin and Irminger Sea may possibly be dust-derived. In the GEOVIDE shipboard aerosol data (Shelley et al, 2017(Shelley et al, , 2018, Pb concentrations were high in the Iceland Basin but low in the Irminger Sea. However, as Pb has a residence time of ∼ 1 year in this region, seasonal changes in the flux also could account for this difference.…”

“…Trace metal enrichment factors of dry aerosols and wet deposition were collected during the GEO-VIDE cruise (Shelley et al, 2017(Shelley et al, , 2018. Results for Pb enrichment indicated atmospheric Pb was predominantly anthropogenic in origin (20-120, median 30).…”

“…Results for Pb enrichment indicated atmospheric Pb was predominantly anthropogenic in origin (20-120, median 30). Using positive matrix factorization of the aerosol concentration data, Shelley et al (2017Shelley et al ( , 2018 estimated that ∼ 60 % of the Pb was from a mineral dust source and only 40 % was of anthropogenic origin. This finding parallels the 2010 study of Pb in the tropical North Atlantic by Bridgestock et al (2016) that found 30 %-50 % of total dissolvable Pb in seawater was from natural mineral dust from the North African dust plume.…”

“…At the latest occupation of BATS in 2011, surface water concentrations were less than 20 pmol kg −1 (Noble et al, 2015) compared to 160 pmol kg −1 observed in 1979 (Schaule and Patterson, 1983) and ∼ 200 pmol kg −1 inferred from coral data from the mid-1970s (Kelly et al, 2009). Despite a dramatic reduction in [Pb], it is still believed that a large fraction of current Pb deposition results from coal and other combustion, and industrial processes, based on positive matrix factorization analysis of aerosols (Shelley et al, 2017(Shelley et al, , 2018Noble et al, 2015). In the tropical Atlantic, another 2010-2011 study found that 50 %-70 % of Pb in the surface ocean was anthropogenic in origin (Bridgestock et al, 2016), with the remaining fraction from natural North African dust.…”