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Abstract. A range of leaching protocols have been used to measure the soluble fraction of aerosol trace elements worldwide, and therefore these measurements may not be directly comparable. This work presents the first large-scale international laboratory intercomparison study for aerosol trace element leaching protocols. Eight widely-used protocols are compared using 33 samples that were subdivided and distributed to all participants. Protocols used ultrapure water, ammonium acetate, or acetic acid (the so-called “Berger leach”) as leaching solutions, although none of the protocols were identical to any other. The ultrapure water leach resulted in significantly lower soluble fractions, when compared to the ammonium acetate leach or the Berger leach. For Al, Cu, Fe and Mn, the ammonium acetate leach resulted in significantly lower soluble fractions than those obtained with the Berger leach, suggesting that categorizing these two methods together as “strong leach” in global databases is potentially misleading. Among the ultrapure water leaching methods, major differences seemed related to specific protocol features rather than the use of a batch or a flow-through technique. Differences in trace element solubilization among leach solutions were apparent for aerosols with different sources or transport histories, and further studies of this type are recommended on aerosols from other regions. We encourage the development of “best practices” guidance on analytical protocols, data treatment and data validation in order to reduce the variability in soluble aerosol trace element data reported. These developments will improve understanding of the impact of atmospheric deposition on ocean ecosystems and climate. 

Abstract. Supply of iron (Fe) to the surface ocean supports primary productivity, and while hydrothermal input of Fe to the deep ocean is knownto be extensive it remains poorly constrained. Global estimates of hydrothermal Fe supply rely on using dissolved Fe (dFe) toexcess He (xs3He) ratios to upscale fluxes, but observational constraints on dFe/xs3He may be sensitive toassumptions linked to sampling and interpolation. We examined the variability in dFe/xs3He using two methods of estimation, forfour vent sites with different geochemistry along the Mid-Atlantic Ridge. At both Rainbow and TAG, the plume was sampled repeatedly and the range ofdFe/xs3He was 4 to 63 and 4 to 87 nmol:fmol, respectively, primarily due to differences in plume age. To account for backgroundxs3He and shifting plume position, we calibrated He values using contemporaneous dissolved Mn (dMn). Applying thisapproach more widely, we found dFe/xs3He ratios of 12, 4–8, 4–44, and 4–86 nmol fmol−1 for the Menez Gwen, LuckyStrike, Rainbow, and TAG hydrothermal vent sites, respectively. Differences in plume dFe/xs3He across sites were not simplyrelated to the vent endmember Fe and He fluxes. Within 40 km of the vents, the dFe/xs3He ratios decreased to3–38 nmol fmol−1, due to the precipitation and subsequent settling of particulates. The ratio of colloidal Fe to dFe wasconsistently higher (0.67–0.97) than the deep N. Atlantic (0.5) throughout both the TAG and Rainbow plumes, indicative of Fe exchangebetween dissolved and particulate phases. Our comparison of TAG and Rainbow shows there is a limit to the amount of hydrothermal Fe releasedfrom vents that can form colloids in the rising plume. Higher particle loading will enhance the longevity of the Rainbow hydrothermal plume withinthe deep ocean assuming particles undergo continual dissolution/disaggregation. Future studies examining the length of plume pathways required toescape the ridge valley will be important in determining Fe supply from slow spreading mid-ocean ridges to the deep ocean, along with thefrequency of ultramafic sites such as Rainbow. Resolving the ridge valley bathymetry and accounting for variability in vent sources in globalbiogeochemical models will be key to further constraining the hydrothermal Fe flux.