An Anthropogenic Iron "Invasion" of the Ocean Followed by Radioactive Polonium.

[NNadir]

The paper I'll briefly discuss is this one: Anthropogenic Iron Invasion into the Ocean: Results from the East Sea (Japan Sea) Hojong Seo and Guebuem Kim Environmental Science & Technology 2023 57 (29), 10745-10753.

I am always interested in nuclear chemistry, and so this paper, which has nothing to do with the last best hope of humanity, nuclear power, but rather focuses on naturally occurring radioactive materials (NORM) and their use in tracing the source of iron in the oceans.

From the text:

Iron (Fe) is an important micronutrient for primary producers, and its low concentration in the surface ocean can limit photosynthesis and nitrogen fixation in half of the global ocean. (1,2) In general, atmospheric transport of natural mineral dust has been regarded as the primary source of bioavailable Fe in the surface ocean. (3,4) However, Fe-bearing anthropogenic aerosol has received considerable attention as another significant source of bioavailable Fe. Although the total mass amount of Fe emitted from anthropogenic sources is an order of magnitude smaller than that from natural mineral dust globally, (5?7) the anthropogenic aerosol may considerably contribute to the water-soluble Fe concentrations in aerosols due to its higher Fe solubility in comparison with that of natural mineral dust. (8?12) The distinct difference in Fe solubility may be associated with the different mineralogical characteristics of Fe between natural mineral dust and anthropogenic aerosols. Fe in natural mineral dust is mainly in the form of coarse crystalline oxides and aluminosilicate minerals. (13,14) By contrast, Fe in anthropogenic aerosols is generally in the form of fine ferric sulfate and aggregated Fe oxides since they go through high-temperature combustion (>1000 °C) by fossil-fuel power plants, exhaustive vehicles, and metal smelting plants. (15?17) In addition, the anthropogenic aerosol releases soluble Fe much more rapidly than natural mineral dust by inorganic acids (e.g., NOx and SO2) formed from anthropogenic pollutants. (18,19) Recently, the contribution of anthropogenic Fe to the water-soluble Fe in the aerosol was estimated to be approximately 10–50% in East Asian air masses and 10–100% in European and North American air masses based on the Fe isotopes (20,21) and other chemical tracers (V and Al). (9,22,23) The water-soluble Fe may become a bioavailable form of Fe in seawater.

The atmospheric depositional flux of anthropogenic Fe to the ocean has been mostly estimated by modeling approaches, although different results have been obtained depending on the study area and the model used. For instance, it was suggested that anthropogenic Fe contributed to the atmospheric supply of water-soluble Fe from 2 to 70% in the Northern Hemisphere. (24) In the Southern Ocean, the atmospheric depositional flux of anthropogenic Fe accounts for from insignificant proportions (less than 5%) to over half of the water-soluble Fe supply (>60%). (24?26) As such, global atmospheric models have shown approximately two orders of magnitude (0.1–30 Tg Fe yr–1) differences in the water-soluble Fe flux (5,27) owing to the large uncertainties in obtaining the emission flux of Fe, the solubility of Fe for different sources and by atmospheric processes, and the lack of observational data for model validation. (28) In the real-world ocean, to our knowledge, Pinedo-González et al. (29) is the only study that provides in situ evidence of anthropogenic Fe contribution (20–60%) to the dissolved Fe in the surface layer of the North Pacific Ocean using isotopic mass balance. However, the isotopic signature of Fe also has large uncertainties in quantifying anthropogenic Fe in the ocean because of isotopic fractionation associated with biological production, scavenging, and ligand complexation. (30)

Thus, in this study, we used various tracers (Al, K, V, Ni, Pb, and 210Pb) to identify the main sources of anthropogenic Fe in 72 aerosol samples collected over one year in the eastern coastal region of Korea, located in the northwestern Pacific Ocean (Figure 1). In addition, we used 210Pb as a tracer of the atmospheric fallout flux of anthropogenic Fe. Furthermore, we evaluated the contribution of anthropogenic Fe to seawater based on a non-steady-state scavenging model, including the 210Pb-derived Fe flux and the measured Fe concentrations in the East Sea (Japan Sea). Since this sea is located in the downwind of Asian aerosol sources and is almost fully closed from 200 to 3000 m, the results in this region may reveal what is happening in the global ocean with the invasion of anthropogenic Fe...

The use of Pb-210 - measured as its daughter Po-210 (polonium) from the natural decay series of U-238 is rationalized thus:

The concentrations of sFeanthro also show a significant correlation with excess 210Pb (half-life: 22.3 years) concentrations (Figure 3b). 210Pb is naturally produced from 222Rn (half-life: 3.8 days), an inert gas that emanates from the continent. However, natural Fe in water-soluble fraction (sFenatural) does not correlate with excess 210Pb (Figure S4). Thus, the unique correlation between sFeanthro and excess 210Pb suggests that sFeanthro is mainly distributed on fine particles (less than 1 ?m) through gas-to-particle conversion during the processes of anthropogenic combustion, as shown for 210Pb produced from 222Rn. (90,91) In addition, as the anthropogenic aerosols age in the atmosphere, the sulfate coating that increases the Fe solubility by forming soluble Fe sulfate may become thicker. (17) The long residence time of aerosols also allows more 210Pb to be produced from increased 222Rn in the atmosphere, and the newly produced 210Pb is subsequently adsorbed to the surrounding fine particles. (92) Thus, although the sources of sulfate and 222Rn are different, there can be a significant correlation between sFeanthro and excess 210Pb in the aerosol.

The measurement technique is interesting:

To determine 210Pb activity in aerosols, all aerosol samples were stored for over two years before analysis to ensure radioactive equilibrium between 210Pb and 210Po. The 210Po in the aerosol filter was extracted using 6 M HCl for 6 h at 180 °C after adding a 209Po spike (1.5 dpm). The leachates were separated from the filter by centrifugation and then evaporated to dryness. The residue was redissolved in 0.5 M HCl, and then Po was plated onto a silver disk at ?80 °C by stirring for 3 h after adding ?0.5 g of ascorbic acid to reduce any Fe to Fe2+. The activity of Po was determined using ? spectrometry (? Analyst, Canberra). The procedural blank of the W41 filter for 210Pb was less than 0.05% of the lowest activity found in this study. As we extracted Po from aerosol filters using 6 M HCl and the activity of 226Ra is known to be less than ?2% of 210Pb in the aerosols, (33,34) the activity of 210Pb in this study can be regarded as an excess 210Pb, which is produced only from 222Rn in the atmosphere. All analytical results are given in Tables S1 and S2.

Some pictures from the text:



The caption:

Figure 1. Location of the sampling site with air mass trajectories. The air mass trajectories were obtained using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model developed at the Air Resources Laboratory of the National Oceanic and Atmospheric Administration (NOAA). (50) Backward trajectories using the Global Data Assimilation System (GDAS) database were calculated for 72 h at 500 m above ground level, characterized by the well-mixed boundary layer in this region. (51)

The caption:

Figure 3. Sources of anthropogenic Fe and its relationship with 210Pb in aerosols. (a) Plot between non-sea-salt potassium-to-aluminum ratios (nss-K/Al) and non-sea-salt vanadium-to-aluminum ratios (nss-V/Al). The error bars for each end-member represent a 1-standard deviation from average values. The color gradient indicates Fe solubility. (b) Anthropogenic Fe in water-soluble fraction versus excess 210Pb. The dashed line represents the slope and intercept of a linear regression of data (r = 0.66 and p < 0.001). The error bars for excess 210Pb are based on 1-standard deviation counting statistics.

The caption:

Figure 4. Schematic illustration of the sources and atmospheric deposition of Fe in the East Sea.

I'm not sure I entirely buy into the method; I'd need to think about it more, which is not to say that I know it doesn't work.

It's an interesting idea however, worth thinking about.

Enjoy Friday.