An original approach to assess the particulate trace metal concentrations in seawater

5 November 2025

Measuring the particulate trace metal (pM) concentrations in seawater is challenging, even though this parameter is essential to understand the trace metal cycles, a GEOTRACES mission. Sohrin and co-workers (2025, see reference below) propose defining pM as the difference between total dissolvable and dissolved metals after a long storage (approximately two years) of filtered and unfiltered seawater acidified to pH 1.9. They observed that pM increased in two stages and reached equilibrium within ~ 1 y for relatively soluble elements as cobalt (Co) and lead (Pb), and took twice as long for aluminium (Al) and iron (Fe). They then compared their results with those obtained by a strong-acid leaching of filtered particles (total particulate metal; tpM) and established that pM determined by their protocol is a good proxy for tpM. Although they acknowledge some drawbacks for their approach -mostly related to the occurrence of refractory minerals in the samples- they consider that, if researchers are patient enough, they can assess a proper concentration of pM via the proposed protocol: pM result is related to the intercalibrated concentration of dM; approximately a hundred grams of filtered and unfiltered seawater subsampled from a sampling bottle are sufficient to determine pM; the risk of contamination is low because strong acids at high concentrations are unnecessary.

**Figure 1:** Time course of total dissolvable metal (tdM) and dissolved metal (dM) concentrations at pH 1.9 in bottom water collected from the western North Pacific. The error bars represent ±sd. The difference between tdM and dM represents the concentrations of particulate metal (pM). The time required for 90% dissolution is 1060 days for pAl and 281 days for pFe, suggesting that pM is more leachable than previously expected.

**Figure 2:** Correlation between pAl and tpAl, and between pFe and tpFe, in seawater samples that were collected from the subarctic Pacific during the R/V Hakuho Maru KH-17-3 cruise. The red lines represent linear regressions, and the doted lines indicate 1:1 relationships. The slope for pAl vs tpAl is close to 1. The slope for pFe vs tpFe is also close to 1 when the three data with pFe > 50 nmol/kg, which were affected by colloids, are excluded. These results suggest that pM is a good proxy for tpM.

Reference:

Sohrin, Y., Zheng, L., Chan, C.-Y., Nakaguchi, Y., Takano, S., Sohrin, R., Liao, W.-H., & Ho, T.-Y. (2025). Acid-leachability of metals from suspended particles in the Pacific Ocean. Marine Chemistry, 273, 104571. Access the paper:10.1016/j.marchem.2025.104571

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