Acid-leachability of metals from suspended particles in the Pacific Ocean

Suspended particles are major carriers of trace metals in seawater, while the cycling mechanisms of particulate trace metals remain largely unclear owing to analytical challenges. In this study, we focused on the acid-leachability of particulate trace metals collected from the Pacific Ocean and investigated their compositional features by measuring the total dissolvable metal (tdM) and dissolved metal (dM) concentrations in unfiltered and filtered seawater samples, respectively. We defined the difference between total dissolvable and dissolved metals as the particulate metal (pM). We investigated the pM time course in acidified bottom waters (pH 1.9) over approximately two years. pM increased in two stages and reached equilibrium within a few years. In the field study, we stored acidified seawater samples at least two years before analysis. We compared pM with total particulate metal (tpM) concentrations in the same samples collected from 11 stations in the subarctic Pacific. The tpM concentrations were determined by particle filtration followed by total digestion using strong acids, including HF. The results indicate that pAl and pFe are nearly equivalent to tpAl and tpFe, respectively. Many metals in the suspended particles, including clay minerals, are highly acid-leachable during long-time storage. By compiling our pM data observed in the Pacific Ocean and its marginal seas (approximately 1500 samples), we found that the average pM/dM ratio across the Pacific is 9 for Al and 4 for Fe. Although pAl and pFe exhibit strong linear correlations, their concentrations and regression slopes are spatially variable, reflecting differences in sources and fluxes.