Lithogenic particles are refractory assemblages of crust-derived minerals. Studying sources and cycling of these particles is of great importance because they can deliver nutrients or pollutants into the ocean, and participate in marine biogeochemical cycling by serving as sites for adsorption-desorption processes or as ballasting agents that facilitate the sinking of organic materials. In this study, we present the first size-fractionated (0.8–51 μm and >51 μm) particulate concentration data for lithogenic tracer elements — aluminum (Al), iron (Fe), and titanium (Ti) — collected by in situ pumps from transects along 20°N (spanning 125°E to 155°E) and 155°E (spanning 12°N to 42°N) across the Northwestern Pacific Ocean.
The distribution patterns of particulate Al, Fe, and Ti were quite similar in the study region, typically exhibiting elevated concentrations at the surface, a minimum at the subsurface, and increasing concentrations at greater depths. Spatial variations were evident, with the highest concentrations of particulate Al, Fe, and Ti observed at Sta. K2b (20°N, 126°E), closest to the continental margin, and at Sta. 41 (37°N, 155°E), where the Kuroshio extension current flows through. At both stations, particulate Fe:Al ratios below the euphotic zone were close to the upper continental crust value of 0.23, whereas stations farther from Sta. K2b and Sta. 41 had Fe:Al ratios approaching the Asian dust value of 0.43. This contrast indicates that, in addition to Asian dust, the relatively high concentrations of lithogenic particles observed at certain stations highly likely result from crustal materials transported by ocean currents. The highest Fe:Al ratios (up to ~ 2.0) were observed in the upper layer of the Northwestern Pacific Ocean, especially in small size-fractionated particles, suggesting an increase in biogenic Fe particles. Similar patterns were observed in Ti:Al ratios, implying that Ti might be biologically active despite no known essential role of Ti in the metabolism of any organisms.