[M-ZZ45] Frontiers in geochemistry: discussing its appeal and future prospects

[MZZ45-07] Seasonal variations in the Fe flux from different sources of atmospheric aerosols in the western subarctic North Pacific

*Minako Kurisu¹, Koji Sugie¹, Fumikazu Taketani¹, Takuma Miyakawa¹, Teruhiko Kashiwabara¹, Katsuhiko Suzuki¹, Kohei Sakata², Jun Nishioka³, Hajime Obata⁴, Tim M. Conway⁵, Yoshio Takahashi⁶ (1.Japan Agency for Marine-Earth Science and Technology, 2.National Institute for Environmental Studies, 3.Institute of Low Temperature Science, Hokkaido University, 4.Atmosphere and Ocean Research Institute, The University of Tokyo, 5.University of South Florida, 6.Graduate school of science, The University of Tokyo)

Keywords:Iron, aerosol, western subarctic North Pacific, stable isotope ratios

The western subarctic North Pacific exhibits high primary productivity and is one of the most significant areas where the ocean absorbs carbon dioxide from the atmosphere. A lack of dissolved iron (Fe) is a limiting factor for primary productivity (Martin and Fitzwater, 1988). Atmospheric aerosols, mainly including mineral dust and combustion aerosols, are one of the major Fe sources, but the deposition flux of Fe to the surface ocean from different aerosol sources, their impact on surface primary productivity, and their seasonal variations are still unknown, partly due to the difficulty of fixed-point observations. In this study, we collected aerosols at a certain point in the western subarctic North Pacific in different seasons and aimed to determine the deposition flux of atmospheric Fe from different sources and Fe solubility (related to the bioavailability) by using Fe stable isotope ratios (δ⁵⁶Fe) as an indicator of Fe sources.
Size-fractionated aerosol particles were collected on a pre-cleaned PTFE filter using a high-volume air sampler during R/V Hakuho-maru KH-14-3 (summer 2014) and KH-17-3 (summer 2017) cruises and R/V Mirai MR-21-01 (winter 2021) and MR-22-03 (spring 2022) cruises. Total metal concentrations were measured using inductively coupled plasma mass spectrometry (ICP-MS) after mixed acid digestion, whereas soluble metal concentrations were measured after extraction with ultrapure water. Then, Fe in the solutions was purified using an anion exchange resin, and isotope measurements were performed with double-spike method using a multi-collector ICP-MS.
Atmospheric Fe concentrations were high in winter and spring, and about an order of magnitude lower in summer. This is consistent with the prevalence of Asian dust in spring. On the other hand, Fe solubility tended to be higher in summer than in winter and spring. As a result, the deposition flux of soluble Fe was estimated to be similar in winter/spring and summer. The δ⁵⁶Fe of the coarse particles was close to that of mineral dust regardless of the season, while that of the fine particles was a minimum of –1‰ in winter and as low as –2.2‰ in summer. These negative δ⁵⁶Fe values suggest the presence of anthropogenic combustion Fe (Kurisu et al., 2016; 2021) and that the fraction of combustion Fe relative to mineral dust is greater in summer than in winter. In the presentation, we will also report the results of δ⁵⁶Fe in the surface seawater collected simultaneously and discuss the impact of atmospheric Fe on the surface ocean.