A unique application of scanning electron microscope-cathodoluminescence (SEM-CL) analysis※1 to a small amount of quartz particles found in seawater has revealed the depositional flux and seasonality of Asian dust※2 in the subarctic region of the western North Pacific.
Approximately 40% of the dissolved iron, an essential micronutrient for oceanic primary production, is transported to the ocean surface from the atmosphere through Asian dust and anthropogenic aerosols※3.
Quantitative evidence supports that Asian dust, which has adverse effects on our livelihoods and health, plays a crucial role in supporting marine ecosystems in the western subarctic Pacific by supplying iron to seawater.
Scanning electron microscope-cathodoluminescence (SEM-CL) analysis: Cathodoluminescence is the emission of light observed when solid materials are irradiated with electron beams, resulting from impurities and native imperfections in the material. SEM-CL analysis allows for the microcharacterization of solids.
Asian dust: Dust particles from arid regions in China and Mongolia. These particles are carried into the atmosphere by strong winds and transported by the prevailing westerly winds to the leeward areas to the east.
Anthropogenic aerosol: Fine particles of sulfates, nitrates, etc. emitted via human activities and suspended in the atmosphere. It has been reported that iron contained in anthropogenic aerosols has a high solubility in water and plays an important role as a source of iron used by phytoplankton in the ocean. (Reported May 2, 2019)
The Earth Surface System Research Center of the Research Institute for Global Change (ESS-RIGC) of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has been conducting atmospheric and oceanographic observations in the subarctic region of the western North Pacific Ocean, centered at K2 (47°N, 160°E), since 2005 to study the effects of various ongoing environmental changes on marine ecosystems. In collaboration with researchers from Hokkaido University, Hiroshima University, and Kyushu University, Kana Nagashima, Researcher at ESS-RIGC, and her colleagues have developed a new analytical method to quantitatively assess the flux of Asian dust deposited in the ocean, and have succeeded in clarifying the deposition flux as 1.0 ± 0.2 mg m-2 per day with a high (up to 6.9 mg m-2 per day) values in May, June, and early July. The authors further quantified the impact of dissolved-iron flux supplied by Asian dust on phytoplankton production in the western subarctic Pacific, since the western subarctic Pacific is a high-nutrient and low-chlorophyll area, where the availability of iron limits phytoplankton production.
During the period between April and July, when oceanic basic productivity is high, dissolved-iron flux supplied by Asian dust was determined as 0.9 ± 0.3 µg m-2 per day. This amount is nearly half of the quantity of dissolved iron supplied from the subsurface ocean to the surface ocean (estimated in this study to be approximately 2.2 µg m-2 per day), which has recently gained attention as a major source in this area. This finding indicates that Asian dust is the second largest source of dissolved iron after subsurface ocean water. Additionally, anthropogenic aerosols are known to supply iron to the ocean via the atmosphere. The quantity of dissolved-iron supplied by anthropogenic aerosols in summer was estimated as approximately half that of Asian dust, and the total amount of “dissolved-iron supplied through the atmosphere,” including that from Asian dust, contributes approximately 40% of the total dissolved-iron supply, including that from the subsurface ocean layer.
This result suggests that Asian dust, which is considered as “bad actors” affecting the health of people living in East Asia, actually plays a significant role in supporting oceanic primary production by supplying iron to the ocean. Future changes in the emission and transport of Asian dust are expected to alter the ecosystem of the subarctic North Pacific and the amount of biological CO2 drawdown. Close monitoring of these changes is crucial.
The results of this study was published in “Scientific Reports” on September 29 (Japan Standard Time). This research was supported by the Japan Society for the Promotion of Science (JP19H05669, JP20H04329, and JP20H04350).
Kana Nagashima1, Hajime Kawakami1, Koji Sugie1, Tetsuichi Fujiki1, Jun Nishioka2, Yoko Iwamoto3, ToshihikoTakemura4, Takuma Miyakawa1, Fumikazu Taketani1, Maki NoguchiAita1
For this study
Kana Nagashima, Researcher, Research Institute for Global Change (RIGC) Earth Surface System Research Center (ESS) Marine Ecosystem Research Group, JAMSTEC
For press release