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Advanced Taiwan Ocean Prediction System

Tseng, Tsu-Lun - ALL News | 2019-09-17 | Count:15


Title: Aerosol Fe biogeochemistry in the Northwestern Pacific Ocean


The supply of Fe to oceanic surface water is a major factor controlling oceanic primary production and global carbon cycling. Aerosol deposition is a dominant process for supplying external Fe to the euphotic zone of the open ocean. Due to their different chemical and physical properties, lithogenic or anthropogenic aerosol Fe may proceed with different transformation processes in the surface water after deposition. Depositing to the surface water, either lithogenic or anthropogenic aerosol Fe would undergo a series of abiotic and biotic transformation processes, including dissolution and precipitation, adsorption and desorption, aggregation and disaggregation, suspension and sinking, and uptake and decomposition. These complicated physical and biogeochemical processes would then decide the retention time and bioavailability of aerosol Fe in the euphotic zone. The Fe availability would then influence phytoplankton growth and community structure, and subsequently carbon and major nutrient cycling in the ocean.  However, the internal transformation processes of aerosol Fe in oceanic surface water still largely remain unclear. 
East Asia has emitted tremendous amount of anthropogenic aerosols to the adjacent oceanic regions. Located downwind site of the strong northeastern monsoon in East Asia, the Northwestern Pacific Oceanic (NWPO) and its marginal seas thus receives large amounts of both anthropogenic and lithogenic aerosols during winter and spring. The feature of relatively high anthropogenic and lithogenic aerosol deposition fluxes with contrasting seasonality turns the NWPO into an ideal region to investigate the transformation and cycling processes of aerosol Fe in the surface ocean. Through Taiwan GEOTRACES process study, we carried out cruises during the representative periods of low and high aerosol deposition seasons. We comprehensively determined Fe concentrations and fluxes in pools associated with aerosol Fe cycling in the surface water, including aerosols, seawater, size-fractionated suspended particles, and sinking particles, particularly emphasizing on investigating the transformation processes from aerosol Fe to size-fractionated suspended particulate Fe. With one order of magnitude higher aerosol Fe contributed to the surface ocean in the spring month, we found that the elevated deposition of aerosols was not reflected in the dissolved pool but the suspended pool. Elemental ratios further indicate that biotic and abiotic suspended particulate Fe exhibited size-dependent distribution patterns. Lithogenic particles originating from aerosol deposition were mainly in particle fraction with sizes ranging from 2 to 25 µm in the surface water. Biotic particles smaller than 2 µm were the major carriers of intracellular and surface precipitated/aggregated Fe. Due to the relatively small size and low density of biotic particles, the surface precipitated/aggregated Fe on small biotic particles shall possess longer residence time and higher bioavailability than the large and dense lithogenic particles in the euphotic zone.
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