[AOS09-P03] Vertical fluxes of nutrients based on radium-228 data in the western subarctic Pacific
Keywords:Vertical flux, Radium-228, western subarctic Pacific, nutrient
It is also well known that the western subarctic Pacific is high nutrient, low chlorophyll area, in which nutrients remain undepleted in surface waters throughout the year. In this area, phytoplankton growth is limited by the availability of Fe in seawater. To improve our understanding of primary production in the subarctic Pacific Ocean, it is important to determine supply processes of both nitrate and iron.
In this study, we revealed the vertical profile of radium-228 (half-life, 5.7 years ) with those of nitrate and iron, and estimated vertical fluxes of nitrate and iron at the K2 station. We will also compare the results in this study with the vertical fluxes of the nutrients by physical parameters.
Seawater samples were collected using acid-cleaned samplers with external springs mounted on CTD-CMS system during a research cruise of TS Oshoro-maru, June 2015. The samples for Fe analyses were collected in low-density polyethylene bottles through a 0.2 µm-pore size filter. Then, dissolved Fe was analyzed at onshore laboratory by using a flow-injection analysis chemiluminescence detection system (Obata et al. 1993).
The samples, for Ra-228 analysis, were pre-treated onboard the ship with the coprecipitation of barium sulfate (Inoue at al., 2006). Briefly, barium and iron carriers were added to seawater samples, and then sodium sulfate solution was added to coprecipitate radium with barium sulfate. Iron hydroxide was then deposited by re-adjustment to pH 7, and gathered together with barium sulfate After bring back the precipitation, the precipitations were dried and compressed for gamma-counting. The gamma-spectrometry of all water samples was performed using large-volume well-type Ge-detectors, specially designed for low-background counting and equipped at Ogoya Underground Laboratory, Japan.
The vertical fluxes were calculated based on the previous studies (Ku et al., 1995; Nozaki and Yamamoto, 2001). The vertical fluxes were calculated as 0.36-2.86 mmol/m2/d for nitrate and 0.01-0.09 mmol/m2/d for iron. The vertical fluxes for the nutrients were higher than those obtained by physical parameters probably because the fluxes based on Ra-228 distribution were averaged during the decay time of Ra-228. To obtain the vertical fluxes of nutrients more precisely, we will need time variation data of Ra-228 with nutrient data.