Biogeochemical properties of sinking particles in the southwestern part of the East Sea (Japan Sea)
Introduction
The East Sea (also known as the Japan Sea) is a marginal sea surrounded by the Asian continent, the Korean peninsula, and the Japanese islands. The East Sea is connected to the Pacific Ocean through shallow straits (Fig. 1), and the surface water in the East Sea, especially south of the subpolar front at ~ 40°N, is mainly supplied by the Tsushima Warm Current flowing through the Korea Strait (Cho and Kim, 2000).
A study of primary production based on satellite observations showed that the southwestern part, the Ulleung Basin (UB), was the most productive region in the East Sea (Yamada et al., 2005). For the UB, the annual primary production determined using monthly in situ measurements (273 gC m− 2 yr− 1; Kwak et al., 2013a) and Moderate Resolution Imaging Spectroradiometer (MODIS-aqua) satellite observations (280 gC m− 2 yr− 1; Joo et al., 2014) is reportedly higher than that of the Kuroshio region (where the Tsushima Warm Current bifurcates), the East China Sea, and the Northwestern Pacific Ocean at similar latitudes (Kwak et al., 2013a, Kwak et al., 2013b).
A few hypotheses have been suggested to explain the high annual primary productivity in this region. Frequent upwelling in the southeast coast of Korea was observed to supply nutrients supporting a local enhancement in primary production, and nutrients and particulate organic matter may be further transported into the basin via the surface current (Yoo and Park, 2009). Another hypothesis points out the importance of the vertical structure of the water column in the UB. Especially in summer when the surface mixed layer is much shallower than the euphotic depth, upward flux of nitrate from the underlying nutrient-abundant water supports the primary production (Kwak et al., 2013b). The high production in summer in the subsurface chlorophyll maximum was suggested as another main cause of the high annual primary productivity in this region (Rho et al., 2012). Another study suggested that the quantity of nutrients supplied by the current flowing into the UB through the Korea Strait is enough to sustain the observed primary productivity (Lee and Rho, 2013).
The organic carbon content in the surface sediment of the UB is > 2% (Cha et al., 2007, Lee et al., 2008), which is unusually high considering its bottom depth of 2300 m. This value is comparable to those observed in highly productive regions such as the Chilean upwelling region (Schubert et al., 2000, Böning et al., 2005). The riverine input of organic matter from the Korean peninsula and the Japanese islands is much smaller when compared to the in situ primary production (Hong et al., 1997). Therefore, the high organic carbon content in the basin sediment implies a high supply of marine organic carbon into the basin sediment. The potential for a high export flux of particulate organic carbon (POC) was implied by the relatively high f-ratio values (~ 0.6) estimated using the nitrate and ammonium uptake rates in the UB (Kwak et al., 2013b). Nevertheless, the sinking particle flux to the basin interior and the biological carbon pump operation in this region are not well understood.
In this paper, we present the one year fluxes and biogeochemical compositions of sinking particles collected at 1040 m and 2280 m on bottom-tethered mooring in the UB. We examined the biogeochemical properties, including the radiocarbon content and excess Mn, in sinking particle samples to better understand the particle supply to the basin. Then, we compare our results for the UB with previously reported data from other major basins in the East Sea to obtain a more comprehensive picture of the particle transport dynamics.
Section snippets
Sample collection
Sinking particles were collected for one year from March 2011 by using time-series sediment traps with a conical type with an aperture diameter of 80 cm and height/diameter ratio of 1.32 (SMD-26S-6000, Nichiyu Giken Kogyo Co., Ltd., Japan) deployed at 1040 m (nominal depth of 1000 m hereafter) and 2280 m (20 m above the seafloor, nominal depth of 2300 m hereafter) on bottom tethered mooring at Station EC1 (37.33°N, 131.45°E; 2300 m water depth; Fig. 1) in the UB. This station has been occupied since
Particle flux
The total particle flux (< 1 mm) ranged between 39 and 817 mg m− 2 d− 1 at 1000 m (Fig. 2a). At 1000 m, the particle flux showed a peak in early May with a dip in late April immediately before the peak. Another peak was observed in the particle flux in the fall. The particle flux decreased gradually from early November to low values of ~ 50 mg m− 2 d− 1. In general, the total particle flux at 2300 m exhibited a similar temporal variation to that at 1000 m. However, the flux at 2300 m was noticeably higher in
Biological carbon pump in the East Sea
The sudden dip in the total particle flux observed in late April during the spring bloom and the abnormally low values in the winter are suspicious and need to be further examined to determine whether the flux measurements were compromised because of low sediment trap efficiency possibly associated with tilting of the mooring line. The ADCP, deployed at a depth of 465 m showed three events with an abrupt increase in tilting to > 10° for 2–10 days in late April, late June, and late January (Fig. 4a
Summary and conclusions
Sinking particle samples at 1000 m and 2300 m (20 m above the seafloor) in the UB were collected and analyzed to investigate the biological carbon pump processes operating in the East Sea. The sinking particle flux and biogenic particle composition of the samples collected at 1000 m generally reflected the biological processes in the surface water. A considerable POC flux was observed during the summer months from June to August accounting for ~ 27% of the annual POC flux, which is consistent with a
Acknowledgements
We thank the captains and the crews of the survey vessel Haeyang 2000 and the R/V Eardo and cruise participants for help at sea; S.C. Hwang, Y.B. Kim, and members of the Current Dynamics Research Laboratory of Seoul National University for sediment trap deployment and recovery; staffs at NOSAMS WHOI for carbon isotopic analyses; staffs at the Korea Basic Science Institute for element and metal analyses. This research was part of the project, East Asian Seas Time series-I (EAST-I), funded by the
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