Biogeochemical properties of sinking particles in the southwestern part of the East Sea (Japan Sea)

doi:10.1016/j.jmarsys.2016.11.001

Journal of Marine Systems

Volume 167, March 2017, Pages 33-42

https://doi.org/10.1016/j.jmarsys.2016.11.001 Get rights and content

Highlights

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POC flux variation at 1000 m shows a good relationship with the primary production.
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The ratio of POC flux at 1000 m to primary production was ~ 3% in the UB.
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The particle flux in the East Sea appears to be controlled by complex sources.

Abstract

This study investigates the biological pump system in the East Sea (Japan Sea) by conducting an analysis of the total particle flux, biogenic material composition, and carbon isotope ratios of sinking particles. The samples were collected for one year starting from March 2011 using time-series sediment traps deployed at depths of 1040 m and 2280 m on bottom-tethered mooring at Station EC1 (37.33°N, 131.45°E; 2300 m water depth) in the Ulleung Basin (UB), southwestern part of the East Sea. The temporal variation in the particulate organic carbon (POC) flux at 1000 m shows a good relationship with the primary production in the corresponding surface water. The ratio of POC flux at 1000 m to satellite-based primary production in the corresponding region in the UB was ~ 3%, which is comparable to the values of 2 to 5% estimated from previous studies of other part of the East Sea. The lithogenic material accounted for > 17% of the sinking particles at 1000 m and for a larger fraction of 40 to 60% at 2280 m. The radiocarbon contents of the sinking POC at both trap depths imply the additional supply of aged POC, with a much greater contribution at 2280 m. Overall, the particle flux in the deep interior of the East Sea appears to be controlled by the supply of complex sources, including aeolian input, the lateral supply of resuspended sediments, and biological production in the surface water.

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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The sources and behaviors of particulate organic carbon (POC) and particulate iron and manganese (pFe and pMn, respectively) in the bottom nepheloid layer (or benthic nepheloid layer, BNL) of the southwestern East Sea, also known as the Japan Sea, along a transect from the shelf to the central Ulleung Basin were investigated. The fluxes of POC, pFe, and pMn from the BNL to the seafloor on the shelf and in the basin were determined based on thorium-234 (²³⁴Th). The influence of resuspended sediment on POC was quantified using radiocarbon isotope ratio of POC (Δ¹⁴C). Sources and behaviors of pFe and pMn in the BNL were investigated by comparison to those of particulate aluminum (pAl). The stable carbon isotope ratios mainly indicated the marine origin of the POC, and the Δ¹⁴C values indicated that the majority of the POC in the BNL (68% ± 22%) was supplied by sediment resuspension. pAl and pFe were lithogenic in origin, whereas pMn was mainly authigenic (89–100%). The deficiency of ²³⁴Th activity relative to that of ²³⁸U increased toward the seafloor in the BNL, implying the efficient removal of ²³⁴Th by adsorption to the resuspended sediment particles. The ²³⁴Th-based settling fluxes of POC, pAl, pFe, and excess Mn (pMn_xs) to the seafloor in the central basin agreed with the results previously obtained from a sediment trap study in the Ulleung Basin. The settling flux of pAl and pFe in the central basin was 2–8% of the lateral transport from the Korea Strait, implying that the transported lithogenic particles mostly settled on the slope before reaching the central basin. In contrast, the settling flux of pMn in the central basin was much larger than that of pMn in the shelf, implying that pMn is further transported toward the central basin or supplied from the local sediments.
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Understanding bathymetric patterns of deep-sea meiofauna is important for elucidating the influence of environmental variables on the characteristics of meiofaunal community structures. Depth-dependent meiobenthic community structures were investigated on the continental shelf, slope and deep basin of the Ulleung Basin in the southeastern East Sea. Meiobenthic samples were collected at 29 stations (water depth: 74–2155 m) from the continental shelf near the Korea Strait to the southern slope and basin areas of the Ulleung Basin during the summer cruise of R/V Eardo (June 2006). In total, 15 meiobenthic taxa (between two and nine taxa at each station) were identified. Nematodes formed the most abundant taxon (53–80%), followed by benthic foraminifera (7–39%). Meiobenthic abundance varied from 295 to 3697 ind. 10 cm⁻² (nematodes, 106–3319 ind. 10 cm⁻²) at each station. The distribution of meiobenthic animals in the Ulleung Basin showed considerable spatial trends, with higher densities on the southern slope (1083 ± 249 ind. 10 cm⁻²) of the Ulleung Basin than the shelf (650 ± 220 ind. 10 cm⁻²) or deep basin (459 ± 214 ind. 10 cm⁻²) regions. Among the environmental parameters, TOC (total organic carbon) concentration was significantly correlated with meiobenthic abundance (r²=0.297), indicating that high food supply promotes meiobenthic abundance. Meiobenthic community structures showed depth-related trends from the continental shelf to the deep basin, with several meiobenthic taxa (e.g., amphipods, bivalves, harpacticoids, kinorhynchs, nauplius, nematodes, ostracods and polychaetes) contributing to these variations in the meiofauna.
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Thus, changes in atmospheric dust deposition were not responsible for the observed seasonal variations in the lithogenic material flux. Excess Mn can be used as an indicator of advective mixing of the water near the seafloor because it is controlled by the diffusion of soluble Mn2+ produced via MnO2 reduction in the sediment, mobilization into the water column, subsequent oxidation into particles, and settling (Yeats et al., 1979; Davison et al., 1982; Kim et al., 2017). Both the content and flux of excess Mn were several times greater at 4950 m when compared with 4500 m. Despite the difference in magnitude of the excess Mn contents at these two depths, its temporal variations at the two depths were similar at both (Fig. 3).
We examined the biogenic and lithogenic particle composition and radiocarbon content of sinking particulate organic carbon to investigate sediment resuspension and its contribution to sinking particles on the deep abyssal plain of the Eastern Tropical Pacific Ocean. Samples were collected using sediment traps from August 2011 to July 2012 at depths of 4500 and 4950 m (50 m above the seafloor); above and within the benthic nepheloid layer (BNL), respectively. Biogenic particles derived from export production were the major source of sinking particles and their flux showed a unimodal temporal distribution, with larger values between February and April. At a depth of 4500 m, the lithogenic material flux was slightly greater than, or similar to, the flux of atmospheric dust deposition. In comparison, lithogenic material and excess Mn consistently showed a greater contribution to resuspended particles at 4950 m. The lithogenic material flux was proportional to the biogenic flux. These observations imply that resuspended particles exist at a background concentration in the BNL throughout the year, and are scavenged by sinking biogenic particles, especially during the high flux period.
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The Nutrient Diol Index (NDI) has been proposed as a paleo sea surface nutrient proxy. However, the NDI's broad applicability needs to be further assessed by examining its temporal variability in association with modern sea surface nutrient concentrations. This study is the first report to evaluate the NDI with respect to sinking particles in the East Sea. The NDI values showed seasonal variations resembling those of the surface phosphate concentrations in the study area. However, the calibration based on the sinking particles at 1000 m water depth (NDI = 1.134 × [PO⁴⁻] + 0.115, R² = 0.59, n = 26) deviated from the newly compiled global surface sediment dataset (NDI = 0.392 × [PO⁴⁻] + 0.023; R² = 0.58, n = 566). This local calibration predicted the phosphate concentrations in surface sediments in the East Sea better than the global calibration. Our study suggests that the difference of the calibration slope between the East Sea trap dataset and the global surface sediment dataset might be associated with different 1,14-diol producers and their sensitivity to nutrient concentrations in the East Sea. Accordingly, the NDI should be further assessed in various oceanic settings before it is routinely used to reconstruct sea surface nutrient conditions.
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Biogeochemical properties of sinking particles in the southwestern part of the East Sea (Japan Sea)

Highlights

Abstract

Introduction

Section snippets

Sample collection

Particle flux

Biological carbon pump in the East Sea

Summary and conclusions

Acknowledgements

Mar. Geol.

J. Asian Earth Sci.

Deep-Sea Res. I

Deep-Sea Res. I

Geochim. Cosmochim. Acta

Deep-Sea Res. II

Atmos. Environ.

Deep-Sea Res. I

Nucl. Instrum. Methods Phys. Res., Sect. B

Deep-Sea Res. I

Mar. Chem.

Cont. Shelf Res.

Org. Geochem.

Geochim. Cosmochim. Acta

Deep-Sea Res. I

Mar. Chem.

J. Mar. Syst.

Lamont radiocarbon measurements VI

Am. J. Sci.

Branching mechanism of the Tsuchima Current in the Korea Strait

J. Phys. Oceanogr.

The dynamics of iron and manganese in a seasonally anoxic lake; direct measurement of fluxes using sediment traps

Limnol. Oceanogr.

The atmospheric input of trace species to the world ocean

Glob. Biogeochem. Cycles

Composition, age, and provenance of organic matter in NW African dust over the Atlantic Ocean

Geochem. Geophys. Geosyst.