Atsuko Amano

Michinobu Kuwae

Tetsuro Agusa

Koji Omori

Hidetaka Takeoka

Shinsuke Tanabe

Takashige Sugimoto

Spatial distribution and corresponding determining factors of metal concentrations in surface sediments of Beppu Bay, southwest Japan

Keywords: surface sediment, metal, organic geochemical property, redox condition, PCA, RDA

the surface sediments of Beppu Bay on the basis of comparisons of the organic geochemical properties and environmental parameters through principal component analysis (PCA) and redundancy analysis (RDA). The results of PCA and RDA showed that the concentrations of V, Cr, Co, and As were closely related to the distances between the sampling sites and the Oita River. This indicated that these metals originated from the river's drainage area. The Mn, Cu, Mo, and Cd concentrations were related to the water depth. These results indicated that the Mo, Cd, and Cu deposition processes were controlled by oxygen depletion, and that these elements accumulated in the deeper parts of the bay under anoxic conditions.

. Introduction

Semi-enclosed coastal areas such as estuaries and bays are characterized by stagnant conditions and the supply with water inflow from land. As a result, materials supplied from the watershed area accumulate in this type of coastal areas, which is likely to cause environmental pollution. In particular, recent human activities have had a strong impact on these semi-enclosed environments. In order to identify the causes of marine pollution, it is important to determine the sources and transfer processes of pollutants. However, the physical transport processes in a coastal area are very complicated because of drastic environmental changes to the border

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3 between land and sea. In particular, the metal elements in sediments can originate from many sources, such as minerals, biogenic materials, and urban and industrial human activities, and they have complex deposition processes. Therefore, it is often difficult to determine the main factor driving the transfer and deposition processes for metals in sediments. In an effort to understand the supply sources and deposition processes of metals in semi-enclosed areas, many studies have attempted to determine the spatial distribution of metal concentrations in surface sediments and explain the controlling factors through simple correlations and principal component analysis (PCA) (e.g., Ergin et al., 1991;[START_REF] Rubio | Geochemistr y of Major and Tr ace Element s in Sediment s of Ria de Vigo (NW Spain ): an Assessm ent of Met al Pollution[END_REF]Cassia et al., 2003;Reid and Spencer, 2009). These studies elucidated the relationships between metal concentrations and other sedimentary parameters, such as grain size, organic/inorganic matter properties, and metals, and discussed the sources and diffusion processes of the metals in sediments. In order to indicate the sources in a drainage area and their relationships with the chemical behavior of seawater, it is essential to correlate the metal concentrations, environmental properties, such as bathymetric and topographic parameters and measurement data. Therefore, we determined the spatial variations and corresponding controlling factors of metal concentrations in the sediments of Beppu Bay and correlated them with environmental parameters using statistical analyses.

Beppu Bay is located on the western part of the Seto Inland Sea in southwest Japan (Fig. 1). It is characterized by a shallow bay mouth with a water depth that increases toward the interior of the bay; as a result, it is hydrodynamically stagnant. These features form a strong thermocline and low to zero oxygen concentration at the bottom during the summer to autumn seasons [START_REF] Shiozawa | Ver t ical[END_REF]. Several rivers flow

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4 into the bay, and their drainage areas are used for human activities such as urbanization, industrialization, and tourism. [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 25 Kuwae | Spat ial distr ibu tion of or ganic and su lfu r geochemical par ameter s of oxic to anoxic sur face sedim en t s in Beppu Bay in sou thwest Japan[END_REF] focused on the sediments of Beppu Bay and reported a wide variation in redox conditions in an effort to understand the oxygen depletion process in semi-enclosed bays. In addition, they explained the relationships between the spatial variations in the organic geochemical properties of sediments and environmental variables, which are mainly due to the supply from the rivers and the redox conditions.

This study focused on 14 metals that are sensitive to the redox conditions, geology, and anthropogenic impacts of the watershed area; we investigated the spatial distributions of the concentrations of surface sediments in Beppu Bay. To determine the controlling factors for variations in metal concentrations, we used PCA, RDA, and simple correlation to investigate the relationships between the geochemical properties, measured bottom condition data, and bathymetric parameters.

Study area

Beppu Bay is located on the western part of the Seto Inland Sea in southwest Japan (Fig. 1). Its basin has a maximum depth of 72 m in the interior with a sill of 53 m depth at the bay mouth. These bathymetric features have caused the bottom conditions to become strongly stagnant. The tidal current velocity near the bottom is so low that it is undetectable, even using a current meter with a detection limit of 2 cm/s [START_REF] Hoshika | Ver tical distr ibu tion of par ticulat e man gan ese and ir on in th e Be ppu Bay[END_REF]. The surface sediments in the interior of the bay were found to mainly contain silty clay, whereas

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those from the area outside the sill mainly contained fine sand sediments (Inouchi, 1990). This hydrodynamic condition is associated with a thermocline that has strong temperature and salinity gradients that form at a water depth of 60 m in the basin from July to November [START_REF] Shiozawa | Ver t ical[END_REF]. Accompanying this process, the dissolved oxygen (DO) of the bottom water varies seasonally. The DO is 6 ml/L from January to March and decreases to almost zero by September; further, hydrogen sulfide has been observed between August and November [START_REF] Shiozawa | Ver t ical[END_REF][START_REF] Hoshika | Man ganese, Iron , Copper, and Zin c in S edimen t Cor es from S easonally S tratif ied Be ppu Bay, the S eto I nland S ea, Japan[END_REF]Kameda and Fujiwara, 1995).

The main rivers flowing into the bay are the Oita and Ono rivers. They flow into the southern area of the bay, and have drainage areas of 650 and 1465 km 2 , respectively. These areas account for approximately 70% of the Beppu Bay watershed. Multiple industrial zones containing steel plants and petrochemical complexes are located in the southern coastal area around the mouths of the Oita and Ono rivers, and electronics plants exist in the watershed area of the Ono River. Beppu City, which is located on the western part of the bay, has numerous hot springs.

Materials and Methods

Sam pl in g an d an al ysis

Surface sediments (the uppermost 1 cm) were collected in August 2006 from Beppu Bay at 16 sites 1). Mostly clay-silt grains were observed in all of the samples. At the same time, undisturbed samples collected by a gravity corer were used to measure the temperature and dissolved oxygen (DO) concentrations of the bottom water and the redox potential (Eh) of the upper surface sediments were measured using a DO/Eh meter (D-55, HORIBA, Ltd., Japan).

Sediment samples were dried at 60 ºC for 24 h and ground to a fine powder. Approximately 0.2 g of a powdered sample was digested by a microwave system (Ethos D, Milestone S.r.l., Sorisole, BG, Italy) with an acid mixture (HNO 3 :HF = 6:1) in a Teflon vial. The digested sample was dried to remove all of the acid. The residue was dissolved with HNO 3 and then diluted with Milli-Q water. The Al, V, Cr, Mn, Fe, Co, Cu, Zn, As, Mo, Ag, Cd, In, and Pb concentrations were determined by inductively coupled plasma-mass spectrometers (ICP-MS; HP-4500, Hewlett-Packard, Avondale, PA, USA and 7500cx, Agilent, Palo Alto, CA, USA). The metal analysis was conducted in duplicate. The analytical quality was assessed using standard reference materials, NIES No. 2 pond sediments, and NIES No. 12 marine sediments (National Institute for Environmental Study, Tsukuba, Japan), and the recovery of metals was 80-110%. In this study, metal concentrations were expressed as µg/g dry wt.

D at a an al ysis

In order to examine the relationships between the metals concentrations and other factors, we used of these consisted of the linear ordination methods such as principal components analysis (PCA) and redundancy analysis (RDA). These analyses were performed to determine the variation in the species data within a narrow range, which could be attributed to linear coupling. Another study used unimodal based methods of correspondence analysis (CA), detrended correspondence analysis (DCA), and canonical correspondence analysis (CCA) to determine the variation in the species data over a wide range. DCA was carried out on the metal data to determine the most appropriate method, with the gradient length as the criterion [START_REF] Ter Braak | CANOCO-a FORTRAN progr am for canon ical communit y ordinat ion by ( oar tial)(detrended) (can onical) cor r esponden ce an alysis, pr in ciple com pon ent s analysis and r edun dan cy an alysis ( ver sion 2.1)[END_REF]. The results of DCA showed that linear methods (PCA and RDA) were preferable for the dataset considered in this study; this was because the gradient length in standard deviation unites appeared to be not larger than two by the analysis (Birks, 1995). In order to identify the primary factors influencing the variations in the geochemical parameters, we performed PCA for two cases. One involved analysesthe 14 metal concentrations investigated in this study (Table 2). The other involved analyses of the metals and seven organic matter parameters measured by Kuwae et al. ( 2007) (Fig. 2; Table 1); these include stable isotope ratios of organic carbon (δ 13 C) and nitrogen ( δ 15 N); total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) content; and ratios of total organic carbon to total nitrogen (C/N) and sulfur (C/S). RDA and a simple correlation analysis were used to examine the relationships between the metal concentrations and the seven environmental variables: water depth; distances from the mouths of Oita, Ono, and Haruki rivers; temperature and DO of the bottom water; and Eh of the uppermost surface sediment, shown in Fig. 2 and Table 1

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PCA and RDA computations were performed with centered and standardized data using CANOCO for Windows 4.5 (ter Braak and Smilauer, 2002). In the RDA computations, the correlation matrix option was selected, and scaling was conducted on a correlation biplot. The significance of each environmental variable was tested using 499 unrestricted Monte Carlo permutations.

Results

D istr ibut ion pat ter n of m etal con cen tr ation s

The spatial distributions of the metal concentrations in the surface sediments of Beppu Bay were classified into four patterns (Fig. 2; Table 3). The concentrations of V, Cr, Co, As, and Ag were relatively high on the southern coast (Sites 1-5) and decreased toward the northern part of the bay. The concentration ranges were 62-95, 34-76, 7.7-15.6, 7.2-22.3, and 0.22-0.46 µg/g, respectively. The distribution pattern for the Fe concentrations (27 800-39 200 µg/g) was similar to these metals, although the value for Site 10 in the northwest was higher than the average for the bay. The indium concentrations were relatively high at >1.1 µg/g at the offshore parts of the Oita and Ono river mouths (Sites 1-3) and <0.2 µg/g at the other sites. In contrast, the distribution patterns for the Al, Zn, and Pb concentrations showed relatively low values on the southern coast and increased toward the north and west. These metals had ranges of 44 100-71 800, 111-144, and 24.7-32.2

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9 µg/g, respectively. The Mn concentration was 375-2200 µg/g, and its distribution pattern showed increases toward the bay mouth from the southwest, which is where the deepest basin lies in the interior of the bay. In contrast, the distribution patterns for Cu, Mo, and Cd showed increases in the deeper parts from the bay mouth with ranges of25.9-37.3, 1.1-12.9, and 0.19-0.47 µg/g, respectively.

Pri nci pal com ponent anal ysi s of geochemi cal paramet ers

The PCA results for the metal concentrations revealed two main axes that explain most of the variations (Fig. 4; Table 3). The first principal component (PC1) accounted for 43% of the variation; when taken together with the second principal component (PC2), this value increased to 71% (Table 3). The loadings of V, Cr, Co, As, and Pb on PC1 were high (0.98, 0.90, 0.95, 0.97, and -0.73, respectively); however, the loadings of Mn, Cu, Mo, Cd, and In were low (-0.08, 0.09, -0.26, -0.06, and 0.44, respectively). In contrast, the loadings of Mn, Cu, Mo, Ag, and Cd on PC2 were high (-0.73, 0.79, 0.83, 0.70, and 0.92, respectively), while the other parameters exhibited low values. Samples from Sites 3-5, which were located near the Oita River mouth on the southern coast of the bay, were characterized by high PC1 and low PC2 values. These samples showed the same trends in the PCA results as the V, As, Cr, Co, and In values. In contrast, samples from Sites 6-8, which were located in the deepest and innermost regions of the bay, were characterized by high PC2 values, and samples from Sites 10 and 13-16, which were located on the northern part of the bay, were characterized by The PCA results for the metals and organic matter parameters revealed two main axes that explained most of the variation (Fig. 5; Table 4). The first principal component (PC1) accounted for 50 % of the variation; when taken together with the second principal component (PC2), this value increased to 77 %. The V, Cr, Co, Zn, As, δ 13 C, δ 15 N, C/N, TN, TS, and C/S loadings on PC1 were high (0.88, 0.86, 0.94, -0.78, 0.88, -0.89, -0.92, 0.94, -0.74, -0.92, and 0.90, respectively). In contrast, the Mn, Cu, Mo, Ag, Cd, and TOC loadings on PC2 were high (0.69, -0.76, -0.70, -0.88, -0.85, and -0.81, respectively), while the other parameters exhibited low values. Samples from Sites 3-5 were characterized by high PC1 and low PC2 values. These samples showed the same trends for the V, As, Cr, Co, In, and C/N values, which were the opposite to those for δ 13 C, δ 15 N, and Pb. In contrast, samples from Sites 6-8 were characterized by low PC2 values, and those from Sites 10 and 13-16 were characterized by high PC2 values. The samples from Sites 6-8 showed the same trends for the Mo, Cd, Cu, and TOC values, while the samples from Sites 10 and 13-16 showed the same trends for the Mn values.

Re dun dan c y an d sim pl e cor rel ation an al ysis of ge och em ical par am eter s an d envir on m en tal var iabl es

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The eigenvalues of the first two RDA axes were high (RDA axis 1 (RD1): 0.35 and RDA axis 2 (RD2): 0.27) when compared with the subsequent axes and accounted for 61.8% of the variance in the geochemical data (Fig. 6; Table 5). The correlations between the geochemical data and environmental variables for RD1 and RD2 were high (0.91 and 0.99, respectively). Both of the axes together accounted for 82.6% of the variance in the relation between the metals and seven environment variables. The results of the permutation tests revealed that two environmental variables had statistically significant effects: the water depth (P = 0.002) and distance from the Oita River mouth (P = 0.002). On the other hand, the results revealed that the following had insignificant effects: the bottom temperature, Eh, DO, and distances from the Ono and Haruki river mouths.

The distance from the Oita River mouth showed a significant correlation with RD1 (r = -0.79) because the absolute t-value of the regression coefficient was greater than two (ter Braak and Smilauer, 2002) (Table 6). In addition, the depth showed a significant correlation with RD2 (r = 0.90, Table 6). The RDA results (Fig. 6) demonstrated a pattern similar to the PCA results (Fig. 4). The trends of the metals and samples along the RDA axes were also similar to those along the PCA axes. This implies that the distribution patterns for the variations in the metals, as revealed in the RDA axes, were reliable. The RDA results revealed two close relationships between the metal concentrations and environmental variables. This indicates that these sample score variations can be mainly attributed to the variations in water depth.

The results of the simple correlation analysis showed trends similar to the RDA results (Table 7); however, there were some differences. The V, Cr, Co, As, and Pb concentrations were correlated with the distances from the Oita River, while In was correlated with the distances from both the Oita and Ono rivers.

These results indicated that the controlling factors for the In concentrations in the sediments were closely related to both of these rivers.

Discussion

. In f luen ces of su ppl y fr om th e dr ain age ar ea

The PCA results for metal concentrations demonstrated that the V, Co, Cr, As, and Pb concentrations were affected by common factors (Fig. 4; Table 3). The other PCA results for metals and organic geochemical parameters showed that these metal concentrations were closely related to C/N, δ 13 C, and δ 15 N, which are

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13 indicators of organic matter sources in coastal sediments (Fig. 4; Table 4). The C/N, δ 13 C, and δ 15 N values for surface sediments in Beppu Bay were closely related to the distance between the Oita River mouth and the sampling sites [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 25 Kuwae | Spat ial distr ibu tion of or ganic and su lfu r geochemical par ameter s of oxic to anoxic sur face sedim en t s in Beppu Bay in sou thwest Japan[END_REF]. By the same token, the RDA results suggested that the spatial variations for these metal concentrations were controlled by the distances from the sampling sites to the Oita River mouth (Fig. 6; Table 5).

The V, Cr, Co, and As concentrations were relatively high around the mouths of the Oita and Ono rivers on the southern coast (Fig. 3); according to the PCA and RDA results (Tables 4 and6), they had positive relations with C/N and their distances from the Oita River mouth and negative relations with δ 13 C and δ 15 N.

These relationships indicate that these metals originated in the drainage area for the Oita River in the same manner as terrestrial organic matter. The V, Cr Co, and As concentrations for the surface sediments in Beppu Bay ranged between several-fold greater and one-third of the global mean values for the crust (Bowen, 1979) and background values of the fluvial sediments in the Oita River [START_REF] Imai | G eoch emical m ap of Japan[END_REF]) (Table 2). Previous studies have explained that the V, Cr, Co, and As sources are closely related to industry (e.g., [START_REF] Pekey | Th e distr ibu tion and sour ces of heavy m et als in Izm it Bay sur face sedimen t s aff ect ed by a pollut ed str eam[END_REF]García et al., 2008); however, this is not very likely for Beppu Bay. Therefore, these metals likely originated from clastic fractions in the drainage area of the Oita River.

The RDA and simple correlation analysis results suggested that In was supplied from both the Oita and Ono rivers (Fig. 6; Table 7). The In concentration in Beppu Bay was very high, over twenty times the crustal value (Table 2); this result implies the influence of special accumulation processes. Previous studies have not

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14 focused on variations in the In concentrations in sediments, and the geochemical behavior of In is not well understood for coastal estuarine systems. In is used industrially as indium tin oxide alloys for manufacturing flat panel displays and semiconductors for high technology (Roskill Information Services Ltd., 1996). The drainage areas for the Oita and Ono rivers have plants producing electronic devices and accurate instruments; therefore, these plants may be the In sources.

The lead concentrations in the surface sediments were low around the Oita River mouth (Fig. 3) and had trends that were opposite to those exhibited by the V, Cr, Co, As, and In concentrations in the PCA and RDA results (Figs. 4 and6). These results suggest that the deposition processes for Pb were likely associated with marine plankton and/or supply from drainage areas of the bay excluding the Oita River. Many studies have described increases in Pb concentrations in coastal sediments as caused by particulates produced during the burning of coal and oil fuel (e.g., Gracía et al., 2008). Lead products exhausted from human activities in the surrounding bay area may have diffused through the atmosphere toward the sea, and the Pb concentrations in the surface sediments may have been diluted by clastic sediments supplied from the Oita and Ono rivers.

Therefore, these statistical results may show the relationship between the Pb concentrations and the distance from the Oita River.

The RDA results showed that the Al, Fe, Zn, and Ag concentrations were weakly correlated with the environmental parameters used in this study (Fig. 6). These results suggest that variations in these metal concentrations were controlled by other factors such as parameters not considered in this study and/or multiple In general, the metal concentrations of sediments vary with grain size (e.g., [START_REF] Imai | Geoch emical beh avior of heavy m etal in coast al m arin e sedim en t s fr om th e east er n m argin of the Japan S ea[END_REF][START_REF] Lin | I nf lu ence of th e Yan gt ze River an d gr ain size on t he spat ial var iation of h eavy m et als an d or ganic carbon in th e East Ch ina S ea con tin en tal sh elf sediment s[END_REF]Neto et al., 2006;[START_REF] Ohta | Elem ent al distribut ion of coast al sea and str eam sedim ent s in t he island-ar c region of Japan and mass tr an sf er processes from t er restrial t o m arin e environm en t s[END_REF]. Aluminum is a structural element of clay and its concentration is relatively constant in the crust (Schropp et al., 1990;Summers et al., 1996). Although the median diameters of the sediments in the Beppu Bay interior were over 6 φ, the mud content (over 4φ) slightly decreased to 85% around the Oita and Ono river mouths from more than 90% in the main part of the bay (Inouchi, 1990). This pattern reflects the deposition process for coarser sediments from the Oita and Ono river drainage areas. To explain the variation in grain size, a trend of decreasing Al concentration may accompany a decrease in mud content.

The Fe concentrations showed middle negative values in RD1 and positive values in RD2 and negative relations with the distances from the Oita River mouth and the water depth (Fig. 6; Table 6). This result implies that the spatial distribution of Fe concentration in the surface sediments was influenced by the following two topographic parameters: supply from the Oita River, as with the V, Cr, Co, and As concentrations, and the redox conditions. Although we explain the relationships between the metal concentrations and redox conditions in the next section, [START_REF] Shiozawa | Ver t ical[END_REF] demonstrated that Fe dissolves from sediments in anoxic conditions during the summer season; they used vertical profiles of seawater from the deepest part of the bay for dissolved and particulate metals. However, no data are available that show the diffusion process in the horizontal direction after materials are dissolved from sediments. Although it is currently difficult to explain

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16 the relationship between the Fe concentrations and redox conditions, if a transfer process for dissolved Fe in seawater can be demonstrated, the reason that the Fe concentrations in the surface sediments decrease toward the deeper part of the bay will be elucidated.

Zinc concentrations increased toward the western part of the bay (Fig. 3). RDA results indicated that the distance from the Ono River was an insignificant factor in the variation of the metal concentrations.

However, the same trend was observed for Zn concentration and the distance from Ono River shown in Fig. 6.

A geochemical map of Japan produced by [START_REF] Imai | G eoch emical m ap of Japan[END_REF] demonstrated that the Zn concentrations in the western part of the bay, including Beppu City, were much higher than in the southern part, which is where the Ono River flows. [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 26 Osawa | D isch arge of u sef ul metal elem ent s fr om th e sprin g to th e river[END_REF] showed that the Zn concentrations in the rivers flowing through Beppu City, including the Haruki River, were very high and explained this phenomenon as a resulted of the discharge from hot springs. Statistical results did not show a relationship between Zn and the distance from the Haruki River in Beppu City, although the RDA results indicated a relationship with the Ono River. To summarize these results, the Zn source in Beppu Bay may be the discharge from hot springs in the western part of the bay, with the Zn concentrations in the surface sediments diluted with clastic sediments supplied from the Ono River containing little Zn.

The PCA results showed that Ag also had the highest loading values in PC2, with trends similar to Mo, Cd, and Cu (Figs. 4 and5; Tables 3 and4). However, these distribution patterns showed a small difference.

The RDA results showed that the trend for Ag was in the middle point between the water depth and opposite

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17 trend in the distance from the Oita River (Fig. 6). The simple correlation results showed that the Ag concentration was related to these topographical parameters. These results suggested that both factors affected the spatial variation of the Ag concentration. In general, most of the Ag in the coastal sediments was from municipal and industrial waste water discharges (e.g., [START_REF] Flegal | Silver in th e east er n Atlant ic Ocean[END_REF]Macdonald et al., 2008). In the same way, Ag was likely supplied from rivers flowing near industrial plants located in the southern coastal area of Beppu Bay.

In order to explain the sources of metals, it is necessary to discuss, not only river pathways, but also wind transport. In the case of Beppu Bay, eolian dust might contain crustics, volcaniclastic material distributed around Mt. Aso in the southwest of the bay and anthropogenic materials like Pb, as described above. This study focused on surface sediment deposition processes and chemical behaviors of seawater. In addition, the supplied amount from wind transport is less than those from rivers in Beppu Bay. Therefore, it is difficult when considering bulk element concentrations to separate the supply from rivers and the atmosphere. A different approach such as one involving element isotopes could resolve this problem more comprehensively.

Rel ation sh ips w ith re dox con dition

The RDA and simple correlation results revealed that the spatial variations in the Mn, Cu, Mo, and Cd concentrations were closely related to the water depth (Fig. 6; Table 6). [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 25 Kuwae | Spat ial distr ibu tion of or ganic and su lfu r geochemical par ameter s of oxic to anoxic sur face sedim en t s in Beppu Bay in sou thwest Japan[END_REF]

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18 that the water depth controlled the temperature and/or redox conditions of the seafloor, and that TOC, TN, TS, and C/S were sensitive to these variations in Beppu Bay. Thus, the spatial variations in these metal concentrations are probably related to the redox conditions.

However, the RDA results showed non-significant relationships between these metal concentrations and the Eh values of the surface sediment and DO of the water at the bottom. It is highly possible that the causes for these results are related to measurement errors and differences in the time scales for the records of the sediment and water. The Eh values around the boundary area between the sediments and seawater varied rapidlly; therefore, it was difficult to standardize the measurements, and the measurement error was large. In addition, the DO and Eh values showed momentary records. When considering the sedimentation rates, which were calculated as 0.20-0.27 g/cm 2 /yr by [START_REF] Hoshika | Man ganese, Iron , Copper, and Zin c in S edimen t Cor es from S easonally S tratif ied Be ppu Bay, the S eto I nland S ea, Japan[END_REF] for Beppu Bay, the parameters of the surface sediments in the uppermost 1 cm were averages for at least several years and reflected the variation in redox conditions for a longer time than the seasonal variations in the seawater. As a result, the RDA permutation test may not have registered dominant factors such as the DO and Eh. However, [START_REF] Shiozawa | Ver t ical[END_REF] clearly demonstrated that the water depth in Beppu Bay influences the vertical and seasonal variations in DO and Eh in the seawater; the long-term variation in the redox conditions may be closely related to the water depth rather than to DO and Eh.

The PCA results showed the same trends for the Mo, Cd, and Cu concentrations (Figs. 4 and5) and the RDA results showed that these parameters and the water depth had similar trends (Fig. 6). These results

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19 indicate that the concentrations of these metals in the surface sediments reflect the average redox conditions over a few years. The Mo and Cd concentrations in Beppu Bay were relatively high at the southwestern deep end with anoxic to euxinic conditions (Fig. 3). Thus, the Mo and Cd had high depositions under anoxic conditions in the deep parts of the bay. Our results were consistent with previous studies that focused on the metal concentrations in surface sediments, which revealed that Mo and Cd were abundant in suboxic to euxinic anoxic sediments (Emerson and Huested, 1991;Calvert and Pedersen, 1993).

Copper is essential for biological processes and shows a high affinity for humic substances, which represent a major component of organic matter in sediments (Calvert et al., 1985). The PCA results for the metals and organic geochemical properties showed that TOC, which indicates the deposition quantity of organic matter, had the same trend as Cu, Mo, and Cd. [START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript 25 Kuwae | Spat ial distr ibu tion of or ganic and su lfu r geochemical par ameter s of oxic to anoxic sur face sedim en t s in Beppu Bay in sou thwest Japan[END_REF] suggested that high depositions of organic matter occurred in the deeper part of Beppu Bay under anoxic conditions. As a result, Cu accumulated with the organic matter and its concentration is indirectly controlled by the variation in the redox conditions.

The trend for Mn was the opposite to that for Mo and Cd, and the RDA results showed a negative correlation between the Mn concentrations and water depth. This suggested that oxic-anoxic and redox conditions also influenced the spatial variations in the Mn concentrations for the surface sediments of Beppu Bay.

Manganese diffuses from sediments under anoxic conditions and is deposited by oxidation in oxic conditions (Calvert and Pederson, 1993). [START_REF] Hoshika | Man ganese, Iron , Copper, and Zin c in S edimen t Cor es from S easonally S tratif ied Be ppu Bay, the S eto I nland S ea, Japan[END_REF] Table 8 shows the results of a comparison of the concentrations of Mo, Cd, Cu, and Mn, which were closely related with redox conditions, in 5 restricted sea areas along the coast. Manganese, Cu, and Mo concentrations in Beppu Bay were in the same range as other areas. While, the Cd concentrations of Beppu Bay were much lower than those in other areas. Although our data could not indicate the reason for these results, perhaps differences in the water quality related elemental behavior or the amounts supplied from surrounding areas are contributing factors. To determine the exact cause, it will be necessary to compare the environmental characteristics of these areas in detail.

Con clusion

In this paper, we compared environmental parameters such as water depth, distances from river mouths, and measured data used in PCA and RDA to determine the supply sources of 14 metals in the surface sediments of Beppu Bay and elucidated the relationships between their concentrations and redox conditions. The

  -McIntyer grab sampler (Fig. 1; Table

  Numerous studies have primarily used two multivariate ordination techniques. One

  . The samples from Sites 6-8, which were located in deeper regions, showed the same trends for the Mo, Cd, and Cu values; on the other hand, samples from Sites 10 and 13-16, which were from shallower areas, showed the same trends for the Mn values.

  One was the relationship between the V, As, Cr, Co, In, and Pb values and the distance from the Oita River mouth. The other was the relationship between the Cu, Cd, Mo, and Mn values and the water depth. The trend of the distribution sample scores associated with Sites 2-5 was similar to the arrows in relation to the distance from the Oita River mouth. Therefore, the sample score variations associated with Sites 2-5 were mainly attributed to the variations in the distance from the river mouth. In contrast, the distribution patterns for the sample scores associated with Sites 6, 7, 10, 14, and 16 were parallel to the arrow trends in relation to the water depth.

  order to explain the depositional processes of these metals, we considered the possibilities of relation with other factors and sources.

  investigated the vertical variations in the Mn concentrations in two core sediments collected under anoxic conditions from the interior part of the bay and oxic bay mouth. These results explained the increases in Mn concentrations for sediments less than 2 cm below the surface in the bay mouth; they were observed to occur through post-depositional mobilization under reducing conditions in the deeper sediments before subsequent precipitation in the oxygenated surficial sediments. As a result, diagenetic behavior responded to redox conditions, which controlled the spatial variation of the Mn concentrations in Beppu Bay.

  that the spatial variations of the 14 metal concentrations of the surface sediments in Beppu Bay were mainly controlled by two factors. One factor was the distance from the mouth of the Oita River; this factor was significant for the V, Cr, Co, and As concentrations. This relationship indicated that these metals were mainly supplied from the drainage area of the Oita River. The supply source for In is also the drainage areas of the Oita and Ono rivers, which empty into Beppu Bay.The Mn, Cu, Mo, and Cd concentrations were closely related to the redox condition parameters, water depth, and the DO and Eh in the bottom water. The concentrations of these metals were controlled by the oxic-anoxic and redox conditions at the bottom. Molybdenum and Cd were found to accumulate in the deeper part under anoxic conditions; in contrast, the Mn concentration increased toward the bay mouth, which was oxic. Copper concentrations were found to increase in the anoxic areas, where organic matter such as humic substances may have been prevented from decomposing. Silver and Pb likely originated from anthropogenic discharges of industrial plants in the southern part of the bay and the burning of fossil fuels over the whole watershed area, respectively. The variations in the Al, Fe, and Zn concentrations were controlled by other factors not considered in this study and/or other multiple factors. Referen ce Adelson , J. M., Helt z, G. R., Miller, C. V., 2001. Recon str u ct ing th e rise of r ecent coast al an oxic; molybdenum in Ch esapeake Bay sedimen t s. Geochem ica et Cosm ochim ica Act a 65,


Jirong Xu

You-Shao Wang

Jinping Yin

Jinping Lin

New Series of Corers for Taking Undisturbed Vertical Samples of Soft Bottom Sediments

Keywords: New Corers, Undisturbed Sediment Cores, Application

A new series of sediment corers-a Single Tube Gravity Corer and a Multiple Tube Gravity Corer-has been developed for taking undisturbed vertical soft sediment samples, and a Hydraulic Pressure Multiple Tube Corer has been preliminarily designed. The Single Tube Gravity Corer can be operated by hand easily without using a winch, and it might be the simplest and the cheapest core sampler available. The Multiple Tube Gravity Corer was developed based on the Single Corer, which normally consists of three coring tubes, however, it is possible to install up to six tubes. The Hydraulic Pressure Multiple Tube Corer has been designed based on the Multiple Corer, which would take undisturbed sediment samples , and would be controlled by a computer and monitored by an underwater video camera. The pore water collection and the sediment Eh measurement-subsampling process through the transparent plastic tube wall is straightforward. Compared to other types of coring devices, such as the famous KC Kajak sediment core sampler made in Denmark; these corers have greater reliability, being simple in structure, relatively light weight, easy and efficient to operate, highly adaptable, and relatively inexpensive to produce.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 1.Introduction

Obtaining representative, undisturbed, vertical samples of soft bottom sediments is a fundamental prerequisite for much research in aquatic biology, ecology, geochemistry and environmental sciences conducted in fluvial, lacustrine, estuarine and marine situations. Unfortunately, the surface sediment may be disturbed during the sampling process; further, most soft sediment samplers (e.g. trawls, sledges, dredges, suction samplers, snap grabs, scoop, bucket, impact or hydrostatic corers) provide only semi-quantitative samples [START_REF] Ankar | Digging profile and penetration of the Van Veen grab in different sediment types[END_REF][START_REF] Riddle | Bite profiles of some benthic grab samplers[END_REF]. Today greater demand for undisturbed benthic samplers is obvious in countless studies. Marine scientists cannot investigate benthic flux of materials, transfer of energy, nutrient and metal dynamics without proper attention to sediment sampling. Similarly, tracing aquatic pollutants, determining the vertical profiles of organisms and monitoring meio-and micro-fauna population density depends upon careful recovery of the upper sedimentary profile in an undisturbed manner.

Since the beginning of the 20th century, many different types of samplers for taking soft bottom sediments have been developed [START_REF] Peterson | Havets bonitering. . Habundens dyreliv, dets naering og maengde[END_REF]Jensen 1911, Ekman 1915). Soft sediment samplers fall into three main categories-grab samplers, box samplers and corers. Grab samplers of various designs are used to obtain surface samples. They perform well in soft, sandy or silty sediments, but are less effective when the substrate is gravely or rocky. Grab samplers disturb samples during digging and closing, so they are unable to recover sediment in an undisturbed way. Box samplers, in their original or modified versions, are the standard equipment for benthic research. They collect undisturbed samples, but, due to their size and weight constraints, whose use is limited to relative large-scale operations. Actually, box samplers also disturb sediment samples to same extent, because its mechanical closing structure is similar with grab samplers. Corers are generally simpler, easier to handle and more economical than other types of soft sediment samplers. Corers are often used to take undisturbed samples for microbial, biological and geochemical investigation incorporating the collection of the sediment-water interface.

Other corer-type sediment samplers have a number of deficiencies relative to the sampler described in this paper. The device described in Barnes (1960) has a diameter of only 1.0 cm, giving it a high wall friction co-efficient, outside the range for which entry characteristics can be calculated. The device described in Zullig (1953) has an internal diameter of 3.2 cm and a wall thickness of 0.4 cm and surprisingly, did not have a sharpened or bevelled edge, meaning it would displace or distort a high proportion of the core in a manner that could not easily be predicted or accounted for. With these deficiencies, it is not surprising that [START_REF] Elmgren | Methods of sampling sublitoral soft bottom meiofauna[END_REF] found that the boxcorer was superior to those with which it was compared. However, our wall thickness (polycarbonate) is 0. 15 cm (1.5 mm), which is efficiently to overcome the problem. Samples were taken in stiff mud at 47 m depth with a grab-type box sampler, a van Veen grab sampler and three gravity core samplers. Results were compared to 'control' cores obtained by SCUBA (self-contained underwater breathing apparatus) divers. The grab-type box sampler was shown to be suitable for collecting undisturbed samples for meiofaunal research, whereas the van Veen grab and three core samplers were not. The validity of studies employing the latter samplers is therefore open to a question. [START_REF] Elmgren | Methods of sampling sublitoral soft bottom meiofauna[END_REF] stressed the importance of sampling methods in all quantitative work on sublittoral meiofauna populations. [START_REF] Blomqvist | Reliability of core sampling of soft bottom sediment-an in suit study[END_REF] examined the reliability of samples obtained using four lightweight, soft sediment sampling devices:Axelsson-Håkanson gravity corer, Kajak gravity corer, Jenkin bottom sampler and Ekman grab

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(box corer). Each of these samplers was intended for use in soft sediments in situ by direct observation, measurement and photographic documentation by SCUBA divers. The results showed the relationship between compaction and sediment depth was dependent on core-tube diameter. These findings highlight the importance of sampler design in preserving stratigraphic integrity. Core description, penetration/compaction will depend largely on the sediment encountered and is not easily predicted in the case of layered sediments of different densities and water contents. [START_REF] Blomqvist | Quantitative sampling of soft-bottom sediments: problem and solutions[END_REF] reviewed the development of samplers from the 1910s to the end of the 1980s, and suggested guidelines for the further design and development of samplers. He also predicted that the future samplers would be based around box and tube-coring samplers, because these would better suit the demand of providing representative quantitative (e.g. undisturbed) samplers. [START_REF] Black | A multiple corer for taking virtually undisturbed samples from shallow water sediments[END_REF] stated: " [START_REF] Blomqvist | Quantitative sampling of soft-bottom sediments: problem and solutions[END_REF] gives an analysis of techniques for sampling soft bottoms and concludes that the instruments best suited for quantitative sampling of soft bottom sediments appear to be those based on the coring principle." After summarizing limitations and biasing factors to be considered, [START_REF] Blomqvist | Quantitative sampling of soft-bottom sediments: problem and solutions[END_REF] indicated that a perfect core should: (1) permit unimpeded water flow through the coring tubes to obviate formation of a bow wave, (2) work on a supporting stand, permitting it to settle down, before the coring tubes enter, (3) avoid disturbing sample during closing operation and withdrawal from the sediment, (4) avoid shorting the sample during the coring tubes entering sediment, and (5) be of adequate and suitable size in relation to the vessels' size and facilities such as winches and frames.

A multiple corer for taking virtually undisturbed sandy and soft samples from shallow water sediments has been designed by them [START_REF] Black | A multiple corer for taking virtually undisturbed samples from shallow water sediments[END_REF]. However, its mechanical structure is relatively complex, compared to the one described in this paper.

Nowadays, with the development of modern science and technology; it is possible to develop a modern corer to avoid factors such as impacts from water movements and from mechanical deficiencies (closing structure) to overcome disturbance of samples.

The purpose of this paper is to introduce a new series of corers for taking undisturbed soft sediment samples; these coreers having advantages of greater reliability, simplicity in structure, light weight, easy and efficient to operate, highly adaptable, and relatively inexpensive to produce.

The New Series of Corers

The Single Tube Gravity Corer was designed and developed first. Then, the Gravity Multiple Tube-Corer was developed based on this design but improved in order to take more cores synchronously.

The Single Tube Gravity Corer and the Multiple Tube Gravity Corer can successfully take undisturbed cores in calm conditions. However, when used in rough conditions such as strong currents, large waves, difficult sediment characteristics (such as when sampling of hard sediments is attempted) or when the equipment is operated incorrectly, some sample disturbance may occur. Therefore, it is always necessary to check if the core sample has been successfully collected in an undisturbed state. If our observations indicted that 1% or more of a core was disturbed, the core was discarded.

It is possible that most gravity corers enter into the sediment relatively quickly, so it is impossible to completely eliminate friction drag and prevent sediment smearing

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against the tube wall, which might compact the sediment, disturb stratigraphy and cause sampling error.

For the reasonsstated above, the Hydraulic Pressure Multiple Tube-Corer has been preliminarily designed, based on the Multiple Tube Gravity Corer. It is possible that this third corer will be become available when or if the program is funded in the near future.

Single Tube Gravity Corer

The structure of the Single Tube Gravity Corer is shown diagrammatically in Fig 1 and as a photograph in Fig 2 . The head blade is made of stainless steel and the tube (inner diameter 50 mm, and length 1 m) is made of perspex (wall thickness 5 mm) or polycarbonate (wall thickness 1.5 mm). The head blade is not necessary if the tube is made of polycarbonate. The main board is made of stainless steel in order to support the whole sampler system. The valve body is made of perspex and the valve ball of ABS(acrylonitrile butadiene styrene). The low density sphere caged in the valve body is one of the most basic types of closing mechanisms. It is used as a directional flow valve to reduce water resistance. So ABS (density 1.16 g/cm 3 ) was selected as the material for the valve ball. The valve and the valve ball are the key parts of the corer, which prevent the sample sliding from the tube when the corer is extracted from sediments. The fixing board is made of stainless steel and is fitted with screws. The lead ring is to keep the coring tube vertical while it descends through the water column and penetrates into the sediment. The total weight of the Single Corer is about 25 kg.

Fig1 Fig 2

The operation of the Single Tube Gravity Corer is quite straightforward because of its mechanical simplicity. A defect of some other samplers is that the mechanism may foul or be triggered before samples have been obtained. Fig 1 shows the Single Tube Gravity Corer assembly prior to sediment collection. A metal handle can be used for sample retrieval in shallow water (< 4 m), and a rope attached in deeper waters.

Firstly, lower the corer into surface water slowly and wait for a moment, to make sure that the tube is vertical. It is an important step to sample sediment successfully. Secondly, lower the corer down slowly to 3 m above the bottom, and then let the corer go down as a free fall. The final step is when the tube penetrates into sediment to take a sample by gravity.

When the corer is cast into the water, the ball valve will float while the core goes down towards the bottom sediment. The valve ball will seat in the rubber O-ring and seal the tube by gravity to hold the sample as the tube is hoisted from the sediment and raised through the water column. Regulating the number of lead weights can control the depth of corer penetration into the sediment, enabling the sediment-water interface to be sampled. The corer has several advantages over other sediment samplers: (1) The structure is very simple, so, it is easy and economical to build,(2) It can be operated without a winch, (3) it is more reliable than others, because there is no mechanical movement during sampling, (4) It can be adapted to various situations and surroundings, (5) If the tube is perspex or polycarbonate, it is easy to drill a hole to collect sediment-water interface (pore water) and insert an electrode to measure sediment Eh (redox potential).

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2.2 Multiple Tube Gravity Corer

The Multiple Tube Gravity Corer is a relatively simple modification of the Single Tube Gravity Corer. A detailed structure of the Multiple Tube Gravity Corer consisting of three tubes is shown in The Multiple Tube Gravity Corer normally consists of three tubes, while it is possible to install up to six tubes. From our past sampling, the Multiple Tube Gravity Corer can efficiently take six cores, so long as the supporting frame is strong enough and spacing of the individual corers is sufficient. Each tube can collect a sediment core on it own without interfering with other corers.

The Multiple Tube Gravity Corer is an assemblage of the Single Tube Gravity Corer, so it has all the advantages of the Single Gravity Tube Corer. However, the Multiple Tube Gravity Corer needs a winch because of its increased weight when containing sediment samples.

Fig3 Fig4 2.3 Hydraulic Pressure Multiple Tube Corer

As stated in the section 2 above, our Hydraulic Pressure Multiple Corer is intended to solve the problems: sample disturbance in rough conditions, reducing friction drag, and preventing sediment smearing against the tube wall and provides almost undisturbed vertical core samples of soft bottom sediment. According to our design, several coring tubes (3-6 tubes) could be assembled on a support stand controlled by a hydraulic pressure device. The hydraulic pressure device will be designed to keep the tubes' movement slow and smooth during the operation, in order to keep the bottom sediment in its original and undisturbed state. A preliminary schematic drawing of the Hydraulic Pressure Multiple Corer (with three tubes) is shown in Fig 5 . As it is stated above the Hydraulic Pressure Multiple Tube Corer is possible to become available when or if it is funded in the near future.

Fig5

The whole sampling process of the Hydraulic Pressure Multiple Tube Corer is controlled by a computer. An under-water video camera, which is fixed on the supporting frame, monitors the whole sampling process including sampling station selection. Firstly, the corer is lowered slowly to the bottom and the corer is prevented from sinking into the sediment by the supporting frame. Secondly, the hydraulic pressure device drives the sampling tubes penetration into the sediment slowly and smoothly controlled by the computer. The valve balls are floated to the top of valves to let water go through, when the sampling tubes enter the sediment. Then, when the sampling is finished, the hydraulic pressure device pulls the coring tubes out off the sediment, when the tubes have reached the predetermined depth and the valve balls fall back onto the rubber ring and seal the valve to prevent samples sliding out. Finally, the winch hoists the corer onboard the sampling vessel. Compared to other multiple corers driven by a hydraulic pressure device, our equipment is much simpler, more reliable and more efficient because there are no opening and sealing operations needed during sampling.

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Application

Several Single Tube Gravity Corers and Multiple Tube Gravity Corers have been produced and used by us to take sediment samples in the Pearl River estuary and by other researchers in other places. All results showed that the corers were able to successfully collect sediment samples.

We would like to provide some details of the Pearl River estuary and the process of the sediment sampling there, as follows.

The Pearl River is the third largest river in China and the largest river in southern China, being 2214 km long and with a delta area of 453,690 km 2 . It is of a considerable economic and environmental importance. The Pearl River passes through the city of Guangzhou, which is one of the largest cities in China and the largest city in southern China with over 7 million inhabitants. Guangzhou is also an important trading centre as well as a busy port and an industrial base. The climate of Guangzhou is sub-tropical. The average year-round temperature is 22℃. August is the hottest month, with an average temperature of 28℃. January is the coldest month, with an average of 13℃.The rainy season falls between April and August. Average annual rainfall is 1,720 mm.

The Peal River estuary receives and carries most of the outflow from the Pearl River. The discharge into the Pearl River estuary is around 1,500 m 3 /s during the dry season, whereas the flow could increase up to 20,000 m 3 /s during the wet season. It has 3.36×10 11 m 3 of freshwater discharge into the South China Sea every year. It is a bell-shaped estuary, nearly 60 km long and 50 km wide at its mouth (outlet). The depth of the estuary averages 9 m, but containing some deeper dredged channels; and its tidal range is 1-2 m.

Industrial and domestic wastewater from the Pearl River Delta area, especially from Guangzhou, has impacted the water quality in the river. Recently it was reported that the mean concentrations of BOD (biochemical oxygen demand) and TN (total nitrogen) in the Pearl River water have both increased by 0.4 mg/L over the last 20 years [START_REF] Xu | Marine environment of Zhujiang River estuary and its integrated treatment issues[END_REF].

During our investigation, water samples were taken from all stations-ST1 to ST9; and only three sampling stations-ST2 (water depth 3.5 m), ST5 (7.7 m) and ST9 (18.7 m)-were chosen for taking sediment cores for the research of nitrification, denitrification and nitrate reduction rate in sediments. The distance from the cental Guangzhou to ST2 is about 14 km, to ST5 38 km, and to ST9 82 km. ST2, ST5 and ST9 represent as the upstream site, mid-stream site and downstream site of the estuary; respectively. All of them are along the main river-course axis in the Pearl River estuary. The details are shown in Fig 6.

Fig 6 Nine sediment cores were collected from ST2, ST5 and ST9 in April 2004 with the Single Tube Gravity Corer and the Multiple Tube Gravity Corer. Each core was examined visually (by eye) through the transparent sampling tube whether it had been disrupted or disturbed by sampling process. We adopted a criterion that the core would be discarded if it showed any sign (1% or more) of stratigraphic disruption as indicated in Section 2 above. However, none of our nine sediment cores showed any sign of disrupted stratigraphy when visually assessed. Six cores were used for measuring nitrification and denitrification rates. Sediment pore waters were extracted . The pore waters were obtained by compressing the whole core line at 0.5 cm intervals. The temperature and Eh were measured with a platinum wire electrode inserted into intact sediments within 6 hours of sampling.

Acknowledgements

Authors are appreciated that the program has been financially supported by Chinese National Funding (No. 40676073). Authors are grateful to our colleagues Dr. Qingji Wang, Lei He, Fengqing Zhang and Cuici Sun for their assistance in testing corer's characteristics; to Dr. Robert Bernard Spies and two anonymous referees for their detailed and helpful comments and thoughtful editing on the manuscript; and especially to Phil Sinclair for assistance with English expression and clarification of some technical terms.

  Fig 3. Compared to the structure of the Single Tube Gravity Corer (Fig 1), the main modified parts of the Multiple Gravity Tube Corer are the head blade (Part A in Fig 3) and the support frame (Part F).

  three cores. The pore water collection process is very simple and is completed by drilling holes through the transparent plastic tube wall and then collecting pore waters for the measurement of NH 4

Fig 1 .♦

 1 Figure captions: