Application of spatial distribution patterns of multi-elements in geochemical maps for provenance and transfer process of marine sediments in Kyushu, western Japan

The Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), has created geochemical maps showing the spatial distribution patterns of the concentrations of multiple elements throughout the country based on stream and marine sediments (Imai et al. 2004; Imai et al. 2010). These maps provide fundamental information on the behaviour of the elements in nature, and are widely used for environmental assessment and mineral exploration (e.g. Webb et al. 1978; Darnley et al. 1995). Several subcontinental and cross-border geochemical mapping projects have been conducted recently to further nationwide geochemical mapping (Xie and Chen 2001; Salminen et al. 2005; De Vos et al. 2006; De Caritat and Cooper 2011; Reimann et al. 2014a, b; Smith et al. 2014).

Japanese geochemical mapping was initially designed to provide natural background concentrations of elements to quantitatively elucidate anthropogenic contamination in urban regions and coastal sea zones (Imai et al. 2004; Imai et al. 2010). Nonetheless, the spatial distribution patterns of elements across land and marine areas may be further applicable to the study of the particle transfer process from land to the coastal sea, as well as in a marine environment. Although Japan is surrounded by oceans, we still have little information about the characteristics of the surface sediment on shelves, slopes and basins in the island arc region in terms of the erosion, transport and deposition of the sediments. Thus, we have applied geochemical maps to the provenance and transfer analyses of marine sediments (Ohta and Imai 2011) to reveal objectively the complex factors affecting the spatial distributions of elements in marine sediments using analysis of variance (ANOVA), cluster analysis and Mahalanobis’ generalized distances (Ohta et al. 2007, 2010, 2013, 2017b).

We successfully identified the long-range transport process of modern silty sediments on the shelf and slope by oceanic and tidal currents using Cr and Ni geochemical maps (Ohta et al. 2007, 2010, 2017a). Silty sediments originating from ultramafic rocks can be easily isolated from those with a different origin because the rocks greatly elevate the Cr and Ni concentrations of stream and marine sediments. In contrast, granule and sandy sediments on the shelf are mainly composed of sediments that formed during the Quaternary regression and transgression cycles (e.g. Park and Yoo 1988; Ikehara 1993); most of them formed under hydrographical conditions that are different to the modern environment. It is therefore difficult to directly identify their origin, evaluate the corresponding relationship between marine and terrestrial materials, and to examine the transportation process of sandy sediments (Ohta et al. 2007, 2010).

Thus, the Kyushu region, southwestern Japan, was selected as a study area in which sandy sediments containing pyroclastic deposits supplied by large-scale eruptions from the Kikai Caldera (7.3 ka) (Maeno and Taniguchi 2007; Fujihara and Suzuki-Kamata 2013) are widely distributed in the southern marine area of Kyushu Island (Ikehara 2014) (Fig. 1). In this region, there are various bedforms such as subaqueous sand dunes, sand ribbons and ripples (Ikehara 1988, 1993; Ikehara and Kinoshita 1994). Shelf sandbodies with these bedforms are presumed to have formed during the transgression after the last glacial maximum and are also presently active owing to the branch currents of the Kuroshio Current (Fig. 1). Therefore, the Kyushu region is a suitable place to apply our geochemical datasets in order to find the range over which the coarse pyroclastic deposit moved, or the boundary separating pyroclastic deposits from the sandy sediments derived from terrestrial areas without volcanic materials using the geochemical data of sediments. In this study, we elucidate in particular the following three items using marine and terrestrial geochemical maps: (1) the relationship between marine sediments and adjacent terrestrial materials; (2) the dispersion processes of volcanic materials erupted by very large eruptions; and (3) and the transportation of granules and sandy sediments by water currents.

Figure 1 shows the geographical names of the features within the study area, as well as a geological map of the area. The water depth in the northern part of the study area is generally less than 200 m. The Goto Nada, Tsushima Strait and Genkai Nada in the northwestern part are continental shelf areas, where ‘Nada’ means an open sea where a strong ocean current, fast tidal current or severe wave makes marine navigation challenging. The Suo Nada is in the western part of the Seto Inland Sea and is generally less than 60 m deep. It passes through the Bungo–Suido Strait to the Pacific side of Kyushu Island (Hyuga Nada). The Hyuga Nada is characterized by a narrow shelf with a depth that increases steeply towards the SE. Submarine landslide surfaces are found along the slope of the Hyuga Nada (Ikehara 2000). The Amakusa and Yatsushiro seas, the largest inner bays of Kyushu mainland, have extreme tidal ranges and connect to the Amakusa Nada. The water depth of the Amakusa Nada gently increases towards the SW from the SW area of Kyushu mainland and connects to the East China Sea. Kagoshima Bay, with a maximum water depth of 80 m, is part of the Aira Caldera and is associated with thermal activity. The Osumi Strait is a marine area between the Osumi Peninsula and the islands of Yakushima and Tanegashima, where the water depth is less than 500 m. The SE area off Tanegashima is a steep slope associated with a submarine canyon. The strong oceanic currents, which are branches of the Kuroshio Current, prevail on the coast of Kyushu mainland. The Kuroshio Current flows from SW to NE in the the SW areas of the Yakushima and Tanegashima islands. A branch of the Kuroshio Current also flows from SW to NE through the Tsushima and Osumi straits.

Most of the marine sediments in the study area are mainly from the Holocene and Pleistocene. Coarse sand and gravel from the Goto Nada, Tsushima Strait, Genkai Nada, Bungo–Suido Channel and Osumi Strait are composed of sediments that formed during Quaternary regression and transgression cycles (Ohshima et al. 1975, 1982; Ikehara 2000, 2013, 2014). Bedforms such as ripples and current lineations have been found in the Tsushima Strait, Genkai Nada, Osumi Strait and NE of Tanegashima Island; these can be explained by changes in sea level after the last glacial maximum and a strong bottom current related to the oceanic current that moves surface sediments (Ikehara 1988, 1992, 1993; Nishida and Ikehara 2013). Submarine topography and bedforms in the Bungo–Suido Channel are influenced by post-glacial sea-level changes and related changes with strong tidal energies (Ikehara 1998). Silt and clay found in the depths of the Hyuga Nada and in the slope and basin off Tanegashima Island are hemipelagic deposits. Modern silty sediments derived from the adjacent terrestrial area are found in the shallows of inner bays such as the Ariake and Yatsushiro seas, the Suo Nada, and nearshore regions. The shelf and upper slope of the Hyuga Nada are covered by sandy deposits, with some turbidites intercalated in the slope sediments (Ikehara 2000).

Mainland Kyushu is mountainous and comprises many active volcanoes, such as Kujusan, Asosan, Unzendake, Kirishimayama, Sakurajima and Satsuma–Iojima (see the small triangles in Fig. 1). It is also associated with many isolated islands such as Iki, Hirado, Fukue, Nakadori, Amakusa, Tanegashima and Yakushima, and its major rivers include the Chikugo, Kuma, Gokase, Oyodo and Sendai. The sediment yield, drainage basin area and river water discharge rate of the major rivers in the study area are summarized in  Appendix Table A1. The sediment yield of each river system is calculated using the drainage basin area and the average rate of erosion in each river basin (after Akimoto et al. 2009). In the current environment, rivers on Kyushu Island supply 49% of the total sediment yield to the western region, including the Ariake Sea, Yatsushiro Sea and Amakusa Nada ( Appendix Table A1). The Hyuga Nada and Suo Nada receive sediment discharges of 29 and 17% from Kyushu Island, respectively ( Appendix Table A1). Little sediment is discharged from the rivers on Kyushu Island to the northern regions of the Genkai Nada, Goto Nada and Tsushima Strait, or to the southern region including the Osumi Strait and Kagoshima Bay ( Appendix Table A1).

Figure 2 shows a geological map of SW Japan that has been simplified from the Geological Map of Japan 1:1 000 000 scale (Geological Survey of Japan, AIST 1992). The details of the geology of the Kyushu region have been provided by Miyazaki et al. (2016). A Permian accretionary complex is located in the NE; and Jurassic, Cretaceous and Paleogene accretionary complexes are located in the central and SE parts of the study area. The Paleogene unit is also found in the Tanegashima and Yakushima islands. These accretionary complexes are exposed across 23% of the study area. The Permian unit mainly consists of mudstone and sandstone associated with blocks of limestone, basaltic rock and chert. The Jurassic unit is associated with exotic rocks such as serpentinite mélange, metamorphic rock and ultramafic rock. The Cretaceous and Paleogene units consist mainly of accreted turbidites (mudstone to sandstone) associated with exotic blocks of basaltic rock. Carboniferous–Permian and Triassic–Jurassic high-pressure-type metamorphic rock is distributed in the north. These metamorphic rocks are dominated by pelitic, mafic and psammitic schists; and their exposed area is less than 4% of the total. Cretaceous high-temperature-type metamorphic rock is found in the central region, although the area of distribution is only 0.5% of the area. Cretaceous granitic rock is intruded in the northern region and comprises only 5% of the area. In contrast, Neogene granitic rock is intruded in the Osumi Peninsula and the island of Yakushima, and crops out sporadically in accretionary complexes in 3% of the area.

Neogene–Quaternary basaltic, andesitic and dacitic volcanic rocks are distributed in both the northern and southern regions of Kyushu; they crop out across 22% of the area. They are associated with huge pyroclastic-flow deposits, debris and tephra that were erupted by large-scale caldera-forming processes; they comprise only 17% of the area. In particular, Aso and Ito pyroclastic rocks erupted from the Aso and Aira calderas mainly at about 90 and 29 ka, respectively; they are distributed widely in the central and the southern parts, respectively (Nakada et al. 2016). The Kikai Caldera to the south of the Kyushu mainland was formed by large-scale violent eruptions at about 7.3 ka. Koya pyroclastic flows from the Kikai Caldera were extended to 40–80 km (Maeno and Taniguchi 2007; Fujihara and Suzuki-Kamata 2013). Asosan, Sakurajima and Satsuma–Iojima are volcanoes associated with the Aso, Aira and Kikai calderas, respectively (see the triangular symbols in Fig. 1).

Pre-Cretaceous non-accretionary sedimentary rocks are found in the central part with an exposure of less than 3% across the total area. Paleogene forearc sedimentary rocks with coal beds are distributed in the northern and northwestern regions of the Kyushu mainland and in the Amakusa islands (5% of the area). Neogene and Quaternary unconsolidated sediments are distributed widely in the downstream basin of the Chikugo and Oyodo rivers but are rather restricted to a small area elsewhere; they comprise 17% of the study area.

Figure 2 also shows the distribution of major and economically mined deposits in Kyushu (Sudo et al. 2003). Many Au and Ag mineral deposits are hydrothermal or disseminated, and are closely related to Neogene and Quaternary volcanic activity. Some skarn deposits bearing Cu, Zn, As and Sn were formed in Paleozoic limestone bedrocks of the Jurassic accretionary complex where the Neogene granitic rocks intruded.

Figure 3 shows the sampling location of the Kyushu region. For a geochemical mapping project carried out during 1999–2004 by the Geological Survey of Japan, AIST, 366 stream sediment samples were collected from locations on Kyushu Island (Imai et al. 2004) (Fig. 3). In addition, 191 stream sediment samples were collected from 23 isolated small islands around the Kyushu mainland from 2013 to 2014 for a high-density geochemical map (Ohta 2018); of these, 25 samples with wide watershed areas were used for this study (see  Appendix B). The collected stream sediments were dried in air and sieved through a 180 µm screen. Magnetic minerals contained in the samples were removed using a magnet to minimize the effect of magnetic mineral accumulation (Imai et al. 2004; Ohta 2018).

A total of 504 marine sediment samples were collected from the marine areas around the Kyushu mainland during cruises GH83-1, GH84-1 and GH85-2 in 1983, 1984 and 1985, respectively (Ikehara 2000, 2001, 2013, 2014). In addition, 102 marine sediment samples were collected from the Tsushima Strait, Seto Inland Sea, Ariake and Yatsushiro seas, and Amakusa Nada in 2005, and from Kagoshima Bay in 2007 (Imai et al. 2010). These 606 samples were collected using a Kinoshita-type grab sampler (RIGO Inc. Co.), and the uppermost 3 cm layer of each sediment sample was separated, air dried, ground with an agate mortar and pestle, and retained for chemical analysis.

The particle sizes of 334 marine sediments were determined on the basis of the median particle diameter of the surface sediments (Ikehara 2013, 2014). The median particle diameter was not measured for the remaining 272 samples; their classification is based on a visual inspection of texture. The elemental concentrations in the marine sediments change with variations in particle sizes. As a general rule, CaO, MnO, total Fe₂O₃(⁠$Fe2O3T$⁠), Sr and Co are high in coarse and medium sand because these soils contain calcareous materials and coatings of Fe–Mn oxides; the trace elements are high in silt and clay because of the less effective dilution effect in the region (Ohta et al. 2010; Ohta and Imai 2011). Thus, marine sediments were classified roughly as coarse sediment comprising pebbles, granules and medium–coarse–very coarse sands; fine sands comprising very fine and fine sand; and silt and clay consisting of coarse silt, fine silt and clay (Fig. 3).

We compiled the geochemical data of 53 elements in 391 stream sediments and 606 marine sediments around the Kyushu region that had been reported in Imai et al. (2004, 2010) and Ohta (2018). The details of the digestion of the samples, the measurement of elemental concentrations and the quality control of measurement values are described in Imai et al. (2004, 2010) and Ohta (2018). Each sample was digested using HF, HNO₃ and HClO₄. The concentrations of Na₂O, MgO, Al₂O₃, P₂O₅, K₂O, CaO, TiO₂, MnO, $Fe2O3T$⁠, V, Sr and Ba were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES), while those of Li, Be, Sc, Cr, Co, Ni, Cu, Zn, Ga, Rb, Y, Nb, Zr, Mo, Cd, Sn, Sb, Cs, the lanthanides (Ln: La–Lu), Ta, Hf, Tl, Pb, Bi, Th and U were determined by using ICP mass spectrometry (ICP-MS) (Imai et al. 2004, 2010; Ohta 2018). The As and Hg analyses of stream sediment samples from the Kyushu mainland were subcontracted to ALS Chemex in Vancouver, BC (Imai et al. 2004). The As concentrations in the stream sediment samples collected from isolated islands were determined using ICP-MS after the digestion of samples using HF, HNO₃, HClO₄ and KMnO₄ (Ohta 2018). The Hg concentrations in stream sediments on isolated islands and marine sediments were determined by using an atomic absorption spectrometer equipped with the direct thermal decomposition of a sample (Imai et al. 2010; Ohta 2018). A summary of the geochemical characteristics of the marine and stream sediments in the Kyushu region is given in Table 1. However, the Zr and Hf concentrations were not used in this study because the heavy mineral fraction, especially that of zircon, was not digested by the HF–HNO₃–HClO₄ solution. Additionally, the Na₂O concentration of marine sediments was also used as a guide because such sediments were not desalinated in this study.

Geochemical maps of the Kyushu mainland have been recreated as mesh maps associated with the data of the surrounding isolated islands. They were created using geographical information system software (ArcGIS 10.5: Environmental Systems Research Institute (ESRI), Japan Corporation, Tokyo, Japan) after Ohta et al. (2004). The stream sediment is considered a composite sample of the products of weathering and soil and rock erosion in the watershed area upstream of the sampling site (Howarth and Thornton 1983). A sampling site is presumed to express the average chemical concentrations in a drainage basin. The watershed area for each sample was calculated in ArcGIS based on a digital elevation model (50 m mesh data) obtained from the Geospatial Information Authority of Japan. The marine geochemical maps were created by interpolating data points using radial basis functions (Aguilar et al. 2005). The resultant marine geochemical maps were combined with the existing land geochemical maps as shown in Figure 4a–l. The elemental concentrations were classified separately for the stream and marine sediments because their chemical and mineralogical compositions differ significantly, as detailed by Ohta et al. (2004, 2010). The following percentile ranges were used for classification of the elemental concentration intervals in the colour image maps: 0 ≤ x ≤ 5, 5 < x ≤ 10, 10 < x ≤ 25, 25 < x ≤ 50, 50 < x ≤ 75, 75 < x ≤ 90, 90 < x ≤ 95 and 95 < x ≤ 100%, where x represents the elemental concentration according to Reimann (2005). This class selection is advantageous in that the same range of percentiles (e.g. 90–95%) implies the same statistical weight even at different numerical scales (Reimann 2005).

Geochemical maps of the 12 elements K₂O, CaO, MnO, TiO₂, Cr, Cs, La, Yb, Cu, Cd, Sb and Pb are shown in Figure 4a–l. They are representative and characteristic distribution patterns of elements that were obtained by considering the similarity of the spatial distribution patterns of 49 elements across land and sea. The Li, K₂O, Rb, Cs and Tl contents are high in the northern, central and southeastern parts of Kyushu mainland and in Yakushima Island, where accretionary complex, granitic rock and felsic volcanic rock crop out. However, the concentrations are low in the central part of Kyushu, where mafic volcanic rock, pyroclastic rock and metamorphic rock are distributed (K₂O and Cs: Fig. 4). The spatial distributions of MgO, CaO, TiO₂, MnO, $Fe2O3T$⁠, Sc, V, Co and Sr are opposite to those of Li, K₂O, Rb, Cs and Tl (CaO and $Fe2O3T$⁠: Fig. 4). Their high concentration areas correspond mainly to areas covered by the mafic volcanic rocks and pyroclastic rocks, particularly near Aso volcano (Fig. 2). The Cr and Ni concentrations are high in the northern-central and central parts of Kyushu mainland. High-pressure-type metamorphic rock and mafic rocks of the accretionary complex crop out sporadically in these areas (Cr: Fig. 4). High enrichment of Be, Na₂O, rare earth elements (REEs), Nb, Ta, Th and U was found in the northern and southern parts of the islands of Kyushu and Yakushima, which are underlain by granitic rocks (La and Yb: Fig. 4). In addition, Y and heavy REEs (HREEs), such as Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, were also enriched in the pyroclastic rock outcrops of central and southern Kyushu Island (Yb: Fig. 4). The metalliferous skarn-type deposits bearing Cu, Zn, As and Sn (circle with a dot and star symbols in Fig. 4) caused extremely high concentrations of Cu, Zn, As, Mo. Cd. Sn, Sb, Hg, Pb and Bi (Cu, Cd, Sb and Pb: Fig. 4). However, the Au–Ag deposits (diamond symbols in Fig. 4) did not appear to contribute to the enrichment of these elements.

Silt and clay sediments in the Suo and Hyuga nadas, southeastern Osumi Strait, and Yatsushiro Bay are commonly enriched in Li, Be, Cr, Ni, Cu, Nb, Mo, Cd, Sn, Sb, Cs, Ta, Hg, Tl, Pb, Bi and U. Silt and clay sediments in the basin SE of Tanegashima Island have high concentrations of Be, K₂O, Cr, MnO, Ni, Cu, As, Rb, Mo, Cd, Sn, Sb, Cs, Ba, light REEs (LREEs), Hg, Tl, Pb, Bi and Th. The enrichment of MnO, Ni, Cu, Mo, Cd, Sn, Sb, Pb and Bi in the deep seas was caused by early diagenetic processes under a static depositional environment; their concentrations have a definite correlation with the MnO concentration (e.g. Klinkhammer 1980; Shaw et al. 1990).

The coarse sediments and fine sands in the Tsushima Strait, Genkai Nada and Goto Nada are rich in CaO and Sr, and poor in other elements, as a result of calcareous deposits such as shell fragments and foraminifera tests. The coarse and fine sands in the Amakusa Nada are somewhat enriched in MgO, Cr and Co, although their chemical compositions are similar to those in the Tsushima Strait, Genkai Nada and Goto Nada. The sandy sediments in the southern part of the Suo Nada near the Kunisaki Peninsula are enriched in MgO, CaO, TiO₂, $Fe2O3T$⁠, Sc, V, Co and Sr, and are poor in K₂O, Rb and Cs; the sediments in the northern part of the Suo Nada are in contrast to those near the Kunisaki Peninsula.

The fine sands in the Hyuga Nada are rich in Li, Be, K₂O, Cr, Ni, Rb, Ba, LREEs and Nb, and poor in P₂O₅, CaO, MnO, Sr, HREEs and Cd. High concentrations of As, Sn, Hg, Pb and Bi, in particular, were found in samples collected within a 16 km radius of the mouth of the Gokasegawa River, where upper river basin skarn-type deposits bearing Cu, Zn, As and Sn occur. Granules and coarse sands in the Osumi Strait are highly enriched in MgO, P₂O₅, TiO₂, MnO, $Fe2O3T$⁠, Sc, V, Co, Zn, Sr, Mo, Cd and HREEs. Marine sediments occurring between the Osumi Peninsula and Tanegashima Island are, in particular, enriched in MgO, P₂O₅, TiO₂, MnO, $Fe2O3T$⁠, Sc, V and Co; however, these elements are less abundant in stream sediments derived from the adjacent terrestrial area in which unconsolidated sediment and sedimentary rock, granitic rocks, and sedimentary rocks of accretionary complexes are the dominant lithologies. In contrast, the fine sands of the Osumi Strait are more enriched in CaO and Sr, and are poorer in MgO, TiO₂, V, $Fe2O3T$⁠, Co and Zn than the coarse sands owing to dilution by biogenic carbonate materials. The sediments in Kagoshima Bay have chemical compositions similar to those of the coarse sediments in the Osumi Strait irrespective of particle size; furthermore, they are highly enriched in As, Cd, Mo, Sb, Hg and Bi owing to hydrothermal activity (Sakamoto 1985). Almost all of the elements are less abundant in the fine sands of the slope to the SE of Tanegashima Island.

The geochemistry of stream sediment is determined predominantly by the dominant lithology distributed in the catchment area, as we have previously suggested (e.g. Ohta et al. 2004). As a first approximation, we assume that when a specific rock type underlies more than half of the area of the river basin, it is representative of the geology of the watershed and mainly controls the chemical composition of the stream sediment (Ohta et al. 2004). Unconsolidated sediment and sedimentary rock (Sed); an accretionary complex comprising mainly sandstone, mudstone and a mélange matrix (Acc); granitic rock (Gr); felsic volcanic rock (Fv); mafic volcanic rock (Mv); a pyroclastic-flow deposit (Py); and high-pressure-type metamorphic rock (Mp) were considered as typical lithologies in the study area. When no specific rock type extensively covered a catchment basin, the sample was classified as other (Oth). Table 2 shows the median of the elemental concentration of stream sediment classified to the parent lithology.

An analysis of variance (ANOVA) and Bonferroni's multiple comparison test were applied to the geochemical datasets to objectively examine the influence of the lithology on the chemical compositions of stream sediments (Hochberg and Tamhane 1987; Nagata and Yoshida 1997; Miller and Miller 2010). These statistical tests assume that the data follow a normal distribution. However, the data distribution for each elemental concentration followed neither a normal nor a log-normal distribution when the Shapiro–Wilk test was applied to our dataset (Shapiro and Wilk 1965). In such cases, approaching data symmetry is the second-best alternative. Accordingly, data distribution with a skewness closer to zero, which indicated a symmetrical distribution, was used for the statistical tests (Ohta et al. 2005). The concentrations of the 42 elements were transformed into common logarithms; those of Al₂O₃, Ga, Rb, Er, Tm and Yb were unchanged for the statistical tests (Table 3).

The null hypothesis of the one-way ANOVA is that no significant difference exists in elemental concentrations among the seven categories: Sed, Acc, Gr, Fv, Mv, Py and Mp. Table 3 shows the significance probability (P) and effect size (η²) of the one-way ANOVA analysis. The effect size (η²) was used to form a plausible estimation of the P value irrespective of sample number because a statistical test with a large amount of input data is highly sensitive to very small differences (Richardson 2011; Fritz et al. 2012). In this study, we concluded that the null hypothesis was rejected for the elemental concentration data with P < 0.01 and η² > 0.14. Table 4 shows that the null hypothesis was rejected for all elements except Cu, Zn, As, Cd, Sn, Hg and Pb, which indicates that most elements are significantly influenced by the parent lithology. The concentrations of these elements were influenced by metalliferous deposits rather than by a specific lithology, as described above.

Next, we examined whether the parent lithology significantly affects the concentrations of elements, except Cu, Zn, As, Cd, Sn, Hg and Pb, in the stream sediment using Bonferroni's test at the 0.01 confidence level (Hochberg and Tamhane 1987). Table 4 presents the results of direct pairwise comparisons. The elements are grouped as felsic element enrichment in rhyolitic–dacitic and granitic rocks, mafic element enrichment in andesite–basaltic and gabbroic rocks, and sulfurous elements concentrated in a sulfide deposit for descriptive purposes. The stream sediments derived from Acc are highly rich in K₂O, Li, Rb, Cs, Ba and Tl compared with those from Mv, Py and Mp. Moreover, they are poor in Na₂O, P₂O₅, CaO, Sc, V, TiO₂, MnO, $Fe2O3T$⁠, Sr, Nb, Eu and HREEs (Gd–Lu) compared with those from Fv, Mv, Py and Gr. The sediments derived from Mv and Py are significantly richer in MgO, CaO, Sc, TiO₂, V, $Fe2O3T$⁠, Co and Sr than those from Sed, Acc and Gr, and are significantly poorer in alkali metal elements Nb, Ba, Tl, Th and U than those from Sed, Acc and Gr. Although the stream sediments derived from Py have chemical compositions similar to those from Mv, they are significantly poorer in Cr and Ni. The stream sediments derived from Gr show greater enrichment in alkali metal elements Be, Ga, REEs, Ta, Th and U than the other samples, although they are poor in Cr and Ni. The sediments derived from Mp are highly enriched in Cr and Ni, and are poorer than the other samples in K₂O, Nb and Ta. The samples derived from Fv have few systematic geochemical features compared with those of other samples. These results are consistent with the visual interpretations of the spatial distribution patterns of elemental concentrations (Fig. 4a–l).

In terms of the sulfurous elements, the samples derived from Fv are significantly more enriched in Sb than those from Mv, Py and Gr; the sediments from Gr are richer in Bi than those from Sed and Mv; and the sediments from Mv and Py are richer in Mo and Bi than those from Sed and Gr. The hydrothermal activity associated with Mv, Fv and Gr may have enhanced the concentrations of the sulfurous elements. However, the spatial distributions of the major hydrothermal-type deposits do not appear to be comparable to areas in which sulfurous elements are highly concentrated (Cd and Sb: Fig. 4).

We examined the supplementary process of clastic deposits from terrestrial to marine environments based on the chemical compositions of stream and marine sediments. Those of the marine sediments were influenced by various factors: (1) particle transport from the land to the sea; (2) the dilution effect of quartz and biogenic calcareous materials; (3) increases in the concentrations of alkali metal ions in silty and clayey sediments; (4) the transportation of sediments by gravity flow or oceanic currents; (5) early diagenetic processes in the deep sea; (6) denudation or resedimentation of basement rocks; and (7) contamination from human activity (Ohta et al. 2010; Ohta et al. 2017a). Thus, we used a step-by-step approach.

The chemical compositions of marine sediments differ between regions because they originate from adjacent terrestrial materials. Some marine sediments are formed by the denudation or resedimentation of basement rocks, which may belong to any of the lithologies in the adjacent terrestrial area. This factor is referred to herein as a regional difference. Additionally, the elemental concentrations in the marine sediments vary with particle size, caused by the dilution of large amounts of biogenic calcareous materials in the coarse grains and by an increase in clay minerals in the fine grains. This factor is referred to herein as the particle size effect. These two effects are common in all marine sediments.

The marine sediments were grouped as those from the Tsushima Strait and Goto Nada, those from the Suo Nada, those from the Amakusa Nada including the Ariake and Yatsuhiro seas, those from the Hyuga Nada, and those from the Osumi Strait; the zonal classification is given in Figure 3. They were further classified into coarse sediments, fine sands and silt–clay. The median concentration of elements in the marine sediments grouped by each region and particle size were also calculated (Table 5). The silty sediments of Kagoshima Bay were separated from the silts and clays of the Osumi Strait because there are clear differences in their chemical compositions.

A two-way ANOVA test was applied to identify the factors that significantly change the chemical compositions of marine sediments. Table 6 presents the variance ratios (F), probabilities (P) and effect size (η²) owing to the regional effect (factor A), particle size (factor B) and the interaction effect (factor A × B). The interaction effect (A × B) refers to the effect that one factor has on another (Miller and Miller 2010). When the estimated probability (P) was lower than 0.01 and the effect size (η²) was larger than 0.14, we concluded that the factor made a significant difference in the chemical composition (Table 4). The ANOVA results suggest that the chemical compositions of 29 elements in marine sediments change significantly between different regions. The regional difference effect was not significant for K₂O, Ni, Rb, Sb, Cs, Ba, Hg, Bi and U, whose concentrations were determined simply by particle size (factor B). The results suggest that these elements cannot be used to understand the influence of terrestrial materials on coastal sea sediments. The concentrations of 17 elements in the marine sediments are strongly controlled by the particle effect; however, 15 elements, such as Cr and La, are also significantly influenced by the regional effect. Therefore, it is more effective to subdivide marine sediments by region and particle size when evaluating their geochemical similarities and differences.