地球科学 70 巻，119 ～ 128（2016 年） 119 Earth Science (Chikyu Kagaku) vol. 70, 119-128. 2016 Geochemical evaluation of present mangrove soil in Okinawa Island, Japan

ISHIGA Hiroaki*,** and DIALLO Ibrahima M’Bemba**

Abstract Mangroves have been stressed by human activities, especially deformation of natural landform and inflow of red soil. Some of the mangroves in the Okinawa Island, Japan, were described with a focus on soil geochemical properties. Soil samples of the common mangrove forests were enriched in heavy metals

(such as Zn, Cu and Fe), TS (total sulfur), and P2O5 related to organic matter. Soils of dominant biome show normally dark color compared to those of soils affected by red soil inflow. On the other hand, CaO contents became higher in deforested mangrove soils due to its derivation from marine carbonate material. Deforest of mangroves and degradation of their soils were related to the rapid land use and agriculture practices after World War II in the Okinawa Island. Although observation was limited in some areas, contrasting geochemical compositions are significant between common mangrove soils and soils affected by inflow of red soils. Observation of mangrove and geochemical data set are expected to be useful to understand property of red soils and planning of conservation of mangroves.

Key Words : mangrove, geochemistry, red soil, Okinawa Island

Introduction 127°40’ 127°50’ 128°00’ 128°10’ 26°50’ Red soil erosion and runoff into the downstream are sever environmental impact on mangroves and coral reefs in the Kunigami Okinawa Islands (Okinawa Prefecture 2013). This environmental Shioya bay issue was caused by formerly intense land use of U. S. military Ogimi (Fig. 6a) 26°40’ 26°40’ base, agriculture practices especially pineapple farm from late Higashi th Taira bay Nago 1950 , and consecutive development of infrastructure in Okinawa Gesashi (Fig. 2) Arume bay (Onaga et al. 1999 ; Okinawa Prefecture 2013). Red soil erosion Ohura (Fig. 3) has been accelerated by the squall (heavy rain) in Okinawa Island, Ohura bay 26°30’ Ginoza Henoko 26°30’ once surface vegetation was peeled off. Also the Okinawa Island is Ginoza (Fig. 5) Kin Kana located at a pass way of typhoons, so that the removal of vegetation Okukubi (Fig. 4) can be unrepaired. Uruma Mangroves are considered an important interface between land Kadena and sea because of their stabilization of the environments for 26°20’ 26°20’ ecosystem with abundant nature (Giril et al. 2011 ; Sandilyan and Ginowan Kathiresan 2012). Mangrove soils have potential of accumulation and immobilization of heavy metals, however few of the statements Naha of heavy metal sink of mangrove soil are expressed (Sandilyan 26°10’ 26°10’ 0 20km and Kathiresan 2012). Therefore the geochemical investigation Itoman of sediments in mangrove could be a benchmark study. In Japan, 127°40’ 127°50’ 128°00’ 128°10’ mangroves occur mainly in islands of the Ryukyu Archipelago Fig. 1 Index map showing study area of mangroves in the Okinawa Island. (Fig. 1), and especially one in the Iriomote Island is well known as huge mangrove habitat of the National Park of Japan. Northern Island (Ishihara et al. 2004). Among these islands, the Okinawa limit of mangrove distribution is observed in the Amami-ooshima Island has been mostly developed and stressed by human induced Fig. 1 Ishiga and Diallo

Received May 12, 2016. Accepted July 16, 2016. Editor KANO Akihiro * San’in Branch ** Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, 690-8504, Japan

（ 1 ） 120 ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba disturbances, which is now facing to crisis of mangrove survival concentrated biome designated as the Natural Monument of Japan, (Okinawa Prefecture 2015b). Ishiga et al. (2016) reported the and the Ohura mangrove as the Natural Monument of Nago City geochemical compositions of mangrove soils at two areas in the (Fig. 1). Rhizophora stylosa forms a zonation at the front zone Okinawa Island. So that much information on the environmental of the estuary facing to saline water even in low tide. Following geochemistry of mangrove soils is necessary to understand the to this zone mixed flora of Kandelia candel and Bruguiera present situation of mangrove. This is because the red soil runoff gymnorhiza are characteristic, and the latter commonly habitats has been accelerated related to development of the lands (Nakasone in the inner zone of the forest (Nakamura and Nakasuga 2001). et al. 2000). For education of ecology and environment, the promenade (wood made observatory route) was established in both Gesashi and Red soils in Okinawa Island Okukubbi mangroves. Kayak tour is also available in both areas, where instructors help participants to learn mangrove environment. The Kunigashira mahji, Shimajiri mahji and Jahgaru were Recently mangrove has been planted by volunteer or throughout discriminated for the soils of the Okinawa Island (Kurotori NGO activities, but a considerable improvement is necessary for its and Kojima 1969). The technical term mahji was used for the management (Nakamura and Nakasuga 2001). This paper presents characteristic soils in Okinawa Island. The source material of these the research results at Okukubi, Ginoza, Ogimi and Kana areas three soils was considered to be Cretaceous to Tertiary sandstone in the Okinawa Island (Fig. 1). Okukubi, Ginoza, and Kana are and phyllite of the Kunchan Group, Pleistocene Ryukyu limestone located in the eastern coast, and Ogimi area is on the western side and Tertiary sedimentary rocks of the Shimajiri Group, respectively of the Okinawa Island.). (Hayashi 1985). The Kunigami mahji is composed of red soil and is characterized by acidification of the soil property (Togashiki 1993). Study areas This soil is widely distributed in the central and the northern parts of the Okinawa Island of which occurrence was at the Kunigami Gesashi area (Fig. 2) is in Higashi Village of Nago City, where plane of the terraces at about 60 m height (Kurotori and Kojima mangrove occurs in the estuary of Gesashi River flowing into 1969). This is a significant source material for red soil runoff the Arume bay (Fig. 1). Gesashi mangrove develops along the (Okinawa Prefecture 2013). The Kunigami Gravel Formation estuary and widens its distribution in the water shed, especially (Furukawa 1992) coincides to the Shimajiri mahji. Matrix of the southwestern side of the river. Total area of the mangrove is Kunigami Gravel Formation is composed of altered red clay which estimated about 10 ha which is the largest mangrove in the Okinawa is also the origin of the red soil runoff. Island (Okinawa Prefecture 2015a). Mangrove forms zonation of The terraces on the Kunigami plane have been widely occupied Rhizophora stylosa in front zone (around 2 m high), Kandelia by U. S. military base (Mitsumoto 2001) and also utilized for plantation of sugar cane, pineapple and fruit, which might have caused large amount of red soil runoff. The protection against the soil runoff is urgent issue for the coastal environment (Okinawa 128°08’42” Prefecture 2013). Red soil occupies about 55.1% of the total land 26°36’28” area (Land Improvement Division LID, Okinawa General Office) soil 1 of which the soil is acidic with pH range of 4.5–5.5 (Onaga 1993). soil 1 So the many researches of red soil effect on coral reef have been reported.

Mangroves in Okinawa Island

Nakasuga and Kobashigawa (1976) reported 34 localities Sampling locality Promenade of mangrove distribution. They described Rhizophora stylosa, Kandelia candel and Bruguiera gymnorhiza and Lumnitzera 0 200m racemose as dominant flora. In this report, limited occurrence of Rhizophora stylosa in Kin, Gesashi and Kana areas (Fig. 1) is Fig. 2 Google image (14 January, 2015) indicating distribution of mangroveFig. 2 Ishiga (surrounded and Diallo by a solid line) in the Gesashi, Higashi Village of considerable. Mangroves commonly distribute along the estuary. Kunigami District, Okinawa Island. Among those, the Gesashi mangrove (Fig. 1) is famous for the Sample locality is indicated as open circle. Blue line is promenade.

（ 2 ） Mangrove in Okinawa Island 121 candel (around 1.5 m) and Bruguiera gymnorhiza in middle to higher LOI (loss on ignition ; weight loss after ignition) values upper zones. Bruguiera gymnorhiza dominates in the central part of 12.10 wt% (in average in Table 1) after Ishiga et al. (2016). of the forest (maximum 5 m high). The mangrove, however, is Kandelia candel (around 2 m high) and Bruguiera gymnorhiza endangered by red soil inflow and the urgent countermeasure has (over 4 m high) occur in the forest. Mangrove soil development been planned (Okinawa Prefecture 2015a ; Tsuchiya 2012). This is considerable for Thalassina anomala formed mound outputting is because the tributary of the river (726.83 ha) has been widely dark color soil (Ishiga et al. 2016). developed. The forest occupies 67.8 % of the total area and cultivated land is about 25.6 % (Okinawa Prefecture 2015a). The total amount of inflow red soil with rate of 9,390 t/year was estimated to have OhuraRiver become 34,860 m3 until 2014. Examined thickness of red soil was 50 cm at the frontal part of the mangrove and 20 cm at the upstream. Red soil is composed of fine-grained particles, and over 90 % was under 128°02’50” grain size of 64 µm (clay size). Accumulation of red soil continues 26°33’33” was observed in March 2015 (Ishiga et al. 2016), which embedded the breathing roots of the mangrove (Okinawa Prefecture 2015a). Within the forest, mounds of Thalassina anomala were observed (Ishiga et al. 2016), suggesting dark color soil underlain red soil. But the true situation could not be examined due to protection of mangrove in the Natural Monument.

Ohura area (Nago City) is in the estuary of the Ohura River 0 200m inflowing to the Ohura bay (Fig. 3). Outer mouth of the southern Ohura bay is known as a wide coral reef called Henoko coast in Fig.1 (NACS-J 2010). The area of mangrove is estimated about Fig. 3 Google image (14 January, 2015) indicating distribution of mangrove (surrounded by a solid line) in the Ohura, Nago City, Okinawa 2 5 . ha. The Ohura mangrove soil shows dark color and is rich in Island. organic matter. This common mangrove soil was evidenced from Sample locality is indicated as open circle. Fig. 3 Ishiga and Diallo

Bridge under construction

127°56’30”

26°27’30”

Okukubi River

Sampling locality 0 100m Promenade

Fig. 4 Google image (14 January, 2015) indicating distribution of mangrove (surrounded by a solid line) in the Okukubi River,Fig. 4 KinIshiga Town and ofDiallo Kunigami District, Okinawa Island. Sample locality is indicated as open circle. Blue line is promenade.

（ 3 ） 122 ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba

Table 1 Element concentrations of Gesashi, Ohara, Okukubi, Ginoza and Ogimi mangrove soil samples in the Okinawa Island. Data set of Ohura and Gesashi is referred from Ishiga et al. (2016). Blank; not detected. (in detail see text)

Trace elements (ppm) Major elements (wt%) wt%

Sample As Pb Zn Cu Ni Cr V Sr TS F Br I Cl TiO2 Fe2O3 MnO CaO P2O5 LOI Gesashi* 1 12 9 35 14 6 51 94 791 2481 173 18 9 5913 0.55 3.77 0.02 7.55 0.08 11.12 2 17 11 43 17 8 62 126 392 2526 196 25 16 6588 0.68 5.35 0.05 3.48 0.11 8.20 3 15 13 39 17 11 57 125 63 2452 89 26 22 5223 0.70 5.35 0.05 0.70 0.09 6.45 4 13 11 37 15 14 54 117 45 1407 235 13 22 1164 0.63 4.56 0.03 0.51 0.07 6.10 5 15 13 42 18 12 63 131 54 1688 122 27 18 5650 0.68 5.47 0.07 0.60 0.09 5.82 average 15 12 39 16 10 57 119 269 2111 163 22 17 4908 0.65 4.90 0.04 2.57 0.09 7.54

Ohura* 1 9 24 79 26 28 76 148 77 3516 296 66 13 11031 0.82 4.95 0.02 0.67 0.12 13.80 2 14 14 60 12 12 54 113 618 1397 15 15 4858 0.61 4.65 0.03 6.35 0.09 9.36 3 13 26 97 32 30 92 177 87 3361 137 81 11264 0.97 6.66 0.03 0.72 0.16 18.34 4 11 25 92 31 30 88 166 79 2590 43 59 4 8468 0.92 6.73 0.04 0.66 0.14 14.01 5 11 25 91 33 31 90 166 75 3061 48 7 7499 0.90 5.84 0.02 0.63 0.14 12.73 6 10 22 81 27 27 92 159 68 2051 13 39 14 6257 0.91 6.06 0.04 0.58 0.12 9.54 7 9 18 73 19 24 76 154 58 1375 43 22 12 3346 0.86 5.35 0.03 0.51 0.08 6.89 average 11 22 82 26 26 81 155 152 2479 76 47 9 7532 0.86 5.75 0.03 1.45 0.12 12.10

Okukubi 1 7 9 22 6 8 35 19 25 933 153 22 38 1534 0.24 1.06 0.49 0.05 2.29 2 16 17 53 12 15 44 66 57 4379 347 65 26 6991 0.45 2.71 0.66 0.14 11.18 3 9 7 30 5 6 32 24 14 3170 42 10 38 0.19 1.20 0.59 0.05 2.15 4 10 8 41 4 5 54 30 24 2788 8 42 0.17 1.39 2.22 0.06 2.72 5 15 12 50 10 12 38 48 37 4378 34 36 5245 0.32 2.54 0.01 0.62 0.11 5.43 6 21 11 55 10 8 40 57 48 1220 247 15 41 1770 0.30 3.20 0.05 2.03 0.14 3.75 7 20 10 52 8 8 41 51 43 1126 12 15 36 3509 0.28 2.83 0.05 1.66 0.13 3.54 8 8 12 34 10 10 43 40 26 2180 25 12 26 1387 0.32 1.77 0.65 0.09 2.58 9 11 12 57 10 12 44 44 39 2328 13 28 35 4848 0.34 2.42 0.01 0.99 0.12 4.38 10 9 12 41 9 9 45 37 32 1667 89 18 38 3041 0.30 1.97 0.01 0.89 0.10 2.82 average 13 11 43 8 9 41 42 34 2417 93 23 36 2833 0.29 2.11 0.02 1.08 0.10 4.09

Ginoza 1a 14 7 21 4 4 70 35 282 798 103 6 38 1973 0.21 1.50 0.03 5.32 0.06 4.11 1b 15 6 22 6 2 53 33 261 801 74 6 36 1078 0.21 1.58 0.03 4.84 0.07 3.99 1c 20 11 58 13 12 55 78 159 1309 152 17 40 5431 0.42 3.88 0.07 3.38 0.16 5.54 average 16 8 34 8 6 59 49 234 969 110 10 38 2827 0.28 2.32 0.04 4.51 0.10 4.54

Ogimi 1a 5 11 98 65 41 152 238 72 755 89 9 5 1.05 8.71 0.11 1.89 0.09 5.27 1b 7 13 97 73 44 153 256 71 1396 13 11 634 1.12 9.25 0.12 1.61 0.12 7.24 2a 7 12 73 52 34 117 201 66 777 14 14 907 0.91 7.49 0.09 1.42 0.07 4.96 2b 11 14 93 70 43 140 241 76 1445 137 30 20 4627 1.05 9.06 0.12 1.36 0.11 8.54 2c 10 13 75 58 35 120 219 62 879 170 21 20 3234 0.95 8.27 0.13 1.17 0.09 6.35 average 8 13 87 64 39 136 231 69 1050 79 17 14 1880 1.02 8.56 0.11 1.49 0.10 6.47

Gesashi soil 1 18 19 26 20 19 79 145 126 604 203 3 4 0.67 6.38 0.03 0.63 0.09 4.50 soil 2 17 10 17 9 6 86 160 50 591 18 19 3359 0.85 6.39 0.00 0.61 0.05 3.80 average 17 15 22 14 12 83 153 88 598 102 11 12 1680 0.76 6.39 0.01 0.62 0.07 4.15

UCJA 7 17 74 25 38 84 110 225 0.62 5.39 0.11 3.90 0.12

Okukubi (Ukukubi) River Mangrove is located in Kin Town, creek in February, 2016 (Fig. 4). The mangroves were covered by Kunigami Gun (Fig. 4). Mangrove distributes along the narrow yellowish grey clay of about 20 cm thick, and at inner part of the creek of the Okukubi river estuary (Fig. 4). The widest part of the forest, dark grey to black soils were observed due to mixture with estuary is about 100 m. Mangroves remain as small patches along decayed leave. Smaller mounds of about 30 cm high were formed both side of the creek of about 770 m length due to the development by mangrove lobster, suggesting existence of much dark color soil of the farm out of the constructed river banks (Fig. 4). Red soil underneath of these soils. inflows to the coastal marine (Yamamoto and Chiba 1994). Main mangrove is Bruguiera gymnorhiza (around 4 m high) and the Ginoza area is in Ginoza Village, Kunigami Gun (Fig. 5). margin of the forest is occupied by smaller Kandelia candel. After Mangrove occurs as small patches along estuary of the Ginoza Nakasuga et al. (1995) minor occurrence of Rhizophora stylosa and River inflowing to the northern Kin Bay (Fig. 5). Medium grained Lumnitzera racemose was reported. Bruguiera gymnorhiza was sand with growth of Kandelia candel (around 2 m high) covers partly damaged and decayed. The Kin dam is located at 1.87 km the mangrove of about 1 m thick. Sand is well sorted and probably upstream of the river. The dam was reconstructed in 2012, starting derived from beach. test flooding and completed in 2014. Now at the upper end of the mangrove distribution, the bridge has been constructed across the Ogimi (Oogimi) area is located in Ogimi Village, Kunigami Gun

（ 4 ） Mangrove in Okinawa Island 123

26°40’27.4”

128°07’33.8”

127°59’24” Miyagi Island

26°29’00” 1a Shioya Bay 1b 1c

Fig. 6b Sampling locality 0 200m

Fig. 5 Google image (14 January, 2015) indicating distribution of mangrove (surrounded by a solid line) in the Ginoza,, Kunigami District, Okinawa Island. Fig.Sample 5 Ishiga locality and isDiallo indicated as open circle.

(Fig. 6a). Mangrove occurs in the inlet of the Shioya Bay (Fig. 6b), 2b 2a and has remained only a small part about 0.8 ha along the Taiho- 1a Ohkawa River. The mangrove flora consists mainly of Bruguiera 2c 1b gymnorhiza (around 3 m high) and small Kandelia candel. The latter species distributes margin of the forest. Rhizophora stylosa 0 100m and Lumnitzera racemose were observed as a minor member. Sampling locality Mangrove soil was dark grey sandy mud of which thickness was Fig. 6 Google image (14 January, 2015) indicating Shioya bay and locality not determined. The soft part of soil is about 20 cm thick. of study area of Ogimi, Kunigami Gun, Okinawa Island (a) above, and distributionFig. 6a, b Ishiga of andmangrove Diallo (surrounded by a solid line) and sample locality Kana area is located in Ginoza Village, Kunigami Gun (Fig. 1). by open circle (b) below. Mangrove was observed along the creak of Fukuji River and dense community was in the river bed under the Kana dam. Kandelia collected on February 25, 2016. Okukubi samples were yellowish candel with propagule (around 3 m high) was the main species, grey mud and 10 samples were collected at tidal flat in the creek and accompanied Bruguiera gymnorhiza and Rhizophora stylosa. (Fig. 4). Ginoza samples were reddish yellow medium grained sand Mangrove soil was dark grey mud with over 1 m thick, and partly and 3 samples were collected at tidal flat (Fig. 5). Ogimi samples covered by thin layer (less than 5 cm thick) of yellowish grey silt. were dark grey sandy mud and 5 samples were collected also at tidal flat (Fig. 6b). For comparison of these mangrove soil samples, Samples for Geochemical analysis two samples of red soil were collected at the outcrop of the left bank in Gesashi creek indicated in Fig. 2. Beside this out crop, Samples were collected at the mangrove to tear off the dark grey phyllite was exposed, and the red soil can be a weathered surface of the soil and to take off fragments of wood and leave. product of the phyllite. At the mangrove monuments, collection was permitted by the organizations. Gesashi and Ohura mangrove soil samples were Analytical procedures referred from Ishiga et al. (2016). Five samples (yellowish grey

mud) in Gesashi mangrove (Fig. 2) had been collected within 400 Abundances of selected major elements (TiO2, Fe2O3* (total iron

m distance along the creek collected during ebb tide on May 16, expressed as Fe2O3), MnO, CaO and P2O5), the trace elements (As, 2015. Seven samples (dark color organic silt) had been collected Pb, Zn, Cu, Ni, Cr, V, Sr, F, Br, I, and Cl), and TS (total sulfur) in in Ohura mangrove (Fig. 3) within 120 m distance in east-west the soils were determined by X-ray fluorescence analysis (XRF) in direction. Okukubi, Ginoza and Ogimi mangrove soils were the Department of Geoscience, Shimane University, using a Rigaku

（ 5 ） 124 ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba

RIX-2000 spectrometer. 120 After removal of roots and any other plant material, Ohura approximately 50 g of each sample was dried in an oven at 160 ℃ 100 Gesashi 48 Okukubi for hrs to remove weakly-bound volatiles. The dried samples Ginoza were then ground for 20 min in an automatic agate pestle and Ogimi 80 mortar grinder. UCJA The XRF analyses were made on pressed powder briquettes 60 (about 5 g sample by a force of 200 kN for 60s), following the Zn (ppm) method of Ogasawara (1987). Average errors for all elements are less than ±10% relatively. Analytical results for GSJ standard JSl- 40 1 were acceptable compared to the proposed values of Imai et al. (1996). 20 red soil LOI (Loss on ignition) was performed on soil samples. A portion (4–5 g) of the powder was transferred into glass vials. The 0 samples were oven dried at 110 ℃ for 24 h. Then gravimetric 0246810

LOI determinations were calculated from the net weight loss after Fe2O3 (wt%) ignition in a muffle furnace at 1,020 ℃ for at least 2 hrs. Fig. 7 Zn-Fe2O3 diagram (Ishiga et al. 2003) showing Ohura, Gesashi, Fig. 7 Ishiga and Diallo All results of these analyses are indicated in Table 1. Okukubi, Ginoza and Ogimi mangrove soil samples in the Okinawa Island (each locality indicated in Fig. 1). Composition of red soil from Gesashi area and UCJA (Upper Crust of Analytical results Japan Arc by Togashi et al. 2000) are also indicated.

Considering the average values, Ohura sample have higher contents of Zn, Cu, V and Fe2O3 than those of Gesashi, Okukubi Geochemical signature of mangrove soil and Ginoza samples (Table 1). Ogimi sample also shows higher values; Zn (87 ppm) and Cu (64 ppm), and this may be related The geochemical analysis clearly demonstrates a feature of to derivation from basalt in the source area (AIST/GSJ 2013). present mangrove soil in the Okinawa Island. Zinc and copper are N-MORB composition was given for this rock with higher of biologically concentrated elements thus they are good indicator concentration of Fe2O3 (11.45 and 10.66 wt% given in Chinen et al. of the soil formation of mangrove ecology (Ramanathan et al.

2004). Ogimi sample records the highest average concentration of 1999). Zn-Fe2O3 diagram (Ishiga et al. 2003) showed 4 groups of Cr, V, Ti and Fe among the five areas (Table 1). mangrove soils (Fig. 7). Ohura and Ogimi samples plotted in the Arsenic and Pb contents do not show significant variation higher part of Zn concentration. They, however, form different among the areas. For TS content in sediment that is considered groups due to relatively high Fe2O3 of Ogimi samples. Okukubi to be related to redox conditions (Berner 1984 ; Calvert and samples are in the low-Zn and low-Fe domain and clearly separated Pedersen 1993), and to contents of organic matter in coastal marine from Ohura cluster group (Fig. 7). This cluster group showed that sediments (Sampei et al. 1997). Samples of Gesashi, Ohura and Zn has strong affinity with Fe bearing material. Gesashi samples Okukubi have high values with a comparable range from 2,111 were plotted in low-Zn and relatively high-Fe domains. To test this ppm to 2,479 ppm, indicating similar sedimentary condition. The consideration, composition of red soil sample from Gesashi area is highest P2O5 content (0.12 wt%) was recorded in Ohura sample, indicated in this diagram (Fig. 7). The red soil has further lower Zn and other samples contain slightly lower P2O5 content of 0.1 - content than the Gesashi sample, but with much higher Fe content. 0.09 wt%. Higher CaO content of Ginoza sample (4.51 wt%) is It can be regarded that Gesashi composition was the mixture of considered to be related to carbonate derivation from the coast Okukubi sample and the red soil. To characterize composition of (Ishiga et al. 2016). Ohura samples, UCJA (Upper Crust of the Japan Arc by Togashi To understand the effect of inflow of red soils from the tributary, et al. 2000) is also indicated. This composition plots in the Ohura analytical result of red soil sample from Gesashi is used (Table 1). cluster group, suggesting similarity of composition of the UCC The result shows lower concentration of Zn (22 ppm), Cu (14 ppm) (Upper Continental Crust) observed by Togashi et al. (2000). and Ni (12 ppm) comparatively to those of mangrove samples. The Cu-Fe2O3 diagram of the samples (Fig. 8) shows trend from

P2O5 content (0.07 wt%) of red soil also has lower than other Okukubi samples with lower concentration to Ogimi samples samples. with higher concentration. Behavior of element correlation in

（ 6 ） Mangrove in Okinawa Island 125

100 10

Ohura Gesashi 80 Okukubi Ginoza Ogimi

60 1

Cu (ppm) 40 Ohura Gesashi Okukubi Ginoza UCJA 20 0.1 red soil As Pb Zn Cu Ni Cr V TS Br Fe Ca P Fig. 9 Element concentrations of Ohura, Gesashi, Okukubi and Ginoza 0 mangrove soil samples in the Okinawa Island normalized by red soil 0246810 composition from Gesashi area.

Fe2O3 (wt%) Fig. 9 Ishiga and Diallo

Fig. 8 Cu-Fe2O3 diagram showing Ohura, Gesashi, Okukubi, Ginoza and Ogimi mangroveFig. 8 Ishiga soil and samples Diallo in the Okinawa Island (Fig. 1). show similar variation for heavy metals, suggesting these mangrove Composition of red soil from Gesashi area and UCJA (Upper Crust of soils were composed of similar material. Zinc contents of these Japan Arc by Togashi et al. 2000) are also indicated. samples still show higher than that of red soil. This is suggestive of contribution of organic matter derived from mangrove, even though the mangroves have been affected by inflow of red soil. this diagram is suggestive of sorting effect for finer material TS values are also higher in mangrove sediments (Fig. 9) and with higher concentrations of Cu and Fe. And addition of source may be absorbed in organic muds (Ahmed et al. 2010). CaO material (basalt as mentioned above) for Ogimi samples can be values of mangrove samples show higher concentrations (Fig. assumed. UCJA plots in the Ohura cluster samples, also suggesting 9). Considering higher CaO values of Gesashi (2.57 wt% in composition of UCC as well as in the Zn-Fe2O3 diagram above. average) and Ginoza (4.51 wt% in average) samples, their origin Red soil composition plots further right hand (higher Fe) of the can be biogenic carbonate fragments from the coast. One sample trend of the mangrove samples. Thus, red soil inflow may be a with highest CaO value of Gesashi area (CaO=7.55 wt%) is also significant alteration of the composition. Acidification by red soil related to carbonate material. Scarce mangrove community in the inflow was observed (Vuai et al. 2003). If our estimation of mixing forest has lost a function as barrier of the current from the sea and of mangrove soil with red soil from geochemical evaluation is permitted the transportation of material from the coast (Sandilyan considerable, chemical effect of red soil inflow may be significant and Kathiresan 2012). as well as physical properties such as burial of mangrove root and LOI value is useful for estimation of content of organic matter, modification of geography of the estuary. and is related to TS values. LOI values of Ohura samples are relatively higher than those of other samples (Fig. 10). LOI values Comparison of mangrove soil with red soil of Okukubi samples are lowest among these (Fig. 10). Impact of the red soil inflow and its sedimentation in the mangrove area To characterize Ohura mangrove sample from other samples, can be resulted in decreased value of LOI. This is because LOI selected element concentrations were normalized by the red soil value of red soil is 4.15 wt% (Table 1). Red soil inflow results composition (Fig. 9). Ohura sample shows significant enrichment in burial of channel, stagnation of creeks, and contamination of in Pb, Zn, Cu, Ni, TS and Br (Fig. 9). Zinc and copper are generally water quality related to less circulation of fresh and marine water concentrated in organic rich sediments (Tribovillard et al. 2006 ; exchanges (Okinawa Prefecture 2015a). Organic matter rich soils Calvert and Pedersen 1993 ; Ahmed et al. 2010), which have been are fundamental of the variety of biome (Sandilyan and Kathiresan used for evaluation of sedimentary environment. This is because, 2012 ; Mitsumoto 2001), thus the protection of the red soil inflow both Zn and Cu may behave as a micronutrient and also present is urgent and indispensable for the mangroves conservation as organometallic ligands (Calvert and Pedersen 19993). Element (Okinawa Prefecture 2015b). distribution patterns of Gesashi, Okukubi and Ginoza samples The Henoko estuary located off shore of the Henoko River

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20 (2010) Abundances, distribution, and sources of trace metals in Nakaumi–Honjo coastal lagoon sediments, Japan, Environ Monit Assess, 167 : 473-491. AIST/GSJ (2013) Seamless Digital Geological Map of Japan 15 (1:200,000), National Institute of Advanced Industrial Science and Technology (AIST) and the Geological Survey of Japan. Berner RA (1984) Sedimentary pyrite formation: An update. 10 Geoch Cosmoch Acta, 48 : 605-615.

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5 record. Marine Geology, 113 : 67-88. Chinen M, Shinjo R and Kato Y (2004) Occurrence and geochemistry of in-situ greenstones from the Shimanto Belt in the Ryukyu Islands. J Miner Petrol Sci, 33 : 208-220. 0 Ohura Gesashi Okukubi Ginoza Ogimi Furukawa H (1992) Ryukyu Group in Okinawa Island. In Regional Fig. 10 Box plot of LOI values of Ohura, Gesashi, Okukubi, Ginoza and Geology of Japan, Kyushu, 203. Editorial Committee of Kyushu,

Ogimi mangrove soil samples in the Okinawa Island. Part 9 of Regional Geology of Japan. Kyoritsu Shuppan Co Ltd Vertical line indicates the range, and box encloses 50 % of data and Tokyo*. illustrates the 25 Fig.10% quantile, Ishiga median and(horizontal Diallo bar), and 75 % quartile. Giril C, Ochieng E, Tieszen LL, Zhu Z, Singh A, Loveland T, Masek J and Duke N (2010) Status and distribution of mangrove develops the wide diversity of biota in coral reef (NACS-J 2010). forests of the world using earth observation satellite data. Global This estuary has been changed to sandy beach in relation to the Ecology and Biogeography, 20 : 154-159. development of the town around the Camp Schuwab of U. S. Hayashi D (1985) Middle and northern part of Okinawa. Geology military base from 1956. Environment of Henoko Sea has been of Ryukyu Arc, 93-106. Kizaki K (ed), Okinawa Times, 278p*. argued for conservation of the coral reefs and the marine ecology Imai N, Terashima S, Itoh S, and Ando A (1996) 1996 compilation (NACS-J 2010 ; Abe 2015). Ohura mangrove has potential of of analytical data on nine GSJ geochemical reference samples, element concentration in soils and can be anticipated stabilization “Sedimentary rock series”. Geostandards Newsletter, 20 : 165- of material outflow to the estuary. Present examination can be 216. useful for the future research, especially comparative evaluation of Ishiga H, Diallo IM, Malick ML, Miguta FN, Magai PJ, Stanley SS mangrove soils and its function of heavy metal sink. (2016) Geochemical approach to evaluate deforest of mangroves. Geoscience Rept Shimane Univ, 34 : 95-104. Acknowledgements Ishiga H, Dozen K, Ahmed F, Bibi MH and Kaita M (2003) Evaluation of sedimentary environment using a Zn-Fe2O3 We appreciate Agency of Cultural Affairs, and Nago City Board diagram. Geoscience Rept Shimane Univ, 22 : 15-22**. of Education for providing permissions of research in Gesashi Ishihara S, Fujimoto K, Kawanishi M, Watanabe R and Tanaka S mangroves of National Monument and Ohura mangroves of (2004) Relationship between mangrove vegetation and micro- Natural Monument of Nago City, Okinawa Prefecture. Thanks are topography, and carbon storage of Kandedelina candel (L.) extended to Higashi-son office for help of procedure of documents Druce forests in Amami-ohshima Island, southwestern Japan. and encouragement for the research. We thank reviewers for Jpn J For Environment, 46 : 9-19**. valuable comments and also handling editor. Kurotori T and Kojima T (1969) General remarks of forest soils in the Ryukyu Islands. Jour For Research Japan, 58 : 270-230*. References Land Improvement Division (LID), Okinawa General Office of Agriculture, Forestry and Fisheries, Cabinet Office of Japanese Abe M (2015) World-class coral reef of Henoko and Ohura bay. Government. Nature of Okinawa Island, 3. Soil. http://ogb.go.jp/ Symposium on informing biodiversity in Henoko sea, Abstract, nousui/nns/c1/page3-3-p.htm (May 1, 2016) *. 8-10. Ichigaya Campus of Hosei University (September 13, Mitsumoto H (2001) Water Pollution by Red Soil Erosion on U. 2015)*. S. military base. Okinawa Prefectural Institute of Health and Ahmed F, Bibi MH, Seto K, Ishiga H, Fukushima T and Roser BP Environment, 35 : 173-184*.

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NACS-J (2010) Urgent and cooperated Investigation of Henoko Koma T and Murata Y (2000) Young upper crustal chemical area, Okinawa (short report) ~Significant Biodiversity of Henoko composition of the orogenic Japan Arc. Geochemistry Sea~. The Nature Conservation Society of Japan (NACS-J), 21p *. Geophysics Geosystems (Electronic Journal of the Earth Nakamura T and Nakasuga T (2001) Introduction of mangrove – Sciences), 1, November 27, Page number 2000GC00083. Green forest in the sea–. 234p. Mekon Co Ltd, Tokyo (3rd edt) *. Togashiki Y (1993) The characteristic properties of the Shimajiri Nakasone K, Omija T, Mitsumoto H, Uehara M and Oshiro T (2000) mahji and Jahgaru soils in Okinawa Prefecture. Pedologist, 37, Monitoring water pollution caused by reddish soil runoff. 29-42*. Okinawa Prefectural Institute of Health and Environment. 34 : Tribovillard N, Algeo TJ, Lyons T and Riboulleau A (2006) 85-95*. Trace metals as paleoredox and paleoproductivity proxies: An Nakasuga T, Kishimoto T and Yang S-C (1995) Studies on stand update. Chemical Geology, 232 : 12-32. structure of mangrove forest in Okukubi River, Okinawa Island. Tsuchiya H (2012) Study on public supports for adoption of red Kyushu J. For. Res., 48 : 105-106*. soil runoff prevention measures in Okinawa, Japan. Doctor Nakasuga T and Kobashigawa Y (1976) Studies on the mangrove Thesis of Kagoshima University, 54p**. community IV - (2) The distribution of mangrove community Vuai SA, Ishiki M and Tokuyama A (2003) Acidification of in Okinawa and Kumejima Islands. Mem Fac Agriculture, Univ freshwaters by red soil in a subtropical silicate rock area, Ryukyu, 23 : 313-337*. Okinawa, Japan. Limnology, 4 : 63-71. Ogasawara M (1987) Trace element analysis of rock samples by Yamamoto S and Chiba T (1994) Sedimentation of terrigenous X-ray fluorescence spectrometry, using Rh anode tube. Bulletin red soils in coastal area adjacent to an estuary and coral reefs, of the Geological Survey of Japan, 38 : 57-68**. Okukubi River estuary, Okinawa Island, Japan. Journal of Okinawa Prefecture (2013) Primary planning of protection of Oceanography, 50 : 423-435*. erosion of red soils and other soils, Okinawa Prefecture. 63p*. Okinawa Prefecture (2015a) Record of environmental evaluation, *: in Japanese. Gesashi River area (draft). Okinawa Prefecture, 12p *. **: in Japanese with English abstract. Okinawa Prefecture (2015b) Guideline for restoration of natural environment in Okinawa. –Recover of the natural environment 地名対照表 and Chura shima Uchina (beautiful island, ourselves) for next Arume 有 銘 generation-. Okinawa Prefecture, 46p.* Gesashi 慶佐次 Onaga K (1993) Local soil of Okinawa and farmland conservation. Ginoza 宜野座 Environmental planning and management in agriculture and Henoko 辺野古 rural development. Ministry of Agriculture, Forestry and Kana 漢 那 Fisheries, Japanese International Cooperation Agency, Institute Kin 金 武 of Irrigation and Drainage, Tokyo, Japan, 1-20*. Kunchan 国 頭 Onaga K, Komesu R and Arakaki A (1999) The Outline of red Kunigami 国 頭 soil loss and erosion control measure in Okinawa. The Science Ogimi 大宜見 Bulletin of the College of Agriculture, University of the Ohura 大 浦 Ryukyus, 46 : 71-82**. Okukubi 億 首 Ramanathan AL, Subramanian R, Ramesh S and Chidambaram Taira 平 良 AJ (1999) Environmental geochemistry of the Pichavaram mangrove ecosystem (tropical), southeast coast of India. Environmental Geology, 37 : 223-233. Sampei Y, Matsumoto E, Kamei T and Tokuoka T (1997) Sulfur and organic carbon relationship in sediments from coastal lakes in Shimane peninsula district, southwest Japan. Geochem J, 31 : 245-262. Sandilyan S and Kathiresan K (2012) Mangrove conservation: a global perspective, Biodiversity and ConservaNakasution. 21 : 3523-3542. Togashi S, Imai N, Okuyama-Kusunose Y, Tanaka T, Okai T,

（ 9 ） 128 ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba

石賀裕明・DIALLO Ibrahima M'Bemba．2016．沖縄のマングローブ土壌の地球化学的検討．地球科学，70， 119-128. ISHIGA Hiroaki and DIALLO Ibrahima M’Bemba. 2016. Geochemical evaluation of present mangrove soil in Okinawa Island, Japan. Earth Science (Chikyu Kagaku), 70, 119-128.

要 旨

沖縄におけるマングローブは人間活動，特に自然の土地の改変や赤土流出により抑圧を受けている．いくつか のマングローブを記載し，多元素組成分析からその土壌の組成を検討した．一般のマングローブの土壌試料は有

機物に関連して，Zn, Cu および Fe などの重金属と TS（全イオウ）と P2O5 に富んでいる．生物群の卓越するマン グローブの土壌では，赤土の流入の影響を受けた土壌に比べ暗色である．一方で，森林破壊を受けているマング ローブ土壌は CaO に富んでおり，これは海洋からの炭酸塩鉱物による．沖縄におけるマングローブの森林破壊 やその土壌の劣化は，戦後の急速な土地利用や農業活動に関連していた．今回の検討は限られた範囲のものであ るが，一般のマングローブ土壌と赤土の流入を受けた土壌では，地球化学的には対照的である．この様なマング ローブの観察と地球化学的データは赤土の組成を理解し，マングローブの保全を計画する上で有用であると期待 される．

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