11:15 AM - 11:30 AM
[BCG05-09] Formation of modern iron-ooidal sands: example at a beach sand from hydrothermal environment at Nagahama Bay, Satsuma Iwo-Jima Island, Japan
Keywords:Iron Oolite, granule type Iron formation, modern hydrothermal condition beach
Modern iron-ooidal sands have been considered as one of the most important tools for investigating the history of the earth as its capacity to provides the analogy of the granular iron sedimentation in the ancient ocean. Interestingly, we found that such an ongoing formation process of iron-ooidal sands can be observed at Nagahama Bay of Satsuma Iwo-Jima Island, 38 km south of Kyushu Island, Japan.
Satsuma Iojima is a volcanic island located on the northwestern edge of the Kikai Caldera, about 50 km south of Makurazaki City, Kagoshima Prefecture. Hot water associated with volcanic activity is supplied to the waters around the island, and when mixed with seawater, brown to milky white precipitates are formed. In particular, weakly acidic hot water (Shikaura and Tasaki, 2001; Sakamoto, 2015), which is rich in dissolved iron and free CO2 and has a pH of about 4.4 to 5.5, is supplied to Nagahama Bay, which is located in the southern part of the island, and precipitates iron. Oxides are deposited on the sea floor, and iron hydroxide chimney has also been found in the hydrothermal vent.
The minimum influence from seawater (e.g. waves) due to the occurrence of breakwaters and the intense hydrothermal activities causes the water to become orange-brown in color with acidic (low pH), hot (high temperature), and containing a large amount of dissolved iron (generated by mixing volcanic fluids and seawater). These conditions lead to the precipitation of Fe-oxyhydroxide sediments on the floor of the bay; and iron-coated sands on the fishing port and along the beach.
Therefore, in order to understand the characteristics and formation of iron-ooidal sands, we collect unlithified sand samples from 15 locations: west and east site of the fishing port, original river, the mouth of the river, 9 samples from the sandy beach, and 2 samples from the rocky beach. Sand grains are spheroidal in shape (ooids) with a rust black-brown color, vary from 0.2 to 2 mm in diameter, and exhibit roundness from sub-angular to sub-rounded. Based on our petrography and FE-SEM/EDS data, we found that ooidal sands are consist of volcanic rock fragments and free crystals (plagioclase, quartz, biotite) on the nucleus; and covered by concentric amorphous Fe-oxide cortex. Three types of Fe-oxide were observed: (1) covering the granules, (2) fracture filling, and (3) void filling. EDS analysis indicates that most of the amorphous Fe-oxide particles are mainly composed of Fe (~58-64%) and Si (~6-9%). Element mapping using FE-EPMA shows that the cortex is composed of Fe-rich which is almost uniform in each layer. The microbial community is well documented in the form of cocci, bacilli, and filamentous morphology, which responsible for triggering the chemical precipitation of Fe-oxide through their metabolic activity hence results in the formation of iron-ooidal sands.
Microbial communities remain in layer depressions in the form of cocci, bacilli, and filamentous forms that may cause iron oxide adsorption through metabolic activity early in particle formation, leading to the formation of sand in iron deposits. After that, it is thought that it adsorbs and grows because it rolls chemically in seawater with a high iron hydroxide water with wave condition.
Satsuma Iojima is a volcanic island located on the northwestern edge of the Kikai Caldera, about 50 km south of Makurazaki City, Kagoshima Prefecture. Hot water associated with volcanic activity is supplied to the waters around the island, and when mixed with seawater, brown to milky white precipitates are formed. In particular, weakly acidic hot water (Shikaura and Tasaki, 2001; Sakamoto, 2015), which is rich in dissolved iron and free CO2 and has a pH of about 4.4 to 5.5, is supplied to Nagahama Bay, which is located in the southern part of the island, and precipitates iron. Oxides are deposited on the sea floor, and iron hydroxide chimney has also been found in the hydrothermal vent.
The minimum influence from seawater (e.g. waves) due to the occurrence of breakwaters and the intense hydrothermal activities causes the water to become orange-brown in color with acidic (low pH), hot (high temperature), and containing a large amount of dissolved iron (generated by mixing volcanic fluids and seawater). These conditions lead to the precipitation of Fe-oxyhydroxide sediments on the floor of the bay; and iron-coated sands on the fishing port and along the beach.
Therefore, in order to understand the characteristics and formation of iron-ooidal sands, we collect unlithified sand samples from 15 locations: west and east site of the fishing port, original river, the mouth of the river, 9 samples from the sandy beach, and 2 samples from the rocky beach. Sand grains are spheroidal in shape (ooids) with a rust black-brown color, vary from 0.2 to 2 mm in diameter, and exhibit roundness from sub-angular to sub-rounded. Based on our petrography and FE-SEM/EDS data, we found that ooidal sands are consist of volcanic rock fragments and free crystals (plagioclase, quartz, biotite) on the nucleus; and covered by concentric amorphous Fe-oxide cortex. Three types of Fe-oxide were observed: (1) covering the granules, (2) fracture filling, and (3) void filling. EDS analysis indicates that most of the amorphous Fe-oxide particles are mainly composed of Fe (~58-64%) and Si (~6-9%). Element mapping using FE-EPMA shows that the cortex is composed of Fe-rich which is almost uniform in each layer. The microbial community is well documented in the form of cocci, bacilli, and filamentous morphology, which responsible for triggering the chemical precipitation of Fe-oxide through their metabolic activity hence results in the formation of iron-ooidal sands.
Microbial communities remain in layer depressions in the form of cocci, bacilli, and filamentous forms that may cause iron oxide adsorption through metabolic activity early in particle formation, leading to the formation of sand in iron deposits. After that, it is thought that it adsorbs and grows because it rolls chemically in seawater with a high iron hydroxide water with wave condition.