5:15 PM - 6:30 PM
[SCG45-P11] Precipitation process and early diagenesis after deposition of iron hydroxide in Nagahama Bay, Satsuma Iwo-Jima Island, Kagoshima, Japan.
Keywords:Satsuma Iwo-Jima, iron hydroxide, siderite
Satsuma Iwo-Jima is a volcanic island located about 50 km south of Makurazaki City, Kagoshima Prefecture, on the northwestern edge of the Kikai caldera, and the size is about 6 km east to west and 3 km north to south. In Nagahama Bay on Satsuma Iwo-jima, iron hydroxide deposition is caused by the supply of acidic hot water rich in dissolved iron and free CO2 (Shikaura and Tasaki, 2001; Sakamoto, 2015). However, there has never been a clear explanation for the oxidation-precipitation process in the bay, and mineral species after deposition. Therefore, in this study, we measured physicochemical properties of seawater, and observed and analyzed sediment samples or sediment porewater. And finally, we summarized a series of morphological changes from iron oxidation to the earliest diagenesis phase.
As a result of infrared thermography drone exploration in the bay, hydrothermal sources were identified at multiple locations, and we could observe that hot water spreads to the sea surface. The results of water quality measurement showed relatively low pH, ORP, EC values and high turbidity values near the water surface, and toward the deep part of the water mass, the former increased and the latter decreased. In particular, the pH and turbidity values showed a strong negative correlation, with the pH being the lowest and the turbidity being the highest near the sea surface. Correspondingly, it was confirmed from the captured water mass image that the sea surface layer was strongly turbid, and that the precipitation progressed toward the deep part (pH ≈ 8) and the turbidity became weaker. Furthermore, DO was higher than the metabolizable concentration of marine microaerobic iron-oxidizing bacteria (zetaproteobacteria) reported in previous studies (Krepski et al., 2013).
As a result of pH and dissolved element measurement of porewater from the sediment surface core sample with a length of about 40 cm collected from Nagahama Bay in 2020, the pH value changed from 7.5(near sea bottom) to 5.8(deepest part of core), and at that time concentrations of elements such as Fe, Mn, and Si increased. As a result of XRD of samples collected from another core sample collected in 2020 with a length of 1m, a peak of goethite was obtained from the upper part of the sediment (10 to 20 cmbsf) and a peak of siderite was obtained from the more deeper part. This siderite represented a rhombic euhedral shape in SEM observation. In addition, as a result of comparing the XRD peaks of the samples (5 sites) from same stratigraphy between above core and the core sample obtained in 2010, a clear siderite peak was identified at the shallowest sample from the new core, but not at the same stratigraphy sample from the past core.
Summarizing these above results, the following precipitation model can be considered in Nagahama Bay. It is suggested that abiotic oxidation of Fe (II) occurs near the surface of the water mass, and that it precipitates as ferrihydrite, which is a precursor of goethite. Also, the increase in pH toward deeper part caused by mixing with seawater is thought to strongly control the aggregation of iron hydroxide during precipitation. In addition, after deposition, parts of sediments are transformed into goethite near the sea bottom (a few cmbsf). In more deep part, dissolved iron derived from the reduction of iron hydroxide deposits and the supply of hydrothermal fluid from more deep part is contained in the pore water and it is thought to be used for the precipitation of siderite.Especially, siderite crystals detected in shallow part are suggested to have grown in sediments over the last decade.
As a result of infrared thermography drone exploration in the bay, hydrothermal sources were identified at multiple locations, and we could observe that hot water spreads to the sea surface. The results of water quality measurement showed relatively low pH, ORP, EC values and high turbidity values near the water surface, and toward the deep part of the water mass, the former increased and the latter decreased. In particular, the pH and turbidity values showed a strong negative correlation, with the pH being the lowest and the turbidity being the highest near the sea surface. Correspondingly, it was confirmed from the captured water mass image that the sea surface layer was strongly turbid, and that the precipitation progressed toward the deep part (pH ≈ 8) and the turbidity became weaker. Furthermore, DO was higher than the metabolizable concentration of marine microaerobic iron-oxidizing bacteria (zetaproteobacteria) reported in previous studies (Krepski et al., 2013).
As a result of pH and dissolved element measurement of porewater from the sediment surface core sample with a length of about 40 cm collected from Nagahama Bay in 2020, the pH value changed from 7.5(near sea bottom) to 5.8(deepest part of core), and at that time concentrations of elements such as Fe, Mn, and Si increased. As a result of XRD of samples collected from another core sample collected in 2020 with a length of 1m, a peak of goethite was obtained from the upper part of the sediment (10 to 20 cmbsf) and a peak of siderite was obtained from the more deeper part. This siderite represented a rhombic euhedral shape in SEM observation. In addition, as a result of comparing the XRD peaks of the samples (5 sites) from same stratigraphy between above core and the core sample obtained in 2010, a clear siderite peak was identified at the shallowest sample from the new core, but not at the same stratigraphy sample from the past core.
Summarizing these above results, the following precipitation model can be considered in Nagahama Bay. It is suggested that abiotic oxidation of Fe (II) occurs near the surface of the water mass, and that it precipitates as ferrihydrite, which is a precursor of goethite. Also, the increase in pH toward deeper part caused by mixing with seawater is thought to strongly control the aggregation of iron hydroxide during precipitation. In addition, after deposition, parts of sediments are transformed into goethite near the sea bottom (a few cmbsf). In more deep part, dissolved iron derived from the reduction of iron hydroxide deposits and the supply of hydrothermal fluid from more deep part is contained in the pore water and it is thought to be used for the precipitation of siderite.Especially, siderite crystals detected in shallow part are suggested to have grown in sediments over the last decade.