5:15 PM - 7:15 PM
[MIS23-P05] Elemental Analysis of alluvial core sesdiments using a XRF Core Scanner obtained in the Naha Lowland, Okinawa Island, southwestern Japan
Keywords:XRF core scanner, Naha Lowland, CNS eleentary analysis, Okinawa Island, alluvium
Along the coast of Okinawa Island, southwestern Japan, there are not only coral reefs but also small alluvial lowlands formed by sedimentation of rivers, wave and tide during the Holocene (Ujiie and Kaneko, 2006). There are few studies on alluvial lowlands and their sedimentary environment and geomorphic development process have not been clarified. This presumably results from that few microfossils in sediments and hardwater effect makes difficult to determine depositional environment and age respectively.
In order to clarify geomorphic development of alluvial lowland between Asato and Kokuba Rivers (Naha Lowland), southwestern part of the Island, we obtained a 7.5 m long core material drilled in Maejima, Naha City. This core includes alluvium with a thickness of about 6.8 m, consisting mainly of sandy mud overlying the Ryukyu Limestone belonging to the Ryukyu Group (Sato et al., 2021). Although the alluvium of this core contains numerous shell fragments and coral gravels, no diatom fossils are preserved. In this study, elemental concentrations were analyzed using an Itrax XRF core scanner of GSJ, AIST. Measurement interval was 2 mm and the X-ray irradiation time was 30 seconds. In addition, CNS analysis was conducted on 7 samples taken from the core material by the Hiruzen Geochronology Laboratory.
In generally, amount of sulfur [S] is an good indicator of marine deposits. This method is based on that marine sediments deposited at reducing environments trends to contain FeS. Amano et al. (2020) confirmed that S values measured by the XRF core scanner of GSJ showed a high positive correlation with the concentration of standard materials. Our results also show strong positive corelation (r = 0.89) between S values of the XRF core scanner and TS values of the CNS analysis at the same horizon. Therefore, it is suggested that higher S values of the XRF core scanner are corresponding to large amount of S content. The S values suggest that the core material is possible to be devided into upper and lower part at the elevation of 0.43 m T.P. The lower parrt shows high S values, generally 1,500 to 2,500 counts with some spikes approximately 20,000 counts, whereas the upper one is characterized by much less S values, about 100 to 200 counts. This indicates that the lower sediments were deposited in a bay or tidal flat environment influenced by seawater, and environmental change to inland area such as freshwater marsh occurred. This environmental change is consistent with that innferred from shell fossils. In addition, the S values shows moderately strong positive correlation with Cl values (r = 0.56), on the other hand, not correlation with the Mo ratio (Mo inc/M coh ratio, r = 0.09) which is considered to indicate organic content (Chagué Goff et al., 2016).
The Sr, Ca, and Mn values of the XRF core scanner fluctuates synchronously, and their correlation coefficient ranging from 0.37 to 0.60. Some spikes of these elements are corresponding to the horizons including coral gravels and/or shell fragments. In the holizons containing shell fragments, only the Ca values often show clear spikes, whereas the Sr values increase less than those in horizons including coral gravels. In addition, the Fe, Al and Ti values show quite strong positive correlations raging from 0.97 to 0.98, which is similar to the trends reported from the present surface soil samples (Yonaha et al., 2008). They show almost no correlation with Sr, Ca and Mn, and are stable values except in organic muddy sediments. Because soil of weathered mudstone of the Shimajiri Group include Fe, Al and Ti (Yonaha et al., 2008), these elements are possibly derived from the Shimajiri Group and may have been supplied stably as background to the alluvial lowland from the inland area.
References
Amano et al. (2020) JpGU-AGU Joint Meeting 2020, MIS12-P09
Chagué Goff et al. (2016) Island Arc, 25, 333-349.
Sato, Y. et al. (2021) Fall Meeting of the Geographical Society of Japan 2021, P010.
Ujiie, H. and Kaneko, N. (2006) Geology of Naha and southern Okinawa City area. Regional Geological Study Report 1:50,000, GSJ, 48 p.
Yonaha, H. et al. (2008) Journal of the Japanese Coral Reef Society, 10, 25-45.
In order to clarify geomorphic development of alluvial lowland between Asato and Kokuba Rivers (Naha Lowland), southwestern part of the Island, we obtained a 7.5 m long core material drilled in Maejima, Naha City. This core includes alluvium with a thickness of about 6.8 m, consisting mainly of sandy mud overlying the Ryukyu Limestone belonging to the Ryukyu Group (Sato et al., 2021). Although the alluvium of this core contains numerous shell fragments and coral gravels, no diatom fossils are preserved. In this study, elemental concentrations were analyzed using an Itrax XRF core scanner of GSJ, AIST. Measurement interval was 2 mm and the X-ray irradiation time was 30 seconds. In addition, CNS analysis was conducted on 7 samples taken from the core material by the Hiruzen Geochronology Laboratory.
In generally, amount of sulfur [S] is an good indicator of marine deposits. This method is based on that marine sediments deposited at reducing environments trends to contain FeS. Amano et al. (2020) confirmed that S values measured by the XRF core scanner of GSJ showed a high positive correlation with the concentration of standard materials. Our results also show strong positive corelation (r = 0.89) between S values of the XRF core scanner and TS values of the CNS analysis at the same horizon. Therefore, it is suggested that higher S values of the XRF core scanner are corresponding to large amount of S content. The S values suggest that the core material is possible to be devided into upper and lower part at the elevation of 0.43 m T.P. The lower parrt shows high S values, generally 1,500 to 2,500 counts with some spikes approximately 20,000 counts, whereas the upper one is characterized by much less S values, about 100 to 200 counts. This indicates that the lower sediments were deposited in a bay or tidal flat environment influenced by seawater, and environmental change to inland area such as freshwater marsh occurred. This environmental change is consistent with that innferred from shell fossils. In addition, the S values shows moderately strong positive correlation with Cl values (r = 0.56), on the other hand, not correlation with the Mo ratio (Mo inc/M coh ratio, r = 0.09) which is considered to indicate organic content (Chagué Goff et al., 2016).
The Sr, Ca, and Mn values of the XRF core scanner fluctuates synchronously, and their correlation coefficient ranging from 0.37 to 0.60. Some spikes of these elements are corresponding to the horizons including coral gravels and/or shell fragments. In the holizons containing shell fragments, only the Ca values often show clear spikes, whereas the Sr values increase less than those in horizons including coral gravels. In addition, the Fe, Al and Ti values show quite strong positive correlations raging from 0.97 to 0.98, which is similar to the trends reported from the present surface soil samples (Yonaha et al., 2008). They show almost no correlation with Sr, Ca and Mn, and are stable values except in organic muddy sediments. Because soil of weathered mudstone of the Shimajiri Group include Fe, Al and Ti (Yonaha et al., 2008), these elements are possibly derived from the Shimajiri Group and may have been supplied stably as background to the alluvial lowland from the inland area.
References
Amano et al. (2020) JpGU-AGU Joint Meeting 2020, MIS12-P09
Chagué Goff et al. (2016) Island Arc, 25, 333-349.
Sato, Y. et al. (2021) Fall Meeting of the Geographical Society of Japan 2021, P010.
Ujiie, H. and Kaneko, N. (2006) Geology of Naha and southern Okinawa City area. Regional Geological Study Report 1:50,000, GSJ, 48 p.
Yonaha, H. et al. (2008) Journal of the Japanese Coral Reef Society, 10, 25-45.