11:00 AM - 1:00 PM
[U05-P02] Study of Benthic Foraminifera Assemblage Variations from Lake Nakaumi During the Past 2000 years
Keywords:Benthic Foraminifera Assemblage, Lake Nakaumi, Paleoenvironment, Holocene
Lake Nakaumi situated in Tottori City, in the southwestern Honshu Island, is the fifth largest lake in Japan and the largest brackish-water lake. Lake Nakaumi is connected with Lake Shinji and has a narrow open into Miho Bay through the Ohashi River and Sakai Strait, respectively. Benthic foraminifera is a natural archive which can be conducted multiple-proxy studies of shallow sea environment changes in the past. Despite the importance mentioned above, there are only few reports of paleoenvironment study in centennial-scale and modern distribution of benthic foraminifera in Lake Nakaumi regions.
In this study, benthic foraminifera collected from sediment core N2015 (35° 28.208'N, 133° 10.894'E. Depth 206.5 to 376.5 cm). In total of 18 samples were chosen (from depth 206.5 to 385.5 cm, in evert 10-cm resolution). Benthic foraminifera were identified, hand-picked and counted to genus level. Averaged genus occuring higher than 2% were used for fauna analysis and examined with principal component analysis (PCA) by R software.
In general, the Nakaumi benthic foraminifera assemblage is represented by Ammonia spp. (45.8-85.6%), Quinqueloculina spp. (1.5-24.1%), Pararotalia spp. (0-19.7%), Triloculina spp. (0-17.7%), Elphidium spp. (5-16.3%), Cibicides spp. (0.7-10.5%). We also find that there is a long-tern faunal overturning between Quinqueloculina spp. and Triloculina spp.. In the period of BC 815 to AD 1073, assemblage of benthic foraminifera in Lake Nakaumi was dominant by Ammonia spp.. Relative frequency of Quinqueloculina spp. gradually increased, and relative abundance of Ammonia spp. was fallen. Our records show that environment of Lake Nakaumi became relative high salinity and a higher oxygen environment. Future studies of benthic foraminiferal assemblage in Nakaumi region could be improved by identifying samples to species level and increasing time resolution to extract detail environment information with higher time resolution.
In this study, benthic foraminifera collected from sediment core N2015 (35° 28.208'N, 133° 10.894'E. Depth 206.5 to 376.5 cm). In total of 18 samples were chosen (from depth 206.5 to 385.5 cm, in evert 10-cm resolution). Benthic foraminifera were identified, hand-picked and counted to genus level. Averaged genus occuring higher than 2% were used for fauna analysis and examined with principal component analysis (PCA) by R software.
In general, the Nakaumi benthic foraminifera assemblage is represented by Ammonia spp. (45.8-85.6%), Quinqueloculina spp. (1.5-24.1%), Pararotalia spp. (0-19.7%), Triloculina spp. (0-17.7%), Elphidium spp. (5-16.3%), Cibicides spp. (0.7-10.5%). We also find that there is a long-tern faunal overturning between Quinqueloculina spp. and Triloculina spp.. In the period of BC 815 to AD 1073, assemblage of benthic foraminifera in Lake Nakaumi was dominant by Ammonia spp.. Relative frequency of Quinqueloculina spp. gradually increased, and relative abundance of Ammonia spp. was fallen. Our records show that environment of Lake Nakaumi became relative high salinity and a higher oxygen environment. Future studies of benthic foraminiferal assemblage in Nakaumi region could be improved by identifying samples to species level and increasing time resolution to extract detail environment information with higher time resolution.