12:00 PM - 12:15 PM
[HQR05-12] Centennial-scale paleoenvironmental changes in Lake Nakaumi during the Late Holocene
Keywords:Ostracod, Late Holocene, Paleoenvironment, Core research
Holocene abrupt climate changes have significantly impacted human development. Among these events, the 4.2 ka event and 3.2 ka event are known to have caused major disruptions to civilizations worldwide [1]. These decadal-scale climate fluctuations may profoundly affect modern societies, driving interest in understanding their mechanisms and impacts. This study aims to reconstruct Late Holocene paleoenvironmental changes in Lake Nakaumi, which is a coastal brackish lake sensitive to climatic influences, by analyzing sediment cores through chemical analyses and ostracod assemblages, with a focus on identifying abrupt environmental shifts.
Lake Nakaumi, located on the border of Shimane and Tottori Prefectures, Japan, is a coastal brackish lake formed by sand spit development that separated it from the open ocean. It transitioned from an inner bay to a brackish lake during the Late Holocene [2]. Paleoenvironmental changes over the past 3,000 years have been extensively studied, revealing centennial-scale environmental fluctuations [3]. However, due to limited records prior to 3,000 years ago, this study collected a new sediment core, NKU23-01. An age model based on two AMS 14C dates indicates that the core spans the last 4,200 years. The core was analyzed for elemental concentrations using an XRF core scanner (ITRAX, MaCRI) at 0.5 mm intervals. For sampling, the lower 1 m of the core was divided into 0.5 cm-thick slices, while the remaining sections were split into 1 cm-thick slices. For ostracod analysis, all samples below 2.9 m depth were examined, and samples above this depth were selected at 10 cm intervals, resulting in a total of 306 samples analyzed.
Based on the variations of Ti, Fe, and Cl among the measured elements, the core can be divided into three main periods. Prior to 2,800 cal yr BP, Ti and Fe showed relatively low concentrations with a decreasing trend upward, suggesting relatively weak river inflow that further diminished over time. The comparatively high Cl concentrations during this period indicate a strong marine influence. Additionally, significant fluctuations in elemental concentrations were observed with centennial-scale variations. During 2,800-900 cal yr BP, Ti and Fe concentrations increased while Cl decreased, accompanied by reduced elemental variability. This suggests a more stable depositional environment dominated by stronger river inflow and weaker marine input. After 900 cal yr BP, Ti and Fe reached their highest concentrations with minimal Cl, reflecting an environment primarily controlled by river inflow with negligible marine influence.
Regarding ostracods, at least 50 individuals were recovered from each sample, and 35 species belonging to 22 genera were identified. Throughout the core, Bicornucythere bisanensis and Spinileberis quadriaculeata, which are species that inhabit muddy bottoms in inner bay and brackish environments [4], were dominant. This indicates that the entire core was deposited in a muddy bottom under inner bay or brackish conditions. Based on Q-mode cluster analysis, the depositional environment transitioned from near the bay mouth to an inner bay around 3700 cal yr BP, and further shifted from an inner bay to a brackish environment around 1200 cal yr BP, consistent with previous paleoenvironmental reconstructions. Additionally, between 3700 and 2500 cal yr BP, centennial-scale environmental fluctuations were inferred from changes in the relative abundance of Propontocypris attenuata [5], a species inhabiting sandy-muddy bottoms in intertidal zones.
Based on the integration of XRF results and ostracod assemblage changes, the fluctuations observed between 4000-2800 cal yr BP suggest alternating centennial-scale environmental shifts between inner-bay environments of increased Fe, Ti, Cl, and environments near bay mouth of reduced Fe and Ti alongside slightly increased Cl. Combined with δ18O data from shells of B. bisanensis,centennial-scale paleoenvironmental changes in Lake Nakaumi during the Late Holocene and its driving mechanisms will be discussed.
References
[1] Hirabayashi, S. & Yokoyama, Y., 2020. The Quaternary research, 59, 129-157. (in Japanese)
[2] Yamada et al., 2015. The Quaternary research, 54, 53-68. (in Japanese)
[3] Yamada et al., 2019. Scientific Reports, 9, 5036.
[4] Ikeya N. & Shiozaki M., 1993. Memoirs of the Geological Society of Japan, 39, 15-32. (in Japanese)
[5] Okubo, I., 1979. In Proceedings of the Japanese Society of Systematic Zoology, 17, 31-37.
Lake Nakaumi, located on the border of Shimane and Tottori Prefectures, Japan, is a coastal brackish lake formed by sand spit development that separated it from the open ocean. It transitioned from an inner bay to a brackish lake during the Late Holocene [2]. Paleoenvironmental changes over the past 3,000 years have been extensively studied, revealing centennial-scale environmental fluctuations [3]. However, due to limited records prior to 3,000 years ago, this study collected a new sediment core, NKU23-01. An age model based on two AMS 14C dates indicates that the core spans the last 4,200 years. The core was analyzed for elemental concentrations using an XRF core scanner (ITRAX, MaCRI) at 0.5 mm intervals. For sampling, the lower 1 m of the core was divided into 0.5 cm-thick slices, while the remaining sections were split into 1 cm-thick slices. For ostracod analysis, all samples below 2.9 m depth were examined, and samples above this depth were selected at 10 cm intervals, resulting in a total of 306 samples analyzed.
Based on the variations of Ti, Fe, and Cl among the measured elements, the core can be divided into three main periods. Prior to 2,800 cal yr BP, Ti and Fe showed relatively low concentrations with a decreasing trend upward, suggesting relatively weak river inflow that further diminished over time. The comparatively high Cl concentrations during this period indicate a strong marine influence. Additionally, significant fluctuations in elemental concentrations were observed with centennial-scale variations. During 2,800-900 cal yr BP, Ti and Fe concentrations increased while Cl decreased, accompanied by reduced elemental variability. This suggests a more stable depositional environment dominated by stronger river inflow and weaker marine input. After 900 cal yr BP, Ti and Fe reached their highest concentrations with minimal Cl, reflecting an environment primarily controlled by river inflow with negligible marine influence.
Regarding ostracods, at least 50 individuals were recovered from each sample, and 35 species belonging to 22 genera were identified. Throughout the core, Bicornucythere bisanensis and Spinileberis quadriaculeata, which are species that inhabit muddy bottoms in inner bay and brackish environments [4], were dominant. This indicates that the entire core was deposited in a muddy bottom under inner bay or brackish conditions. Based on Q-mode cluster analysis, the depositional environment transitioned from near the bay mouth to an inner bay around 3700 cal yr BP, and further shifted from an inner bay to a brackish environment around 1200 cal yr BP, consistent with previous paleoenvironmental reconstructions. Additionally, between 3700 and 2500 cal yr BP, centennial-scale environmental fluctuations were inferred from changes in the relative abundance of Propontocypris attenuata [5], a species inhabiting sandy-muddy bottoms in intertidal zones.
Based on the integration of XRF results and ostracod assemblage changes, the fluctuations observed between 4000-2800 cal yr BP suggest alternating centennial-scale environmental shifts between inner-bay environments of increased Fe, Ti, Cl, and environments near bay mouth of reduced Fe and Ti alongside slightly increased Cl. Combined with δ18O data from shells of B. bisanensis,centennial-scale paleoenvironmental changes in Lake Nakaumi during the Late Holocene and its driving mechanisms will be discussed.
References
[1] Hirabayashi, S. & Yokoyama, Y., 2020. The Quaternary research, 59, 129-157. (in Japanese)
[2] Yamada et al., 2015. The Quaternary research, 54, 53-68. (in Japanese)
[3] Yamada et al., 2019. Scientific Reports, 9, 5036.
[4] Ikeya N. & Shiozaki M., 1993. Memoirs of the Geological Society of Japan, 39, 15-32. (in Japanese)
[5] Okubo, I., 1979. In Proceedings of the Japanese Society of Systematic Zoology, 17, 31-37.