11:15 AM - 11:30 AM
[MIS14-17] Development and application of a water temperature reconstruction method to detect multiyear-scale oceanographic changes: Analysis of the frequency of the Kuroshio Meander
Keywords:planktonic foraminifera, the Kuroshio Meander, Individual Foraminifera Analysis
The Kuroshio Current has two metastable channels, the non-meandering channel and the meandering channel., and the mMeandering channel(KLM) has occurred a total of 6six times since 1962. When the KLMuroshio meanders southward along the Kii Peninsula coast, precipitation and water temperature increases by up to 5 C off in the Tokai region (Sugimoto et al., 2020, ) and precipitation increases in the Kanto and Tokai regions (Sugimoto et al., 2024, etc.). However, the number of years of observation is short, and the frequency and duration of the KLMuroshio meander are not fully understood. In this study, we attempted to analyze the occurrence frequency of the KLMuroshio meander in five stratigraphic levels since the Last Glacial Maximum (LGM) based on individual foraminifera analysis (IFA) of planktonic foraminifera in sediment cores.
IFA-Mg/Ca is a method of analyzing Mg/Ca water temperature of foraminifera shells one by one by picking up about 100 foraminifera from one stratum, and reconstructing the water temperature frequency distribution during the sedimentation period of one stratum (e.g., Khider et al., 2011). In the analysis of foraminiferal shells that record water temperature, it is essential to remove mud, clay particles, organic matter, etc. adhering to the chamber. It requires 100 samples are required for one stratigraphic level., and if this is done for several tens of levels, the number of samples analyzed will exceed 1000. Chemical cleaning of foraminifera shells in 1.5-mL centrifuge tubes is the most common and reliable method (Barker et al., 2003), but however, it is time-consuming and requires considerable labor if the number of samples to be cleaned per day is increased. The presenters We have developed a flow-through washing system for a single foraminifera shell to generate IFA-Mg/Ca ratio data for more than 1000 samples in a short time. The new flow-through washing method produced relatively more samples with higher Al/Ca ratios than the batch washing method, and 31.5% of the data analyzed had to be rejected, which was higher than the batch washing method, but the Mg/Ca ratios of the two methods were almost identical (difference of 0.2 mmol/mol and difference of 0.1 C).
Flow-through washing was used to analyze IFA-Mg/Ca ratios for G. ruber (0.25-0.355 mm) in sediment samples collected during the KH-16-6 cruise. Five stratigraphic levels were analyzed: two Holocene levels (Holocene 4 ka and Holocene 8 ka), Younger Dryas stage (YD), Bolling-Allerod stage (BA), and the coldest stage of the last glacial stage (LGM). IFA was performed for these stratigraphic levels, and the shape of the reconstructed temperature frequency distribution was analyzed. The shape analysis was performed using the R analysis software and the mixtools package. The reconstructed temperature frequency distribution was assumed to be composed of two peaks, and the details of each peak were analyzed.
Among the five stratigraphic levels analyzed in this study, the Holocene 4 ka and 8 ka levels, which have similar climatic conditions similar to the present age, show similar trends to the water temperature distribution with two peaks calculated from the reanalysis data (ORAS5, 1962-2023) at the depth of 20-50 m depth off Tokai. In particular, the water temperature frequency distribution aboutpproximately 8,000 years ago has an area of exhibits a high- water temperature peak similar to that of in the reanalysis data, suggesting that the Kuroshio meander may have occurred as frequently as in the present day. The water temperature frequency distributions of YD, BA and LGM were compared with the reanalysis data, and the results showed revealing an increase in the water temperature fluctuation range and a difference in the shape of the peaks. The lack of a clear bimodal distribution in during the YD and BA periods also makes it difficult to analyze the meandering frequency of the Kuroshio.
IFA-Mg/Ca is a method of analyzing Mg/Ca water temperature of foraminifera shells one by one by picking up about 100 foraminifera from one stratum, and reconstructing the water temperature frequency distribution during the sedimentation period of one stratum (e.g., Khider et al., 2011). In the analysis of foraminiferal shells that record water temperature, it is essential to remove mud, clay particles, organic matter, etc. adhering to the chamber. It requires 100 samples are required for one stratigraphic level., and if this is done for several tens of levels, the number of samples analyzed will exceed 1000. Chemical cleaning of foraminifera shells in 1.5-mL centrifuge tubes is the most common and reliable method (Barker et al., 2003), but however, it is time-consuming and requires considerable labor if the number of samples to be cleaned per day is increased. The presenters We have developed a flow-through washing system for a single foraminifera shell to generate IFA-Mg/Ca ratio data for more than 1000 samples in a short time. The new flow-through washing method produced relatively more samples with higher Al/Ca ratios than the batch washing method, and 31.5% of the data analyzed had to be rejected, which was higher than the batch washing method, but the Mg/Ca ratios of the two methods were almost identical (difference of 0.2 mmol/mol and difference of 0.1 C).
Flow-through washing was used to analyze IFA-Mg/Ca ratios for G. ruber (0.25-0.355 mm) in sediment samples collected during the KH-16-6 cruise. Five stratigraphic levels were analyzed: two Holocene levels (Holocene 4 ka and Holocene 8 ka), Younger Dryas stage (YD), Bolling-Allerod stage (BA), and the coldest stage of the last glacial stage (LGM). IFA was performed for these stratigraphic levels, and the shape of the reconstructed temperature frequency distribution was analyzed. The shape analysis was performed using the R analysis software and the mixtools package. The reconstructed temperature frequency distribution was assumed to be composed of two peaks, and the details of each peak were analyzed.
Among the five stratigraphic levels analyzed in this study, the Holocene 4 ka and 8 ka levels, which have similar climatic conditions similar to the present age, show similar trends to the water temperature distribution with two peaks calculated from the reanalysis data (ORAS5, 1962-2023) at the depth of 20-50 m depth off Tokai. In particular, the water temperature frequency distribution aboutpproximately 8,000 years ago has an area of exhibits a high- water temperature peak similar to that of in the reanalysis data, suggesting that the Kuroshio meander may have occurred as frequently as in the present day. The water temperature frequency distributions of YD, BA and LGM were compared with the reanalysis data, and the results showed revealing an increase in the water temperature fluctuation range and a difference in the shape of the peaks. The lack of a clear bimodal distribution in during the YD and BA periods also makes it difficult to analyze the meandering frequency of the Kuroshio.