11:45 AM - 12:00 PM
[MIS12-20] Orbital-scale Oxygen isotope variations of fossil pollen in Lake Suigetsu
Keywords:Oxygen Isotope Ratios, Fossil pollen, Lake Suigetsu, flow cytometry, cell sorter
It is essential to analyse the interactions between ice sheets, oceans, and terrestrial environments to better understand the climate system and lead to accurate future predictions (e.g., Lowe et al., 2008). Terrestrial palaeoclimate changes have been reconstructed using a wide range of the datasets such as fossil pollen assemblages and the stable isotope ratios of carbonates (e.g., Wang et al., 2008; Turney and Jones, 2010). In particular, fossil pollen assemblage data have been utilised in palaeoclimate reconstruction for a long time because they provide multifaceted information about past environment without the geological limitations.
Fossil pollen grains are mainly composed of sporopollenin, which consists of approximately 60% carbon, 20% oxygen, and 10% hydrogen. Sporopollenin is a robust natural polymer known to be preserved for hundreds of millions of years (Brooks et al., 1971). These characteristics of sporopollenin make fossil pollen grains suitable material not only for radiocarbon dating but also for stable isotope ratio analyses. Loader and Hemming (2004) demonstrated the potential to reconstruct past climates without vegetative inertia (response time of vegetation to the climate change) by using the stable isotope ratios of oxygen, hydrogen, and carbon in fossil pollen. However, paleoclimate reconstruction using the stable isotope ratio has not been realised mainly because it had been difficult to extract fossil pollen grains in high purity.
In recent years, the introduction of cell sorter has significantly advanced the technology for extracting fossil pollen grains from sediments (Tennant et al., 2013). Cell sorter can extract particles with specific optical properties using flow cytometry techniques that can identify each particle based on their fluorescence and intensity of forward scatter and back scatter. Yamada et al. (2021) significantly refined this method and established a technique for extracting fossil pollen grains from sediments in high purity and routinely.
In this study, using this method, we extracted fossil pollen grains from varved sediments of Lake Suigetsu in Fukui Prefecture and measured their stable oxygen isotope ratios to reconstruct palaeoclimate changes. Approximately 0.5 to 2 million pollen grains were extracted from each sample. After washing, their stable oxygen isotope ratios were measured. The measurements of the stable Oxygen isotope ratios were conducted using an Elementar Co. Ltd. manufactured high temperature pyrolysis elemental analyser-stable isotope ratio mass spectrometer (EA-IRMS) owned by the Research Centre for Palaeoclimatology, Ritsumeikan University.
The extracted fraction contained almost exclusively of fossil pollen grains of various genera, including Cryptomeria, Taxodiaceae, Cupressaceae, Alnus etc. The typical diameter of grains was under 50μm due to the limitations of the cell sorter. The oxygen isotope ratios of the fossil pollen obtained in this study showed fluctuations within the range of 15-30‰ VSMOW. Notably, the oxygen isotope ratios obtained from samples dating between 40 and 120ka demonstrated clear periodic variations with a cycle of ca. 20,000 years. These fluctuations were synchronous to the changes in summer insolation at 35 degrees north latitude and with the variations in the oxygen isotope ratios of stalagmites from the Sanbao Cave (Wang et al., 2008). Additionally, the oxygen isotope ratios of modern pollen showed significant correlation with summer temperatures and precipitation.
Although it is premature to draw any conclusion due to the still insufficient number of the data, the variations in the oxygen isotope ratios of fossil pollen grains suggest the potential for reconstructing at least orbital-scale climate changes. This method, using stable isotope ratios of fossil pollen grains, is less susceptible to anthropogenic alterations in vegetation, offering independent method to reconstruct terrestrial palaeoclimates. Furthermore, there is potential for applying this approach to chronological determinations based on orbital tuning of oxygen isotope ratios, as well as comparisons with palaeoclimatic records from marine and polar regions.
Fossil pollen grains are mainly composed of sporopollenin, which consists of approximately 60% carbon, 20% oxygen, and 10% hydrogen. Sporopollenin is a robust natural polymer known to be preserved for hundreds of millions of years (Brooks et al., 1971). These characteristics of sporopollenin make fossil pollen grains suitable material not only for radiocarbon dating but also for stable isotope ratio analyses. Loader and Hemming (2004) demonstrated the potential to reconstruct past climates without vegetative inertia (response time of vegetation to the climate change) by using the stable isotope ratios of oxygen, hydrogen, and carbon in fossil pollen. However, paleoclimate reconstruction using the stable isotope ratio has not been realised mainly because it had been difficult to extract fossil pollen grains in high purity.
In recent years, the introduction of cell sorter has significantly advanced the technology for extracting fossil pollen grains from sediments (Tennant et al., 2013). Cell sorter can extract particles with specific optical properties using flow cytometry techniques that can identify each particle based on their fluorescence and intensity of forward scatter and back scatter. Yamada et al. (2021) significantly refined this method and established a technique for extracting fossil pollen grains from sediments in high purity and routinely.
In this study, using this method, we extracted fossil pollen grains from varved sediments of Lake Suigetsu in Fukui Prefecture and measured their stable oxygen isotope ratios to reconstruct palaeoclimate changes. Approximately 0.5 to 2 million pollen grains were extracted from each sample. After washing, their stable oxygen isotope ratios were measured. The measurements of the stable Oxygen isotope ratios were conducted using an Elementar Co. Ltd. manufactured high temperature pyrolysis elemental analyser-stable isotope ratio mass spectrometer (EA-IRMS) owned by the Research Centre for Palaeoclimatology, Ritsumeikan University.
The extracted fraction contained almost exclusively of fossil pollen grains of various genera, including Cryptomeria, Taxodiaceae, Cupressaceae, Alnus etc. The typical diameter of grains was under 50μm due to the limitations of the cell sorter. The oxygen isotope ratios of the fossil pollen obtained in this study showed fluctuations within the range of 15-30‰ VSMOW. Notably, the oxygen isotope ratios obtained from samples dating between 40 and 120ka demonstrated clear periodic variations with a cycle of ca. 20,000 years. These fluctuations were synchronous to the changes in summer insolation at 35 degrees north latitude and with the variations in the oxygen isotope ratios of stalagmites from the Sanbao Cave (Wang et al., 2008). Additionally, the oxygen isotope ratios of modern pollen showed significant correlation with summer temperatures and precipitation.
Although it is premature to draw any conclusion due to the still insufficient number of the data, the variations in the oxygen isotope ratios of fossil pollen grains suggest the potential for reconstructing at least orbital-scale climate changes. This method, using stable isotope ratios of fossil pollen grains, is less susceptible to anthropogenic alterations in vegetation, offering independent method to reconstruct terrestrial palaeoclimates. Furthermore, there is potential for applying this approach to chronological determinations based on orbital tuning of oxygen isotope ratios, as well as comparisons with palaeoclimatic records from marine and polar regions.