5:15 PM - 7:15 PM
[MIS14-P17] Paleoclimatic reconstruction by algal biomarkers analysis of sediment core in Lake Nakaumi, Japan, during the Little Ice Age
The Little Ice Age (LIA; about 1400-1850 CE) has been proposed as the coldest age in the northern hemisphere over the past about 2000 years by paleoclimatic reconstruction. In particular, climatic cooling was known to take place in Europe during the minimum such as the Maunder Minimum (1645-1715), when were thought to be occurred by reduced solar activity. The decreasing temperatures and durations of the LIA were different in each region. There were few reports for systematic paleoclimatic investigation of the LIA in Japan. In the present study, we analyzed algal biomarkers such as long-chain alkenones and alkyl diols in the sediment core of Lake Nakaumi, Shimane/Tottori prefectures, Japan, to reconstruct the variations in water temperatures and river water input during the periods corresponding to the LIA.
The L. Nakaumi is a brackish lake and lagoon, and the waters in surface to bottom are oligohaline to polyhaline by intrusion of seawater from the Japan Sea. Sediment core used was collected at a Nk3C site, where is located near the center of L. Nakaumi during 2017. The ages model was established by lithostratigraphy and Cs, Pb, and 14C isotope dating and the age of core bottom is estimated to be about 600 years ago (Hirose et al., 2020). Lipids were extracted with dichloromethane/methanol, and separated to aliphatic, aromatic, ketone-ester, and polar fractions. Lipids were identified and quantified via GC-MS and GC-FID.
The alkenone compositions are clearly different between the samples above and below the horizons around 18 cm depth corresponding to the horizon in about 1960s. By comparing with chemotaxonomic features of known alkenone producers as reported previously, we estimate that the main alkenone producers during the timings below and above the horizons in about 1960s are Group III species (including marine producer Gephyrocapsa) and Group II species (coastal and lacustrine producers: e.g. Ruttnera lamellosa), respectively. Thus, the alkenone-based temperatures (UK37, UK’37) were calculated by the indices-temperature calibrations obtained by Gephyrocapsa or R. lamellosa in each horizon. The alkenone-based temperatures rapidly decreased during about 1560-1590 CE, 1670-1700 CE, and 1820-1850 CE, showing rapid cooling events at these timings. The ages of the cooling event during about 1670-1700 CE are correlated with that of the Maunder Minimum. Although the resolution of present data was low, these alkenone temperature data of L. Nakaumi sediments can be one of paleoclimatic records in western Japan, where there were few comparable paleoclimatic data over the past 600 years. In the riverine input indicators using the long-chain diols, %C32 1,15-diol were nearly constant, although the diol values increased gradually, affected strongly by artificial rearrangements such as the widening and dredging constructions of the Ohashi River after 1920s (1924-1933 CE). It is known that fresh water of the Hii River intruded into L. Nakaumi during 1635-1639, but the diol values hardly changed. Thus, the riverine input in this event was not significantly affected in the hydrological conditions of L. Nakaumi.
The L. Nakaumi is a brackish lake and lagoon, and the waters in surface to bottom are oligohaline to polyhaline by intrusion of seawater from the Japan Sea. Sediment core used was collected at a Nk3C site, where is located near the center of L. Nakaumi during 2017. The ages model was established by lithostratigraphy and Cs, Pb, and 14C isotope dating and the age of core bottom is estimated to be about 600 years ago (Hirose et al., 2020). Lipids were extracted with dichloromethane/methanol, and separated to aliphatic, aromatic, ketone-ester, and polar fractions. Lipids were identified and quantified via GC-MS and GC-FID.
The alkenone compositions are clearly different between the samples above and below the horizons around 18 cm depth corresponding to the horizon in about 1960s. By comparing with chemotaxonomic features of known alkenone producers as reported previously, we estimate that the main alkenone producers during the timings below and above the horizons in about 1960s are Group III species (including marine producer Gephyrocapsa) and Group II species (coastal and lacustrine producers: e.g. Ruttnera lamellosa), respectively. Thus, the alkenone-based temperatures (UK37, UK’37) were calculated by the indices-temperature calibrations obtained by Gephyrocapsa or R. lamellosa in each horizon. The alkenone-based temperatures rapidly decreased during about 1560-1590 CE, 1670-1700 CE, and 1820-1850 CE, showing rapid cooling events at these timings. The ages of the cooling event during about 1670-1700 CE are correlated with that of the Maunder Minimum. Although the resolution of present data was low, these alkenone temperature data of L. Nakaumi sediments can be one of paleoclimatic records in western Japan, where there were few comparable paleoclimatic data over the past 600 years. In the riverine input indicators using the long-chain diols, %C32 1,15-diol were nearly constant, although the diol values increased gradually, affected strongly by artificial rearrangements such as the widening and dredging constructions of the Ohashi River after 1920s (1924-1933 CE). It is known that fresh water of the Hii River intruded into L. Nakaumi during 1635-1639, but the diol values hardly changed. Thus, the riverine input in this event was not significantly affected in the hydrological conditions of L. Nakaumi.