5:15 PM - 6:30 PM
[MIS16-P09] Paleoenvironmental variations reconstructed by algal biomarker analysis of a sediment core from Lake Nakaumi, Japan
Keywords:Brackish lake, Lake Nakaumi, Algae biomaker, Alkenone, Alkyl diol, Paleoenvironmental Reconstruction
Diverse microalgae species inhabit brackish and lacustrine environments, and sensitively change their assemblages and compounds they produce in response to environmental changes. Algal biomarkers have been used to reconstruct the paleoenvironment in aquatic system: aquatic ecology and environmental factors such as temperature and salinity. In particular, long-chain alkenones, haptophyte-produced biomarker, have been widely used for temperature reconstruction in ocean, but there are few examples of their discovery in Japanese lakes such as Lakes Toyoni (Hokkaido), Ichinomegata (Akita), and Nakaumi - Shinji (Tottori-Shimane). Long-chain (C28-C32) alkyl diols are widely distributed in marine and lacustrine sediments and recently used as paleoenvironmental indicators. In particular, C32 1,15-diol is considered to originate from freshwater eustigmatophytes and has been proposed as indicator of freshwater input in coastal area (Lattaud et al., 2016). In the present study, we conducted a biomarker analysis of a sediment core from the Lake Nakaumi to evaluate how the algal biomarkers long-chain alkenones and alkyl diols record the changes in algal assemblages and environments in the brackish lake.
The sediment core was collected from central area of the lake (Nk3C) during May 2017. The ages were determined by Cs and Pb isotopes, and the lowermost horizon was estimated to be about 600 years ago. Biomarker analysis was performed as follows; freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column chromatography. Resulted ketone-containing and alcohol-containing fractions were analyzed by GC-MS and GC-FID. The Lake Nakaumi, has been strongly affected by anthropogenic rearrangements during the high economic growth in Japan. Perticularly, the land reclamation and desalination projects in the 1960s to 1980s are known to have had a significant impact on the ecosystem.
The long-chain alkenones and alkyl diols were detected in all the samples from the Nk3C core. The alkenone compositions were clearly different between the samples above and below the horizons affected by the reclamation and desalination project during 1960s. Below the horizons, C37:2 alkenones were dominant and C40 homologues was absent. On the other hand, C37:4 alkenones and C40 alkenones were relatively abundant above the horizons. We presume that the main alkenone-producers in sediments below and above the horizons in 1960s were typical marine (Group III) and lacustrine species (Group II), respectively. The compositions of long-chain alkyl diols are also different between the sediments above and below the horizons in 1960s; C30 1,15-diol, common alkyl diol produced by marine eustigmatophyte, was predominant below the horizons, while C32 1,15-diol, biomarker for freshwater eustigmatophyte, was dominant above the horizons in 1960s. From these results, it is suggested that typical marine species of haptophyte and eustigmatophyte inhabit shallow marine environment in the Lake Nakaumi by the middle 20th century, and subsequently, the freshwater species flourished as the dominant producers under brackish environment in the lake after the anthropogenic rearrangements during 1960s. Furthermore, the paleo-temperature (T) calculated by using lacustrine-specific alkenone unsaturation index (UK’’37; Zheng et al., 2016) gradually increased through the horizon that alkenone-producing species were converted, whereas the common index (UK37)-based T showed unexpected change (sudden 5ºC inclease). Although the resolution of the biomarker data is still low, the variations in UK’’37-based T appear to be broadly consistent with historical record of climate-related events such as the four great famines during the Edo Period and the long-term divisions of climatic periods after 17th century. Thus, the algal biomarker proxies can be expected to apply to reconstructing paleoenvironmental, moreover paleoclimatic variations in lacustrine sediments.
The sediment core was collected from central area of the lake (Nk3C) during May 2017. The ages were determined by Cs and Pb isotopes, and the lowermost horizon was estimated to be about 600 years ago. Biomarker analysis was performed as follows; freeze-dried sediments were extracted by solvents and then separated into fractions by silica-gel column chromatography. Resulted ketone-containing and alcohol-containing fractions were analyzed by GC-MS and GC-FID. The Lake Nakaumi, has been strongly affected by anthropogenic rearrangements during the high economic growth in Japan. Perticularly, the land reclamation and desalination projects in the 1960s to 1980s are known to have had a significant impact on the ecosystem.
The long-chain alkenones and alkyl diols were detected in all the samples from the Nk3C core. The alkenone compositions were clearly different between the samples above and below the horizons affected by the reclamation and desalination project during 1960s. Below the horizons, C37:2 alkenones were dominant and C40 homologues was absent. On the other hand, C37:4 alkenones and C40 alkenones were relatively abundant above the horizons. We presume that the main alkenone-producers in sediments below and above the horizons in 1960s were typical marine (Group III) and lacustrine species (Group II), respectively. The compositions of long-chain alkyl diols are also different between the sediments above and below the horizons in 1960s; C30 1,15-diol, common alkyl diol produced by marine eustigmatophyte, was predominant below the horizons, while C32 1,15-diol, biomarker for freshwater eustigmatophyte, was dominant above the horizons in 1960s. From these results, it is suggested that typical marine species of haptophyte and eustigmatophyte inhabit shallow marine environment in the Lake Nakaumi by the middle 20th century, and subsequently, the freshwater species flourished as the dominant producers under brackish environment in the lake after the anthropogenic rearrangements during 1960s. Furthermore, the paleo-temperature (T) calculated by using lacustrine-specific alkenone unsaturation index (UK’’37; Zheng et al., 2016) gradually increased through the horizon that alkenone-producing species were converted, whereas the common index (UK37)-based T showed unexpected change (sudden 5ºC inclease). Although the resolution of the biomarker data is still low, the variations in UK’’37-based T appear to be broadly consistent with historical record of climate-related events such as the four great famines during the Edo Period and the long-term divisions of climatic periods after 17th century. Thus, the algal biomarker proxies can be expected to apply to reconstructing paleoenvironmental, moreover paleoclimatic variations in lacustrine sediments.