11:15 〜 11:30
[MIS02-02] Changes in and marine primary production recorded in algal biomarker compositions in sediments from the Miocene Kawabata Formation, Hokkaido, Japan
キーワード：Primary productivity, Diatoms, Dinsoflagellates, Organic geochemistry, Biomarkers, Temperature
Changes in marine primary production recorded in algal biomarkers in sediments from the Miocene Kawabata Formation, Hokkaido, Japan
Muhammad Adam bin Ismail1, Satoshi Furota2, Kazuki Okano1, Ken Sawada1Department of Earth and Planetary Science, Graduate School of Science, Hokkaido UniversityInstitute for Geo-Resources and Environment, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)
The global Miocene Cooling Trend (15Ma to 10Ma) created a change in global temperatures as well as a shift in global primary production as indicated by δ18O ratios and changes in the accumulation rates of silica respectively. During this period, the Japan Sea was also evolving and allowed the influx of cooler nutrient-rich waters into the Japan Sea. This combination created a good environment for the increase in diatom-derived marine primary productivity marked by increases in biogenic silica during that period (Tada, 1994). The Soumokumaisawa-gawa River succession, a part of the Miocene Kawabata Formation, was deposited during the Miocene Cooling Trend and the evolution of the Japan Sea, creating an ideal location for studying marine primary productivity during that period. Diatoms, having cell walls composed of opaline silica, may not have been preserved as complete fossils, but algal-derived biomarkers such as steranes, higher branched isoprenoid (HBI) n-alkanes and thiophenes, and the dinoflagellate-derived dinosteranes have been preserved. Thus, the objective of this study is to evaluate changes in marine primary productivity by conducting marine algal, specifically diatom- and dinoflagellate-derived biomarker analysis on the Soumokumaisawa-gawa mudstones deposited during 15Ma to 13Ma.
The Miocene Kawabata Formation in Yubari, central Hokkaido, Japan consists of basin-filled, slope-apron turbidite systems exposed along the Soumokumaisawa-gawa River, Yubari. Mudstones were collected from the Soumokumaisawa-gawa River succession for biomarker analysis. Lipids were extracted with dichloromethane / methanol and separated to aliphatic and aromatic fractions. GC-MS was used for identification and quantification of biomarkers.
Based on the preference of diatoms for higher nutrient conditions, and the preference of dinoflagellates for lower nutrient conditions, three zones of differing dominance between diatoms and dinoflagellates were identified. In the older sediments, diatom-derived biomarkers such as the HBI ratios are low while dinoflagellate biomarkers, dinosteranes, are high. Afterwards, dinosteranes decreased while HBIs increased, indicating a shift from dinoflagellate dominance to diatom dominance, and lastly, in the younger sediments, dinosteranes remained low while HBIs remained high. These results indicate a persistent nutrient-rich environment and a dominance of diatom-derived primary production. Some studies demonstrated that Rhizosolenia requires high amounts of silicate because of their large silica skeletons (e.g. Sinninghe Damste et al., 2004). Thus, we interpret that the elevated values of diatom-derived biomarkers might be attributed to increase of nutrient and silicate supplies to the surface waters around Hokkaido. The timing of 15 to 14 Ma also coincides with the onset of siliceous ooze deposition in the North Pacific. In addition to cooler climates and higher nutrient influx promoting the growth and expansion of diatoms, the shallowing of the Kawabata Formation may have also allowed the transport of more terrestrial-derived nutrients into the area, further promoting the growth of diatoms.
In conclusion, due to the cooler climate caused by the Miocene Cooling Trend, the influx of cooler, nutrient-rich waters into the Japan Sea, and shallowing of the Kawabata Formation, dinoflagellate-derived primary production was replaced by diatom derived primary production.
Muhammad Adam bin Ismail1, Satoshi Furota2, Kazuki Okano1, Ken Sawada1Department of Earth and Planetary Science, Graduate School of Science, Hokkaido UniversityInstitute for Geo-Resources and Environment, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)
The global Miocene Cooling Trend (15Ma to 10Ma) created a change in global temperatures as well as a shift in global primary production as indicated by δ18O ratios and changes in the accumulation rates of silica respectively. During this period, the Japan Sea was also evolving and allowed the influx of cooler nutrient-rich waters into the Japan Sea. This combination created a good environment for the increase in diatom-derived marine primary productivity marked by increases in biogenic silica during that period (Tada, 1994). The Soumokumaisawa-gawa River succession, a part of the Miocene Kawabata Formation, was deposited during the Miocene Cooling Trend and the evolution of the Japan Sea, creating an ideal location for studying marine primary productivity during that period. Diatoms, having cell walls composed of opaline silica, may not have been preserved as complete fossils, but algal-derived biomarkers such as steranes, higher branched isoprenoid (HBI) n-alkanes and thiophenes, and the dinoflagellate-derived dinosteranes have been preserved. Thus, the objective of this study is to evaluate changes in marine primary productivity by conducting marine algal, specifically diatom- and dinoflagellate-derived biomarker analysis on the Soumokumaisawa-gawa mudstones deposited during 15Ma to 13Ma.
The Miocene Kawabata Formation in Yubari, central Hokkaido, Japan consists of basin-filled, slope-apron turbidite systems exposed along the Soumokumaisawa-gawa River, Yubari. Mudstones were collected from the Soumokumaisawa-gawa River succession for biomarker analysis. Lipids were extracted with dichloromethane / methanol and separated to aliphatic and aromatic fractions. GC-MS was used for identification and quantification of biomarkers.
Based on the preference of diatoms for higher nutrient conditions, and the preference of dinoflagellates for lower nutrient conditions, three zones of differing dominance between diatoms and dinoflagellates were identified. In the older sediments, diatom-derived biomarkers such as the HBI ratios are low while dinoflagellate biomarkers, dinosteranes, are high. Afterwards, dinosteranes decreased while HBIs increased, indicating a shift from dinoflagellate dominance to diatom dominance, and lastly, in the younger sediments, dinosteranes remained low while HBIs remained high. These results indicate a persistent nutrient-rich environment and a dominance of diatom-derived primary production. Some studies demonstrated that Rhizosolenia requires high amounts of silicate because of their large silica skeletons (e.g. Sinninghe Damste et al., 2004). Thus, we interpret that the elevated values of diatom-derived biomarkers might be attributed to increase of nutrient and silicate supplies to the surface waters around Hokkaido. The timing of 15 to 14 Ma also coincides with the onset of siliceous ooze deposition in the North Pacific. In addition to cooler climates and higher nutrient influx promoting the growth and expansion of diatoms, the shallowing of the Kawabata Formation may have also allowed the transport of more terrestrial-derived nutrients into the area, further promoting the growth of diatoms.
In conclusion, due to the cooler climate caused by the Miocene Cooling Trend, the influx of cooler, nutrient-rich waters into the Japan Sea, and shallowing of the Kawabata Formation, dinoflagellate-derived primary production was replaced by diatom derived primary production.