5:15 PM - 6:45 PM
[MIS12-P21] Alkenone palaeothermometry for the southern margin of the Sea of Japan during the Pleistocene Calabrian period
Keywords:Quaternary, Omma Formation, Alkenone palaeothermometry
The Pleistocene Calabrian is known to be a period dominated by a 41,000-year glacial-interglacial cycle. The Omma Formation is a marine strata distributed through Kanazawa City, Ishikawa Prefecture to Oyabe City, Toyama Prefecture, and its age is identified as Pleistocene Calabrian based on microfossils and paleomagnetism chronostratigraphies. Studies on molluscan shell fossils, foraminifers and other microfossils, and paleomagnetism have been conducted mainly through the middle part of the Omma Formation. Based on previous studies, a glacial-interglacial cycles are evident in this stratigraphic interval, especially with the evidences of the grain size variation, molluscan fauna, and variation of relative abundance of Globigerinoides ruber suggesting inflow of the Tsushima Warm Current. On the other hand, cyclicity derived from glacial-interglacial cycles in the lower part of the Omma Formation is not evident, and no paleotemperature through the depositional period.
We focus on alkenones, biomarkers of haptophytes. The abundance ratio of alkenone molecules with 37 carbons and with two and three double bonds (UK’37) is known to indicate a linear relationship with surface water temperature, and is often applied on Quaternary oceanic sediments for paleothermometry. This study tries to apply the alkenone paleothemometry to the lower part of the Omma Formation, the sedimentary sequence exposed on land. Alkenone was detected in 32 out of 33 samples. Influence of diagenetic alteration of the molecules based on isomerization confirmed that the samples are suitable for the paleotemperature estimation based on UK’37. Paleotemperature variation between 19.7℃ and 27.0℃ was observed through the lower part of the Omma Formation. As UK’37 is known to reflect SST of mid-June based on core-top studies along the southern rim of Japan Sea, two possibility is suggested: SST in mid-June during the Calabrian was up to 8℃ higher than the present SST, or the season of the alkenone production had been different from the present season. Stratigraphic variation of paleo-SST reconstructed with alkenones is basically consistent with the glacial and interglacial variation estimated by previous studies. Further studies that focus on shell beds are required for detecting exact SST during glacial period.
We focus on alkenones, biomarkers of haptophytes. The abundance ratio of alkenone molecules with 37 carbons and with two and three double bonds (UK’37) is known to indicate a linear relationship with surface water temperature, and is often applied on Quaternary oceanic sediments for paleothermometry. This study tries to apply the alkenone paleothemometry to the lower part of the Omma Formation, the sedimentary sequence exposed on land. Alkenone was detected in 32 out of 33 samples. Influence of diagenetic alteration of the molecules based on isomerization confirmed that the samples are suitable for the paleotemperature estimation based on UK’37. Paleotemperature variation between 19.7℃ and 27.0℃ was observed through the lower part of the Omma Formation. As UK’37 is known to reflect SST of mid-June based on core-top studies along the southern rim of Japan Sea, two possibility is suggested: SST in mid-June during the Calabrian was up to 8℃ higher than the present SST, or the season of the alkenone production had been different from the present season. Stratigraphic variation of paleo-SST reconstructed with alkenones is basically consistent with the glacial and interglacial variation estimated by previous studies. Further studies that focus on shell beds are required for detecting exact SST during glacial period.