09:45 〜 10:00
[MIS03-04] Changes in primary productivity and terrestrial input of the Miocene Japan Sea recorded in the sediments of south-central Hokkaido, Japan
キーワード：Biomarkers, Organic Matter, Global Cooling, Asian Monsoon, Primary Productivity, Climate Change
The global Miocene Cooling Trend created a decline in climatic temperatures as well as a biogeochemical shift associated with global primary production, as indicated by δ18O ratios and changes in the accumulation rates of biogenic silica, respectively. During this period, the Japan Sea was also evolving and allowed the influx of cooler nutrient-rich waters. Concurrently, the East Asian Monsoon system also evolved, with climates generally becoming more arid in the East Asian region. This combination created a good environment for the increase in diatom-derived marine primary productivity (Tada, 1994). Precipitation rates also fell due to the weakening East Asian Summer Monsoon, which reduced precipitation and continental weathering (Clift et al., 2008).
The Miocene Kawabata Formation (Yubari, south-central Hokkaido, Japan) was deposited during the above climatic events, creating an ideal location for studying marine primary productivity variations, as well as variations in paleovegetation and terrigenous organic matter (OMterr) in response to changes in atmospheric temperature and aridity. Lipid biomarkers derived from both terrestrial higher plants as well as marine algae, can be preserved in sediments for a long period of time, allowing us to reconstruct changes in Earth’s biosphere throughout geological time, especially when microfossils such as diatoms have not been well preserved. Thus, the objective of this study is to evaluate changes in OMterr characteristics and the controls on marine primary productivity by conducting biomarker analysis on the Miocene Kawabata Formation.
The Miocene Kawabata Formation, deposited between 15 and 9 Ma, consists of basin-filled, slope-apron turbidite systems exposed along the Soumokumaisawa and Higashiyama rivers. Mudstones were collected from both areas 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.
Long chain n-alkanes derived from leaf waxes show a slight reduction in concentration in younger horizons, suggesting a reduction in OMterr delivery as precipitation and riverine discharge weakened. Gymnosperm-derived diterpenoids increase in concentration in the younger horizons and increase slightly in comparison to angiosperm-derived triterpenoids, suggesting in an increase in gymnosperm growth in the hinterland, possibly due to global cooling. However, n-alkane derived indices such as carbon preference index (CPI), average chain length (ACL) and n-C31/n-C27 ratios remain constant throughout the Kawabata Formation, suggesting that the effects of global cooling and aridity variations during this period may not have been intense in the Japanese Island between 15 and 9 Ma, generally mimicking those seen in the central Asia, where aridity strengthened after 10 Ma (Peng et al., 2016).
Diatom-derived highly branched isoprenoid (HBI) alkanes also increase in concentration in younger sediments. Statistical analysis shows a very poor correlation between HBIs and long-chain n-alkanes (R2= -0.12) while a better correlation is seen with lower pristane/phytane ratios and higher gammacerane concentrations at R2 = -0.66 and R2 = 0.55 respectively. These show that diatom growth was more dependent on upwelling of dysoxic/anoxic saline waters derived from the inflow of oxygen minimum zone (OMZ) waters rather than terrigenous derived nutrients. Fluctuations in diatom concentration could have been due fluctuations of the inflow of OMZ due to changing eustatic sea-levels (Tada, 1994).
In conclusion, the north Japan Sea showed a reduction in OMterr delivery and an increase in gymnosperm vegetation in the hinterland between 15 and 9 Ma, probably due reduced precipitation and cooler climates respectively with limited effects of aridity. Diatom growth was more dependent on upwelling of OMZ waters rather than terrigenous material.
The Miocene Kawabata Formation (Yubari, south-central Hokkaido, Japan) was deposited during the above climatic events, creating an ideal location for studying marine primary productivity variations, as well as variations in paleovegetation and terrigenous organic matter (OMterr) in response to changes in atmospheric temperature and aridity. Lipid biomarkers derived from both terrestrial higher plants as well as marine algae, can be preserved in sediments for a long period of time, allowing us to reconstruct changes in Earth’s biosphere throughout geological time, especially when microfossils such as diatoms have not been well preserved. Thus, the objective of this study is to evaluate changes in OMterr characteristics and the controls on marine primary productivity by conducting biomarker analysis on the Miocene Kawabata Formation.
The Miocene Kawabata Formation, deposited between 15 and 9 Ma, consists of basin-filled, slope-apron turbidite systems exposed along the Soumokumaisawa and Higashiyama rivers. Mudstones were collected from both areas 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.
Long chain n-alkanes derived from leaf waxes show a slight reduction in concentration in younger horizons, suggesting a reduction in OMterr delivery as precipitation and riverine discharge weakened. Gymnosperm-derived diterpenoids increase in concentration in the younger horizons and increase slightly in comparison to angiosperm-derived triterpenoids, suggesting in an increase in gymnosperm growth in the hinterland, possibly due to global cooling. However, n-alkane derived indices such as carbon preference index (CPI), average chain length (ACL) and n-C31/n-C27 ratios remain constant throughout the Kawabata Formation, suggesting that the effects of global cooling and aridity variations during this period may not have been intense in the Japanese Island between 15 and 9 Ma, generally mimicking those seen in the central Asia, where aridity strengthened after 10 Ma (Peng et al., 2016).
Diatom-derived highly branched isoprenoid (HBI) alkanes also increase in concentration in younger sediments. Statistical analysis shows a very poor correlation between HBIs and long-chain n-alkanes (R2= -0.12) while a better correlation is seen with lower pristane/phytane ratios and higher gammacerane concentrations at R2 = -0.66 and R2 = 0.55 respectively. These show that diatom growth was more dependent on upwelling of dysoxic/anoxic saline waters derived from the inflow of oxygen minimum zone (OMZ) waters rather than terrigenous derived nutrients. Fluctuations in diatom concentration could have been due fluctuations of the inflow of OMZ due to changing eustatic sea-levels (Tada, 1994).
In conclusion, the north Japan Sea showed a reduction in OMterr delivery and an increase in gymnosperm vegetation in the hinterland between 15 and 9 Ma, probably due reduced precipitation and cooler climates respectively with limited effects of aridity. Diatom growth was more dependent on upwelling of OMZ waters rather than terrigenous material.