25th International Meeting on Organic Geochemistry IMOG 2011

P-459
Long-term variations of palaeovegetation recorded by
angiosperm and gymnosperm biomarkers in the Late
Cretaceous sequence of Kotanbetsu, Hokkaido, Japan
Hideto Nakamura 1 , Ken Sawada 1 , Reishi Takashima 2
1 Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapppro, Japan, 2 Tohoku
University Museum, Sendai, Japan (corresponding author:hideton@mail.sci.hokudai.ac.jp)
The Cretaceous is an interval of exceptional
interest for understanding the early evolution and
expansion of angiosperm. The reconstruction and
ecological interpretation of the Cretaceous flora,
however, are far from complete because of some
disadvantages of classical palaeobotanical and
palynological studies (e.g. Rare occurrence of
morphologically well-preserved fossils). Meanwhile,
terrestrial plant biomarkers preserved in ancient
sediments have been proposed as useful tool for
reconstruction of palaeovegetational and
palaeoclimatic conditions (e.g. [1, 2]). However, there
are few biomarker studies focused on reconstruction
of the early evolution of angiosperms [3, 4].
Palynological studies reports that angiosperm pollen
explosively advanced in diversity in the Late
Cretaceous sequences in Japan [5]. Thus, we
analyzed terrestrial plant biomarkers from sediments
of the Lower Cenomanian to the Lower Campanian of
the Yezo group, Hokkaido, Japan, in order to
reconstruct palaeovegetation and to examine the
applicability for its indicator.
Mudstones, sandy siltstone and sandstone
samples were collected from the middle-upper part of
the Yezo Group exposed along the Kotanbetsu River
and its tributary in northern Hokkaido, Japan. The
high-resolution litho-, bio-, and carbon-isotope
stratigraphy of studied section have been established
in [6], and the samples range from the latemost Albian
to the Early Campanian.
The maturity indices ������������� of
hopanes show gradual decrease toward the bottom of
the sequence (0.49–0.05), while 20S/(20S+20R) of
C29 steranes show low values (