Sci. Rep. Kanazawa Univ.
Vol. 54, pp.49-62, 2010                             

49High resolution carbon isotope stratigraphy across the
Cenomanian/Turonian boundary in the Tappu area,
Hokkaido, Japan: correlation with worldreference sections
1

4

Takashi HASEGAWA, 2Sohhei SEO, 3Kazuyoshi MORIYA,
Yoshito TOMINAGA, 4Toshifumi NEMOTO and 4Takahiro NARUSE

1. School of Natural System, Institute of Science and Engineering, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192 Japan
Email: jh7ujr@kenroku.kanazawa-u.ac.jp
Phone: +81-76-264-6508, Fax: +81-76-264-6545
2. Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192 Japan
3. Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences,
Waseda University, Tokyo 169-8050 Japan
4. Graduate School of Natural Science and Technology, Kanazawa University
Kakuma-machi, Kanazawa 920-1192 Japan(Received March 2, 2010 and accepted in revised form March 11, 2010)

Abstract High resolution carbon isotope stratigraphy encompassing the
Cenomanian/Turonian boundary in the Tappu section, Hokkaido, Japan enables
correlations to be made with Eastbourne and Pueblo world reference sections. The
organic carbon in the sedimentary rocks of the Yezo Group is derived
predominantly from terrestrial higher plants. The stratigraphic profiles of carbon
isotope values in the Tappu section show remarkable variations. A brief and small
peak in the lower part, an extended plateau-shaped peak with larger magnitude in
the upper part, and a trough dividing the two peaks observed in the Tappu section
are well known patterns for the isotope profiles of the reference sections. Close
similarity of the patterns among these regions validates a precise correlation of theJapanese Cenomanian/Turonian boundary sequence with the international standard
reference sections in North Atlantic and Western Interior Sea regions.
Key words: Cenomanian, Turonian, OAE, Carbon isotopes, Cretaceous, Organiccarbon50     

C-isotope stratigraphy across the C/T boundary in Japan

Introduction
Cretaceous oceanic anoxic events (OAEs) are well known oceanic events typically
represented by organic-rich dark sediments (e.g. Arthur and Premoli Silva 1982, Arthur etal. 1987, Schlanger et al. 1987). During OAEs, large quantities of organic carbon havebeen deposited and have not been recycled because of expanded anoxic bottom water
masses (Schlanger and Jenkyns 1976, Arthur and Schlanger 1979). We can interpret an
OAE as a short-term departure from the steady state of carbon cycling that fluctuates witha wavelength of several million years (Kump and Arthur 1999). The OAE recorded acrossthe Cretaceous Cenomanian/Turonian (C/T) boundary is called Oceanic Anoxic Event 2(OAE2; Arthur and Schlanger 1979) and is known for its wide-reaching oceanic anoxia
that affected most of the world (Arthur et al. 1987, Schlanger et al. 1987). The intensity of
OAE2 is believed to have been the strongest among the Cretaceous OAEs in terms of its
oxygen deficiency and global extent (e.g. Schlanger et al. 1987, Arthur and Schlanger1979). A positive 2-3 ‰ excursion of carbon isotope ratios (G13C) derived from theperturbation of carbon cycling characterizes the OAE2 well, and it often is employed as
an interregional chemostratigraphic marker (e.g. Hasegawa 1997, Paul et al. 1999, Pratt
1985, Voigt et al. 2007). Carbonate carbon has been employed for isotope studies forpelagic carbonate sequences in Tethys, the North Atlantic and the Western Interior Sea ofNorth America; whereas, terrestrial organic carbon is the target material for clastic
sequences of the Western Pacific (e.g. Hasegawa 1997, Hasegawa et al. 2003).
Recent progress in C-isotope stratigraphy in European sections shows a detailed
pattern of OAE2 fluctuation through the uppermost Cenomanian (Paul et al. 1999, Voigt
et al. 2007). Previous studies of C-isotope stratigraphy on the Yezo Group along the
Kanajiri River (Tappu section) showed a conspicuous positive excursion across the C/Tboundary (Hasegawa and Saito 1993, Uramoto et al. 2009). At the Hasegawa Laboratoryof Kanazawa University, we have studied C-isotope stratigraphy across the C/T boundary
through the Tappu section and published preliminary results (Moriya et al. 2008). We
could correlate the section with the reference section at Dover, UK, and with the highmass accumulation rates (~400 m/myr) of the Tappu section, we were able to achieve
higher stratigraphic resolution. From a global perspective, high resolution C-isotope
stratigraphy, along with detailed correlations with the world reference sections, isfundamental for investigations on OAE2 from the Yezo Group.
The objective of this study is to establish the C-isotope stratigraphy through therange equivalent to the OAE2 interval with the world’s highest resolution from Japaneseland sections, and to describe the isotope profiles. Basic lithology near the C/T boundary
also is described.
Materials and methods
Stratigraphically dense samples with stratigraphic intervals of 2-5 m along theFig. 1

Index map showing the study area along the Kanajiri River, a tributary of the
Obirashibe River in the Tappu area, Obira Town, Hokkaido, Japan.

44N

Fig. 1

Tappu area

Tappu area

O
s
ra
bi
be
hi

Study area

r
ve
Ri

500 m

Kana

jiri River

Index map showing the study area along the Kanajiri River, a tributary of the
Figure 1 Hasegawa et al.
Obirashibe River in the Tappu area, Obira Town, Hokkaido, Japan.

142E

T. Hasegawa et al.
51

࿑㧝㧔fig 1㧕ࠍᝌ౉ ࠠࡖࡊ࡚ࠪࡦ❑ᦠ߈52     

C-isotope stratigraphy across the C/T boundary in Japan

Kanajiri River in the Tappu area, Hokkaido, Japan (Fig. 1) were collected from the lower
part of the Saku Formation (Takashima et al. 2004) (Fig. 2). The lower part of the section
is characterized by mudstone to sandy mudstone. Common intercalations of fine to veryfine sandstone layers of distal turbidites are observed through 120 m from the base of thecolumn. The upper part of the section is mainly composed of bioturbated sandy mudstone
with occasional intercalation of thin, very fine sandstone to sandy mudstone layers. Aninterval of about 3.5 m of coarsening with indurated, stratified, very fine sandstone tosandy siltstone was observed around 270 m in the upper part. The C/T boundary marker
bed described below is located about 0.5 m below this interval. Tuff layers thicker than 20
cm are common in the upper part of the section. See Moriya et al. (2008) for details of thelithology of the study area.
Organic compositions of 16 selected mudstone samples (indicated with * on Table 1
and with light-blue circular highlights in Fig. 2) were checked by visual observation of
kerogen under reflected light and in fluorescent light. Crushed mudstone material was
made into polished blocks following standard preparation procedures (Bustin et al. 1983).
Polished pellets were examined under the microscope to identify organic particles.
Powdered mudstones were obtained from fresh rock surfaces using a dental grinder
and were treated with 5M HCl for 12 hours to remove carbonate minerals. After repeatedrinsing with deionized water to remove Cl-, each neutralized sample was dried andapproximately 2-4 mg of the sample was placed in a tin film cup and weighed. The
samples were then introduced into a Thermo Finnigan elemental analyzer (EA) Flash1112 at the Center for Advanced Marine Core Research of Kochi University. Each sample
was combusted at 900 ºC and the sample-derived CO2 was then transferred into an
EA-connected Delta Plus Advantage mass spectrometer for G13C analyses. The results
reported herein were obtained using reference CO2 calibrated by ANU-sucrose directly
and NBS-19 indirectly. Each data point is an average of multiple (more than three)analyses for each sample, and is expressed in the conventional delta notation with respect
to the VPDB standard, where G13C(‰)=[(13C/12C)sample/ (13C/12C)standard-1]X1000. The
isotopic values were checked by isotopically well-characterized laboratory standards.
Repeated analysis of a laboratory standard indicates ±0.2‰ as the instrumental
reproducibility for a single analysis.

Results
The composition of kerogens in all selected samples shows the predominance of
semifusinite or vitrinite, from terrestrial origins. Associated with these kerogens, minor(<1 %) content of cutinite and liptinite, including sporinite and possible alginite with
weak yellow fluorescence, was observed in some samples.
All G13C values obtained in this study are summarized in Table 1. The G13C values
are consistent around -24.7‰ at the lower part of the section, between 43.6 m (KZZ-016)

53

Turonian

T. Hasegawa et al.

CenomanianOccurrence ofRotalipora
brotzeni     

࿑㧞㧔fig 2㧕ࠍᝌ౉ ࠠࡖࡊ࡚ࠪࡦᮮᦠ߈
Fig. 2 Geologic column and carbon isotope stratigraphy through the studied section. Data
points highlighted by light-blue circles indicate samples on which microscopic
observations were performed to corroborate the terrestrial origin of organic matter.
Note rapid positive shift around 150 m and negative shift-back around 175 m.
Occurrence of planktonic foraminifera (Rotalipora brotzeni) is indicated with anarrow. The species does not indicate latest Cenomanian dates. Details of theforaminiferal study will be published elsewhere.

54     

SampleKZZ064
KZZ063*
KZZ062
KZZ061
KZZ060*KZZ059KZZ058KZZ057
KZZ056*
KZZ055
KZZ054
KZZ052
KZZ051*
KZZ049KZZ048KZZ047
KZZ045
KZZ044*
KZZ043
KZZ042
KZZ039KZZ038KZZ033
KZZ031
KZZ025
KZZ019KZZ016
KZZ004KZZ003
KZZ001

C-isotope stratigraphy across the C/T boundary in Japan

Thickness13
separation ǅ C org.
from KZZ- (‰㻌VPDB)
001 (m)
154.9
152.2
149.5
146.9
146.1
143.7
140.9
140.8
138.9
136.6
134.9
131.5
129.6
123.4
120.9
119.0
113.3
111.6
108.9
106.2
98.5
95.7
83.2
77.9
63.9
50.3
43.6
3.0
2.7
0.0

-23.09-22.89-24.02-23.83-25.26-24.94-24.25-24.90-24.76-24.66-24.69-24.81-24.55-24.82-25.52-24.93-24.97-24.20-24.86-24.55-24.43-24.44-24.71-24.35-24.79-24.54-24.96-25.48-24.25-25.68SampleKZZ096
KZZ095
KZZ094
KZZ093*
KZZ092
KZZ091
KZZ090*
KZZ089*KZZ088
KZZ087
KZZ086
KZZ085
KZZ084
KZZ083
KZZ082*
KZZ081
KZZ080KZZ079KZZ078
KZZ077
KZZ076
KZZ073
KZZ072*
KZZ071
KZZ070KZZ069KZZ068
KZZ067
KZZ066
KZZ065*Thickness13
separation ǅ C org.
from KZZ- (‰㻌VPDB)
001 (m)
230.0
228.2
226.5
223.7
221.7
220.2
217.6
214.6
212.0
209.9
207.3
205.1
202.8
200.4
198.0
195.7
193.2
190.8
188.8
187.1
184.9
177.3
174.6
172.3
169.6
166.2
164.7
164.2
162.9
157.6

-22.92-23.21-22.76-22.48-22.71-23.25-24.77-22.78-23.37-22.76-22.59-22.49-23.05-23.29-22.56-23.25-24.51-23.68-23.77-23.88-24.09-24.49-24.09-24.37-24.01-23.20-23.28-23.46-23.21-23.62

SampleKZZ-142
KZZ-140
KZZ-139
KZZ-138
KZZ-135
KZZ-132
KZZ-131
KZZ-125
KZZ-121
KZZ-120
KZZ119*KZZ117*
KZZ116
KZZ115
KZZ114
KZZ113
KZZ112
KZZ111*
KZZ110KZZ109*
KZZ108
KZZ107
KZZ105
KZZ103
KZZ101*
KZZ100
KZZ099KZZ098
KZZ097

Thickness13
separation ǅ C org.
from KZZ- (‰㻌VPDB)
001 (m)
372.6
348.6
340.3
336.7
326.7
317.0
313.5
293.1
283.2
280.7
279.8
276.3
274.0
271.2
269.9
268.1
265.6
263.4
261.8
259.6
257.4
255.5
251.1
246.0
240.8
238.4
235.8
233.1
232.2

Each sample horizon is indicated in thickness separation from KZZ-001.
Sample names with astalisks indicate samples that are micriscopically observed for organic composition.

Table 1 Carbon isotope values of studied samples in the Tappu section, Hokkaido, Japan.

-24.38-23.29-22.61-22.83-22.86-23.24-22.80-23.02-22.47-22.63-22.90-23.55-23.34-23.18-22.78-22.64-23.04-23.10-22.77-22.47-22.61-22.94-23.24-23.30-22.81-23.00-23.62-22.69-22.24

T. Hasegawa et al.

55

and 146.1 m (KZZ-060). Then the values shift in the positive direction and reach -22.89‰
at 152m (KZZ-063). The values gradually shift back in the negative direction and reachthe lowest point at 177.3 m (KZZ-073). The trend reverses here, where the G13C values
start a gradual positive shift toward 198.0 m (KZZ-082). Once G13C values reach around-22.9‰, they remain similar through the extended range. Thus the trend is that of aplateau through the upper half of the studied section. However, at the top of the section,
the G13C values drop significantly and reach -24.38‰. Some horizons (i.e. at 2.7 m, 193.2
m and 217.6 m; KZZ-003, -080 and -090 respectively) have irregular positive or negative
spikes, larger than 1‰, which are not concordant with the general trend of the G13C curve.
The profile of carbon isotope values obtained in this study (Fig. 2) shows thecharacteristic double-peaked positive excursion of 2.5-3‰ with a dividing trough. A
relatively sharp peak near 150 m comprises the first peak, and a plateau-like stable range
constitutes a second peak. The trough drops to two-thirds to three-fourths of the value ofthe first peak. The second peak is four or five times more expanded in stratigraphic
thickness than the first peak or the trough. The maximum G13C value in this section wasrecorded within the second peak interval.

Discussion
The organic matter in each sample was derived predominantly from terrestrial woody
plants. All previous studies, which have analyzed organic components in mudstones of the
Yezo Group, including some along the Kanajiri River (Hasegawa 2001, Hasegawa andSaito 1993, Uramoto et al. 2009), show a predominance of material from terrestrial
origins, and a high marine organic content has never been reported. Therefore, the isotope
stratigraphy in this study reflects fluctuations of G13C of higher plants, whichphotosynthesized from atmospheric CO2 during the depositional period. Because
atmospheric CO2 is interpreted to have been C-isotopically equilibrated with dissolved
CO2 in sea surface water, the carbonate carbon precipitated from CO32-, which also was
C-isotopically equilibrated with the dissolved CO2, should show the same fluctuations(Hasegawa 1997, Hasegawa 2003).
Regardless of some irregular spikes, the general trend of the G13C curve clearly
exhibits a positive excursion composed of double peaks. The first peak reflects anapparent prompt onset, but the duration is brief and the peak value is not as positive asthat of the second peak. The peaks are comparable to a diagnostic feature for G13C  patterns of the OAE2 horizon (Paul et al. 1999, Pratt 1985, Snow et al. 2005), andindicate that our G13C profile can be correlated to the world reference sections (Fig. 3).Paul et al. (1999) subdivided the isotope profile across the C/T boundary into six
segments, which included the Pre excursion (PrE), First build-up (1BU), Trough interval(TI), Second build-up (2BU), Plateau (PL) and Recovery (RC). All of these phases can berecognized in the Tappu section, as mentioned by Moriya et al. (2008). In this study, we࿑㧟㧔fi 3㧕ࠍᝌ౉ ࠠ ࡊࠪ ࡦ❑⟎߈㧔એਅࠍ S iR

3 fi 3 dߩ߽ߩߣᏅߒ

2BU

PL

Fig. 3 Comparisons of carbon isotope profiles from world reference sections, specifically Eastbourne,
UK (Paul et al. 1999) and Pueblo, USA (Pratt 1985, Snow et al. 2005) and the Tappu section(this study). Note that all G13C phases (Paul et al. 1999) are recognized through the Tappu
section. The phase boundaries are alphabetically marked and tied between the sections.

❑⟎߈↪ࠠࡖࡊ࡚ࠪࡦ㧔࿑㧟㧕

CTB marker bed

Turonian
Cenomanian56     
C-isotope stratigraphy across the C/T boundary in Japan

T. Hasegawa et al.

57

could designate the correlated horizon of each phase boundary more precisely with ahigher resolution data set (lines a-e in Fig. 3). Sageman et al. (2006) established an orbital
time scale across the C/T boundary at the Pueblo section. They estimated that the durationof the carbon isotope excursion (from 1BU to near the top of PL) was 538 kyr.
Correlation between the Tappu and Pueblo sections enables us to extrapolate this duration
onto the Tappu section. It is inferred that the mean time resolution of our samples isgreater than 10 kyr.
Hasegawa and Saito (1993) studied C-isotope stratigraphy across the C/T boundary
along the Kanajiri River, and recognized double peak excursions of which the “first peak”
was composed of a single data point. Recently, Uramoto et al. (2009) studied the same
section, but did not collect samples from the stratigraphic range of the “first peak”(recognized by Hasegawa and Saito, 1993). Uramoto et al. (2009) plotted their original
data with data of Hasegawa and Saito (1993) to construct a C-isotope profile.
Consequently, the “first peak” is found in their profile and three peaks are recognizedwithin it. Conversely, we collected a planktonic foraminiferal species (Rotalipora
brotzeni) above this “first peak” horizon. There are two reasons why the “first peak”
observed by Hasegawa and Saito (1993) is not correlated to the real first peak in thereference section. These reasons are: (1) The Last Appearance Datum of this species islocated in the middle of the Rotalipora cushmani Zone (Caron 1985). (2) The first peakrecognized at the Eastbourne section is observed at the uppermost part of R. cushmani
Zone. Furthermore, our G13C data across the “first peak” horizon (corresponding to ~95 min this study) did not show any positive excursions. Therefore, we conclude that the single
data point that made the “first peak” in Hasegawa and Saito (1993) was erroneous. The
real first peak in the Tappu section is represented by samples KZZ-063-069 (152.2-166.2m) in this study, and appears to correspond to the second peak of Uramoto et al. (2009).
We tried to estimate the horizon of the C/T boundary in the studied section from theC-isotope stratigraphy as it could not be identified by biostratigraphy alone (Uramoto et al.
2009). The biostratigraphically defined C/T boundary at the Pueblo section appears to belocated at the top of the PL (Sageman et al. 2006) or within the PL (Keller et al. 2004,
Snow et al. 2005); whereas, it is located just above a ~0.4‰ positive excursion within the
PL in the Eastbourne section (Jarvis et al. 2006). In the Tappu section, we could not
identify the positive 0.4‰ excursion since there are four positive excursions within the
PL. Consequently, the C/T boundary could not be extrapolated based on C-isotope
stratigraphy, even though the “range” in which the real C/T boundary was located has
been narrowed. Nevertheless, we draw the provisional C/T boundary just above “CTB
(C/T boundary) marker bed”. The bed is stratigraphically located in the middle of the PL
and it is interpreted to be very close to the real C/T boundary (Fig. 3). The marker bed isconvenient for further studies across the C/T boundary along the Kanajiri River section
since the beds are very conspicuous, unique, and easy to recognize (Figs. 4, 5).

Turonian࿑㧠㧔fig 4㧕ᝌ౉ ࠠࡖࡊ࡚ࠪࡦ❑⟎߈㧔એਅࠍ SciRep_3_fig4.doc ߩ߽ߩߣᏅߒ
ᦧ߃ࠆ㧕

Sandy siltstone with occasional parallel black laminations.

Sand-dominated feather-like lamination.
Mud-dominated feather-like lamination of mudst. & m.s.s.

Sandy siltst. with patchy black silty parts.
Green mineral grains.
Green mineral grains.
Mud-dominated or equal content of sand & mud.
❑⟎߈↪ࠠࡖࡊ࡚ࠪࡦ㧔࿑㧠㧕
Parallel but discontinuous laminae.
Sand contains darkgreen mineral grains.

Bioturbated part

Amalgamatedturbidite units

Provisional C/T boundary

Coase siltst.
(Ichnofossils ~2-3 mm in diameter.)
Sandy siltst.
(Easily weathered into fine particles)
Coase siltst.
Sandy siltst. with patchy muds.
Uneven sandy siltst. with parallel laminae of sand grains.

Clear parallel lamination
Clear cross lamination

Parallel lamination
Compressed calcareous nodule (major axis: ~10 cm).
Calcified part (5 mm thick) at the bottom of a turbidite unit.

Sandy siltst.
Bioturbated sandst. (N10E50W)
Gray coase siltst.
White mud layer with ichnofossils.
Brownish red-colored compressed nodule bed.

Fig. 4 Lithology of “CTB (C/T boundary) marker bed”. A remarkable, characteristic sedimentary sequence isobserved in the middle of PL (“Plateau” phase of C- isotope profile). Stratigraphic position of the marker
bed is interpreted to be close to the real C/T boundary. A provisional C/T boundary along the Kanajiri River
is drawn just above a conspicuous, compressed nodular bed.

10cm

Cenomanian58     
C-isotope stratigraphy across the C/T boundary in Japan

59

T. Hasegawa et al.

a Provisional C/T boundary
Nodular bed

Close-up photo
below

b

Provisional C/T boundary
Nodular bed

Amalgamated turbidites࿑㧡㧔fig 5㧕ࠍᝌ౉ ࠠࡖࡊ࡚ࠪࡦᮮᦠ߈
Fig. 5

Outcrop of “CTB marker bed” described in Fig. 4. a) Entire sequence of “CTB marker
bed” and b) Close-up photo of amalgamated turbidites.

60     

C-isotope stratigraphy across the C/T boundary in Japan

Some negative and positive events observed in the middle of the PL are composed ofmultiple data points and appear to be meaningful events rather than errors or noises (Fig.
2). We suggest that smaller G13C events observed in the Eastbourne and Pueblo sections
could be correlatable to the Tappu section. Higher resolution studies in both of the worldreference sections and the Tappu section could increase tie points between the sections toestablish an orbital scale of global correlations.

Summary
Carbon isotope stratigraphy across the C/T boundary in the Yezo Group was studied
with higher resolution than any previous studies. As a result, the data set obtained in thisstudy enables us to correlate OAE2-corresponding intervals of the Pacific Tappu section
with those of well-studied European sections with five tie lines (Fig 3. a-e). Such precise
correlation, along with a description of the CTB marker bed, provides fundamental
information that is beneficial for further comparative OAE2 studies from a global view
point. LIP formation was discussed as a causal event of OAE2 from other sections of theworld, based on Pb and Os isotopes and other geochemical data with stratigraphic control
of detailed carbon isotope stratigraphy (Kuroda et al. 2007, Snow et al. 2005, Turgeon andCreaser 2008). On the other hand, Voigt et al. (2006) considered the extensive coolingderived from perturbation of carbon circulation during OAE2. The Pacific Tappu section
is an excellent section for international correlation and potentially provides much higher
stratigraphic resolution than counterpart world reference sections. Thus, this section may
provide a valuable reference for studying events associated with OAE2, and may help
identify their causal relationships within the OAE2 interval.
Acknowledgments
The authors express their deep appreciation to Prof. T. Kamiya of the Kanazawa
University for their constructive suggestions during the review process, and Dr. Y. Suzukiof the Geological Survey of Japan for his kind assistance and helpful suggestions during
microscopic observations at his laboratory. This study was performed under thecooperative research program of the Center for Advanced Marine Core Research (CMCR), Kochi University (Accept No.06B021, 07A008, 07B007). This study was
financially supported by Grants-in-Aid for Scientific Research nos. 18340164, 20403016,
20340144 from JSPS (to T. Hasegawa).

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