Sci. Rep. Kanazawa Univ.
Vol. 55, pp.1–16, 2011

Testing significance of total organic carbon isotope
values for chronostratigraphy: Application to theCretaceous Yezo Group in Hokkaido, Japan
Yoshito T OMINAGA1 , Takashi H ASEGAWA2
1 Graduate School of Natural Science and Technology, Kanazawa University

Kakuma, Kanazawa 920-1192, Japan
Email: tominaga@stu.kanazawa-u.ac.jp
2 School of Natural System, Institute of Science and Engineering, Kanazawa University

Kakuma, Kanazawa 920-1192, Japan
Email: jh7ujr@staff.kanazawa-u.ac.jp(Received December 8, 2011 and accepted in revised form February 17, 2012)

Abstract To realistically interpret chronostratigraphic correlation based on carbon
isotope stratigraphy, we clarified deviation of total organic δ 13 C values within a single
sample, deviations of total organic δ 13 C values within identical horizons, and therelationship of δ 13 C values between total organic matter and wood fragments. The
deviation (2σ ) of δ 13 C values of total oraganic matter (δ 13 CTOM values) within asingle sample was 0.4‰. Maximum differences of δ 13 CTOM values from identicalhorizons were smaller than 0.4‰. Although cross-plots of δ 13 CTOM and δ 13 C valuesof wood fragments (δ 13 Cwood values) certify their proportional relationship, generally
negative values of δ 13 CTOM relative to the δ 13 Cwood were also evident. Differences
between δ 13 Cwood and δ 13 CTOM were between 0.2 and 1.9‰. Regarding this characterof δ 13 CTOM distribution, its stratigraphic variations are applicable for stratigraphic andcorrelation purposes. This result provides important criteria for application of δ 13 C
stratigraphy to the Yezo Group.
Keywords: Cretaceous, carbon isotope, chronostratigraphy, organic carbon

1 Introduction
The Cretaceous period (145.5-65.5 Ma) is generally considered warmer than other periods, compared with recent climate in particular (Huber et al., 2002). Knowledgeof Cretaceous ocean-climate systems provides important information about forthcoming global warming. To improve knowledge of temporal and spatial dimensions of2

Testing significance of total organic carbon isotope values for chronostratigraphyenvironmental changes in the Cretaceous world, international correlation for the Cretaceous System is prerequisite. The Cretaceous is currently subdivided into 12 stages
and 32 substages (Ogg et al., 2004; 2008). Global Boundary Stratotype Sections andPoints (GSSPs) of Cretaceous stages and substages are located in Europe and America. The GSSPs provide primary and secondary criteria for interregional correlation of
the stages (Kennedy et al., 2005).
International chronostratigraphic correlation of the Upper Cretaceous Yezo Group,
especially the Cenomanian through Coniacian, has been extensively addressed throughthe occurrence of marine macro-micro fossils (Matsumoto et al., 1991; Toshimitsu etal., 1995; Nishi et al., 2003). However, the occurrence of international zoned marker
fossil species or supplementary age-indicative species is generally rare or absent inUpper Cretaceous sequences of Japan.
On the other hand, stratigraphic fluctuations of carbon isotope values observed incarbonate sequences, as well as secular variation of the isotope value of marine andterrestrial organic carbon, have been used for global correlation (Jenkyns et al., 1994;
Jarvis et al., 2006; Voigt, 2000). Therefore, carbon isotope stratigraphy of total organic
matter has been established for the Yezo Group over the past decade (Hasegawa andSaito, 1993; Hasegawa, 1997, 2003a; Hasegawa et al., 2003; Uramoto et al., 2009).
Because organic matter in each sample was derived predominantly from terrestrial
plants in the previous studies, carbon isotope fluctuations of total organic matter havebeen interpreted as those of terrestrial plants. However, a minor part of the organiccarbon appeared to be marine algal and/or bacterial in origin, and an analytical result from bulk rock did not necessarily show carbon isotopic fluctuations of terrestrial
plants. Recently, Takashima et al. (2010) reported carbon isotope values of the Yezo
Group, but using a different technique. They extracted and picked wood fragmentsfrom each mudstone sample, and analyzed them instead of bulk mudstone. However,
the results occasionally showed large deviations when some adjacent horizons werecompared, suggesting the potential predominance of signals without correlational importance.
The objective of this study is to clarify how to discriminate δ 13 C global eventsin the Yezo Group from those representing regional, local or other sedimentologicalprocesses, for use in global correlation. Therefore, deviation of total organic δ 13 C
values within a single sample, deviations of total organic δ 13 C values within identicalhorizons, and the relationship of δ 13 C values between total organic matter and wood
fragments were clarified.

Y. Tominaga and T. Hasegawa

3

2 Geologic setting
The Yezo Group is exposed along the Horotate River in the Kotanbetsu area of Hokkaido,
Japan, and is interpreted as a forearc basin sequence (Okada, 1983). The group ismainly composed of hemipelagic and shallow marine mudstone and sandstone, and it
yields well-preserved, abundant marine macro and microfossils. The kerogen composition in the sediments of this group shows a remarkable predominance of terrestrial
woody kerogens over marine algal ones (Hasegawa, 2001).
Samples used for analyses were collected from the Saku and Haborogawa Formations of the middle-upper part of the Yezo Group exposed along the Horotate River
section (Figs. 1-3). The Saku Formation consists of alternating beds of sandstone andsiltstone. The Haborogawa Formation is mainly characterized by dark grey massive
siltstone, with intercalated sandstone. See Wani and Hirano (2000) for details of thelithology of the study area. The ages of the studied interval range from Cenomanian to Coniacian, based on macrofossil biostratigraphy and carbon isotope stratigraphy (Wani and Hirano, 2000; Tsuchiya et al., 2003; Hasegawa, 2003a; Takashima
et al., 2004). Occurrences of global age-diagnostic macrofossils such as Collignoniceras woollgari Mantell and Inoceramus pictus minus Matsumoto characterize the
Cenomanian-Turonian sequences (Wani and Hirano, 2000; Tsuchiya et al., 2003).

Figure 1: (a) Index map showing Kotanbetsu area of Hokkaido, Japan. (b) Location and geologic map of
study area along Horotate River, Kotanbetsu. Boxed area shows sampling area. Geologic map is modified
from Wani and Hirano (2000).

4

Testing significance of total organic carbon isotope values for chronostratigraphy

Figure 2: Geologic column and horizons of samples used for carbon isotope analyses and visual observation of kerogen. Assignment of stratigraphic units follows Wani and Hirano (2000). Macrofossil dataand stage boundary follow Wani and Hirano (2000) and Tsuchiya et al. (2003).

Y. Tominaga and T. Hasegawa

5

3 Samples
Fresh siltstone and sandy siltstone samples were collected from 39 horizons for the
Horotate section (Figs. 1-3). All studies of stable carbon isotope stratigraphy of the
Yezo Group have used siltstone and sandy siltstone samples (e.g., Hasegawa andSaito, 1993; Hasegawa, 1997, 2003a; Hasegawa et al., 2003; Uramoto et al., 2009;
Takashima et al., 2010).
For clarifying deviation of total organic δ 13 C values within a single sample, an
OL144 sample was collected (Fig. 2). Most samples were siltstones. Those sediment grain sizes are similar. However, the OL144 sample was sandy siltstone. Thosesediment sizes are mixed. Therefore, sandy siltstones like the OL144 sample are considered to have the largest variation in total organic δ 13 C values. Therefore, stablecarbon isotopic values of total organic matter (δ 13 CTOM ) within the OL144 (about 20
cm × 40 cm) sample were measured every about 2 cm (Fig. 4).
To make known deviations of total organic δ 13 C values within identical horizons,
28 samples (OMA010, OMA031, OMA041, OMA055, OMA061, OMA071,
OMA080, OMA090, OMA098, OMA104, OMA110, OMA123, OM051, OM063,
OM072, OM082, OM093, OM102, OM111, OM121, OM131, OM152, OM162,
OM171, OM182, OM191, OM201 and OM213) were collected (Fig. 3). The outcrops
from which these samples were extracted have the stable dip and strike of geologic
stratum. Therefore, it is possible to pursue identical horizons on both banks of the
Horotate River. Those identical horizons are about 6 m away horizontally from 4 m.
In order to clarify the relationship of δ 13 C values between total organic matterand wood fragments of terrestrial plants, 10 samples (THO031, THO057, THO104,
THO130, THO148, THO153, THO174, THO193, THO206 and THO230) were collected at 40 to 200 m stratigraphic intervals (Fig. 2). Moreover, nine of these samples(THO057, THO104, THO130, THO148, THO153, THO174, THO193, THO206 andTHO230) were used to show clearly that the organic matter in each sample was derived
predominantly from terrestrial plants.

4 Methods
To obtain the δ 13 C value of total organic matter, each siltstone sample was powderedwith a micro grinder, avoiding contamination from ichnolite and turbidite units. Then,the powdered samples were acidified by 1N and 5N solution of HCl to remove carbonates, then neutralized and dried. A small aliquot (3-5 mg) of the powder was weighedin a tin capsule and put in an elemental analyzer connected with a mass spectrometer,
to analyze the δ 13 C value in the same manner described below for analysis of wood
fragments. Some samples were analyzed with a mass spectrometer (Delta plus Advan-

6

Testing significance of total organic carbon isotope values for chronostratigraphy

Figure 3: (a) Map showing locations of samples used for clarifying lateral deviation of δ 13 C values
within a single horizon. (b) Geologic column and horizons of samples used for lateral deviation of δ 13 C
values within a single horizon.

Y. Tominaga and T. Hasegawa

7

Figure 4: (a) Photograph and line drawing of OL144 used for clarifying deviation of δ 13 C values withinsingle sample. (b) Photograph and line drawing of OL144 showing localities of subsamples for analyses.

8Testing significance of total organic carbon isotope values for chronostratigraphy

tage, Thermo Finnigan) in line with an elemental analyzer (Flash EA1112, Thermo
Finnigan) at the Center for Advanced Marine Core Research of Kochi University.
Each selected sample of about 150 g was treated to extract wood fragments, following the method of Takashima et al. (2010). All wood fragments larger than 63µ m were handpicked under a microscope. Then the wood fragments of each sample
were weighed in a tin capsule, put in the furnace of an elemental analyzer (NCS2500,
Thermo Quest at Kanazawa University) and burned at 1000◦ C under helium flow, toconvert organic carbon to CO2 . The CO2 was transferred to a mass spectrometer (Delta
V advantage, Thermo Electron at Kanazawa University) to analyze the δ 13 C value.
Both mass spectrometers used were inter-calibrated with a lab standard (triphenylamine of Indiana University) and international standard (ANU-sucrose). The results
reported herein were obtained using a reference CO2 gas calibrated by ANU-sucrose
directly and NBS-19 indirectly. Each data point was an average of multiple (morethan three) analyses for each sample, and expressed in the conventional delta notation
with respect to the VPDB standard, where δ 13 C (‰) = [(13 C/12 C) sample/(13 C/12 C)
standard − 1] × 1000. Isotopic values were checked by isotopically well-characterizedlaboratory standards. Repeated analysis of laboratory standards indicated instrumental
reproducibility to be ±0.1‰ for both mass spectrometers.
Organic compositions of selected samples 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 etal., 1983). Polished pellets were examined under the microscope to identify organic
particles.

5 Results
5.1 Deviation of δ 13 C values within a single sample
All δ 13 C values obtained are summarized in Table 1. Stable carbon isotopic values of
total organic matter (δ 13 CTOM ) within a single sample (OL144) were from −25.0 to−23.9‰, with an average of −24.5‰ (Fig. 5). No systematic difference associatedwith subsample localities within a lump of OL144 siltstone (Fig. 3) was observed.
Eighty-three percent of all data points from the OL144 sample were within −24.5‰± 0.2‰. If those values are based on the D’Agostino and Pearson K2 test (K2 = 5.3044< 5.991), it may be a normal distribution. The standard deviation (1σ ) of a normal
distribution is 0.2.

9

Y. Tominaga and T. Hasegawa

Sample name
THO031
THO057*
THO104*
THO130*
THO148*
THO153*
THO174*
THO193*
THO206*
THO230*
OL144aOL144b
OL144c
OL144dOL144e
OL144f
OL144g
OL144hOL144i
OL144j
OL144k
OL144lOL144m
OL144n
OL144oOL144p
OL144q
OL144r
OL144s
OL144t
OL144uOL144v
OL144w
OMA010OMA031OMA041OMA055
OMA061OMA071OMA080
OMA090
OMA098
OMA104
OMA110OMA123
OM051
OM063
OM072
OM082
OM093
OM102
OM111
OM121
OM131
OM152
OM162
OM171
OM182
OM191
OM201
OM213

δ13CTOM (‰ vs. VPDB䋩
-23.4
-22.5
-23.0
-24.8
-24.7
-24.6
-25.1
-26.3
-24.2
-25.0
-24.8
-24.7
-24.6
-24.6
-24.7
-24.6
-24.4
-24.7
-24.5
-24.5
-24.4
-23.9
-24.3
-24.2
-24.5
-24.4
-24.6
-24.7
-24.7
-24.4
-24.7
-24.4
-25.0
-25.0
-25.1
-25.2
-24.9
-24.9
-24.8
-24.9
-24.9
-25.1
-24.7
-24.5
-24.1
-24.9
-25.04-25.07-24.95-24.87-24.61-25.06-24.65-24.84-24.61-24.82-24.09-24.33-23.97-24.28-24.57

δ13Cwood (‰ vs. VPDB䋩
-22.8
-21.5
-21.1
-23.9
-23.5
-23.2
-24.6
-26.1
-23.4
-23.9
-

Stratigraphic level (m)
211.4
343.0
571.5
745.3
953.6
991.2
1194.21341.31462.11636.9-

Table 1: Carbon isotope values of studied samples from Horotate River, Kotanbetsu, Hokkaido, Japan.
Sample names with asterisks indicate samples that are microscopically observed for organic composition.

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Testing significance of total organic carbon isotope values for chronostratigraphy

Figure 5: Deviation of δ 13 C values within a single sample (OL144).

5.2 Lateral differences of δ 13 C values between identical stratigraphic
range
Two sequences of δ 13 C value of total organic matter within a stratigraphically limited(∼10 m) but identical range showed variations between −25.2 and −24.0‰ (Fig. 6).
Stratigraphic trends of the two sequences were positive upwards. This was especiallyclear in upper parts of the successions (from the 5 to 10 m stratigraphic level). The
sections showed a trend of 1‰ increase from bottom to top.
However, in lower parts of the successions (from the 2 to 5 m stratigraphic level),lateral differences of δ 13 C values were conspicuous. The maximum lateral differenceof δ 13 C values observed within a single horizon (about 3 m stratigraphic level from
OM051) was about 0.3‰.

5.3

Visual observation of total organic matter

The composition of kerogens in all selected samples shows the predominance of semifusinite, vitrinite or non-fluorescent amorphous kerogens of terrestrial origin. Alginiteof possible marine origin with weak yellow fluorescence was observed in all samples,
although it represented a small amount (5% or less of total organic matter).

5.4

δ 13 C values of total organic matter and handpicked wood fragments

Stable δ 13 C values of total organic matter (δ 13 CTOM ) from 10 selected samples were
between −26.3 and −22.5‰, with an average of −24.4‰. On the other hand, δ 13 C
values of bulk wood fragments (δ 13 Cwood ) from the same sample set were between−26.1 and −21.1‰, with an average of −23.4‰. δ 13 Cwood were generally heavier
than δ 13 CTOM in the studied samples.

Y. Tominaga and T. Hasegawa

11

Figure 6: Lateral differences of δ 13 C values within identical horizons.

6 Discussion
6.1 Relationship of δ 13 C values between total organic matter and wood
fragments
Previous studies have clearly documented that the majority of kerogens in siltstoneof Cenomanian and Turonian sequences in the Upper Cretaceous Yezo group originated from terrestrial higher plants; a high marine organic content has not been reported (Hasegawa, 1997; Tsuchiya et al., 2003; Yamamoto et al., 2006; Uramoto etal., 2009; Hasegawa et al., 2010). The organic matter in each sample was derived predominantly from terrestrial woody plants, but contributions from other sources werealso observed. Total organic matter contains minor amounts of cutinite, sporinite andamorphous kerogen that are derived from higher plant tissues, but not from lignin, asource of woody particles in the samples. This is a possible reflection of the fact thatδ 13 CTOM values are generally lower compared with δ 13 Cwood values, since organic
matter other than wood tissues (such as leaves) is known to have lighter δ 13 C values(Tyson, 1995). Differences between δ 13 Cwood and δ 13 CTOM were between 0.2 and
1.9‰.
Nevertheless, stratigraphic carbon isotope variations from the bulk analysis technique are believed to be applicable for stratigraphic/correlation purposes, since thecarbon isotope values of wood fragments and total organic matter in the same sample
are highly correlated (r = 0.96) (Fig. 7). The slope of the regression line is 1.2. The
standard error of the slope is 0.13. The intercept of the regression line is 6.43. The

12

Testing significance of total organic carbon isotope values for chronostratigraphy

standard error of the intercept is 3.17. This indicates that δ 13 C stratigraphies basedon both bulk organic matter and wood fragments are controlled by identical factors.
As a result, it is believed that carbon isotope fluctuations of total organic matter havebeen interpreted as those of terrestrial plants. These findings demonstrate that the bulk
analysis technique is adequate for carbon isotope stratigraphy of the Yezo Group. In
addition, the tested variation of δ 13 CTOM within a single rock lump and single horizon was very stable in the Horotate River section (most δ 13 CTOM differences from an
identical horizon were smaller than 0.4‰). This deviation is much smaller than thatreported for wood fragments. For example, Takashima et al. (2010) indicated deviations as large as 0.5-1.1‰ in δ 13 C values of wood fragments (> 120 m in size) inmudstones collected from the identical horizons in the Kotanbetsu River section, 4 km
from the Horotate River section. Similar results are reported in Yans et al. (2010).
Moreover, differences of δ 13 C values from neighboring horizons (vertically conjunct)obtained with the bulk rock analysis technique in this and previous studies (Uramoto
et al., 2009; Nemoto and Hasegawa, 2011) are considerably smaller than those produced by the method of picking wood fragments (Takashima et al., 2010). This isconsidered to likely originate from total organic matter becoming mixed; comparedwith wood fragments (of only huge sizes, especially ≥ 120 µ m), δ 13 CTOM values arestable compared with δ 13 Cwood values. The wood fragment technique is also reasonable. However, the bulk analysis aids acquisition of a stable dataset, and is especiallyadvantageous when closely-spaced, high-resolution samples are analyzed.

Figure 7: Cross-plots of δ 13 C values of total organic matter (TOM) and wood fragments from selected
samples.

Y. Tominaga and T. Hasegawa

13

Figure 8: Comparison of Upper Cretaceous δ 13 C profiles within Yezo Group. Data of the Obira area
are from Uramoto et al. (2007) and Uramoto et al. (2009). Data of Naiba area are from Hasegawa etal. (2003). Correlative carbon isotope fluctuations recognized by Uramoto et al. (2009) are connectedwith grey bands. Macrofossil data for Obira area are after Tanabe et al. (1977), Matsumoto et al. (1981),
Sekine et al.(1985) and Funaki and Hirano (2004). Red dashed lines are first occurrences of regional
marker inoceramids of the Yezo Group.

6.2

δ 13 C events in the Yezo Group available for correlation: A perspective based on results of this study

Based on comparison of δ 13 C curves established with total organic matter, Uramoto etal. (2009) selected some δ 13 C events for correlation of Cenomanian and Turonian sequences of the Yezo Group (and its equivalent of the Russian Far East) (Fig. 8). Theyevaluated their “tie zones” with intrabasinal biostratigraphy of regional marker inoceramids. However, most of the events employed for the correlation were represented by
few data points. As our study shows that correlation cannot be safely made unless theevents are larger than 0.4‰, up-dating of the correlation proposed by Uramoto et al.
(2009) is required. Carbon isotope stratigraphy based on δ 13 CTOM should be appliedwith caution about this deviation size (0.4‰) that is potentially non-correlative use.

7 Conclusions
To realistically interpret chronostratigraphic correlation based on carbon isotope stratigraphy, we tested potential uncertainty sequestered in a dataset of organic carbon isotope values with the bulk rock analysis method. The size of the potential signal derived

14

Testing significance of total organic carbon isotope values for chronostratigraphy

from factors other than “long-term secular variation of carbon isotope values of atmospheric CO2 reservoir” was determined to be 0.4‰ or smaller, for mudstone from theYezo Group of Hokkaido, Japan. Such “noise” does not affect recognition of long-term
secular variation of Late Cretaceous carbon isotope values, based on bulk rock analysis of organic carbon. Caution is required for event correlation when the magnitudesof δ 13 C events are 0.4‰ or smaller. The data here lead to the recommendation that
only δ 13 C events larger than 0.4‰ be used for rigid chemostratigraphic correlation.

Acknowledgements
The authors express their deep appreciation to the referees for their critical suggestions, which helped the authors improve the paper. We also thank Profs. T. Kamiya
and M. Kato of Kanazawa University for their constructive suggestions during groupseminars. Acknowledgments are also due to Dr. Y. Suzuki of the Geological Surveyof Japan for his kind assistance and helpful suggestions during microscopic observation at his laboratory. This study was financially supported by a research grant from
Fukada Geological Institute (awarded to YT), Grants-in-Aid for Scientific Research(Nos. 18340164, 20403016, 20340144, and 23340158) from the JSPS (awarded toTH). The study was performed under the cooperative research program of the Center for Advanced Marine Core Research (CMCR) of Kochi University (Accept No.
08A011, 08B009, 09A009, 09B009).

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