Strontium and carbon isotope stratigraphy of the Late Jurassic shallow marine limestone in western Palaeo-Pacific, northwest Borneo

Journal of Asian Earth Sciences 73 (2013) 57–67

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Strontium and carbon isotope stratigraphy of the Late Jurassic shallow marine limestone in western Palaeo-Pacific, northwest Borneo Yoshihiro Kakizaki a,⇑, Helmut Weissert b, Takashi Hasegawa c, Tsuyoshi Ishikawa d, Jun Matsuoka e, Akihiro Kano a a

Division of Earth Sciences, Graduate School of Social and Cultural Studies, Kyushu University, Fukuoka 819-0395, Japan Geological Institute, Department of Earth Sciences, ETH Zürich, CH-8092 Zürich, Switzerland Department of Earth Sciences, Faculty of Natural Systems, Institute of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan d Kochi Institute for Core Sample Research, Japan Agency for Marine Science and Technology, Nankoku 783-8502, Japan e Kochi Marine Research Support Section, Department of OD Science Technical Support, Marine Works Japan Ltd., Nankoku 783-8502, Japan b c

a r t i c l e

i n f o

Article history: Received 26 November 2012 Received in revised form 26 February 2013 Accepted 17 April 2013 Available online 28 April 2013 Keywords: Bau Limestone Oxfordian–Kimmeridgian Sr isotopic age Carbon isotope Palaeoceanograpy Palaeo-Pacific

a b s t r a c t Strontium and carbon isotope stratigraphy was applied to a 202 m-thick shallow marine carbonate section within the Late Jurassic Bau Limestone at the SSF quarry in northwest Borneo, Malaysia, which was deposited in the western Palaeo-Pacific. Strontium isotopic ratios of rudist specimens suggest that the SSF section was formed between the latest Oxfordian (155.95 Ma) and the Late Kimmeridgian (152.70 Ma), which is consistent with previous biostratigraphy. The d13Ccarb values of bulk carbonate range from 0.10 to +2.28‰ and generally show an increasing upward trend in the lower part of the section and a decreasing upward trend in the upper part of the section. A comparable pattern is preserved in the d13Corg isotope record. Limestone samples of the SSF section mainly preserve the initial d13Ccarb values, except for the interval 84–92 m, where an apparent negative anomaly likely developed as a result of meteoric diagenesis. Comparing with the Tethyan d13Ccarb profile, a negative anomaly in the lower SSF section can be correlated with the lowered d13C values around the Oxfordian/Kimmeridgian boundary. In addition, d13Ccarb values of the Bau Limestone are generally 1‰ lower than the Tethyan values, but comparable with the values reported from Scotland and Russia, located in Boreal realm during the Late Jurassic. This suggests that either the Tethyan record or the other records have been affected by the d13C values of regionally variable dissolved inorganic carbon (DIC). The Late Jurassic d13CDIC values are thought to have been regionally variable as a result of their palaeoceanographic settings. This study shows that d13C chemostratigraphy of the Palaeo-Pacific region contributes to an improved understanding of global carbon cycling and oceanography during this time period. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction During the Late Jurassic, carbonate deposition was actively expanded in shallow marine environments (e.g. Leinfelder et al., 2002; Philip, 2003). The stratigraphy and palaeontology of Upper Jurassic limestones have been studied in the Tethys Region, and subsequent studies during the last decade have demonstrated that chemostratigraphy based on carbon isotope geochemistry is a valuable stratigraphic tool (e.g. Padden et al., 2001, 2002; LouisSchmid et al., 2007a,b,c; Rais et al., 2007). These Tethyan studies outlined how high d13C values corresponded to accelerated carbon cycling with elevated organic carbon burial rates and probably elevated atmospheric pCO2 levels (e.g. Weissert and Mohr, 1996; Weissert and Erba, 2004; Weissert et al., 2008). However, in order ⇑ Corresponding author. Tel.: +81 92 802 5790; fax: +81 092 802 5791. E-mail address: [email protected] (Y. Kakizaki). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.04.020

to test the global trend of the carbon isotope record from Tethyan sections, carbon isotope stratigraphies from the Palaeo-Pacific are urgently needed (Fig. 1A). Palaeo-Pacific carbonates of Jurassic age outcrop in Japan as small lenticular bodies of less than 100 m in thickness; they were deposited in a fore-arc basin setting (Matsuoka, 1992). One example is the ‘‘Torinosu-type Limestone’’ in SW Japan (Fig. 1A: Kano et al., 2006) whose Kimmeridgean to Berriasian age was determined through radiolarian biostratigraphy (e.g. Yao et al., 1982; Kano, 1988; Matsuoka, 1992, 1995). Chronostratigraphic resolution was later improved within a 2.0 Myr age error, using Sr isotope stratigraphy (e.g. Shiraishi et al., 2005; Kakizaki et al., 2012). In these later studies, researchers realized the difficulty in acquiring continuous isotopic profiles from the small limestone bodies. In the present study, we have selected a limestone succession located in northwest Borneo (Sarawak, Malaysia) for the

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Torinosu-type Limestone (Japan)

A

Bau Limestone (Borneo Is.)

Palaeo-Pacific Ocean

B

116 E

112 E

Tethys Ocean

4N

Kuching

Studied shallow-marine carbonate

Paku

Kuching City Central

C

Equator

Borneo Is.

Kuching Airport

Bau

Sarawak Kiri

River

4S

Cenozoic

Jurassic - Cretaceous

Alluvial deposits

N

10km

Plateau Sandstone

Pedawan Formation (Lower Cretaceous)

Intrusive rocks

Bau Limestone

Pre-Jurassic

Kedadom Formation (Upper Jurassic)

Fig. 1. (A) Location of the Bau Limestone in the palaeogeography of the Late Jurassic, showing the distribution of shallow-marine carbonate platforms (Leinfelder et al., 2002) including Torinosu-type Limestones (Tazawa, 2004). Outline of Southeast Asia refers to Metcalfe (2011). Locations of Switzerland (white star; Padden et al., 2002), Scotland (black star; Nunn et al., 2009; Nunn and Price, 2010) and Russian platform (doubled diamond; Podlaha et al., 1998; Price and Rogov, 2009) are also shown for discussion. (B) Map of Borneo Island. (C) Geological map around the Kuching area (modified after Wilford, 1955) indicating the Paku limestone body of the Bau Limestone.

Pg

Eocene Paleocene

Cretaceous

LOWER UPPER

AGE

THICKNESS

LITHOFACIES

Plateau Sandstone Fm.

MAX 3900m

Sandstone, Conglomerate, Alternation

Pedawan Fm.

MAX 500m

Shale, Sandstone, Conglomerate, Limestone, Tuff

Kim.

Tit.

Reefal limestone, Pelagic limestone,

Bau Limestone Formation

Oxf.

UPPER

Jurassic

TYPE COLUMN

?

?

Pre-Jurassic

Kedadom Fm. Serian Fm. (Triassic)

Palaeozoic Plutonics

Ave. 300m

Limestone breccia,

(100-1000m)

Calcareous shale

MAX 300m

Sandstone, Conglomerate, Shale, Limestone, Acidic tuff Volcanic clasts Granodiolite

Fig. 2. General stratigraphy around the Kuching area, compiled from Wilford (1955), Wilford and Kho (1965), Wolfenden (1965), Kho and Wilford (1967), and Sato (1975). The following abbreviations are used for ages; Pg (Palaeogene), Oxf. (Oxfordian), Kim. (Kimmeridgian), and Tit. (Tithonian).

construction of a Palaeo-Pacific carbon-isotope curve. The investigated Bau Limestone (Fig. 1B) has a maximum thickness of >1000 m (Fig. 2; Wilford, 1955; Wolfenden, 1965; Kho and

Wilford, 1967). Based on fossil occurrences, the approximate depositional age of the Bau Limestone is estimated to be Late Jurassic to Early Cretaceous, but has not been specified to the stage level.

Y. Kakizaki et al. / Journal of Asian Earth Sciences 73 (2013) 57–67

Therefore, first objective of this study is to provide an improved stratigraphy by applying strontium isotope stratigraphic techniques. Secondly, we present a new carbon isotope stratigraphy of the Bau Limestone, and discuss aspects of palaeoceanography based on a comparison with the Tethyan-Boreal d13C profiles of comparative ages. 2. Geological setting and studied section The Bau Limestone is widely distributed across an area of 320 km2 around the town of Bau, located 30 km SW of Kuching city in northwest Borneo (western Sarawak, Malaysia; Fig. 1C). It conformably overlies the Upper Jurassic marine sandstone and conglomerate (Kedadom Formation), and underlies the Lower to Upper Cretaceous marine shale and sandstone (Pedawan Formation). These lithologic transitions are gradual and the two lithologies show an interfingering relationship (Fig. 2; Kho and Wilford, 1967; Sato, 1975). These Jurassic-Cretaceous sedimentary units unconformably overlie Triassic volcaniclastics (Serian Formation) and Palaeozoic plutonics, and they are intruded by quartz porphyry of Quaternary age (Fig. 2; Wilford, 1955; Wilford and Kho, 1965; Wolfenden, 1965; Kho and Wilford, 1967).

59

During the Late Jurassic and the Early Cretaceous, Borneo was located in the western Palaeo-Pacific, near the gateway of the Tethys (Fig. 1A). The area is of considerable palaeogeographical interest, and the Jurassic-Cretaceous units were subjected to palaeontological studies during the 1960s and 70s (e.g. Wilford and Kho, 1965; Hashimoto and Tamura, 1968; Lau, 1973; Tamura, 1973; Sato, 1975). Reported fossils from the Bau Limestone include some age-diagnostic fauna, such as Upper Jurassic foraminifera (Bayliss, 1966) and Oxfordian to lower Kimmeridgian brachiopods (Yanagida and Lau, 1978). The underlying Kedadom Formation yields Kimmeridgian ammonoids (Lithacoceras and Subplanites sp.; Wilford and Kho, 1965) and the overlying Pedawan Formation yields ammonoids of the Late Tithonian to the Early Cretaceous (e.g. Berriasella or Micracanthoceras sp.; Hashimoto and Tamura, 1968). This study focuses on the Paku limestone body (Fig. 1C) exposed at the SSF Quarry (N1°250 52.000 , E110°110 12.000 ; Fig. 3A) located 4 km east of Bau. The Paku limestone body covers an area of 2 km2, and occurs in an anticline with an axis trending in a NW– SE direction (Fig. 3A; Wilford, 1955; Wolfenden, 1965). Limestone beds in the quarry generally dip 40–50° to the SW, and are best exposed on the eastern face from which we sampled a continuous limestone sequence 202 m in thickness (SSF section; Fig. 3B and C).

Fig. 3. Geology, columnar section, and outcrop of the SSF section. (A) Geological map of Paku limestone body showing location of the SSF section (modified after Wolfenden, 1965). Legend follows Fig. 1C. (B) Lithostratigraphic column of the SSF section, showing horizons of samples in Figs. 4 and 5. (C) Full view of the outcrop in the SSF section. A car (arrowed) is shown for scale.

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Table 1 The results of Sr-isotopic age study of four rudists estimated by LOWESS Look-up Table version 4: 08/03 (revised from McArthur et al., 2001). 87Sr/86Sr ratios were calibrated by measuring the standard sample (NIST 987). Horizon (m)

Sample

Sr (ppm)

Mn (ppm)

Mn/Sr

87

Sr/86Sr ratio

11 24 90 90 188

BC-0035 BC-0075 BC-0245a BC-0245b BC-0525

152 266 – – 245

4.2 6.2 – – 8.0

0.028 0.023 – – 0.032

0.7068738 0.7068873 0.7069151 0.7069138 0.7069432

2s (107)

Absolute Age (Ma) (Oldest-Mean-Youngest)

Range (Myr)

Geological time

37 45 38 39 30

156.05–155.45–154.95 155.20–154.75–154.35 154.00–153.65–153.40 154.05–153.70–153.45 153.05–152.85–152.65

1.10 0.85 0.60 0.60 0.40

Latest Oxfordian – Early Kimmeridgian Early Kimmeridgian Early Kimmeridgian Early Kimmeridgian Early Kimeridgian – Late Kimmeridgian

Table 2 Results of geochemical analyses of the SSF section. The following abbreviations are used for facies; Bind. (Bindstone), Pel. (Peloidal packstone), Onc. (Oncoidal pack-grainstone), Bio. (Bioclastic pack-wackestone), Frame-Baffle. (Framestone and Bafflestone). Horizon (m)

Facies

d13Ccarb(‰)

STDEV

d18Ocarb (‰)

STDEV

IR (wt.%)

TOC (wt.%)

d13Corg (‰)

1 9 13 17 20 24 28 32 35 39 42 46 50 54 58 61 65 69 73 77 80 84 88 92 96 99 103 106 113 117 121 125 128 132 135 139 143 146 150 153 161 164 168 176 183 186 190 199 202

Bind. Bind. Bind. Bind. Bind. Bind. Bind. Bind. Bind. Pel. Pel. Pel. Pel. Onc. Bio. Bio. Bio. Bio. Bio. Pel. Onc. Pel. Pel. Pel. Onc. Onc. Bio. Pel. Bio. Pel. Pel. Pel. Pel. Bio. Pel. Bio. Pel. Pel. Pel. Frame-Baffle Frame-Baffle Frame-Baffle Frame-Baffle Frame-Baffle Pel. Frame-Baffle Frame-Baffle Frame-Baffle Frame-Baffle

1.20 1.44 0.04 1.57 1.84 1.36 1.77 1.95 1.93 1.41 1.97 1.70 1.82 2.24 2.17 2.17 2.10 1.75 1.10 2.04 1.79 1.31 0.09 0.83 1.95 0.91 2.28 1.57 0.91 1.86 1.87 2.00 1.40 1.81 2.11 1.44 1.75 1.81 1.72 1.26 1.25 1.11 1.29 1.23 0.75 0.47 0.50 0.10 0.43

0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.02 0.02 0.02 0.03 0.02 0.11 0.03 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.00 0.03 0.01 0.02 0.02 0.01 0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.22 0.10 0.19 0.16 0.27

6.85 9.66 7.07 8.91 6.72 8.70 7.07 5.11 5.27 7.90 9.01 8.20 10.22 8.81 4.47 8.50 6.90 8.61 8.45 7.59 10.14 10.07 11.75 10.41 9.54 7.96 5.01 7.20 5.97 6.86 5.99 9.87 9.75 6.45 5.78 8.65 7.49 7.05 4.60 6.07 7.31 5.92 6.20 9.25 4.38 4.63 4.78 6.28 5.87

0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.02 0.02 0.04 0.02 0.26 0.04 0.03 0.02 0.01 0.01 0.01 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.02 0.01 0.04 0.01 0.02 0.02 0.01 0.02 0.03 0.02 0.02 0.01 0.01 0.02 0.02 0.16 0.08 0.20 0.23 0.32

0.04

0.009

26.24

0.01

0.007

26.70

0.07

0.009

26.18

0.04

0.009

25.52

0.16

0.003

24.30

0.06

0.005

23.59

0.13

0.003

23.71

5.68

0.033

24.60

0.08

0.005

25.25 25.09

0.09

0.012

0.10

0.004

24.91 25.07

0.02

0.011

25.56

0.01

0.007

25.94

3. Materials and methods 3.1. Materials Limestone samples were collected from 49 horizons in the SSF section at intervals of 2–5 m (Fig. 3B). After making a lithofacies description, following the terminology of Dunham (1962) and Em-

25.35 25.20

bry and Klovan (1972), material for carbon and oxygen isotopic analysis was carefully extracted using a micro-drill, taking care to avoid large fossils, cements, and veins. We also collected four well-preserved paired shells of rudists for Sr isotopic analysis (Table 1; Fig. 3B). They are equivalved and likely assigned to Diceratidae sensu Lau (1973). We drilled and powdered subsamples from the outer layer for the

Y. Kakizaki et al. / Journal of Asian Earth Sciences 73 (2013) 57–67

measurement: two for specimen BC-0245 and one for each of the other three samples. The outer layer of the rudist shell is ideal material for the measurement of Sr isotopic compositions, as it consists primarily of low-Mg calcite, which is the carbonate mineral most resistant against diagenetic alteration (e.g. Steuber, 2003; Steuber et al., 2005; Frijia and Parente, 2008; Huck et al., 2011). We confirmed the extent of diagenetic alteration by measuring strontium and manganese contents for three subsamples of a large sample size, using an absorption spectrophotometer (Shimadzu AA-646). The manganese contents and Mn/Sr ratios are low (4.2–8.0 ppm and 0.023–0.032, respectively; Table 1) and satisfied the criteria indicating insignificant alteration (Mn <50 ppm: Jones et al., 1994; Mn/Sr ratio <2: Jacobsen and Kaufman, 1999).

3.2. Sr isotope stratigraphy The rudist subsamples were dissolved in 3 M purified nitric acid and centrifuged. Strontium was separated from the sample solution with an ion-exchange resin column (0.2 mL Eichrom Sr resin: Horwitz et al., 1992), cleaned with 3 M HNO3. After loading the sample solution, 3 mL of 3 M HNO3 was loaded into the column to elute the other cations, such as Ca, Ba, and Rb. The Sr-containing fraction was collected by adding 2 mL of 0.05 M HNO3 to the solution. The 87Sr/86Sr ratio was measured using a thermal ionization mass spectrometer (Thermo Finnigan Triton), using a tungsten single filament with tantalum oxide activator at the Kochi Core Center, Kochi University. The procedural blank of Sr during column separation was <1.13 ng, which was much lower than the Sr content used for isotopic analysis (100–150 ng) in this study. The repeated analyses of NIST SRM 987 (standard reference material of

61

strontium carbonate provided by NIST) obtained an average value of 0.7102608 ± 81  107 (n = 16, 2r). The measured 87Sr/86Sr ratios were normalized to the recommended ratio of NIST987 in an age-isotope curve (87Sr/86Sr = 0.7102480, Look-up Table Version 4: 08/03 revised from McArthur et al., 2001). Minimum and maximum ages are calculated with consideration of the uncertainties of both the measurement (2r) and the age-isotope curve, which is tied to the Geologic Time Scale of Gradstein et al. (2004) (Table 1). For details see Kakizaki et al. (2012).

3.3. Carbon and oxygen isotope analysis Carbon and oxygen isotope analyses of 49 bulk carbonate samples were performed using a mass spectrometer (Thermo Fisher Delta V plus) with GasBench II system at the Isotope-Geochemistry Laboratory of Geological Institute, ETH Zürich. Approximately 300 lg of sample powder was reacted with 100% phosphoric acid at 70 °C within a 12 ml vial tube filled with He, and generated CO2 gas was introduced into a mass spectrometer for measurement of isotopic composition. The reproducibility of the measurements of the standard (n = 19) was ±0.04‰ (2r) for d13C, and ±0.08‰ (2r) for d18O. The isotopic values are expressed in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) (d13Ccarb and d18Ocarb, Table 2).

3.4. Total organic carbon and organic carbon isotope Total organic carbon (TOC) and organic carbon isotopes were measured for samples from 16 horizons where negative or positive anomalies were recognized in the d13Ccarb profile.

Fig. 4. Fossil occurrence and microscopic view of the SSF section (I). Arrow indicates stratigraphic upward. (A) Diceratidae rudist on the outcrop (ca. 15 m horizon). Length of paired shells is approximately 20 cm. (B) Thin-section image of bindstone in the lowermost horizon of the SSF section (28 m), which contains corals, Bacinella (Bac), and Lithocodium (Lit). (C) Nerineoid gastropod on the outcrop (ca. 94 m horizon). (D) Concentration of gastropods and rudists (ca. 79 m horizon).

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Powdered sample was reacted with 5 N HCl for 24 h to allow complete decarbonation. After neutralization, insoluble residue (IR) was freeze-dried and weighed out to within the range of 0.1–2.1 mg, in order to obtain more than 6.5 lg of carbon, which is the minimum amount required for the isotopic analysis. IR set in a tin capsule was burned, and the generated CO2 gas was quantified by elementary analyzer (Flash EA 1112, Thermo Scientific) at the Isotope Laboratory of Geological Institute, ETH Zürich. CO2 gas was also introduced into the mass spectrometer (Delta V plus, Thermo Scientific) for isotopic measurement. In this study, atropine (C17H23NO3) with a known isotopic composition was used for the standard. Repeated measurements (n = 9) obtained a reproducibility (2r) of ±0.10‰. Organic carbon isotopes are measured in per mil (‰) relative to the VPDB (d13Corg, Table 2). 4. Results 4.1. Lithostratigraphy of the SSF section The SSF section exposes a 202 m-thick shallow marine limestone sequence (Fig. 3C), which can be divided into three units based on lithofacies associations (Fig. 3B). We have identified five different facies types (bindstone, peloidal packstone, oncoidal pack-grainstone, bioclastic pack-wackestone, and frame-bafflestone). The lower unit 1 (37 m thick) consists of bindstone, which is a dark gray color in outcrop. This facies yields abundant megafossils,

including rudists (Fig. 4A), nerineoid gastropods, and dendritic corals. The dark-colored matrix of the bindstone contains microencrusters, such as Lithocodium aggregatum and Bacinella irregularis, which encrust corals and rudists or trap bioclasts (Fig. 4B). Unit 2, in the middle of the section (37–152 m interval) exposes a repetition of the peloidal packstone, oncoidal pack-grainstone, and bioclastic pack-wackestone facies, (Fig. 3B). These sediments are dark gray to light gray colored in outcrop, and yield rudists and nerineoids (Fig. 4C), which are concentrated in some horizons (e.g. in the 79 m horizon; Fig. 4D). Peloidal packstone mainly consists of fine-grained peloids (<0.2 mm in diameter) and micrite, and contains larger bioclasts up to 1 mm in diameter (Fig. 5A). Oncoidal pack-grainstone is developed in three horizons of unit 2. This facies contains oncoids, peloids, and bioclasts. Oncoids generally consist of a nucleus of bioclasts and a cortex showing irregular lamination. Their diameter reaches up to 3 mm. The intergranular spaces are filled mainly with sparite (Fig. 5B). Bioclastic pack-wackestone is developed in five horizons of this middle section. This facies mainly consists of micrite and poorly-sorted bioclasts, but also contains peloids and oncoids. The characteristic bioclasts are sponge spicules and rudists fragments (Fig. 5C). The uppermost unit 3 (50 m thick) is mainly composed of the frame-bafflestone facies (Fig. 3B), which shows a reefal framework constructed by corals, microbial crust (Lithocodium and Bacinella), and stromatoporoids (Fig. 5D). Peloidal packstone occurs at the 183 m horizon.

Fig. 5. Fossil occurrence and microscopic view of the SSF section (II). All images are right side up. (A) Peloidal packstone in the lower part of unit 2 (50 m). (B) Oncoidal packstone from unit 2. The intergranular space is occupied by sparite. (C) Bioclastic packstone in the lower part of unit 2 (69 m) including meshwork of sponge spicules (on the right). (D) Framestone in unit 3 (168 m), consisting of dendritic corals and microencrusters (Lithocodium; Lit).

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4.2. Strontium isotopes and Sr isotopic age The rudist 87Sr/86Sr ratios from the four sampled horizons; 11 m (BC-0035), 24 m (BC-0075), 90 m (BC-0245), and 188 m (BC-0525), indicate an upward-increasing trend (Table 1; Fig. 6A), which is consistent with Upper Jurassic strontium isotope stratigraphy (Jones et al., 1994; McArthur et al., 2001). Comparison with the age-isotope curve (Look-up Table Version 4: 08/03; revised from McArthur et al., 2001) reveals that the ratio of the lowermost

Oxf Late

Kimmeridgian Early Late ca.152.70Ma

Unit 3

200

horizon (0.7068738) gives an oldest age of 156.05–154.95 Ma (the latest Oxfordian to the Early Kimmeridgian; Fig. 6A). The ratio in the 24 m horizon (0.7068873) indicates an age of 155.20– 154.35 Ma (Early Kimmeridgian; Fig. 6A). The two ratios obtained from the specimen from the 90 m horizon are almost identical (0.7069151 and 0.7069138) and cover an age interval of 154.05– 153.40 Ma (Early Kimmeridgian; Fig. 6A). The ratio in the uppermost horizon (0.7069432) gives the youngest age of 153.05– 152.65 Ma (Early to Late Kimmeridgian; Fig. 6A).

61.4m/myr

Unit 2

100

118.8m/myr

150

yr m/m 18.6

Unit 1

50

ca.155.95Ma

0m

156 155 154 153 152

(A) Sr-isotope age (Ma)

0

1.0

(B)

2.0 carb

3.0

-12

(C)

-10 18

Ocarb

-8

-6

-4

-27

(D)

-25

-23

org

Fig. 6. Chemostratigraphic profiles of the SSF section. (A) Age model based on Sr-isotopic age of four horizons. The ages of the section base and top were evaluated 155.95 and 152.70 Ma, respectively. (B) Carbonate carbon isotope (d13Ccarb) and (C) oxygen isotope (d18Ocarb) profiles. For the horizons shown with a black diamond, organic carbon isotopes (D; d13Corg) were measured.

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4.3. Bulk carbonate carbon and oxygen isotope data Results of isotopic analyses for bulk carbonate are shown in Table 2 and Fig. 6B and C. The d13Ccarb values range from 0.10 to +2.28‰, and the established curve is punctuated by several negative excursions. Three distinct low values (around 0‰) were recorded at 13 m, 88 m, and 199 m horizons. The highest value was recorded at 103 m. In addition to these anomalous values, the d13Ccarb gradually increases from the bottom of the section (+1.20‰) to the lower part of unit 2 (54 m horizon; +2.24‰). The d13Ccarb values remain high for a 20 m-thick interval, before beginning to fluctuate between 73 m and 113 m. In the upper part of unit 2 (117–152 m interval), the d13Ccarb values show a slightly decreasing trend from 2.0‰ to 1.7‰ and the values fluctuate with smaller amplitudes. In unit 3, the d13Ccarb values decrease monotonously to the minimum (0.10‰) recorded at level 199 m (Fig. 6B). The d18Ocarb values range from 4‰ to 12‰ with a larger amplitude of fluctuation. The d18Ocarb values are around 8‰ in unit 1. From a high value at the 58 m horizon (4.47‰), d18Ocarb decreases upwards in the lower part of unit 2 to a minimum value at the 88 m horizon (11.75‰). The d18Ocarb values in the upper horizons are generally higher, around 6‰, and the maximum value is at the 183 m horizon (4.38‰; Fig. 6C). 4.4. Insoluble residue and organic matter Limestone in the SSF section is very pure, and contains only a minor proportion of IR (mostly below 0.2 wt.%; Table 2). In only one sample (at 88 m) where both d13Ccarb and d18O reveal a negative anomaly, the limestone contains elevated amounts of IR (5.68 wt.%; Table 2). TOC measured throughout the section remains low (mostly below 0.012 wt.%) and the maximum value is recorded at the 88 m horizon (0.033 wt.%). Organic carbon isotopic values (d13Corg) show a relatively simple trend. Values increase upwards in the lower 70 m (from 26.70 to 23.59‰). In the upper part, a decreasing upward trend is observed (to 25.94‰). The maximum value is recorded in the lower part of unit 2 (23.59‰ at 65 m). Although it is of lower resolution, the d13Corg profile lacks high amplitude fluctuations, unlike the d13Ccarb curve (Fig. 6D). 5. Discussion 5.1. Age model based on the Sr isotopic ratio The 87Sr/86Sr ratios obtained from well-preserved rudists all include a small analytical error that provides a narrow age range. Even for BC-0075, with the largest analytical error (2r, 45  107), the age can be evaluated to within a range of 0.85 Myr (Table 1). Accuracy of the analysis is also supported by the reproducibility of two sub-specimens of BC-0245, which represent almost identical results. In addition, the ages from the four horizons are stratigraphically in the correct order (Table 1, Fig. 6), and are consistent with ages of reported brachiopods (Yanagida and Lau, 1978) and foraminiferal fauna (Bayliss, 1966) from the Bau Limestone. We propose an age model for the SSF section based on the measured data, connected with three lines (Fig. 6). Based on this model, the lowermost 24 m of the section covers the age interval from 155.95 to 154.75 Ma, the section from 24 m to 90 m is from 154.75 to 153.675 (average of two Sr mean ages from BC-0245) Ma, and the upper 112 m represents the interval from 153.675 to 152.70 Ma. The lines intersect the Oxfordian–Kimmeridgian boundary (155.60 Ma) around the 7 m horizon, and the boundary

between Lower and Upper Kimmeridgian (153.00 Ma) at the 165 m horizon (Fig. 6). For each age-horizon line, the depositional rate can be evaluated by the gradient, namely, 18.6 m/Myr for the lower line, 61.4 m/Myr for the middle line, and 118.8 m/Myr for the upper line (Fig. 6). These rates are much larger than those evaluated with the same method for the Upper Jurassic-Lower Cretaceous Torinosutype limestone deposited in mid-latitudes of the northwestern Palaeo-Pacific (6.3–14.5 m/Myr; Kakizaki et al., 2012). The Torinosutype limestone is a shallow marine limestone, and is lithologically similar to the Bau Limestone in terms of abundance of reef-builders and calcified microbes (Kano and Jiju, 1995; Shiraishi and Kano, 2004; Kakizaki and Kano, 2009). Differences in the sedimentation rate are therefore related to the difference in regional tectonic setting (passive margin vs. fore-arc basin) and/or global sea-level change, rather than the differences in the palaeo-latitude of the near-equator vs. mid-latitude environments (Fig. 1A). The early Kimmeridgian is a time of second-order sea level rise (Haq et al., 1987) which, in addition to continuous subsidence, may explain the high sedimentation rate recorded by the Bau Limestone. The strontium isotope stratigraphy seems to be reliable and provides ages with higher resolution than biostratigraphy. For the Jurassic strata in the Palaeo-Pacific region, the radiolarian biozones are biostratigraphic tools that possess the highest time resolution. However, the resolution of three radiolarian zones covering the Oxfordian and Kimmeridgian (Pseudodyctiomitra primativa, Hsuum maxwelli, Stylocapsa spiralis zones) is 5–9 Myr (Matsuoka, 1995), which is one order lower than the resolution of strontium isotopic ages of this study. 5.2. Carbon isotope stratigraphy Based on the Sr-isotope age model, the new carbon isotope stratigraphy (Fig. 6) can be correlated with other carbon isotope stratigraphies established from successions in Europe. 5.2.1. Examination of diagenetic alteration Prior to attempting an inter-regional correlation, possible diagenetic alteration affecting measured isotope signatures should be examined. Because the d18Ocarb values of the SSF section are clearly lower than those of carbonate and belemnites from Europe (>3‰ in Weissert and Erba, 2004; Nunn et al., 2009; Nunn and Price, 2010), limestone in the SSF section was likely subjected to meteoric diagenesis. We first examined the facies dependency of the d18Ocarb value by analysis of variance (ANOVA) between the five facies types (bindstone, peloidal packstone, oncoidal pack-grainstone, bioclastic pack-wackestone, and frame-bafflestone), which would have initially had different porosity and different potential for diagenetic alternation. Results of the ANOVA test yield insignificant variance among the five facies groups (F(4, 44) = 2.62, p < 0.01). Thus, the hypothesis of facies dependency of diagenetic alternation is not statistically supported. It has been suggested that meteoric diagenesis lowers the d13Ccarb values by incorporating cementation from fluids containing 13C-depeleted dissolved inorganic carbon (DIC) (Allan and Matthews, 1982). Such meteoric diagenesis is often associated with soil development in a subaerial setting. Fluid passing through a soil layer is normally depleted in 18O as well as in 13C. The presence of meteoric cements therefore lowers the initial values of both d13C and d18O, and generates an apparent covariance in the carbonate section (e.g. Allan and Matthews, 1982; Marshall, 1992). However, the cross-plots of d13Ccarb and d18Ocarb values for the SSF section indicate no significant correlation between these two parameters (Fig. 7), except for the interval 84–92 m, where both isotopic values decrease upwards to the minimum value (Fig. 6).

65

δ Ccarb ( 13

-0.5 -2

0

0.5

VS PDB)

1.0

1.5

2.0

2.5

153

VS PDB)

-4

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Y. Kakizaki et al. / Journal of Asian Earth Sciences 73 (2013) 57–67

153.00 Ma

L3 E3

-6

154

-10

Bindstone

-12

Peloidal packstone Bioclastic pack-wackestone

-14

Oncoidal pack-grainstone Frame-Bafflestone

Fig. 7. Plot of carbon isotopes (d13Ccarb) and oxygen isotopes (d18Ocarb) for different lithofacies. Approximate line drawn by the least square method for all data. No covariation was obtained between the two isotopic values (R2 = 0.00).

It has been suggested that the d13Ccarb value is more resistant to diagenetic alteration than the d18Ocarb value, which has been observed in shallow marine carbonates of Mesozoic age (Vahrenkamp, 1996; Grötsch et al., 1998; Burla et al., 2008). Diagenetic alteration of the d13Ccarb values was found to be generally insignificant. This is also supported by the overall covariation between the d13Ccarb and d13Corg profiles, showing the lowest value at 13 m horizon, the highest in the middle of unit 2, and a decreasing trend in the upper section. The smoothed trend in the measured d13Corg profile is partly ascribed to the lower data resolution, and/or to the less altered C-isotope values of organic matter. The lowered d13Ccarb values in the interval of 84–92 m could be diagenetically altered, however. In addition to the covariation between d13Ccarb and d18Ocarb (Fig. 7), the high IR content of this interval (Table 2) could have originated from a mixture of terrigenous material during aerial exposure. It should be mentioned that two other intervals of lowered d13Ccarb (around 13 and 199 m) lack features suggesting intensive diagenetic alternation.

5.2.2. Comparison with Tethyan d13C profiles For comparison with d13C stratigraphy of the SSF section (Fig. 8), we have selected two Upper Jurassic sections reported from Switzerland by Padden et al. (2002); the Gemmi pass and Montsalvens sections, both of which were formed along the continental margin of the northwestern Tethys (their locations are shown in Fig. 1A). These two sections, consisting of pelagic and hemipelagic carbonates, represent relatively high resolution data, and the d13C profiles of bulk carbonate are regarded as representative Tethyan d13C records (Weissert and Erba, 2004; Weissert et al., 2008). The two Swiss sections contain the interval corresponding to the SSF section (uppermost Oxfordian–Upper Kimmeridgian), and are characterized by a narrow range of d13C values (+1.7 to 2.8‰). Although there are no distinct C-isotope anomalies with an amplitude >1‰, lowered and elevated d13C values (by >0.5‰) were recognized in several horizons in the higher-resolution profile from the Montsalvens section (Fig. 8). Lowered d13C values are recognized below the Oxfordian/Kimmeridgian boundary (L1; at 155.75 Ma), in the middle Lower Kimmeridgian

Lower Kimmeridgian

y=-0.08x-7.34 R2=0.00

L2 E2

155

E1

156

Upper Oxf.

δ18Ocarb (

-8

155.60 Ma

L1

0

1.0

2.0

3.0

δ13Ccarb (‰ vs VPDB) SSF Quarry (This study)

Montsalvens

Gemmi pass

Fig. 8. Correlation of age-profiles of carbon isotopes (d13Ccarb) from the SFF section in the western Palaeo-Pacific, and two Swiss sections: Montsalvens and Gemmi pass (Padden et al., 2002) which are representative Tethyan records. Sr-isotopic age ranges are shown beside the time scale. A constant depositional rate was assumed for the two Swiss sections. Horizons indicated by squares represent horizons with lowered d13C values (L1, L2, L3), and elevated d13C values (E1, E2, E3) recognized in Tethyan records. The anomalous d13Ccarb values in the interval 84 m–92 m were excluded from profile of the SSF section.

(L2; 154.4 Ma), and around the Lower/Upper Kimmeridgian boundary (L3; 153 Ma). In comparison, elevated values occur above the Oxfordian/Kimmeridgian boundary (E1; at 155.4 Ma), below L2 (E2; 154.5 Ma) and widely in the upper Lower Kimmeridgian (E3; around 153.3 Ma; Fig. 8). Because Sr-isotopic ages from this study include considerable uncertainty, reliable correlation of these lowered and elevated values to the SSF section is not attempted. However, a distinct negative anomaly at 13 m in the SSF section can be correlated to L1 of the Montasalvens section. If so, E1 is correlated to high values above the 20 m horizon. Correlation is difficult for the other anomalies (E2, L2, E3, and L3) observed in both the Gemmi pass and Montsalvens sections, however. In particular, the horizon corresponding to L3 can hardly be seen in the d13Ccarb profile of the SSF section, in which the values continuously decrease throughout the relevant interval. It is worthy of note that the Tethyan profiles lack any excursion that can be correlated with the most distinct negative anomaly in the 84–92 m interval of the SSF section (153.75 Ma). This may

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support the hypothesis that the d13Ccarb values of this interval have been lowered by intensive meteoric diagenesis. Because of this, the low d13Ccarb values from the 84–92 m interval are excluded from profiles during comparison (Fig. 8). 5.2.3. Regional variation in the Late Jurassic d13C Importantly, results from the SSF section indicate d13Ccarb values that are consistently 1‰ lower than the Swiss sections for the relevant interval. This could represent regional variation resulting from the difference in oceanographic conditions (e.g. carbon cycling). In fact, such regional variation has been suggested in a study of a d13C profile from Scotland, located in the Boreal realm (Fig. 1A; Nunn et al., 2009; Nunn and Price, 2010). Although the Scottish profile has a lower time resolution, a decreasing trend is observed from the lower Kimmeridgian (+2‰ on average) through the upper Kimmeridgian (+1‰ on average) to the lower Tithonian (0‰ on average) (e.g. Nunn and Price, 2010). This trend resembles those of the d13C profiles reported from the Russian platform (Fig. 1A; Podlaha et al., 1998; Price and Rogov, 2009). The Russian d13C profiles also show a decreasing trend from the lower Kimmeridgian (+2.5‰ on average) through the upper Kimmeridgian (+1‰ on average) to the lower Tithonian (+ 0‰ on average) (e.g. Price and Rogov, 2009). The Late Jurassic oceanography and carbon cycling has been discussed, largely based on geochemical profiles from the Tethys Seaway (e.g. Weissert and Mohr, 1996; Weissert and Erba, 2004; Weissert et al., 2008). However, the global trend of d13C values should also appear in the Palaeo-Pacific sections, such as the Bau Limestone and Torinosu-type Limestone, because the Palaeo-Pacific largely surpassed the Tethys Ocean in area during the Late Jurassic (Fig. 1A). The Tethys Ocean could have been affected by a locally controlled d13CDIC gradient changing along the Seaway from the Pacific into the Tethys Ocean. The higher d13C values in the inner Tethyan profiles may be explained by elevated productivity and higher nutrient input from the surrounding continents (Weissert and Mohr, 1996). For instance, widespread occurrences of planktonic crinoids (e.g. Saccocoma) in Kimmeridgian–Tithonian pelagic limestones of the alpine Tethys could serve as evidence for slightly elevated biological productivity (e.g. Nicosia and Parisi, 1979; Price and Sellwood, 1994; Kroh and Lukeneder, 2009; Jach et al., 2012).

6. Conclusions The investigation of strontium and carbon isotopic compositions has revealed the following information from the 202 m-thick SSF section of Bau Limestone exposed in northwest Borneo, Malaysia. 1. Careful analysis of 86Sr/87Sr of rudist specimens determined the age of the limestone to within substage level (Table 1). Depositional age of the SFF section was estimated to range from 155.95 (the latest Oxfordian) to 152.70 Ma (the Late Kimmeridgian). 2. The d13Ccarb values of the SSF section range from 0.10 to +2.28‰ (Table 2) and show a decreasing upward trend in unit 3 (Fig. 6). Distinct negative anomalies (around 0.0‰) appear in two horizons (at 155.4 and 153.75 Ma), but the younger anomaly is interpreted as diagenetically altered signature. The d13Ccarb values generally maintain a high level (around +2‰) in several intervals (Fig. 8). 3. The older negative anomaly of the SSF section can be correlated with more widespread lowered d13C values around the Oxfordian/Kimmeridgian boundary, which were also recognized in the inner Tethys (Padden et al., 2002).

4. The d13Ccarb values of the SSF section are 1‰ lower than the Tethyan values of the same interval. The d13Ccarb trend of the SSF section seems to resemble Boreal records from Scotland and Russia during the Oxfordian to Kimmeridgian (e.g. Nunn and Price, 2010) in terms of the upward-decreasing pattern observed in the upper SSF section. Hence, the Tethyan record may represent a global trend, but local changes in productivity could have resulted in C-isotope gradients from the Pacific into the Tethys Seaway. This study is the first isotopic investigation of the Bau Limestone that provides new oceanographic information of the marginal Palaeo-Pacific during the Late Jurassic. Further study aiming to extend the stratigraphic coverage will provide valuable new insight for understanding oceanography and carbon cycling at this time.

Acknowledgments Prof. Y. Isozaki (The University of Tokyo) and an anonymous reviewer provided comments that greatly helped us to improve the manuscript. Measurement of Sr isotope in this study was performed in the cooperative research program of Kochi Core Center (No. 09B040) with the support by Dr. K. Okamura (Kochi University) and Dr. K. Nagaishi (Marine Works Japan Ltd.). Analyses in ETH were financially supported by the Japan Society for the Promotion of Science (JSPS), through the Institutional Program for Young Researcher Overseas Visits proposed by Prof. S. Arai and Prof. S. Umino (Kanazawa University). We thank M. C. Strasser, S. E. Bishop, Dr. M. I. Millán (ETH), and Dr. M. Giorgioni (Australian National University) for assisting with the isotope measurements in ETH. Dr. M. Sone (Malaya University) provided useful information on fieldwork in Borneo Island. Y. Kakizaki was financially supported by the Sasakawa Scientific Research Grant from the Japan Science Society, and T. Hasegawa by JSPS KAKENHI Grant (No. 20340144).

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