Ammonite biostratigraphy and organic carbon isotope ...

Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 531–542

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Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo

Ammonite biostratigraphy and organic carbon isotope chemostratigraphy of the early Aptian oceanic anoxic event (OAE 1a) in the Tethyan Himalaya of southern Tibet Xi Chen a,⁎, Vyara Idakieva b, Kristalina Stoykova c, Huiming Liang a, Hanwei Yao a, Chengshan Wang a a b c

State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China Dept. of Geology, Paleontology and Fossil Fuels, Sofia University “St. Kl. Ohridski”, 15 Tsar Osvoboditel Blvd., BG-1504 Sofia, Bulgaria Dept. of Paleontology, Stratigraphy and Sedimentology, Geological Institute Bulgarian Academy of Sciences, 24 G. Bonchev Str., BG-1113 Sofia, Bulgaria

a r t i c l e

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Article history: Received 30 December 2016 Received in revised form 10 July 2017 Accepted 10 July 2017 Available online 15 July 2017 Keywords: Early Cretaceous Ammonite zones Dark shale Organic matter Eastern Tethys

a b s t r a c t The early Aptian oceanic anoxic event (OAE 1a) is well known in the western Tethys, the North Atlantic and the Pacific Ocean, but has not been reported in the eastern Tethys to date. In this paper, we present bulk organic carbon isotope data and ammonite biostratigraphy of a lower Aptian succession from the Gucuo area (southern Tibet). These findings document the occurrence of the OAE 1a for the first time from the eastern Tethys. The studied sequence can be attributed to the D. forbesi and D. deshayesi ammonite zones of the lower Aptian. The δ13Corg data can be correlated with published early Aptian carbon isotope records from the western Tethys and the Pacific. A distinctive negative carbon isotope excursion of 2.4‰ in the upper part of the section corresponds to segment C3 of the OAE 1a, and the following positive excursion correlates to segment C4. The absolute values of the carbon isotope ratio in the Gucuo area are higher than those of known sections in the western Tethys and equatorial Pacific. We suggest that diagenetic alteration is the major cause of the higher absolute values in the Gucuo area. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Oceanic anoxic events (OAEs) were initially described by Schlanger and Jenkyns (1976) as transient periods of marine anoxia and the widespread deposition of organic carbon-rich sediments in different basins during the Cretaceous Period. There were at least seven episodes of global or regional OAEs during the Cretaceous, as compiled by Jenkyns (2010). The early Aptian oceanic anoxic event (OAE 1a, ca. 120.5 Ma) is one of the most studied OAEs (e.g., Menegatti et al., 1998; Arthur et al., 1990; Erba, 1994; Méhay et al., 2009; Erba et al., 2010; Stein et al., 2011). The OAE 1a is characterized by a positive carbon isotope excursion (CIE) in marine carbonates and sedimentary organic matter that is preceded by a pronounced negative CIE (see Menegatti et al., 1998; Bralower et al., 1999; de Gea et al., 2003; Ando et al., 2008; Heldt et al., 2012; Kuhnt et al., 2011; Elkhazri et al., 2012; Hu et al., 2012; Föllmi, 2012). The CIEs have been identified in both marine (e.g., Bellanca et al., 2002; Dumitrescu and Brassell, 2006; de Gea et al., 2003; Kuhnt et al., 2011; Luciani et al., 2001; Millán et al., 2009; Yamamoto et al., 2013) and terrestrial deposits (e.g., Ando et al., 2002; Heimhofer et al., 2003; Suarez et al., 2013; Zhang et al., 2016) at various ⁎ Corresponding author. E-mail address: [email protected] (X. Chen).

http://dx.doi.org/10.1016/j.palaeo.2017.07.010 0031-0182/© 2017 Elsevier B.V. All rights reserved.

localities around the globe and are thought to reflect globally significant perturbations in the carbon cycle. The early Aptian time interval is associated with major global paleoceanographic and climatic change (Millán et al., 2009; Heldt et al., 2012; Mutterlose and Bottini, 2013; Mutterlose et al., 2014) including prevailing anoxic conditions in the water column and increased continental weathering and runoff (e.g., Michalík et al., 2008; Najarro et al., 2010; Mutterlose et al., 2014). The OAE 1a corresponds to sea-level rise and a significant change in nannofossil assemblages (Erba, 1994; Menegatti et al., 1998; Bralower et al., 1999; Erba et al., 1999), which are indicative of a period of ocean acidification (Erba et al., 2010) and/ or changes in the trophic levels (Föllmi, 2012). These paleoenvironmental changes are associated with an increase in the atmospheric CO2 concentration which is most likely due to enhanced volcanic activities (Menegatti et al., 1998; Larson and Erba, 1999; Weissert and Erba, 2004; Föllmi et al., 2006; Millán et al., 2009; Méhay et al., 2009; Tejada et al., 2009; Charbonier and Föllmi, 2017) or an extensive release of methane from gas hydrates (Jahren et al., 2001; Beerling et al., 2002). The deposition of OAE-related sediments in southern Tibet was first documented by Wang et al. (2001). Since then, studies of OAEs have mainly focused on the OAE 2 (Cenomanian/Turonian boundary event), including its characteristic biostratigraphy, carbon isotope stratigraphy and redox conditions (Wang et al., 2001; Li et al., 2006; Wendler et al.,

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X. Chen et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 531–542

2009; Bomou et al., 2013). Recently, Li et al. (2015) conducted a study on the OAE 1b. So far, however, no reliable identification or detailed study of the OAE 1a in southern Tibet has yet been accomplished. Early Aptian ammonite faunas from southern Tibet are poorly understood. Recently, Lukeneder et al. (2013) reported the first discovery of the genus Deshayesites in the Tethyan Himalaya from Spiti Valley, India, identifying a “Cleoniceras assemblage”. The family Deshayesitidae Stoyanow, 1949 represents a major component of early Aptian ammonite faunas and has a key significance for the ammonite biostratigraphy of the early Aptian (Bogdanova and Michailova, 2004; Reboulet et al., 2011; Lehmann et al., 2015). The Deshayesitidae have a wide paleogeographic distribution, ranging from the Mediterranean province of the western Tethys eastward to the Transcaspian area. They are also found in the Boreal Realm including northwest Europe and the Russian Platform. The first occurrence (FO) of the genus Deshayesites in the Alpine Tethys and the FO of the genus Prodeshayesites in northwest Europe and northeast Greenland mark the base of the Aptian Stage (Rawson et al., 1999; Kelly and Whitham, 1999; Bogdanova and Prozorosvky, 1999; Reboulet et al., 2011). The primary purpose of this study is 1) to establish a biostratigraphic frame based on ammonites and 2) to identify the δ13C CIEs and confirm

the presence of the OAE 1a in the lower Aptian sedimentary succession of southern Tibet. 2. Geological setting The study area is situated approximately 30 km northwest of Old Tingri in southern Tibet and approximately 5 km south of the village of Gucuo. Tectonically, this area belongs to the southern zone of the Tibetan Tethyan Himalaya (Fig. 1), which is situated between the Higher Himalayan Crystalline Zone and the Indus-Yarlung Zangbo Suture Zone (Burchfiel et al., 1992). During the Early Cretaceous, the Tethyan Himalaya zone was surrounded by the Neo-Tethys Ocean, connected the Pacific Ocean to the east and the Alpine Tethys to the west (e.g., Scotese, 1991). The Mesozoic strata of this area belong to two different tectonic domains: the passive continental margin of the Indian continental plate and the adjacent deep oceanic basin (Yu and Wang, 1990; Liu and Einsele, 1994). The Lower Cretaceous sediments of the study area are attributed to Gucuo Formation, which consists of dark gray shales, intercalated with volcaniclastics or punctuated by clastic dykes (Hu et al., 2010). The Aptian–early Albian age of the Gucuo Fm. is based on ammonites, foraminifers and detrital zircon U-Pb ages (Hu et al., 2008). A fault separates the Lower Cretaceous sediments from the overlain Upper Jurassic

Fig. 1. Geological map of southern Tibet (A and B) (modified from Li et al., 2006) and location of section and geological settings of the studied area (C) (modified from Zhu et al., 2002).


Menkadun Formation. The latter is composed of dark gray shales and quartz sandstones with abundant ammonites, and was deposited in shelf to near-shore environments (Jadoul et al., 1998; Yin and Enay, 2004; Hu et al., 2010). The studied interval was logged at two sections positioned next to each other: Gucuo East (GPS: 28°44′36″N, 86°18′39″E) and Gucuo II (GPS: 28°44′58″N, 86°18′38″E). These two sections, which have a total

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thickness of approximately 480 m, correspond to the middle part of the Gucuo Fm. (Fig. 2). 3. Materials and methods Gucuo II is a well-exposed 78-m-thick sequence, which was sampled in 2014 for biostratigraphy and geochemical studies. Sixty-seven

Fig. 2. Lithological log and position of the ammonite findings of Gucuo East and Gucuo II sections.

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samples were collected for carbon isotope and total organic carbon (TOC) content analysis. The Gucuo East section, which is 400 m thick, was sampled for ammonites in 2015, providing some additional material from the lower levels. Forty-two ammonite specimens were collected from five different levels of Gucuo II and one level from Gucuo East (Fig. 2). The preservation of the ammonites varies from poor to moderate, improving in the mudstone and siltstone lithologies. The specimens recovered from the shales are usually strongly compressed and are thus difficult to identify taxonomically. Nevertheless, twelve ammonite species from all of the lithologies were determined. For geochemical analysis (TOC and δ13Corg) 66 samples were collected from the Gucuo II section. Approximately 0.5 g rock sample was acidified with 1 N HCl, rinsed repeatedly with deionized water, and then dried at 50 °C. The TOC content was measured with a TOC analyzer (Multi N/C, Analytik, Jena, Germany). The carbon isotope values of organic matter were analyzed using a total organic carbon cavity ring-down spectroscopy (TOC-CRDS), which is composed of wavelength scanning optical cavity ring-down spectroscopy (WS-CRDS, Picarro G2121-i) unit, a solid combustion front processing module (CM, Costa), and a vacuum pump. The CM is used to transform solid organic carbon into CO2 by combusting 200– 600 μg of carbon-containing samples at 980 °C. Through the CM, the produced CO2 is transmitted online to the wavelength scanned optical cavity ring down spectroscopy (WS-CRDS) unit, which is used to measure the carbon isotope ratio (δ13C). International organic carbon isotope standards used in the experiments were USGS 40 (δ13C = − 26.39‰), IAEA CH6 (δ13C = − 10.45‰), and IAEA 600 (δ13C = − 27.77‰). The maceral analysis of kerogen was conducted for ten of the samples taken from the upper part of the Gucuo II section. To prepare the kerogens, rock chips were leached in 6 mol/L HCl, heated to 60 °C for 2 h to remove carbonates and then washed several times with distilled water. The residues were treated with 40% hydrofluoric acid (HF) and heated to 60 °C for 2 h to remove silicates. The samples were again rinsed several times with distilled water. The above procedure was repeated to ensure the complete removal of carbonates and silicates. The visual estimation of the relative abundance of the maceral content was determined using an Axioskop2 plus microscope. The maceral analyses were performed in the Organic Geochemistry Laboratory, Research

Institute of Exploration and Development, Huabei Oilfield Branch Company of PetroChina. 4. Results 4.1. Lithostratigraphy The Gucuo II section is divided into seven informal lithostratigraphic units (Fig. 3). Unit 1 (0–29 m) is composed of couplets of dark thin-bedded mudstones and shales (Fig. 3A). At the base, the mudstones yield abundant bivalves and ammonite remains. Unit 2 (29–56 m) is characterized by dark gray shales with thin-bedded mudstone intercalations (Fig. 3A). Very few macrofossils were found in this unit and clastic dikes are common. Unit 3 (56–60 m) consists of dark shales and silty shales with a few lenticular volcaniclastic sandstones (Fig. 3B). Unit 4 (60–63 m) is composed of dark shales with fine to medium bedded mudstone intercalations (Fig. 3B). The mudstones yield rich ammonites. Unit 5 (63–71 m) is composed of dark shales and a large sandstone block (Fig. 3B). This unit is overlain by an interval of silty shales ~ 3 m thick (Unit 6) and coarsening-upward silt and sandstone intervals with thin dark shale intercalations (Unit 7) (Fig. 3C). 4.2. Ammonite biostratigraphy The ammonite associations in the Gucuo II and Gucuo East sections yield deshayesitid faunas of both Mediterranean and Boreal provinces. In the Gucuo II section, ammonites were recovered from the lower and middle part of Unit 1, middle part of Unit 4 and from the base of Unit 5 (Fig. 2). In the Gucuo East section ammonites were found within mudstone intercalations at a depth of ~360 m (Fig. 2). Various biostratigraphic schemes based on the genus Deshayesitids, have been applied to the subdivisions of the early Aptian, including within the Boreal area of northwest Europe (Casey, 1961; Kemper, 1971, 1995; Casey et al., 1998), the Russian Platform (Baraboshkin and Mikhailova, 2002), and the western Mediterranean province up to the Transcaspian region (Hoedemaeker et al., 1990, 2003; Reboulet et al., 2011, 2014; Bogdanova and Michailova, 2004). Recent studies focused on the ontogeny, phylogeny and evolution of the deshayesitids, including taxonomic revisions and biostratigraphic interpretations

Fig. 3. Field photos, showing the lithological units of the studied section. White arrow marks a person as a scale (height = 1.7 m).


(Bogdanova and Michailova, 2004; Ropolo et al., 2006; Bersac and Bert, 2012, 2015; Bersac et al., 2012; Moreno-Bedmar et al., 2014). Bersac and Bert (2012) noted a common similarity between the genera Prodeshayesites Casey, 1961, Paradeshayesites Kemper, 1967, Obsoleticeras Bogdanova and Michailova, 1999 and Deshayesites Kasansky, 1914. They combined all four of these genera into one, known as Deshayesites Kasansky, 1914. However, we consider Prodeshayesites and Deshayesites as clearly distinguishable genera. 4.2.1. Deshayesites forbesi Zone Casey (1961) initially defined this zone in England. In its upper part, it includes the Deshayesites callidiscus Subzone. The zone was later subdivided into four subzones by Casey et al. (1998) that are applicable to the Boreal Realm. For the Alpine Tethys province, Reboulet et al. (2011) proposed the replacement of the former D. weissi Zone with the D. forbesi Zone, by considering the species D. weissi as a nomen dubium (see comments in Reboulet et al., 2006, 2009, 2011). D. forbesi was identified in the lower stratigraphic levels and was exposed at the Gucuo East section (approximately 40 m below the base of the interval sampled for δ13C and TOC content) (Fig. 4A, C). The recovered ammonite fauna yield D. cf. consobrinus (Fig. 4B), Prodeshayesites fissicostatus (Fig. 4D, E), P. aff. germanicus (Fig. 4F) and D. euglyphus (Fig. 4I). Of particular interest are the occurrences of P. fissicostatus

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(Fig. 4D, E, G, H) and P. aff. germanicus (Fig. 4F) along with ammonites typical of the D. forbesi Zone is of particular interest. The latter two species are of boreal affinity, which are characteristic of the preceding P. fissicostatus Zone of the northwestern European zonal schemes (Casey, 1961; Casey et al., 1998; Lehmann et al., 2012). Their co-occurrence implies a partial overlap of their ranges at the basal part of the D. forbesi Zone. The presence of Boreal elements can be explained by putative migration routes eastward to the Tethyan Himalaya area. In the Gucuo II section, the base of the D. forbesi Zone is marked by the FO of the index species D. forbesi. In the lower part of Unit 1 (approximately 5 m above the base), the mudstones yield an abundance of rich deshayesitid ammonites. The following species were recorded: D. cf. lavaschensis (Fig. 5A, B) and D. cf. luppovi (Fig. 5C, D, E). Several of the Deshayesites spp. could not be precisely identified due to their poor preservation state. It is noteworthy that some of the ammonite specimens possess a specific morphology, probably due to endemic evolution. From the upper levels of Unit 1, we identified D. sp. juv. (Fig. 5F) and D. euglyphus (Fig. 5G), which is a characteristic species for the D. forbesi Zone. At the base of Unit 3, two specimens were found: the typical D. callidiscus (Fig. 5H) and one specimen of D. cf. callidiscus (not shown here). Its occurrence in the Gucuo II section substantiates the presence of the upper part of the D. forbesi Zone.

Fig. 4. Ammonites from the Gucuo East section. Scale bar is 1 cm. A – Deshayesites forbesi Casey, 1964, 31f, D. forbesi Zone; B – Deshayesites cf. consobrinus (d'Orbigny, 1841), 37f, Ibid.; C – Deshayesites forbesi Casey, 1964, 35f, Ibid; D, E – Prodeshayesites fissicostatus (Phillips, 1829), 39f, Ibid. (D, E are the two halves of a single fossil); F – Prodeshayesites aff. germanicus (Casey, 1964), 32f, Ibid.; G, H – Prodeshayesites fissicostatus (Phillips, 1829), 41f, Ibid. (G, H are the two halves of a single fossil); I – Deshayesites euglyphus Casey, 1964, 36f, Ibid.

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Fig. 5. Ammonites from the Gucuo II section. Scale bar is 1 cm. A, B– Deshayesites cf. lavaschensis Kazansky, 1914, f22, f10, D. forbesi Zone; C–E – Deshayesites cf. luppovi Bogdanova, 1983, f 20, f 24, f 17, D. forbesi Zone; F – Deshayesites sp. juv., f 21, D. forbesi Zone; G – Deshayesites euglyphus Casey, 1964, f 36, D. forbesi Zone; H– Deshayesites callidiscus Casey, 1964, 10f2, D. forbesi Zone; I – Deshayesites sp., f 37, D. deshayesi Zone; J – Deshayesites cf. dechyi (Papp, 1907), f 38, D. deshayesi Zone; K – Deshayesites cf. consobrinoides (Sinzow, 1898), f 39, D. deshayesi Zone; L – Deshayesites cf. grandis (Spath, 1930), f 41a, D. deshayesi Zone; M – Deshayesites sp., f 41b, D. deshayesi Zone; N – Deshayesites kudrjavzevi Mikhailova, 1958, f 40, D. deshayesi Zone.

Unit 1 through Unit 3 of the Gucuo II section, along with the interval below (Gucuo East section) are assigned here to the D. forbesi Zone. The lower boundary of the zone was not observed. The upper boundary equates to the boundary between Unit 3 and Unit 4. The D. forbesi Zone of southern Tibet corresponds to the D. forbesi Zone of the

standard Mediterranean scheme (Reboulet et al., 2011, 2014) and Casey's scheme for northwest Europe (Casey, 1961; Casey et al., 1998). It is well correlated with the D. weissi Zone in the Transcaspian region (Bogdanova, 1978, 1999; Bogdanova and Michailova, 2004) and probably corresponds to the D. volgensis Zone of the Russian

Fig. 6. Correlation of the lower Aptian ammonite zonations in the Tethys and the Boreal Realm.


Platform in the Middle Volga region (Baraboshkin and Mikhailova, 2002) (Fig. 6). 4.2.2. Deshayesites deshayesi Zone/D. grandis Subzone This zone was defined by Kilian (1907–1913). In its modern use, it comprises the stratigraphic interval between the FO of Deshayesites deshayesi and the FO of Dufrenoyia furcata. The lower boundary of the zone corresponds to the base of Unit 4. Deshayesites sp. (Fig. 5I) was reported for the mudstones of Unit 4. Moreover, D. cf. dechyi (Fig. 5J), D. cf. consobrinoides (Fig. 5K), D. cf. grandis (Fig. 5 L) and D. kudrjavzevi (Fig. 5N) were registered in the sandstones of the lower part of Unit 5. The recognition of the subzone D. grandis was based upon the occurrence of D. cf. grandis, although the zonal index species D. deshayesi is missing. In the western Tethys, the D. grandis Subzone corresponds to the upper part of the D. deshayesi Zone. The upper boundary of the D. deshayesi Zone was not precisely traced and no specimens of Dufrenoyia were found in the section. The FO of the genus Dufrenoyia marks either the base of the overlying D. furcata Zone or the base of the transitional interval between Deshayesites deshayesi and Dufrenoyia furcata, which is well documented in northern Spain where the two genera co-occur (Raisossadat, 2011). The Deshayesites deshayesi Zone, including the D. grandis Subzone, was recognized in the upper part of the studied section. It spans the entire Unit 4 and the lower part of Unit 5. It can be correlated with the sequences described in the Isle of Wight (Casey, 1961; Casey et al., 1998), the Alpine Tethys (Reboulet et al., 2006, 2009, 2011, 2014), the Middle Volga region (Baraboshkin and Mikhailova, 2002; Mikhailova and Baraboshkin, 2002) and the Transcaspian area (Bogdanova, 1978; Bogdanova and Michailova, 2004). Biostratigraphic analysis allows us to document the geographically expanded occurrence of the Deshayesitidae during the early Aptian. Therefore, we can speculate upon a putative marine connection between the western Tethys, the Transcaspian area, and the eastern Tethys Himalaya region. Previous studies of the Tethyan Himalaya (Bordet et al., 1971) reported a few findings of Deshayesites and Prodeshayesites spp. in the “green sandstones” (lower Aptian) in the Thakkhola region of central Nepal. According to this study, the sequence of black shales and green sandstones exactly corresponds to the Guimal sandstones (the Spiti Valley, India). From this latter location, Lukeneder et al. (2013) published the first finding of the genus Deshayesites by describing a new speciesism thereafter named D. fuchsi. Regionally, the studied interval of the Gucuo Fm. can be correlated to the upper part of the Guimal Fm. (the Spiti Valley, India). 4.3. δ13Corg stratigraphy The stable carbon isotopic composition of organic matter (δ13Corg) throughout the studied succession varies from − 26.3‰ to − 23.3‰ (Table 1). The δ13Corg record can be subdivided into six successive intervals (Fig. 7). For Interval 1 (0–23 m), the δ13Corg shows a significant and progressive increase from around −26‰ to −24.5‰. In Interval 2 (23– 51 m), the δ13Corg values are less variable and range between − 25‰ and − 24.5‰. In Interval 3 (51–56 m), the δ13Corg values rapidly increase from − 24.5‰ to − 23.3‰. A notable negative excursion from − 23.3 to − 25.7‰ is observed in Interval 4 (56–61 m). Upwards, the δ13Corg values increase and reach − 24.4‰ at ~ 73 m. This interval is punctuated by a large lenticular sandstone block in the lower part, which results in an ~8 m gap in the δ13Corg curve. At the top of the sequence (73–78 m), the δ13Corg values are stable at −24.5‰. 4.4. TOC content The TOC content, which ranges from 0.5% to 1.5%, shows a good relationship with the lithology. In the lower part (0–23 m), the TOC

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Table 1 Lower Aptian carbon isotope data of the Gucuo II section, southern Tibet. Samples

Meters

δ13C (‰ PDB)

Samples

Meters

δ13C (‰ PDB)

14GCII 01 14GCII 02 14GCII 03 14GCII 04 14GCII 05 14GCII 06 14GCII 07 14GCII 08 14GCII 09 14GCII 10 14GCII 11 14GCII 12 14GCII 13 14GCII 14 14GCII 15 14GCII 16 14GCII 17 14GCII 18 14GCII 19 14GCII 20 14GCII 21 14GCII 22 14GCII 23 14GCII 24 14GCII 25 14GCII 26 14GCII 27 14GCII 28 14GCII 29 14GCII 30 14GCII 31 14GCII 32 14GCII 33

0.8 1.65 2.45 3.25 4.1 4.9 5.7 6.5 7.35 8.15 8.95 9.8 10.6 11.4 12.2 13.05 13.85 14.65 15.5 16.30 17.1 17.95 18.75 19.55 20.35 21.2 22.0 22.65 23.70 24.75 25.25 26.8 28.25

−26.23 −26.03 −26.21 −26.33 −26.06 −25.68 −25.58 −25.74 −25.62 −25.69 −25.81 −25.28 −25.30 −24.95 −25.24 −25.31 −25.47 −25.48 −25.72 −25.30 −25.02 −25.03 −25.16 −25.52 −24.96 −24.80 −25.17 −25.08 −24.35 −25.14 −24.84 −24.95 −25.12

14GCII 34 14GCII 35 14GCII 36 14GCII 37 14GCII 38 14GCII 39 14GCII 40 14GCII 41 14GCII 42 14GCII 43 14GCII 44 14GCII 45 14GCII 46 14GCII 47 14GCII 48 14GCII 49 14GCII 50 14GCII 51 14GCII 52 14GCII 53 14GCII 54 14GCII 55 14GCII 56 14GCII 57 14GCII 58 14GCII 59 14GCII 60 14GCII 61 14GCII 62 14GCII 63 14GCII 64 14GCII 65 14GCII 66

29.9 31.4 32.95 34.0 35.05 36.05 37.1 38.65 40.2 41.7 43.25 44.8 46.35 47.9 49.45 51.0 52.0 53.05 54.3 55.5 56.75 58.0 59.2 59.25 60.0 60.7 61.7 70.4 71.85 73.3 74.75 76.15 77.6

−24.94 −24.82 −24.75 −24.72 −24.78 −24.49 −24.62 −24.53 −24.59 −24.84 −24.84 −24.45 −24.66 −24.47 −24.53 −24.61 −23.76 −24.18 −24.20 −23.30 −24.18 −24.47 −24.69 −24.90 −25.10 −25.73 −25.19 −24.82 −24.69 −24.39 −24.58 −24.58 −24.66

contents are normally less than 1%, while they are ~0.5% higher in the upper part (25–78 m). In Units 3 and 6, the TOC contents decline by 0.5%. The shale intercalations in Unit 7 yield higher TOC content values of ~1% to 1.5%. 4.5. Maceral composition of kerogen The maceral composition was analyzed for 10 samples covering the upper part of Interval 3, all of Interval 4 and the lower part of Interval 5 (45–70 m). The kerogen is mainly composed of amorphous organic matter (~80%) with minor inertinite (~20%) (Table 2). The kerogen is Type II and the organic matter is mainly of marine origin. The maceral composition is stable throughout the investigated interval, suggesting that the negative excursion was not caused by changes in the organic matter composition. 5. Discussion 5.1. Records and correlation of OAE 1a The marine carbon isotope composition has been considered a useful tool for interpreting the stratigraphy of both pelagic sequences (e.g., Jarvis et al., 2006; Wendler, 2013) and shallow water successions (e.g., Adabi, 1997; Ferreri et al., 1997; Jenkyns, 1995), as well as for global correlation. Accordingly, we correlate the carbon isotope data obtained in this work with those published for the Aptian OAE 1a. According to the pattern of the δ13C curve for bulk carbonates and organic matter, the δ13C curve representing the OAE 1a can be divided into six segments (C3 to C8 of Menegatti et al., 1998). The most distinctive feature of the OAE 1a is a large negative excursion of the δ13C values (C3 segment). This negative δ13C anomaly defines the onset of the OAE 1a and is believed to reflect a major carbon cycle perturbation that was caused by a massive release of 13C–depleted carbon into the ocean-

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Fig. 7. Lithology column, δ13C and TOC values of the Gucuo II section.

atmosphere reservoir. The magnitude of the negative δ13C spike (CIE) reaches as much as − 3‰ in the marine carbonates and − 4‰ to −5‰ in the organic carbon. Segment C4 was initially described as an abrupt step-like positive excursion (Menegatti et al., 1998). The duration of segment C4 is 300–320 kyr (Li et al., 2008; Kuhnt et al., 2011), thereby lasting 10 times longer than C3, which had a duration of 27–44 kyr (Li et al., 2008). The high-resolution record of the C4 segment in the Camping section reveals a division into four subsegments (Kuhnt

Table 2 Maceral composition of the kerogens from the Gucuo II section, southern Tibet. Samples

Amorphous organic matter (%)

Inertinite (%)

Vitrinite (%)

14GCII 45 14GCII 48 14GCII 50 14GCII 52 14GCII 54 14GCII 56 14GCII 58 14GCII 59 14GCII 60 14GCII 61

84 86 80 88 80 72 85 87 78 73

15 14 19 12 20 28 15 13 22 26

1 0 1 0 0 0 0 0 0 1

et al., 2011): an initial slow increase (C4a, lasting 124 kyr), followed by a rapid increase (C4b, lasting 59 kyr), an intermittent plateau (C4c, lasting 71 kyr), and the final stepped increase (C4d, extending over 76 kyr). The boundary between segments C1 and C2 coincides with the Barremian/Aptian boundary (e.g., Menegatti et al., 1998). The C2 segment is characterized by a long-term negative trend in the lower part followed by a positive trend upward (Li et al., 2008; Bottini et al., 2012; Graziano et al., 2013). In the Gucuo II Section, a negative CIE can be observed in Interval 4 (Fig. 7). The δ13C value declines from −23.3‰ to −25.7‰, which corresponds to the segment C3 (Fig. 8). The δ13C values in the Gucuo II section are similar to those in the Santa Rosa Canyon and La Boca Canyon in NE Mexico (Bralower et al., 1999). They are significantly higher than those in the western Tethyan and Pacific sections (Fig. 8). Based on the ammonite biostratigraphy, our studied succession does not include the lowermost Aptian ammonite zone (i.e., D. oglanlensis). We interpret the δ13Corg data from the lower part of the succession (Intervals 1–3) as the upper C2 segment (Fig. 8), because the boundary between segments C1 and C2 coincides with the Barremian/Aptian boundary (e.g., Menegatti et al., 1998). Between 61 and 72 m (Interval 5 on Fig. 7) of the Gucuo II section, the δ13Corg values increase from − 25.7‰ to − 24.4‰. Despite the


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Fig. 8. Stratigraphic comparison of the δ13C records for OAE 1a sequences from southern Tibet, Mexico, western Tethys, and the equatorial Pacific Ocean. The blue shaded area marks segment C3 of the reference sections for the OAE 1a (e.g., Menegatti et al., 1998), and the yellow shaded area marks segment C4. The red dashed line is the boundary of C4 a + b and C4 c + d (Kuhnt et al., 2011).

~8 m gap, an increasing trend in the δ13Corg values is observed through Interval 5. An ~0.5‰ positive excursion is found at the base of this interval. Above the gap, a further rapid increase of +0.5‰ occurs. In the uppermost part of the section, high δ13Corg values are sustained at ~− 24.5‰ (Interval 6 on Fig. 7). The thickness of Interval 5 is ~ 12 m, which is approximately two times thicker than Interval 4. Considering that the depositional environment has not changed dramatically throughout the section, we assume that the sedimentation rates are similar during Intervals 5 and 4. This implies that the duration of this positive excursion is around two times longer than segment C3. Although the stratigraphic record in southeast France indicates that the duration of C3 could be longer than 100 kyr, astrochronological studies in other representative sections suggest that it is ~10 times shorter than C4. Therefore, we suggest that our Interval 5 of Gucuo II section reflects the subsegments C4a and C4b, whereas our Interval 6 represents the lower part of subsegment C4c (Fig. 8).

5.2. Factors influencing δ13Corg values The δ13Corg values in the Gucuo II Section are by ~2‰ higher than in the other known European and Pacific OAE 1a sections. On the other hand, they are slightly lower than the records in northeast Mexico where diagenetic influences were envisioned. The minimum δ13Corg value for the Cau section in Spain is ~ − 27‰, which is slightly lower than our result. Meanwhile, the absolute values in the Cau section are higher than those in Italy and Switzerland. Trends of discrepancies in the curves of δ13Corg and δ13Ccarb can be observed in the Cau section (de Gea et al., 2003), which implies the possible influences of diagenetic processes on the carbon isotope values. It appears that the diagenetic alteration of the Gucuo II section samples could be one of the main reasons for the higher observed values of δ13Corg. Changes in carbon isotope fractionation (εp) by marine phytoplankton could provide another explanation for the variation of the δ13Corg

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values among the different basins. A positive relationship between εp and the ambient temperature has been experimentally observed (Hinga et al., 1994). During the Early Cretaceous, the OAE 1a sections in Europe and in the Pacific Ocean were located at low northern latitudes. In contrast, the studied Gucuo area was situated in the southern hemisphere, between the mid to high latitudes at that time. According to recent paleogeographic reconstructions, the northern continental margin of India was still between 40 − 50°S during Aptian − early Albian (~125–110 Ma, e.g., Stampfli and Borel, 2002; van Hinsbergen et al., 2012; Metcalfe, 2013) even though the Indian continent had already broken away from Gondwana at ~132 Ma (e.g., Zhu et al., 2009). This provides a paleogeographically more feasible scenario for the lower temperatures of surface seawater at the study area than in the western Tethys and equatorial Pacific Ocean. Therefore, we propose that the lower εp led to less 12C absorbed by organisms in the Gucuo area and consequently higher δ13Corg values. However, the temperature gradient from equator to pole was significantly reduced during the OAE 1a (e.g., Mutterlose et al., 2014), so the effect of εp on the δ13C values remains to be evaluated. The δ13Corg values principally reflect the isotopic composition of carbon sources (e.g., Hayes, 1993). Dissimilarities among the carbon sources of different basins could result in a spatial variation of the δ13Corg values. Although many factors can influence the values of δ13Corg, and considering that the Gucuo II section is located within an orogenic belt, we speculate that diagenetic alternations are the major cause for the higher δ13Corg values. 6. Conclusions The lower Aptian succession in the Gucuo area yields abundant ammonites. Consequently, for the first time, we document the presence of the lower Aptian D. forbesi and D. deshayesi zones in the eastern Tethys region. Biostratigraphic analysis of the recovered ammonites allows us to substantiate the expanded occurrence of the Deshayesitids group during the early Aptian. Our new data shed light on a putative marine connection between the western Tethys, the Transcaspian area, and the eastern Tethys Himalayan region. Therefore, the data support a reliable long-distance correlation between these two remote regions. Moreover, they can be used in paleobiogeographic modelling and constraints for the early Aptian. In this study, we present a high-resolution carbon isotope curve for the lower Aptian succession in southern Tibet. The δ13Corg curve can be correlated with early Aptian carbon isotope records in the western Tethys and Pacific Ocean areas. A distinct negative excursion of 2.4‰ in the upper part of the section is identified as the segment C3 of the OAE 1a and the subsequent positive excursion corresponds to segment C4. The absolute carbon isotope values in the Gucuo area are higher than those of other known sections in the western Tethys and in the equatorial Pacific Ocean. We suggest that diagenetic alteration is the main cause for these higher absolute values. Acknowledgments We thank Prof. K. Föllmi and one anonymous reviewer for their constructive comments. Prof. J. Mutterlose and T. Algeo made important contribution for improving the manuscript, helping us very much in preparing the final version of this paper. We are grateful to Dr. Jiawei Zhang and Xinyu Qian for their help with the fieldwork. This study was financially supported by the National Natural Science Foundation of China (41672104), the National Basic Research Program of China (973 Project, 2012CB822005) and Zhongba 1:50,000 geological mapping projects by the Chinese Bureau of Geological Survey (12112011086037, 1212011121229). References Adabi, M.H., 1997. Application of carbon isotope chemostratigraphy to the Renison dolomites, Tasmania: a Neoproterozoic age. Aust. J. Earth Sci. 44, 767–775.

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