Evolution of mid-Cretaceous radiolarians in response to oceanic anoxic events in the eastern Tethys (southern Tibet, China)

Journal Pre-proof Evolution of mid-Cretaceous radiolarians in response to oceanic anoxic events in the eastern Tethys (southern Tibet, China) Tianyang Wang, Guobiao Li, Jonathan C. Aitchison, Lin Ding, Jiani Sheng PII:

S0031-0182(19)30558-9

DOI:

https://doi.org/10.1016/j.palaeo.2019.109369

Reference:

PALAEO 109369

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received Date: 8 June 2019 Revised Date:

5 September 2019

Accepted Date: 5 September 2019

Please cite this article as: Wang, T., Li, G., Aitchison, J.C., Ding, L., Sheng, J., Evolution of midCretaceous radiolarians in response to oceanic anoxic events in the eastern Tethys (southern Tibet, China), Palaeogeography, Palaeoclimatology, Palaeoecology, https://doi.org/10.1016/ j.palaeo.2019.109369. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

Evolution of mid-Cretaceous radiolarians in response to oceanic anoxic

2

events in the eastern Tethys (southern Tibet, China)

3

Tianyang Wanga, b, c, Guobiao Lia, b, Jonathan C. Aitchisonc, Lin Dingd, Jiani Shengc

4 5

a

6 7

b

8 9

c

10 11

d

12 13 14 15 16 17

Corresponding author at: State Key Laboratory of Environmental Geology and Biogeology, China University of Geosciences, Beijing 100083, China.

18

* Corresponding

State Key Laboratory of Environmental Geology and Biogeology, China University of Geosciences, Beijing 100083, China School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China School of Earth and Environmental Sciences, University of Queensland, Brisbane 4072, Australia Institute of Geology of Qinghai-Tibet Plateau, Chinese Academy of Sciences, Beijing 100101, China

E-mail addresses: [email protected] (T.Y. Wang), [email protected] (G.B. Li), [email protected] (J.C. Aitchison), [email protected] (L. Ding), [email protected] (J.N. Sheng) author. Email address: [email protected] (G.B. Li)

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20

Abstract

21

Oceanic anoxic events (OAEs) are well-known from their widespread

22

black shale and carbon isotopic excursions in the (western and eastern) Tethys

23

and Atlantic Ocean. However, the weakest link in the studies of the OAEs is

24

biological evolution. Sedimentological, biostratigraphical and geochemical data

25

from Albian to Coniacian strata in southern Tibet recorded turnover events and

26

fluctuations in diversity of the radiolarian fauna within the eastern Tethys

27

during OAE 1d and OAE 2. Abundant radiolarian fossils were obtained from

28

the Gyabula Formation, with 93 species from 43 genera identified and assigned

29

to the mid to Upper Cretaceous Acaeniotyle umbilicata, Archaeospongoprunum

30

tehamaensis, Crucella cachensis, Alievium superbum, and Dictyomitra formosa

31

zones. The association of carbon isotopic excursions, black shale and

32

radiolarian turnover indicates extensive changes in the ocean-climate system.

33

Nutrients are made increasing available to the marine plankton through

34

submarine volcanic activity and rising sea-level, which were a likely cause of

35

radiolarian turnover at or near the OAEs. Active submarine tectonism-

36

volcanism leads to the expansion of the hypoxic zone and may cause many

37

deeper dwelling forms to become extinct whereas most of the shallower

38

dwelling radiolarians survive. Radiolarian evolution thus provides a useful

39

record with which to seek understand relationships between climate,

40

paleoceanographic processes and plankton evolution.

41

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Keywords ：biostratigraphy, turnover events, black shale, OAEs, biological

43

evolution

44

45

1. Introduction

46

Global deposition of organic-rich black sediments occurred in oceanic and

47

marginal basins during the mid-Cretaceous interval. Discrete events of this kind

48

are commonly referred to as “oceanic anoxic events” (OAEs). They profoundly

49

affect the evolution and productivity of the marine biosphere (Schlanger and

50

Jenkyns, 1976; Leckie et al., 2002; Jenkyns, 2010; Sabatino et al., 2018). The

51

two widespread OAEs that occurred during the late Albian to Coniacian are the

52

late Albian OAE 1d and the Cenomanian-Turonian boundary OAE 2. They are

53

characterized by pronounced positive carbon isotope excursions (CIE) in both

54

marine carbonates and terrestrial realms (Tsikos et al., 2004; Gröcke et al., 2006;

55

Wu et al., 2009; Barclay et al., 2010; Li et al., 2017; Yao et al., 2018; Laurin et

56

al., 2019). Both these OAE horizons provide excellent opportunities to

57

investigate dynamic ocean-atmosphere interactions during globally significant

58

perturbations in the carbon cycle. How biostratigraphic, carbon isotope

59

sedimentary and geochemical processes interact and evolve through a major

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perturbation

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Cenomanian/Turonian boundary is of particular interest (Erbacher and Thurow,

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1997; Bjerrum et al., 2006; Bottini and Erba, 2018).

to

the

climate

and

global

carbon

cycle

across

the

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An improved understanding of the disruption to pelagic eco-systems

64

caused by OAE 1d and 2 may be obtained through studies of the biodiversity

65

and abundance of planktonic organisms, especially those plankton groups

66

capable of generating biogenic sediments (e.g. Erbacher et al., 1996; Leckie et

67

al., 2002). Among the fossilized plankton, radiolarians are globally utilized as

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good bioindicators for palaeoceanographic variations and appear to have

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responded to the environmental perturbation of OAE with significantly altered

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abundance and assemblage compositions. Evolutionary events have been

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described around OAEs such as OAE la (e.g. Coccioni et al., 1992; Bralower et

72

al., 1994; Chen et al., 2017; Li et al., 2019), OAE 1d (e.g. Bornemann et al.,

73

2005; Yao et al., 2018) and especially OAE 2 (e.g. Jarvis et al., 1988; Musavu-

74

Moussavou et al., 2006; Navidtalab et al., 2019). More is known about

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extinction/radiation events of planktonic foraminifers, nannofossils and

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ammonites around OAEs. The evolution of mid-Cretaceous (Aptian-Turonian)

77

radiolarians is also marked by numerous extinction/radiation events, which

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correlate well with most of the OAEs. Previous investigations of radiolarians

79

around these events concentrated mainly on OAE 2, the Cenomanian/Turonian

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Boundary Event with less investigation of the older OAEs in the mid-

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Cretaceous (Marcucci Passerini et al., 1991; Erbacher et al., 1996; O’Dogherty

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and Guex, 2002).

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The Yongla study section in southern Tibet represents a part of the Neo-

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Tethys. Previous research on the C/T interval in southern Tibet focused mainly

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on foraminiferal biostratigraphic records, carbon-isotopes and responses of

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marine biota to the OAE 2 (Wan et al., 2003a, b; Wendler et al., 2009; Wang et

87

al., 2011; Li, 2012; Li et al., 2017). Because there is a general lack of

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macrofossils, radiolarians were typically used as one of the most important age-

89

indicators.

90

paleoceanography, and paleogeography. Radiolarian deposition from the

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Middle Triassic to early Eocene was widespread at pelagic depths in southern

They

also

play

an

important

role

in

biostratigraphy,

92

Tibet (e.g. Wu and Li, 1982; Ziabrev et al., 1999; 2003; Ding et al., 2003; Wu,

93

2007; Li et al., 2009, 2011; Wang et al., 2017; Li and Li, 2019). However,

94

previous studies have focused on Jurassic to Lower Cretaceous or Paleogene

95

radiolarian biostratigraphy and somewhat less on the Albian-Coniacian interval

96

(e.g. Matsuoka et al., 2002; Liu and Aitchison, 2002; Liang et al., 2012).

97

Cretaceous radiolarian community evolution can be used as palaeoclimate

98

and palaeoecological representative during OAEs because of the high sensitivity

99

of radiolarians. Therefore, the main objectives of this study are 1) to establish

100

radiolarian biostratigraphic zonations, 2) to define the bioevents that occurred

101

during the Albian to Coniacian interval in southern Tibet, and 3) to explore the

102

linkages between radiolarian turnover events, the cycling of carbon and the sea-

103

level change.

104

105

2. Geological setting

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The Yongla section (28°58′14″N, 89°46′41″E) is situated approximately 23

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km E of Gyangze in southern Tibet. This area belongs tectonically to the

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northern Tethyan Himalayan Belt (Fig. 1), which is located between the

109

Yarlung-Tsangpo Suture Zone (YTSZ) to the north and the Higher Himalaya

110

zone to the south. In southern Tibet, the YTSZ contains remnants of what once

111

existed within Neo-Tethys. It is marked by discontinuous ophiolite complexes

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and melange, delineating the E-W trending contact between India and Eurasia

113

for 1000s of kms (Gansser, 1980; Searle et al., 1987; Aitchison et al., 2000;

114

Hebert et al., 2012). Mesozoic strata provide a record that can be used to

115

constrain and reconstruct the tectonic evolutionary history of Neo-Tethys

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(Wendler et al., 2009; Li et al., 2011; Chen et al., 2017). The North Tethys

117

Himalayan Belt throughout the Cretaceous is characterized by semi-pelagic to

118

pelagic sediments and contains plankton assemblages including belemnites,

119

ammonites, planktonic foraminifers, and radiolarians.

120

Lower Cretaceous sedimentary strata in the study area are assigned to the

121

Gyabula Formation, characterized mainly by black siliceous/calcareous shales,

122

cherts and lenticular marls. It conformably overlies the Weimei Formation.

123

Ammonites, belemnites, foraminifers and radiolarians indicate a Berriasian-

124

Early Santonian age. Abundant planktonic foraminiferal and radiolarian fossils

125

have been recognized including, Hedbergella spp., Rotalipora sp., Theocampe

126

sp., and Whiteinella archaeocretacea (Li et al., 2005). The overlying lower

127

Chuangde Formation is composed of red shales intercalated with thin marlstone

128

beds (CORBs). Two age-diagnostic foraminiferal taxa Dicarinella asymentrica

129

and Globotruncana ventricose indicate Santonian-lower Campanian affinity.

130

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3. Materials and methods

132

In this study, fossil data cover the stratigraphic distribution of 93 Albian to

133

Coniacian radiolarian taxa in southern Tibet, and extend over a time interval of

134

about 20 Ma (Table S1). The authors undertook a regional geologic survey in

135

the Gyangze area, and detailed measurement of the Cretaceous Yongla section

136

(28°58′14″N, 89°46′41″E) was made for biostratigraphic study, with a total of

137

162 samples collected. The samples, of approximately 1 kg weight, were

138

collected at 0.5 m intervals for chert and 1 m intervals for shale (Fig. 2). Species

139

identification is chiefly based on taxonomic studies of mid-Cretaceous Tethyan

140

radiolarians (Wu, 1982 and 1986; O’Dogherty, 1994; Bak, 2011). Evolutionary

141

rates were calculated following the method of Wei and Kennett (1986). We

142

compared the biotic change as expressed by evolutionary rates amongst

143

radiolarians (this study) and, benthic and planktonic foraminifers with the

144

temporal and spatial distribution of the OAEs, as well as other proxies of global

145

change, including carbon isotopes (Jia et al., 2013; Li et al., 2017; Yao et al.,

146

2018).

147

Most skeletons of Mesozoic radiolarians are have been replaced by

148

chalcedony or other polymorphs of silica over geologic time. Such changes in

149

the radiolarian skeletons occur to varying degrees in different parts of the host

150

rock. Differences in geochemical composition, porosity and dissolution patterns

151

may explain the diverse preservational conditions in various lithologies.

152

Compared to the coarse fraction (weathered arenite) of the rock, radiolarians are

153

less numerous in the fine fraction (argillites or shales). However, they have

154

better preserved and spines and show a greater variety on the external

155

morphology (DeWever et al., 2001). In this study, radiolarians were most

156

commonly preserved in chert and siliceous shale. To free the radiolarians from

157

the siliceous rocks, the standard procedures (Dumitrica 1970), Pessagno and

158

Newport 1972) were followed. Samples were broken into 5 cm3 pieces and

159

placed in plastic beakers. A diluted (5%) hydrofluoric acid solution was added

160

to cover the rocks for 20–24 hours. The samples were wet sieved and the 63–

161

180 µm fraction was examined. Representatives of each species were measured,

162

gold-coated and imaged using a Zeiss Supra 55 Scanning Electron Microscope

163

(SEM). Only when radiolarian skeletons are sufficiently well preserved and

164

different from their surrounding matrix does this method produce useful

165

radiolarian fossils.

166

The extraction and identification of radiolarians was carried out in the

167

Micropalaeontology Laboratory of the China University of Geosciences

168

(Beijing) and the School of Earth and Environmental Sciences of the University

169

of Queensland, Australia.

170

171

4. Results

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4.1. Radiolarian biostratigraphy

173

Established Cretaceous radiolarian biostratigraphy was applied to this

174

study using age assignments for radiolarian zones of Moore (1973), Pessagno

175

(1976, 1977), Taketani (1982), Sanfilippo and Riedel (1985), Vishnevskaya and

176

Kazintsova (1990), Thurow (1990), O’Dogherty (1994) and Bragina (2016) (Fig.

177

3).

178

Building the chronology required to calculate the rates of changes is a

179

challenge when studying the evolution of organisms during any given period in

180

geological history. Well-preserved and abundant radiolarian fossils were

181

recovered from the Gyabula Formation in the Yongla section. Ninety-three

182

species from forty-three radiolarian genera were extracted from the cherts and

183

siliceous shales of the Gyabula Formation and identified (Figs. 4-7, S1, and S2).

184

Five radiolarian zones recognized in this study are assignable to the mid-

185

Cretaceous:

186

Archaeospongoprunum tehamaensis Zone (lower Cenomanian), Crucella

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cachensis Zone (upper Cenomanian), Alievium superbum Zone (Turonian), and

188

Dictyomitra formosa Zone (Coniacian).

189

4.1.1. Acaeniotyle umbilicata Zone

Acaeniotyle

umbilicata

Zone

(mid-upper

Albian),

190

This zone includes the co-occurrence of A. umbilicata, Holocryptocanium

191

barbui, and Thanarla conica. It corresponds to samples YA 2-YB 13, which

192

contain abundant and moderately preserved radiolarian tests. Representative

193

species of this zone include Acaeniotyle longispina (Squinabol), A. sp., A.

194

tribulosa Foreman, A. umbilicata (Rust), A. vitalis O’Dogherty, Crucella

195

messinae Pessagno, Dicerosaturnalis amissus (Squinabol), Dictyomitra

196

communis (Squinabol), Dispongotripus acutispina Squinabol, Hiscocapsa

197

grutterinki (Tan), H. uterculus (Parona), H. verbeeki (Tan), Holocryptocanium

198

japonicum Nakaseko and Nishimura, H. barbui Dumitrica, Holocryptocapsa

199

hindei Tan, Loopus ?nuda (Schaaf), Orbiculiforma tuberculata Wu, Praecaneta

200

mimetica Dumitrica, Praeconocaryomma jiangzeensis Wu, Pseudoeucyrtis

201

hanni (Tan), Pseudodictyomitra primitiva (Matsuoka and Yao), P. hornatissima

202

(Squinabol), P. lilyae Tan, P. nuda (Schaaf), Quinquecapsularia parvipora

203

(Squinabol),

204

(Nakaseko and Nishimura), Thanarla lacrimula (Foreman), T. brouweri (Tan),

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T. conica (Squinabol), Triactoma cellulosa Foreman, T. hybum Foreman,

206

Trimulus fossilis (Squinabol), and Ultranapora durhami Pessagno.

Spongostichomitra

elatica

(Aliev),

Stichomitra

japonica

207

The first occurrence (FO) of A. tehamaensis, C. irwini, Halesium

208

quadratum, Orbiculiforma belliatula, O. depressa, P. macphersoni, and P.

209

putahensis define the top of this zone. However, the base of the zone cannot be

210

defined herein due to the lack of suitable outcrop. The present zone includes

211

strata characterized by the co-occurrence of H. barbui and T. conica. H. barbui

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extends from the A. umbilicata Zone into the O. somphedia Zone

213

(approximately Albian to Cenomanian), and has been reported from Japan, NW

214

Pacific, eastern Atlantic and southern Tibet (Sanfilippo and Riedel, 1985). H.

215

uterculus was reported from the Lower Cretaceous of the Maiolica Formation at

216

Cittiglio (northern Venetian Alps, North Italy) and extends from the Asseni

217

Zone to the Turbocapsula Zone (upper Barremian to lower Albian)

218

(O’Dogherty, 1994). T. brouweri may be one of the most illustrated species

219

among the Early Cretaceous radiolarians. It is reported from Barremian to

220

middle Albian strata from Italy and Spain (O’Dogherty, 1994). The lower part

221

of the Turbocapsula Zone of O’Dogherty is characterized by the FO of the H.

222

verbeeki and the extinction of H. hindei, which make their last occurrence in the

223

lower part of the A. umbilicata Zone of this study. In conclusion, the A.

224

umbilicata Zone is assigned to upper Albian by correlation with Foreman’s

225

zonation.

226

4.1.2. Archaeospongoprunum tehamaensis Zone

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A. tehamaensis Zone corresponds to Samples YB 13-29, which contain a

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well-preserved and diverse fauna including Acaeniotyle diaphorogona Foreman,

229

A. longispina (Squinabol), A. tribulosa Foreman, A. vitalis O’Dogherty,

230

Archaeospongoprunum

231

O’Dogherty, Crucella messinae Pessagno, Dactyliodiscus lenticulatus (Jud),

232

Dispongotripus acutispina Squinabol, H. quadratum Pessagno, Orbiculiforma

233

belliatula

234

Pessagnobrachia

235

Pseudoeucyrtis

Wu,

O.

tehamaensis

depressa

?fabianii hanni

Pessagno,

Wu,

(Squinabol), (Tan),

Cavaspongia

Patellula P.

sphaerica

cognata

O’Dogherty,

macphersoni

O’Dogherty,

Pseudodictyomitra

nuda

(Schaaf),

236

Pseudoaulophacus

putahensis

Pessagno,

Quinquecapsularia

parvipora

237

(Squinabol), Stichomitra stocki (Campbell et Clark), Thanarla pulchra

238

(Squinabol), T. cellulosa Foreman, Ultranapora crassispina (Squinabol), and

239

Xitus triangularis Wu.

240

The base of the A. tehamaensis Zone is characterized by the FO of A.

241

tehamaensis, H. quadratum, O. belliatula, O. depressa, P. macphersoni, and P.

242

putahensis. The top of this zone is delimited by the LO of A. tehamaensis. A.

243

diaphorogona ranges from below the S. septemporatus Zone through most of

244

the A. umbilicata Zone (approximately Tithonian to Turonian), and has been

245

reported from Japan, the NE Atlantic, California, North Pacific and northern

246

Italy (Pessagno, 1976, O’Dogherty, 1994). S. japonica has been reported from

247

the late Albian-Cenomanian (H. barbui-H. geysersensis assemblage) of the

248

Suzaki Formation (Shimanto belt, SW Japan) (Taketani, 1982). One of the most

249

important radiations occurring during the mid-Cretaceous seems to have its

250

origin in S. japonica, from which evolved the genera Phalangites, Trimulus and

251

the lineage Rhopalosyringium-Prodromus (O’Dogherty, 1994). Previously, P.

252

nuda was known from an Aptian radiolarian zone (Turbocapsula costata Zone)

253

in the Xialu Chert, southern Tibet. However, Wu (2007) renamed this zone as

254

Tricapsula costata Zone by comparing “Turbocapsula costata from the western

255

Mediterranean regions with Tricapsula costata of the Congdu Formation.

256

Meanwhile, the “Turbocapsula costata” from the Xialu Chert should be

257

Tricapsula costata. Therefore, it is better to assign the Xialu Chert to the

258

Aalenian to Cenomanian interval at present. C. messinae was reported from the

259

lower Cenomanian Fiske Creek Formation, Californian Coast Ranges (Pessagno,

260

1976) and evolved from C. euganea, which accompanied it for much of its

261

range, and gave rise to C. irwini. Based mainly on correlation with Pessagno’s

262

zonation, the A. tehamaensis Zone is assigned to lower Cenomanian.

263

4.1.3. Crucella cachensis Zone

264

The representative elements of this fossil zone include (Samples YC 15

265

and 27) A. diaphorogona Foreman, Archaeospongoprunum pontidum Bragina,

266

C. cachensis Pessagno, Crucella irwini Pessagno, C. latum (Lipman),

267

Cryptamphorella conara (Foreman), Dactyliodiscus cayeuxi Squinabol,

268

Diacanthocapsa ancus (Foreman), Dispongotripus acutispina Squinabol,

269

Excentropylomma cenomana Dumitrica, Godia ornata Wu, Godia floreusa Wu,

270

H. quadratum Pessagno, Thanarla pulchra (Squinabol), Triactoma micropora

271

Bragina,

272

Pseudoaulophacus cf. putahensis Pessagno, Pseudoeucyrtis hanni (Tan),

273

Pessagnobrachia

274

Quadrigastrum oculus O’Dogherty, Rhopalosyringium elegans (Squinabol), T.

275

cellulosa Foreman, Wrangellium versum Wu, Xitus decorus Wu, and Xitus

276

triangularis Wu.

P.

cognata

fabianii

O’Dogherty,

(Squinabol),

Pseudodictyomitra

Q.

parvipora

rigida

Wu,

(Squinabol),

277

Amongst them, G. floreusa, G. ornata, P. rigida, P. lilyae, X. decorus, and

278

X. triangularis have been reported from the Upper Cretaceous Cenomanian

279

Congdu Formation in southern Tibet (Li and Wu, 1985; Wu, 1986). A

280

remarkable feature of this zone is the presence of the stratigraphically

281

significant species C. cachensis, which characterizes the upper Cenomanian to

282

Turonian sections of the Moscow Basin and represents an index species of the

283

synonymous C. cachensis- A. superbum Zone (Vishnevskaya and Kazints,

284

1990). A. pontidum ranges from middle to upper Cenomanian in northern

285

Turkey. T. micropora has been reported from Cenomanian to Turonian of Spain

286

and northern Turkey (Bragina, 2004). T. pulchra appears to be stratigraphically

287

restricted to the middle Aptian-uppermost Cenomanian, and has been recorded

288

from Japan, NW Pacific, California, Costa Rica, southern Europe and southern

289

Tibet (Matsuoka et al., 2002). The co-occurrence of A. diaphorogona, C.

290

conara, C. irwini, D. ancus, E. cenomana, H. quadratum and Q. oculus is

291

concordant with this zone assignment. In conclusion, the interval characterized

292

by the above-listed radiolarian taxa is named as the C. cachensis Zone, which is

293

roughly correlated with the upper Cenomanian.

294

4.1.4. Alievium superbum Zone

295

Taxa in the A. superbum Zone were collected from the upper part (Samples

296

YC 38, 42, and 48) of the Gyabula Formation. The top of this zone is defined by

297

the FO of A. superbum. Abundant species include A. rebellis O’Dogherty,

298

Angulobracchia portmanni Baumgartner, Archaeodictyomitra squinaboli

299

Pessagno, Cavaspongia antelopensis Pessagno, C. californiaensis Pessagno,

300

Crucella euganea (Squinabol), C. irwini Pessagno, C. latum (Lipman), C.

301

conara (Foreman), D. ancus (Foreman), Dictyomitra napaensis Pessagno,

302

Dictyodedalus

303

Holoctyptocanium tuberculatum Dumitrica, Paronaella solanoensis Pessagno,

304

Pseudoeucyrtis hanni (Tan), Pseudoaulophacus cf. putahensis Pessagno,

305

Pessagnobrachia

306

Praeconocaryomma

307

O’Dogherty, Q. parvipora (Squinabol), Sciadiocapsa euganea Squinabol,

308

Stichomitra manifesta Foreman, Triactoma parva (Squinabol), T. hexeris

309

O’Dogherty, T. cellulosa Foreman, T. micropora Bragina, and Ultranapora

310

cretacea (Squinabol),

cretaceus

O’Dogherty,

fabianii lipmanae

(Squinabol), Pessagno,

H.

P.

quadratum

?fabianii

Pessagno,

(Squinabol),

Quinquecapsularia

panacea

311

The best age-diagnostic taxon is A. superbum, occurrence of which is

312

restricted to the Turonian Zone in the western Tethys and California (Pessagno,

313

1976; O’Dogherty, 1994). H. quadratum is a well-known globally-distributed

314

taxon and has a range from lower Cenomanian to Turonian. A. squinabol, C.

315

californiaensis, P. solanoensis, and P. lipmanae have been reported from the

316

Turonian to Coniacian (A. superbum-A. praegallowayi Zone) of the Venado

317

Formation, California (Pessagno, 1976). D. ancus is known from the middle to

318

upper Cenomanian of northern Turkey, lower Turonian of the Crimean

319

Mountains and upper Maastrichtian of Moreno Formation, California. The co-

320

occurrence of A. stocktonensis, D. cretaceus, Q. panacea, S. euganea, T. hexeris,

321

T. parva and U. cretacea indicates a Turonian age for the A. superbum Zone. In

322

summary, the A. superbum Zone is assigned a Turonian age.

323

4.1.5. Dictyomitra formosa Zone

324

D. formosa Zone corresponds to the chert samples YC 48-58 collected

325

from the uppermost part of Gyabula Formation. The representative radiolarian

326

species recovered in this study include Archaeodictyomitra squinaboli Pessagno,

327

Archaeospongoprunum salumi Pessagno, A. stocktonensis Pessagno, C.

328

antelopensis Pessagno,

329

Holoctyptocanium tuberculatum Dumitrica, Orbiculiforma monticelloensis

330

Pessagno, Praeconocaryomma universa Pessagno, Praeconocaryomma sp. and

331

S. manifesta Foreman. This zone is characterized by the co-occurrences of

332

numerous species including D. formosa, P. solanoensis and S. manifesta.

333

Among which, D. formosa is a globally well-known taxon and has a range from

334

middle Cenomanian to lower Maastrichtian (Pessagno, 1976; O’Dogherty, 1994;

335

Bandini et al., 2008). S. manifesta is reported to range from the Turonian to

336

Coniacian (D. formosa Zone) of Japan (Taketani, 1982). P. solanoensis is

337

known from the Alievium praegallowayi Zone (Coniacian) with the Yolo

338

Formation, California. Besides, A. stocktonensis, A. salumi, C. conara, and L.

339

hexaxyphophorus have also been reported from the Coniacian to Campanian (A.

C. conara (Foreman), D. formosa Squinabol,

340

praegallowayi -C. espartoensis Zone), California (Pessagno, 1976). The first

341

occurrence of P. universa defines the base of the lower Coniacian S. fossilis

342

Zone (Taketani, 1982). A. stocktonensis is known from the Cenomanian to

343

Coniacian worldwide. It has been reported from the upper Cenomanian to lower

344

Turonian in the Crimean Mountains, and from the middle to upper Cenomanian

345

in Turkey (Tekin et al., 2015). O. monticelloensis was reported to occur near the

346

top of D. formosa Zone (upper Turonian-lower Coniacian) in Japan (Taketani,

347

1982). P. universa and O. monticelloensis have been reported from the

348

Coniacian to lower Maastrichtian (A. praegallowayi-D. koslovae zone) of

349

western Serbia and California (Pessagno, 1976). The co-occurrence of S.

350

manifesta is concordant to this zone assignment. Ultimately, the interval

351

characterized by the above-listed radiolarian taxa is named the D. formosa Zone,

352

which is roughly assigned to the Coniacian, based on the correlation with the

353

zonation of Taketani (1982).

354

355

4.2. Evolutionary pattern of mid-Cretaceous radiolarians

356

The highest turnover rates amongst mid-Cretaceous radiolarians examined

357

as part of this study occurred in the latest Albian, the mid-Cenomanian and the

358

Cenomanian/Turonian boundary, which highlights the environmental stress

359

associated with OAEs in the area (Fig. 8). A major faunal change occurred

360

during sea-level rise in the late Albian. Ten forms (23%) disappeared, such as T.

361

brouweri, T. hybum, U. crassispina, and U. durhami. A total of thirteen

362

Cenomanian taxa (43%) including T. pulchra, X. triangularis, G. floreusa, H.

363

quadratum and O. belliatula, appeared. These forms have been previously

364

reported from Gyangze, Saga and Zhongba in southern Tibet (Wu and Li, 1982).

365

On the other hand, OAE 1d is widely preserved as one black shale bed across

366

Tethys during R. appenninica foraminiferal biozone. It is associated with

367

marine organic matter. Another decrease in radiolarian diversity started from

368

the bottom of the R. reicheli zone to the bottom of R. cushmani foraminifer zone

369

(mid-Cretaceous) and lasted for nearly 1 Ma. Seven typical lower Cretaceous

370

radiolarian taxa (30%) like O. belliatula, C. meesinae and D. lenticulatus

371

disappeared. At the same time, the planktonic foraminifera suffered great rates

372

of extinction (Leckie et al., 2002). This extinction event amongst radiolarians

373

and foraminifers is well correlated with a positive carbon excursion, which

374

occurred within the R. reicheli Zone. The decline in the diversity of radiolarians

375

and planktonic foraminifers and the positive excursion of δ13C are concentrated

376

at the mid-Cretaceous OAE 1d level suggesting extensive environmental

377

changes.

378

High-resolution foraminiferal biostratigraphy reveals an expanded

379

Cenomanian–Turonian (C/T) boundary interval (Li et al., 2009a; Li, 2012) and

380

the δ13C record includes the main features of the classical positive carbon-

381

isotope excursion that characterizes the OAE 2 in southern Tibet (Wang et al.,

382

2001; Bomou et al., 2013; Li et al., 2017). The drastic mid-Cretaceous

383

radiolarian faunal change event is characterized by an accelerated decrease in

384

diversity species and illustrates the transition from the Cenomanian to the

385

Turonian. This event occurred during the transgressive anoxia in the latest

386

Cenomanian and correlates well with the OAE 2 (Pessagno, 1976; Thurow and

387

Kuhnt, 1986), which marks the

388

Cretaceous radiolarian taxa including C. cachensis, A. squinaboli, P. putahensis

389

and C. antelopensis. Sixteen taxa (48%) disappeared and twenty forms (42%)

390

first occurred during OAE 2. Radiolarian evolutionary patterns clearly show

391

that the highest rates of speciation occurred during the early Turonian. Typical

392

Turonian faunas consist of new forms like C. euganea, T. hexeris, P.

393

solanoensis and A. portmanni (Fig. 7). The interval between the C. cachensis

394

and the A. superbum zones is characterized by important development of the

395

genera Diacanthocapsa, Halesium and Cavaspongia. The radiolarian fauna in

396

the western Tethys (O’Dogherty, 1994; Musavu-Moussavou et al., 2007) and

397

California (Pessagno, 1977) also developed in the same way, which reflects an

398

apparent global pattern of OAE 2. Investigations of radiolarian data from the

399

east coast of North America (Thurow, 1988), California (Pessagno, 1976; 1977),

400

Italy (O’Dogherty, 1994) and Japan (Taketani, 1982) show similar evolutionary

401

patterns for radiolarians of the eastern Tethys, which demonstrates the global

402

character of this faunal turnover event. Therefore, the Cenomanian/Turonian

403

boundary should be placed between the radiolarian zones C. cachensis and A.

first appearance of several typical Late

404

superbum, which would place the CTB in the upper Gyabula Formation of the

405

Yongla section in Gyangze, southern Tibet.

406

407

5. Discussion

408

5.1. Biostratigraphic links with the isotope record and black shales

409

Due to environmental controls on the plankton community structure, the

410

high sensitivity of radiolarians to external changes can be used as

411

paleoceanographic and paleoclimate proxy during episodes of evolutionary

412

turnover. This investigation of mid-Cretaceous radiolarians from Tibet reveals

413

that major evolutionary events correlate strongly with carbon isotope changes

414

and black shale sedimentation.

415

Carbon isotope records have been established in the western and eastern

416

Tethys, eastern Pacific and North Atlantic Oceans for the latest Albian to

417

Cenomanian time (Wilson and Norris, 2001; Kennedy et al., 2004; Melinte-

418

Dobrinescu et al., 2015; Yao et al., 2018), which further confirms the global

419

nature and synchronicity of the C cycle perturbation during the OAE 1d.

420

Evolutionary patterns amongst radiolarians and planktonic foraminifers indicate

421

that the OAE 1d marked the overall diversity of this period (Wilson and Norris,

422

2001). During the Cenomanian-Turonian period, a positive excursion of δ 13C in

423

southern Tibet correlates well with those observed globally (Tsikos et al., 2004;

424

Meyers et al., 2012; Ma et al., 2014; Batenburg et al., 2016; Li et al., 2017). The

425

same seems true for the planktonic biostratigraphy. The FAD of A. superbum

426

appears to be synchronous compared to other sections, such as Italy and

427

California (Pessagno, 1976, 1977b; O’Dogherty, 1994; Musavu-Moussavou et

428

al., 2007). In the Yongla section, the A. superbum FAD is observed 6 m above

429

the black shales. Thus, the first appearance of A. superbum is stratigraphically

430

above the top of the organic-rich beds and corresponds to the beginning of the

431

Turonian in southern Tibet.

432

433

5.2. Sea-level change, submarine volcanism and productivity during OAEs

434

Several researchers have suggested that the OAE 2 may have been

435

associated with active submarine volcanism and increased marine productivity

436

(Bralower et al., 1997; Sinton and Duncan, 1997; Kerr, 1998). In the early stage

437

of anoxic events, active submarine tectonism-volcanism causes abrupt warming

438

of seawater and structural instability. Anomalous volcanism that occurred

439

during the late Cenomanian to Turonian may thus have played a decisive role in

440

a worldwide environmental disturbance. In the eastern Tethyan Ocean, available

441

age data indicated an intra-oceanic island arc subduction system had begun by

442

the Early Cretaceous (Aitchison et al., 2000). Arc volcanism continued through

443

the mid-Cretaceous and likely ceased with emplacement of the arc assemblage

444

onto the Indian passive margin in Paleocene time (Aitchison et al., 2007a, b).

445

The activity of submarine volcanoes in the Tethys Ocean (Zedong terrane) may

446

have resulted in the emission of large quantities of CO2 into the atmosphere,

447

concluding to a greenhouse effect. Furthermore, the injection of warm saline

448

intermediate or deep waters may have triggered OAE 2 and created favourable

449

conditions for the vertical advection of nutrients, widespread productivity,

450

expansion of oxygen minima, and the accumulation of organic matter (Huber et

451

al., 1999, 2002).

452

The abiotic events at the Albian-Cenomanian boundary include a eustatic

453

maximum, which terminated the Albian global transgression (Haq, 2014).

454

Radiolarians diversified rapidly during the sea-level and temperature rise

455

associated with this boundary (O’Dogherty and Guex, 2002). On the one hand,

456

the upper part of the A. umbilicata Zone represents the beginning of an

457

important

458

Neosciadiocapsidae, including species such as Sciadiocapsa euganea and

459

Dictyodedalus cretaceus. This development, together with the occurrence of

460

numerous other species, may be directly related to the more oxygenated water

461

mass (O’Dogherty, 1994). Simultaneously, the appearance of keeled

462

morphotypes and the continued increase in diversity, size and morphological

463

complexity amongst the planktonic foraminifers may also be related to sea-level

464

rise and higher temperatures (Hart 1990, Leckie et al., 2002). On the other hand,

465

the change in the lower part of the Gyabula Formation from black shales to

466

reddish chert indicates a transition to a well-oxygenated environment around the

467

Albian/Cenomanian boundary (Fig. 2). It has been shown that this steady

radiation

of

the

spumellarian

families,

Rotaformidae

and

468

scenario favoured the rapid development of new species in a well-oxygenated

469

environment. This suggests that sea-level rise and global temperature change

470

are two important factors for the diversification of new plankton genera during

471

the OAE 1d (Haq et al., 1988; Erbacher and Thurow, 1997).

472

Both global sea-level rise and climate warming are trademark

473

characteristics of the Cenomanian to early Turonian (Haq et al., 1987; Bice et

474

al., 2006; O’Brien et al., 2017). Radiolarian faunas showed a period of stability

475

after the OAE 1d and the mid-Cretaceous extinction event. However,

476

productivity associated with OAE 2 (Cenomanian/Turonian boundary) had an

477

important effect on the radiolarians and other plankton (Thurow et al., 1992;

478

Gale et al., 1993; Jenkyns et al., 1994; Sugarman et al., 1999; Wang et al.,

479

2001). Long-term sea-level rise and global warming during the Albian-Turonian

480

period resulted in: (1) flooded coastal plains and increased marine productivity;

481

(2) upwelling of deep (intermediate) waters and continental weathering which

482

provided nutrients for marine plankton; (3) enhanced opportunities for

483

diversification and evolutionary innovation for plankton through increased

484

access to nutrients and adaptation; (4) production of marine organic matter and

485

burial of black shale deposition. Thus, trends in global sea-level tend to run

486

parallel with marine productivity and plankton diversification.

487

One of the most important ecological characteristics of the radiolarians is

488

their vertical distribution. According to the data from both the water column and

489

surface sediments of the northeastern Pacific Ocean, living forms and skeletons

490

of spumellarians dominated at depths of 50-150 m, while empty nassellarians

491

skeletons increase rapidly in abundance and diversity, and dominate at depths of

492

150-2000 m (Gowing and Coale, 1989; Gowing, 1993). Cretaceous radiolarian

493

species displayed a wide spectrum of ecological preferences. Preservation

494

factors, such as selective dissolution in the water column and in sediments, are

495

also important reasons that lead to the different spumellarian/nassellarian ratios

496

in samples (Blome and Reed, 1993; O’Dogherty and Guex, 2002). However, the

497

analysis of radiolarian faunas in this study reveals that about 75% of the total of

498

16 species crossing OAE 2 are spumellarians (12 species), which seem to have

499

higher potential for survival of OAE. Moreover, in the Umbria-Marche and

500

Outer Carpathian basins of the western Tethys, Bak (2011) divided the

501

Cenomanian to Turonian radiolarian taxa into 25 groups, related to specific

502

water masses. Radiolarian species from groups E3-5 and D1-3 were seldom

503

influenced by OAE 2, and were surface and subsurface dwellers that lived in

504

shallow waters (Fig. 9).

505

Analysis of existing data (Erbacher et al., 1996; Erbacher and Thurow,

506

1997; Leckie et al., 2002; Bragina, 2004; Friedrich et al., 2006; Musavu-

507

Moussavou et al., 2007; Jenkyns, 2010), suggests that, during OAE 2, (1)

508

anoxia involved only bottom waters; (2) micro- and macrofauna inhabiting the

509

upper part of the water column did not experience the suppressing effort of

510

anoxia; (3) habitats of deep-dwelling forms disappeared and extinctions

511

occurred (Fig. 9). This deprives the ocean water of oxygen and extends the

512

oxygen minimum zone (OMZ), which promotes the preservation of marine

513

organic matter as black shales and generates positive carbon isotope excursions

514

(Erbacher et al., 1996; Erbacher and Thurow, 1997). Due to the expansion of

515

the OMZ, the extinction of planktonic foraminifers at the Cenomanian/Turonian

516

boundary can also be described by a similar pattern. The loss of the genus

517

Rotalipora shows that deeper-dwelling planktonic foraminifers were the most

518

severely affected by the OAE 2 (Hart, 1980; Caron and Homewood, 1983,

519

Leckie, 1989, 2002). A similar model also applies to the end-Permian mass

520

extinction (Algeo et al., 2013; He et al., 2013). A poleward expansion of the

521

OMZ during the Permian-Triassic boundary (PTB) crisis caused deep-water

522

radiolarian taxa to decline, with some survivors migrating to shallower

523

environments (Algeo et al., 2011; Feng and Algeo., 2014).

524

525

5.3. Dinoflagellate, nutrient availability and primary producers

526

Ando et al. (2017) evaluated the marine primary producers during OAEs

527

based on A-ring methyl triaromatic steroids and desmethyl and suggested that

528

dinoflagellates dominate as primary producers under eutrophic circumstances,

529

whereas coccolithophorids had the likelihood to flourish under more

530

oligotrophic conditions. During the mid-Cretaceous, radiolarian diversity had a

531

strong association with the nannoplankton diversification. The nannoplankton

532

diversity reaches a maximum peak when radiolarian diversification drops. The

533

marine nutrient availability may explain this inverse correlation (O’Dogherty

534

and Guex, 2002). Erba (1994) used this mechanism to explain the possible

535

competition that occurred between nannoconids and coccolithophorids during

536

the Cretaceous. It is suspected that radiolarians and dinoflagellates were

537

restricted to the lower euphotic zone, whereas coccolithophorids and other

538

nannoplankton had the same preferences for the upper euphotic zone (Casey et

539

al. 1979; Takahashi 1991).

540

In the Vocontian Basin, OAE 1d was triggered by an excess input of

541

terrestrial matter through sea-level rise, the nannofossil data indicated an

542

oligotrophic environment (Bornemann et al., 2005; Ando et al., 2017). A major

543

radiolarian evolutionary radiation event occurred when there was a low number

544

of dinoflagellate during the latest Albian (Fig. 8). As previously stated, during

545

OAE 2, sea-level rise and global warming result in marine productivity increase

546

and a eutrophic environment. The dinoflagellates presumably increased in

547

diversity as the predominant marine primary producers under eutrophic

548

conditions and stratified (Ando et al., 2017). Therefore, the deeper-dwelling

549

radiolarians were the most affected by the increasing number of dinoflagellates

550

during the OAE 2. By contrast, in the upper euphotic zone, coccolithophorids

551

were seldomly affected by eutrophic conditions. This may explain why

552

radiolarians inhabiting the upper part of the water column did not experience the

553

suppressing effort of anoxia. Taken together, radiolarian turnover events were

554

controlled by the cumulative effects of sea-level fluctuations, marine nutricline,

555

ocean fertility, climate and submarine volcanism during mid-Cretaceous.

556

557

6. Conclusions

558

Micropaleontological studies in southern Tibet show that the turnover

559

(extinction/radiation) events amongst mid-Cretaceous (Albian-Coniacian)

560

radiolarian faunas are correlated with oceanic anoxic events (OAEs).

561

Stratigraphic and geochemical studies document evolutionary events amongst

562

radiolarians that correlate with the appearance of black shales and positive

563

oxygen isotope excursions.

564

1) The Gyabula Formation records a diverse, abundant, well-preserved

565

radiolarian fauna that can be assigned to five Albian-Coniacian radiolarian

566

zones, including A. umbilicata, A. tehamaensis, C. cachensis, A. superbum and

567

D. formosa zones.

568

2) Cretaceous (Albian-Coniacian) plankton display the greatest rates of

569

evolution at or near the major OAEs. Amongst them, radiolarian faunas were

570

affected by the Albian/Cenomanian boundary OAE 1d and Cenomanian

571

/Turonian boundary OAE 2. Those events may have been connected with active

572

submarine volcanism and increased productivity with rising sea-level.

573

3) The expansion of the OMZ, caused a layer of poorly oxygenated bottom

574

and intermediate waters to develop within the Tethys. As a result of the loss of

575

deep habitats, numerous deeper dwelling radiolarian forms became extinct.

576

However, most of the shallower dwelling radiolarians survived. At the same

577

time, once the methane is saturated in the water, it will be released into the

578

atmosphere and rapidly converted into CO2 by oxygen oxidation. The direct

579

geological effect is climate warming.

580 581

Acknowledgments

582

This research is supported by the State Scholarship Fund of China

583

(41272030, 40972026), the Strategic Project of Science and Technology of

584

Chinese Academy of Sciences (XDB050105003), the National Basic Research

585

Program of China (2012CB822001), IGCP 608 and 679 and The University of

586

Queensland. We appreciate the constructive comments from the reviewers and

587

the editor that helped improve the manuscript significantly. Special thanks are

588

due to Qiubei Gu who provided invaluable help to the first author.

589 590

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591 592 593 594

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Fig. 1. Sketch map showing the tectonic setting of southern Tibet and the location of the study area. A red star shows the location of the Yongla section.

Fig. 2. Measured stratigraphic column for mid Cretaceous sequences of the Gyabula Formation in the Yongla area (N28°58′14″, E89°46′41″), Gyangze, southern Tibet. Grey areas show the OAEs.

Fig. 3. Correlation of mid-Cretaceous radiolarian zones and integrated radiolarian biostratigraphy used in this study.

Fig. 4. Scanning electron photomicrographs of radiolarians from the Gyabula Fm. in the Yongla section, Gyangze, southern Tibet. 1, 2. Pseudoeucyrtis hanni (Tan); 3. Dictyomitra communis (Squinabol); 4. Crucella latum (Lipman); 5. Dictyomitra formosa Squinabol; 6. Rhopalosyringium elegans (Squinabol); 7. Godia floreusa Wu; 8. Paronaella solanoensis Pessagno; 9. Archaeospongoprunum salumi Pessagno; 10. Diacanthocapsa ancus (Foreman); 11. Holoctyptocanium tuberculatum Dumitrica; 12, 20, 25. Thanarla brouweri (Tan); 13. Godia floreusa Wu; 14, 17, 36. Pseudodictyomitra primitiva (Matsuoka & Yao); 15. Pseudoaulophacus cf. putahensis Pessagno; 16. Patellula cognata O’Dogherty; 18. Thanarla lacrimula (Foreman); 19. Pseudodictyomitra rigida Wu; 21. Pessagnobrachia ? fabianii (Squinabol); 22. Godia ornata Wu; 23. Praeconocaryomma sp.; 24. Wrangellium versum Wu; 26. Triactoma micropora Bragina; 27. Quinquecapsularia panacea O’Dogherty; 28. Hiscocapsa grutterinki (Tan); 29. Halesium quadratum Pessagno; 30. Stichomitra manifesta Foreman; 31. 37. Praeconocaryomma jiangzeensis Wu; 32. Hiscocapsa uterculus (Parona); 33. Dispongotripus acutispina Squinabol; 34. Triactoma hexeris O’Dogherty; 35. Angulobracchia portmanni Baumgartner; 38. Praecaneta mimetica Dumitrica; 39. Orbiculiforma depressa Wu; 40, 41. Archaeospongoprunum stocktonensis Pessagno; 42. Crucella cachensis Pessagno; 43. Archaeospongoprunum salumi Pessagno. All scale bars are 100 µm. A: 21, 26, 29, 33, 34; B: 4, 7, 16, 22, 24, 35, 39−43; C: 1−3, 6, 8, 9, 11, 13−15, 17, 19, 28, 30, 31, 36, 37; D: 5, 10, 12, 18, 20, 23, 25, 27, 32, 38.

Fig. 5. Scanning electron photomicrographs of radiolarians from the Gyabula Fm. in the Yongla section, Gyangze, southern Tibet. 1. Acaeniotyle diaphorogona Foreman; 2, 3. Triactoma hybum Foreman; 4−6. Acaeniotyle vitalis O’Dogherty; 7. Triactoma parva (Squinabol); 8. Acaeniotyle umbilicata (Rust); 9. Dictyodedalus cretaceus O’Dogherty; 10. Xitus decorus Wu; 11. Hiscocapsa uterculus (Parona); 12. Acaeniotyle tribulosa Foreman; 13. Acaeniotyle rebellis O’Dogherty; 14, 15. Spongostichomitra elatica (Aliev); 16. Psuedodictyomitra lilyae Tan; 17, 18. Pseudodictyomitra nuda (Schaaf); 19, 20, 21. Orbiculiforma tuberculata Wu; 22. Triactoma cellulose Foreman; 23. Orbiculiforma belliatula Wu; 24. Dactyliodiscus cayeuxi Squinabol; 25. Godia floreusa Wu; 26. Cavaspongia sphaerica O’Dogherty; 27. Orbiculiforma monticelloensis Pessagno; 28. Sciadiocapsa euganea Squinabol; 29. Dactyliodiscus lenticulatus (Jud); 30. Quadrigastrum oculus O’Dogherty; 31. Alievium superbum (Squinabol); 32. Ultranapora cretacea (Squinabol); 33. Praeconocaryomma jiangzeensis Wu; 34, 35. Dicerosaturnalis amissus (Squinabol); 36. Dactyliodiscus cayeuxi Squinabol; 37. Cavaspongia

antelopensis Pessagno. All scale bars are 100µm. A: 12−15, 23, 24, 27, 29, 31, 36; B: 10, 26, 30, 34, 35; C: 1, 4−8, 16−22, 25, 33, 37; D: 2−3, 9, 11, 28, 32.

Fig. 4

Fig. 5

Fig. 6. Occurrence chart of Albian radiolarians encountered as part of this study. Lines = occurrence.

Fig. 7. Oc cur ren ce cha rt of Al bia n− Co nia cia n rad iol ari ans enc ou nte red as part of this study. Lines = occurrence.

Fig. 8. Summary of the stratigraphy, geochemical, temperature, sea level, foraminifer and radiolarian evolutionary events associated with mid−Cretaceous oceanic anoxic events in southern Tibet. Note the concentration of speciation and extinction events associated with the OAE 1d, MCE, and OAE 2.

Fig. 9. Simplified scheme of sea level change links with black shales and evolution of radiolarians during mid−Cretaceous. A) time interval before the OAE 2; B) time interval within OAE 2. Sea−level rise leads high nutrients input, preservation of black shales and high productivity; active submarine volcanism leads expansion of OMZ, deeper dwelling radiolarians extinct, shallower dwelling radiolarians survive and speciate, and temperature rise.

Fig. S1. Scanning electron photomicrographs of radiolarians from the Gyabula Fm. in the Yongla section, Gyangze, southern Tibet. 1, 2. Dactyliodiscus cayeuxi Squinabol; 3. Godia floreusa Wu; 4, 5. Orbiculiforma tuberculata Wu; 6. Thanarla conica (Squinabol); 7. Pseudodictyomitra hornatissima (Squinabol); 8. Xitus triangularis Wu; 9−11. Dictyomitra formosa Squinabol; 12, 13. Stichomitra stocki (Campbell et Clark); 14. Psuedodictyomitra lilyae Tan; 15. Dictyomitra communis (Squinabol); 16, 17. Pseudodictyomitra nuda (Schaaf); 18, 19. Ultranapora crassispina (Squinabol); 20. Ultranapora durhami Pessagno; 21. Acaeniotyle tribulosa Foreman; 22. Acaeniotyle rebellis O’Dogherty; 23. Crucella latum (Lipman); 24. Acaeniotyle vitalis O’Dogherty; 25. Quinquecapsularia parvipora (Squinabol); 26. Holocryptocapsa hindei Tan; 27. Trimulus fossilis (Squinabol); 28, 30. Cryptamphorella conara (Foreman); 29. Holocryptocanium barbui Dumitrica; 31. Holocryptocanium japonicum Nakaseko and Nishimura; 32, 33. Hiscocapsa verbeeki (Tan); 34. Xitus decorus Wu; 35. Loopus ? nuda (Schaaf). All scale bars are 100µm. A: 1, 2, 21, 22; B: 12, 13, 23, 25, 28, 30, 34, 35; C: 3−7, 14−17, 20, 29, 31−33; D: 8, 9−11, 18, 19, 24, 26, 27. Fig. S2. Scanning electron photomicrographs of radiolarians from the Gyabula Fm. in the Yongla section, Gyangze, southern Tibet. 1. Crucella messinae Pessagno; 2, 3. Crucella euganea (Squinabol); 4, 5. Crucella irwini Pessagno; 6. Pessagnobrachia macphersoni O’Dogherty; 7−9. Pessagnobrachia fabianii (Squinabol); 10−12. Halesium amissum (Squinabol); 13. Cavaspongisa californiaensis Pessagno; 14. Praeconocaryomma universa Pessagno; 15. Praeconocaryomma lipmanae Pessagno; 16. Orbiculiforma tuberculate Wu; 17. Holoctyptocanium tuberculatum Dumitrica; 18. Excentropylomma cenomana Dumitrica; 19, 20. Dictyomitra communis (Squinabol); 21, 22. Dictyomitra napaensis Pessagno; 23. Thanarla pulchra (Squinabol); 24. Stichomitra japonica (Nakaseko & Nishimura); 25, 26. Archaeodictyomitra squinaboli Pessagno; 27. Becus sp.; 28−30. Acaeniotyle longispina (Squinabol); 31, 32. Archaeospongoprunum pontidum Bragina; 33. Acaeniotyle sp.; 34. Archaeospongoprunum sp.; 35, 36. Archaeospongoprunum tehamaensis Pessagno; 37. Pseudoaulophacus putahensis Pessagno; 38−41. Pseudoeucyrtis hanni (Tan); 42. Crucella euganea (Squinabol). All scale bars are 100µm. A: 13−15; B: 1−12, 18, 27−37, 42; C: 16, 17, 19−23, 25, 26, 38−41; D: 24.

Fig. S1

Fig. S2

Key Points • Well-preserved and abundant mid-Cretaceous radiolarians were recovered from Tibet • A model to explain the relationship between the radiolarian turnover events and the OAEs • Rising sea level and submarine volcanism may connected with plankton evolution