Paleoclimate evolution and aridification mechanism of the eastern Tethys during the Callovian-Oxfordian:evidence from geochemical records of the Qiangtang Basin,Tibetan Plateau

Yongyao Zeng·Lei Gao·Wenqing Zhao

Abstract Global climate during the Jurassic has been commonly described as a uniform greenhouse climate for a long time.However,the climate scenario of a cool episode during the Callovian-Oxfordian transition following by a warming trend during the Oxfordian(163.53 to 157.4 Ma)is documented in many localities of the western Tethys.It is still unclear if a correlatable climate scenario also occurred in the eastern Tethys during the same time interval.In this study,a detailed geochemical analysis on the 1060 m thick successions(the Xiali and Suowa formations)from the Yanshiping section of the Qiangtang Basin,located in the eastern Tethys margin during the Callovian-Oxfordian periods,was performed.To reveal the climate evolution of the basin,carbonate content and soluble salt concentrations(SO42-,Cl-)were chosen as climatic indices.The results show that the overall climate patterns during the deposition of the Xiali and Suowa formations can be divided into three stages:relatively humid(～164.0 to 160.9 Ma),dry(～160.9 to 159.6 Ma),semi-dry(～159.6 to 156.8 Ma).A similar warming climate scenario also occurred in eastern Tethys during the Callovian-Oxfordian transition(～160.9 to 159.6 Ma).Besides,we clarify that the Jurassic True polar wander(TPW),the motion of the lithosphere and mantle with respect to Earth’s spin axis,inducing climatic shifts were responsible for the aridification of the Qiangtang Basin during the Callovian-Oxfordian transition with a review of the paleolatitude of the Xiali formation(19.7+2.8/-2.6° N)and the Suowa formation(20.7+4.1/-3.7° N).It is because the TPW rotations shifted the East Asia blocks(the North and South China,Qiangtang,and Qaidam blocks)from the humid zone to the tropical/subtropical arid zone and triggered the remarkable aridification during the Middle-Late Jurassic(ca.165-155 Ma).

Keywords Qiangtang basin·The Callovian-Oxfordian·Geochemistry·Paleolatitude·Paleoclimate

1 Introduction

The global climate of the Jurassic has been commonly described as a uniform greenhouse climate by a wealth of proxy data for a long time(Hallam 1993;Sellwood and Valdes 1997;Sellwood et al.2000).The data include coral reefs,widespread bauxites,paleosols(Price and Sellwood 1997),extensive distributions of evaporites,and desert deposits.However,this view,a uniform greenhouse climate,is increasingly challenged by paleontological and oxygen isotope studies(Abbink et al.2001;Jenkyns et al.2002;Dromart et al.2003a,b;Brigaud et al.2008).More specifically,an obvious temperature fall was revealed by δO of belemnite rostra from Russia(Barskov and Kiyashko 2000),Poland(Gruszczynski 1998),and England(Jenkyns et al.2002)during the Callovian-Oxfordian transition(166.08 to 157.4 Ma).Additionally,a significant temperature fall was also indicated by palynological data from the North Sea(Abbink et al.2001)and boreal ammonite fauna invaded western Tethys(Mediterranean and Submediterranean)during this time interval(Fortwengler et al.1997;Fig.1).The concept of Tethys,firstly introduced by Eduard Suess in 1893,is a superorogenic complex stretching from southern Europe to the south and eastern Asia(Fig.1).Also,a substantial warming climate starting in the Early Oxfordian(Abbink et al.2001;Brigaud et al.2008),or in the Middle Oxfordian(Riboulleau et al.1998;Lécuyer et al.2003;Wierzbowski 2004),or in the Late Oxfordian(Malchus and Steuber 2002;Bartolini et al.2003),is documented in the most regions of western Tethys(Fig.1)based on the oxygen isotopic data.At present,it is still unclear if a correlatable climate scenario also occurred in eastern Tethys.To improve our understanding of the evolution of paleoclimate in eastern Tethys during the Callovian-Oxfordian transition,we traced the sedimentary archives from the Qiangtang Basin located in the eastern Tethys margin in this time interval(Metcalfe 2009;Fig.1).

Fig.1 Paleogeography of the Earth during the Middle Jurassic(at～165 Ma)showing relative positions of the Western and Eastern Tethys,which is adapted from Brunetti et al.(2015).Most regions of the Western and Eastern Tethys are enclosed in the red and black squares,respectively.Abbreviations,Nam,North America;SAm,South America;Ant,Antarctica;Aus,Australia;G,Greenland;I,Iberia;A,Adria;T,Taurus;AT,Alpine Tethys;BN,Bangong-Nujiang;ES,Elise Sea.QT,Qiangtang Terrane;LT,Lhasa Terrane

The Qiangtang Terrane or the Qiangtang Basin(QB),the largest basin in the Tibetan Plateau(Fig.2a),is an appropriate area to study the Middle-Late Jurassic paleoclimatic change,as continuous sediments are widely exposed and well-dated biostratigraphy are preserved.And the biostratigraphy(Bai 1989;Chen et al.2005)and magnetostratigraphy(Song et al.2016)had been established as a chronology frame of the basin.Carbonate content and soluble salt concentrations(SO,Cl)were chosen as climatic indices for paleoclimatic reconstruction because they are sensitive indicators to climatic change(Birch 1981).Thus,in this study we performed detailed geochemical analysis on the 1,060 m thick Middle-Late Jurassic successions(the Xiali and Suowa formations)from a well-dated section,the Yanshiping(YSP)section of the QB(Fig.2c),to reveal the climate evolution during the Callovian-Oxfordian intervals.We also reviewed the paleolatitude and the megamonsoon records to clarify the mechanism for the climatic change of the QB.

2 Geologic setting

2.1 Regional geology

The Tibetan Plateau,as a major part of the Tethys orogenic collage,is formed by the Qaidam Terrane,the Hoh Xil-Songpan Flysch Complex,the Qiangtang,and Lhasa terranes from north to south(Yin and Harrison 2000;Fig.2a,c).These terranes are separated by EW striking suture zones from Paleozoic to Mesozoic in age(Yin and Harrison 2000;Fig.2a,c).All the terranes record a history of rifting from Gondwana,drifting northward,and amalgamation.This process resulted in the closure of the Paleotethys Ocean,the opening of the Neotethys Ocean,and the accretion of the Asian(Metcalfe 2009).

2.2 Geology of the QB

The QB,located in the Tibetan Plateau(Fig.2a),is considered to have been a contiguous part of the Cimmerian Continent along the eastern Tethys margin of Gondwana during the Permian(Metcalfe 2009;Guynn et al.2012;Zhu et al.2013;Fig.2b).Cimmerian Continent included tectonic terranes ranging from Afghanistan,Tibet,Myanmar,western Thailand to northwest Sumatra,which was firstly proposed by Sengör in 1979(Sengör 1979;Fig.2b).

Fig.2 a Simplified tectonic map of the Tibetan Plateau showing the major blocks,sutures,and the location of the QB(adapted from Song et al.2016).b Paleogeographic reconstructions of the Tethys region for Late Permian(Changhsingian,at～253 Ma)showing the relative position of the Cimmerian Continent in the Gondwana(compiled from Metcalfe 2013).c Simplified geologic map showing the tectonic and stratigraphic frames of the QB and the location of the studied area(compiled from Fang et al.2016).Abbreviations,HJSZ,Hoh Xil-Jinsha Suture Zone;BNSZ,Bangong-Nujiang Suture Zone;IYSZ,Indus-Yalung Zangpo Suture Zone;CUB,Central Uplift Belt;NQB,North Qiangtang Basin;SQB,South Qiangtang Basin

Originated from the Cimmerian Continent(Fig.2b),but the QB was disassembled from the continent in the Early Permian(Zhu et al.2013).And then the QB drifted northward across the Tethys Ocean and collided with the Songpan-Ganzi-Hoh Xil Terrane along the Hoh Xil-Jinsha Suture Zone(HJSZ)during the Late Triassic-Early Jurassic.This process formed a syntectonic foreland basin and a thrust-fold system in the southern margin of the HJSZ(Yin and Harrison 2000;Li et al.2002;Metcalfe 2011;Fig.2c).So this suture zone represents the Paleo-Tethys Ocean between Eurasia and the Qiangtang Terrane(Kapp et al.2003).Later,the QB was joined by the Lhasa Terrane along the Bangong-Nujiang Suture Zone(BNSZ)during the Late Jurassic-Early Cretaceous in a diachronous fashion(commence from east to west),ending the Jurassic epicontinental sea basin and uplifting it as an erosion region(e.g.Sengör 1987;Yin and Harrison 2000;Fig.2c).So,extending for more than 2000 km in the central Tibetan Plateau,the BNSZ with a broad belt of ophiolite fragments and mélange is labeled as the closure of the Meso-Tethys Ocean(Fig.2c).Accordingly,the QB is bounded by the HJSZ to the north and the BNSZ to the south(Yin and Harrison 2000;Fig.2c).

Bounded by the Songpan-Ganzi-Hoh Xil Terrane to the north and the Lhasa Terrane to the south(Fig.2a),the QB is approximately 1,600×300 km and has an average elevation of more than 5,000 m(Yin and Harrison 2000).The QB is also divided into two subbasins(Fig.2c),the North Qiangtang Basin(NQB)and the South Qiangtang Basin(SQB),by the Longmu Co-Shuanghu Suture Zone(LSSZ)(Fig.2c).The suture zone,situated in the central QB,consists of ophiolitic mélange and high-pressure metamorphic rocks(BGMRXAR 1993;Pan et al.2004;Fig.2c).

2.2.1 North Qiangtang Basin(NQB)

The NQB is bounded by the LSSZ to the south and the HJSZ to the north(Fig.2c).The crystalline basement of the NQB might have been tectonically eroded and replaced by the central Qiangtang mélange underthrust southward along the HJSZ(Kapp et al.2003;Pullen et al.2011).The oldest sedimentary sequence in the NQB,the Ningduo Group,consisting of biotite-amphibolite mylonite with mica plagioclase gneiss,biotite-quartz schist,and pyroxene granulite rocks in the lower part and limestone,limestone breccias,gneiss,and quartzite flakes in the upper part.The exposed abundant fusulinid and coral fossils indicate a warm and likely Cathaysian affinity during the Permian(Pan et al.2004).

Scattered across the southern margin of the NQB,the Early and Middle Triassic sediments are upwards purplebrown conglomerates to mudstones intercalated with some coals in the river delta plain environment(BGMRXAR 1993).In the NQB,the Late Triassic sediments are mostly dominated by greenish sandstones to black sandy mudstones intercalated with coals of marine-terrestrial interbedded facies(shelf,near-shore,tidal-flat,river delta,and coast marsh environments)(BGMRXAR 1993;Yan et al.2016).

Only a small amount of Early Jurassic volcanic rocks were deposited in the NQB.The Middle-Late Jurassic strata,the Yanshiping Group(YSP),cover the whole QB.Thus,the QB is sometimes called the Jurassic basin.The Cretaceous and Cenozoic sediments,consisting of continental red beds ranging from conglomerates to mudstones intercalated with some coals or gypsum(Fang and Li 2005),are subject to strong faulting and folding indicating the closure of the Neo-Tethys and the collision of India with Asia(Fang et al.2016).

2.2.2 South Qiangtang Basin(SQB)

The SQB is bounded by the BNSZ to the south and the LSSZ to the north(Fig.2c).The Meso-to Neo-Proterozoic Gemuri Group,exposed in the west of Shuanghu and the Duguer Range,is considered to be the basement of the SQB(Pullen et al.2011).The Gemuri Group,including strongly tectonized metasedimentary rocks(e.g.,phyllite,quartzite,metasandstone,schist,and paragneiss),were intruded by orthogneisses dated～476-471 Ma which indicating the presence of an Ordovician and older crystalline basement beneath the SQB(Pullen et al.2011).The basement in the SQB is covered with an unfossiliferous Lower Ordovician siltstone and shale interbedded with limestone,which is overlying the Middle Ordovician-Devonian fossiliferous sequence mainly consisting of marine carbonate rocks and some siltstones(Li et al.2009;Zhu et al.2013).These rocks are shallow marine sediments deposited on a stable carbonate platform in a continental margin(Li et al.2009;Zhu et al.2013).

The Late Paleozoic strata from the Upper Carboniferous to the Middle Permian,including abundant glaciomarine diamictites,cold-water biota,and basalt interlayers,are widespread(Li et al.2009).The glaciomarine deposit and cold-water biota indicate a cold Gondwana affinity of the SQB.The diametrical sediment in the NQB(the warm fusulinid and coral fossils)and the SQB(cold water biota),suggests the existence of a small ocean between the two subbasins during the Permian(Metcalfe 2002;Pan et al.2004;Li et al.2006).The Upper Paleozoic strata are conformably overlain by Upper Triassic strata of carbonate rocks and a small number of sandstones to black shales intercalated with marls and limestones(Li et al.2002).

The Jurassic sedimentary sequences consisting of Lower Jurassic turbiditic shales,siltstones,and shallow-sea limestones,Middle Jurassic shale and siltstone,and Upper Jurassic shallow-sea limestone(Yan et al.2016).Above these strata,it is angular unconformably overlain by a limited distribution of Lower Cretaceous molasse deposits(BGMRXAR 1993;Li et al.2002).

2.3 Geology of the YSP section

The YSP Sect.(33°3319.9N-33°3511.3N,92°0157.3E-92°0344.3E)is located at the YSP town in the eastern NQB(Figs.2c;3a).The Lower Jurassic sequence is rare in the region.But the Middle to Upper Jurassic sediment(Fig.3a),the YSP Group,is widely exposed due to the prevalent folding and thrusting which originated from the collision of Lhasa Terrane and later the indentation of the India Plate into the Eurasia Plate(GSC and CIGMR 2004).From base to top,the YSP Group is subdivided into the Quemo Co(J),Buqu(J),Xiali(J),Suowa(J),and Xueshan(J)formations(GSC and CIGMR 2004;Fig.3b).And there is a conformable contact between every two adjacent formations.The five formations are characterized by rhythmic cycles of sandstone and limestone sequences(Fig.3b).Specifically,the Quemo Co,Xiali,and Xueshan formations are dominated by clastic sedimentary sequences of marine-terrestrial interbedded siliciclastics deposited in delta environments,whereas the Buqu and Suowa formations are dominated by limestone sequences deposited in a shallow sea carbonate platform environment(GSC and CIGMR 2004).There is an angular unconformity between the Middle-Upper Jurassic YSP Group and the overlying Lower Cretaceous molasse sequence in the YSP region.

The Quem Co Formation(Fm.),the lowest part of the section,consists of purplish-red and grayish-green sandstones,siltstones(Fig.4a),and mudstones with paleosols.Characterized by distinct Bt horizons with striking Bk horizons(Fig.4b),the paleosols are drab brown luvic cambisols to luvisols.Also,abundant fresh water and marine-brackish bivalves are found in the lower and upper members of the unit.

Fig.3 a Simplified geologic map of the YSP area compiled from 1:250 000 scale geologic map(GSC and CIGMR 2004),showing the sampling locations(b,c);b Cross-section of a-d

The Buqu Fm.comprises numerous thick layers of dark gray microcrystalline limestone(Fig.4d)and bioclastic limestone(Fig.4c)intercalated with some black shale,which is occasionally punctuated by fine sediments of grayish-green calcareous sandstones and siltstones.The formation contains numerous marine fossils,e.g.,marine bivalves,brachiopods,echinoids,and foraminifera.

The Xiali Fm.(Fig.5a),a measured thickness of～608 m(0-608 m)in the section,mainly composed of purple-red siltstone,sandstone,and multicolored(dark red,grey,grayish-green)mudstone that is intercalated with thin dark grey beds of microcrystalline limestone and bioclastic limestone.Horizontal,ripple,lenticular,and herringbone cross-bedding(Fig.5b)are well developed in this unit.Thinly bedded gypsum(Fig.5c),mud cracks(Fig.5d),and ripple marks are observed in the upper member of this succession(Fig.5e).Besides,a few bivalve assemblages of Anisocardia ex gr.Islipensis/tenera-Modiala biparta-Isognomon cf.Bergeroni(Song et al.2016;Fig.5f),a typical Callovian genus(Hayami 1975;Bai 1989),is found at～40-140 m in the formation.The observed bivalves plausibly indicate a Callovian age for the Lower Xiali Fm.

The Suowa Fm.(Fig.5g),a thickness of～449 m(at stratigraphic intervals of～608-1057 m),consists of dark grey microcrystalline limestone,greyish marl,light brown sandy limestone,and a few dark grey thin beds of biolithite limestone.Marl and limestone with some bivalves,brachiopods,cephalopoda,and spores are common in the middle-upper member of the formation,whereas siltstone and mudstone are present in the lower member.Two bivalve assemblages of Gervillella aviculoides-Radulopecten fibrosus(Fig.5h)and Entolium-Myopholas(Fig.5i)are identified at～640-800 m and～940-1040 m in the unit,respectively(Song et al.2016).The bivalve assemblage of Gervillella aviculoides-Radulopecten fibrosus,a typical Callovian-Oxfordian genus(Hayami 1975;Bai 1989),indicates a Callovian-Oxfordian age for the Lower Suowa Fm.And the assemblage of Entolium-Myopholas,which occurred during the Oxfordian(Johnson 1984),suggests an Oxfordian age for the Upper Suowa Fm.

The Xueshan Fm.,the youngest unit,is only～10 m thick in the section.The unit mainly contains dark grey mudstone with clear laminations which is intercalated with two～5 cm thick yellowish ostracum beds at the bottom.

In this study,the chronology frame of the Xiali and Suowa formations is based on biostratigraphy and magnetostratigraphy from Bai(1989),Chen,et al.,(2005),and Song et al.,(2016),respectively(Fig.5).Magnetostratigraphic age of the Xiali and Suowa formations is～164.0-160.2 Ma(Late Callovian-Middle Oxfordian)and 160.2-156.8 Ma(Middle Oxfordian-Early Kimmeridgian),respectively(Song et al.2016).

Fig.4 Representative field photos of the Quem Co and Buqu formations.a Field shot of the Quem Co Fm.;b Paleosols in the Quem Co Fm.;c The biogenic limestone of the Buqu Fm.;d Field shot of the Buqu Fm

3 Sampling,method,and proxies

3.1 Sampling

The outcrop samples were taken at ca.0.5-1 m intervals in the freshly-exposed faces,which were removed the weathering and oxidized materials of the outcrop.The sample interval was reduced to ca.0.1-0.3 m if gypsum is appearing in the outcrops.Most of the Xueshan Fm.has been eroded.No samples were collected from this unit.Finally,a total of 1526 outcrop samples were collected from the Xiali and Suowa formations in the YSP section.In this study,all the outcrop samples are oven-dried at 36 °C and then crushed to the powdered samples(a fine＜200 mesh powder)using an agate mortar and pestle at the Key Laboratory of Western China’s Mineral Resources of Gansu Province,Lanzhou University.

3.2 Method

The carbonate content and soluble anion(SO)were respectively measured using the Karbonat-bombe method with errors of ca.0.5 wt%(Birch 1981)and EDTA titration method in the above-mentioned laboratory.For soluble ion measurements(Cl),samples(～0.5 g)were dissolved in ultrapure water for about 12 h.Then the chloride samples were measured by ion chromatography(IC)at the Institute of Tibetan Plateau Research,CAS.The IC system employed in this work was a Dionex ICS 2100 system(Sunnyvale,CA,USA),equipped with an integrated eluent(potassium hydroxide)generator,model RFICEG(EGC III KOH cartridge),and an AERS 500 2 mm membrane conductivity suppressor.Finally,a total of 1,363 powdered samples are analyzed,and 163 samples are excluded for further analysis due to test error.Replicate analyses of samples show that relative standard deviations(RSDs)from the mean value were＜2%.

Fig.5 Lithology,magnetic stratigraphy,representative field photos of the sedimentary structures and fossils for the Xiali and Suowa formations.The observed magnetic polarities and the reference geomagnetic polarity time scale(GPTS)are cited from Song et al.,(2016)and Gradstein et al.,(2012),respectively.a Field shot of the Xiali Fm.;b Herringbone cross-bedding of the Xiali Fm.;c Thinly bedded gypsum of the Xiali Fm.;d Mud cracks of the Xiali Fm.;e Field shot of the Upper Xiali Fm.;f Bivalve fossils identified in the Lower Xiali Fm.;g Field shot of the Suowa Fm.;h Bivalve fossils of Gervillella aviculoides-Radulopecten fibrosus identified in the Lower Suowa Fm.;i Bivalve fossils of Entolium-Myopholas identified in the Upper Suowa Fm

3.3 Proxies used in this study

Carbonate,sulfates,and chlorides are chemical sediments of various degrees of the arid climate.With an increasingly arid climate,a freshwater body turns into a brackish water body,and carbonate is deposited.With further aridification,the brackish water body will be condensed,and sulfate will be deposited.Finally,a long term and extremely arid climate will further condense the brackish water body,and chloride will be deposited(Warren 2010).Thus,the depositional order(carbonates→sulfates→chlorides)and ion contents(SOand Cl)can be used as proxies to evaluate the various degrees of aridity(Williams-Stroud and Paul 1997).

4 Results

Figure 6 displays the variations in carbonate and soluble anions(SOand Cl)in the Xiali and Suowa formations.The overall pattern of these records can be divided into three stages(Phases I-III)(Fig.6).

(1)Phase I(0-460 m;～164.0 to 160.9 Ma)

In Phase I,the carbonate content(Fig.6a)and soluble anion concentrations(SO,Cl;Fig.6b-c)maintain relatively low values with average values of 9.38 wt.%,0.48 mg/g,and 18.82 μg/g,respectively.The maximum of carbonate content and soluble anion concentrations(SO,Cl)are 80.92 wt.%,1.79 mg/g,and 74.38 μg/g,respectively.

(2)Phase II(～460-700 m;～160.9 to 159.6 Ma)

Fig.6 The variations of carbonate content(a),the soluble anion of SO42-and Cl-(b,c)in the Xiali and Suowa formations versus a stratigraphic level and geologic time

In Phase II,the carbonate content slightly increases to an average value of 14.98 wt.%.The maximum value is 84.04 wt.%in 574 m(Fig.6a).The concentrations of SOand Cldramatically increase in this stage to average values of 0.75 mg/g and 23.71 μg/g,respectively(Fig.6b-c).The maximum concentrations of SOand Clare 2.37 mg/g in 474 m and 124.48 μg/g in 554 m,respectively.Also,the highest values of SOand Clappear in this stage(Fig.6b-c).

(3)Phase III(～700-1057 m;～159.6 to 156.8 Ma)

In Phase III,the carbonate content(average value=55.7 wt.%;Fig.6a)relative increases to Phase II.The concentrations of SO(average value=0.34 mg/g)and Cl(average value=18.96 μg/g)obviously decrease relative to Phase II.The maximum concentrations of SOand Clare 1.03 mg/g in 716 m and 47.04 μg/g in 778 m,respectively.The maximum of carbonate content is 96.16 wt.% in 773.5 m.

5 Discussion

5.1 The climate changes in the QB during the Callovian-Oxfordian transition

Based on the variations of the proxies in the Xiali and Suowa formations,the overall climate patterns can be divided into three stages(Phases I-III)(Fig.6d).

(1)Phase I:relative humid(the Middle and Lower Xiali Fm.,0-460 m;～164.0 to 160.9 Ma)

The relatively low values of carbonate content and soluble anion(SOand Cl)concentrations indicate more diluted water that did not reach the solubility limit of carbonate or evaporites(gypsiferous and halite deposits)(Dettman et al.2003).Thus,based on this theoretical knowledge,low values of carbonate content and soluble anion(SOand Cl)concentrations represent a lowersalinity water body and a relatively humid climate(Warren 2010).

The average values of carbonate content(9.38 wt.%)and Clconcentrations(18.82 μg/g)are the lowest in Phase I than in Phases II and IIII.The average value of SOconcentrations(0.48 mg/g)is very lower in Phase I than in Phases II and IIII.The relatively low values and little changing trend of carbonate content(Fig.6a)and soluble anion concentrations(SO,Cl;Fig.6b-c)are showing in the Lower Xiali Fm.(0-250 m).Thus,we infer the climate was humid in Phase I(Fig.6d).The evidence that supports this conclusion is that the presence of hematite and goethite in the Xiali Fm.is indicative of a humid climate(Zeng et al.2014).Besides,Tan et al.,(2010)also suggested a humid climate based on the analysis of REEs,TOC,EF,Sr/Cu from the wider region of the QB.

(2)Phase II:dry(the Upper Xiali Fm.and the Lower Suowa Fm.,460-700 m;～160.9 to 159.6 Ma)

Since carbonates having lower solubility than sulfate and chloride,COanions were consumed firstly to balance the Caions with the gentle increase of aridity(Fang et al.1997).Later,SOand Clanions were used to balance Naand Kions with the obvious increase of aridity(Dettman et al.2003).Thus,the clear increase of soluble anion(SOand Cl)concentrations represents a high-salinity water body and a dry climate(Dettman et al.2003).

The average values of the concentrations of SOand Cldramatically increase in Phase II,which are approximately 2 times and 1.3 times higher in Phase II than in Phase I(Fig.6b-c).And the carbonate content(Fig.6a)slightly increases in Phase II,which meaning carbonate is saturation.Moreover,the scatter plot also shows more significant correlation coefficients of Ca-SO(r=0.985)and Na-Cl(r=0.8974)in this Phase(Song et al.2017).Therefore we infer the climate was dry in Phase II(Fig.6d).Other evidence bolsters this conclusion:(1)the numerous gypsum outcrops(Wu et al.2010)and salt springs with obviously abnormal sodium(Na)levels(Niu et al.2014)in the QB reveal that the basin evolved from the carbonate phase to the sulfate or early chloride phase;(2)the variations of NaO/AlOand NaO/KO and the Chemical Index of Alternation(CIA)reveal that a dry climate in this time interval(Pan et al.2015).Furthermore,pollen and isotope records also reveal that a dry climate belt was present in the Tethys region during the Callovian-Oxfordian transition(Wierzbowski 2004).

(3)Phase III:semi-dry(the Middle and Upper Suowa Fm.,700-1057 m;～159.6 to 156.8 Ma)

With a slight increase in aridity,the contents of carbonate,sulfate,and chloride increase(Fang et al.1997).However,carbonate with the lowest solubility will be the first to precipitate,which means that the increase in the carbonate content will be larger than that of the sulfate and chloride contents(Williams-Stroud and Paul 1997).Thus,the clear increase in carbonate content and the slight increase in soluble anion(SOand Cl)concentrations represent moderate-salinity water and a semi-dry climate(Dettman et al.2003).

The average values of the carbonate content dramatically increase from Phase II(14.98 wt.%)to Phase III(55.7 wt.%)(Fig.6a).The average values of the concentrations of SOand Clobviously decrease from Phase II(0.75 mg/g and 23.71 μg/g)to Phase III(0.34 mg/g and 18.96 μg/g)(Fig.6b,c).Thus,we infer the climate in Phase III was semi-dry(Fig.6d).The evidence that supports this conclusion is that the spores and pollen assemblages reveal that the climate of the Suowa Fm.was semidry(Zeng et al.2012).And some gypsum outcrops were founded in the Shenglihe area of the QB,adjoining the YSP,by He and Bai(2011),which reveal a semi-dry and dry climate of the Suowa Fm.

5.2 Mechanism for aridification of the QB during the Callovian-Oxfordian transition

A striking climate change(from an extremely wet to a drastically arid)during the Middle-Late Jurassic occurred in the East Asia blocks(EABs),consisting of North and South China,Amuria,Junggar,Tarim,the Qiangtang,Songpan-Ganzi,and Qaidam blocks(Boucot et al.2009;Fig.2b).The mechanism for this aridification is contentious(Yi et al.2019).Previous explanations for this aridification of the EABs included rain-shadow effects deriving from the uplift of the eastern margin of Asia(Boucot et al.2009)or a change from a megamonsoon to zonal climate pattern(Fang et al.2016).

Recently,a new view,TPW inducing climatic shifts from an extremely wet to a drastically arid climate,is reported to explain this aridification of the EABs(Yi et al.2019).The arid event is also called the Great Jurassic East Asian Aridification by Yi et al.(2019).TPW is the motion of the lithosphere and mantle with respect to Earth’s spin axis,which transfers mass excesses toward the equator,leading to the redistribution of solar insolation and regional environmental changes(Gold 1955).Due to the lack of the study on the rain-shadow effects of the QB,we can not discuss if it is the trigger for the aridification of the QB during the Callovian-Oxfordian transition.So in this study,we discuss the mechanism for this aridification of the QB based on the following two views:(1)a change from a megamonsoon to zonal climate pattern;(2)The TPW inducing the aridification in the QB during the Callovian-Oxfordian transition.

5.2.1 The termination of the megamonsoon trigger the QB aridification

The Pangea continent formed from the conjunction of the Laurasian and Gondwana continents by the Hercynian Movement during the Late Permian,which resulting in the greatest heat contrast between the continent and ocean and triggering the strongest monsoon,the megamonsoon,in the Earth’s history(Rais et al.2007).

The schematic diagram(Fig.7)is showing how the movement of the global blocks triggering the climatic change in the winter and summer seasons during the Triassic and Jurassic periods(Boucot et al.2009).In summer,the Laurasian would have created the strongest heating and the lowest air pressure in its interior of the tropic-subtropic zones,while the Gondwana would have formed the highest pressure in its interior of the subtropic-midlatitudes zones(Fang et al.2016;Fig.7b,d).This configuration of the air pressures would have led to the lowest pressure center(CL)in the Laurasian and the highest pressure center(CH)in the Gondwana.And the lowest pressure in the Laurasian will exert a great attraction on the flows of the highest pressure from the Gondwana,which will lead to flew northward and across the equator and turned to form the trans-equator megamonsoon.The trans-equator megamonsoon will bring a large amount of warm moist and precipitation and form a tropic rain forest climate in the coastal areas of the Tethys(Fang et al.2016;Fig.7b,d).In winter,this pattern of air pressure and the climate was reversed,with precipitation along the east and northeast coasts of Gondwana(Fang et al.2016;Fig.7 a,c).

According to the above model of global blocks triggering the climatic change,Fang et al.(2016)interpreted the aridification of the QB as a result of the termination of the megamonsoon during the Callovian-Oxfordian transition,which was derived from the dismission of the Pangean continent(Fig.7c,d).Such a climate scenario is unlikely because Pangea was still largely intact during these time intervals(Torsvik et al.2012).Therefore,as for the reason of the aridification of the QB during the Callovian-Oxfordian transition,we disapprove of the view of the termination of the megamonsoon triggering the QB aridification.

5.2.2 The TPW inducing climatic shifts responsible for the QB aridification

The motion of the lithosphere and mantle with respect to Earth’s spin axis,called TPW,transfers mass excesses toward the equator and has lead to the redistribution of solar insolation and regional environmental changes(Gold 1955).In short,the TPW will lead to a change in the regional climate.For example,current compilations of a large-scale TPW event during the Jurassic were offered as an explanation for aridification in the Tethys realm,the Persian Gulf,and the Gulf of Mexico(Mattei et al.2014;Muttoni and Kent 2019).

As mentioned above,an arid climate also occurred in the EABs,consisting of North and South China,Amuria,Junggar,Tarim,Qiangtang,Songpan-Ganzi,and Qaidam blocks,during the Middle-Late Jurassic(Boucot et al.2009;Fig.2b).The reason for this aridification is the TPW rotations shifted the EABs from the humid zone to the tropical/subtropical arid zone and triggered a remarkable climate change over an area of～10,000,000 kmin EABs during the Middle-Late Jurassic(ca.165-155 Ma)(Yi et al.2019).The EABs,comprising of the QT block,are amalgamated by the Late Triassic(Zhao et al.2018).The reconstruction of the drift history of the EABs by Jurassic paleomagnetic poles shows that the EABs were located between～10°N and 35°N,a tropical/subtropical arid zone,during the Late Jurassic(ca.161-153 Ma;Yi et al.2019).

Recently,the new paleomagnetic results reveal a paleopole at 66.1° N/332.1° E(dp=2.7°,dm=4.6°)with a paleolatitude of 19.7+2.8/-2.6°N for the Xiali Fm.and a paleopole at 72.4°N,318.6°E(dp=3.9°,dm=6.7°)with a paleolatitude of 20.7+4.1/-3.7°N for the Suowa Fm.(Yan et al.2016).Note that the Xiali and Suowa formations were located at～20°N during the Callovian-Oxfordian transition(ca.160.9-159.6 Ma)and reached the tropical/subtropical arid belt at～10°N-35°N.Therefore,we also ascribe the arid climate in the QT during the Late Jurassic to the Jurassic TPW motion.

Fig.7 Paleozoic-Mesozoic movement of the global blocks,climatic records,and coupling(Boucot et al.2009).Global blocks and climatic records come from Boucot et al.(2009).The QT Block(c,d),marked in black,is cited from Fang et al.(2016).Note the megamonsoon region in the Late Triassic was remarkably expanded northeastwards compared to that in the Late Permian to the Middle Triassic(maps not attached).Atmospheric isobar,represented by Arabic numerals,is a relative value(without unit).Abbreviation CL,Continental Low;CH,Continental High;ITCZ,Intertropical convergence zone;SH,Subtropic High;MM,Megamonsoon;SSM,Southeast subtropic monsoon

6 Conclusions

(1)The overall climate patterns in the Xiali and Suowa formations,QB,during the Callovian-Oxfordian transition(～164 to 156.8 Ma)can be divided into three stages:relatively humid(the Middle and Lower Xiali Fm.,～164.0 to 160.9 Ma),dry(the Upper Xiali Fm.and the Lower Suowa Fm.,～160.9 to 159.6 Ma),semi-dry(the Middle and Upper Suowa Fm.,～159.6 to 156.8 Ma).

(2)A similar warming climate scenario also occurred in eastern Tethys during the Callovian-Oxfordian transition,which is documented by carbonate content and soluble salt concentrations(SO,Cl)records in the QB,Tibetan Plateau.

(3)The mechanism for the aridification of the QB during the Callovian-Oxfordian transition may be ascribed to the Jurassic TPW motion.

Acknowledgements

This study was supported by the National Basic Research Program of China(Grant No.2011CB403003),the College Innovation Research Program of Gansu Province(Grant No.2020B-320),the College Innovation Foundation of Gansu Province(Grant No.S202013933013).We are grateful to two anonymous reviewers for constructive and thoughtful comments,and to the editor Binbin Wang for kind editorial handling,which significantly improved the manuscript.Xiaohui Fang,Gang Niu,Sa Zhang,Song Wu,Jing Bao,and Jiwei Yang are thanked for laboratory assistance and fieldwork assistance.