Temporal calibration and quantification of Early Trias-sic climatic disturbances through high-precision U-Pb zircon dating and an improved chemical abrasion procedure

Thesis

Reference

Temporal calibration and quantification of Early Trias-sic climatic disturbances through high-precision U-Pb zircon dating and an

improved chemical abrasion procedure

WIDMANN, Philipp

Abstract

Resolving the timing of climate disturbances and carbon cycle fluctuations in the geological past is a pre-requisite to understand the nature of triggers of global change and the timing of environmental and biological feedback. This thesis aims to improve and implement methods to build accurate timeframes, in order to decipher the temporal relationships between triggers and feedbacks of climate disturbances and their environmental impact. This is realized by combining high-precision U-Pb geochronology (CA-ID-TIMS), with the geological, biological and chemical record integrated into a Bayesian age-depth model. Moreover, the conditions of the chemical pre-treatment (“chemical abrasion”) of the zircon ID-TIMS technique are optimized to mitigate the age bias due to loss of radiogenic lead from the crystal structure and to improve the accuracy of the method. The improved protocol is applied to single zircon of volcanic ash beds interspersed with Early Triassic marine sediments (Nanpanjing basin, South China).

WIDMANN, Philipp. Temporal calibration and quantification of Early Trias-sic climatic disturbances through high-precision U-Pb zircon dating and an improved chemical abrasion procedure . Thèse de doctorat : Univ. Genève, 2019, no. Sc. 5360

DOI : 10.13097/archive-ouverte/unige:121676 URN : urn:nbn:ch:unige-1216769

Available at:

http://archive-ouverte.unige.ch/unige:121676

Disclaimer: layout of this document may differ from the published version.

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Département des sciences de la Terre Prof. Dr. Urs Schaltegger Prof. Dr. Hugo Bucher

“Temporal calibration and quantification of Early Triassic climatic disturbances through high-precision U-Pb zircon

dating and an improved chemical abrasion procedure ”

THÈSE

Présentée à la Faculté des Sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention sciences de la Terre.

par

Philipp Alexander Widmann

de

Esslingen (Neckar) (Allemagne)

Thèse N° 5360

GENÈVE 2019/21/6

The research presented in this thesis was accomplished with financial support through the Swiss National Science Foundation (SNSF) under project no. [156424]. The research was carried out in the Department of Earth Sciences, University of Geneva, within the framework of the Doctoral Program in Mineral Sciences (DPMS).

Thesis director:

Prof. Dr. Urs Schaltegger Department of Earth Sciences, University of Geneva, Switzerland

Thesis co-director:

Prof. Dr. Hugo Bucher

Paleontological Institute and Museum, University of Zurich, Switzerland

Members of the Dissertation committee:

Prof. Dr. Torsten W. Vennemann Institute of Earth Surface Dynamics, University of Lausanne, Switzerland

Dr. Jörn-Frederik Wotzlaw

Institute of Geochemistry and Petrology ETH Zürich, Switzerland

CONTENT

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1

C^HAPTER 2 1

C^HAPTER 3 2

C^HAPTER 4 3

R

CHAPITRE 2 4

CHAPITRE 3 5

C^HAPITRE 4 6

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1.1 CLIMATIC IMPACT OF LARGE IGNEOUS PROVINCES

AND MASS EXTINCTIONS 9

1.2 CLIMATE FLUCTUATION IN THE GEOLOGICAL RECORD 11

1.2.1 Changes in Biodiversity 11

1.2.2 Geochemical record 11

1.3 ZIRCON U-PB DATING BY CA-ID-TIMS 13

1.4 ZIRCON IN THE STRATIGRAPHIC RECORD 17

1.5 AGE-DEPTH MODELS 19

1.6 AIM OF THE THESIS 21

1.7 R^EFERENCES 22

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2.1 ABSTRACT 32

2.2 INTRODUCTION 33

2.3 MATERIAL AND METHODS 34

2.3.1 Sample material and preparation 34

2.3.2 Imaging of internal textures 34

2.3.3 Raman spectrometry 35

2.3.4 Electron probe microanalysis 36

2.3.5 Trace element analysis by LA-ICP-MS 36

2.3.6 U-Pb-CA-ID-TIMS 37

2.4 RESULTS 38

2.4.1 CL images 38

2.4.2 Trace element concentrations 39

2.4.3 Calculated α-dose 40

2.4.4 Raman spectrometry 40

2.4.5 Partial dissolution at 180 °C and 210 °C 42

2.4.6 Uranium-lead ages 43

2.5.1 Calibrating the chemical

abrasion procedure 43

2.5.2 Age domains in Plešovice zircon 45

2.5.3 Proposing “confidence zones” for U-Pb ages 46

2.5.4 Geological implications 47

2.6 C^ONCLUSION 48

2.7 REFERENCES 49

2.8 SUPPLEMENTARY 54

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EXTINCTION 85

3.1 ABSTRACT 85

3.2 INTRODUCTION 86

3.2.1 Geological context 88

3.2.2 Biostratigraphy 89

3.3 RESULTS 90

3.3.1 U-Pb and age-depth model 91

3.3.2 Carbon isotope compositions of carbonates 92

3.3.3 Organic carbon and palynofacies 92

3.4 D^ISCUSSION 92

3.4.1 Age of the Smithian-Spathian Boundary and

duration of the CIE 92

3.4.2 The role of volcanogenic gas emissions 93

3.4.3 The role of silicate weathering 94

3.4.4 Carbon cycle and biological pump: Interpretation of

inorganic and organic CIE 94

3.4.5 Timeframe for the late Smithian cooling

and feedback mechanism 95

3.5 CONCLUSIONS 97

3.6 MATERIALS AND METHODS 97

3.6.1 Mineral separation and CA-ID-TIMS U-Pb dating 98

3.6.2 Age depth model 98

3.6.3 Carbon and oxygen isotope measurements

of carbonates 98

3.6.4 Organic geochemistry and palynofacies 98

3.7 REFERENCES AND NOTES 99

3.8 SUPPLEMENTARY 103

3.8.1 Stratigraphy and section description 103

3.8.2 Sampled ash layers 103

3.8.3 Mineral separation and CA-ID-TIMS U-Pb dating 104

3.8.4 Age depth model 105

3.8.5 Rcode “rBacon” for age-depth modelling

South China Sections 106

3.8.6 Carbon and oxygen isotope measurements

of carbonates 107

3.8.7 Organic geochemistry 108

3.8.8 Ammonoid age control and definition of the SSB 108

3.8.10 How reliable is the FO of Novispathodus

pindingshanensis as a base-Spathian index? 110

3.8.11 References 111

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BAYESIAN AGE DEPTH MODELLING 129

4.1 ABSTRACT 129

4.2. INTRODUCTION 130

4.2.1. Geological setting 132

4.2.2. Sampling locations 133

4.2.3. Carbon isotope record 134

4.3. M^ETHODS 134

4.3.1. Samples 134

4.3.2. Hafnium isotopes by solution MC-ICP-MS 134

4.3.3. Zircon CA-ID-TIMS 134

4.3.4. Bayesian age-depth model 135

4.4. R^ESULTS 136

4.4.1. U-Pb-CA-ID-TIMS dates 136

4.4.2. Zircon Hafnium values 137

4.5. DISCUSSION 137

4.5.1. Early Triassic substages 137

4.5.2. Age-depth model unconformities 138 4.5.3. Origin of climatic deteriorations local or global? 141 4.5.4. The tectonic context of volcanism 142

4.6. CONCLUSION 144

4.7 REFERENCES 145

4.8 SUPPLEMENTARY 155

4.8.1 Sampling location 155

4.8.2 Sample preparation and CA-ID-TIMS

geochronology 155

4.8.3 Bulk grains solution Hf isotopes by MC-ICP-MS 157

4.8.4 Age Depth model code Bchron 160

4.8.5 References 159

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S UMMARY

This thesis, entitled “Temporal calibration and quantification of Early Triassic climatic disturbances through high-precision U-Pb zircon dating and an improved chemical abrasion procedure”, comprises three independent studies that intend to improve and implement methods to build accurate timeframes. These timeframes will allow to quantify short-lived climatic processes in Earth’s history by combining high precision U-Pb chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) dating with the litho-, chemo, and biochronological record built into a Bayesian age- depth model.

The first chapter will give a general introduction describing the background and problem statement. The second chapter presents an experimental approach to improve the conditions for the chemical pre-treatment (“chemical abrasion”) of the zircon ID-TIMS technique that aims to mitigate the age bias due to loss of radiogenic lead from the crystal structure and improve the accuracy of this method.

The third and fourth chapters contain two geochronological studies of the Early-Triassic sedimentary records from the marine Nanpanjiang Basin, South China. The third chapter aims for a precise temporal calibration and quantification of the Smithian-Spathian boundary and associated environmental upheavals as well as to unravel the potential underlying mechanism. For this purpose, a Bayesian age- depth model was implemented on the basis of high precision U-Pb zircon dates that are intercalibrated with conodont- and preliminary ammonoid Unitary Association zones. The fourth chapter will present a composite age-depth model for the entire early Triassic. For this purpose, published and new high precision U-Pb zircon dates from ash layers recovered in South China were integrated within a Bayesian age-depth model and compared to the carbonate carbon isotope record, allowing a comprehensive model on the timing of the recovery dynamics after the end-Permian mass extinction. Additionally, hafnium isotopic systematics in zircon are reported that are integrated into the overall tectonic setting and evolution.

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Reliable and meaningful U-Pb dates require the control of factors that cause age dispersion within a zircon population. However, especially the recognition and mitigation of post-crystallization loss of radiogenic lead (Pb*) is a still an unsolved problem in high-precision U/Pb-CA-ID-TIMS dating.

The loss of radiogenic lead preferentially occurs along altered zones of the crystal lattice as a consequence of the structural damage produced by Uranium decay that results in too young ages. Yet, the rejection of young ages from a zircon population due to suspected Pb-loss can be difficult to justify since it is hard to prove that Pb-loss is the cause of the young age. The introduction of procedures such as chemical abrasion (CA) on single zircon grains has significantly reduced the impact of Pb-loss.

However, there is no guarantee that Pb-loss can be completely removed from a zircon. Unfortunately, the CA technique is applied by different laboratories using slightly different procedures making comparisons between laboratories more difficult. Furthermore, this method is mostly empirical and used

without a detailed understanding of how (1) the applied annealing temperature and (2) the temperature and duration of partial dissolution affect more or less decay-damaged zones. Previous studies have investigated the effect of annealing on radiation damaged zones at different time and temperature conditions, or the effect of temperature at a fixed duration of the partial dissolution step on the final reproducibility of U-Pb dates. However, no study has investigated the effect of the chemical abrasion procedure on the zircon trace element chemistry compared to the state of the crystal structure and the U- Pb date.

This chapter presents an experimental approach to quantify the effects of chemical abrasion on the zircon chemistry and its crystal structure. For this purpose, the natural reference zircon Plešovice is chosen, due to its known variation in trace element concentrations and the presence of domains rich in actinides. Plešovice grain fragments were annealed for 48 h at 900 °C. Eight aliquots were separated and attacked in concentrated hydrofluoric acid at 180 °C and 210 °C for 6 h, 12 h, 18 h, and 24 h. Raman spectroscopy, EMPA, CL imaging and LA-ICP-MS trace element analysis was performed on each of these fractions. The zircon treated by CA are compared to untreated and only annealed zircons.

The study aims to propose improved protocols for the chemical pre-treatment of zircon based on the state of the crystal structure and chemistry. Further it is demonstrated that Plešovice zircon cannot be considered as a homogenous standard at the current level of precision achieved by CA-ID-TIMS dating due to natural age variation on a sub-million-year scale. This chapter is published in Chemical Geology.

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Within the entire Triassic, the most important extinction of ammonoids and conodonts occurred during the latest Smithian. The late Smithian was characterized by a global positive carbon isotope excursion (CIE), which culminated at or very near to the SSB. Late Smithian times were usually considered to be associated with hot sea surface temperatures accompanied by the expansion of the oxygen minimum zone as expected during sea level rises is questioned by studies documenting (i): a cooling during the latest Smithian and basal Spathian, (ii): a major change from lycopod- to gymnosperm-dominated vegetation indicating a change toward drier climate (iii): a global eustatic sea- level drop, and variations of chemical weathering. The late Smithian was further accompanied by an enhanced rate of organic carbon burial of terrestrial origin on continental shelves.

This chapter presents a new temporal calibration and quantification of the Smithian-Spathian boundary, the global late Smithian CIE and the associated faunal turnover by using high precision, chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dating technique from single zircon crystals recovered from volcanic ashes of the Nanpanjiang Basin (South China). These U-Pb ages are intercalibrated with conodont- and preliminary ammonoid Unitary Association zones and serve as basis for a Bayesian age-depth model. The new timeframe allows to distinguish between potential causal mechanisms operating at different time scales that can have a sufficient serve impact on the carbon cycle to cause a climate cooling. With respect to the faunal changes we propose a new time resolved model for cascading events that lead into the late Smithian extinction.

This chapter is in review in Science Advances.

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Following the Permian/Triassic mass extinction (PTME), the Early Triassic biotic recovery underwent several setbacks of the nekton and terrestrial plants during its 5 myr time span. The first ~3 myr are characterized by high amplitude fluctuation in the δ¹³C and δ¹⁸O records, associated with changes in global climate and their consequences in terms of sea level, weathering rates, redox conditions, and organic carbon burial. Episodic pulses of volcanism of the Siberian Traps Large Igneous Province (STLIP) are commonly proposed as a trigger for these Early Triassic disturbances, but lack a complete confirmation by reliable radio-isotopic dating. The temporal link is further complicated by a large number of out-dated ⁴⁰Ar/³⁹Ar, ⁴⁰K-⁴⁰Ar and U-Pb data, mostly prone to erroneous interpretation of ages. Yet, it is uncertain how the diminishing magnitude and frequency of STLIP volcanic episodes may have contributed to these Early Triassic climatic drastic disturbances and associated biotic setbacks.

This chapter presents high precision zircon CA-ID-TIMS U-Pb ages, δ¹³Ccarb isotope record and Hf isotope systematics spanning from the latest Permian until the earliest Middle-Triassic. The ash layers are recovered from shallow to deep marine sediments located in the Nanpanjiang Basin (South China) and offer a high resolution ammonoids and conodonts biochronological age control. The U-Pb ages serve as basis for a composite Bayesian age-depth model of the complete Early Triassic that is correlated to the δ¹³Ccarb isotope record. This approach allows to infer durations of δ¹³Ccarb isotope excursions, and likewise, on the dynamic forces of the early Triassic recovery. Moreover, the U-Pb-ages are correlated to the ɛHf isotope systematics in dated zircon grains, permitting to extract information on the timing, petrogenesis and the provenance of magma sources and on the tectonic setting. This chapter is in preparation for submission to an international journal.

.

R ESUME

Cette thèse intitulée, « Improved CA-ID-TIMS U-Pb protocol applied to a temporal calibration and quantification of Early Triassic climate disturbances » a pour but d’améliorer et de mettre en œuvre des méthodes précisent de quantification des réponses climatiques de courtes durées, dans l’histoire de la Terre, en combinant la datation U-Pb de haute précision par dilution isotopique, abrasion chimique en spectrométrie de masse par thermo-ionisation (CA-ID-TIMS), avec la lithostratigraphie, chimiostratigraphie, et le registre biochronologique intégrés dans un modèle Bayésien âge/épaisseur stratigraphique. Le premier chapitre est une introduction générale décrivant le contexte et l’énoncé du problème. Le deuxième chapitre porte sur une approche expérimentale visant à améliorer les conditions du pré-traitement chimique (« abrasion chimique ») de la technique ID-TIMS sur zircon. Cette méthode vise à atténuer le biais d’âge dû à la perte de plomb radiogénique de la structure cristalline et à améliorer ainsi la précision. Les troisième et quatrième chapitre contiennent deux études géochronologiques des enregistrements sédimentaires du début du Trias dans le bassin marin du Nanpanjiang, en Chine du Sud.

La troisième étude porte sur l’étalonnage et une quantification temporelle précise de la frontière Smithien-Spathien et sur les perturbations environnementales associées, afin de déterminer les mécanismes potentiels sous-jacents. À cette fin, un modèle Bayésien d’âge-profondeur a été mis en œuvre sur la base de datations U-Pb de haute précision sur zircons, qui sont intercalées avec des conodontes et les zones d’associations unitaires préliminaires d’ammonites. Le quatrième chapitre présente un modèle composite âge-profondeur pour tout le Trias inférieur. À cette fin, des zircons provenant de couches de cendres récupérées dans le même bassin ont été analysés pour des dations U- Pb de haute précision. Ces nouvelles données et celles déjà existantes dans la littérature ont été intégrées dans un modèle bayésien d'âge-profondeur avec un enregistrement des isotopes du carbone carbonate, permettant ainsi de disposer d'un modèle complet de l’histoire de la dynamique de rétablissement de l’environnement suivant la crise du Permien. A cela vient s’ajouter la systématique isotopique du zircon du hafnium dans l’évolution du système magmatique à l’origine de ces cendres et son contexte géodynamique.

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Les âges U-Pb fiables et significatifs nécessitent le contrôle des facteurs responsables de la dispersion des âges dans une population de zircon. Cependant, la reconnaissance et l'atténuation de la perte de plomb radiogénique (Pb *) après la cristallisation restent un problème non résolu dans la datation U/Pb-CA-ID-TIMS de haute précision. La perte de plomb radiogénique se produit préférentiellement le long des zones altérées du réseau cristallin en raison des dommages structurels produits par la désintégration de l’uranium qui a pour conséquence des âges trop jeunes. Toutefois, il peut être difficile de justifier le rejet des âges trop jeunes d'une population de zircon en raison d'une suspicion de perte de plomb, car il est difficile de prouver que la perte de plomb est la cause du jeune âge. L'introduction de procédures telles que l'abrasion chimique (AC) sur des grains de zircon simple a considérablement réduit l'impact de la perte de Pb. Cependant, rien ne garantit que la perte de plomb puisse être complètement

éliminée du zircon. Malheureusement, la technique de l'analyse est appliquée par différents laboratoires avec différentes procédures, ce qui rend les comparaisons entre laboratoires plus difficiles. De plus, cette méthode est principalement empirique et utilisée sans une compréhension détaillée de la manière dont (1) la température de recuit appliquée et (2) la température et la durée de la dissolution partielle affectent plus ou moins les zones métamictes. Des études antérieures ont étudié l'effet du recuit sur des zones endommagées par le rayonnement dans différentes conditions de temps et de température. L’effet de la température pour une durée déterminée de la dissolution partielle influe sur la reproductibilité finale des âges U-Pb. Cependant, aucune étude n'a examiné l'effet de la procédure d'abrasion chimique sur la chimie des éléments traces de zircon par rapport à l'état de la structure cristalline et de l’âge U-Pb. Ce chapitre présente une approche expérimentale pour quantifier les effets de l’abrasion chimique sur la chimie du zircon et sa structure cristalline. À cette fin, le zircon de référence naturel de Plešovice est choisi en raison de la variation connue de ses concentrations en éléments traces et de la présence de domaines riches en actinides. Les fragments de grain de Plešovice ont été recuits pendant 48 heures à 900 °C. Quatre aliquotes ont été séparées et attaquées dans de l’acide fluorhydrique concentré à 180 °C et 210 °C pendant 6 h, 12 h, 18 h et 24 h. Nous avons utilisé la spectroscopie Raman, des analyses microsonde de l’imagerie CL et l’analyse des éléments traces par LA-ICP-MS sur chacune des fractions.

Les zircons traités par CA ont été comparés aux zircons non traités et uniquement recuits. L’étude vise à proposer des protocoles améliorés pour le pré-traitement chimique du zircon, basés sur l’état de la structure cristalline et la chimie. De plus, il est démontré que le zircon de Plešovice ne peut pas être considéré comme un standard homogène au niveau de précision actuel atteint par la datation CA-ID- TIMS en raison de la variation naturelle de l’âge sur une échelle inférieure à un million d’années. Ce chapitre est publié dans Chemical Geology.

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Dans l’ensemble du Trias, l’extinction la plus importante des ammonites et des conodontes s’est produite lors de la fin du Smithien. La fin du Smithien a été caractérisée par une excursion positive globale des isotopes du carbone (ECI) qui a culminé à la limite Smithien-Spathien (SSB) ou très près de celle-ci. La fin du Smithien est habituellement considérée comme étant associées à des températures de surfaces chaudes de l’océan, accompagnées par l’expansion de la zone minimale d’oxygène, comme on peut s’y attendre lors de l’élévation du niveau de la mer. Ceci est remis en question par des études documentant (i): un refroidissement à la fin du Smithien et à la base du Spathien, (ii): un changement majeur des communautés de plantes terrestres indiquant un changement vers un climat plus sec (iii): une baisse du niveau marin et des variations de l’altération chimique. La fin du Smithien a également été accompagnée d’un taux accru d’enfouissement du carbone organique d’origine terrestre sur les plateaux continentaux résiduels. Ce chapitre présente un nouvel étalonnage et une nouvelle quantification temporelle de la limite Smithien-Spathien, de la CIE globale du Smithien supérieur et du renouvellement des faunes marines et flores terrestres associées en utilisant la spectrométrie de masse à haute précision et à dilution isotopique par abrasion chimique (CA-ID-TIMS). C’est une technique de datation à U-Pb à partir de cristaux simples de zircon récupérés dans des cendres volcaniques du bassin de Nanpanjiang

(sud de la Chine). Ces âges U-Pb sont intercalés avec des zones d'association unitaire d'ammonoïdes de conodontes et d'ammonoïdes préliminaires et servent de base à un modèle bayésien d'âge-profondeur.

La nouvelle période permet de distinguer les mécanismes potentiels opérant à différentes échelles de temps et pouvant avoir un impact suffisant sur le cycle du carbone pour générer un refroidissement du climat. En ce qui concerne les changements de la faune, nous proposons un nouveau modèle à résolution temporelle pour les événements en cascade menant à la dernière extinction du Smithien. Ce chapitre est soumis à Science Advances et est en cours de révision.

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À la suite de l’extinction de masse Permien/Trias (PTME), la récupération biotique du Trias inférieur a subi plusieurs remises à zéro au cours d’une période de 5 Ma. Les premiers ~ 3 Ma sont caractérisés par une forte fluctuation d'amplitude des enregistrements du δ¹³C et δ¹⁸O. Ceci est associé aux changements du climat mondial et à leurs conséquences en termes de variation du niveau marin, du taux d’altération, des conditions d'oxydo-réduction et d'enfouissement du carbone organique. Les impulsions épisodiques du volcanisme de la grande province ignée de Sibérie (STLIP) sont communément proposées comme déclencheurs de ces perturbations du début du Trias, mais manquent d’une réelle confirmation par datation radio-isotopique fiable. Le lien temporel est encore compliqué pour un grand nombre de données ⁴⁰Ar/³⁹Ar, ⁴⁰K-⁴⁰Ar et U-Pb obsolètes, généralement sujettes à une interprétation erronée des âges. Cependant, on ne sait pas comment la diminution de l'ampleur et de la fréquence des épisodes volcaniques du STLIP pourraient avoir contribué à ces bouleversements climatiques dramatiques du début du Trias, et à l’extinction biotique associée. Ce chapitre présente la systématique de haute précision du zircon CA-ID-TIMS, de la chaîne de l'isotope du carbone, δ¹³C et de l'isotope de l’Hf, du Permien tardif au début du Trias moyen. Les couches de cendres sont récupérées dans des sédiments marins tant profonds que peu profonds situés dans le bassin de Nanpanjiang (sud de la Chine) et offrent un contrôle biochronologique haute résolution grâce aux ammonites et aux conodontes. Les âges U-Pb servent de base à un modèle bayésien composite d'âge-profondeur du début du Trias, qui est corrélé à l'enregistrement de l'isotope δ¹³Ccarb. Cette approche permet de déduire les durées des excursions isotopiques de δ¹³Ccarb, ainsi que la dynamique de la récupération du début du Trias. De plus, les âges U-Pb sont corrélés à la systématique des isotopes ɛHf dans les grains de zircon datés, ce qui permet d'extraire des informations temporelles sur la pétrogenèse des magmas et sur la tectonique régionale. Ce chapitre est en préparation pour soumission à une revue internationale.

C HAPTER 1: I NTRODUCTION

At the present time, Earth climate is changing at high pace. The current change of Earth climate and environment is recently referred to the 6^th mass extinction (Anthropocene extinction) accompanied by a large number of species going extinct because of the harmful activities of humans (Wagler, 2013).

However, little is known about future short- and long-term effects on the Earth dynamic system as well as the related feedback processes to the current modification of the carbon cycle. As geoscientists, we are capable to study the geological record and to acquire information on past climate changes. We in- vestigate potential natural analogues in Earth history and identify their basic mechanisms in order to understand how the current climatic change will affect the Earth system, environment, and our society in future.

The integration of high-precision U-Pb zircon geochronology with litho-, bio- and chemostrat- igraphic records at the <100 ka temporal resolution can provide valuable information to answer some of the current scientific questions such as:

 factors that directly trigger biotic crises (e.g., injection of volcanic gases into the atmos- phere related to Large Igneous Provinces (LIPs) and causing severe climate change),

 factors that can indirectly cause biotic crises (e.g., cooling due to the drawdown of atmos- pheric CO2 by weathering of LIP basalts),

 the ecological consequences of the disturbance of the carbon cycle,

 its environmental response and timing of feedback processes,

 on what timescales extinction and the consecutive recovery of biotic crises took place.

These questions will be addressed in the third and fourth chapter of this thesis, with an example from the Early Triassic in South China. These chapters will combine high precision U-Pb dating, bio- chronology and the isotopic record built into a Bayesian age-depth model, with the purpose to assess the timing and duration of potential trigger mechanisms, to quantify the timing of abiotic and biotic feedback processes and their temporal relation to volcanic activity. This approach will, as well, allow a first ap- proximation for the impact of different processes on the global carbon cycle.

South China offers some exceptional fossil-rich sections in shallow and deep marine environ- ments, with abundant volcanic ash layers, especially in the Upper Permian and Lower Triassic (Yin et al., 1992), which allow a detailed integration of U-Pb dates into, the litho-, bio- and chemostratigraphic record. The most severe extinction occurred at the Permian-Triassic boundary (afterwards called Permo- Triassic Mass Extinction, PTME) (Fig. 1.1) that erased more than 90% of marine species and 70% of terrestrial vertebrates on land (Hallam and Wignall, 1997; Erwin, 1994). Due to the temporal coincidence and the severe climatic impact, the volcanic activity of the Siberian Traps LIP, is generally considered as the ultimate cause for the PTME and Early Triassic climate fluctuations (Algeo et al., 2015; Burgess et al., 2014, 2017; Burgess and Bowring, 2015; Svensen et al., 2009). The biotic recovery following the PTME during the Early Triassic is traditionally described as delayed, due to protracted harsh environ- mental conditions (Chen and Benton, 2012; Song et al., 2011). Though, some taxa such as conodonts

and ammonoids appear to recover rapidly in the Early Triassic (Song et al., 2011; Brayard et al., 2009;

Orchard, 2007), others such as reef builders, and benthos show a rather slow and protracted recovery (Hautmann et al., 2015; Chen and Benton, 2012; Song et al., 2011). The Early Triassic is characterized by high frequency / high amplitude isotopic fluctuation in the δ¹³C and δ¹⁸O record, showing multiple rapid but short-lived recovery pulses that are thought to be linked with climate disturbances linked to volcanic activity with global impact (Goudemand et al., 2019; Romano et al., 2013; Sun et al., 2012;

Galfetti et al., 2007; Payne et al., 2004). The Early Triassic received more scientific interest within the last years, yet, the basic causes, the tempo, and mechanism for the carbon cycle disturbances and rapid consecutive extinction-radiation pulses of the biota are controversially debated (e.g., Goudemand et al., 2019; Hochuli et al., 2016; Tian et al., 2014; Romano et al., 2013; Sun et al., 2012; Pruss and Bottjer, 2005).

To build quantitative, time-resolved models highest accuracy and precision of the acquired U- Pb ages are required, which can be only achieved by absolute control and understanding of the potential factors that can bias the age of the dated sample (e.g., partial loss of radiogenic lead after the crystalli- zation of the host mineral). The mitigation of loss of radiogenic lead from the zircon crystal structure is instrumental and commonly achieved by the so-called “chemical abrasion technique” (Mattinson, 2005) prior to complete dissolution and analysis of a sample. The second chapter will cover an experimental approach to investigate and improve conditions for the chemical pre-treatment of the zircon ID-TIMS technique, this study will additionally contribute to harmonise laboratory procedures to allow a better reproducibility of acquired U-Pb dates within the same laboratory and better comparability between different high-precision U-Pb laboratories. The following introduction will give a brief outline of triggers

Figure 1.1. Diversity of marine faunas throughout the Phanerozoic, showing the ‘big five’ extinctions.;

(1) Ordovician-Silurian, (2) Late Devonian, (3) Permian-Triassic, (4) Triassic-Jurassic and (5) Cre- taceous-Palaeogene. The greatest decrease in the number of marine families occurred during the Per- mian-Triassic mass extinction. Modified after Raup and Sepkoski (1982).

(e.g., periods of intense volcanism) and feedbacks (e.g., warming, cooling, changes in weathering rates, anoxia) of climatic and biotic fluctuations in the geological past. As well, the applied techniques and the current challenges that arise with modern high-precision U-Pb dating will be introduced.

1.1 C

L

I

P

M

E

Several major and minor mass extinctions occurred during the Phanerozoic. These events lasted for a rather short time on a geological timescale but had substantial impact on the biota (Bond and Grasby, 2017; Hallam and Wignall, 1997; Raup and Sepkoski, 1982). Mass extinctions are characterised by the rapid disappearance of taxa from the fossil record that is followed by rapid rediversification of the surviving taxa (Stanley, 2016; Hull, 2015). These biotic events are usually considered to correlate, among others, with global cooling, global warming, ocean acidification, toxic gases, anoxia, sea level changes, bolide impacts and periods of intense volcanic activity (Bond and Grasby, 2017; White and Saunders, 2005; Wignal, 2001). The biggest five documented mass extinction events occurred during the end-Ordovician (∼444 Ma), in the Late Devonian (∼370 Ma), at the Permian-Triassic boundary (∼251 Ma), at the Triassic-Jurassic boundary (∼200 Ma), and at the Cretaceous-Tertiary boundary (∼65

Figure 1.2. Age-correlation between LIPs and mass extinctions. Taken from Courtillot and Renne (2003).

Ma) (Fig. 1.1). However, minor extinction events are as well recognized in the sedimentary record (Ernst and Youbi, 2017; Courtillot and Renne, 2003) (Fig. 1.2).

Due to their close coincidence, the emplacement of LIPs is commonly suggested as ultimate triggers for environmental disturbances and mass extinctions. LIPs are large magmatic provinces that formed from multiple short igneous pulses of mantle or lithosphere-derived magma, lasting for a pro- longed period of time (1–5 Ma) (e.g., Bryan and Ernst, 2008) and are predominantly emplaced as conti- nental flood basalt or as oceanic plateaus (Saunders, 2005) through a system of feeder dykes and sills from mantle depth. Even though their nature remains under debate, LIPs are commonly attributed to mantle plumes, continental breakup, meteorite impact, or to lithospheric delamination (Saunders, 2005).

Though, the commonly reviewed temporal coincidence of mass extinctions and LIPs is only at a resolu- tion of ca. 1 Ma (see Fig. 1.2; Courtillot and Renne, 2003). Yet at a closer look, the temporal association to biotic crises and/or geochemical anomalies appears more complex, see, e.g., Davies et al. (2017) and Wignall (2001), that primarily arises from the improved precision at the <100 ka level and accuracy of U-Pb dating. And not all LIP eruptions coincide with extinctions, e.g., such as the large Paraná–Etendeka Province (~133 Ma), Columbia River Flood Basalts (~16 Ma) or the North Atlantic Igneous Province (~62 Ma) that are characterized by low extinction rates (Bond and Grasby, 2017; Wignall, 2005). Addi- tionally, oceanic plateaus such as the Ontong Java (~120 Ma) or the Caribbean–Colombian Plateau (~90 Ma) had only minor impact on the marine environment due to anoxia and calcification crises (Wignall, 2005). In turn, this raises new questions of their environmental impact and on the fundamental triggers of past environmental disturbance. As pointed out by Bond and Wignall (2014) and Wignall (2001) the geodynamic situation (continental or oceanic), their eruption style, as well as the paleogeographic posi-

Figure 1.3. Proposed triggers, environmental impact and feedbacks of the emplacement and volcanic ac- tivity of the Siberian traps Large Igneous Province. Taken from Guex (2016).

tion (mainly the paleo-latitude) are critical prerequisites for their environmental impact. The end-Per- mian (Siberian Traps) and Triassic-Jurassic (CAMP) extinction are both temporally linked to intrusions of sills into evaporite-rich and/or coal-rich sedimentary basins, therefore their heating and degassing might be a substantial factor for environment impact of LIPs (Svensen et al., 2009). As the volcanic activity of LIPs injects large amounts of climate modulating gases into the atmosphere during eruption or emplacement of dykes and sills (e.g., Burgess et al., 2017), these periods attracted much scientific interest, as they are considered as valid analogue to the recent climate change (see, e.g., Wignall, 2005).

The emission of volcanic gases from LIPs is commonly stated as main trigger of environmental fluctu- ations (Bond and Wignall, 2014; Wignall, 2001, see Fig. 1.3). Especially if related intrusions imprint a contact metamorphism onto their sedimentary host, large amounts of climate modulating gases such as SO2, CH4 or CO2 are released into the atmosphere (e.g., Svensen et al., 2009; Self et al., 2005). Different gases can have varying impact on the global climate and, due to their different residence times in the atmosphere, are active on different timescales. As the injection of CO2 and CH4 promote global warming over periods of 100 ka, the emission of SO2 aerosols promotes rather a brief cooling restricted to hun- dreds to thousands of years (Guex et al., 2016; Buggisch et al., 2010; Saunders and Reichow, 2009;

Robock, 2000). Harmful feedbacks associated with the injection of such volcanic gases include oceanic anoxia, ocean acidification, sea level changes, toxic metal input, essential nutrient decrease or global cooling (Bond and Grasby, 2017; Ernst and Youbi, 2017; Bond and Wignall, 2014; Buggisch et al., 2010) (Fig. 1.3). But also global cooling promoted by CO2 drawdown through enhanced weathering of LIP-related basalts, can act as potential feedback with major impact on the carbon cycle acting on geo- logical timescales, significant longer than, e.g., the injection of SO2 (Yang et al., 2018; Schaller et al., 2012; Dessert et al., 2003, 2001).

Feedback processes can either enhance (positive feedback) or weaken (negative feedback) the effect triggers such as volcanic gases, that in turn force either global cooling or warming. A positive feedback process to the release of volcanic CO2 will therefore be, e.g., the rise in global temperature (Lüthi et al., 2008; Jouzel et al., 2007), the release of CH4 from, e.g., permafrost (Schaefer et al., 2014) or melting of glaciers and lowering of the albedo (Roscher et al., 2011; Gardner and Sharp, 2010). In contrast, a negative feedback would draw down the CO2 by enhanced bioproductivity (Schippers et al., 2004) or silicate weathering (Liu et al., 2011; Gislason et al., 2008; West et al., 2005).

1.2 C

Though, little is known about the exact mechanism, nor the timing and duration of these pro- cesses in the geological past. To investigate the temporal relationships between triggers and feedback processes and their impact on the environment, highest precision and accuracy of the temporal calibra- tion with absolute dates is instrumental. This thesis will demonstrate how a precise and accurate timeframe established by U-Pb dates allows to link different biotic and climatic processes, which in turn leads to an integrated and quantitative understanding of underlying triggers and feedbacks and their du- ration. This will be exemplified by a case study from the Early Triassic.

1.2.1 C

B

Ammonoids and conodonts are important organisms for biochronological studies especially in Paleozoic and Mesozoic marine strata, due to their abundance, extensive paleogeographic distribution and high evolutionary rates (Bai et al., 2017; Sweet and Donoghue, 2001; House, 1985). Ammonoids nearly died out during the Permian-Triassic mass extinction but recovered rapidly during the Early Tri- assic (Brayard et al., 2009). Therefore conodonts and ammonoids are important to study the post-extinc- tion recovery dynamics during the Early Triassic (Brayard et al., 2009; Orchard, 2007).

1.2.2 G

The δ¹³C values of inorganically and biologically precipitated carbonate in the oceans is close to that of the dissolved inorganic carbon (DIC), e.g., CO2(aq), in the oceans (Maslin and Swann, 2005) that provides evidence about the isotopic composition of ancient oceans and in turn of the atmosphere.

However, photosynthesis involves a large negative fractionation of atmospheric carbon by preferential uptake of ¹²CO2 and therefore organic carbon compounds are strongly depleted in ¹³C relative to atmos- pheric CO2 (e.g., Fogel and Cifuentes, 1993). The long-term climate change is controlled primarily by the balance between CO2 sources from volcanic and metamorphic degassing and CO2 sinks like silicate weathering and organic carbon burial (Kump and Arthur, 1999). The deterioration of the carbon cycle by volcanic activity is preserved in the geological record as geochemical anomalies. Perturbations in the global carbon budget are reflected as fluctuations in carbon isotope ratios measured in marine carbonates (δ¹³Ccarb) and/or organic matter (δ¹³Corg). Analysis of δ¹³C isotope values is a key to study environmental change and is widely used as a parameter for stratigraphic correlations (Saltzman and Thomas, 2012), within regional but also global context.

Positive ¹³Ccarb excursions are generally related to an increase of burial of organic carbon either due to anoxic events or cooling, while negative ¹³Ccarb are related to injection of a deleted carbon source and typically coincide with climate warming (Berner, 2005; Kump and Arthur, 1999) (Fig. 1.4). The covariation of carbonate and organic carbon isotope compositions are furthermore anticipated to reflect a change in the global carbon reservoir (Meyer et al., 2013; Kump and Arthur, 1999) or reflect relative sea level changes (e.g., Bachan et al., 2017; Zhao et al., 2017; Jenkyns, 1996). A gradual increase in δ¹³Ccarb values is interpreted to reflect enhanced sequestration, and burial of organic matter that conse- quently led to a reduction in atmospheric CO2 and cooling (Berner, 2005). In contrast, a δ¹³Ccarb decrease results from CO2 and CH4 release to the atmosphere causing an increase of the global temperatures (e.g., Saltzman and Thomas, 2012; Payne and Kump, 2007; Berner, 2002). Episodes of extinction are associ- ated with carbon isotope excursions (CIEs) (e.g., Stanley, 2010; Hallam and Wignall, 1997). The com- bination of numerous proxies offers the virtuous potential for paleo-environmental and paleo-climatic reconstructions (Cui and Kump, 2015).

Especially if ¹³C values are combined with ¹⁸O, we can draw conclusions on paleo-climate fluctuations. The ¹⁸O isotope ratios serve as an indicator for paleo Sea Surface Temperatures (SST) (Grossman, 2012; MacLeod, 2012). However, ¹⁸O of marine carbonates (¹⁸Ocarb) are prone to diage- netic alteration (Grossman, 2012; MacLeod, 2012). In contrast, ¹⁸O of biogenic apatite, such as from

conodonts (¹⁸Oapatite) is considered as a more reliable proxy due to its resistance to alteration (Grossman, 2012; MacLeod, 2012; Trotter and Eggins, 2006). Fluorapatite precipitates in oxygen-isotopic equilib- rium with seawater, assuming no species-vital effects (MacLeod, 2012). However, ¹⁸O variations can be altered by ice-sheet size and/or local salinity variations (Cui and Kump, 2015), and may also reflect the different life habitats of conodonts (Goudemand, 2014). Conodont ¹⁸Oapatite composition have been successfully applied to reconstruct Permo-Triassic temperature variations (e.g., Goudemand et al., 2019;

Romano et al., 2013; Sun et al., 2012).

As these proxies provide only a relative time control, the intercalibration with absolute high- precision U-Pb dates is critical and will allow a quantification of different processes and their feedbacks in the geological record in, on the basis of duration and rates. As well, high-precision - high accuracy time constraints offer the potential to explore diachroneity in the fossil record (Brosse et al., 2016), and determine the temporal relationship of ecological and climatic deteriorations to volcanic eruptions (Bowring et al., 2006).

1.3 Z

U-P

CA-ID-TIMS

Zircon (ZrSiO4) is by far the most commonly utilized mineral for U–Pb dating, that is frequently found in the above-mentioned ash layers. It allows determining the age of a targeted stratigraphic horizon Figure 1.4. Evolution of carbonate carbon isotopic composition (δ¹³Ccarb) and oxygen isotopic composition of biogenic phosphate (δ¹⁸Ophos) during the Smithian and Spathian (Early Triassic) in Pakistan. Taken from Goudemand et al. (2019), red dashed line from Romano et al. (2013). Positive δ¹³Ccarb values coincide with higher δ¹⁸Ophosvalues, and negative δ¹³Ccarbvalues coincide with lower δ¹⁸Ophosvalues, Fluctuations in δ¹⁸Ophos

indicating changes in sea surface temperatures, variation in δ¹³Ccarb reflecting changes in the isotopic composi- tion of marine carbonate. VPDB, Vienna PeeDee Belemnite; VSMOW, Vienna Standard Mean Oceanwater.

at an optimal precision and accuracy. Zircon appears to be resistant to chemical and physical alteration in various geological settings (e.g., Scherer et al., 2007; Finch and Hanchar, 2003). In conjunction with low diffusion rates, its content of minor- and trace elements, chemical- as well as isotopic information are retained (Scherer et al., 2007; Finch and Hanchar, 2003). Additionally, high concentrations of acti- nides (e.g., Uranium, Thorium) and exclusion of common Pb during crystallization makes zircon a val- uable tool in high precision dating and for detailed studies of magmatic processes. However, the chem- ical and physical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure (Finch and Hanchar, 2003, and references therein). However, due its high concentration in actinides, zircon suffers from radiation damage due to α-decay causing a disor- dered crystal lattice, that provides pathways for Pb to diffuse through the lattice (Nasdala et al., 2005) (Fig. 1.5). The loss of radiogenic lead occurs along altered zones of the crystal lattice as a consequence of the structural damage (Fig. 1.5b), which is not following the rules of temperature-activated (“Fickian”) volume diffusion (Cherniak and Watson, 2000; Lee et al., 1997) but occurring along “short circuit”

pathways (Bowring et al., 2006), and is facilitated by its high solubility in diagenetic or hydrothermal fluids percolating the zircon lattice (Geisler et al., 2003, 2002). The loss of radiogenic lead from the zircon crystal structure biases the apparent U-Pb age to younger ages.

The age of zircon can be determined on the basis of its U-Pb ratio. Natural uranium contains three isotopes: ²³⁸U (99.3 wt. %), ²³⁵U (0.72 wt. %) and ²³⁴U (0.006 wt. %) (Priest, 2001), with their corresponding decay constants λ²³⁸ = 1.55125 x 10^-10 yr^-1, and λ²³⁵ = 9.8485 x 10^-10 yr^-1 (Jaffey et al., 1971). Among all geochronologic decay schemes, the U-decay constants are the most precisely and ac- curately determined (Jaffey et al., 1971; Mattinson, 2010; Schoene et al., 2006). Furthermore, the natural uranium isotope ratio had been redetermined to ²³⁸U/²³⁵U = 137.818 ± 0.045 (2σ) (Hiess et al., 2012).

The parent uranium decays to stable daughter isotopes Pb by emitting alpha particles;

(1) ²³⁸U  ²⁰⁶Pb + 8 𝛼 (⁴He) (2) ²³⁵U  ²⁰⁷Pb + 7 𝛼 (⁴He) (3) ²³²Th  ²⁰⁸Pb + 6 𝛼 (⁴He)

Respectively, this allows calculating U-Pb age by the following equations,

Eq. 1: ^𝑃𝑏

207

235𝑈 = 𝑒𝑥𝑝^𝜆235𝑡− 1 Eq. 2: ^𝑃𝑏

206

238𝑈 = 𝑒𝑥𝑝^𝜆238𝑡− 1 Eq. 3: ^𝑃𝑏

207

206𝑃𝑏=²³⁵238^𝑈_𝑈 ×^𝑒𝑥𝑝_𝑒𝑥𝑝^𝜆235𝑡_𝜆238𝑡⁻¹₋₁

with ²⁰⁷Pb and ²⁰⁶Pb being only of radiogenic origin. ²⁰⁶Pb/²³⁸U plotted against ²⁰⁷Pb/²³⁶U is called a Concordia diagram (Wetherill, 1965). The Concordia diagram allows the recognition of potential complications such as inheritance, Pb loss (“open-system”), or initial daughter product disequilibrium that all result in a discordant analysis (Schoene, 2014). A concordant analysis plotting within error on this curve indicates a “closed system” of the dated mineral. However the discordance between the

206Pb/²³⁸U and ²⁰⁷Pb/²³⁵U isotopic systems is recently challenged as a valid indicator for Pb-loss, due to

small sample size, very low concentration of Pb* and/or by low radiogenic/common lead ratios of single zircon analysis resulting in elevated analytical uncertainties of ²⁰⁷Pb/²³⁵U ratios (e.g., Schoene et al 2014).

The analysis of U and Pb isotope ratios in zircon (ZrSiO4) by thermal ionization is considered the most precise and accurate geochronological dating technique. The technique enables to acquire ab- solute ages via tracer solutions (EARTHTIME tracer; ²⁰²Pb-²⁰⁵Pb-²³³U-²³⁵U (ET2535); ²⁰⁵Pb-²³³U-²³⁵U (ET535)) that are calibrated against a series of certified isotopic reference material and are traceable to SI units (Condon et al., 2015; McLean et al., 2015). U–Pb analysis by isotope-dilution thermal ionization mass spectrometry (ID-TIMS) involves a chemical pre-treatment prior dissolution of single grains or grain fragments, mixing with an isotopically-enriched tracer solution (e.g., Schoene and Baxter, 2017;

Schoene et al., 2010), ion exchange chromatography and TIMS analysis. Lead and uranium isotopes are separated from the dissolved sample by ion exchange chromatography, in order to improve ionization and to eliminate polyatomic or isobaric interferences from other elements (Krogh, 1973). The sample is then loaded with a Si-gel emitter onto a Re filament to improve the ionization efficiency of U and Pb (Gerstenberger and Haase, 1997). In an additional step, geochemical or isotopic information on exactly the same volume of dated material can be obtained such as concentrations of REE (TIMS-TEA), or the isotope composition of Hf, which allows to extract additional information of the crystallization history of a zircon grain, and/or of the tectonic setting of magmatism (Samperton et al., 2015; Schoene et al., 2012; Schoene et al., 2010; Schaltegger et al., 2002, 2009).

Reliable and meaningful ages require control on those factors that cause dispersion of ages, beside precision these concern accuracy, within-laboratory repeatability and interlaboratory reproduci- bility. The uncertainty on a measurement comprises non-systematic- (e.g., raw isotope measurements) and systematic uncertainties (e.g., tracer composition, decay constants) (Schoene, 2014).

Fig 1.5. a) Schematic illustration of radioactive decay in zircon. 1) ²³⁸U and ²³⁵U isotope decays to 2) stable ²⁰⁶Pb and ²⁰⁷Pb, respectively. Decay constants λ²³⁸ = 1.55125 x 10^-10 yr^-1, and λ²³⁵ = 9.8485 x 10^-10 yr^-1, after Jaffey et al.

(1971). b) crystal structures of zircon, including those be- fore and after radiation damage. Radioactive decay trans- fers a non metamict crystal with well-ordered crystal lattice to a metamict zircon with disordered crystal lattice, provid- ing pathways for Pb to diffuse through the lattice. c) molec- ular-dynamics simulation animation. b) and c) taken from Xu et al. (2012).

Considerable advances of ID-TIMS technique in the last 15 years concern the improved accu- racy and repeatability at a precision of about 0.1% (2) (Schoene, 2014) using single zircon crystals (usually <300 µm) or fragments. This has been achieved by lowering of procedural blanks, more precise and accurate, empirical calibration of the U-decay constants (Mattinson, 2010; Schoene et al., 2006), measuring the UO2 oxygen isotopic composition (Condon et al., 2015), improved understanding of initial daughter product disequilibrium (Schärer, 1984), as well as the correction for mass-dependent isotope fractionation during mass spectrometric measurements using double-isotope tracer solutions (McLean et al., 2015).

It is instrumental to minimize systematic uncertainties to allow a direct comparison of dates produced in different laboratories or using different protocols. The long term goal of an intercalibration between U-Pb-laboratories at the level of 0.1% of ²⁰⁶Pb/²³⁸U ages has been achieved by 1) the distribu- tion of the gravimetric EARTHTIME ²⁰²Pb-²⁰⁵Pb-²³³U-²³⁵U tracer solution calibrated against certified isotopic reference materials (Condon et al., 2015; McLean et al., 2015); 2) introduction of commonly used statistical data reduction software such as Redux and Tripoli (Bowring et al., 2011); and 3) the improved understanding of error contribution to the final age and its propagation (Schmitz and Schoene, 2007). Furthermore, repeated analysis of synthetic reference solutions (e.g., EARTHTIME 100, 500 and 2000) and/or well-characterized natural reference materials (such as Plešovice, Temora, R33, GJ-1) have been established as quality control of laboratory reproducibility and accuracy (Schaltegger et al., 2015).

This is indispensable to directly compare dates at the same precision from different high precision U- Pb-laboratories (Condon et al., 2015; McLean et al., 2015) or between different TIMS instruments (Schaltegger, 2018) without systematic error.

A breakthrough of the ID-TIMS technique to mitigate the impact of Pb-loss, has been the im- plementation of the so-called chemical-abrasion (Mattinson, 2005), that involves an annealing step at >

900 °C for 48 − 60 hours to restore the zircon crystal structure that is followed by a chemical treatment with hydrofluoric acid in order to remove domains compromised by Pb-loss, leaving an undisturbed (closed-system) residue. The method was successfully applied by various studies (e.g., Schoene et al., Figure 1.6. Schematic procedure of the CA-ID-TIMS technique. Zircon picture taken from Rioux et al. (2007).

2010; Mundil et al., 2004). However, the CA-technique is based on an empiric calibration and a clear understanding of the underlying mechanisms is still lacking. Moreover, the proposed protocol by Matti- son (2005) has been modified by different laboratories in terms of the applied temperature and duration of the chemical abrasion step. This makes the comparison of U-Pb dates difficult between laboratories and is a problem for the accuracy of the acquired dates. Therefore, a more thorough and quantitative, albeit empirical calibration of the experimental conditions during chemical abrasion is an important step forward for the U-Pb dating community. In the second chapter of the thesis an empirical approach is undertaken that will contribute to harmonise laboratory procedures to allow improved comparsion of the acquired U-Pb dates between high-precision U-Pb laboratories.

1.4 Z

The intercalibration of U-Pb ages with the lithological, biochronological, or geochemical record is commonly realized by sampling closely spaced volcanic ash layers within stratigraphic successions deposited in marine settings (e.g., Ovtcharova et al., 2015; Bowring et al., 2006), see c) in Fig. 1.8. This approach assumes that the age of the youngest zircon grain or population equals the time of crystalliza- tion during emplacement of an intrusive complex or a volcanic eruption and ash bed deposition (Schaltegger et al., 2015; Guex et al., 2012; Schoene et al., 2012; Wotzlaw et al., 2012; Bowring et al., 2006). Zircon is a common accessory mineral in volcanic rocks ranging from lavas to air-fall tuffs (a in Fig. 1.8) (Bowring et al., 2006). It can be dated to the required precision and accuracy by chemical- Figure 1.7. Different interpretations of a scattering, high-precision U–Pb geochronological data set. 1) Weighted-mean zircon age calculated from a complete data set, yielding a statistically unacceptable mean square of weighted deviates (MSWD). 2) Youngest zircon records magma emplacement and older zircons are attributed to magmatic residence or inheritance. 3) A weighted-mean zircon age calculated from a complete data, yielding a statistically acceptable MSWD. Modified after Samperton et al. (2015).

abrasion isotope dilution thermal ionization uranium-lead mass spectrometry (“CA-ID-TIMS”). Im- proved age precision in U–Pb CA-ID-TIMS geochronology frequently results in complex dispersion of

206Pb/²³⁸U dates within a single zircon population (Fig. 1.7). The observed age spread in one single sam- ple can be caused by analytical bias and underestimated uncertainties, post-crystallization loss of radio- genic Pb from zircon, pre-eruptive growth in a magma reservoir (b in Fig. 1.8) and/or inheritance (Schaltegger et al., 2015, 2014; Schoene, 2014; Schoene, et al., 2010). Recent studies revealed that zircon can crystallize over a 10⁴–10⁶ year timescale, within multiple magmatic systems, and at different crustal depth (e.g., Samperton et al., 2015; Schoene et al., 2010; Schaltegger et al., 2009; Michel et al., 2008;

Miller et al., 2007; Matzel et al., 2006; Charlier et al., 2005). These findings lead to more complex crystallisation models, as the age dispersion is considered not only to reflect protracted zircon growth, but mainly the temporal and spatial heterogeneity of magma bodies, alternating between near-solidus and partially molten states, through periodic magma recharge (Schaltegger and Davies, 2017; Barboni et al., 2013; Wotzlaw et al., 2013; Miller et al., 2007), reaching zircon saturation at different places and at different times. Several terms have been implemented to describe zircon in terms of their crystalliza- tion history (Miller et al., 2007). Thereafter, autocrysts are considered to have crystallized late in the magmatic history, at or near the emplacement and sampling location. Antecrystic zircons are considered to reflect transport from depth to the level of emplacement and can predate solidification to up to 500 ka (Schaltegger and Davies, 2017). In contrast, xenocrystic zircon are inherited from the fully crystallized country rock. Further complications arise from reworking of volcanic rock during deposition, or from post-depositional sedimentary reworking, incorporating pre-existing volcanic debris of significantly older age. It is not uncommon to find antecrystic or even xenocrystic zircon within one single air-fall ash bed or find volcanic deposits containing exclusively reworked zircon. Therefore, a careful preselec- tion of zircon grains via, e.g., cathodo-luminescence or backscatter imaging, to exclude zircon grains

Figure 1.8. Illustrating principles and applications of zircon U-Pb geochronology. a) eruption of zircon- bearing ash cloud, b) reconstruction of magma reservoir evolution in magmatic systems, c) volcanic ash bed deposition in marine environment and calibration of the litho-, chemo- and biochronostrati- graphic record. Modified after Schaltegger et al. (2014).

with a clear sign of inheritance from (e.g., xenocrysts or antecrysts), that pre-date the actual eruption and therefore depositional age of an ash bed (Fig. 1.8a).

Particularly critical is the assessment of the validity of the youngest occurring date within a zircon population, as it might be influenced by post-crystallisation loss of radiogenic lead. However, recognizing zircon grains suffered from loss of radiogenic lead is challenging. Despite state-of-the-art procedures using chemical abrasion on single zircon (Mattinson, 2005) there is no guarantee to com- pletely mitigate the loss of radiogenic Pb. In consequence, the rejection of zircon grains with suspected loss of radiogenic lead, while reviewing a complex data set, is often difficult to justify. Currently, those samples can be only recognized as they do violate the stratigraphic sequence, (e.g., Ovtcharova et al., 2015; Schoene et al., 2010).

As the post-crystallization loss of radiogenic Pb from the crystal lattice bias the age to younger ages, the prolonged residence of a zircon crystal in the magma chamber or magmatic recycling result in biased ages which are too old. These complex patterns of age distribution may occur within one sample and in consequence question the commonly used data interpretation of a weighted-mean dates (Samperton et al., 2015).

1.5 A

-

The improved protocol of the “chemical abrasion” procedure has been applied to most of the dated ash beds by the ID-TIMS technique. This led to practically an entire mitigation of the occurrence of single zircon grain analysis, that appear respectively too young and would have been rejected due to suspected Pb-loss. These U-Pb dates serve as basis for the Bayesian age-depth models to build a quanti- tative timeframe to infer rates of climatic reorganizations within the Early Triassic. However, the usual low sample density of U-Pb dates through a stratigraphic section, in some cases, hinders an accurate and precise determination of, e.g., a targeted stage boundary or an isotopic excursion. Within the past years,

Figure 1.9. Example for two different models based on the same dataset. a) Including a prior known discon- formity from the stratigraphic record. b) Same dataset without including a prior known information. The one date that appears to be too young in respect to the following older age will be treated as outlier (modified after Blaauw (2010)).

particularly the use of Bayesian statistics in interpolating age-depth relationships of dated samples has become increasingly common (De Vleeschouwer and Parnell, 2014; Blaauw, 2010; Haslett and Parnell, 2008; Parnell et al., 2011, 2008). Despite their uncertainty and error associated with fitted age-depth models (e.g., Trachsel and Telford, 2016; Telford et al., 2004), these models can significantly improve age constraints of stage boundaries or timing of important biological and chemical proxies recorded in the sediments, and as well deliver an uncertainty. In addition, these models allow a more robust compar- ison between different sections in terms of absolute age and allow to detect sudden discontinuities in sedimentation (e.g., hiatus) (Fig. 1.9). Yet, this approach may also be used as an external check for consistency in the stratigraphic record e.g. if a sample violates the stratigraphic record. However, the primary use for these age-depth models has been the modelling of age-depth relationships derived from

14C data, that has to deal with an assembly of effects induced from external parameters (e.g., fluctuations and systematic variability in ¹⁴C production), see e.g., Chiu et al. (2007) and Damon et al. (1978). There- fore, to be capable to use these codes for U-Pb ages, available codes have to be adapted and some com- promises have to be made. This includes e.g., the assumption that the uncertainty of a U-Pb age is nor- mally distributed around the mean, which in most cases is not valid. As well, the low number of available U-Pb dates compared to ¹⁴C data, frequently challenges the detection of sudden changes in sedimentation rate, as expected, if hiatus are present in the studied section, as they will be smoothed out (Fig. 1.9).

Therefore, external a priori known information, such as abrupt lithological changes or unconformities, are essential to build an age-depth model based on U-Pb dates. It is important to point out that none of the currently available codes does satisfy all the requirements that are currently needed to respond to the previously stated scientific challenges. Yet, several codes are available that serve different purposes (CLAM (Blaauw, 2010), OxCal (Ramsey, 2008; Bronk Ramsey, 1995), Bacon (Blaauw and Christeny, 2011), and Bchron (Parnell et al., 2008)). Since each code has is strength and weakness, it is crucial to choose the most suitable model to a specific question. In this thesis, the codes Bchron and Bacon have been used. As stated by Trachsel and Telford (2016), the setting of model-specific parameters used in Bacon influences the estimated uncertainties, whereas age uncertainties produced by Bchron are often too large. The variability of sediment accumulation rates is overestimated in both codes Bacon and Bchron. Contrasting Bchron, Bacon allows to include a priori known information, such as discontinuities and allows to adjust model parameters.

1.6 A

Despite the increased scientific interest in past climate deteriorations and biotic crises, the basic causes, causalities, and impacts on the environment remain controversial. The integration of high-preci- sion and accurate U-Pb zircon geochronology with litho-, bio- and chemostratigraphic records at the

<100 ka timescale can provide valuable evidence on underlying processes and feedbacks. As the chem- ical pre-treatment of zircon prior to ID-TIMS analysis appears to have a major contribution to the accu- racy of the acquired ages, the second chapter will aim to develop improved protocols for chemical pre- treatment of zircon. In the third chapter, the U-Pb ages acquired by the improved protocol will serve as basis for Bayesian age-depth models. The age-depth models will allow the temporal quantification and distinction of triggers and feedbacks that were active during the latest Smithian. On the base of precise

temporal constraints, this approach will aim to deduce and/or exclude processes, leading to the biggest intra Triassic extinction around the Smithian Spathian boundary. The fourth chapter aims to develop a composite Bayesian age-depth model of the complete Early Triassic, that is correlated to the δ¹³Ccarb

isotope record. This approach allows inferring durations of δ¹³Ccarb isotope excursions. Additionally, hafnium isotope systematics of U-Pb dated zircon from the volcanic ash beds are used to trace their origin and deduce the evolution of the magmatic and tectonic setting during the Early Triassic in the Southeast China.