Establishing a robust timeframe for environmental change in the geological past through combined use of high-precision u-pb geochronology of zircon from volcanic ash layers and age-depth modelling

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Thesis

Reference

Establishing a robust timeframe for environmental change in the geological past through combined use of high-precision u-pb geochronology of zircon from volcanic ash layers and age-depth

modelling

FORTES DE LENA, Luis Otavio

Abstract

This thesis comprises a detailed study of practices on how to precisely and accurately establish long-distance correlations of biozones and evaluate causality between Large Igneous Provinces (LIPs) and biotic and environmental crisis. More specifically, this thesis shows the importance of high-precision U-Pb geochronology when coupled with age-depth modelling, chemostratigraphy, and high-resolution biostratigraphy. A clear division of the geological time scale relies on accurate and precise long-distance correlations in order integrate geological information from various stratigraphic sections generally in disparate localities with the goal of piecing together Earth's history.

FORTES DE LENA, Luis Otavio. Establishing a robust timeframe for environmental change in the geological past through combined use of high-precision u-pb

geochronology of zircon from volcanic ash layers and age-depth modelling. Thèse de doctorat : Univ. Genève, 2019, no. Sc. 5415

DOI : 10.13097/archive-ouverte/unige:127563 URN : urn:nbn:ch:unige-1275637

Available at:

http://archive-ouverte.unige.ch/unige:127563

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

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UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES

Département des Sciences de la Terre Prof. U. SCHALTEGGER

Establishing a robust timeframe for environmental change in the geological past through combined use of high-precision U-Pb geochronology of zircon from volcanic ash layers and

age-depth modelling

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

Luis Otavio FORTES DE LENA de

Belo Horizonte

Thèse N^o 5415

GENÈVE le 20 novembre, 2019

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I would like to dedicate this thesis to my loving wife Thanise

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Acknowledgements

Financial support for the development of this thesis was provided by the Coordenação de Aperfeiçoamento de Pessoa de Nível Superior (CAPES - Brazil) under grant agreement no. 1130-13-7 and by the Department of Earth Sciences, University of Geneva.

I would like to express my deepest gratitude to Prof. Urs Schaltegger who gave me the once-in-a-life-time opportunity to work under him and in his laboratory. His dedication to all his students, the lab, and the department is really inspiring. His corrections and comments throughout my PhD were always very thought provoking. Careful and close attention to the development of this research has enabled me to grow as a scientist. This has been and will continue to be the very best academic experience of my life.

I would like to thank the entire Department of Earth Sciences at the University of Geneva, with specially emphasis to Rossana Martini, who provided financial aid for 18 months. Without the financial support from the department this thesis would not have been possible to finish.

Enormous thanks my co-authors Dave Taylor, Jean Guex, Thierry Adatte, Elias Samankassou Annachiara Bartolini, Jorge Spangenberg, Torsten Vennemann, Beatriz Aguirre-Urreta, Victor A. Ramos, Rafael López-Martínez, the late Márcio Pimentel.

A special thanks to Phil & Christy Saint Clair for welcoming us and letting us into their land.

The UNIGE clean lab is warmly thanked: Josh, Maria, Federico, Philipp, Adam and Nick. Sharing the lab and mass spec has been a great pleasure over three years.

Colleagues from MIT: Samuel Bowring, Jahan Ramezani, Michael Eddy, Ann Bauer, Eric Barry, Ben Mandler, Christine, Danielle Gruen, Christine Chen, and Neesha Schepf.

And finally to my family who was instrumental for my career.

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iv Thesis Supervisor:

Prof. Urs SCHALTEGGER Department of Earth Sciences University of Geneva, Switzerland Members of the Thesis Committee:

Prof. Bas VAN DE SCHOOTBRUGGE Department of Earth Sciences

Utrecht University, Netherlands Prof. Thierry ADATTE

Department of Earth Sciences University of Lausanne, Switzerland Dr. Elias SAMANKASSOU

Department of Earth Sciences University of Geneva, Switzerland Prof. Sébastien CASTELLTORT Department of Earth Sciences University of Geneva, Switzerland

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Summary

This thesis comprises a detailed study of practices on how to precisely and accurately establish long-distance correlations of biozones and evaluate causality between Large Igneous Provinces (LIPs) and biotic and environmental crisis. More specifically, this thesis shows the importance of high-precision U-Pb geochronology when coupled with age-depth modelling, chemostratigraphy, and high-resolution biostratigraphy. A clear division of the geological time scale relies on accurate and precise long-distance correlations in order integrate geological information from various stratigraphic sections generally in disparate localities with the goal of piecing together Earth’s history. To this aim, I have chosen three case studies where the temporal correlations have been elusive. In each chapter of the thesis I investigate a case where previous temporal connections breakdown when measured against an accurate and precise timescale and suggest some different scenarios than previously accepted.

CHAPTER 3 – High-precision U-Pb ages in the early Tithonian to early Berriasian and implications for the numerical age of the Jurassic-Cretaceous boundary

The numerical age of the Jurassic/Cretaceous boundary has been controversial and difficult to determine. In this study, I present high-precision U-Pb geochronological data around the Jurassic/Cretaceous boundary in two distinct sections from different sedimentary basins: the Las Loicas, Neuquén Basin, Argentina, and the Mazatepec, Oriental Sierra Madre, Mexico. These two sections contain primary and secondary fossiliferous markers for the boundary as well as interbedded volcanic ash horizons allowing new radio-isotopic dates in the Late Tithonian and Early Berriasian. I also present the first age determinations in the Early Tithonian and tentatively propose a minimum duration for the stage as a cross check for our ages in the early Berriasian. Given our radio- isotopic ages in the Early Tithonian to Early Berriasian, we discuss the implications for the numerical age of the boundary, which significantly differ from the agreed age of the boundary by the ICS. This chapter has been fully accepted for publication at Solid Earth, EGU on 8^th January 2019.

CHAPTER 4 – The driving mechanisms of the environmental crisis in the late Pliensbachian (Early Jurassic)

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The Early Jurassic (late Pliensbachian-early Toarcian) was a period marked by extinctions, climate fluctuations, and ocean anoxia. Although the causes of early Toarcian- Ocean Anoxia Event (OAE) have been fairly well studied, the events that lead to the Toarcian-OAE, i.e. the events in the late Pliensbachian, have not been well constrained.

Scenarios of the driving mechanism of biotic and environmental changes of the late Pliensbachian has ranged from LIP volcanism (the Karoo-Ferrar LIP), ocean stagnation, changing ocean circulation, to orbital forcing. The temporal relationship between the Karoo LIP and the late Pliensbachian (Kunae-Carlottense ammonite zones) are investigated in an effort to evaluate a causal relationship. We present the first absolute timescale on the Kunae and Carlottense Zones based on precise high-precision U-Pb geochronology, and additional geochemical proxies for a range of environmental factors such as bulk organic carbon isotopes, Hg concentration and Hg/TOC ratios, and Re-Os isotopes to further explore their causal relationship. The data presented here show that causality between the Karoo-Ferrar LIP and the late Pliensbachian events cannot be established contrary to what was previously thought. This chapter is under revision in Scientific Reports since 13^th of February 2019 and is currently under the second round of review.

CHAPTER 5 - A precise temporal correlation between the early Wuchiapingian (Lopingian, late Permian) volcanic ash beds and the Emeishan LIP

Over the years, the Emeishan LIP has been broadly related to the extinction in the middle to late Capitanian stage. This connection has been speculated based on a few occurrences of basaltic flows of the Emeishan LIP interbedded with Capitanian limestone of the Maokou Fm. In this Chapter, I show that the Emeishan LIP does not hold a strong temporal relation with the latest Capitanian, but rather holds a strong temporal connection to early Wuchiapingian stage. Therefore, this results presented here show that the common assessment that the Emeishan LIP has been believed to be the main driver of the extinction and climate variability of the late Capitanian as still speculative, and is more likely related to the extinctions and climate fluctuations of the early Wuchianpingian. This chapter will be submitted to the journal Geology.

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Résumé

Cette thèse comprend une étude détaillée, alliant justesse et précision, des techniques de corrélations longue distance entre différentes biozones, et l’évaluation des causes liant les grandes provinces magmatiques (LIPs en anglais) et les crises environnementales et biologiques. Cette thèse montre également l’importance du couplage entre la géochronologie U-Pb de haute précision, la modélisation âge-profondeur (age-depth modelling), la chemostratigraphie et la biostratigraphie haute précision. Une division claire de l’échelle des temps géologiques demande justesse et précision, en plus de pouvoir être corrélée sur de grandes distances afin d’intégrer des informations géologiques provenant de sections stratigraphiques variées autant géologiquement qu’en terme de localisation, afin de les relier dans l’histoire de la Terre. C’est pourquoi j’ai choisi trois études de cas pour lesquelles les corrélations temporelles manquaient. Dans chaque chapitre, j’étudie un cas où la datation haute-précision montre une différence notable avec les datations relatives et/ou absolues déjà existantes, menant à revoir les hypothèses préexistantes.

CHAPITRE 3 – Ages U-Pb haute précision du Thitonien inférieur au Berriasien inférieur et les implications pour l’âge numérique de la limite Jurassique/Crétacé.

L’âge numérique de la limite Jurassique/Crétacé a longtemps été controversé et difficile à déterminer. Dans cette étude, je présente des données d’U-Pb haute précision autour de cette limite dans deux sections distinctes provenant de deux bassins sédimentaires différents : Le Las Loicas, bassin du Neuquèn en Argentine et le Mazatepec dans la Sierra Madre orientale au Mexique. Ces deux sections montrent des marqueurs fossilifères primaires et secondaires pour cette limite ainsi que des couches de cendres inter stratifiées dans les sections permettant de nouvelles datations radio-isotopiques dans le Thitonien inférieur et le Berriasien supérieur. Je présente également la première détermination d’âge pour le Thitonien inférieur et je propose une durée minimumpour cette période comme double vérification des âges pour le Berriasien supérieur. Ces âges radio-isotopiques, nous permettent de proposer un nouvel âge numérique pour la limite, qui change significativement par rapport à l’âge précédemment accepté par l’ICS. Ce chapitre a été accepté pour la publication chez Solid Earth EGU le 8 janvier 2019.

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CHAPITRE 4 –Le mécanisme moteur de la crise environnementale au Pliensbachien supérieur (Jurassique inférieur)

Le jurassique inférieur (Pliensbachien supérieur- Toarcien inférieur) futune période marquée par des extinctions, des fluctuations climatiques et une phase d’anoxie océanique.

Bien que les causes de l’épisode d’anoxie océanique (T-OAE) au Toarcien soient bien connues, la chronologie des événements qui ont conduits au T-OAE, c’est-à-dire les évènements de la fin du Pliensbachien, n’est pas bien contrainte. Les scénarios envisagés vont du volcanisme de grande province magmatique (Karoo-Ferrar LIP), à la stagnation océanique, en passant par le changement de la circulation océanique et le forçage orbital.

L’étude se concentre sur la relation temporelle entre la province magmatique du Karoo et le Pliensbachien supérieur (Zone à ammonites du Kunae-Carlottense) afin de trouver un lien de causalité. Nous présentons la première échelle de temps absolue pour les zones Kunae et Carlottense basée sur de la géochronologie U-Pb de haute précision. En complément, nous avons utilisé des proxies pour une gamme de facteurs environnementaux tels que les isotopes du carbone organique, la concentration en Hg, le rapport Hg/TOC et les isotopes du couple Re-Os. Les données présentées ici, montre qu’il n’est pas possible de trouver un lien de cause à effet entre le magmatisme du Karoo-Ferrar et les évènements du Pliensbachien supérieur, ce qui est en désaccord avec les hypothèses précédentes. Ce chapitre a été soumis à Scientific Reports le 13 février 2019 et est en ce moment dans la deuxième phase de révision.

CHAPITRE 5 – Une corrélation temporelle précise entre les couches de cendres du Wuchianpingien inférieur (Lopingien, Permien supérieur) et la grande province magmatique de l’Emeishan.

L’extinction du milieu-fin du Capitanien a été essentiellement reliée à la province magmatique de l’Emeishan. Cette connexion repose sur la présence de coulées basaltiques de l’Emeishan inter stratifiées dans les calcaires de la formation du Maokou. Dans ce chapitre, je démontre que la province magmatique de l’Emeishan n’a pas de relation temporelle forte avec la fin du Capitanien mais plutôt avec le début du Wuchianpingien.

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Ces résultats montrent donc qu’il est peu probable que le magmatisme de l’Emeishan soit responsable de l’extinction et des changements climatiques au Capitanien mais plutôt qu’il ait impacté les fluctuations climatiques et les extinctions du début du Wuchianpingien. Ce chapitre sera soumis au journal Geology.

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List of Figures

Figure 1.1 – Illustration of a cross section showing the relative volume of zircon sizes

which each mass spectrometry technique. ... 29

Figure 1.2 – Illustration showing the idea behind the U-Pb dating of the sedimentary record by dating volcanic horizons. ... 33

Figure 1.3 – Age correlation between the Phanerozoic Large Igneous Provinces and mass extinction events... 35

Figure 1.4 – Illustration between Large Igneous Provinces, extinction, kill mechanisms, and environmental impacts. ... 36

Figure 2.1 – Cartoon illustrating a few steps of the Chemical Abrasion laboratory procedures. ... 64

Figure 2.2 – Radioactive decay chains used in U-Th-Pb geochronology. ... 66

Figure 2.3 – Illustration of the Concordia diagram. ... 69

Figure 2.4 – Illustration of the intermediate daughter product disequilibrium. ... 74

Figure 2.5 – A plot of percentage of uncertainty contribution from oxide correction, common Pb correction (Pbc), and Pb mass fractionation. ... 76

Figure 2.6 – Illustration of the Nier geometry mass spectrometry configuration. .... 79

Figure 2.7 – Cartoon illustrating the relative abundance of Pb and U isotopes for a spiked ID-TIMS sample. ... 82

Figure 2.8 – Repeatability of measured U-Pb measurements within the Geneva Laboratory. ... 84

Figure 2.9 – Illustration of a typical zircon age dispersion from igneous and volcanic samples. ... 88

Figure 3.1– Geological location of Las Loicas, La Yesera, and Mazapetec sections107 Figure 3.2– Temporal correlation between the J-K sections from Argentina and Mexico. ... 109

Figure 3.3– Microphotographs of nannofossils from the Mazatepec section. ... 112

Figure 3.4– U-Pb weighted mean plot of dated ash beds in J-K sections. ... 113

Figure 3.5– Global biostratigraphic and magnetostratiphic correlation of the J-K boundary ... 121

FS. 3.6 – Stratigraphic distribution of calcareous nannofossils of Mazatepec section, Mexico. ... 135

FS. 3.7 – Field figures from Las Loicas, La Yesera, and Mazatepec. ... 137

Figure 4.1 – Stratigraphic sections and composite section spanning the Suplee, Nicely, and Hyde formations (East Oregon, USA). A ... 148

Figure 4.2 – Geochemical data for the Suplee, Nicely, and Hyde formations. ... 150

Figure 4.3 – Global correlation of the carbon isotopic record of the late Pliensbachian. ... 153

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Figure 4.4 – Temporal correlation between late Pliensbachian to the Early Toarcian ammonite zones, carbon isotope variations, Hg/TOC anomalies, and the duration of

the Karoo-Ferrar LIP. ... 157

FS. 4.5 – Location map of the studied sections. ... 169

FS. 4.6 – Ammonite micrograph ... 172

FS. 4.7 – U-Pb weighted mean plot of the Nicely Fm. ash beds ... 173

FS. 4.8 – Supplementary Rock Eval Data plot I ... 175

FS. 4.9 – Supplementary Rock Eval Data plot II. ... 177

FS. 4.10 – Field figures of the Nicely Fm. ... 180

Figure 5.1 – Regional geology and location of Mapojiao section in Southern China. ... 210

Figure 5.2 – Geochronological U-Pb weighted mean data on the Mapojiao section ash beds. ... 214

Figure 5.3 – The Mapojiao section ... 215

Figure 5.4– Age comparison between the Guadalupian-Lopingian transition and the Emeishan LIP ... 219

Figure 5.5 – Global correlations of the events around to the end-Guadalupian extinction ... 222

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List of Tables

Table 3.1 – U-Pb geochronology data of Las Loicas, Mazatepec, and La Yesera

sections ... 138

Table 3.2 – Age-depth Model for the Las Loicas section... 140

Table 4.1 – U-Pb geochronology of the St Clair, Sterrett, Rosebud, and Garden of Concretion sections, Oregon, USA ... 186

Table 4.2 – Age-depth model of the Nicely-Hyde Fm, Oregon, USA ... 190

Table 4.3 – Geochemical data of the Nicely-Hyde Fm, Oregon, USA ... 197

Table 4.4 – Re-Os data table of the lower Nicely Fm, Oregon, USA ... 203

Table 5.1 – U-Pb geochronology data table of the Mapojiao section ... 233

Table 5.2 – Age-depth model data table of the Mapojiao section ... 235

Table 5.3 – Lu-Hf data table of the Mapojiao section ... 237

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Chapter 1 – Introduction

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1.1 Preface

This thesis is a detailed study about the importance of geochronology to foster our understanding of geological processes stored in the stratigraphic record, in particular by investigating contemporaneity, synchronicity, and inferring causality of geological phenomena using high-precision U-Pb geochronology. Dating the age and duration of oceanic anoxic and euxinic events, prominent carbon and oxygen isotope excursions, periods of reduction of biodiversity, speciation, recovery, emplacement of Large Igneous Provinces (LIPs), permits the quantification in absolute time of the rate at which these Earth processes operate. Understanding the rate of these processes is of fundamental importance because they have shaped the evolution of the oceans, the atmosphere, and life on Earth.

For instance, age-bracketing these processes in various sedimentary sequences around the world allows determining their global extent, understanding the timescale and geographical scale at which these important evolutionary and environmental changes happened, i.e., local vs. global. This is of particular importance when calibrating the age of a GSSP, which is thought as being a time-equivalent reference point in the stratigraphic record. Thus proving their global synchronicity of appearance is a primary condition for establishing a GSSP. Furthermore, another essential reason for investigating the contemporaneity of geological events is because it gives a first-order insight into evaluating the causality between geological phenomena; the relationship between mass extinctions and LIPs is prime example.

One of the aims of this thesis is to test at the highest temporal resolution the temporal coincidence between LIPs and periods where geochemical disturbances and biotic turnover takes place. To illustrate, for a causal relationship to be established between LIPs and biotic/environmental crises requires that the emplacement of the LIPs has to take place at the same time or slightly before than the given biotic/environmental crisis. In practice, this implies that their numerical age needs necessarily to overlap within the analytical uncertainty. However, establishing a temporal connection does not necessarily imply causality. Testing contemporaneity only gives a first-order examination as to the plausibility of causality, but it is, nonetheless, a fundamental condition for causality.

Ecological changes that do not have the same age as LIPs cannot be contemplated as being connected, i.e., the cause and effect break down. This important first-order test of causality

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can only be achieved by dating both the emplacement age of an LIP and the age of the extinction or environmental disturbance through accurate and precise radio-isotopic dating.

However, testing synchronicity, causality, and rates of geological phenomena in Earth history are highly dependent on the accuracy and precision of the geochronological methodology employed. The most widespread geochronometres used to date geological events are the U-Pb and Ar-Ar techniques. However, the majority of geochronological tools available today lack sufficient temporal resolution to investigate processes that take place on a 10⁴-10⁵ year timescale such as extinctions, ocean anoxia, ocean acidification, continental weathering, global warming, global cooling, extinctions, recovery and LIPs emplacement. For instance, U-Pb Secondary Ionisation Mass Spectrometry (SIMS), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Ar-Ar geochronology yield 1-5% precision on final dates, which for the majority of the Phanerozoic results in dates with a ± 1-5 Myr uncertainty. Precision on this order of magnitude is much greater than the duration of the events being studied; therefore, evaluating contemporaneity, synchronicity, and causality of these geological phenomena is often not possible or made elusive. Broadly, the low precision of these methodologies stems from the following: i) instrumental analytical precision, usually related to low beam intensity and counting statistics; ii) correction of measured ratios from inhomogeneous natural reference standard materials; iii) lack of accurate and precise determination of decay constants, iv) the lack of unified error propagation and age calculation practices; and finally v) mechanisms to evaluate accuracy of dates. The only geochronological methodology available today that has a strong hold on all of these parameters is U-Pb Chemical Abrasion Isotope Dilution Thermal Ionisation Mass Spectrometry (CA-ID-TIMS), or high-precision U-Pb geochronology as it is also referred, which routinely yields 0.1%-0.05% precision of the ²⁰⁶Pb/²³⁸U chronometre as well as high accuracy.

In summary, this thesis brings three case studies where the central approach to solving or understanding the issues surrounding each case study is to establish a high- temporal resolution timescale for a geological time at which they take place and test their synchronicity.

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1.2 The importance of the sedimentary record

Sedimentary rocks are an important archive of Earth’s history, recording the significant events that shaped the Earth. For example, the oxygenation of Earth’s atmosphere, known as the Great Oxidation Event (Canfield et al., 2013; Holland, 2006). This important event saw the disappearance of mass independent fractionation of sulphur (MIF-S)(Farquhar et al., 2000; Ohmoto et al., 2006; Strauss, 2003), which is attributed to the change in oxidative state of atmospheric conditions recorded in sedimentary pyrites of Palaeoproterozoic sedimentary sequences of South Africa at ~2.5 Ga. The disappearance of the MIF-S is followed by a prominent positive carbon isotopic excursion known as the Lomagundi excursion at ~2.2 Ga recorded in the carbonate record (Bekker et al., 2008, 2006; Canfield et al., 2013; Ohmoto et al., 2006). The excursion is attributed to the rise of photosynthetic cyanobacteria resulting in a rise in the partial pressure of atmospheric O2. The important Neoproterozoic Snowball Earth glaciations that took place at least in three instances (Hoffman et al., 1998), ca 700-720 Ma (Sturtian), the Marionoan at 635 Ma (Hoffmann et al., 2004), and the Gaskiers at 582 (Bowring et al., 2007). These glaciations are unique in Earth’s history because they represent rapid and extreme climate changes where polar icecaps stretched to equatorial latitudes and are believed to have enabled the evolution of complex life (Hoffman and Schrag, 2000). The Cambrian explosion with the sudden appearance of complex life forms with complex mineralized skeletons at ~521 Ma (Bowring and Erwin, 1998) is one of the most important evolutionary events in Earth’s history. Five important mass extinctions occur in the Phanerozoic, termed the “Big Five”, which are 1) the end-Permian, 2) end-Ordovician, 3) the end-Triassic, to 4) the end- Cretaceous 5) Late Devonian. All of which hold a temporal connection to endogenous processes such as the emplacement of LIPs linked to global enhanced greenhouse effect (Bond and Wignall, 2014; Ernst and Youbi, 2017). In the Mesozoic drastic changes to the ocean chemistry took place known as Oceanic Anoxic Events (Jenkyns, 2010 for a review on the subject) such as the one in the early Toarcian (Farrimond et al., 1988; Jenkyns and Clayton, 1986), early Aptian (AOE 1) (Arthur et al., 1987), and Cenomanian-Turonian (OAE 2) (McArthur et al., 1987) characterised by a global deposition and preservation of organic-rich carbon shales (Arthur and Sageman, 1994). In summary, significant events have taken place throughout Earth’s history, which have profoundly altered the atmosphere, the oceans, and the biosphere. They impacted fundamental cycles operating

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on Earth’s surface such as the carbon, oxygen, nitrogen, and sulphur on a global scale and are all recorded in sedimentary rocks.

In addition to these processes that affected and shaped Earth’s surface environments, the sedimentary record also serves as a proxy for reconstructing crustal evolution.

Sedimentary rocks that are evolving close to plate margins such as subduction zones, rifts, continental collisional have a lot of the material being formed during these processes that end up being deposited in the surrounding sedimentary basins. The deposited detritus material is often used as geochronological and geochemical snapshots of the processes from which they formed. For example, U-Pb detrital zircon age spectra from siliciclastic sequences are routinely used to infer on the nature of the crustal building processes taking place in the nearby cratons (Cawood et al., 2012; Gehrels et al., 2008). This technique is especially more robust when coupled with isotope systems such as Lu-Hf or oxygen isotope composition of zircon serving as geochemical tracers of the crustal and mantle processes (Frei and Gerdes, 2009; Gerdes and Zeh, 2006). The provenance of detrital zircon allows the comparison of known cratonic sources, temporally correlate sedimentary units based on similar provenance patterns and assign a relative age to the sedimentary sequence, i.e., maximum depositional age (Ireland et al., 1998). The approach is also widely used to reconstruct palaeogeographic scenarios. In summary, the wealth of information contained in sedimentary rocks is extraordinary and allows studying a vast array of Earth processes.

Even though the sedimentary record is an important repository of Earth’s history, it is, nevertheless, incomplete. A variety of factors affecting the quality of the geological information recorded in sedimentary rocks: erosion, hiatuses, metamorphism, crustal recycling, etc. This poses certain challenges because what we know about Earth past atmosphere, oceans, and biosphere is only as good as the quality and the extent of the information preserved in the stratigraphic record. Therefore, a combination of tools and proxies is usually required to improve the quality and the reliability of the information we know. Moreover, a full understanding of the mechanisms that shaped the Earth’s surface commonly requires the integration of a wide range of geological information from different geological contexts on a global scale.

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1.3 Reconstructing the stratigraphic record at high temporal resolution

1.3.1 The choice of geochronological methodology

The processes operating on Earth occur on a variety of different timescales and have taken place at different moments in the deep time. For instance, major plate tectonic processes such as mountain building, plate motion, subduction, underplating, sedimentary basin evolution, all occur on long timescales of several tens of millions of years. Other processes take place on a much shorter timescale, i.e., in the order of tens of 10⁴-10⁵ years.

For instance, the Milankovitch cycles operate at 10⁴ to 10⁶ years’ timescales, biogeochemical cycles such as biological pump also operate on a similar timescale, ocean acidification occurs on a 100 kyr (Honisch et al., 2012), whereas globally warm and cool periods lasted for 100’s kyr time scales. Similarly, biological evolutionary rates and extinction also occur in on a multi-millennial scale. To fully understand the exact rate at which these processes take place, it is of paramount importance that the geochronological tool provides sufficient temporal resolution to decipher these processes on a timescale at which they have happened.

The most widely used geochronometres to date Earth processes are U-Pb and Ar- Ar methodologies. The U-Pb systematics has always had an advantage over Ar-Ar systematics because of its ability evaluate the accuracy of its dates based on the concordance between two independent radioactive decay systems, from ²³⁸U and ²³⁵U to stable ²⁰⁶Pb and ²⁰⁷Pb, respectively. The lack of concordance between the two parent- daughter pairs (²⁰⁶Pb /²³⁸U and ²⁰⁷Pb/²³⁵U) denotes open system behaviour (Tilton, 1960;

Wasserburg, 1963). As such, the U-Pb isotope system has a built-in cross-check mechanism to evaluate accuracy of its dates. However, open system behaviour is a known problem within U-Pb systematics and is attributed to post-crystallisation loss of radiogenic Pb from the mineral lattice (Holmes, 1954; Krogh, 1982; Tilton, 1960). It is a known fact that U- Th-rich minerals such as zircon are damaged by α decay of U and Th (Meldrum et al., 1998;

Nasdala et al., 1996). The high energy decay rates result in a build-up of crystal lattice defects, which results in amorphous domains within the crystals that facilitate the release of radiogenic Pb during post-crystallisation processes (Nasdala et al., 1998). Pb does not

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favourably substitute the major chemical constituents in zircon minerals due to its large radius, contrarily to Hf⁴⁺, U⁴⁺, or heavy REE³⁺, which are replacing Zr⁴⁺. Zircon tends to lose Pb²⁺ much easier via diffusion than the 3+ or 4+ cations, because they fit the lattice better (Kober, 1987). Ultimately, the loss of radiogenic Pb will result in biased Pb/U ratios and thus younger U-Pb ages.

Over the last 10 years, the CA-ID-TIMS methodology has seen an unprecedented development towards more reliable and precise results. The most important one was to mitigate the effects of Pb-loss. The most successful of which is the chemical abrasion (CA) technique, which was developed by Mattinson, (2005) and empirically calibrated by Widmann et al., (2019). Broadly, the technique consists of two steps: 1) a high temperature annealing step of the zircon grains in an attempt to restore the parts of the crystal lattice that have undergone weak damage by radioactive decay; 2) a partial dissolution step that involves submitting single grains to high temperature and high pressure partial dissolution under hydrofluoric acid. The latter step aims to preferentially remove the domains affected by radiation damage and thus affected by Pb-loss. As a result, the chemical abrasion technique aims to remove the effects of Pb-loss and yield more accurate dates (Please see section 2.2.2.3 for further detail).

Currently, there are three analytical approaches that are routinely used in U-Pb geochronology: TIMS, SIMS, and LA-ICP-MS. The latter two are often referred to as high- spatial resolution or in-situ geochronology and use a micro-beam that allow minute parts of the analysed material to be dated. However, high-spatial resolution methodologies have limited use in dating processes on the 10⁴-10⁵ year timescale because of their inherently low analytical precision. Precision of U-Pb dates from these methodologies is a function of the stability of the analyte signal, the number of detected ions, the sensitivity of the collectors, and corrections to the measured ratios from reference standards, and correction for common Pb (Ireland and Williams, 2003; Košler and Sylvester, 2003; Schaltegger et al., 2015). The analytical instrumentation of LA-ICP-MS and SIMS displays large ionisation efficiencies (up to 100%) compared to TIMS (10%). On the other hand, the ion transmission is much lower in in-situ techniques.

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Figure 1.1 – Illustration of a cross section showing the relative volume of zircon sizes which each mass spectrometry technique.

Modified after Schaltegger et al. (2014)

In the case of Pb, the ion transmission is in the order of 10% for TIMS (Gerstenberger and Haase, 1997), whereas it is 0.39-1.72% in LA-ICP-MS (Amelin and Davis, 2006; Black et al., 2003; Thirlwall, 2002). Moreover, the thermal ionisation technique delivers a much longer-lasting beam (up to 4h) when compared to LA-ICP-MS (1-3 min) and SIMS (20 min). Consequently, the precision in TIMS is improved by sheer counting statistics.

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Correction of measured isotope ratios against a naturally occurring reference material such as the Temora (Black et al., 2003) or Plešovice zircons (Sláma et al., 2008) is a major source of uncertainty; mainly because naturally occurring materials are not homogeneous at the scale of analysis and even less so on a grain-to-grain scale. The poor geochemical characterization of naturally occurring standards leads to corrections in the measured ratios that comprise accuracy and precision of the final dates. Conversely, the high-precision U-Pb geochronology community has developed a high purity tracer solution with an accurately and precisely calibrated concentration of each Pb and U isotopes needed.

The tracer solution is a gravimetrically weighed solution that contains precise and accurate molar quantities of U and Pb (Condon et al., 2015; Krogh and Davis, 1975; Parrish and Krogh, 1987). The tracer solution is calibrated against metrological standards and traceable to the units of the SI system. As such, ID-TIMS U-Pb ages are considered to be absolute whereas in-situ techniques are considered relative ages. Finally, the use of a double-isotope tracer solution for both Pb and U, such as the EARTHTIME ²⁰²Pb-²⁰⁵Pb-²³³U-²³⁵U spike allows for accurate correction of mass fractionation thus further improving accuracy (Condon et al., 2015; Parrish et al., 2006). Moreover, a jointly calibrated and world-wide distributed tracer solution contributes to minimize the inter-laboratory bias which is now at the level of 0.1%. Control over other chemical parameters are also of importance in high- precision U-Pb geochronology. A precise and accurate re-determination of the natural

238U/²³⁵U (Condon et al., 2010; Hiess et al., 2012) has contributed to the accuracy of the methodology as well.

The correction for common Pb (Pbc) present in the analysis is a major challenge in U-Pb geochronology. Since Pb²⁺ is not readily incorporated into the zircon lattice during crystallisation, the Pbc present is due to contamination (“blank”). High-precision geochronology laboratories have consistently lowered the Pbc contamination, i.e., that amount of laboratory contamination added to the sample, which has been a major factor in improving precision while analysing evermore smaller samples. Currently the amount of laboratory blanks in high-precision U-Pb laboratories is in the order of 0.1 to 0.8pg per sample. Furthermore, the Pb isotopic composition of laboratory contamination is precisely measured in high-precision geochronology unlike with the U-Pb techniques, contributing largely to the superior precision and accuracy of high-precision U/Pb dating using ID- TIMS. As a result, these combined efforts lead to age uncertainties below 0.1% in

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206Pb/²³⁸U that permit the resolution of processes taking place on timescales of hundreds of thousands of years. Contrarily, in-situ techniques such as SIMS or LA-ICP-MS are not capable of quantifying ²⁰⁴Pb present in the sample, therefore not offering a full isotopic analysis of Pbc present in the sample necessary for precise and accurate correction

In addition to issues compromising precision, in-situ techniques lack accuracy because of its inability to address Pb-loss. Theoretically, zircons should be concordant and the source of uncertainty should solely derive from analytical issues (counting statistics), and precision could be improved by repeated analysis of a large zircon population.

However, small proportions of Pb-loss are camouflaged by the low precision (1-5%). As a result, high-spatial resolution techniques are often inaccurate and imprecise.

The standardisation of laboratory procedures between different laboratories (such as, e.g., for the chemical abrasion suggested by Widmann et al., (2019)) is a continuous effort under the EARTHTIME initiative to reduce inter-laboratory bias, and the long-term reproducibility and repeatability of the dates produced. Additionally, the standardization of the statistical algorithms and error propagation by the use of the same open-source software (Tripoli and Redux) for reporting, reducing and treating U-Pb data (Bowring et al., 2011;

McLean et al., 2011; Schmitz and Schoene, 2007) is major advancement.

The mitigation of a systematic off-set in the order of 1% between U-Pb and

40Ar/³⁹Ar ages on the same rock has become a major target of the high-precision geochronology community (Min et al., 2000; Schmitz and Bowring, 2001). The Ar-Ar chronometre relies on radioactive decay of ⁴⁰K to ⁴⁰Ar; however, the inaccurate knowledge of the ⁴⁰K decay constant (Min et al., 2000; Renne et al., 2010), as well as other physical constants such as the ⁴⁰Ar/⁴⁰Ca branching ratio and the natural ⁴⁰K/Ktot ratio contribute to this offset. Additionally, as with in-situ techniques, the Ar-Ar methodology relies on correction of the measured isotope ratios to inhomogeneous natural reference material, which, as with in-situ U-Pb techniques, is a major contributor to the overall uncertainty (Renne et al., 2009; Villeneuve et al., 2000). The widespread use of the Ar-Ar chronometre to date Earth processes is partially due to the widespread presence of K-bearing minerals such as plagioclase, sanidine, biotite, micas, and amphiboles in crustal rocks.

Intercalibration between U-Pb and Ar-Ar ages have been tried through direct comparison

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of U/Pb vs. Ar/Ar age pairs (Renne et al., 2010; Villeneuve et al., 2000) and through intercalibration via astronomical cycles (e.g., Kuiper et al., 2008)

In this thesis, I used exclusively high-precision U-Pb geochronology using CA-ID- TIMS techniques applied to zircon from volcanic layers within the sedimentary record, in order to obtain a precision and accuracy to study processes at the 100 kyr timescales, which only this methodology can deliver.

1.3.2 Absolute numerical calibration the stratigraphic record

The general approach to applying high-precision U-Pb geochronology to the stratigraphic record is to date volcanic horizons interbedded within sedimentary sequences (Bowring, 1998; Bowring et al., 1993; Grotzinger et al., 1995; Isachsen et al., 1994;

Schaltegger et al., 2008). In sedimentary basins close to active margins, where volcanism is widespread and continuous, it is quite common to find volcanic horizons such as ash fall deposits. They serve as important key stratigraphic markers for regional correlation in sedimentary basins, but most importantly they allow dating the stratigraphic record because of the presence of zircon within these volcanic deposits. (Fig. 1.2). They are the result of successive volcanic eruptions by nearby volcanos over a relatively short period of time (a few thousand years or less) (Barboni and Schoene, 2014). The time between volcanic eruption, airborne transport, and deposition is considered very small relative to the analytical precision of 0.1 to 0.05% of the high-precision U-Pb dates (See section 2.5.2.3 for further discussion). To preserve ash deposits requires a fast sedimentation rate and thus their deposition is largely considered instantaneous with respect to the rate of sedimentation of their neighbouring sediments and the evolution of a sedimentary basin. Therefore, the age of eruption can be considered the depositional age of the ash beds. The combination of high-precision U-Pb geochronology with biostratigraphy or chemostratiography as well as sedimentary facies leads to the reconstruction of climatic and environmental processes in deep time. The approach consists of age-bracketing of relevant episodes recorded in the stratigraphic record such as, isotopic excursions, biotic crises, mass extinctions, stage or substage boundaries, oceanic anoxic events, just to name a few. The benefit of age- bracketing is that it imposes certain age constraints (or boundary conditions) to the events

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being studied with the ultimate goal of constraining their durations and age in an attempt to quantify their rate of change.

Figure 1.2 – Illustration showing the idea behind the U-Pb dating of the sedimentary record by dating volcanic horizons.

a) demonstration of the volcanic ash fall deposit bearing zircons; b) reconstruction of magma reservoir prior to eruption; c) location of the ash beds in the sedimentary record showing potential chemostratigraphic proxies for environmental factors; modified after Schaltegger et al. (2014).

For instance, age-bracketing important carbon isotopic excursions allows us to understand when and at which timescales the carbon cycle is disturbed. Similarly, age- bracketing an interval of a mass extinction allows quantification of the rate of extinction;

age-bracketing climate proxies permit the evaluation of cause and effect of climate fluctuations to specific key evolutionary episodes through simple age comparison. In the past decade, there are several examples that highlight the importance of high-precision geochronology to our understanding of the rate at which significant extinctions to place.

For instance, Earth’s most severe mass extinctions have recently been constrained such as the end-Permian extinction in the Meishan locality, South China to be 60±48 kyr (Burgess et al., 2014); the end-Triassic extinction interval in North America to ca 5 kyr (Blackburn et al., 2013), the latest Devonian (Famennian) Hangenberg event to 50-100 kyr (Myrow et

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al., 2014); the end-Cretaceous extinction to ca. 185 kyr (Clyde et al., 2016). These studies show how far we have come to understand the rate of change of important evolutionary changes. This is especially relevant when considering that in the seminal paper Wignall (2001), the author suggested that mass extinction events were short-lived events with durations of ca. 1 Ma. Today we have come to understand that these processes are indeed short-lived, but rather their duration occurred on a timescale that was ten times smaller than what has thought less than 20 years ago.

Not only does high-precision U-Pb geochronology offer the possibility of quantification of the rates of various Earth processes, but it also permits high-resolution temporal correlation of important geological information contained in different stratigraphic sections using age comparison. It is commonly the case to be able to correlate platform with platform-margin and deeper marine sequences, as well as to correlate different geochemical proxies with each other (isotopes of O, C, Sr, or Hg/TOC and other chemical values). Key fossil taxa occur in different ecological environments and need to be temporally correlated to arrive at precise biostratigraphic correlation schemes.

Geochemical signals need to be temporally correlated in different areas across the globe to demonstrate global contemporaneity and to distinguish them from local events.

1.3.3 Evaluating cause and effect between Large Igneous Provinces and environmental and biotic crises

The temporal coincidence between the occurrences of LIPs and biotic crises in the Phanerozoic is a known fact (Bond and Wignall, 2014; Courtillot, 2003; Courtillot and Renne, 2003) (Fig. 1.3) Consequently, LIPs have been widely considered as the main driving force of environmental collapse and key biotic turnovers in the Phanerozoic (Bond and Grasby, 2017; Bond and Wignall, 2014; Courtillot, 2003; Wignall, 2005, 2001). All of the most significant Phanerozoic mass extinctions share similarities such as: (i) high atmospheric CO2 concentrations, leading to fluctuations of the carbon cycle and the global bioproductivity; (ii) periods of global warming triggered by high pCO2 in the atmosphere;

(iii) periods of global cooling due to of CO2 drawdown by, e.g., absorption into biomass, or weathering of large masses of flood basalts from LIPs, or from volcanic SO2 emissions;

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(iv) oceanic acidification through volcanic gases; (v) ocean anoxia leading to black shale deposition (Fig. 1.4), with the latter not being a typical feature of the end-Cretaceous extinction (Bond and Grasby, 2017).

Figure 1.3 – Age correlation between the Phanerozoic Large Igneous Provinces and mass extinction events.

Taken from Courtillot and Renne (2003)

It is widely accepted that LIPs are the main driving force for the majority of mass extinction events because of their impression frequency of association (Fig. 1.3 & Fig. 1.4).

Even though the temporal link between the two is compelling, open questions still remain.

For instance, for much the Phanerozoic it is a known fact that the extinction losses occur at a much shorter timescale (<100 kyr) when compared to the average duration of LIPs eruption and emplacement (~500-800 kyr). Evidence for magmatism before, during, and after extinctions events are widespread. To illustrate, in the case of the end-Triassic extinction, the first occurrences of the CAMP start ca. 100 kyr before the onset of extinction (Blackburn et al., 2013; Davies et al., 2017). In Blackburn et al., (2013) the extinction event and the biotic recovery period are both coincident with CAMP occurrences. A similar scenario is postulated for the end-Cretaceous extinction, which has been related to a second of three main pulses of the Deccan Traps (Schoene et al., 2019). The main extinction event is constrained to ca. 50-100 kyr with the recovery period to have started no later than 71 kyr after the main extinction event (Clyde et al., 2016). Therefore, extinction and recovery

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usually occur within 2-3 orders of magnitudes less time than of a LIP lifespan. In the case of the Siberian Traps, the magmatism precedes the extinction by ca. 300 kyr and continues ca.500 kyr after the extinction interval (Burgess and Bowring, 2015). This begs the question as to why there is a mismatch between the timescales of important evolutionary turnovers and LIPs. To reconcile this scenarios, it has been suggested that CO2 injections are not continuous throughout LIP’s lifespan and that biotic catastrophes are linked to specific short-lived pulses. Admittedly, in the case of the Deccan Traps the magmatic pulse temporally related to the extinction accounts for 80% of the Deccan volcanism (Schoene et al., 2019). In the case of the Siberian Traps, an intermediate dyke phase is believed to be the main trigger (Burgess et al., 2017). Nevertheless, why certain pulses are contemporaneous with mass extinctions and others are not is still a major limitation to invoking LIPs as the main driving force of extinction and environmental change. Another important drawback is that only a small number of LIPs seem to be linked to a specific mass extinction event, with the Columbia Flood Basalts, North Atlantic Igneous Province, having no link to any significant biological or environmental collapse of any sort.

Figure 1.4 – Illustration between Large Igneous Provinces, extinction, kill mechanisms, and environmental impacts.

Taken from Bond and Grasby (2017)

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1.3.3.1 Potential deleterious effects of LIPs

Volcanic CO2 greenhouse: The single most important feature among of all of the major mass extinction events is the increased greenhouse effect from volcanic CO2, which is believed to be the trigger of a series of cascading effects in Earth’s biosphere (Fig. 1.4). By far the most important greenhouse gases that cause global warming are CO2 and CH4

delivered into the atmosphere-ocean system. The continuous and prolonged injection of these gases into the atmosphere is believed to have caused global temperatures to rise above global averages, up to +4 to +10^oC (Beerling and Berner, 2002; McElwain, 1999;

McElwain et al., 2005; Royer et al., 2001; Steinthorsdottir Margret et al., 2011). The increase in atmospheric CO2 levels is coincident with significant δ¹³C excursion towards negative values (e.g., 4‰ to -7‰) in the inorganic and organic carbon isotope reservoirs, which usually overlaps with the extinction intervals. Since the global carbon isotope composition is shifted towards negative values, the injected carbon driving global warming is believed to have negative δ¹³C values, albeit the exact source and its isotopic composition is unknown. An important positive feedback loop to increased atmospheric CO2 is ocean acidification by ocean CO2 uptake forming carbonic acid (H2CO3) (Kump et al., 2000).

Ocean acidification of shallow marine water leads to calcification crisis directly affecting the global carbonate and organic carbon production (van de Schootbrugge et al., 2007). The rate at which CO2 is injected into the atmosphere is what ultimately controls the rate of change in ocean chemistry, notably ocean acidification (Grasby et al., 2013; Honisch et al., 2012; Lau et al., 2016). Smaller protracted injections of CO2 to the atmosphere will result in a small build-up and will generate smaller perturbations. Rain acidification also holds a positive feedback loop with increasing atmospheric CO2 levels as well as SO2 (Black et al., 2014; Bond et al., 2010b). Apart from the direct deleterious effects of acidification of rain to life on earth, the acidification of rain also facilitates the chemical breakdown of minerals during continental weathering. Furthermore, enhanced continental weathering is directly related to increased atmospheric CO2 levels, but as negative feedback loop as enhanced continental weathering constitutes an efficient mechanism of CO2 sink counteracting the increased pCO2 (Kump et al., 2000, 1999). Continental weathering, in turn also triggers enhanced productivity by increased delivery of nutrients from the continents to the oceans by riverine input; notably, N and P (Kump et al., 2000). Global warming also leads to oxygen deficiency in the oceans (Meyer and Kump, 2008). As increasing seawater

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temperature decreases the solubility of oxygen in surface ocean water leading to ocean anoxia (Calvert and Pedersen, 1993; Kerr, 1998). The depletion of oxygen in the deep ocean decreases the efficiency of organic matter oxidation in the deep ocean, which effectively shuts down the biological pump. This creates the condition for the enhanced preservation of organic matter, thus increasing the fraction of organic carbon being buried, which crate impact the carbon cycle (Kump and Arthur, 1999). Additionally, under ocean anoxia phosphate is rarely buried and is put back into the water column creating feeding a positive feedback loop with enhanced productivity maintaining the production of organic matter in the shallow ocean and its export to the deep ocean (Meyer and Kump, 2008). If the system continues for long enough, euxinia might ensue, and the appearance of sulphur- reducing organisms become widespread (Lyons et al., 2009; Meyer and Kump, 2008). The release of methane is potentially even worse than that of CO2 for global warming. Global warming can induce the melting of marine methane hydrates (clathrates) that can significantly amplify the effects of global warming (Beerling et al., 2002; Hesselbo et al., 2000).

Volcanic SO2 icehouse: SO2 is also believed to compose a major part of the volatile budget of volcanic eruptions. Contrary to CO2, SO2 is believed to cause global cooling at least on a 2-3 yr timescale (Self et al., 2006, 2006). The rapid conversion of SO2 to sulphate aerosols such as H2SO4 and H2O (Zhao et al., 1995) is viewed as an important cooling mechanism (Jones et al., 2016; Robock, 2000); however, for this mechanism to sustain global cooling for a prolonged time, it is necessary that SO2 is continuously delivered to the atmosphere for a protracted period because of the short residence time of sulphur in the atmosphere. Such a scenario might increase planetary albedo if SO2 can reach the stratosphere (Robock, 2000); a positive feedback loop might trigger an expansion of the polar icecaps (Macdonald and Wordsworth, 2017).

Halogens: Other deleterious effects to marine life arise from the halogen (Cl, F) and toxic metal (Hg, Zn, Cu, Ni, Pb, As, Cd) poisoning delivered directly by volcanism (Font et al., 2016; Grasby et al., 2016; Sanei et al., 2012; Sial et al., 2013), which also contributing to acid rain formation and ozone depletion (Black et al., 2014). In the terrestrial realm, ozone depletion, wildfires, and acid rain may have been important killing mechanisms (Belcher et al., 2010; He et al., 2012; Pausas and Keeley, 2014) although the majority of mass extinctions are limited to marine realms.

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The long-term temperature regulation on Earth is controlled by CO2 balance between several sources: volcanic input, metamorphic degassing, silicate weathering, primary productivity, and organic burial (Kump, 1989; Kump and Arthur, 1999). It is thought that the intrusive and extrusive members of LIPs are capable of injecting massive volumes of CO2 into the atmosphere, mainly from two sources: 1) volatiles of magmatic origin (Self et al., 2014 and references therein); 2) so-called thermogenic volatiles, where the injection of CO2 into the atmosphere would be the result of the contact metamorphism between LIP intrusions and organic-rich shales within the surrounded country rock (Svensen et al., 2009). The interaction of the LIP dyke complexes with sediments would induce the sediments to degas carbon-based species (CO2, CO, CH4) into the atmosphere via venting pipes. The sill-dyke complexes of many LIPs cross-cut organic-rich strata or source rocks in sedimentary basins. For instance, the CAMP has dykes and sills that intruded into the Amazonas basin, in Brazil (Milani and Zalan, 1999); the Karoo LIP into the Karoo Basin in South Africa (Aarnes et al., 2011); the Siberian Traps intruded into the coal-rich deposits of the Tunguska basin (Retallack and Jahren, 2008; Rothman et al., 2014; Svensen et al., 2009). The thermogenic release model conditions the release of CO2 into the atmosphere to the nature of the surrounding crust onto which the LIP is emplaced, and thus the LIPs is thought to be merely a mediator of the green-house gases and not their actual source.

Implied in this model is the randomness of an LIP to encounter carbon-rich sediments while penetrating the crust, which would explain why some LIPs are connected to biotic and/or environmental crises and others are not. Furthermore, the carbon isotopic composition of the thermogenic released gas would be highly negative, facilitating the shift of global carbon budget to negative values.

1.3.3.2 Tracing LIP activity in the marine record

One of the central limitations to fully comprehending the relationship between LIPs severe climatic changes, extinctions, drastic changes to ocean chemistry, and variations to global temperatures has been the lack of a direct geochemical proxy for volcanism in marine record where these palaeonenvironmental changes are recorded. This has left the connection only made on a first-order temporal bases. Nevertheless, temporal connections do not imply causality. The lack of a geochemical proxy for volcanism that could be tied with δ¹³C changes in the same stratigraphic section, allowing cause and effect of volcanism

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and climate change to be assessed accurately. In the past decade, the use of Hg/TOC as a geochemical proxy for LIP activity in the marine record has shown considerable promise in some biotic and environmental crises (Grasby et al., 2016; Percival et al., 2017, 2015;

Sanei et al., 2012; Thibodeau et al., 2016). Since Hg is put into the environment via volcanic emissions, the unusual enrichment of Hg in the marine record coeval with other shits for environmental factors was considered as a strong proxy of LIP activity. However, the long- term enrichment of Hg in the marine environment is possible by other pathways than solely via oxidation of volcanic Hg⁰ delivered via rainfall. Volcanic Hg is readily absorbed and accumulated into the terrestrial reservoir (biomass and soils) and is a viable source of recycled Hg into the marine environment during environmental crises (Grasby et al., 2017;

Them et al., 2019). Furthermore, Percival et al., (2018) suggest that local sedimentary processes could bias the Hg/TOC signal, such as abrupt changes to the organic matter input into the sedimentary environment could mute the Hg/TOC ratio. It has also been suggested that different marine environments preferentially record different sources of Hg, with proximal marine facies preferentially recording a higher proportion of remobilized terrestrial Hg, whereas distal marine environments preferentially recording volcanic Hg (Grasby et al., 2017). Alternatively, ¹⁸⁷Os/¹⁸⁸Os(i) has been shown to be a much more reliable proxy in recording the effect of LIP volcanism than Hg/TOC in the marine record (Fantasia et al., 2019; Percival et al., 2017; Schoene et al., 2019). Another issue with the Hg/TOC proxy is that anomalies are much shorter than the LIP life span, usually coeval with extinction intervals and climate fluctuation and do not last the entirety of the LIP eruption. Therefore, the relationship between LIPs and environmental changes continues to be elusive, albeit for some cases the connection seems to be fairly well demonstrated.

1.3.3.3 The importance of high-resolution timescales

The degree to which LIPs can impact the Earth’s ecosystems remains to be fully understood. Changes to biodiversity and environmental equilibrium involve a series of complex processes in a series of positive and negative feedback loops that operate at different timescales. For instance, wildfires may operate on some months to years timescale; cooling due to sulphate aerosol degassing, acid rain, and ozone depletion operate on a scale of years to tens of years; ocean acidification, ocean anoxia, global warming due to increased pCO2 operate on a scale of hundreds of thousands of years. Therefore, sorting