Nitrogen in the Earth System: Planetary Budget and Cycling During Geologic History
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Nitrogen in the Earth System: planetary budget and cycling during geologic history by Benjamin William Johnson B.Sc., University of Puget Sound, 2006 M.Sc., University of Utah, 2009 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the School of Earth and Ocean Sciences © Benjamin William Johnson, 2017 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author. ii Nitrogen in the Earth System: planetary budget and cycling during geologic history by Benjamin William Johnson B.Sc., University of Puget Sound, 2006 M.Sc., University of Utah, 2009 Supervisory Committee Dr. Colin Goldblatt, Supervisor (School of Earth and Ocean Sciences) Dr. Dante Canil, Departmental Member (School of Earth and Ocean Sciences) Dr. Michael Whiticar, Departmental Member (School of Earth and Ocean Sciences) Dr. Rana El-Sabaawi, Outside Member (Department of Biology) iii ABSTRACT The distribution and geologic history of nitrogen on Earth is poorly known. Tra- ditionally thought to be an inert gas, with only a small but important biologic cycle, geochemical investigation highlights that it can also be present in rocks and minerals. Even at low concentrations, the great mass of the solid Earth allows for the possi- bility of substantial N mass and cycling in the geosphere over Earth history. Thus, the assumption that N on the surface of the Earth has remained in steady state over Earth history can be questioned. The research goals of this thesis are to investigate the Earth System N cycle using both large- and small-scale approaches. I present a comprehensive literature compilation to ascertain the N budget of Earth. Determining the total abundance of N in all reservoirs of the Earth, including the atmosphere, oceans, crust, mantle, and core is crucial to a discussion of its cycling in the past. This budget study suggests that the majority of planetary N is likely in the core, with the Bulk Silicate Earth a more massive reservoir than the atmosphere. I also present experimental data and data from lunar samples as added context. As quantification of geologic N is difficult, I present research detailing the adapta- tion of a fluorometric technique common in aquatic geochemistry for use on geologic samples. I compare fluorometry analysis of geochemical standards to several other techniques: colourimetry, elemental analyzer mass spectrometry, and neutron activa- tion analysis. Fluorometry generally behaves well for crystalline samples, and is a relatively quick and easy alternative to more expensive or intensive techniques. As a preliminary application, I have determined a N budget estimate for the continental crust based on analysis of crystalline crustal rocks and glacial tills from North Amer- ica. This budget is consistent with published work, suggesting about 2 × 1018 kg N, or half a present atmospheric mass of N, is in the continental crust. I also present a geochemical study measuring N-isotopes and redox sensitive trace elements from a syn-glacial unit deposited during the the Marinoan Snowball Earth. Snowball Earth events were the most extreme glaciations in Earth history. The measurements presented herein are the first to quantify biologic activity via N-isotopes as well as the redox state of the atmosphere and ocean using trace elements from this intriguing time period in Earth history. The data suggests that there was active N- fixing in the biosphere, persistent but limited O2, nitrification, and nearly quantitative denitrification during the glaciation. After the glacial interval, O2 levels increased and denitrification levels dropped, indicated by near-modern δ15N values. The combined iv use of N-isotope with redox sensitive trace elements provides a more nuanced and comprehensive view in reconstructing past ocean and biologic conditions. Lastly, I present an Earth-system N cycle model with nominal results. Previous modelling efforts have agreed with the traditional notion that atmospheric N-levels have remained constant over geologic time. This is in contrast with modern geochem- ical evidence suggesting net transport of N from the surface into the mantle. The aim, in turn, of this model is to model N cycling over Earth history by explicitly incor- porating both biologic and geologic fluxes. The model is driven by a mantle cooling history and calculated plate tectonic speed, as well as a prescribed atmospheric O2 evolution history. This approach is the first of its kind, to my knowledge, and pro- duces stable model runs over Earth history. While tuning and sensitivity studies may be required for publishable results, nominal runs are compelling. In model output, atmospheric N varies by an factor of 2 − 3 over Earth history, and the availability of nutrients (i.e., PO4) exerts a strong control on biologic activity and movement of N throughout the Earth system. Such a planetary perspective on N serves as an entry point into discussions of planetary evolution as a whole. With the great increase in the number of discovered exoplanets, the scientific community is charged with developing models of planetary evolution and factors that promote habitability. Comparison of Earth to its solar system neighbours and future data on exoplanets will allow a system of evolution pathways to be explored, with the role of N expected to be prominent in discussions of habitability and planetary evolution. v Contents Supervisory Committee ii Abstract iii Table of Contents v List of Tables ix List of Figures xii Acknowledgements xiv Dedication xv 1 Introduction 1 1.1 Background and motivation . 1 1.2 Research goals and dissertation outline . 4 2 The Nitrogen Budget of Earth 6 2.1 Abstract . 6 2.2 Introduction . 7 2.3 Nitrogen speciation in geologic materials, experimental results, and budget tools . 10 2.3.1 Nitrogen speciation in the solid Earth . 11 2.3.2 Experimental results . 12 2.3.3 Database of geologic N measurements . 17 2.4 \Top-down" Budget: Accretion through Core formation . 18 2.4.1 Initial N composition and planetary comparison: missing N? . 18 2.4.2 Core Formation, N sequestration, and remaining BSE N content 26 2.4.3 A Lunar analogue for the Early Mantle? . 27 vi 2.5 \Bottom-up" approach: terrestrial analyses . 29 2.5.1 Atmosphere . 29 2.5.2 Oceans . 30 2.5.3 Biomass . 31 2.5.4 The Crust . 31 2.5.5 The Mantle . 42 2.6 Discussion . 60 2.6.1 Key uncertainties . 60 2.6.2 Evolution of the atmosphere-mantle system . 61 2.6.3 Bulk Earth δ15N and N delivery during accretion . 63 2.7 Conclusions . 64 3 Measurement of geologic N using mass spectrometry, colourime- try, and a newly adapted fluorometry technique 66 3.1 Abstract . 66 3.2 Introduction . 67 3.3 Methods . 69 3.3.1 Rock standards and samples . 69 3.3.2 Rock Sample Preparation . 70 3.3.3 Method 1: Elemental analyzer mass spectrometry . 70 3.3.4 Method 2: Colourimetric . 71 3.3.5 Method 3: Fluorometric . 72 3.4 Results . 74 3.4.1 Method 1: Mass spectrometry . 74 3.4.2 Method 2: Colourimetric . 75 3.4.3 Method 3: Fluorometric method . 75 3.4.4 Rock standards . 75 3.4.5 Continental Crust . 77 3.5 Discussion . 79 3.5.1 Fluorometry . 79 3.5.2 Methods comparison: pros and cons . 80 3.5.3 Suggestions for fluorometry improvement . 85 3.5.4 Preliminary application - continental crust . 89 3.6 Conclusions . 90 3.7 Author Contributions . 93 vii 3.8 Acknowledgements . 93 4 Marine primary productivity and oxygen production during Snow- ball Earth 94 4.1 Abstract . 94 4.2 Snowball Earth biogeochemistry . 95 4.3 Geologic nitrogen isotopes and redox-sensitive trace elements . 96 4.4 Geologic Setting and Sample Description . 98 4.5 Geochemical data supporting periodic oxygenation . 100 4.5.1 Nitrogen isotopes record primary values . 100 4.5.2 Trace element concentrations controlled by redox variations . 104 4.6 Palaeoenvironmental implications and context . 109 4.7 Acknowledgments . 112 4.8 Methods . 113 4.8.1 Rock powder preparation . 113 4.8.2 Nitrogen and carbon . 113 4.8.3 Trace elements . 114 4.9 Supplementary Information . 116 5 Earth system nitrogen cycle model 120 5.1 Motivation and background . 120 5.2 Model setup . 123 5.2.1 Brief element cycle descriptions . 125 5.2.2 40K-decay . 127 5.2.3 Atmosphere . 127 5.2.4 Ocean . 129 5.2.5 Geologic model . 134 5.2.6 Sediments . 134 5.2.7 Crust . 139 5.2.8 Mantle . 141 5.2.9 Differential equations . 142 5.3 Details on code structure . 145 5.4 Nominal runs . 146 5.4.1 Model performance check . 146 5.4.2 High initial atmosphere N2 . 147 viii 5.4.3 Low initial atmosphere N2 . 149 5.4.4 High initial atmosphere N2, low PO4 . 150 5.4.5 Initial conclusions . 151 5.5 Future development . 152 5.5.1 O2 cycle . 152 5.5.2 Low nutrient levels . 152 5.5.3 Nutrient excursions . 153 5.5.4 Isotopic fractionations . 153 5.6 Summary . 153 6 Conclusions 155 A Nitrogen budget of Earth supplemental data 160 B Measurement of geologic N supplemental data 235 C Snowball Earth geochemical supplemental data 253 Bibliography 261 ix List of Tables Table 2.1 Previous estimates for the N budget of the silicate Earth. 10 Table 2.2 Estimated volatile concentrations for C, H2O, Ne, Ar, and Kr in chondrites. 23 Table 2.3 Concentrations of K and Rb in carbonaceous chondrites (CC) and enstatite chondrites (EC), compared to their abundance in the BSE (BE). 24 Table 2.4 Total Earth, core, and BSE N masses based proxies.