Experiencing an international childhood, and an innate curiosity of learning the origin of himself and his surroundings, Steve Mojzsis set out to try his best to make sense of the universe he was born into. This quest will never be fulfilled, but it doesn’t prevent him from trying. The ultimate prize is to figure out how the biosphere came into being. The path towards learning began with adventures mostly in North America and Central and Western Europe. To find himself still further meant going, to Africa, Asia and Australia, with expert guidance from the best mentors along the way. Ever the bridge builder, he was not satisfied with staying within one narrow discipline of science for very long before trying a hand at something else, albeit related. In this way he stumbled into biogeodynamics and geoastronomy; we will probably not see spectacular developments in either of these nascent fields until the end of 21st century or long after he is returned to some mineral salts, CO2 and H2O. Back on his home world, it is evident that in its first 500 Myr (q.v. Hadean eon) Earth’s crust co-existed with liquid water; it was affected by late accretion bombardments and harboured chemically diverse hydrothermal systems. Globally active volcanism powered by a hotter mantle, impact generated topography and inherent differential buoyancy of the crust means that scattered emergent land masses existed irrespective of different estimates for plausible Hadean ocean volumes. Consequently, land surfaces in the first few hundred million years – with diverse subaerial and (sub-)aqueous environments – could bring prebiotic chemical ingredients to reactive concentrations. This active chemistry led to the first life. Yet, the inferred complexity for even the minimum biological entity probably means that both operative and persistent biology in a planetary environment is the most difficult developmental stage to reach. Soon after Earth’s formation, its surface had the capacity to host fully fledged (i.e. living) biochemistry. While life emerged at the dynamic interface of the planet’s primordial geosphere, hydrosphere and atmosphere, nobody knows whether any Hadean environment was ideally suited, or merely good enough, to facilitate its emergence. Given that we do not have time machines to go back and see what happened on Earth, or anywhere else for that matter, we are compelled to tell stories that fit the data. That said, comfortable fictions about what “might” have happened, just because somebody said something (without evidence), ought to be taken with a grain of salt. If we ever are to come up with a plausible pathway to a life giving world, it will be more metaphorical than literal. All of this will likely change if we find an independent origin of life, either on purpose or, more likely in Mojzsis’ opinion, by accidental discovery of something strange in the data.
The long term evolution of Earth’s atmosphere and climate has been an active topic of investigation for at least the last 60 years. My own participation in this investigation goes back more than 45 years, and this monograph relates that story from my personal perspective. One major thread concerns the rise of atmospheric O2 from near-zero levels initially to the 21 percent mixing ratio that we observe today. Photochemical models developed by me and my students, along with some close colleagues, have helped to better constrain the prebiotic O2 concentration and to interpret the constraints imposed by the record of mass independent fractionation of sulfur isotopes. Most geochemists now agree that a so called Great Oxidation Event (GOE) occurred between 2.4 and 2.2 Ga and that the atmosphere has been O2-rich since that time. However, the exact level of O2 during the ensuing Proterozoic Era remains controversial, as do the timing and magnitude of subsequent O2 increases. The corresponding development of the ozone layer is also of interest because of its moderating influence on surface solar UV fluxes and their effect on biological evolution. This can also be studied with photochemical models. A second thread concerns the gradual decline in atmospheric CO2 in response to slowly increasing solar luminosity. The early Earth would have been frozen had the atmosphere not contained high concentrations of greenhouse gases, most importantly CO2. A negative feedback in the carbonate-silicate cycle that controls CO2 over long time scales has ensured that Earth’s surface has remained habitable during most of Earth’s history, despite occasional forays into Snowball Earth conditions. Evidence from palaeosols provides support for this hypothesis. CH4 is an additional greenhouse gas that may have supplemented surface warming prior to the GOE. The increase in O2 at that time may have caused CH4 to decrease, possibly triggering the Huronian glaciations. The same feedback mechanism that controls long term CO2 evolution on Earth could operate on Earth-like planets orbiting other stars, increasing the probability that some of them may harbour life. Large direct imaging space telescopes currently under development may eventually allow us to test this hypothesis and to learn whether we have company in this part of our galaxy.
Thwarted in pursuit of a career in aviation, academic underachiever Mark Harrison then kicked around the world for a couple of years.Doing so he stumbled into a series of geological technician jobs in the southern hemisphere that motivated a return to school in Canada to prepare for a research career.A series of inspirational mentors at Australian, Canadian, and American universities imbued him with an outsider's perspective that encouraged intellectual grazing across several fields, including geochronology, tectonics, and early Earth evolution. The connective tissue between these disciplines was the development of thermochronology - the release of temperature history information stored in minerals to infer geophysical mechanisms acting in the distant past.Since most geodynamic processes involve heat flow discontinuities, this new science could image ancient events that might otherwise go undetected. A second unifying theme, reflective of both Harrison's contrarian nature and his visionary mentors, was a pronounced mistrust of received wisdom. His early kinetic calibrations established Ar-40/Ar-39 thermochronology as a key tool in documenting epeirogenic histories. Of particular note, development of the multi-diffusion domain model required a natural test bed with the highest possible dynamic range of geologic rates, taking him first to Tibet and then the Himalaya. Together with UCLA colleague An Yin, they developed a widely emulated, hybridised application of geochemistry and tectonics to address longstanding questions in the evolution of that mountain system.On hand as a graduate student in Australia to witness development of the high sensitivity ion microprobe, Harrison and colleagues commissioned the first high sensitivity ion microscope, which could determine both in situ ages and stable isotope compositions. This unique capability permitted them to find microscale evidence that life likely emerged prior to 3.8 billion years (Ga) ago - 300 million years (Ma) earlier than previously thought. Harrison's research groups, both at UCLA and the Australian National University, set out to create an unprecedented archive of >4 Ga zircons from which they documented evidence most simply interpreted as reflecting Hadean eon oceans, continental crust, plate boundary interactions, and the emergence of terrestrial life as early as 4.1 Ga. These interpretations challenged the longstanding paradigm that Earth had not evolved stable continental crust or life over the first 500 to 1000 million years of its history. That dialectic led Harrison to ponder the intellectual and philosophical underpinnings of historical geology, which he sees as underdeveloped. Ultimately, the field needs to adopt a multiple working hypothesis credo along with the scientific humility to acknowledge the limitations imposed by the fragmental and biased rock record. Doing so shouldn't dampen our enthusiasm for understanding Earth history but instead only make its pursuit more intriguing, challenging, and communally rewarding.
The following personalised narrative aims to document the highlights of my involvement in some of the ground breaking developments in Economic Geology and their direct application to mineral exploration and discovery over the past half century. The story begins with my introduction to geology at secondary school and university, followed by doctoral research based on fieldwork in the Andes of South America. Then, as an employee of the Chilean Geological Survey, I got my introduction to porphyry copper deposits before returning to the UK to take up a post-doctoral research fellowship. This formative period concluded with my starting out as an independent geological consultant to the global exploration and mining industry. These early years happened to coincide with the plate tectonics revolution and its radical implications for metallogeny. I realised that porphyry copper and related deposits are integral parts of volcano-plutonic arcs generated during subduction of oceanic lithosphere, and volcanogenic massive sulphide deposits in ophiolite complexes must have formed at oceanic spreading centres. At approximately the same time, application of K-Ar dating to copper deposits led to definition of metallogenic belts and corresponding epochs in the Andes, and then established the timing of their economically important supergene oxidation and enrichment. Subsequently, using more modern and precise U-Pb zircon and Re-Os molybdenite methods, collaborative attempts were made to determine porphyry copper deposit lifespans and ages of various copper belts, deposits and prospects around the world, including the Zambian Copperbelt. The focus on porphyry copper deposits led first to an appreciation of the linkage between them and subaerial volcanism and the importance of potassic alteration as a major host of hypogene copper mineralisation, and then to geological characterisation of the increasing number of gold-rich examples. Appreciation of the importance of hydrothermal breccias in porphyry copper deposits, including recognition of mineralised diatremes, resulted in a classification scheme for breccias that may be extended to related deposit types. Extensive fieldwork showed that zones of advanced argillic alteration, termed lithocaps, constitute the shallow parts of porphyry copper systems. The role of tectonic uplift in both porphyry copper formation and subsequent supergene modification was also charted. The end result of this body of work was a porphyry copper model that can be used as a basic exploration guide. In response to a marked increase in the world gold price in the late 1970s, more effort was devoted to gold concentrations in magmatic arc terranes, commencing with epithermal gold deposits in the shallow lithocaps of porphyry copper systems. This led to an input to classification schemes for epithermal precious metal deposits and, eventually, to assignment of the three main epithermal types to specific tectono-magmatic settings. After years of speculation, porphyry gold deposits were recognised for the first time in northern Chile, followed by definition of a new gold deposit class in association with relatively reduced granitic intrusions. A magmatic-hydrothermal origin for Carlin-type gold deposits was steadfastly supported over many years, notwithstanding its unpopularity until relatively recently, culminating in a proposal for modern analogues. For decades, metallogenic provinces and corresponding epochs have been widely appreciated, but debate concerning their origin(s) persists. The nature of accompanying magmatism could well provide an adequate explanation for at least some provinces (e.g., tin, molybdenum and possibly silver), but precursor metal enrichment in the lowermost crust and/or subcontinental lithospheric mantle may well be required in the case of gold and copper provinces. The story concludes with brief commentary on mineral exploration, which has been my lifelong (pre)occupation. Requirements for success in mineral exploration are discussed, based primarily on familiarity with the circum-Pacific region, followed by analysis of the types of companies and individuals involved and the burgeoning challenges to the exploration process, which, if not remedied, will have dire consequences for future metal production and global plans for the energy transition. My involvement as a lone practitioner in both mineral exploration and metallogenic research probably says something about my independent character traits, but nonetheless has been largely unstructured and certainly unplanned. I have no hesitation in recommending a similar career path for any recent graduate who relishes adventure and is prepared to endure significant work-life imbalance.
Anthropogenic carbon emissions have overwhelmed the natural carbon cycle, leading to a dramatic increase in atmospheric CO2 concentration. The rate of this increase may be unprecedented in Earth’s history and is leading to a substantial increase in global temperatures, ocean acidification, sea level rise and potentially human health challenges. In this Geochemical Perspectives we review the natural carbon cycle and its link to global climate. Notably, as directly observed by field observations summarised in this volume, there is a natural negative feedback loop between increasing global temperature, continental weathering rates, and CO2 that has tended to limit Earth climate changes over geological time scales. Due to the rapid increase in atmospheric carbon concentrations, global average temperatures have increased by more than 1.2 °C since the start of the industrial revolution. One way to slow or even arrest this increasing global average temperature is through Carbon Capture and Storage (CCS). Carbon dioxide can be captured either from large industrial point sources or directly from the atmosphere. Taking account of the natural carbon cycle, the most secure approach to storing captured CO2 is by reacting it with mafic or ultramafic rocks to form stable carbonate minerals, a process referred to as “mineral carbonation”. Although mineral carbonation can occur and be accelerated at the Earth’s surface, due to the required scale and required time frames it is most effective in the subsurface. This subsurface mineralisation approach was developed into an industrial scale process through an academic-industrial collaboration called CarbFix. The history of CarbFix, from its beginnings as a concept through its installation as an industrial process is presented in detail. This Geochemical Perspectives concludes with an assessment of the future of subsurface mineralisation as a means to help address the global warming challenge, as well as a detailed list of potential research directions that need to be addressed to further upscale and optimise this carbon storage approach.
I started my journey in science by studying noble gases implanted by the solar wind in dust grains on the surface of the Moon, and with many colleagues I have studied solar wind implanted noble gases in natural and artificial samples throughout my career, the latter exposed primarily by the Genesis space mission. Major questions are what noble gases in the solar wind can tell us about the present and the past Sun, and how they can contribute to understanding the formation and history of the planets and their building blocks, represented, for example, by meteorites. Since my early years as a postdoc, I have also been interested in noble gases (and radioactive nuclides) produced in meteorites and other extraterrestrial samples by interactions with energetic elementary particles from galactic cosmic radiation (and the Sun). These so called “cosmogenic” nuclides allow us to study the transport of meteorites to Earth, and the dynamics of the top surface layers (“regoliths”) on the Moon, asteroids, and comets. Cosmogenic noble gases are also crucial for studying even more exotic topics such as the history of tiny presolar grains that formed in the cooling envelopes of earlier generations of stars towards the end of their lives and were eventually incorporated into the meteoritic matter where they are found today. Cosmogenic noble gases in some tiny phases in meteorites are also likely tracers of our highly active Sun at a very early stage in its history. A few years later, I started my third major research topic in cosmochemistry, the study of primordial noble gases in meteorites and other extraterrestrial samples. These noble gases were incorporated into meteorites or their precursors in the early solar system or even in a presolar environment. I also participated in studies by colleagues of isotopic anomalies of other elements important in cosmochemistry, my expertise being mainly in aspects of the influence of cosmic rays on these elements. Although working in an Earth Science institution, it took quite a while before I started to also study noble gases (and radionuclides) in terrestrial samples. This is described in the second part of this contribution. A major focus was on cosmogenic noble gases and radionuclides produced in samples near the Earth’s surface. Although production rates of cosmogenic nuclides on Earth are several orders of magnitude lower than in space, making their analysis more challenging, they have become an important tool in geomorphology. Because stable noble gas nuclides are particularly well suited to the study of ancient landscapes, much of our work focused on areas with arid climates, such as Antarctica and the Andes in Chile, in collaboration with geoscience colleagues. We also participated in the large multinational CRONUS collaboration, funded by the European Union, a community effort to improve our knowledge of nuclide production rates at the Earth’s surface. In another major collaboration with external colleagues we are involved in noble gas analyses of water samples, ranging from lakes to aquifers to tiny inclusions in stalagmites. This research focuses on studying lake and groundwater dynamics, including contributions of mantle-derived noble gases such as in volcanic lakes. Atmospheric noble gases dissolved in suitable samples are also palaeotemperature indicators, supplementing information from other proxies such as oxygen isotopes.
Lourens Baas Becking (1895-1963) was a Dutch plant physiologist, trained in the Botanical Laboratory of Utrecht University. After graduating in 1919, he worked in America at Stanford University, where he obtained his Doctor’s degree in 1921. From 1928, he was Herzstein Professor of Biology and Director of the Jacques Loeb Physiological Laboratory at the Hopkins Marine Station in Palo Alto. In 1931, he became Professor of General Botany at the University of Leiden. There, he and his staff and students continued to work on the research of microorganisms under extreme saline conditions. In 1939, he was appointed Director of the institutes of the Botanic Garden at Buitenzorg (Bogor) in the Dutch East Indies (Indonesia). In May 1940, when the war broke out, he was in Leiden to retire from his professorship. The war prevented his return to his family and the institutes in the East Indies. Baas Becking made several failed attempts to escape to England. These resulted in imprisonments by the German occupying authorities in Scheveningen (1940-1941) and in Utrecht and the German Zuchthaus in Siegburg (1944-1945). An ordeal that he barely survived due to the inhuman situation in the penitentiary and typhus. In July and August 1944, as a prisoner of the German Kriegsmarine in Utrecht, he wrote in seven weeks a manuscript of Geobiology, an essay on the relationship between living organisms and the earth. It was an update of his earlier ideas. Baas Becking had been inspired by Lawrence Henderson’s The Fitness of the Environment (1913), Victor Moritz Goldschmidt's Der Stoffwechsel der Erde (1922) and Grundlagen der quantitativen Geochemie (1933), Alfred J. Lotka’s Elements of Physical Biology (1924) and Vladimir Vernadsky’s La Géochimie (1924). They were with Frank W. Clarke’s The Data of Geochemistry (1916), sources for his perception of The Universality of Life in 1927, which integrated Vernadsky’s concepts of biosphere and geosphere. Long before James Lovelock and Lynn Margulis defined the Gaia hypothesis in the early 1970s, Baas Becking discussed Gaia or Life and Earth in his inaugural address in 1931. In this tract he also succinctly summarised the ubiquity hypothesis, borrowed from the work of Martinus Beijerinck, as “Everything is everywhere, but the Milieu selects.” The biological “law” was further elaborated in Geobiologie of inleiding tot de milieukunde (1934, English version 2016, Baas Becking’s Geobiology). In the Utrecht prison Baas Becking wrote his scientific testament. In the ten years since the publication of Geobiologie, he “wished to do justice to the work that was performed in Leiden by so many workers”, in an English textbook. With a limited access to scientific literature, he wrote the manuscript Geobiology in a ledger in a barely legible handwriting. The document reflected his vast biological knowledge and his idea of mutual dependence of vital-units (cells, tissues, organs, organism, communities), either of a parasitic, mutualistic or commensalistic character. This relationship was elaborated in his model of symbiosis. His description of the role of man in Geobiology is a personal complaint of a geobiologist over the disastrous treatment of the earth by man. With his concept of “dissipation”, he introduced a material analogue for “the entropy lowering capacity of living systems”. It summarised his conviction that the human intellect and life condition were attributes of free will. Although Geobiology (1944) remained unfinished and had major gaps, it still is an inspiring memoir of a scientist who records his enlightened vision on the relationship between life and earth. In this issue of Geochemical Perspectives the manuscript of Geobiology is integrally transcribed, annotated, edited and introduced by Dr. Alexander J.P. Raat, who graduated in 1974 in Leiden as a plant physiologist. The transcript is published with the original illustrations. A sketch of Baas Becking’s life and works is part of the introduction. The annotation and introduction refer to many of his published and unpublished studies. Among these is an unpublished, further updated and revised version of Geobiology, which he completed in 1953 in Australia.
Complex interactions between microbial communities and geochemical processes drive the major element cycles and control the function of marine sediments as a dynamic reservoir of organic matter. Sulfate reduction is globally the dominant pathway of anaerobic mineralisation and is the main source of sulfide. The effective re-oxidation of this sulfide at the direct or indirect expense of oxygen is a prerequisite for aerobic life on our planet. Although largely hidden beneath the oxic sediment surface, the sulfur cycle is therefore critical for Earth’s redox state. This Geochemical Perspectives begins with a brief primer on the sulfur cycle of marine sediments and a description of my own scientific journey through nearly fifty years of studies of sulfur geochemistry and microbiology. Among the main objectives of these studies were to quantify the main processes of the sulfur cycle and to identify the microbial communities behind them. Radiotracers in combination with chemical analyses have thereby been used extensively for laboratory experiments, supported by diverse molecular microbiological methods. The following sections discuss the main processes of sulfate reduction, sulfide oxidation and disproportionation of the inorganic sulfur intermediates, especially of elemental sulfur and thiosulfate. The experimental approaches used enable the analysis of how environmental factors such as substrate concentration or temperature affect process rates and how concurrent processes of sulfate reduction and sulfide oxidation drive a cryptic sulfur cycle. The chemical energy of sulfide is used by chemolithotrophic bacteria, including fascinating communities of big sulfur bacteria and cable bacteria, and supports their dark CO2 fixation, which produces new microbial biomass. During the burial and aging of marine sediments, the predominant mineralisation processes change through a cascade of redox reactions, and the rate of organic matter degradation drops continuously over many orders of magnitude. The main pathways of anaerobic mineralisation and the age control of the organic matter turnover are discussed. In the deep methanic zone, only a few percent of the entire degradation process remains, which provides a small boost of substrate for sulfate reduction through the process of anaerobic methane oxidation. The stable isotopes of sulfur provide an additional tool to understand these diagenetic processes, whereby the combination of microbial isotope fractionation and open system diagenesis generate a differential diffusion flux of the isotopes. In relation to the organic carbon cycle of the seabed and the contribution of methane, the paper discusses the global sulfur budget and the role of sulfate reduction for organic matter mineralisation in different depth regions of the ocean - from coast to deep sea. The published estimates of these parameters are evaluated and compared. Finally, the paper looks at future perspectives with respect to gaps in our current understanding and the need for further studies.
This article has three major components that include, in addition to the technical aspects, reminiscences of my academic upbringing, my move to the USA from India, and my professional career. I have recounted many stories that I hope convey some sense of time, especially in these two countries with vastly different cultures, my personal journey with its ups and downs and how I made the transition to an academic career path in USA even though that was not in my future plan as a young man. The development of the field of thermobarometry and its integration with diffusion and crystal kinetic modelling of compositional zoning (or lack thereof) and cation ordering in minerals have led to important quantitative constraints on the pressure-temperature-time evolution of terrestrial rocks and meteorites. I review the historical developments in these areas and a segment of my own research spanning the period of 1964-2021. The foundational works of the thermometry of metamorphic rocks and palaeothermometry were laid at the University of Chicago around 1950. Subsequently, the synergetic growth of thermodynamics and experimental studies in petrology in the 1960s and 1970s, along with the introduction of electron microprobe as a nondestructive analytical tool with micron scale resolution, gave a major boost to the field of thermobarometry. There were also significant new developments in the field of thermodynamics of solid solutions in the petrology community and demonstration from observational data, countering strong scepticism, that the principles of classical thermodynamics were applicable to “complex natural systems”. The section on thermodynamic basis of thermobarometry concludes with a discussion of the thermodynamics of trace element and single mineral thermometry. I further deal with the experimental protocols, along with selected examples, for phase equilibrium studies that provide the bedrock foundation for the field of thermobarometry based on elemental compositions of coexisting minerals in a rock. It is followed by an account of the controversies and international meetings relating to the aluminum silicate and peridotite phase diagrams that play crucial roles in the thermobarometry of metamorphic rocks and mantle xenoliths, respectively. The construction of quantitative petrogenetic grids to display stability relations of minerals in multicomponent–multiphase systems came into play in the field of metamorphic petrology in the mid-1960s and early 1970s. Augmented by experimental data, these petrogenetic grids led to important discoveries about the P-T-f(O2) and bulk compositional controls on the stability of certain “index” minerals that are used to define metamorphic isograds and different types of regional metamorphism; one such grid also opened up a new field that came to be known as ultra-high temperature metamorphism. The construction of petrogenetic grids has now evolved to computer based calculations of complex equilibrium P-T phase diagrams, commonly referred to as “pseudosections”, by minimisation of Gibbs free energy of a system with fixed bulk composition. I discuss these historical developments and modern advancements. Subsequently I highlight some aspects of thermobarometry and diffusion kinetic modelling of selected natural samples along with their broader implications and present a critical discussion of different protocols for thermobarometry of natural assemblages. Following up on the introductory historical perspective of development of palaeothermometry, I discuss the modern advancements using density functional theory (DFT). Examples of DFT based calculations have been shown for hydrogen isotope fractionation in mineral-water/hydrogen systems and “clumped isotope” thermometry. The hydrogen isotope fractionation data led the development of new low temperature palaeothermometers using serpentine-talc/brucite mineral pairs. These results enable simultaneous solutions of both temperature and source of fluid in the serpentinisation process of rocks. The final section is devoted to high temperature thermochronology dealing with the problems of closure temperature of decay systems in minerals and the use of bulk and spatial resetting of mineral age according to a specific decay system to determine cooling rates of the host rocks. Complications arise in the interpretation of mineral ages determined by such decay systems as 176Lu-176Hf or the short- lived system 53Mn-53Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the 176Lu-176Hf and 147Sm-143Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.
