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.

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.

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-<em>f</em>(O<sub>2</sub>) 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 <sup>176</sup>Lu-<sup>176</sup>Hf or the short- lived system <sup>53</sup>Mn-<sup>53</sup>Cr in which the parent nuclide has a much lower closure temperature than the corresponding daughter product. Numerical simulations help explain the discrepancy between the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd ages of garnets in metamorphic rocks and enable construction of the entire T-t cycle from the discrepant ages and some additional constraints.

My journey in science began with the study of volcanic gases, sparking an interest in the origin, and ultimate fate, of the volatile elements in the interior of our planet. How did these elements, so crucial to life and our surface environment, come to be sequestered within the deepest regions of the Earth, and what can they tell us about the processes occurring there? My approach has been to establish geochemical links between the noble gases, physical tracers par excellence, with major volatile elements of environmental importance, such as water, carbon and nitrogen, in mantle-derived rocks and gases. From these analyses we have learned that the Earth is relatively depleted in volatile elements when compared to its potential cosmochemical ancestors (e.g., ~2 ppm nitrogen compared to several hundreds of ppm in primitive meteorites) and that natural fluxes of carbon are two orders of magnitude lower than those emitted by current anthropogenic activity. Further insights into the origin of terrestrial volatiles have come from space missions that documented the composition of the proto-solar nebula and the outer solar system. The consensus behind the origin of the atmosphere and the oceans is evolving constantly, although recently a general picture has started to emerge. At the dawn of the solar system, the volatile-forming elements (H, C, N, noble gases) that form the majority of our atmosphere and oceans were trapped in solid dusty phases (mostly in ice beyond the snowline and organics everywhere). These phases condensed from the proto-solar nebula gas, and/or were inherited from the interstellar medium. These accreted together within the next few million years to form the first planetesimals, some of which underwent differentiation very early on. The isotopic signatures of volatiles were also fixed very early and may even have preceded the first episodes of condensation and accretion. Throughout the accretion of the Earth, volatile elements were delivered by material from both the inner (dry, volatile-poor) and outer (volatile-rich) solar system. This delivery was concomitant with the metals and silicates that form the bulk of the planet. The contribution of bodies that formed in the far outer solar system, a region now populated by comets, is likely to have been very limited. In that sense, volatile elements were contributed continuously throughout Earth’s accretion from inner solar system reservoirs, which also provided the silicates and metal building blocks of the inner planets. Following accretion, it likely took a few hundred million years for the Earth’s atmosphere and oceans to stabilise. Luckily, we have been able to access a compositional record of the early atmosphere and oceans through the analysis of palaeo-atmospheric fluids trapped in Archean hydrothermal quartz. From these analyses, it appears that the surface reservoirs of the Earth evolved due to interactions between the early Sun and the top of the atmosphere, as well as the development of an early biosphere that progressively altered its chemistry.

Some scientists and journalists, and many members of the general public, have been led to believe that the world is rapidly running out of the metals on which our modern society is based. Advocates of the peak metal concept have predicted for many decades that increasing consumption will soon lead to exhaustion of mineral resources. Yet, despite ever-increasing production and consumption, supplies of minerals have continued to meet the needs of industry and society, and lifetimes of reserves remain similar to what they were 30-40 years ago.In this volume, we discuss the reasons for this apparent paradox using our broad experience and expertise on both academic and industrial sides of the minerals sector. Many misconceptions arise from flawed estimates of the size of global mineral resources which stem from a lack of understanding of the critical difference between reserves and resources. Some authors use quoted reserves the amount of metal proven to exist and to be economic for mining at present - when predicting imminent shortages. Resources - the amount that may be accessible in the upper few kilometres of the crust - are far larger.Over the last 150 years, improved technologies, economies of scale and increased efficiency have combined to reduce costs hence allowing lower-grade ore to be mined economically. The net result is that the long-term inflation-adjusted price of most metals has decreased more or less in parallel with increasing production, a second apparent paradox that frequently is not well understood.Using copper as the principal example and other metals as appropriate, we summarise the latest research on ore deposits and the activities of the minerals industry. Following a description of the numerous geological processes that form ore deposits, we outline the scientific methods used by the minerals industry to explore for new deposits. We also discuss how resources are mined and how minerals are processed, as well as recent efforts to reduce related environmental impacts. Economic and societal factors influence supply, and these are as important as the actual presence of a resource. Finally, we discuss the critical roles that geoscientists will play in assuring continued supplies of minerals. These include the development of new concepts and techniques that will assist the discovery, mining, processing, remediation, and management of mineral resources. It is essential that researchers help to educate the general public about the need for continued exploration to find new resources to meet growth in world living standards.We demonstrate that global resources of copper, and probably of most other metals, are much larger than most currently available estimates, especially if increasing efficiencies and higher prices allow lower-grade ores to be mined. These observations indicate that supplies of important mineral commodities will remain adequate for the foreseeable future.

This perspective is a very personal account of the history and evolution of experimental trace element partitioning, although I cannot hope to exhaustively cover all aspects of this discipline. Therefore, I emphasise issues with which I am most familiar: (i) partitioning between mafic silicates and silicate melt; (ii) solid metal-liquid metal partitioning – especially the effects of non-metals; and (iii) metal-silicate liquid partitioning. I first entered the field of experimental partitioning as a grad-student in the mid-1970’s and so was able to see some of the growing pains of this discipline up close and personal. Also where appropriate, I will mention applications of experimental partitioning data to geologic and planetary problems.