Annual Review of Earth and Planetary Sciences - Volume 40, 2012

This is a memoir relating how the author became a geomicrobiologist and how he practiced his specialty. Born in Germany and receiving his early schooling in Berlin, he completed his secondary education, followed by college and graduate school training, after emigration to the United States in 1940. After attaining a PhD degree in 1951, he spent his entire professional career as a faculty member of the Department of Biology at Rensselaer Polytechnic Institute (RPI) in Troy, New York. He was introduced to geomicrobiology in 1959 by a question from a colleague in the Department of Geology at RPI concerning the recent discovery of acidophilic iron-oxidizing, autotrophic bacteria in acid coal mine drainage. This led him to investigate bacterial interaction with metal sulfides, Mn(II) and Mn(IV) on land and in the sea, chromate, and bauxite; to teach a course in geomicrobiology; and to write a textbook on the subject, first published in 1981.

Isotopic abundances of short-lived radioisotopes such as ²⁶Al appear to provide precise chronometers of events in the early Solar System, assuming that they were initially homogeneously distributed. However, both ⁶⁰Fe and ²⁶Al were likely formed in a supernova and then injected into the solar nebula in a highly heterogeneous manner. Conversely, the abundances in primitive meteorites of the three stable oxygen isotopes exhibit mass-independent fractionations that somehow survived homogenization in the solar nebula. Both the presence of refractory particles in Comet 81P/Wild 2 and the anomalously high crystallinity observed in protoplanetary disks may require large-scale outward radial transport from the hotter inner disk regions, even as disk gas accretes onto the central protostar. We examine here theoretical efforts to solve these seemingly disparate cosmochemical puzzles and conclude that the mixing and transport produced by a phase of marginal gravitational instability appears to meet all of these constraints.

The two cosmogenic isotopes ¹²⁹I and ³⁶Cl have half-lives and geochemical characteristics that make their application of interest in the tracing of crustal fluids, oil field brines, and geothermal fluids. The focus of this review is to compare ¹²⁹I data from volcanic fluids with those from mud volcanoes and gas hydrate locations associated with the same subduction zone in order to demonstrate that fundamentally different hydrologic systems are present in active margins. Whereas ¹²⁹I/I ratios in volcanic fluids are site dependent and show a relation to the ages of subducting marine sediments, ratios in fore arc fluids are similar in all sites investigated and are independent of the ages of the host sediments and the age of the subducting slab. Volcanic fluids contain iodine transported in sediments from the trench to the main volcanic zone, whereas iodine in fore arc fluids is derived from organic material stored in the upper plates of subduction zones.

Volunteer computing, also known as public-resource computing, is a form of distributed computing that relies on members of the public donating the processing power, Internet connection, and storage capabilities of their home computers. Projects that utilize this mode of distributed computation can potentially access millions of Internet-attached central processing units (CPUs) that provide PFLOPS (thousands of trillions of floating-point operations per second) of processing power. In addition, these projects can access the talents of the volunteers themselves. Projects span a wide variety of domains including astronomy, biochemistry, climatology, physics, and mathematics. This review provides an introduction to volunteer computing and some of the difficulties involved in its implementation. I describe the dominant infrastructure for volunteer computing in some depth and provide descriptions of a small number of projects as an illustration of the variety of projects that can be undertaken.

The greatest loss of biodiversity in the history of animal life occurred at the end of the Permian Period (∼252 million years ago). This biotic catastrophe coincided with an interval of widespread ocean anoxia and the eruption of one of Earth's largest continental flood basalt provinces, the Siberian Traps. Volatile release from basaltic magma and sedimentary strata during emplacement of the Siberian Traps can account for most end-Permian paleontological and geochemical observations. Climate change and, perhaps, destruction of the ozone layer can explain extinctions on land, whereas changes in ocean oxygen levels, CO2, pH, and temperature can account for extinction selectivity across marine animals. These emerging insights from geology, geochemistry, and paleobiology suggest that the end-Permian extinction may serve as an important ancient analog for twenty-first century oceans.

Theory and observations point to the occurrence of magma ponds or oceans in the early evolution of terrestrial planets and in many early-accreting planetesimals. The apparent ubiquity of melting during giant accretionary impacts suggests that silicate and metallic material may be processed through multiple magma oceans before reaching solidity in a planet. The processes of magma ocean formation and solidification, therefore, strongly influence the earliest compositional differentiation and volatile content of the terrestrial planets, and they form the starting point for cooling to clement, habitable conditions and for the onset of thermally driven mantle convection and plate tectonics. This review focuses on evidence for magma oceans on planetesimals and planets and on research concerning the processes of compositional differentiation in the silicate magma ocean, distribution and degassing of volatiles, and cooling.

Humans are continuing to add vast amounts of carbon dioxide (CO2) to the atmosphere through fossil fuel burning and other activities. A large fraction of the CO2 is taken up by the oceans in a process that lowers ocean pH and carbonate mineral saturation state. This effect has potentially serious consequences for marine life, which are, however, difficult to predict. One approach to address the issue is to study the geologic record, which may provide clues about what the future holds for ocean chemistry and marine organisms. This article reviews basic controls on ocean carbonate chemistry on different timescales and examines past ocean chemistry changes and ocean acidification events during various geologic eras. The results allow evaluation of the current anthropogenic perturbation in the context of Earth's history. It appears that the ocean acidification event that humans are expected to cause is unprecedented in the geologic past, for which sufficiently well-preserved records are available.

The existence of extraterrestrial life was heralded by controversial claims made in 1996 that the Martian meteorite ALH84001 harbored relics of ancient microorganisms. We review here the accumulated evidence for and against past extraterrestrial life in this Martian meteorite. The main pro-life arguments—the presence of polycyclic aromatic hydrocarbons, magnetite crystals, carbonate globules, and structures resembling terrestrial life-forms known as nanobacteria—can be deemed ambiguous at best. Although these criteria are compatible with living processes, each one of them can be explained by nonliving chemical processes. By undergoing amorphous-to-crystalline transformations and binding to multiple substrates, including other ions and simple organic compounds, minerals—especially those containing carbonate—have been shown to display biomimetic properties, producing forms that resemble bacteria. This simple and down-to-earth explanation can account fully for the existence of mineral entities resembling putative nano- and microorganisms that have been described not only in the ALH84001 meteorite but also in the human body.

Subduction drives plate tectonics and builds continental crust, and as such is one of the most important processes for shaping the present-day Earth. Here we review both theory and observations for the viability and style of Archean subduction. High Archean mantle temperature gave low mantle viscosity and affected plate strength and plate buoyancy. This resulted in slower or intermittent subduction, either of which resulted in Earth cooling profiles that fit available data. Some geological observations are interpreted as subduction related, including an “arc” signature in various igneous rocks (suggesting burial of surface material to depths of 50–100 km), structural thrust belts and dipping seismic reflectors, and high-pressure–low-temperature and low-pressure–high-temperature paired metamorphic belts. Combined geodynamical and geochemical evidence suggests that subduction operated in the Archean, although not, as often assumed, as shallow flat subduction. Instead, subduction was more episodic in nature, with more intermittent plate motion than in the Phanerozoic.

Hydrogen-isotopic abundances of lipid biomarkers are emerging as important proxies in the study of ancient environments and ecosystems. A decade ago, pioneering studies made use of new analytical methods and demonstrated that the hydrogen-isotopic composition of individual lipids from aquatic and terrestrial organisms can be related to the composition of their growth (i.e., environmental) water. Subsequently, compound-specific deuterium/hydrogen (D/H) ratios of sedimentary biomarkers have been increasingly used as paleohydrological proxies over a range of geological timescales. Isotopic fractionation observed between hydrogen in environmental water and hydrogen in lipids, however, is sensitive to biochemical, physiological, and environmental influences on the composition of hydrogen available for biosynthesis in cells. Here we review the factors and processes that are known to influence the hydrogen-isotopic compositions of lipids—especially n-alkanes—from photosynthesizing organisms, and we provide a framework for interpreting their D/H ratios from ancient sediments and identify future research opportunities.

This article reviews our current understanding of terrestrial planet formation. The focus is on computer simulations of the dynamical aspects of the accretion process. Throughout the review, we combine the results of these theoretical models with geochemical, cosmochemical, and chronological constraints to outline a comprehensive scenario of the early evolution of our solar system. Given that the giant planets formed first in the protoplanetary disk, we stress the sensitive dependence of the terrestrial planet accretion process on the orbital architecture of the giant planets and on their evolution. This suggests a great diversity among the terrestrial planet populations in extrasolar systems. Issues such as the cause for the different masses and accretion timescales between Mars and Earth and the origin of water (and other volatiles) on our planet are discussed in depth.

Solid, liquid, and gaseous products of life's metabolic processes have a profound effect on the chemistry of Earth and its fluid envelopes. Earth's mantle has been modified by the ubiquitous influence of life on recycled lithosphere, with dramatic changes resulting from subduction of redox-sensitive minerals following the rise of photosynthetic oxygen approximately 2.5 billion years ago. Throughout geological time, production and degradation of organic carbon affected minor-element, trace-element, and isotopic systems in the mantle. Carbon in the mantle decreased as carbonate sediments sequestered CO2, but nitrogen concentrations were augmented by subduction of biologically derived ammonium structurally bound in diagenetic minerals. The biologically modulated mantle vents its biosignatures through island arc and oceanic volcanoes, with fractionated sulfur isotopes providing a durable record. Deeply subducted CO2-rich domains are the source of carbonatite melts, as well as eclogitic diamonds with low ¹³C/¹²C ratios and remnant biologically derived nitrogen. The mantle preserves a concentrated biological record throughout Earth history, thus giving expectation of finding a Hadean record of life.

Molecular data on relationships within angiosperms confirm the view that their increasing morphological diversity through the Cretaceous reflected their evolutionary radiation. Despite the early appearance of aquatics and groups with simple flowers, the record is consistent with inferences from molecular trees that the first angiosperms were woody plants with pinnately veined leaves, multiparted flowers, uniovulate ascidiate carpels, and columellar monosulcate pollen. Molecular data appear to refute the hypothesis based on morphology that angiosperms and Gnetales are closest living relatives. Morphological analyses of living and fossil seed plants that assume molecular relationships identify glossopterids, Bennettitales, and Caytonia as angiosperm relatives; these results are consistent with proposed homologies between the cupule of glossopterids and Caytonia and the angiosperm bitegmic ovule. Jurassic molecular dates for the angiosperms may be reconciled with the fossil record if the first angiosperms were restricted to wet forest understory habitats and did not radiate until the Cretaceous.

The recently reinvigorated field of infrasonics is poised to provide insight into atmospheric structure and the physics of large atmospheric phenomena, just as seismology has shed considerable light on the workings and structure of Earth's solid interior. Although a natural tool to monitor the atmosphere and shallow Earth for nuclear explosions, it is becoming increasingly apparent that infrasound also provides another means to monitor a suite of natural hazards. The frequent observation of geophysical sources—such as the unsteady sea surface, volcanoes, and earthquakes—that radiate energy both up into the atmosphere and down into the liquid or solid Earth and transmission of energy across Earth's boundaries reminds us that Earth is an interconnected system. This review details the rich history of the unheard sound in the atmosphere and the role that infrasonics plays in helping us understand the Earth system.

Conditions in Titan's troposphere are near the triple point of methane, the second most abundant component of its atmosphere. Our understanding of Titan's lower atmosphere has shifted considerably in the past decade. Ground-based observations, Hubble Space Telescope images, and data returned from the Cassini and Huygens spacecraft show that Titan's troposphere hosts a methane-based meteorology in direct analogy to the water-based meteorology of Earth. What once was thought to be a quiescent place, lacking in clouds or localized weather and changing only subtly on long seasonal timescales, is now understood to be a dynamic system with significant weather events regularly occurring against the backdrop of dramatic seasonal changes. Although the observational record of Titan's weather covers only a third of its 30-year seasonal cycle, Titan's atmospheric processes appear to be more closely analogous to those of Earth than to those of any other object in our solar system.

Recent studies suggest the existence of a global atmospheric teleconnection of extratropical cooling to the tropical rainfall climate, mediated through the development of a thermal contrast between the hemispheres—an interhemispheric thermal gradient. This teleconnection has been largely motivated by studies that show a global synchronization of rapid climate change during abrupt climate changes of the last glacial period, in addition to attribution studies of twentieth-century Sahel drought and studies that examined the climate impacts of anthropogenic aerosols. This research has led to interesting developments in atmospheric dynamics of the underlying mechanisms and in applications toward understanding past and present tropical climate change. The emerging teleconnection hypothesis promises to offer new insights into understanding future patterns of tropical rainfall changes due to interhemispheric thermal gradients from greenhouse warming, aerosols, and land-use change.

Water is a key ingredient in the generation of magmas in subduction zones. This review focuses on the role of water in the generation of magmas in the mantle wedge, the factors that allow melting to occur, and the plate tectonic variables controlling the location of arc volcanoes worldwide. Water also influences chemical differentiation that occurs when magmas cool and crystallize in Earth's continental crust. The source of H2O for arc magma generation is hydrous minerals that are carried into Earth by the subducting slab. These minerals dehydrate, releasing their bound H2O into overlying hotter, shallower mantle where melting begins and continues as buoyant hydrous magmas ascend and encounter increasingly hotter surroundings. This process is controlled by plate tectonic variables that ultimately influence the location of the active volcanic arc above subduction zones. Water also modifies the thermodynamic properties of melts, leading to the unique chemical composition of arc volcanic rocks and Earth's continental crust.

Observations of Earth's magnetic field from space began more than 50 years ago. A continuous monitoring of the field using low Earth orbit (LEO) satellites, however, started only in 1999, and three satellites have taken high-precision measurements of the geomagnetic field during the past decade. The unprecedented time-space coverage of their data opened revolutionary new possibilities for monitoring, understanding, and exploring Earth's magnetic field. In the near future, the three-satellite constellation Swarm will ensure continuity of such measurement and provide enhanced possibilities to improve our ability to characterize and understand the many sources that contribute to Earth's magnetic field. In this review, we summarize investigations of Earth's interior and environment that have been possible through the analysis of high-precision magnetic field observations taken by LEO satellites.

Objects in the Kuiper belt are difficult to study in detail, even with the best telescopes available. Therefore, for many years, studies of the compositions of these objects were relegated to collections of moderate-quality spectroscopic and photometric data that remained difficult to interpret. Much early effort was put into simple correlations of surface colors and identifications of spectral features, but connecting these observations to a larger understanding of the region remained elusive. The past decade, however, has seen a blossoming in our understanding, a product of the discoveries of larger—and thus easier to study—objects, continued collection of high-quality photometric and spectroscopic observations, and continued work at the laboratory and theoretical levels. Today, we now know of many processes that affect these objects' surface compositions, including atmospheric loss, differentiation and cryovolcanism, radiation processing, the effects of giant impacts, and the early dynamical excitation of the Kuiper belt. I review the large quantity of data now available and attempt to build a comprehensive framework for understanding the compositions and their causes.