Annual Review of Earth and Planetary Sciences

The thermal conductivity of iron alloys at high pressures and temperatures is a critical parameter in governing ( a) the present-day heat flow out of Earth's core, ( b) the inferred age of Earth's inner core, and ( c) the thermal evolution of Earth's core and lowermost mantle. It is, however, one of the least well-constrained important geophysical parameters, with current estimates for end-member iron under core-mantle boundary conditions varying by about a factor of 6. Here, the current state of calculations, measurements, and inferences that constrain thermal conductivity at core conditions are reviewed. The applicability of the Wiedemann-Franz law, commonly used to convert electrical resistivity data to thermal conductivity data, is probed: Here, whether the constant of proportionality, the Lorenz number, is constant at extreme conditions is of vital importance. Electron-electron inelastic scattering and increases in Fermi-liquid-like behavior may cause uncertainties in thermal conductivities derived from both first-principles-associated calculations and electrical conductivity measurements. Additional uncertainties include the role of alloying constituents and local magnetic moments of iron in modulating the thermal conductivity. Thus, uncertainties in thermal conductivity remain pervasive, and hence a broad range of core heat flows and inner core ages appear to remain plausible.

The discovery of the first chemically produced mass-independent isotope effect in 1983 by Thiemens & Heidenreich opened a broad variety of applications, including physical chemistry studies, atmospheric chemistry, paleoclimatology, biologic primary productivity assessment, Solar System origin and evolution, planetary atmospheres (Mars), and the origin and evolution of life in Earth's earliest environment. This chapter reviews the history of the field as well as all of the various applications since the first report of the mass-independent isotope effect.

▪ Abstract  Earthquake triggering is the process by which stress changes associated with an earthquake can induce or retard seismic activity in the surrounding region or trigger other earthquakes at great distances. Calculations of static Coulomb stress changes associated with earthquake slip have proven to be a powerful tool in explaining many seismic observations, including aftershock distributions, earthquake sequences, and the quiescence of broad, normally active regions following large earthquakes. Delayed earthquake triggering, which can range from seconds to decades, can be explained by a variety of time-dependent stress transfer mechanisms, such as viscous relaxation, poroelastic rebound, or afterslip, or by reductions in fault friction, such as predicted by rate and state constitutive relations. Rapid remote triggering of earthquakes at great distances (from several fault lengths to 1000s of km) is best explained by the passage of transient (dynamic) seismic waves, which either immediately induce Coulomb-type failure or initiate a secondary mechanism that induces delayed triggering. The passage of seismic waves may also play a significant role in the triggering of near-field earthquakes.

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 CO₂is 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 thermal structure of the lithosphere controls many properties and processes of Earth's crust. The total ∼47-TW heat loss of Earth is key to understanding and modeling this thermal structure, as is partitioning the various sources of that heat into heat entering the base of the lithosphere, heat generated within the lithosphere by radioactive decay (primarily within the continental crust), and secular cooling of the mantle lithosphere (primarily in oceanic lithosphere). A set of framework geotherms for the continental lithosphere explains deep crustal melting in high heat flow regions, metamorphic pressure-temperature (P-T) space in the crust, partial melting at the base of the lithosphere to produce an S-wave low-velocity zone in Phanerozoic and younger terranes, and the P-T fields inferred from mantle xenoliths. Important perturbations to a standard thermal state are produced by orogenic overprints, transient thermal regimes, and exhumation.

Prevailing opinion assigns the Tibetan Plateau a crucial role in shaping Asian climate, primarily by heating of the atmosphere over Tibet during spring and summer. Accordingly, the growth of the plateau in geologic time should have written a signature on Asian paleoclimate. Recent work on Asian climate, however, challenges some of these views. The high Tibetan Plateau may affect the South Asian monsoon less by heating the overlying atmosphere than by simply acting as an obstacle to southward flow of cool, dry air. The East Asian “monsoon” seems to share little in common with most monsoons, and its dynamics may be affected most by Tibet's lying in the path of the subtropical jet stream. Although the growing plateau surely altered Asian climate during Cenozoic time, the emerging view of its role in present-day climate opens new challenges for interpreting observations of both paleoclimate and modern climate.

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo₂ with depth, which appears to result principally from the effect of pressure on the controlling Fe³⁺/Fe²⁺ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo₂ will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe₂O₃-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe³⁺/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe³⁺ into (Fe,Mg)SiO₃ perovskite results in very high perovskite Fe³⁺/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle should contain approximately 1 weight% metallic Fe formed through FeO disproportionation, if the bulk oxygen content is the same as the upper mantle. Loss of disproportionated metallic Fe from the lower mantle during core formation could explain the higher Fe³⁺/ΣFe ratio of the present-day upper mantle when compared to that expected during core formation. The influence of pressure on mantle fo₂ has important implications for the speciation of C-O-H-S volatile phases in Earth today and during its early evolution.