Elements Covers

Thematic Articles

Borate and the Origin of RNA: A Model for the Precursors to Life

According to the RNA World hypothesis, ribonucleic acid (RNA) played a critical role in the origin of life. However, ribose, an essential component of RNA, is easily degraded: finding a way to stabilize it is critical to the viability of the hypothesis. Borate has been experimentally shown to have a strong affinity for ribose, and, thus, could have protected ribose from degradation in the formose reaction, a potential process for prebiotic ribose formation. Accumulation of borate on Hadean Earth (prior to ~4,000 Ma) might have been a key step in the chemical evolution of the biotic sugar. Proto-arcs are suggested as a geological setting sufficiently rich in borate to stabilize ribose during the Hadean.

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Tripping the Light Fantastic: Organoboron Compounds

Small molecules containing boron can play all sorts of roles in chemistry, biology and materials science. Molecular boron compounds display a wide range of unusual and fascinating structures, and their chemical reactivity can be very different from that of boron’s next-door neighbour carbon. Some of the reasons for this will be considered and illustrated through applications in energy, medicine and new materials. The boron dipyrrins, also known as BODIPYs, are a prime example. They are strongly fluorescent when excited by illumination and are widely used as fluorescent tags in biology and as biosensors. More recently, they have been studied for their energy transfer properties in light-harvesting applications.

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Origin and Distribution of Evaporite Borates: The Primary Economic Sources of Boron

Naturally occurring borates are the major economic source of boron. Borates were first used over 4,000 years ago in precious-metal working and are now essential components of modern industry. Although borates have been exploited from other sources, three minerals from non-marine evaporites now form the major commercial sources of borate – borax, colemanite and ulexite. These major commercial deposits are associated with Neogene volcanism in tectonically active extensional regions at plate boundaries. The most important continental borate provinces are located in the USA, Argentina, Chile, Peru, and China, with the largest borate reserves in the world being found in western Anatolia (Turkey).

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Boron Isotopes: A “Paleo-pH Meter” for Tracking Ancient Atmospheric CO2

The boron isotope composition of calcium carbonate shells of marine organisms has the unique potential to record surface ocean pH, allowing the calculation of atmospheric pCO2 due to the established relationship between pH and the partial pressure of (atmospheric) CO2 (pCO2). This “paleo-pH meter” allows scientists to produce a record of the natural fluctuations of atmospheric pCO2 over geologic time, which will help us better understand the impacts of the recent anthropogenic addition of CO2 to Earth’s atmosphere. Towards this end, a tremendous effort to understand the systematics of boron uptake in marine carbonates is underway. Here, we review the potential of boron isotopes to constrain ocean pH and, thus, atmospheric pCO2.

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Boron Cycling in Subduction Zones

Subduction zones are geologically dramatic features, with much of the drama being driven by the movement of water. The “light and lively” nature of boron, coupled with its wide variations in isotopic composition shown by the different geo-players in this drama, make it an ideal tracer for the role and movement of water during subduction. The utility of boron ranges from monitoring how the fluids that are expelled from the accretionary prism influence seawater chemistry, to the subduction of crustal material deep into the mantle and its later recycling in ocean island basalts.

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Boron Behavior During the Evolution of the Early Solar System: The First 180 Million Years

The behavior of boron during the early evolution of the Solar System provides the foundation for how boron reservoirs become established in terrestrial planets. The abundance of boron in the Sun is depleted relative to adjacent light elements, a result of thermal nuclear reactions that destroy boron atoms. Extant boron was primarily generated by spallation reactions. In the initial materials condensing from the solar nebula, boron was predominantly incorporated into plagioclase. Boron abundances in the terrestrial planets exhibit variability, as illustrated by B/Be. During planetary formation and differentiation, boron is redistributed by fluids at low temperature and during crystallization of magma oceans at high temperature.

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Boron: From Cosmic Scarcity to 300 Minerals

Boron is rare in the cosmos because its nucleus is “fragile.” So, how does one get from the interstellar medium, where boron was first produced, to Earth’s upper continental crust where boron is concentrated in deposits containing remarkably diverse suites of boron minerals? Processes that led to the formation of continental crust also concentrated boron, which is preferentially incorporated into melts and aqueous fluids. Deposits with high boron-mineral diversity include granitic pegmatites, peralkaline intrusions, boron-enriched skarns, and evaporite deposits. Despite the loss of boron minerals from the geologic record due to their ready solubility in water and breakdown at low temperatures, the increase in boron-mineral diversity with time is real, and is accelerated during supercontinent assembly.

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Rock Coatings and the Potential for Life on Mars

Rock coatings observed on the surface of Mars since the Viking landers of the mid-1970s continue to generate considerable interest in the field of astrobiology. Terrestrial rock coatings are associated with diverse microbial life, raising the question as to whether martian rock coatings might also be partly of biogenic origin. Rock coating formation can be mediated by microbes, which in turn are shielded from harmful radiation and can obtain nutrients from the coating. In addition, coatings may be indicative of life and can preserve its signatures. Pragmatically, rock coatings are surface environments accessible by Mars rovers. This article focuses on the habitability, preservation capability, and practicality of rock coatings as astrobiological targets on Mars.

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Climate Change, Rock Coatings, and the Archaeological Record

Rock varnish commonly occurs in arid environments. It can coat not only rock surfaces but also any exposed prehistoric stone artifacts and rock art. Thin sections of varnish coatings reveal microlaminations that reflect major changes between wet versus dry paleoclimates. These microlaminations can be used to assign minimum ages to the underlying rock surface, providing dates for otherwise undateable stone artifacts and rock art. This dating approach has provided important information about the peopling of the Americas (North and South America), including how and when the native American populations adapted to changing climatic conditions.

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Marine Ferromanganese Encrustations: Archives of Changing Oceans

Fungi are ubiquitous inhabitants of rock and mineral surfaces and are significant geoactive agents. Capable of numerous transformations of metals and minerals, fungi can prosper in the most adverse of environments, their activities underpinned by growth form and metabolism. Free-living filamentous species, microcolonial fungi and lichens can significantly change a rock’s surficial structure and appearance, ranging from discolouration and staining to biodeterioration and the formation of new biogenic minerals and rock coatings. The presence and activity of fungi should be considered in any study of rock and mineral transformations that seeks to understand the biotic and abiotic processes that underpin geochemical change in the biosphere.

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