Nanoscale Structure and Dynamics in Geochemical Systems

Neutron scattering is a powerful tool to elucidate the structure and dynamics of systems that are important to geochemists, including ion association in complex aqueous solutions, solvent-exchange reactions at mineral–water interfaces, and reaction and transport of fluids in nanoporous materials. This article focusses on three techniques: neutron diffraction, which can reveal the atomic-level structure of aqueous solutions and solids; quasi-elastic neutron scattering, which measures the diffusional dynamics at mineral–water interfaces; and small-angle neutron scattering, which can show how properties of nanoporous systems change during gas, liquid, and solute imbibition and reaction. The usefulness and applicability of the experimental results are extended by rigorous comparison to computational simulations.

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Where is the Hydrogen?

How is hydrogen distributed among minerals and how is it bonded in their crystal structures? These are important questions, because the amount of hydrogen and the bonding configuration of hydrogen in crystalline materials governs many of that material’s properties: its thermal and compressional behavior, P–T phase stability, rheology, and electrical conductivity. A reliable reconstruction of the Earth’s interior, or the prediction of mineral transformations in complex industrial processes, must account for these parameters. Neutron diffraction can locate hydrogen sites in mineral structures, reveal any static or dynamic hydrogen disorder, help define the libration regime of hydrogen, and elucidate hydrogen-bonding configurations. Thus, that most elusive element for X-ray probes is perfectly detectable using neutrons.

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Neutrons “101” – A Primer for Earth Scientists

The fundamental properties of the neutron make it a powerful tool for Earth science investigations because neutrons provide information that cannot be obtained by any other research method. This is because neutrons are magnetically sensitive, nondestructive, and sensitive to the lighter elements, such as hydrogen. They provide a unique, nondestructive method for obtaining information ranging from Ångstrom-scale atomic structures (and related motions) to micron-scale material strain, stress, and texture, and even up to meso-scale porous matrices and defects in materials and functional components. In this article, we introduce neutrons and their unique properties, neutron production and sources, and provide an overview of the different types of neutron methods applicable to the Earth sciences.

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Attraction in the Dark: The Magnetism of Speleothems

No matter how quiet and pristine a cave setting may appear, all speleothems contain assemblages of magnetic minerals. These iron oxide minerals are derived largely from overlying soils, though minor fractions may come from the residuum of dissolved bedrock, reworked sediment carried by episodic floods, geomicrobiological activity, and even windblown dust. Regardless of their origin, these minerals become aligned with Earth’s ambient magnetic field before they are fixed within a speleothem’s growing carbonate matrix. Here, we describe how the magnetism of stalagmites and flowstone can be used to chronicle high-resolution geomagnetic behavior and environmental change.

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Cave Decorating with Microbes: Geomicrobiology of Caves

Microorganisms are important for the formation and biogeochemistry of caves. Some caves are energy-rich systems with abundant organic or inorganic chemical energy inputs that support robust microbial ecosystems, but most are extremely oligotrophic settings with slow-growing microbial communities that rely on limited energy resources. Microorganisms are catalysts for element cycling in subterranean environments and act as agents of mineral precipitation and dissolution. Microbes can contribute to cave formation by producing acids and corroding limestone bedrock, and they can form secondary mineral deposits by catalyzing metal oxidation and inducing carbonate precipitation. We describe the energy sources for microbial life in caves, and we review three situations in which microorganisms may play a direct role in mineral deposition and bedrock corrosion.

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Temperature Reconstructions Using Speleothems

Methods for reconstructing past temperatures from speleothems have only recently been developed. Advances in quantitative temperature proxies for speleothems are now allowing critical knowledge gaps to be filled, given the outstanding age control and wide geographical distribution of the speleothem archive. The methods of reconstructing temperatures from speleothems are diverse: they rely on concepts from geochemistry, biology, and physics, and are based on different aspects of speleothems, including water inclusions, calcite, and organic molecules. Combining the different approaches makes temperature reconstructions more robust, affords further insights into the methodologies, and provides constraints on other climate variables.

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Tales from the Underground: Speleothem Records of Past Hydroclimate

Geochemical records from speleothems have significantly advanced our understanding of natural climate variability over the last ~600,000 years. Speleothems are sensitive recorders of past changes in hydroclimate because they can be precisely dated and contain multiple hydrologically sensitive geochemical proxies. Oxygen isotope records from speleothems tell us about the timing and mechanisms of past changes in precipitation amount, temperature, atmospheric circulation, and/or global monsoon intensity. Variations in speleothem carbon isotope ratios or trace element concentrations reflect changes in local water balance, vegetation, and karst hydrology. Speleothem paleoclimate records represent a window into the past that can provide crucial information for understanding how anthropogenic climate change and natural climate variability will impact future water resources on Earth.

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Uranium–Thorium Dating of Speleothems

Speleothems are important timekeepers of Earth’s climate history. A key advantage of speleothems is that they can be dated using U–Th techniques. Mass spectrometric methods for measuring U and Th isotopes has led to vast improvements in measurement precision and a dramatic reduction in sample size. As a result, the timing of past climate, environment, and Earth system changes can be investigated at exceptional temporal precision. In this review, we summarize the principles and history of U–Th dating of speleothems. Finally, we highlight three studies that use U–Th dated speleothems to investigate past changes to the Asian monsoon, constrain the timing of sociopolitical change in ancient civilizations, and develop a speleothem-based calibration of the 14C timescale.

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Cave and Speleothem Science: From Local to Planetary Scales

Caves occur everywhere on our planet, from the tropics to the high latitudes and from below sea level to alpine settings. Cave morphologies provide clues to their formation mechanisms, and their iconic mineralogical features—stalagmites and stalactites—carry a wealth of paleoenvironmental information encoded in their geochemistry and mineralogy. Recent work demonstrates a striking improvement in our ability to decode these paleoenvironmental proxies, and dramatic geochronological advances enable higher resolution records that extend further back in geologic time. Cave research addresses an ever-increasing range of geoscience problems, from establishing the timing and mechanisms of climate change to uncovering detailed records of geomagnetic field behavior.

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How Climate, Uplift and Erosion Shaped the Alpine Topography

Decades of scientific research on the European Alps have helped quantify the vast array of processes that shape the Earth’s surface. Patterns in rock exhumation, surface erosion and topographic changes can be compared to sediment yields preserved in sedimentary basins or collected from modern rivers. Erosion-driven isostatic uplift explains up to ~50% of the modern geodetic rock uplift rates; the remaining uplift reveals the importance of internal processes (tectonics, deep-seated geodynamics) and external processes (glacial rebound, topographic changes). We highlight recent methodological and conceptual developments that have contributed to our present view of the European Alps, and we provide suggestions on how to fill the gaps in our understanding.

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