April 2021 - Volume 17, Number 2

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Speleothems

Kathleen R. Johnson and Joshua M. Feinberg Guest Editors

Table of Contents

Overview

Growing slowly drip by drip through the millennia, stalagmites, stalactites, and flowstone—collectively known as speleothems—are some of the most fantastic mineral features in nature. Speleothems are also critical archives of past environments, and their study incorporates expertise from groundwater hydrogeology and geochemistry, atmospheric chemistry, climate science, geobiology, and even geophysics. Research on speleothem trace element and isotopic geochemistry, constituent organic compounds, noncarbonate minerals, and morphology can help illuminate paleoenvironmental conditions and document historical anthropogenic land-use changes. This issue of Elements will introduce the many ways that speleothems are used within the geoscience community to learn about natural Earth processes and our role in modifying them.

  • Cave and Speleothem Science: From Local to Planetary Scales
  • Uranium–Thorium Dating of Speleothems
  • Tales from the Underground: Speleothem Records of Past Hydroclimate
  • Temperature Reconstructions Using Speleothems
  • Cave Decorating with Microbes: Geomicrobiology of Caves
  • Attraction in the Dark: The Magnetism of Speleothems
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2021 Topics

Thematic Articles

Cave and Speleothem Science: From Local to Planetary Scales

By and

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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Uranium–Thorium Dating of Speleothems

By , and

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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Tales from the Underground: Speleothem Records of Past Hydroclimate

By

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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Temperature Reconstructions Using Speleothems

By , and

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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Cave Decorating with Microbes: Geomicrobiology of Caves

By and

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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Attraction in the Dark: The Magnetism of Speleothems

By and

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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