Boulder, Colo., USA - The close of the Permian Period around 250 million years ago saw Earth's biggest extinction ever. At this time large volcanic eruptions were occurring in what is now Siberia. The volcanoes pumped out gases that led to acid rain. Falling on the supercontinent Pangaea, the acid rain killed off end-Permian forests. The demise of forests led to soil erosion and the production of organic-rich sediments in shallow marine waters.
The sediments are now rocks in cliff faces in the Italian Dolomites, and studying them provides insight into the mechanisms of Permian ecosystem decline. Scientists have proposed a first-ever, organic compound-based, quantitative recorder of acidity for the geological record. Knowledge from the food industry, where vanillin ("vanilla") is used as a flavoring ingredient, shows that oxidation of vanillin to vanillic acid is reduced under acidic conditions.
Ratios of vanillic acid to vanillin in end-Permian organic matter reveal soil acidity close to that of vinegar or lemon juice. Acidification events occurred not once but several times as volcanism hit the land with repeated pulses of acid rain. An acid-induced decline in plant life would have caused a collapse in the food chain, sealing the fate of end-Permian life on land.
Terrestrial acidification during the end-Permian biosphere crisis?
Mark A. Sephton et al., Imperial College, London, South Kensington Campus, UK. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
Other recently posted GEOLOGY articles (see below) cover such topics as
- 1. Human impacts on soil erosion;
2. Precise chronology of the Little Ice Age expansion; and
3. Excavating the "slab graveyard" in Tibet.
GEOLOGY articles published online ahead of print can be accessed online at http://geology.
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Quantifying human impacts on rates of erosion and sediment transport at a landscape scale
Lucas Reusser et al. (corresponding author: Paul Bierman), University of Vermont, Burlington, Vermont, USA. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
Humans impact Earth in many different ways. One of the most visually dramatic impacts is erosion caused by deforestation and intensive agriculture -- but does such erosion really matter? To answer that question, Reusser and colleagues used exquisitely sensitive analyses to measure how quickly Earth's surface changes in the absence of humans. The team estimated natural rates of erosion by collecting sand from rivers draining the southeastern United States and measuring a rare isotope of beryllium produced near Earth's surface. Their measurements show that before the arrival of Europeans, erosion was slow -- only a fraction of an inch every thousand years. Then, the settlers came, cleared the land, and erosion increased more than 100-fold. These measurements clearly demonstrate the dramatic effects that humans can have on the landscape. So much soil sluiced off hillslopes that the rivers could move less than 10% of what was eroded. The rest was left behind on valley bottoms. Background erosion rates are critical for developing realistic landscape management strategies. For example, the technique used in this study can determine whether rates of soil loss are sustainable and help regulators make decisions about allowable levels of sediment pollution in rivers.
Precise chronology of Little Ice Age expansion and repetitive surges of Langjökull, central Iceland
Darren J. Larsen et al., University of Iceland, Reykjavi´k, Iceland, and Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA; current: University of Pittsburgh, Pittsburgh, Pennsylvania, USA. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
The nonlinear and complex behavior of glacier dynamic processes (e.g., surging and ice calving) presents major challenges for future estimates of runoff and sea-level change. Because direct observations are temporally limited, reconstructions of past fluctuations from glaciers that undergo dynamic advance and/or retreat are valuable. This study presents a detailed record of dynamic Langjökull outlet glacier fluctuations through the past 300 years, constructed from annually laminated sediment archives in Hvítárvatn, a large proglacial lake in Iceland's central highlands. We demonstrate a novel combination of geophysical survey techniques (cm-scale multibeam bathymetric mapping and high-resolution seismic profiling) and multiple sediment cores taken from targeted locations to accurately constrain glacier fluctuations at unprecedented temporal resolution. We are able to track the behavior of both Langjökull's lake-terminating outlet glaciers during their occupation of Hvítárvatn and show that while Norðurjökull remained relatively stable, Suðurjökull's terminus repeatedly surged and rapidly disintegrated through stagnate ice collapse and iceberg melting. Our precise estimates of the timing, duration, and magnitude of Suðurjökull's surges, provide ideal targets for the next generation of glacier surge models that may improve our ability to estimate future glacier mass loss rates if summer temperatures continue to rise.
Tibetan chromitites: Excavating the slab graveyard
Nicole M. McGowan et al., ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), and GEMOC, Macquarie University, Sydney, New
South Wales, Australia. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (Tibet) contain diamonds and a highly reduced trace-mineral association that suggests that the chromitites formed at ultra-high pressure (UHP) corresponding to the Transition Zone (>400 km; >12.5 GPa). However, trace-element signatures of the chromites are indistinguishable from those of typical ophiolitic chromitites (e.g. Antalya Complex, Turkey), implying primary crystallization from typical arc-type melts at shallow depths. New data on geochronology and Fe oxidation state may explain this conundrum. We propose that a lithospheric mantle slab containing the crystallized chromitite was subducted to the Transition Zone, where chromite inverted to a high-pressure polymorph and reacted locally with reducing fluids to form the highly reduced trace-mineral assemblage. Thermo-mechanical modeling suggests a rapid (?10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.
The source of gypsum in Mammoth Cave, Kentucky
J. Garrecht Metzger et al., Washington University in St. Louis, St. Louis, Missouri, USA. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
Mammoth Cave, Kentucky, is the longest cave system in the world. Many of the dry passages of the cave are lined with gypsum (CaSO4 * 2H2O), yet despite nearly a century of research, the source of the gypsum sulfur remains uncertain. Identifying the sulfur source is important because it reveals how fluids move through the cave, which helps geologists understand cave formation and engineers understand chemical transport in karst terrains. To constrain the gypsum sulfur source, we measured the stable sulfur isotope ratios (d34S) of different geological sources and compared them with that of the gypsum. The entire range in gypsum d34S values falls within that of pyrite d34S found in the cave hosting formations. Sampling along the crystal growth axis suggests that the source of sulfur remained the same during the entire growth period of the crystal while d34S of dissolved SO42 in modern seeps suggests the sulfur source is the same today. These results suggest that the dominant source of gypsum sulfur is pyrite found locally (within ~ 20 m) and disseminated within the limestone cave walls.
The enigma of crustal zircons in upper-mantle rocks: Clues from the Tumut ophiolite, southeast Australia
Elena A. Belousova et al., ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Macquarie University, Sydney, New South Wales, Australia. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
Zircon, the mineral most widely used to date rocks by the U-Pb method, is common in crustal rocks, but is increasingly being found in rocks from the upper mantle. Several studies have concluded that this reflects the deep subduction of crustal rocks into the mantle. This paper presents a new explanation for the presence of crustal zircons in the upper mantle rocks. In this case, granitoid-related melts/fluids, injected into already-emplaced mafic-ultramafic rocks, apparently transported pre-existing zircons and possibly crystallized new grains. This study sends a cautionary message about the interpretation of zircon ages from mafic-ultramafic rocks and emphasizes that integrated isotopic and trace-element information on zircons (not U-Pb age data alone) is critical to the interpretation of events in upper-mantle rocks, including their emplacement and subsequent tectonic history. A clear understanding of the origin of zircons and their relationship with the host rock increases the probability of correct conclusions about the history of upper mantle rocks, and interpretations of the dynamics of crust-mantle interaction.
Slip re-orientation in oblique rifts
M. Philippon et al., Utrecht University, Utrecht, Netherlands, and Université des Antilles Guyane, Pointe-à-Pitre, French West Indies. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
In the course of Earth's dynamic evolution, the drifting of continents mostly initiates along oblique rifts. Whereas oblique extension is expected to result in a combination of (i) dip-slip along faults with strike orthogonal to the extension direction and (ii) strike-slip displacement along oblique faults, Philippon and colleagues show that in oblique rifts, faults show dip-slip kinematics indicating pure extension irrespective of the fault strike with respect to the regional extension direction. These findings question the use of paleo-stress reconstructions to constrain plate kinematics in oblique extensional tectonic settings.
Re-Os dating of pyrite confirms an early diagenetic onset and extended duration of mineralization in the Irish Zn-Pb ore field
Danny Hnatyshin et al., University of Alberta, Edmonton, Alberta, Canada. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
ABSTRACT: The Irish Midlands region contains one of the world's largest hydrothermal Zn-Pb ore districts, but uncertainty exists in the timing of mineralization relative to host rock ages. Consequently, genetic models for ore formation are poorly constrained and remain controversial. Here we use Re-Os geochronology to show that ore-stage pyrite from the Lisheen deposit formed at 346.6 ± 3.0 Ma, shortly after host rock deposition. Pyrite from the Silvermines deposit returns an age of 334.0 ± 6.1 Ma, indicating that at least some mineralization occurred during later burial. These age determinations show that the much younger paleomagnetic ages reported for the Irish Zn-Pb deposits reflect remagnetization during the Variscan orogeny, a process that we suggest affects paleomagnetic dating more widely. The Re-Os ages overlap with the ages of lower Carboniferous volcanic rocks in the Midlands, which are the product of magmatism that has been invoked as the driving force for hydrothermal activity. The relatively low initial Os ratios for both Lisheen (0.253 ± 0.045) and Silvermines (0.453 ± 0.006) are compatible with derivation of Os from these magmas, or from the Caledonian basement that underlies the ore deposits.
Late Holocene sea-level fall and turn-off of reef flat carbonate production: Rethinking bucket fill and coral reef growth models
Daniel L. Harris et al., University of Sydney, Sydney, New South Wales, Australia, and ZMT (Leibniz Center for Tropical Marine Ecology) and MARUM (Center for Marine Environmental Research), The University of Bremen, Bremen, Germany. Published online 7 Jan. 2015; included in the February issue of Geology; http://dx.
ABSTRACT: Relative sea-level rise has been a major factor driving the evolution of reef systems during the Holocene. Most models of reef evolution suggest that reefs preferentially grow vertically during rising sea level then laterally from windward to leeward, once the reef flat reaches sea level. Continuous lagoonal sedimentation ("bucket fill") and sand apron progradation eventually lead to reef systems with totally filled lagoons. Lagoonal infilling of One Tree Reef (southern Great Barrier Reef) through sand apron accretion was examined in the context of late Holocene relative sea-level change. This analysis was conducted using sedimentological and digital terrain data supported by 50 radiocarbon ages from fossil microatolls, buried patch reefs, foraminifera and shells in sediment cores, and recalibrated previously published radiocarbon ages. This data set challenges the conceptual model of geologically continuous sediment infill during the Holocene through sand apron accretion. Rapid sand apron accretion occurred between 6000 and 3000 calibrated yr before present B.P. (cal. yr B.P.); followed by only small amounts of sedimentation between 3000 cal. yr B.P. and present, with no significant sand apron accretion in the past 2 k.y. This hiatus in sediment infill coincides with a sea-level fall of ~1-1.3 m during the late Holocene (ca. 2000 cal. yr B.P.), which would have caused the turn-off of highly productive live coral growth on the reef flats currently dominated by less productive rubble and algal flats, resulting in a reduced sediment input to backreef environments and the cessation in sand apron accretion. Given that relative sea-level variations of ~1 m were common throughout the Holocene, we suggest that this mode of sand apron development and carbonate production is applicable to most reef systems.