September 2011 Geology highlights: New research posted Aug. 5
Highlights from the September issue of GEOLOGY, which is now online, include debris flow hazard assessment; dune migration in the "greatest desert on Earth": Antarctica; Luizi, the first confirmed meteorite impact structure in Central Africa; a determination of the likelihood that fungal disease had accelerated latest Permian woodland deterioration; the first comprehensive compilation of sedimentary and historical records of ash-fall events in northern Europe; and high-speed video of ash fall from the 2010 Eyjafjallajökull eruption.
Keywords: Mariana arc, New Caledonia, autogenic processes, Zagros fold belt, subducting seamounts, Roanoke River, debris flows, Antarctica, McMurdo Dry Valleys, Bering plate, Yellow River, fluvial terraces, Upper Rhine Graben, serpentinite, Luizi impact structure, Democratic Republic of Congo; Canary archipelago, Henry Seamount, Coats Land, Laurentia, mafic magma, Jakobshavn Glacier, Flinders Ranges, South Australia, Asian Monsoon system, Llanfawr Mudstones Lagerstaette, Rhizoctonia, Reduviasporonites, northern Europe, volcanic ash, Eyjafjallajökull
Highlights are provided below. Representatives of the media may obtain complimentary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
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High SO2 flux, sulfur accumulation, and gas fractionation at an erupting submarine volcano
David A. Butterfield et al., University of Washington, Joint Institute for the Study of the Atmosphere and Ocean (JISAO), Box 354925, Seattle, Washington 98195, USA; doi: 10.1130/G31901.1.
Volcanic eruptions in the deep ocean have been directly observed on very few occasions. NW Rota-1 submarine volcano in the Mariana arc has been erupting slowly and persistently since it was first observed with a remotely operated vehicle in 2004. The processes of magma degassing, acidic alteration of volcanic rock, and formation of elemental sulfur have been captured. High concentrations of sulfur dioxide have been measured in hydrothermal solutions for the first time by David A. Butterfield of the University of Washington and colleagues. The chemistry of submarine volcanic arc hydrothermal systems is influenced by the high volatile content and slightly oxidized nature of magmas produced on volcanic arcs as compared to mid-ocean ridge systems. Highly acidic fluids produced during submarine eruptions on volcanic arcs may have a significant impact on the chemical balance of the oceans, notably for sulfur and aluminum.
New Caledonian carbon sinks at the onset of Antarctic glaciation
Douglas N. Reusch et al., Dept. of Geology, University of Maine, 173 High Street, Farmington, Maine 04938, USA; doi: 10.1130/G31981.1.
The partial pressure of atmospheric carbon dioxide (pCO2), a factor in the energy balance of Earth's climate system, reflects the competitions between oxidative weathering of organic carbon and burial and between volcanic-metamorphic carbon dioxide sources and weathering reactions with silicate materials that yield carbonates at the expense of carbon dioxide. At the time of Antarctic glaciation 34 million years ago, proxy geochemical estimates of pCO2 indicate a decrease coincident with global cooling and ice-sheet growth. Also at this time, a large tract of basic and ultrabasic seafloor, composed of silicate materials with an uncommonly high capacity to sequester carbon, breached sea level in the New Caledonian region of the southwest Pacific Ocean. The timing and amplitude of silicate carbonation and organic carbon burial, based on estimates of paleo-area, paleo-erosion rate, and paleo-sedimentation rate, allow Douglas N. Reusch of the University of Maine at Farmington and colleagues to suggest that sufficient carbon dioxide may have been removed from the atmosphere in this region to lower pCO2 on the order of 100 parts per million by volume, thus triggering growth of the Antarctic ice sheet and a host of related environmental changes.
Scale-dependent compensational stacking: An estimate of autogenic time scales in channelized sedimentary deposits
Yinan Wang et al., Dept. of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA; doi: 10.1130/G32068.1.
Ancient sedimentary basins are archives of past climate, tectonic, and land-surface changes on Earth. These deposits also contain important energy and water reserves and will serve as hosts for carbon capture and storage. In order to understand sedimentary deposits and manage these resources, improved methods are needed for interpreting and predicting stratigraphic patterns. Internally generated (autogenic) dynamics in sedimentary systems can generate stratigraphy that mimics patterns produced by tectonic, climate, and sea-level changes. Yinan Wang of Tulane University and colleagues present statistical methods that can help stratigraphers filter autogenic signals from sedimentary deposits in order to fully understand and model stratigraphy. The team determines a time scale that predicts when autogenic sedimentation patterns average out in stratigraphy. Through a combination of experiments, numerical modeling, and fieldwork, they map and measure fluviodeltaic stratigraphic organization. This work will advance scientists' ability to recover meaningful data about autogenic processes from stratigraphic datasets, isolate preserved signals of changing environmental conditions in ancient deposits, and generate predictive stratigraphic models in alluvial basins. This study was supported in part by a grant from the U.S. National Science Foundation.
Dynamic constraints on the crustal-scale rheology of the Zagros fold belt, Iran
Philippe Yamato et al., Geosciences Rennes, UMR CNRS (Unite Mixte de Recherche, Centre National de la Recherche Scientifique) 6118, Universite de Rennes 1, 35042 Rennes Cedex, France; doi: 10.1130/G32136.1.
In the Zagros fold belt (Iran), folds are spaced in a regular manner with a wavelength of ~15 km. These folds were commonly considered the result of the folding of a sedimentary sequence above a salt basal layer. Here, Philippe Yamato of UMR CNRS and colleagues show that by using realistic values of thicknesses for these two layers, this simplistic view is mechanically impossible. The models show that faults develop instead of folds, which is inconsistent with observations. However, a careful analysis of the lithologies constituting the sedimentary sequence reveals that at least three additional decollements exist. Once these additional weak layers were taken into account, their model finds folds rather than faults. Next, they developed a new method to determine the fold wavelength as a function of input parameters, and show that the fold wavelength is mainly sensitive to the friction angle (brittleness) of the crust and to the viscosity (weakness) of the weak layers. If applied to the Zagros, Yamato and colleagues' results show that the friction angle should be small (~5 degrees), which means that crust is significantly weaker than previously thought, possibly due to the effect of fluids.
Do subducting seamounts generate or stop large earthquakes?
Kelin Wang and Susan L. Bilek, Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia, V8L 4B2, Canada; doi: 10.1130/G31856.1.
In the study of large earthquakes at subduction zones, such as the recent events in Japan and Chile, it is important to understand what controls the location and dimension of the rupture zone. Subducting seamounts have been widely viewed as candidates for fault asperities that cause large earthquakes. Kelin Wang and Susan L. Bilek of the Pacific Geoscience Centre point out that the role of seamounts is likely opposite of this popular view. That is, seamounts tend to act as barriers to stop large earthquakes. Wang and Bilek's reasoning is based not only on the remarkable lack of evidence for subducting seamounts causing large earthquakes, but also on the recognition that a subducting seamount, as a major geometrical irregularity, modifies the structure and stress field of the fault zone to give rise to an environment that favors small earthquakes and aseismic creep but not large ruptures. As the seamount forces its way through the subduction zone, it generates and continues to modify a fracture network around and within itself. The extreme heterogeneity of the stress field causes frequent failure in many parts of this evolving fracture system, and the complex structure makes it difficult for ruptures to propagate to become large earthquakes. This understanding has profound implications relating to the fundamental mechanics of subduction earthquakes.
Stream capture as driver of transient landscape evolution in a tectonically quiescent setting
Philip S. Prince et al., Dept. of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA; doi: 10.1130/G32008.1.
Philip S. Prince of Virginia Tech and colleagues show that stream capture, a process through which one stream erodes into the basin of another and gains new tributaries, drives localized rapid erosion along the margin of the Blue Ridge Plateau of the southern Appalachians (North America). Unique "stranded" river gravels preserved atop the New River-Roanoke River divide indicate that the Roanoke once followed a gently sloping course across the elevated plateau surface to the New River. Capture by a steep headwater stream of an adjacent river system suddenly diverted the Roanoke off of the plateau surface into an approximately 250-m-lower valley, steepening its course and forcing rapid downcutting as it adjusted to its new, lower outlet. The preservation of the stranded gravels on the plateau surface adjacent to the Roanoke River gorge indicates that a captured stream basin erodes much more rapidly than the surrounding landscape. Topographic analysis indicates rapid erosion has spread to the headwaters of some Roanoke tributaries, which are eroding further into the plateau and producing additional captures of small New River basin streams. Collectively, the results presented by Prince and colleagues indicate that repeated stream capture can drive spreading "waves" of rapid erosion through otherwise stable landscapes that are not experiencing surface uplift.
Dynamic controls on erosion and deposition on debris-flow fans
Peter Schuerch et al., Dept. of Geography and Institute of Hazard Risk and Resilience, Durham University, South Road, Durham DH1 3LE, UK; doi: 10.1130/G32103.1.
Recent fieldwork in Switzerland has provided new insight into the behavior of debris flows. Until now, there were only poor data available about erosion in debris flows, which was a major problem in debris-flow hazard assessment. Peter Schuerch of Durham University and colleagues used a terrestrial laser scanner to monitor debris flows and showed how debris-flow erosion relates to the maximum flow depth. Debris flows are avalanches of rock, soil, woody debris, and water that occur in steep mountain torrents around the world. They pose a serious threat to communities because of their ability to grow in volume by picking up more and more material as they move down valleys. After some distance, the debris flow may have grown so much that it breaks out from the normal stream channel, causing destruction to property and nearby infrastructure. Effective debris-flow hazard assessment relies on accurate information about the size of flows that are to be expected in a particular location. This research provides guidance on how estimates of debris-flow volumes can be improved.
Is climate change affecting rates of dune migration in Antarctica?
Charlie S. Bristow et al., Dept. of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK; doi: 10.1130/G32212.1.
Thinking about deserts and sand dunes usually conjures up visions of desert nomads with their camels struggling over sun-baked sand dunes toward a distant oasis ringed by palm trees. But there is another type of desert--the cold desert--and the greatest desert on Earth is Antarctica. Antarctica is the coldest, driest, and windiest continent on Earth, as well as the highest, and the wind plays an important role in its surface processes. In those areas free of snow and ice, which are a very small portion of the continental land mass, the wind has built sand dunes. The most extensive field of dunes in Antarctica, as well as the largest dune on that continent, are found in the Victoria Valley, one of the McMurdo Dry Valleys. These dunes are being studied as examples of cold climate dunes to investigate the controls on sand transport in a cold climate, and to understand their age and development. Toward the end of the last Ice Age, cold-climate dunes were much more extensive, covering large areas of northwest Europe. In addition, Antarctic dunes are being studied as analogues for sand dunes on other planets such as Mars, which is also very cold, dry, and free of surface vegetation. Charlie S. Bristow of the University of London and colleagues used ground-penetrating radar to image layers of sand within the dunes, which shows how they have been built up over time. They then selected layers for optical dating, which determines when sand was last exposed to daylight and when it was buried. Based on optical ages and sample locations relative to the front of the dune, Bristow and colleagues worked out how far the dunes have moved and calculated end-point migration rates. These rates of dune migration are less than would be expected from similar-sized dunes in a hot desert because ice cements are holding the dune sands in place. In addition, their migration rates are less than the recent rates derived from historic aerial photographs and field measurements. Bristow and colleagues suggest that the apparent increase in the rates of dune migration might indicate a geomorphic response to a warming climate in the Dry Valleys of Antarctica.
Kinematics of a diffuse North America-Pacific-Bering plate boundary in Alaska and western Canada
E.S. Finzel et al., Dept. of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA; doi: 10.1130/G32271.1.
Profuse seismicity that extends ~1000 km inboard from the Pacific-North America plate margin characterizes much of Alaska and western Canada. However, inconsistencies between regional seismicity and block tectonic models, the recent recognition of the Bering plate, and the availability of global positioning system data motivate a large-scale kinematic analysis of Alaska and western Canada. E.S. Finzel of Purdue University and colleagues present the first attempt to develop a spatially continuous kinematic model for Alaska and western Canada. Their results provide new insight on questions like (1) how important is diffuse accommodation of relative plate motions in the neotectonics of the region?; (2) is southern Alaska being extruded westward?; (3) what is the role of the newly discovered Bering plate?; and (4) what are the locations and relationships of the various plate boundaries in the region? They conclude that a wide zone of diffuse deformation defines the boundaries between the North America, Pacific, and Bering plates, and that the relative motion between these plates may be the source for much of the modern deformation.
Cosmogenic burial ages reveal sediment reservoir dynamics along the Yellow River, China
Xiaofei Hu et al., Key Laboratory of Western China's Environmental Systems, Ministry of Education, Lanzhou University, Lanzhou 730000, P.R. China; doi: 10.1130/G32030.1.
Understanding the source, transport, and ultimate fate of sediment eroded from active mountain belts remains a first-order challenge to geoscientists. Recently, the development and application of cosmogenic isotopes, produced in rocks near Earth's surface, has begun to provide new tools with which to track the erosion and transport of sediment. Xiaofei Hu of Lanzhou University and colleagues show that discrepancies between the age of fluvial terraces along the Yellow River, near Lanzhou, China, and the expected concentrations of the cosmogenic isotopes 26Al and 10Be, provide a glimpse into the source regions over the past two million years of erosion. Ancient sediments trapped within basins along the margin of the Tibetan Plateau developed reduced 26Al/10Be ratios during burial due to radioactive decay. Rapid incision at approximately 1.7 million years ago by the ancestral Yellow River led to excavation of debris, some of which was sequestered on terrace surfaces. By measuring these ratios today, Hu and colleagues were able to infer where in the source region such sediment was generated. Their results suggest a new way to track sediment over long time scales in active mountain belts such as the Tibetan Plateau.
Reworked microfossils as a paleogeographic tool
Claudius Pirkenseer et al., Dept. of Geosciences, Section Geology-Paleontology, University of Fribourg, Chemin du Musee 6, CH-1700 Fribourg, Switzerland; doi: 10.1130/G32049.1.
Reworked microfossils are generally considered a hindrance when interpreting microfossil assemblages, falsifying biostratigraphic and paleoecologic analysis. Claudius Pirkenseer of the University of Fribourg and colleagues use the inherent biostratigraphic information of reworked microfossils as a tool for reconstructing sedimentary pathways and hinterland evolution. The abundant occurrence of reworked planktonic foraminifera within Oligocene marine deposits in the Upper Rhine Graben, the central part of the European Continental Rift System, represents a unique opportunity for studying the provenance of these allochthonous microfossils in a paleogeographically and lithostratigraphically well-constrained setting. The biostratigraphic ranges of the reworked planktic foraminifera taxa indicate Cretaceous and Eocene ages for the sediment sources. Consequently, the reworking originated from related west alpine source deposits via a northward-draining fluviatile and deltaic system.
Syndeformational antigorite dehydration produces stable fault slip
Linda J. Chernak and Greg Hirth, Dept. of Geological Sciences, Brown University, Box 1846, 324 Brook Street, Providence, Rhode Island 02912, USA; doi: 10.1130/G31919.1.
For more than 30 years, scientists have hypothesized that dehydration reactions in subducting lithosphere promote intermediate-depth earthquakes (quakes that occur at depths from 70 to 300 km for which slip instability is difficult to explain with standard frictional sliding models). This interpretation was strongly influenced by the results of experiments conducted at temperatures above the thermal stability of serpentinite--a rock type found near the surface of oceanic lithosphere. Linda J. Chernak of Brown University and colleague Greg Hirth present new experimental results that challenge the direct link between dehydration reactions and earthquakes. They conducted experiments during which samples were first deformed within the serpentine stability field; temperature was then increased (at constant pressure) to cross the dehydration temperature while samples continued to deform. They found that sample strength decreased significantly at the onset of dehydration; however, the weakening was stable and did not promote unstable fault slip. Chernak and Hirth documented that the sample unloading slope depends on the ratio between the rate of temperature increase (which controls the rate of reaction) and the deformation rate--a parameter that they suggest can be scaled to geologic conditions. Importantly, they also document that the sample strength increases with increasing deformation rate, a scenario that inhibits unstable fault slip.
The newly confirmed Luizi impact structure, Democratic Republic of Congo--Insights into central uplift formation and post-impact erosion
Ludovic Ferriere et al., Natural History Museum, Burgring 7, A-1010 Vienna, Austria; doi: 10.1130/G31990.1.
Ludovic Ferriere of the Austria's Natural History Museum and colleagues report on the large, ~17-km-diameter Luizi structure, located in the remote and politically tumultuous Democratic Republic of Congo. Based on their expedition, the first to this site in almost a century, they were able to find shatter cones and shocked quartz grains, which are rock features only found in impact structures, thus, allowing them to confirm the meteorite impact origin of the crater. Luizi is the first confirmed meteorite impact structure in Central Africa, and also the largest, best-preserved impact crater to be discovered in the past several years. This finding brings the number of known impact craters on Earth to 182. Because of its preservation state and the shape of the structure, with an inner ring, the Luizi crater also provides insights into the formation of mid-sized impact craters on Earth. Ferriere and colleagues estimate that the crater was made by the collision of a kilometer-wide asteroid with a speed of about 72,000 km per hour. However, based on their investigations, they can only say that the crater is younger than about 575 million years, the age of the rocks that have been excavated.
Holocene fluid venting at an extinct Cretaceous seamount, Canary archipelago
Andreas Kluegel et al., Universitat Bremen, Fachbereich Geowissenschaften, Postfach 330440, D-28334 Bremen, Germany; doi: 10.1130/G32006.1.
Andreas Kluegel of Universitat Bremen and colleagues describe samples retrieved from greater than 3000-m depth from Henry Seamount, a 126-million-year-old submarine volcano near the western Canary Islands. The samples include shells from clams known to live at fluid discharge sites, a deep-sea coral, volcanic rocks, and fragments of barite (barium sulfate). These samples and the rock compositions provide evidence for the discharge of hydrothermal fluids during the past 10,000 years, which is surprising, because Henry Seamount is long since extinct. This is the first documented finding of hydrothermal activity and related shells at the Canary archipelago. Recharge probably occurred through the submarine flank of a nearby volcanic island (25-30 km distant), where seawater can migrate into the old oceanic crust and become heated. The driving force for this hydrothermal circulation may be the increased heat flow from Canary archipelago magmatism. Kluegel and colleagues confirm earlier concepts of the importance of seamounts for cooling of the oceanic crust, chemical exchange between crust and ocean, and marine ecosystems. Their findings also contribute to the quest for hydrothermally active seamounts and represent a significant advance in the existing knowledge of the extent of such activity.
Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia?
S.L. Loewy et al., Dept. of Geology, California State University, Bakersfield, California 93311, USA; doi: 10.1130/G32029.1.
S. L. Loewy of California State University and colleagues use new lead (Pb) isotopic data from 1.1-billion-year-old rocks from Coats Land, East Antarctica, to constrain the positions of Laurentia (ancestral North America) and Kalahari (ancestral southern Africa) in the 1-billion-year-old supercontinent, Rodinia. These Coats Land rocks are identical in age to both the Keweenawan and Umkondo large igneous provinces of North America and southern Africa, respectively. Comparison of the isotopic compositions of the Coats Land rocks with those of the Keweenawan and Umkondo rocks link Coats Land with the Keweenawan province, and suggests that the Coats Land block was a piece of Laurentia near west Texas 1.1 billion years ago. Furthermore, the Coats Land block was transferred to Kalahari during a 1-billion-year-old collision between Laurentia and Kalahari. Paleomagnetic data permit the Coats Land block to lie close to this part of Laurentia 1.1 billion years ago, and allow juxtaposition of Kalahari and southern Laurentia 1.0 billion years ago. Based on this reconstruction, Laurentia collided with Kalahari along Antarctica's Maud mountain belt, which would represent a continuation of the 1-billion-year-old Grenville mountain belt of eastern and southern Laurentia.
Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so
E.S. Jennings et al., Dept. of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK; doi: 10.1130/G32037.1.
Detrital zircon from sediments is frequently used in large-scale crustal evolution studies because it can be precisely dated and is highly resistant against weathering and abrasion. However, the original rocks in which the detrital zircon crystallized are no longer preserved. This significantly restricts scientists' ability to read the geologic archive, especially for early periods of Earth's history for which zircon grains are the only witnesses. Different host rock types cannot be identified by the chemical composition of zircon. E.S. Jennings of the University of Bristol and colleagues demonstrate that inclusions of other minerals in the zircon (i.e., apatite and biotite) are indeed diagnostic for the composition of the magmatic host rock. Apatite and biotite inclusions in zircon can tell whether the zircon originated from a mafic rock or from granite. Mafic magmas are typically derived from the mantle and form the most primitive crust (similar to the modern ocean floor), while the formation of granite requires several steps of melting and/or crystallization, as seen in the modern continents. These results therefore provide the tools to find evidence for or against the existence of granitic crust early in Earth's history, and hence for the birth of the continental crust.
A 100-yr record of ocean temperature control on the stability of Jakobshavn Isbrae, West Greenland
Jeremy Lloyd et al., Dept. of Geography, Durham University, South Road, Durham DH1 3LE, UK; doi: 10.1130/G32076.1.
Recent observations have identified a significant acceleration in the production of icebergs and the retreat of many glaciers from the Greenland Ice Sheet that currently reach the sea. This recent increase in ice discharge has led to an increase of greater than 0.25 mm/yr in the Greenland Ice Sheet contribution to sea-level rise. A strong link to climate change has been suggested with evidence for changes in ocean temperatures reaching the ice sheet margin in several of Greenland's fjords over the past few years. However, an important question is whether this recent behavior is indeed a response to recent global warming or part of natural climate variability. Jeremy Lloyd of Durham University and colleagues investigated Jakobshavn Glacier, one of Greenland's largest glaciers, that drains ~7% of the ice sheet. Based on marine organisms preserved in ocean sediments, Lloyd and colleagues show a close correlation between ocean temperature and the position of the ice margin of Jakobshavn Glacier over the past 100 years. They also show a close coupling between ocean temperatures in West Greenland and the broader ocean circulation of the North Atlantic, and their results suggest that this close coupling between the oceans and the ice sheets should be fully assessed in future projections of sea-level change.
Neoproterozoic aragonite-dolomite seas? Widespread marine dolomite precipitation in Cryogenian reef complexes
Ashleigh v.S. Hood et al., School of Earth Sciences, University of Melbourne, Parkville, Victoria 3051, Australia; doi: 10.1130/G32119.1.
The oceans in which the first animals evolved, around 650 million years ago, were very different from those of today. Those ancient oceans were almost certainly rich in elements like iron, but lacking in oxygen. However, there is much that is not known about the chemistry of past oceans. Recently, 650-million-year-old reefs have been discovered in the northern Flinders Ranges, South Australia. New research has uncovered minerals in cavities within these reefs that have grown directly from ancient seawater. Dolomite, a calcium magnesium carbonate mineral, appears to have precipitated from these oceans. This is very different from the situation scientists see in modern environments, where dolomite almost never forms in coral reefs. The reasons for the growth of dolomite in this ancient seawater are not well understood, but may be related to the peculiar climatic conditions that Earth experienced at this time. These ancient reefs grew in the period of geological history known as the Cryogenian when the Earth was largely covered in ice.
Plant-wax hydrogen isotopic evidence for postglacial variations in delivery of precipitation in the monsoon domain of China
Osamu Seki et al., Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, Sapporo 060-0819, Japan; doi: 10.1130/G32117.1.
The Asian Monsoon system, which constitutes one of the most active atmosphere-ocean-land interactions on Earth, dominates the climate and weather of eastern Asia and has shaped its prehistoric and historic cultures. Therefore, better understanding of the mechanisms that force variation in monsoon behavior is a prime interest both scientifically and socially. For this purpose, numerous studies have been conducted to reconstruct precipitation change in the Asian monsoon domain during the Holocene interval. However, different proxies have yielded different paleoclimate records of long-term monsoon rainfall variability. This apparent discrepancy among the monsoon proxies remains an open question. Osamu Seki of Hokkaido University and colleagues apply a novel approach, hydrogen isotope compositions of the plant wax in peat sequence in Eastern Tibet, to reconcile the previous paleoclimate records. Their new data suggest that different proxies record different aspects of monsoonal climate. They conclude that the variability of Holocene monsoon precipitation in south China in the Holocene is rather asynchronous among regions with variable wetness, suggesting that responses of summer precipitation to precessional change in summer insolation in the Northern Hemisphere are regionally variable.
Welsh gold: A new exceptionally preserved pyritized Ordovician biota
Joseph P. Botting et al., Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China; doi: 10.1130/G32143.1.
Few Konservat-Lagerstaetten are known from the Ordovician, and most preserve atypical marginal marine communities. Thus, scientists have little idea of how animals with a low preservation potential evolved during the Great Ordovician Biodiversification Event. Joseph P. Botting of the Chinese Academy of Sciences and colleagues report the newly discovered Llanfawr Mudstones Lagerstaette from the basal Sandbian (Late Ordovician) of central Wales, UK. This biota, which has been studied through X-ray radiography and microtomography, is dominated by sponges. It also includes cnidarians (the oldest known solitary hydroid), arthropods, priapulids, various worm-like forms, entoproct-like organisms, and a variety of enigmatic fossils. The fauna includes taxa that are rarely preserved even in exceptional fossil biotas, and offers the potential for a new perspective on Ordovician ecosystems. The dominantly filter-feeding assemblage resembles modern abyssal sponge-dominated communities, although it was formed in much shallower water. The unusual Llanfawr Mudstones fauna shows that Ordovician ecological development was considerably more advanced in offshore environments than the mineralized fossil record implies.
Fungal virulence at the time of the end-Permian biosphere crisis?
Henk Visscher et al., Laboratory of Palaeobotany and Palynology, Faculty of Science, Utrecht University, Budapestlaan 4, 3584CD Utrecht, Netherlands; doi: 10.1130/G32178.1.
One of the most remarkable phenomena related to worldwide ecological crisis and associated mass extinction at the Permian-Triassic transition, about 250 million years ago, is the prolific occurrence of filamentous microfossils in latest Permian sediments. Earlier, these chains of cells, known as Reduviasporonites, have been controversially identified as either fungi or algae. A new appraisal of the microfossils now clarifies the enigma. Henk Visscher of Utrecht University and colleagues have discovered that the fossil remains show near-identity to resting structures of modern soil-borne fungi, such as Rhizoctonia. These fungi produce hyphae of robust-walled cells, which may fuse together into aggregates called sclerotia. The end-Permian crisis is marked by a dramatic decline of woody vegetation. Previously, mass occurrences of Reduviasporonites had been ascribed to wood-rotting fungi utilizing the excessive abundances of dead wood. Visscher et al.'s findings of Rhizoctonia-like resting structures now suggest a much more aggressive role for fungi. Modern Rhizoctonia include ubiquitous plant pathogens, causing root, stem, and foliar diseases in many plants. Pathogenic fungi are particularly aggressive on plants already weakened by environmental stress. Therefore, in harmony with patterns of present-day forest mortality, it is likely that fungal disease had accelerated latest Permian woodland deterioration, initially triggered by the adverse environmental effects of massive volcanic activity in Siberia.
A 7000-yr perspective on volcanic ash clouds affecting northern Europe
Graeme T. Swindles et al., School of Geography, University of Leeds, Leeds LS2 9JT, UK; doi: 10.1130/G32146.1.
The ash cloud resulting from the A.D. 2010 eruption of Eyjafjallajökull caused severe disruption to air travel across Europe, but as a geological event, it is not unprecedented. Graeme T. Swindles of the University of Leeds and colleagues present the first comprehensive compilation of sedimentary and historical records of ash-fall events in northern Europe, spanning the past 7000 years. In the past 1000 years, volcanic ash clouds reached Northern Europe with a mean return interval of 56 plus or minus 9 years (the range of return intervals is between 6 and 115 years). Probabilistic modeling using the ash records for the last millennium indicates that for any 10-year period there is a 16% probability of a tephra fallout event in Northern Europe. These values must be considered as conservative estimates due to the nature of tephra capture and preservation in the sedimentary record.
Aggregation-dominated ash settling from the Eyjafjallajökull volcanic cloud illuminated by field and laboratory high-speed imaging
J. Taddeucci et al., Istituto Nazionale di Geofisica e Vulcanologia, Dept. of Seismology and Tectonophysics, Via di Vigna Murata 605, 00143 Rome, Italy; doi: 10.1130/G32016.1.
The 2010 Eyjafjallajökull eruption in Iceland strikingly underlined the vulnerability of our globalized society to ash-loaded clouds from volcanic eruptions. Hazards posed by such clouds are mitigated by forecasting their path in the atmosphere, but limited knowledge on how ash settles from the cloud often frustrates this task. J. Taddeucci of Italy's National Institute of Geophysics and Volcanology and colleagues show how, for the first time, high-speed videos collected under the Eyjafjallajökull eruption cloud (at thousands of frames per second) can be used to study the settling of volcanic ash from eruption clouds. Analysis of the videos revealed ash particles settling as small clusters. Clusters fall from the plume earlier than single particles, increasing the amount of ash accumulating on the ground close to the volcano and reducing the ash load of the wind-blown cloud. Comparing the videos collected on the volcano with similar videos performed in the laboratory, Taddeucci and colleagues conclude that, at Eyjafjallajökull, clustering was the dominant process removing particles from the cloud, increasing ash settling close to the volcano by a factor of ten. Monitoring future eruptions by high-speed video will significantly improve scientists' capability to forecast and mitigate the risks posed by eruption clouds.
Provided by Geological Society of America