|16 April 2014
GSA Release No. 14-28
Tubular features in 15 million-year-old impact glass from the Ries Impact Structure, Germany. See related article by H.M Sapers et al., http://dx.doi.org/10.1130/G35293.1.
Scratching the Surface: Microbial Etchings in Impact Glass and the Search for Life on Mars
New GEOLOGY articles posted online ahead of print 10 and 15 April 2014
Boulder, Colo., USA – Haley M. Sapers and colleagues provide what may be the first report of biological activity preserved in impact glass. Recent research has suggested that impact events create novel within-rock microbial habitats. In their paper, "Enigmatic tubular features in impact glass," Sapers and colleagues analyze tubular features in hydrothermally altered impact glass from the Ries Impact Structure, Germany, that are remarkably similar to the bioalteration textures observed in volcanic glasses.
The authors note that mineral-forming processes cannot easily explain the distribution and shapes of the Ries tubular features; therefore, they suggest that the tubules formed by microbes etching their way through the impact glass as they excreted organic acids.
A meteorite impact into a water-rich target such as Earth or Mars has the potential to generate a post-impact hydrothermal system. Impact structures, especially post-impact hydrothermal systems, represent an understudied habitat with potential relevance to early life and the evolution of early life on Earth. Understanding the biological significance of impact products such as impact glass on Earth will better inform the search for evidence of life and past life on other terrestrial planets such as Mars.
Highlights of all the latest GEOLOGY postings are provided in detail below. Topics include
1. A second paper discussing the results of meteorite impacts, this time finding preserved flora and other organic compounds;
2. The downfall of Cahokia, the largest Native American settlement north of Mexico;
3. How the 2011 Mineral, Virginia, USA, earthquake (5.8 magnitude) has shed light on the fact that the Eastern North American Margin is not as tectonically "dead" as you might think;
4. Discovery of a "living fossil";
5. "Middle-aged" Earth; and
6. Glacial events in Africa.
GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Enigmatic tubular features in impact glass
H.M. Sapers et al., Centre for Planetary Science and Exploration, Dept. of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35293.1.
Preserved flora and organics in impact melt breccias
P.H. Schultz et al, Dept. of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. Published online 15 Apr. 2014, http://dx.doi.org/10.1130/G35343.1.
Impacts generate intense shocks that heat, melt and vaporize rock. Yet impact products also can capture and preserve fragile plant matter down to the cellular level, only a few microns across. These plant materials were discovered trapped inside impact melts generated by separate impacts into thick, soft sediments dating back nine million years. Even more surprising, they retained organic compounds, including remnants of chlorophyll and related photosensitive pigments in chloroplasts. Similar sediments also cover much of Mars, and authors P.H. Schultz and colleagues propose that impact materials accumulated there may hold key signatures for past life, otherwise lost over billions of years.
A record of sustained prehistoric and historic land use from the Cahokia region, Illinois, USA
Samuel E. Munoz et al., Dept. of Geography, University of Wisconsin–Madison, 550 North Park Street, Madison, Wisconsin 53706, USA. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35541.1.
The downfall of Cahokia, the largest Native American settlement north of Mexico, likely coincided with a large flood of the Mississippi River. Researchers had previously believed that flooding may have played a role in Cahokia’s history, but little evidence of flooding was present in archaeological contexts. Samuel E. Munoz and colleagues at the University of Wisconsin-Madison and Washington University analyzed lake sediment cores from Horseshoe Lake next to Cahokia. The authors examined changes in sediment characteristics alongside botanical and geochemical remains. They found that distinct layers of sediment were deposited in the lake by large floods from the Mississippi River, with the largest flood over the last two thousand years coinciding with the beginning of Cahokia’s decline around 1200 AD. They also found that the clearance of forests for agriculture began nearly 600 years before the emergence of Cahokia, much earlier than previously thought. The authors suggest that prehistoric peoples had profoundly shaped vegetation of the Cahokia region for centuries before the arrival of European explorers, but that the ancient inhabitants of the Midwestern U.S. were also vulnerable to flooding and other natural disasters.
Volcanoes of the passive margin: The youngest magmatic event in eastern North America
Sarah E. Mazza et al., Dept. of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall (0420), Blacksburg, Virginia 24061, USA. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35407.1.
The 2011 Mineral, Virginia, USA, earthquake (5.8 magnitude) has shed light on the fact that the Eastern North American Margin (ENAM) is not as tectonically "dead" as one might traditionally think. Preserved in the Appalachian Valley and Ridge Province of Virginia and West Virginia is a suite of Eocene (about 48 million years old) volcanic rocks aligned structures reactivated in the Cenozoic These volcanic rocks are the youngest known magmatic even that has occurred along the ENAM, and provide the only window into the most recent mantle processes. This study presents new findings that confirm an age of 48 million year. The authors suggest that melting occurred due to delamination of dense instabilities in the lithosphere which allowed for the mantle to rise and melt.
Living fossils in the Indo-Pacific warm pool: A refuge for thermophilic dinoflagellates during glaciations
Kenneth N. Mertens et al., Research Unit for Palaeontology, Ghent University, Krijgslaan 281 S8, Ghent 9000, Belgium. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35456.1.
Kenneth N. Mertens and colleagues report the unique discovery of the marine dinoflagellate Dapsilidinium pastielsii from Southeast Asia, notably the Indo-Pacific Warm Pool (IPWP). This unicellular species, with planktonic and benthic stages, was previously thought to have become extinct within the early Pleistocene. It evolved more than 50 million years ago and is the last survivor of a major early Cenozoic lineage. The discovery of living D. pastielsii in the IPWP suggests that this stable environment served as an important refuge for thermophilic dinoflagellates, and its disappearance from the Atlantic following the early Pleistocene implicates cooling. This study highlights the importance of refugia in plankton biogeography and broadens their relevance as hotspots of biodiversity. It explains how biostratigraphic ranges in the Pacific can significantly extend those established for the Atlantic. Our discovery of D. pastielsii as a "living fossil" in the IPWP highlights the potential for refugia to harbor species likely to expand their distributional range in a warming world.
Thick sediments beneath Greenland's ablation zone and their potential role in future ice sheet dynamics
Fabian Walter et al., Institut des Sciences de la Terre, Université de Grenoble I, CNRS, BP 53, 38041 Grenoble Cedex 9, France. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35492.1.
In this study, we present an analysis of ground shaking of far-away earthquakes recorded on the Greenland ice sheet. On their way from the earthquake source to the seismometer, the earthquake shocks are modified when they pass interfaces between the Earth's mantle and crust and the overlying ice sheet cover. We can thus use the on-ice seismic records to study the ice sheet bed. This reveals for the first time the presence of a thick sediment layer beneath the ice sheet periphery. We suggest that these sediments may sustain a widespread 'soft bed', across which the ice sheet can move with relatively little resistance. However, the deformability of such a soft layer strongly depends on bed topography and the presence and pressure of basal water. Since surface runoff to the ice sheet base is increasing during current climate change, we suggest that basal sediments play an important role in future developments of ice sheet dynamics.
Earth's middle age
Peter A. Cawood and Chris J. Hawkesworth, Dept. of Earth Sciences, University of St Andrews, Irvine Building, North Street, St. Andrews, Fife KY16 9AL, UK. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35402.1. Open Access.
Earth's middle age, extending from 1.7 to 0.75 billion years ago, is characterized by environmental, evolutionary, and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of preserved passive continental margins, relatively stable ocean and atmospheric compositions, a lack of orogenic gold and volcanic-hosted massive sulfide mineral deposits, and an absence of glacial deposits and iron formations. In contrast, certain types of intrusive igneous rocks (anorthosites and kindred bodies) are well developed and major pulses of molybdenum and copper mineralization, including the world's largest examples of these deposits, are features of this period. These trends are attributed to a relatively stable continental assemblage that was initiated by about 1.7 billion years ago that continued until its dispersal at around 75 million years ago. The overall low abundance of passive continental margins is consistent with a stable continental configuration, which also provided a framework for environmental and evolutionary stability. A series of convergent plate margins developed along the edge of the stable continental assembly. Abundant anorthosites and related rocks developed inboard of the plate margin. Their temporal distribution appears to link with the secular cooling of the Earth’s mantle in which the overlying continental lithosphere was strong enough to be thickened and to support the emplacement of large plutons into the crust, yet the underlying mantle was still warm enough to result in widespread melting of the lower thickened crust.
Expanded glaciers during a dry and cold Last Glacial Maximum in equatorial East Africa
Meredith A. Kelly et al., Dept. of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03750, USA. Published online 15 Apr. 2014, http://dx.doi.org/10.1130/G35421.1.
Glaciers atop the world's highest tropical mountains are among the most sensitive components of the cryosphere, yet the climatic controls that influence their fluctuations are not fully understood. The research by Meredith Kelly and colleagues focused on glaciers in the Rwenzori Mountains, located on the border of Uganda and the Democratic Republic of Congo. They applied a dating method known as "surface exposure dating" based on the measurement of the nuclide 10Be to determine the ages of ridges of sediment (moraines) that mark past glacial extents. It is the first time that 10Be surface exposure dating has been applied to examine past glacial extents in Africa. The results show that Rwenzori glaciers experienced significant growth between ~24,300 and 20,100 years ago during the height of last ice age. A comparison with nearby paleoclimate records shows that glaciers advanced contemporaneously with regionally dry and cold conditions. Glacial recession began by ~20,100 years ago when paleoclimate records show an increase in temperature. These results suggest that, on millennial time scales, past fluctuations of Rwenzori glaciers were strongly influenced by air temperature.
Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings
Peter Koděra et al., Dept. of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia. Published online 10 April 2014, http://dx.doi.org/10.1130/G35270.1.
Porphyry gold deposits are a relatively rare and are hosted by magmatic intrusions in volcanic terrains. In contrast to more common and important porphyry deposits, they are devoid of economically mineable copper and molybdenum. This paper by Peter Koděra and colleagues discusses findings via microanalytical data on tiny inclusions of paleofluids in quartz veinlets from the recently discovered Biely Vrch porphyry gold deposit (Western Carpathians, Slovakia).
Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt
Barbara J. Tewksbury et al., Dept. of Geosciences, Hamilton College, Clinton, New York 13323, USA. Published online 15 Apr. 2014, http://dx.doi.org/10.1130/G35362.1.
Seismic investigations over the 15 last years in marine basins around the world have revealed the common occurrence of sets of extensional faults that intersect to form networks of large polygons, each hundreds of meters to more than a kilometer across. Although polygonal faults been studied remotely in over 100 basins worldwide, extensive on-land exposures that lend themselves to field study have remained elusive. We report on discovery of a polygonal fault system in chalk near Farafra Oasis, Egypt. A unique combination of regional structure, topography, and a hyperarid climate has resulted in almost continuous exposure of polygonal faults over an area of nearly 1000 square kilometers. We first discovered the faults in high-resolution satellite imagery in Google Earth, where their polygonal pattern is spectacularly revealed. Our subsequent field work established contemporaneous activity of faults of different orientations, provided evidence for the role of elevated fluid pressures in initiating the faults, and documented the small-scale features of polygonal faults that are difficult, if not impossible, to study remotely using seismic data. Our field observations provide new insights into polygonal fault systems in marine environments and may assist in the recognition of on-land exposures elsewhere.
The contribution of mountains to global denudation
Isaac J. Larsen et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35136.1.
It has been proposed that mountain uplift regulates global climate on geological timescales because high erosion rates in mountains lead to high rates of chemical weathering of minerals, which can lead to sequestration of atmospheric carbon dioxide. However, mountains occupy a small fraction of Earth’s land area. Hence it is unclear whether changes in the proportion of mountains can substantially change global-scale erosion and weathering rates. Understanding the role mountains currently play in global erosion and weathering can yield insight into whether the uplift of mountain ranges in the geological past may have influenced the carbon cycle and climate. Global-scale denudation rates were modeled using slope data from a digital elevation model. The results predict that more than half of the total denudation and 40% of chemical denudation occur on the steepest 10% of Earth's surface. Mountains, though small in total land area, are the dominant location for erosion and weathering on Earth's surface. These results are consistent with the view than mountains are an important link in the geological carbon cycle and suggest that small changes in the proportion of mountains on Earth's surface had the potential to perturb global biogeochemical cycles.
Reducing time-scale uncertainty for the Devonian by integrating astrochronology and Bayesian statistics
David De Vleeschouwer, Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium; and Andrew C. Parnell, School of Mathematical Sciences, Complex and Adaptive Systems Laboratory, University College Dublin, Dublin 4, Ireland. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35618.1.
Geologists use geological timescales day in, day out. To know when a particular dinosaur went extinct, or to determine when the first forests appeared on Earth. It is therefore in the interest of every geologist that geologic time scale is precise and accurate. In this paper, we demonstrate that the geologic time scale that is used by the majority of geologists has overoptimistic uncertainties. To overcome this problem, we apply an alternative method, Bchron, to the Devonian time scale. Bchron explicitly takes into account the correct estimation of the time scale uncertainties. The Devonian was chosen as an extremely interesting case study, as three quarters of all species on Earth died out during the Frasnian-Famennian mass extinction event. In this study, we come up with a new, and better constrained, numerical age estimate of the Frasnian-Famennian boundary about 374 million years ago.
Magmatic activity and plate motion during the latent stage of Midcontinent Rift development
Nicholas L. Swanson-Hysell et al., Dept. of Earth and Planetary Science, University of California–Berkeley, Berkeley, California 94720, USA. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35271.1.
Volcanic rocks in the Lake Superior region of North America records a significant tectonic event that nearly split the continent apart 1.1 billion years ago and created a feature known as the Keweenawan Midcontinent Rift. Understanding this history of rifting and of plate motion requires precise determinations of rock units. Despite the importance of rocks on the eastern shores of Lake Superior at a location known as Mamainse Point to resolve this history, geologists have not been able to get a handle on its age. We report the discovery of a new rock unit containing zircon crystals that we dated precisely with U-Pb geochronology. This new age demonstrates that some of the lava flows at Mamainse Point erupted during an interval of time during which there was reduced volcanic activity elsewhere in the rift. This result clarifies the history of reversals of the geomagnetic field and this time and supports interpretations that North America was moving rapidly towards the equator throughout the time period of rifting.
Timing of the Brunhes-Matuyama magnetic polarity reversal in Chinese loess using 10Be
Weijian Zhou et al., State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China; and Xi'an AMS Center, Institute of Earth Environment and Xi'an Jiaotong University, Xi'an 710061, China. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35443.1.
Thousands of studies in geology, ocean and climate science have used the timing of the Brunhes-Matuyama (B-M) geomagnetic reversal for age control. Chinese loess is a major archive of Quaternary terrestrial climate records, yet this reversal occurs there about 25-40 thousand years prior to the axial dipole reversal age found in marine sediments. This has led to a longstanding debate on how to correlate the climate records found in these two major sediment archives. This asynchroneity has been attributed by some to post-depositional magnetic overprinting of loess, while others have argued that it is due to errors in the loess timescale. In our manuscript, we present a new method to extract reproducible records of geomagnetic field intensity using loess 10Be. The result shows that a pronounced minimum in field intensity -- a requirement for dipole field reversal -- is recorded in two separate loess records about 780,000 years ago. This timing is synchronous with the B-M reversal timing seen in marine records, verifying the standard loess timescale as correct. Because of the very long half-life of 10Be (about 1.4 million years), the new method shows promise as a means for generating long high-resolution records of geomagnetic field intensity.
How collision triggers backarc extension: Insight into Mediterranean style of extension from 3-D numerical models
Valentina Magni et al., Dept. of Earth Sciences, University of Durham, Durham DH1 3LE, UK. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35446.1.
Backarc basins are features that form in a subduction system and are associated with extensional regime in the overriding plate. Their formation and evolution is linked to the dynamics of subduction. A well-documented example of backarc basins opening is the Central Mediterranean area, which is characterized by short-lived episodes of fast spreading. The underlying reasons for this episodicity are obscured by the complexity of this subduction system, with the presence of multiple continental blocks entering the subduction zone. We present results from three-dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and its episodic style of spreading. The presence of continental blocks within the subducting plate has an important effect on the subduction velocity because where a continent (that is less dense than the surrounding material) enters the trench it resists to sinking in the mantle and locally subduction slows. These results show that these lateral variations produce efficient backarc extension, because they cause deformation to be very localized within the overriding plate. We observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin and, later, to the rupture of the slab at the depth (formation of slab windows). We suggest that the observed episodic trench retreat behavior in the Central Mediterranean is caused by the formation of slab windows.
Experimental evidence for fluvial bedrock incision by suspended and bedload sediment
Joel S. Scheingross et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Published online 10 Apr. 2014, http://dx.doi.org/10.1130/G35432.1.
River erosion of bedrock shapes the surface of the Earth and other planets, creating landscapes which awe the public from raging waterfalls to deep, narrow canyons. Water alone is not enough to erode rivers; instead, bedrock erosion in rivers occurs primarily through impacts from transported sediment. Sediment can be transported in two distinct ways, either as bedload, where particles roll and hop along the channel floor, or as suspended load, where particle motion is controlled by the water turbulence. We performed controlled laboratory experiments to test the ability of suspended sediment to erode bedrock. Our experimental results show clear evidence for the erosion by suspended sediment (counter to the most commonly used river erosion theory), and suggest that suspended-sediment erosion will dominate the total erosion signal during large floods and in steep mountain streams. These results have significant implications for the rate and style at which landscapes evolve over geologic time.
Aspect-dependent variations in regolith creep revealed by meteoric 10Be
Nicole West et al., Dept. of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Published online 15 Apr. 2014, http://dx.doi.org/10.1130/G35357.1.
From the abstract: Although variations in insolation and emergent feedbacks among soil moisture, vegetation, and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect, few studies have directly quantified the efficiency of regolith transport along hillslopes of opposing aspect. We utilize meteoric 10Be concentrations in regolith to determine mass flux along equatorial-facing and polar-facing hillslopes in three forested upland watersheds in and adjacent to the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania (USA). In combination with regolith depth measurements and high-resolution topography, these fluxes allow us to evaluate transport rate laws and the efficiency of regolith creep. Concentrations of meteoric 10Be in regolith along six separate transects imply that regolith flux is similar along all hillslopes, despite differences in topographic gradient and regolith thickness. Comparison of flux with regolith depth and topographic gradient reveals that transport depends on regolith depth, and that regolith creep is twice as efficient along low-gradient, south-facing slopes with thin regolith as compared to steep, north-facing slopes mantled with thicker regolith. We suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freeze-thaw events between hillslopes of opposing aspect.
A record of the Jurassic massive plate shift from the Garedu Formation of central Iran
Massimo Mattei et al., Dipartimento di Scienze, Università degli Studi Roma Tre, 00146 Rome, Italy. Published online 15 Apr. 2014, http://dx.doi.org/10.1130/G35467.1.
From the abstract: Modern generations of apparent polar wander paths (APWPs) show the occurrence in North American and African coordinates of a major and rapid shift in pole position (plate shift) during the Middle to Late Jurassic (175 to 145 million years ago) that alternative curves from the literature tend to underestimate. This Jurassic massive polar shift (JMPS), of vast and as-yet unexplored paleogeographic implications, is also predicted for Eurasia from the North Atlantic plate circuit, but Jurassic data from this continent are scanty and problematic. Here we present paleomagnetic data from the Kimmeridgian-Tithonian (upper Jurassic) Garedu Formation of Iran, which was part of Eurasia since the Triassic… Our results illustrate (1) the potent, but often neglected, control that plate motion (continental drift and/or true polar wander) across zonal climate belts exerts on the genesis of sedimentary facies; and (2) the importance of precisely controlled paleogeographic reconstructions for tectonic interpretations, especially during times of fast plate motion like the Jurassic. As a suggestion for future research, we predict that the adoption of Eurasian reference paleopoles incorporating the JMPS may lead to a reconciliation (or reinterpretation) of existing geologic and paleomagnetic data regarding the deformation history of central Asia.