May 2009 Geology media highlights

Boulder, CO, USA – Hot topics include (1) opposition to the idea that chevron-shaped dunes are indicative of mega-tsunamis; (2) discovery of a complex microbial community that extends the fossil record of cavity-dwelling life by more than 1.5 billion years; (3) documentation of nanoscale, respirable cristobalite fibers in volcanic ash from Chaiten volcano and the likely adverse health effects; (4) closing the gap between Earth’s first animals and fossil and geochemical evidence; and (5) the largest trilobites ever found.

New evidence from seismic imaging for subduction during assembly of the North China craton
Tianyu Zheng et al., Seismological Laboratory (SKL-LE), Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Pages 395-398.

The “frozen-in” information in the crust plays an important role in improving our understanding of cratonic formation and evolution, and of plate tectonics in the Precambrian. The Trans-North China orogen (TNCO) is a continental-to-continental collision belt generated by the assembly of the North China craton (NCC). The mechanism and modality of the collision are disputed. Here, Zheng et al. present a seismic image of the Western Block and the TNCO of the NCC-derived using receiver function analysis of the teleseismic records from a dense array. A low-velocity zone extending from the middle crust to the Moho is interpreted as a remnant of upper-middle crustal material associated with westward-dipping subduction beneath the Western Block of the NCC. Crustal uplifting and magmatic underplating resulting from subsequent tectonic events were responsible for modifying the remaining subduction architecture. The western boundary of the TNCO is located west of the boundary earlier identified by surface investigation. The results, combined with previous seismic imaging in the eastern NCC, provide insight into the amalgamation of the Eastern and Western Blocks and the subsequent tectonic deformation of the NCC.

Reconstructing Earth’s surface oxidation across the Archean-Proterozoic transition
Qingjun Guo et al., State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China. Pages 399-402.

Earth’s atmosphere experienced a substantial increase in free oxygen during the Archean-Proterozoic transition (2650-2100 million years ago), termed the Great Oxidation Event. Although the atmospheric oxygen content was still far less than today, surface environments were affected by oxidation. This change in redox conditions was associated with changes in ocean biogeochemistry. Interestingly, observed geochemical changes appear, in part, to be related also to Earth’s earliest ice age. Guo et al. pursued a multi-geochemical study of a respective rock succession, in order to reconstruct the history of atmospheric oxygenation. Time-series data for multiple sulfur isotopes from carbonate-associated sulfate, as well as sulfides in sediments of the Transvaal Supergroup, South Africa, capture the significant rise of atmospheric oxygen (as well as the protective ozone layer), its subsequent consequences for ocean chemistry and biology, and the loss of atmospheric mass-independent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event.

“Chevrons” are not mega-tsunami deposits — A sedimentologic assessment
Joanne Bourgeois and Robert Weiss, Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Pages 403-406.

Bourgeois and Weiss challenge the debated hypothesis that impact-generated tsunamis have deposited giant chevron-shaped dunes (“chevrons”) on many of the world’s coastlines. This highly speculative interpretation made one of its first splashes with a New York Times article (14 Nov. 2006) by Sandra Blakeslee and continues to capture public fascination. The “Holocene Impact Working Group” has argued, primarily outside the peer-reviewed literature, that there are many large, v-shaped dunes in the world that were deposited by mega-tsunamis, generated by oceanic asteroid impacts in the past 10,000 years. Bourgeois and Weiss make a counter-argument using the basic physics of tsunamis and sediment transport. They present three lines of argument: (1) the dunes are not limited to coastlines where mega-tsunamis could strike; (2) the orientation of the dunes does not match the travel direction of modeled tsunami waves; and (3) the giant scale of tsunamis proposed by researchers such as those of the Holocene Impact Working Group would wash out any bedforms such as chevron dunes.

Subglacial bedforms reveal complex basal regime in a zone of paleo-ice stream convergence, Amundsen Sea embayment, West Antarctica
Robert D. Larter et al., British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Pages 411-414.

Larter et al. present the most extensive, continuous area of seafloor sonar imagery collected to date on the Antarctic continental shelf, which reveals the “footprint” of a formerly much more extensive ice sheet. The swath bathymetry data collected on expeditions by the British Antarctic Survey and the German Alfred Wegener Institute show details of former subglacial features on parts of the seabed that are now up to 1600-m deep. The trends and characteristics of these features allow reconstruction of past ice flow paths and provide clues about the processes that enabled fast ice flow. Understanding the processes that control fast ice flow in “ice streams” is important because they account for most ice discharge from the large ice sheets on Antarctica and Greenland, and knowledge of how large ice sheets responded to warming at the end of the last glacial period will help predict how the ice sheets will change in response to future warming. The new data indicate that conditions at the base of the former ice sheet were more complex than previously thought, suggesting much more research will be necessary to reliably predict the future behavior of modern ice sheets.

Sulfur content at sulfide saturation in oxidized magmas
Pedro J. Jugo, Dept. of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6, Canada. Pages 415-418.

Although sulfur is a trace element in most magmas, it influences important magmatic processes. For example, sulfides (reduced sulfur) control the behavior of base and precious metals (such as copper, nickel, gold, palladium, platinum, etc.). Therefore, the ability to predict the chemical precipitation of sulfides from a magma is essential in the prediction of where mineral deposits could be located. On the other hand, if magmas are sufficiently oxidized, sulfide is unstable, sulfates become dominant, and no mineral deposits will form. In addition, magmas will tend to accumulate sulfur and metals and release them to the atmosphere during volcanic eruptions. Loss of metals to the atmosphere precludes the formation of ore deposits, but is otherwise inconsequential for geological processes. However, explosive, high-sulfur emissions reaching the stratosphere have strong, short-term impact on global climate, and assessing the impact of high-sulfur volcanism on climate through geologic history requires understanding of how often high-sulfur emissions occur. The study by Jugo links, for the first time, the change in sulfur content as a sulfide-saturated magma becomes progressively oxidized until sulfides are no longer stable. In contrast to long-held views, the study demonstrates that sulfur content increases exponentially with oxidation because of the increasing contribution of dissolved oxidized sulfur (sulfates). The implications are that most magmas related to volcanism in subduction zones have the potential to transport and release more sulfur than previously estimated; therefore, high-sulfur magmatism in geologic history is likely more common than currently assumed. For the formation of mineral deposits, the main implication is that, rather than having an “on-off” system for sulfide or sulfate stability, the transition is continuous. In addition, magma generation from oxidized mantle will enhance the metal transfer from the deep Earth to the crust.

Downdip segmentation of strike-slip fault zones in the brittle crust
Eliza S. Nemser and Darrel S. Cowan, Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. Pages 419-422.

Segmentation within fault zones is widely recognized and can influence earthquake rupture behavior. Both along-strike and downdip segmentation have been observed along fractures and within dip-slip fault zones, whereas only along-strike segmentation has been documented within strike-slip fault zones. Nemser and Cowan report new field evidence of downdip segmentation within a strike-slip fault from the western Salton Trough region of Southern California. In this outcrop example, steeply dipping fault segments step over along mechanically weak stratigraphic units that accommodate bedding-parallel slip. They infer similar geometries at a much larger scale in the form of vertically restricted steep seismicity streaks defined by relocated crustal seismicity data along the nearby southern San Jacinto fault zone. Nemser and Cowan interpret these streaks as evidence of vertically restricted steep fault segments that may be linked by low-angle fault segments (décollements). A lack of spatial correspondence between earthquakes on low-angle fault planes and the locations of inferred décollements suggests that these structures are weak and behave aseisimically. Downdip segmentation of strike-slip faults may be common at a variety of scales; this result may lead to the reinterpretation of fault geometry and kinematics in many strike-slip fault zones.

Evidence for microbial life in synsedimentary cavities from 2.75 Ga terrestrial environments
Birger Rasmussen et al., Dept. of Applied Geology, Curtin University of Technology, Kent St, Bentley, WA 6102, Australia. Pages 423-426.

Sediments deposited in lakes about 2.75 billion years ago preserve finger-like microbial deposits that grew in the roof of cavities that developed shortly after deposition. Analysis of sulfur isotopes in the structures suggests the presence of sulfur-metabolizing bacteria, while the isotopic composition of carbon in organic debris in the adjacent sediments indicates that methane-cycling microbes were present. Evidently, Earth’s ancient land surface supported a complex microbial community, including microbes that inhabited gas-filled hollows in shallow sediments, extending the fossil record of cavity-dwelling life by more than 1.5 billion years. This discovery by Rasmussen et al. provides a new habitat in the search for early life on Earth and an analogue for life on Mars.

Delta-37Cl systematics of a backarc spreading system: The Lau Basin
Graham D. Layne et al., Dept. of Earth Sciences, IIC 1047, Memorial University, St. John’s, Newfoundland A1B 3X5, Canada. Pages 427-430.

Although chlorine is an extremely important agent for the dissolution and mass transfer of many elements by natural waters and brines in Earth’s crust, the details of its recycling into the deep crust and mantle by plate tectonic action are very poorly understood. Of particular interest are the processes by which chlorine trapped in ocean sediments and in seawater-altered ocean floor basalt may be recycled to the lower crust and mantle as part of the “Subduction Factory,” in systems such as the Lau Basin of the western Pacific Ocean. Layne et al. use advanced microanalytical techniques to study the chlorine isotope compositions of lavas returned to the surface during subduction-related volcanism in the Lau Basin in order to learn details of these deep processes, despite the fact that the lavas have often been erupted directly in to the chlorine-rich waters of the modern ocean.

Schwertmannite in wet, acid, and oxic microenvironments beneath polar and polythermal glaciers
R. Raiswell et al., Earth and Biosphere Institute, School of Earth and Environment, Leeds University, Leeds LS2 9JT, UK. Pages 431-434.

Chemical conditions beneath glaciers are difficult to observe and are usually studied by sampling meltwaters emerging from glacial oulets or drill holes. These waters average chemical signals over a large area and cannot record small-scale spatial and temporal variability. By contrast, subglacial sediments contain minute (nanometer-sized) particles of iron oxides that can only have formed in transient geochemical “hotspots.” Raiswell et al. have found minute crystals of schwertmannite, an iron hydroxy-sulfate mineral, in glaciers from the Antarctic and Arctic. Schwertmannite is typically found in acid mine drainage, where it forms by the oxidation of pyrite at low pH. These conditions can also be created in subglacial environments, but only in minute hotspots or “microenvironments.” There, schwertmannite forms rapidly but is very unstable, and its survival requires freezing into ice within 100 years. So, frozen nanoparticle schwertmannite indicates the presence of transient geochemically active microenvironments deep within glacial ice. The formation, preservation, and delivery of nanoparticles of schwertmannite (and iron oxides) into the Southern Ocean may partially relieve iron-limited photosynthesis and assist in the removal of manmade carbon dioxide from the atmosphere.

Formation of cristobalite nanofibers during explosive volcanic eruptions
Martin Reich et al., Departamento de Geologia, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. Pages 435-438.

On 2 May 2008, the Chaiten volcano in Patagonia, Chile, started its first historical eruption. Eruption plumes and ash accumulation have forced the complete and permanent evacuation of about 4,000 people from the town of Chaiten and the closure of airports in both Chile and Argentina. Explosive volcanic eruptions, such as at Chaiten, release large amounts of fine particles into the atmosphere, and previous studies have reported the adverse health effects of “respirable” crystalline silica-ash particles (i.e., particles thinner than a human hair that enter directly into the lungs). Reich et al. used high-magnification microscopy techniques (transmission electron microscopy, TEM) to image the “respirable” particles present in volcanic ash sampled during the first week of the eruption. They obtained high-resolution, near-atomic scale images of crystalline silica identified as cristobalite. The presence of cristobalite is especially relevant, because this mineral has been previously identified as a health threat, raising concerns about adverse health effects of long-term exposure to ash. Reich et al.’s detailed TEM images document, for the first time, that “respirable” cristobalite in volcanic ash forms fibers, wires, and needles at the nanoscale. This information is of utmost importance to better assess the health risks of volcanic ash (and its contained nano-fibrous silica) associated with the short-term inhalation of ash during explosive volcanic eruptions, and also long-term exposure in ash-covered areas.

Chlorine enrichment in central Rio Grande Rift basaltic melt inclusions: Evidence for subduction modification of the lithospheric mantle
M.C. Rowe and J.C. Lassiter, Dept. of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Pages 439-442.

The study by Rowe and Lassiter draws important conclusions regarding magmatism in the central Rio Grande Rift based on microanalytical techniques. Rowe and Lassiter identify a new potential method for identifying the source of the “geochemical fingerprint” of basaltic lavas, and suggest that approx. 40-million-year-old subduction beneath the western United States altered the source for present-day volcanics.

Giant trilobites and trilobite clusters from the Ordovician of Portugal
Juan C. Gutierrez-Marco et. al., Departamento de Paleontologia, Instituto de Geologia Economica (CSIC-UCM), Facultad de Ciencias Geologicas, 28040 Madrid, Spain. Pages 443-446.

Fossils from 465 million years ago recently discovered in Portugal have revealed the huge size reached by trilobites, the most diverse group of extinct marine arthropods. Gutierrez-Marco et al. describe the largest trilobites ever found, which, in life, would have reached up to 90 centimeters (35 inches). This remarkable record suggests evidence of polar gigantism in an area of Gondwana close to the South Pole during the Ordovician. The Portuguese trilobites also show an astounding array of behavioral clustering — with some patches reaching groups of over a thousand specimens — revealing a very diverse social conduct, including hiding from predators and sexual aggregations. This could have played a major role in the undisputed success of this group through the Paleozoic Era. The original discovery site and its fossils are one of the main attractions of the recently established Arouca Geopark in northern Portugal.

Blake Nose stable isotopic evidence against the mid-Cenomanian glaciation hypothesis
Atsushi Ando et al., Dept. of Paleobiology, MRC NHB 121, Smithsonian National Museum of Natural History, Washington, D.C. 20013-7012, USA. Pages 451-454.

A recent debate in paleoclimatic research concerns whether a transient glaciation event occurred across the mid-Cenomanian time interval (about 96 million years ago) despite the prevalence of an extensive greenhouse condition. In a study by Ando et al., this greenhouse glaciation hypothesis is rigorously tested through detailed stable isotope analyses of multiple calcareous microfossil groups from a Blake Nose deep-sea section off northern Florida. A significant advancement is achieved by the establishment of carbon isotope stratigraphy, facilitating precise correlation of their northwest Atlantic oxygen isotope trends with the European sedimentary records of sea-level change. Ando et al. find that the Cenomanian upper ocean oxygen isotope composition remained constant across the duration of rapid sea-level fall. This observation contradicts what is predicted from the glaciation hypothesis.

Low-angle collision with Earth: The elliptical impact crater Matt Wilson, Northern Territory, Australia
Thomas Kenkmann and Michael H. Poelchau, Museum fur Naturkunde, 10115 Berlin, Germany. Pages 459-462.

Nearly all meteorite impact craters on Earth are circular. Elongated crater structures are expected only at impacts at angles lower than 12 degrees from the horizontal. Kenkmann and Poelchau document the first elliptical crater on Earth that provides insights into the mechanisms of crater formation at low angles. The diameter of the Proterozoic Matt Wilson impact structure (Northern Territory, Australia) is 7.5 by 6.3 kilometers, with its long axis trending northeast-southwest. The exposed crater floor shows a preferred stacking of thrust sheets within the central uplift, indicating a material transport top-to-southwest. This is explained by remnant horizontal momentum transferred from the impacting projectile to the target rocks. The Matt Wilson structure provides evidence for the usefulness of structural asymmetries as a diagnostic tool to infer the direction of impact.

Field evidence for climate-driven changes in sediment supply leading to strath terrace formation
Theodore K. Fuller et al., National Center for Earth-Surface Dynamics, University of Minnesota, Minneapolis, Minnesota 55414, USA. Pages 467-470.

Along the South Fork of the Eel River in northern California, the widest and most extensive bedrock terraces are covered with alluvial sediment, which was deposited when the landscape was eroding twice as fast as modern erosion rates. This study by Fuller et al. supports the hypothesis that extensive bedrock terraces are carved during conditions of elevated sediment supply. In addition, dating of terrace deposits reveals that the period of elevated sediment supply correlates in time with increased rates of annual precipitation during the late Pleistocene. Thus, their data suggest a link between climate-driven increases in sediment supply and the carving of bedrock terraces. Finally, a comparison between basin-wide erosion rates and estimated rates of vertical channel incision indicate that local hillslope relief has been increasing over the past 20,000 years.

Early Neoproterozoic origin of the metazoan clade recorded in carbonate rock texture
Fritz Neuweiler et al., Departement de Geologie et Genie geologique, Universite Laval, 1065, Avenue de la Medecine, Quebec, QC G1V 0A6, Canada. Pages 475-478.

The Neoproterozoic interval of “hidden” evolution refers to a gap of unknown duration between the time when animals first evolved (uncertain) and the oldest known fossil or geochemical evidence of animals (latest Neoproterozoic, about 600-650 million years ago). Neuweiler et al. now propose to fill this gap. They describe distinctive, microscopic features in early Neoproterozoic limestone (between 779 and 1083 million years old) from the Northwest Territories of Canada, consisting of highly structured zones with multi-generational arrays of carbonate minerals, secondary voids, and internal sediment. Today, such a texture develops when aragonite crystals precipitate on the decaying connective tissue (collagen) of sponges in sediment on the sea floor. As sponge decay progresses, a complex fabric of calcareous material and voids is produced, which is identical to fabrics in Phanerozoic limestones (less than 542 million years old) containing known sponge body fossils and to the fabrics now reported from the early Neoproterozoic. Collagenous connective tissue is a fundamental character of all metazoans (animals), and so the presence in early Neoproterozoic rocks of a microscopic fabric directly associated with it implies that metazoan-grade organisms existed at that time. These purported ancestral metazoans did not have a canal system that would relate them to sponges, the simplest form of animal known today. Instead, they likely represent a structured consortium of protists in a shared collagenous scaffold. These results push back the earliest geologic evidence for animals by around 200 million years. This timing corroborates results of an integrated phylochronology and supports the concept of a biosphere that persisted through Snowball Earth.

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