Boulder, CO, USA - GEOLOGY articles extract information on forces shaping Earth's surface, solve the puzzle of LIPs on land, trace the leading edges of dispersing continents, expose magmatic plumbing, argue over gold deposits, show how fungi break down rocks, unveil tightly kept secrets about the Amazon River, investigate deep geological structures associated with the 2004 Sumatran earthquake, settle the debate over porphyroblasts, and define neptunian eruptions. GSA Today uncovers the impact of Cenozoic snow melt in the Rockies.
Beyond threshold hillslopes: Channel adjustment to base-level fall in tectonically active mountain ranges
William B. Ouimet et al., Dept. of Geology, Colorado College, Colorado Springs, Colorado 80903, USA. Pages 579-582.
Extracting information about the forces shaping Earth's surface directly from topography is an invaluable tool for all earth scientists, as well as engineers, planners, and managers who are faced with preparing for and dealing with the erosion of mountainous landscapes. Ouimet et al. utilize concentrations of cosmogenic nuclides extracted from river sand on the eastern margin of the Tibetan Plateau to explore relationships between millennial erosion rate, hillslope gradients, and channel gradients. The unique setting of the eastern margin allowed them to cover the majority of the globally known ranges for these topographic metrics with erosion rates ranging over two orders of magnitude, from about 0.03 to 3 mm/year. Their dataset offers insight into how hillslope and river channel morphology changes to accommodate higher rates of erosion and provides a critically needed test for advancing theoretical models of hillslope evolution and bedrock river incision in tectonically active settings.
The 132 Ma Comei-Bunbury large igneous province: Remnants identified in present-day southeastern Tibet and southwestern Australia
Di-Cheng Zhu et al., State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China. Pages 583-586.
A large igneous province (LIP) refers to a large area of Earth's surface covered with great volumes of igneous rocks of basaltic composition (with or without rhyolitic varieties) emplaced in a relatively short time period as a result of extensive decompressive melting of ascending mantle plume heads. Although there are exceptions, generation of many LIPs on land has been linked to breakups of supercontinents. A debate exists regarding the role of the Kerguelen mantle plume initiation in the India-Australia breakup. Zhu et al. report new zircon U-Pb age dates of the extensive Cretaceous igneous rocks exposed around the Comei area in southeastern Tibet. These data allow the recognition of a new 132-million-year-old LIP (i.e., the Comei LIP) that can be readily reconstructed in time and space to be related to the Bunbury basalts in present-day southwestern Australia. Until now, the expected LIP associated with the Kerguelen mantle plume and India-Australia breakup has been missing, and the best candidate was the volumetrically limited Bunbury basalts in southwestern Australia. With the recognition of the Comei LIP in southeastern Tibet, Zhu et al. have resolved the puzzle. Both the Comei LIP and the Bunbury basalts are tectonically isolated remnants of a previously unknown LIP, which is called the Comei-Bunbury LIP. These results are important in (1) resolving the above puzzle; (2) providing a broader context for understanding the Kerguelen mantle plume event, which appears to be a second stage of the Comei-Bunbury LIP; and (3) exploring the development of the 132-million-year-old Weissert oceanic anoxic event.
The 1500-year climate oscillation in the midlatitude North Pacific during the Holocene
Dai Isono et al., Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan. Pages 591-594.
In this study by Isono et al., evidence of 1,500-year climate variability during the past 11,000 years was found in sediment cores taken from the northwestern Pacific Ocean off central Japan. Based on the periodicity and the timing, this variability is interpreted as an extension of the Dansgaard-Oeschger cycle seen in the last glacial period. The regular pacing at 1,500-year intervals seen throughout both the Holocene and the last glacial period suggests that the oscillation was a response to external forcing.
Supercontinent reconstruction from recognition of leading continental edges
J. Brendan Murphy et al., Dept. of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada. Pages 595-598.
There is a general consensus that repeated cycles of supercontinent amalgamation and dispersal have occurred for the past two billion years, and that these cycles have profoundly influenced the evolution of Earth's crust, climate, and life. Amalgamation of supercontinents is characterized by widespread mountain-building events. Breakup of supercontinents, on the other hand, is characterized by the development of continental shelves along the trailing edges of the dispersing continents. Both these events can be recognized in the geologic record and aid in the reconstruction of the changing geography of the Earth. Nevertheless, because Earth is a dynamic planet, preservation of these features is fragmentary, so that Earth's ancient geography is poorly constrained and often controversial. Pangea, which formed about 300 million years ago and began to break up about 200 million years ago, is the most recent of these supercontinents. Its geography is reasonably well constrained, but uncertainties increase substantially as we try to investigate deeper into Earth's history. Murphy et al. show that the leading edges of dispersing continents may be recognized by the diagnostic chemical composition of magmas derived from a young mantle that preferentially occurs along its margins. Using the magmatism of western North America since the breakup of Pangea as an analogue, they show that the leading edges of dispersing continents have diagnostic isotopic characteristics that can identify these margins. For example, the isotopic signatures of 600-450-million-year igneous rocks along the northern margin of Gondwana indicate that they were derived by melting a 700-million- to 1.1-billion-year mantle. This mantle must have originated in an ocean surrounding the ancient supercontinent Rodinia. The mantle was subsequently attached to northern Gondwana in response to the breakup of Rodinia, and provided a source for subsequent magmatism. Murphy et al. propose that such features should be common along the leading edges of dispersing continents following supercontinent breakup. This recognition provides an additional aid in the reconstruction of Earth's changing geography.
Mid-Pliocene Asian monsoon intensification and the onset of Northern Hemisphere glaciation
Yi Ge Zhang et al., State Laboratory for Mineral Deposits Research, Institute of Surficial Geochemistry, Dept. of Earth Sciences, Nanjing University, Nanjing 210093, China. Pages 599-602.
The late Pliocene onset of major Northern Hemisphere glaciation (NHG) is one of the most important steps in the Cenozoic global cooling. Although most attempts have been focused on high-latitude climate feedbacks, no consensus has been reached in explaining the forcing mechanism of this dramatic climate change. Here, Zhang et al. present a key low-latitude climate record, the high-resolution Asian monsoon precipitation variability for the past five million years, reconstructed from South China Sea sediments. Their results, with supporting evidence from other records, indicate significant mid-Pliocene Asian monsoon intensification, preceding the initiation of NHG at about 2.7 million years ago. The correlation between the monsoon record and marine calcium isotopes indicates that the 1.4-million-year-long monsoon intensification probably enhanced Asian continental erosion and chemical weathering. Elevated chemical weathering and organic carbon burial probably lowered the contemporary atmospheric CO2, and the CO2 drop may have triggered the NHG onset as suggested by a recent study. As opposed to previous studies, here Zhang et al. propose a low-latitude-driven NHG glaciation scenario.
Magmatic plumbing of a large Permian caldera exposed to a depth of 25 km
J.E. Quick et al., Southern Methodist University, Dallas, Texas 75275, USA. Pages 603-606.
Large volcanic calderas, aka supervolcanoes, are enormous craters tens of kilometers in diameter produced by giant, explosive eruptions that rank among the most violent geologic events. Geophysical studies of recently active calderas and investigations of their eruption products suggest that their magmatic systems are driven by intrusion of mantle-derived basalt in the deep crust, a process commonly referred to as magmatic underplating. However, direct confirmation of this connection and our understanding of the processes involved have been limited by lack of a crustal section exposing rocks deeper than about 5 km beneath a caldera. Quick et al. report evidence for a 285-million-year-old fossil caldera, more than 13 km in diameter, in northwest Italy, situated atop a tilted crustal section that was exposed by uplift and erosion to reveal the caldera's magmatic plumbing system from the surface to a depth of greater than 25 km. This unprecedented exposure of magmatic plumbing provides a model for interpreting geophysical profiles and magmatic processes beneath active calderas, and direct confirmation of the cause-and-effect link between intrusion mantle-derived basalt in the deep crust and explosive volcanism.
Mafic dikes displacing Witwatersrand gold reefs: Evidence against metamorphic-hydrothermal ore formation
Dimitri L. Meier et al., ETH Zurich, Isotope Geochemistry and Mineral Resources, Dept. of Earth Sciences, Clausiusstrasse 25, 8092 Zurich, Switzerland. Pages 607-610.
The origin of Earth's greatest gold reefs, in the Witwatersrand sedimentary basin in South Africa, has been debated since they were discovered a hundred years ago. Meier et al. describe an new geological argument based on the nature of cross-cutting magmatic dikes, indicating that the gold deposition into the ancient sedimentary rocks probably occurred by mechanical sedimentation into loose river gravels, not by hot fluids flowing through cracks in the strata millions of years after the deposition of the gold-bearing rocks.
Variations in 238U/235U in uranium ore deposits: Isotopic signatures of the U reduction process?
Charles John Bopp IV et al., University of Illinois at Urbana-Champaign, Dept. of Geology, 245 Natural History Building, 1301 West Green St., Urbana, Illinois 61801, USA. Pages 611-614.
Variations in uranium isotope ratios have been both hinted at and refuted in the geological literature. Recent work has shown, however, that uranium isotope ratios do vary by geologic setting. Bopp et al. analyzed a subset of the samples first analyzed by Cowan and Adler in their 1976 paper on variations of 235U content in uranium ores. They confirm their results by modern multicollector-inductively coupled plasma-mass spectrometer and double isotope tracer methods, showing an approx. one part per mil difference between samples drawn from lower-temperature reduced (sandstone) uranium deposits and higher-temperature (magmatic) uranium deposits. They go further and hypothesize that this is due to an unorthodox isotope effect called the nuclear field shift. Finally, they propose that this effect could be used as a tracer to monitor the remediation of uranium contamination.
Plant-driven fungal weathering: Early stages of mineral alteration at the nanometer scale
Steeve Bonneville et al., Earth and Biosphere Institute, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Pages 615-618.
For the first time, the boundary between fungi and rock has been imaged on a nanoscale --unraveling the fundamental processes by which fungi break down rocks into soil whilst extracting essential nutrients. Bonneville et al. show that fungi launch a dual attack to decompose rocks, first weakening them through mechanical strain and then by chemical alteration. This process is extremely common (occurring in 90% of trees in the Northern Hemisphere where roots are in symbiosis with fungi), but up to this point little understood. In exchange for the delivery of nutrients, the fungi receive carbon that has been fixed by the trees during photosynthesis, which effectively links the carbon cycle with the formation of soil from rocks. The implications of this work are therefore very broad, from soil fertility and agriculture to the chemistry of river water, the atmosphere, and Earth's climate.
Late Miocene onset of the Amazon River and the Amazon deep-sea fan: Evidence from the Foz do Amazonas Basin
J. Figueiredo et al., Dept. of Earth and Ocean Science, University of Liverpool, Liverpool, UK. Pages 619-622.
Oceanic margins provide a unique insight into paleoclimate and paleogeography on land. Although it seems contradictory, sediment aprons in the proximity of major rivers hold a continuous record, whereas evidence from land is mostly scattered. Until recently, the Amazon Fan kept its secrets on the history of the Amazon River tightly. A sediment column of over 10 kilometers in thickness proved a hard nut to crack, and scientific drilling expeditions such as the Ocean Drilling Program could only reach a fraction of it. However, recent exploration efforts by Petrobras -- the national oil company of Brazil -- lifted the veil and for the first time permitted an insight into the history of one of the mightiest rivers on Earth. Sedimentological and paleontological analysis showed that the Amazon River was initiated as transcontinental river around 11 million years ago and took its present shape around 2.4 million years ago. Prior to this time, a carbonate shelf characterized this part of the Atlantic coast. This research by Figueiredo et al. has huge implications for our understanding of South American paleogeography and the evolution of species in the Amazonian Region and the Atlantic coast of South America.
Facies model for fluvial systems in the seasonal tropics and subtropics
Christopher R. Fielding et al., Dept. of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Pages 623-626.
Geologists routinely recognize the deposits of ancient river systems in the rock record. The models that geologists utilize to help interpret these rocks are biased toward the deposits of perennial rivers that characterize temperate and ever-wet climate regimes, and to a lesser extent those of desert systems. Fielding et al. propose that a third, intermediate type of fluvial system, characteristic of the subhumid to semiarid (seasonal) tropics, can be readily recognized in the rock record. This distinctive "fluvial style" describes rivers that experience strongly seasonal flow, and elevated levels of evaporation over precipitation such that trees preferentially colonize the riverbed as a specialized ecological niche. Major flow events are short-lived and intense, leading to abrupt rises and falls of water level. The resulting deposits of these rivers preserve abundant, in situ tree fossils, complex lateral transitions from coarse-grained to fine-grained sediments, and an abundance of sedimentary structures formed under fast-flowing water. Recognition of this fluvial style may aid those researchers concerned with interpreting climate from ancient rocks, and those exploring for natural resources in ancient fluvial strata.
Biogenic origin for Earth's oldest putative microfossils
Bradley T. De Gregorio et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-1404, USA. Pages 631-634.
Microbes and bacteria were the first living organisms on Earth, and they can be preserved in Archean silica-rich rocks. One such outcrop from western Australia, dated to 3.5 billion years ago, may hold the oldest "microfossils." However, their authenticity has been called into question, and it is possible the microbe-like features may have been formed by non-biological processes inside an ancient hydrothermal vent. De Gregorio et al. have compared carbonaceous matter in this rock with that from the 1.9-billion-year-old Gunflint Formation, which contains textbook examples of microfossil features. They found striking similarities between the carbon bonding and nitrogen, sulfur, and phosphorus composition of both ancient carbonaceous materials. This similarity suggests that biological processes may be responsible for the formation of the 3.5-billion-year-old carbonaceous matter, and that hydrothermal microorganisms existed at that time on Earth.
How long was Meridiani Planum wet? Applying a jarosite stopwatch to determine the duration of aqueous diagenesis Megan E. Elwood Madden et al., School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd St., Suite 810, Norman, Oklahoma 73019, USA. Pages 635-638.
Jarosite, a metastable iron-sulfate salt commonly found as an ephemeral phase in acid mine drainage environments on Earth, can be used as a stopwatch to determine the maximum duration of liquid water in environments in which it is preserved, including Meridiani Planum, Mars. Jarosite forms quickly in aqueous solutions, but then gradually transforms into hematite or other iron oxide minerals. This study by Elwood Madden et al. measured the rate of jarosite dissolution and used the data to determine the maximum lifetime of jarosite -- how long the mineral lasts before it completely dissolves -- in dilute water and salty brine. The results show that jarosite likely would not be preserved after 1-2 years in warm, dilute waters, but may last up to 100,000 years in very salty, cold waters. The preservation of jarosite within rocks at Meridiani Planum, Mars, suggests that the area was wet for only a small fraction of the billions of years since jarosite formed. Since life as we know it on Earth requires liquid water, this may limit the timeframe for biological activity at the site as well.
Products of neptunian eruptions
Sharon R. Allen and Jocelyn McPhie, School of Earth Sciences and Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 639-642.
Allen and McPhie define a newly recognized kind of explosive eruption, termed "neptunian," that is restricted to seafloor volcanoes. These eruptions are sustained and driven by gas exsolved from magma. The explosions inject large volumes of hot pumice clasts into the seawater above the vent. The hot pumice clasts rapidly absorb water and sink, forming density currents that flow across the seafloor. Vast areas of the modern seafloor are covered by these pumice-rich neptunian deposits. Neptunian eruptions differ dramatically from magmatic-gas-driven explosive eruptions on land, reflecting the important influence of confining pressure and the higher heat capacity, density, and viscosity of water compared to air.
Morphometry and evolution of arc volcanoes
Pablo Grosse et al., CONICET and Fundacion Miguel Lillo, Miguel Lillo 205, (4000) San Miguel de Tucuman, Argentina. Pages 651-654.
Superficial appearance often does not indicate internal character, but in the case of volcanoes, their shape contains clues to their inner life. Grosse et al. show that by studying the morphometry of volcanoes -- that is, measuring their shapes -- it is possible to identify groups of volcanoes that grow and mold themselves in different ways. There are the graceful "cones," the classic ideal, but most volcanoes end up with a geological version of middle-age spread: getting fat around the middle -- these are the "sub-cones." There is also a group of bulky, wide volcanoes called "massifs" that grow from the sub-cones. Lastly, there are "super cones" that reach for the sky in frantic growth spurts. Grosse et al. argue that each shape type could be caused by different conditions in their geological environment, as well as by different rates of magma rise. They claim that by knowing the shape of a volcano you can understand its past history and predict what it will do next.
Rugged crater ejecta as a guide to megaregolith thickness in the southern nearside of the Moon
Thomas. W. Thompson et al., Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail-Stop 300-227, Pasadena, California 91109-8099, USA. Pages 655-658.
There is a significant difference in the population of radar-bright craters, 1 to 16 km and larger in diameter, between regions of the southeastern nearside lunar highlands north and south of about 48 degrees south latitude. This is consistent with a lunar megaregolith thickness of about 1.5 km in the north and about 2.5 km in the south, a difference Thompson et al. attribute to South Pole-Aitken basin ejecta.
Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event
J.-X. Dessa et al., GeoAzur, UPMC, CNRS-IRD-UNS, 2 quai de la Darse, BP 48, F-06235 Villefranche-sur-Mer, France. Pages 659-662.
Dessa et al. have investigated the deep geological structures offshore Sumatra, where the giant 2004 earthquake triggered a tsunami that killed over 220,000 people. Current models predict that fault rupture associated with such events can only occur under specific physical conditions that restrict them to a depth range controlled by temperature and rock types. They demonstrate that, in this case, the rupture started and partly propagated in a zone that was not expected to promote brittle failure. This finding requires a revision of some geological models and a reassessment of the size of earthquakes and resulting tsunamis that could occur in similar geodynamic settings elsewhere in the world, as each could prove larger in some places than currently anticipated.
Porphyroblast rotation and strain localization: Debate settled!
Scott E. Johnson, Dept. of Earth Sciences, University of Maine, Orono, Maine 04469-5790, USA. Pages 663-666.
Porphyroblasts are relatively large metamorphic minerals that commonly show chemical zoning and trap preexisting structural fabrics as inclusion trails during their growth. The information contained in porphyroblasts makes them centrally important for studies that examine deformation and metamorphic histories, rates of diffusion and chemical reaction, deformation kinematics, finite strain, kinematic vorticity, pluton emplacement, and folding mechanisms, among other topics. A long-running debate has centered on the rotational behavior of porphyroblasts. The debate is important, because porphyroblasts that have undergone little or no rotation, relative to a fixed kinematic reference frame, may preserve the original orientations of structural fabrics present at the time of porphyroblast growth. The paper by Johnson provides a convincing case of rotation of plagioclase porphyroblasts relative to one another and a fixed kinematic reference frame during ductile deformation, thus settling the debate.
Were deep cratonic mantle roots hydrated in Archean oceans?
Dante Canil and Cin-Ty A. Lee, School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia V8W 3P6, Canada. Pages 667-670.
The mantle beneath ancient parts of Earth's continents, called "cratons," is old, hosts diamonds, and has a deep root (like an iceberg) that differs in composition from mantle underlying other parts of the crust. The study by Canil and Lee shows that the unique composition beneath the cratons may be due to chemical exchange when such mantle rocks resided on the seafloor billions of years ago. This mantle rock chemically exchanged with the old seawater and was then thrust under to become the deep mantle root to the cratons. The study shows how some chemical features in rocks deep in the mantle today owe their existence to geological processes that happened on the surface of our planet billions of years ago.
July GSA Today Science Article
The impact of snow melt on the late Cenozoic landscape of the southern Rocky Mountains, USA
Jon D. Pelletier, Dept. of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth St., Tucson, Arizona, 85721-0077, USA. Pages 4-11.
Is it possible that something as insubstantial and transitory as snow could be responsible for large scale vertical movements of Earth's surface and the excavation of deeply incised gorges? Extensive regions of the southern Rocky Mountains of the Southwestern United States have experienced more than 1.5 km of erosion over the past 10 million years, including the development of deeply incised canyons almost a kilometer deep. And while climate change has been suspected of having a role in the removal of vast volumes of Earth's crust, determining the specific processes responsible for the large scale erosion has proved problematic. In this month's GSA Today article, John Pelletier of the University of Arizona has identified the likely culprit -- snow. Pelletier demonstrates that as the global climate system cooled, the fraction of total river discharge derived from snowmelt increased significantly. The result was a huge increase in the magnitude and frequency of highly erosive floods. Snowmelt descending down from heights of 1.5 to 3.0 km swept across the Intermontane basins, removing kilometers of rock and cutting deep gorges into the large, flat-lying basins, while the surrounding mountain peaks were left largely intact. Pelletier's research, demonstrating that something as fragile as snowflakes could be responsible for the chasms that slice through the Bighorn and adjacent basins, highlights the challenges involved in understanding our finely balanced Earth system.
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Geology