Boulder, CO, USA - GEOLOGY topics include a new model for Grand Canyon erosion; opal formation on Mars; the beautiful and delicate preservation of soft tissues in the Burgess Shale of Canada; Ediacaran atmosphere, ocean, and fossil preservation, including that of an eight-armed ocean-dweller in both China and Australia; a new technique for seismic hazard studies; and arsenic contamination of drinking water. GSA TODAY cautions researchers against using sheeted dikes in ophiolites to investigate mid-ocean ridge spreading processes.
Model for tectonically driven incision of the <6 Ma Grand Canyon
Karl E. Karlstrom et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Pages 835-838.
The Grand Canyon is not only one of the world's most famous scenic wonders, it is also an iconic geologic landscape for understanding the earth processes of uplift, erosion, and canyon carving. A new study by Karlstrom et al. uses new and existing data to propose a model for incision of the Grand Canyon. This model proposes that canyon incision is not a passive process driven solely by river erosion and geomorphic responses to a fall of base level, but rather the rugged topography reflects an erosional setting influenced by ongoing tectonic uplift of the edge of the Colorado Plateau driven by dynamic mantle flow. Karlstrom et al. directly counter recent research that claims the Grand Canyon is more than 17 to 65 million years old. Synthesizing all available literature, Karlstrom et al. open the way for a consensus among geologists about the age of the Grand Canyon by evaluating recent studies and models in the context of over 100 years of careful geologic work. Precursor canyons existed in older landscapes on the Colorado Plateau -- for example, those formed by northeast-flowing Tertiary drainages that may have existed in the now-eroded Mesozoic strata -- but both geoscience education efforts at the Park and scientific clarity require that older paleo-drainages not be confused with the Grand Canyon itself.
Opaline silica in young deposits on Mars
R.E. Milliken et al., Jet Propulsion Laboratory-California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California, USA. Pages 847-850.
Milliken et al.'s paper focuses on the detection of opal (hydrated silica) on Mars. Iron sulfates are also associated with some of the opal deposits, indicating that the minerals were precipitated out of water under acidic conditions. These minerals are found in finely layered deposits in young terrains on Mars, some of which are associated with inverted channels, suggesting they may have been deposited by rivers or in shallow standing bodies of water. These findings suggest that interaction of the near surface with water took place over a much longer period of Mars' history than previously recognized.
Ubiquitous Burgess Shale-style "clay templates" in low-grade metamorphic mudrocks
A. Page et al., Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. Pages 855-858.
The 500-million-year-old fossils of the Burgess Shale in Canada, discovered over a century ago, still provide one of the most remarkable insights into the dawn of animal life. The beautiful silvery fossils show the true nature of life of that time, just after the "Cambrian explosion" of animal life. Yet, their existence is a paradox: The fossils have been buried deep in Earth's crust and heated to over 300 °C (~600 °F) before being thrust up by tectonic forces to form a mountainous ridge in the Rocky Mountains. Usually such extreme conditions are thought to destroy fossils. But, in the Burgess Shale, the most exquisite detail of soft tissues has been preserved. By careful restudy of its fossils, Page et al. have solved this riddle. They have shown that as the delicate organic tissues of these fossils were heated deep within the Earth, they became the site for mineral formation. The new minerals, forged at these tremendous depths, picked out intricate details such as gills, guts, and even eyes.
Carbon isotope excursions and the oxidant budget of the Ediacaran atmosphere and ocean
T. Bristow and M. Kennedy, Department of Earth Sciences, University of California-Riverside, Riverside, California 92521, USA. Pages 863-866.
Puzzlingly large and long perturbations in the oceanic carbon cycle have been inferred from stable isotope signals recorded in carbonate rocks from the Ediacaran Period (635 to 543 million years ago). These negative isotope excursions are difficult to reconcile with carbon budgets and with the current understanding of the way the carbon cycle has operated over the past 500 million years. It has been suggested that the Ediacaran ocean contained hundreds to thousands of times more dissolved organic carbon (DOC) than the modern ocean. Combined with other lines of evidence that indicate oxygen levels in the oceans increased over the same period of time, the oxidative decay of this isotopically depleted carbon pool has been proposed as a mechanism potentially capable of driving negative isotope excursions. However, the size of the DOC pool needed to produce geochemical trends in the rock record and consequent demand for oxidants has not been quantified. Bristow and Kennedy, using a simple box model to test the plausibility of the DOC mechanism, show that a pool of DOC thousands of times greater than that found in the modern ocean is required, and at the consequent rates of oxidative decay, even conservatively high estimates of available oxygen and sulfate are exhausted over time scales shorter than the current multimillion-year estimates for the duration of isotope excursions. Therefore, independent evidence that levels of oxygen and sulfate were maintained during excursions argues against DOC decay as a driver of isotope excursions. The covariation of carbon and oxygen isotopes that accompany excursions indicates that local processes or post-depositional alteration offer alternative explanations for these signals, and are worthy of further investigation.
An eight-armed Ediacara fossil preserved in contrasting taphonomic windows from China and Australia
Maoyan Zhu et al., LPS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. Pages 867-870.
Ediacara fossils are widely known from 575- to 542-million-year-old rocks around the world. They are among the earliest known macroscopic and complex life forms that lived in the oceans immediately before the Cambrian explosion, thus providing key fossil evidence for the early evolution of animals. However, many Ediacara fossils were only known as casts and molds preserved in coarse-grained sandstones, and their evolutionary relationships are poorly understood. Zhu et al. report an eight-armed Ediacara fossil that is preserved as carbonaceous films in black shale in South China, and as casts and molds in sandstone in South Australia. The new discovery represents the first Ediacara fossil that occurs in two drastically different environments and preservational styles. The researchers interpret this eight-armed Ediacara fossil as an ancient animal with two germ layers, possibly similar to modern jellyfishes and comb jellies.
Miocene to present kinematics of fault-bendfolding across the Huerguosi anticline, North Tianshan (China), derived from structural, seismic, and magnetostratigrahic data
Julien Charreau et al., Tectonics Observatory, California Institute of Technology, Mail Code 100-23, Pasadena, California 91125, USA. Pages 871-874.
A number of techniques are available to estimate crustal deformation over time scales ranging from seconds (coseismic) to tens of thousands of years, but quantitative descriptions of crustal deformation over the million-year time scale -- particularly relevant to processes like fault propagation, interactions of faults, folding mechanisms, mountain building, and rift development -- remains a challenge. Charreau et al. develop a new approach that combines surface structural measurements, subsurface seismic imaging, and magnetostratigraphic dating to retrieve (through geometric modeling) the detailed history of fold growth and related sedimentation. They apply this approach to the Huerguosi anticline, a structure in western China's northern Tianshan piedmont that has beautifully exposed growth strata. Their analysis shows that, over the past 10 million years, the fold has been growing remarkably steadily, absorbing horizontal shortening of about 1 mm/yr. This is in part the result of repeating seismic slip events on the associated thrust fault, which last ruptured during the Mw 7.8 Manas earthquake in 1906. The technique can thus be used in seismic hazard assessment studies in areas that are deforming slowly but that can produce large infrequent earthquakes (with return period >1000 yr), such as the Mw 7.9 Beichuan earthquake that recently struck the Sichuan province of China.
Chlorine isotope variations across the Izu-Bonin-Mariana arc
Jaime D. Barnes et al., Department of Earth and Planetary Sciences, MSCO3-2040, University of New Mexico, Albuquerque, New Mexico 87131-0001, USA. Pages 883-886.
Subduction zones are unique localities where surface materials of the subducting oceanic plates (sediments, water, and gases) are recycled back into the Earth's mantle. Volatiles, in particular water, that are released from the subducting oceanic plates are responsible for the explosive and hazardous nature of arc volcanoes that threaten millions of people. The Izu-Bonin-Mariana arc is a particularly well studied subduction zone and the locality where the oldest oceanic crust (180 million years old) is recycled back into the mantle. Barnes et al. analyzed chlorine isotopes of the oceanic crust and the volcanic rocks in the Izu-Bonin-Mariana arc to trace the source of chlorine and its carrier, seawater, from the ocean to arc volcanoes. Their results show that water is released from the subducting plate from both oceanic sediments and the altered oceanic crust (serpentinite). The relative contributions of these two sources of water to arc magmas depends on the depth of the subducting plate below the volcanoes. This result is significant in that it helps to further constrain the global chlorine and water cycles, and may allow future workers to better correlate volcanic activity with large-scale subduction processes.
Effects of fluid circulation in subducting crust on Nankai margin seismogenic zone temperatures
Glenn A. Spinelli and Kelin Wang, Earth and Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Pages 887-890.
Seismologists widely believe that earthquakes in subduction zones, where one tectonic plate moves under another, occur in the region where temperatures are between 150 and 350 C. This study by Spinelli and Wang shows that water rapidly circulating in an aquifer in seafloor rocks moves a tremendous amount of heat in subduction zones, and affects where those key temperatures for earthquake generation occur; they are much farther landward than previously thought. In the subduction zone off southern Japan, that heat transfer cools the underground rock, thus shifting the area where earthquakes can be generated more than 50 km (~30 miles) farther landward, and closer to coastal cities, than previously believed. This research could impact estimates of the hazard due to ground shaking, and therefore earthquake preparedness plans, for similar locations around the Pacific.
Geomorphic controls on groundwater arsenic distribution in the Mekong River Delta, Cambodia
Nicholas C. Papacostas et al., Dartmouth College, Department of Earth Sciences, Hanover, New Hampshire 03755, USA. Pages 891-894.
Arsenic is a contaminant in drinking water that affects as many as 100 million people in areas such as Bangladesh, India, Vietnam, and Cambodia. While environmental contamination is often caused by humans, the arsenic in drinking water is derived from natural sources and reflects the unique chemical and physical properties of those sources. The large river deltas of these arsenic-impacted areas exhibit significant and heterogeneous groundwater contamination. Papacostas et al. examine the connection between the regional geology and the young sediments in the Mekong delta of Cambodia, a populous region severely at risk for arsenic-related disease. They observed the rapid migration of the Mekong River and correlated the localized regions of active sediment deposition to the region's elevated groundwater arsenic concentrations. The heterogeneous and rapid deposition of sediment in specific areas adjacent to large rivers explains much of the regional heterogeneity in drinking-water arsenic levels. In most sediments, arsenic is released through the microbially mediated dissolution of iron minerals in the sediments. This microbial process requires organic matter, which is most prevalent in these young, rapidly deposited sediments. Papacostas et al.'s data are useful for identifying of areas of concern and also point to possible solutions to a looming arsenic health crisis. In some cases, placing drinking-water wells in nearby older sediments may be a viable option to minimize the impact of arsenic on an affected population.
Dispersal and biogeography of marine plankton: Long-distance dispersal of the foraminifer Truncorotalia truncatulinoides
P. Sexton and R.D. Norris, National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK. Pages 899-902.
Evolutionary processes are notoriously difficult to study because of the large spatial and temporal scales at which these processes operate. This is particularly true for the evolutionary processes of marine plankton, given the vast expanses of their open-ocean habitat. It is commonly assumed that tectonic barriers and water mass fronts act as potent isolating mechanisms, driving evolutionary divergence of the separated populations. Sexton and Norris show that these barriers to dispersal are, in some cases, very weak, and that plankton are able to disperse long distances, enabling them to opportunistically colonize normally inhospitable environments as soon as they become favorable. It appears that dispersal and speciation in plankton may be controlled more by changes in the physical and chemical structures of the water masses themselves, rather than the ease of physical exchange between them.
Mid-Cretaceous Hawaiian tholeiites preserved in Kamchatka
Maxim Portnyagin et al., IFM-Geomar, Division of the Ocean Floor, Wischhofstrasse. 1-3, D-24148 Kiel, Germany. Pages 903-906.
Portnyagin et al. report geochemical data on 120- to 93-million-year-old magmatic rocks from the Kamchatsky Mys peninsula (Kamchatka, Russia) that may represent older products of the Hawaiian hotspot than are preserved on the northwest Pacific seafloor, which were accreted to the fore-arc of Kamchatka. The presence of similar compositional components in modern and Cretaceous Hawaiian hotspot lavas suggest a persistent yet heterogeneous composition of the mantle plume, which may have sampled an area of more than or equal to 15% of the core-mantle boundary over the past ~100 million years.
** GSA Today Science Article
The significance of sheeted dike complexes in ophiolites
Paul T. Robinson et al., Dept. of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong. Pages 4-10.
How do large intact fragments of exotic oceanic crust end up being emplaced onto continents? Although these bits of sea floor, or "ophiolites," are common in mountain systems, we still struggle to explain their presence. One of the great initial successes of the plate tectonic revolution was to explain ophiolites as relics of ancient oceans that closed during the collision of continents. However, as demonstrated in a recent paper in GSA Today by Paul Robinson of Dalhousie University, Nova Scotia, and his colleagues, ophiolites are probably not relics of the crust that underpins the bulk of our oceans, and their origin remains enigmatic. Based on the study of ophiolites from mountain belts around the globe, they show that not only are ophiolites geochemically distinguishable from typical oceanic crust, but that they are structurally distinct, lacking the sheeted dikes considered characteristic of oceanic crust. Ophiolites probably originate above subduction zones, but explaining exactly where they develop and how they are subsequently emplaced on continents remain topics of ongoing research.
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