Boulder, CO, USA The July issue of GEOLOGY presents studies on several aspects of temperature and climate change, a new river dataset examining whether the sedimentological record can help document floods, new data estimating motion of the Sagaing fault, active development of the NW British continental margin, how rivers react to earthquakes, and enigmatic volcanism of the Colorado Plateau. GSA TODAY looks at individual mineral grains as key to understanding the rise of the Andes.
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View abstracts for the complete issue of GEOLOGY at http://geology.gsapubs.org/.
Can we distinguish flood frequency and magnitude in the sedimentological record of rivers?
Gregory H. Sambrook Smith et al., School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Pages 579-582.
Geological deposits record the passage of time as an alternating set of sedimentary increments and gaps. Earth scientists have long debated whether these sediments are primarily the result of ordinary day-to-day processes that acted uniformly through time or are mostly extraordinary processes that acted spasmodically. The problem with answering this question is that it requires detailed quantitative data of how modern deposits evolve and are shaped by a range of events with different return periods. Here, Gregory Sambrook Smith and an international team of scientists present the first such dataset from a river - demonstrating that while the morphological response to a large flood was significant, in contrast, the magnitude of scour and style of deposition was the same as that associated with much smaller annual floods. Smith et al. conclude that where flood waters can spread across extensive, low-relief floodplains, river channel deposits will be a product of a range of larger and smaller floods. However, discriminating between these different-scale floods in the rock record will be challenging, if not impossible.
Cryogenic origin for brine in the subsurface of southern McMurdo Sound, Antarctica
Tracy D. Frank et al., Dept. of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Pages 587-590.
Evaporative concentration of seawater is perhaps the most commonly cited mechanism for the formation of natural brines. In settings where direct geological evidence for large-scale evaporation is absent, however, alternative mechanisms must be considered. One such alternative proposed to explain the presence of brine in high-latitude Precambrian shields of the Northern Hemisphere involves the concentration of seawater via freezing along the margins of Pleistocene ice sheets. This mechanism, however, has gained little traction, primarily because the process has not been documented in modern glacial environments. In this paper, Tracy Frank, along with Zi Gui and the ANDRILL SMS Science Team, report on the discovery of brine in the subsurface of southern McMurdo Sound, Antarctica, during deep coring by the ANDRILL (Antarctic Drilling) Program. Brine geochemical data fall along pathways expected to occur during progressive freezing of seawater. The Neogene strata in which the brine occurs show no evidence of large-scale evaporation, but instead record repeated advance and retreat of glaciers through the Ross Sea embayment. In this environment, sea ice formation along ice sheet margins produced dense brines that infiltrated permeable subglacial sediment. In demonstrating the viability of the cryogenic model for brine formation, their results highlight the potential for the same process operating in similar settings during past ice ages.
First global positioning system results in northern Myanmar: Constant and localized slip rate along the Sagaing fault
Thomas Maurin et al., Equipe Geodynamique d'Echanges Recherche Industrie Enseignement, CEREGE, CNRS, UMR 6535, Aix en Provence, France. Pages 591-594.
Thomas Maurin and colleagues from both France and Myanmar present global positioning system data from northern Myanmar used to estimate the rate of motion at the northern tip of the 1000-km-long, right-lateral strike-slip Sagaing fault. The Sagaing fault is one of the biggest strike-slip faults in the world but also one of the least studied. Their results reveal that the right-lateral motion along the fault is rather constant from central to northern Myanmar at ~20 mm/yr. The geometry and mechanical properties of the fault are more variable along-strike, which might be the result of significant rheological contrasts along the fault trace.
Cenozoic post-rift sedimentation off northwest Britain: Recording the detritus of episodic uplift on a passive continental margin
Martyn S. Stoker et al., British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK. Pages 595-598.
Hypothesis-driven research has dominated scientific methodology since the early twentieth century. In a geological context, this is well-exemplified by the concept of "passive continental margins," so-called due to the general assumption that they undergo only decaying thermal subsidence and increasingly reduced sedimentation subsequent to continental breakup and the formation of a new ocean basin. However, this assumption is being increasingly challenged by datasets acquired through large-scale observational programs, such as the Frontier Marine Mapping Program, undertaken by the British Geological Survey (BGS), along the Atlantic margin of northwest Britain. Four decades of seismic data acquisition and geological sampling have enabled the construction of regional thickness maps and the quantification of the solid-grain sediment volume for the post-breakup sedimentary basins on the northwest British continental margin. Contrary to its so-called passive nature, data collected by Martyn Stoker of the BGS and colleagues from the University of Adelaide and Geotrack International reveal a remarkable record of tectonically influenced sedimentation along the northwest British margin, from continental breakup to the present day. This record of ~60 million years of near-continual sediment pulsing implies repeated rejuvenation of the passive margin sedimentary system, which is incompatible with the conventional view of decreasing sedimentation. Their results indicate that the northwest British continental margin has been anything but passive in its development.
Large-scale climatic fluctuations in the latest Ordovician on the Yangtze block, south China
Detian Yan et al., State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Pages 599-602.
The Ordovician-Silurian transition (about 443.7 million years before present) was a critical interval marked by dramatic climatic, oceanic, and biological turnovers in Earth's history, of which the abrupt glaciation in Gondwana was the most remarkable in the context of a long-term greenhouse climate. This team of scientists, lead by Detian Yan of the Chinese Academy of Sciences, use the Chemical Index of Alteration (CIA) of sediments as an indicator of changes in intensity of chemical weathering to explore the climatic changes during this time interval on the Yangtze block, south China. Their data show a high degree of chemical weathering of sediments, and a hot and humid climate under which the sediments were deposited in the Late Ordovician. This condition was followed by a sharp climatic overturn to an overall cold and arid climate. The temporal coincidence of two-phase massive biotic demises with the beginning and end of the cold climate epoch during the latest Ordovician, coeval with fluctuations in sea level and oceanic anoxicity, suggests that the large climatic changes could be one of the main triggering factors in the south China mass extinction.
Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies
A.P. Ballantyne et al., Dept. of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA. Pages 603-606.
Arctic temperatures are currently increasing at an astounding rate primarily due to increased atmospheric CO2 levels; however, A.P. Ballantyne of the University of Colorado at Boulder and colleagues suggest that current CO2 levels may be sufficient to produce significantly warmer temperatures in the Arctic. They investigated Arctic climate during the Pliocene (2.6 to 5.3 million years ago), the most recent period in Earth's history when atmospheric CO2 levels were comparable to levels in today's atmosphere (about 400 parts per million). Their results indicate that temperatures on Ellesmere Island in the High Arctic during the Pliocene may have been considerably warmer than present-day temperatures. Based on three independent measures of past temperature from fossils, Ballantyne et al. estimated that the mean annual temperature of the Arctic during the Pliocene was approximately zero degrees Celsius. Although this does not seem that warm, it is ~19 degrees Celsius warmer than present-day Ellesmere Island, and comparable to Anchorage, Alaska. Therefore the Arctic climate system maybe much more sensitive to greenhouse gas warming than previously thought and current CO2 levels may be sufficient to bring about significant and irreversible shifts in Arctic ecosystems.
Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere
J.W. van Wijk et al., Dept. of Earth and Atmospheric Sciences, University of Houston, 312 Science and Research Bldg. 1, Houston, Texas 77204, USA. Pages 611-614.
Many aspects of the southwestern U.S. Colorado Plateau remain enigmatic. The province behaved as a strong coherent block in the Neogene, apparently unaffected by regional extension tectonics that formed the neighboring Rio Grande rift and the Basin and Range province. If the plateau is indeed a stable tectonic block, what is the source of the abundant young and very recent magmatism along its margins? The San Francisco Peaks and Sunset Crater volcano in Arizona, as well as Mount Taylor, Jemez Mountains, and Valles Caldera in New Mexico, are examples of this enigmatic volcanism. How can this volcanism be explained? J.W. van Wijk of the University of Houston and colleagues present evidence of erosion of the lithospheric base of the Colorado Plateau, including seismic tomography and geodynamic models. The eroding lithosphere allows warm mantle to well up to shallower depths, resulting in melting of the rocks and volcanism at the surface. The Colorado Plateau is thus slowly shrinking; this is visible at the surface as volcanism on the margins of the plateau, and at large depths as erosion of the lithospheric base.
Origin of large-volume pseudotachylite in terrestrial impact structures
Ulrich Riller et al., School of Geography and Earth Sciences & Origins Institute, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada. Pages 619-622.
Pseudotachylite rocks are millimeter- to kilometer-size rock bodies that consist of rock fragments set in a fine-grained groundmass that crystallized from a melt. Pseudotachylite bodies are found in large meteorite impact structures and form intricate networks below crater floors. The melt portions of pseudotachylyte bodies are generally believed to have formed during the impact process and at the positions where they are observed today. Ulrich Riller and colleagues show that the melt portions are derived from large, super hot pools of impact melt ponded in the crater, and that they travelled, likely within minutes, many kilometers away from the crater floor to their present location during impact crater formation. This has important consequences for the formation of large impact craters on Earth and other planets.
Constraints on early Cambrian carbon cycling from the duration of the Nemakit-Daldynian-Tommotian boundary delta-13C shift, Morocco
Adam C. Maloof et al., Dept. of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. Pages 623-626.
The early Cambrian (542-511 million years ago) represents the transition from four million years of Precambrian Earth history dominated by bacteria and algae to a world rapidly populated by nearly all the animal phyla that exist today. During the early Cambrian radiation, the first appearance of calcite biomineralizers and reefs constructed by animals is associated with the largest perturbation to the carbon cycle in the Phanerozoic Eon (542-0 million years ago). A team of scientists from the USA and Morocco led by Adam Maloof report sedimentological and carbon-isotope data from a new 2.5-km-thick stratigraphic section that spans the early Cambrian evolutionary explosion in the Moroccan Anti-Atlas Mountains. Three new uranium-lead zirconages from tuffs within the stratigraphy place the events of the Cambrian explosion in a framework of absolute time, and two of the tuffs constrain the duration of the carbon isotope excursion coincident with the first animal reefs. With a simple model, Maloof et al. evaluate the ramifications of such a rapid carbon-isotope shift, and conclude that metamorphic and/or volcanic fluxes of carbon must have been sustained at levels 4 to 16 times higher than today for millions of years.
A 600 k.y. record of El Nino-Southern Oscillation (ENSO): Evidence for persisting teleconnections during the Middle Eocene greenhouse climate of Central Europe
Olaf K. Lenz et al., Institut fur Angewandte Geowissenschaften, Angewandte Sedimentgeologie, Technische Universitat Darmstadt, Schnittspahnstrasse 9, D-64287 Darmstadt, Germany. Pages 627-630.
In a study by Lenz et al., annually laminated oil shales from the maar lake of Messel (Germany) provide high-resolution sedimentological and paleoenvironmental data of a time interval of about 600,000 years during the Eocene greenhouse phase. Individual laminae consist of a light spring and summer algal layer and a dark autumn and winter layer composed of terrigenous background sediment. Different sections were selected from the oil shale in order to count varves and to measure total varve thickness and the thickness of light and dark laminae. Time series analyses were done in order to detect possible cyclic fluctuations in varve thickness. Lenz et al. found fluctuations in the range of 2 to 6 years which closely resemble the modern El NinoSouthern Oscillation (ENSO) periodicities of 2 to 7 years in the tropical Pacific, thus showing that algal growth, as well as the background sedimentation, was controlled by ENSO effects. The consistent evidence of these effects over a time interval of 600,000 years supports the assumption of a robust Eocene ENSO state, thus confirming that ENSO was the dominant global climate phenomenon during the Eocene epoch.
Reduced ice extent on the western Antarctic Peninsula at 700� cal. yr B.P.
B.L. Hall et al., Dept. of Earth Sciences and the Climate Change Institute, University of Maine, Orono, Maine 04469, USA. Pages 635-638.
Recent rapid warming and ice-shelf collapse have focused attention on the glacial record of the Antarctic Peninsula. Hall and colleagues developed the first record of terrestrial plant remains exposed by recently retreating ice. Ages of the plants range from 700 to 970 calendar years before present, and indicate times when ice extent must have been similar to (or less) than it is at present. These data indicate that the present reduced ice extent on the western Antarctic Peninsula is not unprecedented in the Holocene.
How rivers react to large earthquakes: Evidence from central Taiwan
Brian J. Yanites et al., Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan, 48109, USA. Pages 639-642.
Earthquakes, such as the 1999 Chi-chi earthquake in central Taiwan, are a fundamental process in building mountains and topography around the world. Brian Yanites of the University of Michigan and colleagues show how rivers in central Taiwan react to such events, and reveal that earthquakes may leave lasting imprints on the landscape due to their effects on river processes. Near a fault, changes in the elevation of a riverbed can generate instantaneous waterfalls that accomplish centuries-worth of erosion in just a few years as they migrate and leave the former riverbed elevated above the new river path. Far from the fault, rivers take decades to centuries to evacuate a large pulse of sediment from the thousands of landslides generated by an earthquake. By studying river reaction to the 1999 Chi-chi event, Yanites et al. suggest that rivers may leave records of previous earthquakes that go back thousands of years in central Taiwan, and potentially much longer in other regions.
Normal faulting driven by denudational isostatic rebound
Fermin Fernandez-Ibanez et al., GeoMechanics International, 5373 West Alabama, Houston, Texas 77056, USA. Pages 643-646.
The Neogene-Quaternary Guadix-Baza basin in southeast Spain offers a good scenario to analyze the links between fluvial erosion, uplift, and fault activity. The basin is divided in two sub-basins by a major active structure, the Baza normal fault. The Guadix sub-basin is located on the footwall block of the fault, and the Baza sub-basin on the hanging-wall. In this area, fluvial erosion is especially intense since the Late Pleistocene when the lacustrine drainage network of the basin was captured by a major river: the Guadalquivir River. The capture point was located in the Guadix sub-basin, which meant that a higher amount of sediment was removed from the footwall block of the fault. However, erosion on the hanging-wall basin was not as intense. Sediment remobilization caused load release on the basin, and subsequent surface uplift (isostatic rebound). Differential erosion between sub-basins resulted in differential uplift, as well as deformation concentration along the boundary between both sub-basins (the Baza fault), and the fault slip. Fernandez-Ibanez et al. show the importance of erosion in basin evolution, and the controls on fault activity. This has important implications for the analysis of old basins and the quantification of basin extension rates.
Climate and Antarctic Intermediate Water coupling during the late Holocene
Christine Euler and Ulysses S. Ninnemann, Dept. of Earth Science, University of Bergen, 5007 Bergen, Norway. Pages 647-650.
The ratio of oxygen isotopes in shells of planktonic and bottom-dwelling foraminifera (single celled marine organisms) were used to document natural variations in the southeast Pacific Ocean temperature and salinity during the first 1300 years A.D. The data presented by Christine Euler and Ulysses Ninnemann of the University of Bergen is based on sediments from offshore Chile (Ocean Drilling Program Site 1233). They provide the first ever decadally resolved paired records of temperature changes in the southeast Pacific surface and intermediate water (about 800 meter depth). Their surface water record reveals high-amplitude variations in ocean properties suggesting ocean temperature could have changed by as much as three degrees Celsius in less than a century. This level of natural variability is significant in the context of modern trends and, if active in the future, could alternatively moderate and enhance anthropogenic influences on climate. The study also reveals that intermediate ocean properties covary with those at the surface on centennial timescales, raising the possibility that they are part of a broader Pacific cycle linked through intermediate ocean circulation. The study points out the potential importance of intermediate ocean properties in modulating, and hence predicting, climate over large areas.
Is the trace fossil Macaronichnus an indicator of temperate to cold waters? Exploring the paradox of its occurrence in tropical coasts
Luis I. Quiroz et al., Dept. of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada. Pages 651-654.
Diminutive animal trails preserved in 23-million-year-old rocks reveal unusual cold and turbid waters in the Caribbean coast of South America, according to Luis Quiroz and coworkers from Canada, Panama, and Venezuela. The fossil trails, known as trace fossils, are the evidence of feeding behavior of animals living in sand beaches affected by strong wave action. Identical structures are constructed today by a group of worms living in vast numbers along the Pacific coast of North America and Japan. These structures have also been found around the world, in rocks that range in age from 300 million years to 10,000 years old, and formed in ancient beaches with temperate to sub-arctic waters. The geographic and environmental distribution of the fossil trails mimics that of its modern producers, which has led to suggestions that such feeding trails are indicative of ancient coasts in temperate to high latitudes. This new study reveals that such structures are not related to high latitudes, but to cold water. Their occurrence in tropical coasts are related to an oceanographic phenomenon, known as coastal upwelling, that involves wind-driven motion of dense, nutrient-rich cooler water towards the sea surface, replacing the warmer, nutrient-depleted surface water. According to geologic evidence, this phenomenon was extended in the Caribbean region 23 million years ago, when the darker and cooler Pacific waters flowed into the Caribbean through a deep strait; and lasted until 3.5 million years ago when this strait was closed by the rise of the volcanic mountains of Panama and western Colombia. This study has implications for our understanding of ancient oceanographic conditions based on trace fossils, particularly in the absence of more common lines of evidence.
Glacial conditioning as an erosional driving force in the Central Alps
Kevin P. Norton et al., Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland. Pages 655-658.
Some of the most rapid erosion rates measured on modern landscapes is from mountain ranges that were glaciated during the Pleistocene. While potentially more effective glacial erosion could account for rapid glacial period erosion rates, high modern rates require an alternative explanation. This is especially true in the Central European Alps where modern GPS and fault plane solutions suggest that convergence has all but ceased. Despite the lack of measureable convergence, rock uplift rates in the Central Alps are strongly correlated with rates of erosion. Here, Kevin Norton and colleagues from the University of Bern propose that the cumulative effect of the Pleistocene glaciations has been the creation of over-steepened streams, which exhibit elevated stream power, thereby enhancing local erosion rates. This interpretation is supported by an analysis of stream gradients, rock uplift, and bedrock erodibility. Where streams are graded, channel steepness is a function of both the rock uplift rate and bedrock erodibility. Channel gradients in transient segments are steeper than in the graded areas, and are independent of bedrock erodibility. These transient zones of focused erosion remove significant mass and drive similarly high rock uplift rates through isostatic response of the thick Alpine lithosphere to surface erosion.
Does the Paleoproterozoic Animikie Basin record the sulfidic ocean transition?
Peir K. Pufahl et al., Dept. of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada. Pages 659-662.
The sulfidic ocean transition occurred about 1.8 billion years ago and was one of the most significant changes in seawater chemistry in Earth's history. It was the result of oxygen buildup in Earth's atmosphere and oceans, and is thought to have led to a nearly one-billion-year hiatus in the evolution of early life. Results in the Animikie Basin of North America presented by Peir Pufahl and colleagues provide new information regarding the nature of this change. The Animikie Basin has become the benchmark for understanding the sulfidic ocean transition. However, Pufahl and the team show that the Animikie Basin does not contain an accurate record of this oceanographic change, calling into question the timing of this important event in early Earth history.
Dynamic subsidence and uplift of the Colorado Plateau
Lijun Liu and Michael Gurnis, Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA. Pages 663-666.
How the Colorado Plateau rose from below sea level in the Cretaceous to its 2-km present elevation remains a mystery. Lijun Liu and Michael Gurnis of the California Institute of Technology use an inverse convection model, observationally calibrated, to calculate the vertical evolution of the plateau. As flat Farallon subduction causing broad surface subsidence during the Cretaceous terminates, the plateau is predicted to experience successive stages of uplift between 80 and 40 million years, with a cumulative 1.2 km elevation gain, consistent with a recent independent paleo-altimetry study. The predicted northeast, downward tilting of the plateau may be consistent with a northward flowing drainage system. More recently, buoyant mantle upwellings during the Late Cenozoic may have caused the local surface to rise by an additional 700 m. The presumably thick lithosphere associated with the plateau increases mechanical coupling with the upper mantle, modulates the plateau's high surface topography, and gives rise to sharp topographic gradients around its periphery. The combination of buoyant upwellings and variations in lithospheric thickness may have been responsible for reversing the plateau's tilt during the Miocene, which, in turn, reversed the Colorado River drainage to the southwest.
Resolving uplift of the northern Andes using detrital zircon age signatures
B.K. Horton et al., Dept. of Geological Sciences and Institute for Geophysics, Jackson School of Geosciences, The University of Texas, Austin, Texas 78712, USA. Pages 4-9.
One of the most significant earth-science events of geologically recent time was the rise of the Andes. The growth of Earth's second greatest mountain chain has been linked to the onset of global glaciation; the growth of some of Earth's greatest river systems including the Amazon, the Orinoco, and Magdalena rivers; and the biological evolution and growth of Neotropical rainforests. Resolving the exact role played by growing Andes in the evolution of the Earth System, requires a detailed understanding of the timing of mountain growth and landscape evolution. It turns out that pinning down the timing of growth of the Andes, however, is not that simple. As is demonstrated by Brian Horton of the University of Texas at Austin and colleagues, the key may lie in detailed study of individual mineral grains shed from the growing Andes and which subsequently collected as sediments in adjacent basins and rivers. Working in sedimentary successions of the northern Andes, Horton and his research team have been able to demonstrate that, 25 million years ago, rivers draining west to the Pacific and carrying mineral particles eroded from the ancient cratonic core of South America were inundated and over-run by a new set of rivers draining to the east. These new rivers carried mineral grains eroded from rock units unique to the Andes, and their development can be attributed to the very first stages of development of the Andes. Apparently, little more than a grain of sand is required to unravel the history of one of Earth's greatest mountain belts.
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