Boulder, CO, USA - Geology includes studies of Mars' Hesperian period and associated thermokarst-style landscapes; plant-leaf compounds and soil microbes in ancient Sierra Nevada river sediments; drowned carbonate reefs in Australia; fossil evidence for the floating fern Azolla in the Eocene Arctic Ocean; the spread of mineral dust from Earth's largest natural source, the Sahara; 44-million-year-old microfossils near Salzburg; sub-seafloor microbes; and the history of the Jurassic Sundance Sea. The GSA Today study uses analog models to study erosion.
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Rates of sediment delivery from the Fennoscandian Ice Sheet through an ice age
Julian A. Dowdeswell et al., Scott Polar Research Institute, Univ. of Cambridge, Cambridge CB2 1ER, UK. Pages 3-6.
Debris eroded from land masses by ice sheets is transported by ice flow and eventually deposited in the adjacent ocean. Glacier-derived sediments found on high-latitude margins are often 1 to 3 km thick. Their volume represents the total amount of erosion from beneath an ice sheet aggregated over the age of the deposits. The most deeply buried glacial sediments mark the initial build-up of ice on land. A series of cycles of ice-sheet growth and decay, or glacial-interglacial cycles, has continued to deliver glacial debris. Here, Dowdeswell et al. examine the three-dimensional record of deposition from the Scandinavian Ice Sheet along 600 km of the continental margin west of Norway over the past 2.7 million years, derived from reflection seismic evidence. They investigate variations in rates of sediment delivery through the entire ice age. Mean sediment delivery is two to three times higher for the most recent 600,000 years than for the previous two million years. The hypothesis that glacial erosion is most rapid early in an ice age, when there was presumably much weathered bedrock and pre-glacial sediment available, is not accepted for the study area. More important are changing ice-sheet dimensions and dynamics, varying intensity of individual glacial cycles, and complex ice-sheet dynamics in single deglaciations.
Biomarker reconstruction of the early Eocene paleotopography and paleoclimate of the northern Sierra Nevada
Michael T. Hren et al., Dept. of Geology and Geophysics, Yale Univ., New Haven, Connecticut 06511, USA. Pages 7-10.
The formation of large mountain ranges is an important driver of regional and global climate change over geologic time. At present, there is considerable debate over when the modern topography of major mountain belts, such as the Sierra Nevada of California, formed, which fundamentally impacts our interpretation of the climate history of North America. Fifty million years ago the Earth was considerably warmer than at present, with palm trees and crocodiles as far north as Montana. At that time, large rivers drained westward across the ancient California landscape, carving deep channels that filled with soil and leaves from an ancient subtropical forest, and the gold that would drive the California Gold Rush. Compounds from plant leaves and soil microbes trapped in these ancient river sediments preserve a record of the topography and climate of the California landscape during this period of intense global warming. Chemical analyses of fossil leaf waxes and ancient soil microbe compounds by Hren et al. show that 50 million years ago, the Sierra Nevada formed a high-elevation mountain range (more than 2 km) with temperatures considerably warmer than today (6-8 degrees Celsius). These results show that the Sierra Nevada has been a high-standing feature, shaping the climate of North America, for at least 50 million years.
A dynamic process for drowning carbonate reefs on the northeastern Australian margin
Lydia DiCaprio et al., EarthByte Group, School of Geosciences, Madsen Building F09, The Univ. of Sydney, NSW 2006, Australia. Pages 11-14.
Drowned carbonate reefs on passive margins are puzzling because of their enormous growth potential compared to typical rates of passive-margin subsidence and moderate sea-level fluctuations. A possible solution to this paradox is that slow processes acting over geologic time weaken reefs and contribute to their ultimate demise. The Australian northeastern marginal plateaus, known for their drowned reefs, underwent a period of accelerated tectonic subsidence during the Late Miocene to Pliocene which, combined with a sequence of second-order global sea-level rises, outpaced reef growth and drowned the once thriving Miocene carbonate platforms. However, the mechanism for the observed anomalous subsidence of this relatively mature passive margin 1000 km from the nearest plate boundary is uncertain. DiCaprio et al. use a coupled plate, kinematic-mantle flow model to show that in the Late Miocene northeastern Australia overrode subducted slabs from Eocene Melanesian subduction north of Papua New Guinea. They find that the rate of surface subsidence induced by the sinking slabs increases the likelihood that relative sea-level rises outpaced late Miocene reef growth. In addition to the well-known effects of long-term plate processes and short-term global sea level and climate change, their results demonstrate that deep earth processes can play a substantial role in driving the evolution of passive margins and coral reefs.
How wet was the Arctic Eocene rainforest? Estimates of precipitation from Paleogene Arctic macrofloras
David R. Greenwood et al., Dept. of Biology, Brandon Univ., 270 18th Street, Brandon, Manitoba R7A 6A9, Canada. Pages 15-18.
In the early Eocene, 55-49 million years ago, Earth was globally warm due in part to atmospheric carbon dioxide levels much higher than present levels. At that time, the Arctic Ocean was ice-free, and forests existed throughout polar regions. Fossil evidence reveals that the floating fern Azolla grew in large mats in the Arctic Ocean at this time. Modern Azolla does not tolerate salt water, and therefore it is difficult to explain its presence in the Arctic Ocean. This study used plant fossils to estimate precipitation in the Arctic during the early Cenozoic. Precipitation can be estimated based on climatic tolerances of modern plant species that are closely related to fossils, and also from the average size of leaves. Greenwood et al. found that precipitation was very high during the early Eocene in the Arctic, similar to levels in modern temperate rain-forests. High precipitation would have produced a significant amount of run-off from the surrounding landscape into the Arctic Ocean. This run-off would have provided a vast influx of fresh water into the Arctic basin, potentially freshening surface waters of the Arctic Ocean enough to support the observed Azolla blooms. The high moisture levels would have contributed to warm Arctic climates.
Climate-controlled multidecadal variability in North African dust transport to the Mediterranean Tom Jilbert et al., Dept. of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht Univ., P.O. Box 80.021, 3508 TA Utrecht, Netherlands. Pages 19-22.
The world's largest desert, the Sahara, is also its largest natural source of mineral dust. Every year, millions of tons of microscopic clay and sand, uplifted as winds blow across the desert, are transported around the globe. The particles eventually fall back to Earth, or are washed out of the air by rainfall, painting automobile windscreens as far afield as northern Europe and the Caribbean a distinctive red-brown. This study by Jilbert et al. uses the Mediterranean seabed as a natural archive of dust transport northward from the Sahara. Applying cutting-edge laser technology, they measure the composition of mineral dust in a finely layered seabed core covering the last 140 years. They show that the amount of dust in the atmosphere, and from where in the desert it comes, are related to natural changes in wind and rainfall patterns from one decade to the next. This archive can now be used to improve our knowledge of rapid shifts in climate prior to the pre-instrumental era.
The missing link in the evolutionary origin of the foraminiferal genus Hantkenina and the problem of the lower-middle Eocene boundary
Fred Rögl and Hans Egger, Geological Survey of Austria, Neulinggasse 38, 1030 Vienna, Austria. Pages 23-26.
Close to the town of Salzburg (Austria), 44-million-year-old microfossils were found, which belong to a hitherto unknown species of foraminifera. The morphological features of this newly discovered species makes it the missing link in the evolutionary origin of the foraminiferal genus Hantkenina. This genus is of great stratigraphic importance, as its first appearance has been used for about fifty years to recognize the base of the middle Eocene. However, the new data by Rögl and Egger herald that this genus has developed about 4 million years later than originally supposed. Consequently, other markers for the determination of the onset of the middle Eocene have to be found, and the stratigraphic framework for this episode in Earth's history has to be revised.
Uplift of Oahu, Hawaii, during the past 500 k.y. as recorded by elevated reef deposits Gary M. McMurtry et al., School of Ocean and Earth Science and Technology, Univ. of Hawaii, Manoa, Honolulu, Hawaii 96822, USA. Pages 27-30.
In a study by McMurtry et al., direct information on paleo-sea level and past climate can be provided by ancient shorelines, sometimes preserved as raised or submerged reef deposits in tropical areas. Paleo-shorelines need to be constrained by accurate tectonic history because these environments and their marine deposits can be confused with past tsunami deposits and vice versa. In Hawaii, loading of the massive shield volcanoes on Hawaii island causes subsidence along the Hawaiian chain until the island of Oahu is reached. Elevated reef deposits on Oahu indicate it is rising linearly at a rate of 0.06 mm/yr over the past 500,000 years, in contrast with submerged reefs off Hawaii and Lanai islands that indicate linear subsidence rates of a few mm/yr over the same period. Five of the six major emerged interglacial highstand reefs on Oahu have been identified, and these data suggest little evidence for past maximum sea levels significantly greater than 2 m above the sea level datum at that time. These results also suggest that in this time period, interglacial high stands were never much greater than at present.
Thermal barriers and the fate of perched faunas
Steven M. Stanley, Dept. of Geology and Geophysics, Univ. of Hawaii, Honolulu, Hawaii 96822, USA. Pages 31-34.
In a study by Stanley, thermal barriers provide an explanation for the geologically sudden extinction of benthic faunas of epeiric seas when these seas disappeared by contracting to the open ocean. Biotic interactions could not have caused the sudden extinctions, and neither could reduction of the regional area of seafloor, because substantial areas of shallow seafloor remained along neighboring continental shelves when epeiric seas drained. Instead, temperature contrasts must have been responsible. Epeiric seas had strongly seasonal climates, and when some receded to continental margins, many of their species would have encountered waters that failed to provide the maximum or minimum temperature required for reproduction. When epeiric seas receded poleward, equatorward, or into Coriolis-driven currents, many species faced lethal temperatures. The history of the Jurassic Sundance Sea provides a striking example of the fate of a warm-adapted fauna driven westward into an area dominated by a cool, Coriolis-driven current.
Apatite fission-track data for the Miocene Arabia-Eurasia collision
Aral I. Okay et al., Istanbul Technical Univ., Eurasia Institute of Earth Sciences, Maslak 34469, Istanbul, Turkey. Pages 35-38.
The collision between Eurasian and Arabian plates isolated the Mediterranean from the Indian Ocean and has been linked to the Aegean extension, Red Sea rifting, and the formation of the North and East Anatolian fault systems. Yet the timing of the collision is poorly constrained and the estimates range from Late Cretaceous to late Miocene. Here, Okay et al. report the first apatite fission-track (AFT) ages from the Arabia-Eurasia collision zone. The AFT samples are distributed over the 450 km length of the collision zone in southeast Turkey and include metamorphic rocks and Eocene sandstones. Despite the disparate lithology and large distance, the AFT ages point consistently to exhumation between 18 and 13 Ma. The AFT ages, along with a critical appraisal of the regional stratigraphy, indicate that the last oceanic lithosphere between the Arabian and Eurasian plates was consumed by the early Miocene (about 20 million years ago). The results imply that Aegean extension predates the Arabia-Eurasia collision.
Fluvial form in modern continental sedimentary basins: Distributive fluvial systems
G.S. Weissmann et al., Dept. of Earth and Planetary Sciences, Univ. of New Mexico, MSC03 2040, Albuquerque, New Mexico 87131-0001, USA. Pages 39-42.
In a review of over 700 continental sedimentary basins, Weissmann et al. found that distributive fluvial systems (DFSs) dominate the fluvial deposits in every basin, with a minor portion of the fluvial sediments deposited by the axial fluvial system. This observation holds true for sedimentary basins in all tectonic settings. DFSs have been called alluvial fans, fluvial fans, megafans, and inland deltas in the past. Deposits of DFS rivers will ultimately form fluvial sedimentary rocks since they exist in active sedimentary basins. Rivers on DFS are different than rivers in degradational settings (often used to build facies models) in that they form a radial pattern of channel deposits from the DFS apex, commonly decrease in channel size down-DFS, display a down-DFS grain size decrease, and lack lateral channel confinement on the DFS except when the river is incised into the DFS. This will lead to potentially significant differences in sedimentary patterns on DFSs relative to those observed in degradational settings. Through observation of fluvial form in modern sedimentary basins, Weissman et al. can develop new facies models that place observed facies geometries into a basinal context. This basin-scale architectural interpretation is necessary in order to predict large-scale facies distributions in fluvial aquifers and petroleum reservoirs.
Postglacial changes in El Niño and La Niña behavior
Matthew C. Makou et al., Byrd Polar Research Center, The Ohio State Univ., 1090 Carmack Road, Scott Hall 108, Columbus, Ohio 43210, USA. Pages 43-46.
Reconstructions of past climate suggest distinctive changes in the El Niño-Southern Oscillation (ENSO), the dominant mode of tropical climate variability, since the last glacial period. However, little is known about changes in the relationship between El Niño and La Niña over this period of dramatic global climate variability, thus complicating determination of the climate forcing mechanisms that drive ENSO activity and the mean climate state of the tropical Pacific. Makou et al. provide parallel El Niño and La Niña histories from the past 16,000 years, generated using organic molecular productivity proxies in a Peru margin marine sediment core. Past changes in surface ocean productivity suggest enhanced La Niña-like conditions alone during deglaciation, but strong parallel increases in both El Niño and La Niña activity during the past 2,000 years. They propose that the preponderance of La Niña-like conditions prior to 11.5 thousand years ago reflects the influence of the global glacial climate state, while the co-intensification of El Niño and La Niña during recent millennia was likely caused by changes in tropical insolation. Their results suggest that tropical insolation controls on ENSO variability gained expression as climate influences from high-latitude glacial processes waned.
Using meteoric 10Be to track fluvial sand through the Waipaoa River basin, New Zealand
Lucas J. Reusser and Paul R. Bierman, Rubenstein School of Environment and Natural Resources and Dept. of Geology, Univ. of Vermont, Burlington, Vermont 05405, USA. Pages 47-50.
Erosion swamps rivers with sediment - filling channels, killing fish, and impacting water quality. Tracing and remediating sources of sediment pollution is a difficult but important problem that often vexes land managers. Here, Reusser and Bierman present a new, rapid method for identifying sediment sources and tracking sediment as it moves downstream. They measured a rare isotope, 10-beryllium (10Be), in sand carried by the Waipaoa River on New Zealand's North Island. The Waipaoa landscape, cleared during the first half of the 1900s for agriculture and timber, presents some of Earth's most extreme examples of erosion. In the steep northern basin, large gullies feed prodigious amounts of sediment to river channels; this sediment contains little 10Be because it is sourced deep below the surface. The more stable eastern and western regions of the basin supply sediment with up to 10 times more 10Be. The concentration of 10Be in samples collected down the Waipaoa River increases regularly, demonstrating just how well this isotope tracks sediment mixing as it moves downstream. Today, the gullied headwaters produce sediment about 20 times faster than more stable regions of the Waipaoa basin. Reusser and Bierman's work shows that meteoric 10Be is an effective tool for the rapid assessment of river sediment dynamics.
Paleozoic vegetation and the Siluro-Devonian origin of fluvial lateral accretion sets
Neil S. Davies and Martin R. Gibling, Dept. of Earth Sciences, Dalhousie Univ., Halifax, Nova Scotia B3H 4J1, Canada. Pages 51-54.
In modern environments, vegetation can play an important role in river systems, helping to determine whether they adopt a braided or meandering form, and influencing the geomorphology of fluvial landscapes. For this reason, it is reasonable to expect that river systems prior to the evolution of rooted vegetation would be different from those we see today. Davies and Gibling's research surveys the evidence for lateral accretion sets (fossilized meander bends) in the Paleozoic rock record using a literature compilation and fieldwork, and demonstrates that lateral accretion sets first appear in the latest Silurian (about 418 million years ago), after which they rapidly become more abundant. Their analysis shows that (1) lateral accretion sets appear to have "evolved" at about the same time as rooted vegetation; and (2) lateral accretion sets became larger as trees developed and roots penetrated more deeply during the Devonian. Although this trend may in part reflect improved preservation of meander bends in vegetated terrains, the analysis provides strong evidence that river‐deposited sedimentary rocks exhibit increased complexity from the Cambrian to the Devonian, highlighting the co‐evolution of land plants and river landscapes.
Mechanisms of low-flux intraplate volcanic fields - Basin and Range (North America) and northwest Pacific Ocean
Greg A. Valentine and Naoto Hirano, Dept. of Geology, Univ. at Buffalo, 411 Cooke Hall, Buffalo, New York 14260, USA. Pages 55-58.
Volcanism at mid-ocean ridges, subduction zones, and above mantle hot spots is clearly related to melting caused by the introduction of new heat or new chemical components to the upper mantle, or by decompression. However, many volcanic fields are located far away from tectonic plate boundaries and are not, in any obvious way, actively caused by these factors. Valentine and Hirano compare two such "intraplate" volcanic fields in very different tectonic settings - the floor of the northwest Pacific Ocean, and the central Basin and Range province of North America. Despite their different settings, volcanism in the two fields has relatively similar physical and geochemical characteristics, and both are related intimately to regional tectonic deformation. The magmas in both fields appear to be sourced in heterogeneous upper mantle that might contain pockets of partial melt under ambient pressure and temperature conditions. The authors suggest that such intraplate volcanic fields, particularly those that produce very low volumes of magma over long periods of time, are caused by the mechanical focusing of preexisting partial melts in response to regional deformation.
Asymmetric ocean basins
Giuliano Panza et al., Dipartimento di Scienze della Terra, Universita di Trieste, 34127 Trieste, Italy, and Abdus Salam International Centre for Theoretical Physics (ICTP), Strada Costiera 1, 34151 Trieste Italy. Pages 59-62.
After the observation that mountain building and subduction zones are asymmetric, as evident when comparing the western and eastern margins of the Pacific Ocean, Panza et al. have discovered that ocean basins are also asymmetric. However, this is less evident at the surface. Mantle tomography is the main tool for showing that rift zones such as the Pacific, Mid-Atlantic, and Indian ridges have faster seismic shear waves on the western side than the eastern side. An opposite asymmetry holds for the underlying asthenosphere. This points to an asymmetric Earth, where the asymmetry is visible moving along the tectonic equator, which describes the flow of lithospheric plates and makes an angle of about 30 degrees relative to the geographic equator. The tidal despinning generating the westerly directed displacement of the lithosphere relative to the mantle is inferred as the mechanism for generating this pattern.
Magma fingers and host rock fluidization in the emplacement of sills
Nick Schofield et al., School of Geography, Earth and Environmental Sciences, Univ. of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Pages 63-66.
Constraining the geometry and architecture of sub-volcanic intrusions is fundamental to understanding the transport, delivery, and storage of magma in the upper crust and its eventual eruption at Earth's surface. Interconnected laterally extensive complexes of magmatic sills are increasingly being shown to play a major role in the movement of magma around the upper crust in volcanic terranes. Most of the current models dealing with sill/magma emplacement assume that magma intrudes by the fracturing of host rock alone; however, this may be an oversimplification. Schofield et al. report that a prevalent fluid/fluid or fluid/ductile relationship between host rock and intruding magma can develop in high-level intrusions in certain situations. Once this occurs, the evolution of a given sheet intrusion becomes distinctly different from that produced by normal brittle fracture alone. The breakdown in brittle fracture often leads to the development of magma fingers, which accelerate ahead of the main sheet of magma. It is important to note that it is ultimately the host rock and its coupled response to intrusion of magma that dictates the ongoing evolution of the morphology of sheet intrusions in high-level magmatic systems.
Quantifying the geomorphic impacts of a lake-breakout lahar, Mount Ruapehu, New Zealand
Jonathan Procter et al., Institute of Natural Resources, Massey Univ., P.B. 11 222, Palmerston North, New Zealand. Pages 67-70.
On 18 March 2007, a dam of 0.01 x 106 m3 of tephra collapsed within several minutes to release 1.4 x 106 m3 of acidic water from Crater Lake on Mount Ruapehu, New Zealand. Through direct flow measurements at sites along the >200 km flow path, and by calculating changes between pre- and post-event LiDAR surveys of the topography, volumetric constraints for this non-cohesive lahar were deduced. Unusual regular alternations of sediment erosion and deposition occurred along the channel. The initial flood mobilized a net 2.5-3.1 x 106 m3 of boulders, gravel, sand, and water over its first 5 km, to generate a flowing volume of up to 4.4 x 106 m3 passing 6.9 km. From about 9 km, a braided deposition/erosion pattern developed, with dominantly short-lived, reach-scale motion of coarse sediment shown by destruction and reconstruction of bars and levees. After crossing a broad fan, the lahar was about 3.7 x 106 m3 as it passed 28 km. The data by Procter et al. provide a high-resolution understanding of a catchment-wide geomorphic response to a single lahar event. The dataset enables calculation of rates, and elucidation of mechanisms, of sediment entrainment and deposition by volcanic floods or lahars.
Hesperian equatorial thermokarst lakes in Ares Vallis as evidence for transient warm conditions on Mars
Nicholas Warner et al., Dept. Earth Science & Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK. Pages 71-74.
The Hesperian period on Mars is considered to have been very cold and hyper-arid, with conditions unsuitable for stable liquid water at the surface. The only evidence for water are the huge outflow channels thought to have been carved by giant floods from subsurface aquifers, which likely only flowed for short bursts of time. Here, Warner et al. describe flat-floored depressions in the equatorial region of Mars, which resemble thermokarst landscapes formed by degradation of near-surface ground ice. The origin of these landforms is controversial: they have been attributed to the removal of ground ice by either sublimation or melting; however, no conclusive morphologic evidence has been provided to resolve their origin. Warner et al. discovered small sinuous channels connecting the depressions, indicating that there was fluid connection between depressions and that they were filled with liquid water. The depressions resemble terrestrial permafrost lakes and suggest that the ground ice was melted during a phase of climate warming. Crater statistics indicate that the depressions formed 3.6-3.0 billion years ago. Warming during this period may have been controlled by episodic events (volcanism, impacts, floods, orbital shifts) that formed a temporary thick atmosphere. The occurrence of transient warm conditions during the Hesperian has implications for astrobiology.
40Ar/39Ar dating of oil generation and migration at complex continental margins
Darren F. Mark et al., NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre, Rankine Ave., Scottish Enterprise Technology Park, East Kilbride G75-0QF, UK. Pages 75-78.
Standard methodologies aimed at predicting the movement of oil use indirect evidence, theoretical modeling, and a bit of guesswork. These techniques have proved very successful in identifying simple source-trap oil provinces such as the North Sea. However, as natural resources continue to dwindle, exploration is now targeting geologically complex areas that have witnessed events such as multi-stage rifting and continental collision. This regional complexity makes it harder to predict using standard tools the timing of oil generation, migration, and accumulation. Mark et al. report a new case study from the rifted UK Atlantic Margin, which utilizes a novel exploration technique aimed at determining the temperature, composition, and timing of past fluid flow. The technique uses a mineral (potassium feldspar) which occurs as a natural cement in many oil reservoirs, filling pore spaces and coating sand grains. The presence of oil during mineral precipitation is recorded in tiny bubbles of oil trapped inside the cement. The mineral can be dated by measuring the amount of argon produced by radioactive decay of potassium. Dating the mineral also dates the occurrence of oil in the reservoir. Mark et al. have shown that the UK Atlantic Margin has witnessed several episodes of oil generation and migration, and they constrain them in time. The technique will aid future exploration in geologically complex regions all around the world.
Milankovitch-scale correlations between deeply buried microbial populations and biogenic ooze lithology
Ivano W. Aiello and Barbara Bekins, Moss Landing Marine Laboratories, Moss Landing, California 95039-9647, USA. Pages 79-82.
The deep biosphere is one of Earth's largest (up to 30% of the global biomass) yet poorly understood "extreme" environments for life. Recent studies have indicated that active populations of microbes in the sub-seafloor impact global biogeochemical cycles and raise important questions concerning the functioning of these extreme environments. Sub-seafloor microbes are unevenly distributed, and cell abundances and metabolic activities are commonly independent of sediment depths with increased prokaryotic activity at geochemical and/or sedimentary interfaces. In this study, Aiello and Bekins demonstrate that microbial populations vary at the scale of individual beds in the biogenic oozes of a drill site in the eastern equatorial Pacific (Ocean Drilling Program Leg 201, Site 1226). They relate bedding-scale changes in biogenic ooze sediment composition to organic carbon and microbial cell concentrations using high-resolution color reflectance data as a proxy for lithology. Their analyses demonstrate that microbial concentrations could be up to two orders of magnitude greater in the more organic-rich diatom oozes than in the nannofossil oozes. The variations follow small-scale variations in diatom abundance and organic carbon indicating that the modern distribution of microbial biomass is ultimately controlled by Milankovitch-frequency variations in past oceanographic conditions.
Noble gas signatures of high recharge pulses and migrating jet stream in the late Pleistocene over Black Mesa, Arizona, United States
Chen Zhu and Rolf Kipfer, Dept. of Geological Sciences, Indiana Univ., Bloomington, Indiana 47405, USA. Pages 83-86.
Many semi-arid and arid regions of the world are almost entirely dependent on groundwater for water supply, yet many of these aquifers were primarily recharged during the late Pleistocene. Here, Zhu and Kipfer show that new noble gas data record the high pulses of recharge to the Navajo Sandstone Aquifer at Black Mesa, Arizona, between 14 and 17 ka, associated with the passing of the southern branch of the jet stream. Higher excess neon coincided with high water levels independently deduced from numerical modeling of groundwater flow and 14C data in previous studies. This development allows the use of noble gasses not only as a tool to reconstruct paleo-temperatures but also as a new tool to reconstruct paleo-hydraulic conditions and to verify the conceptual and hydraulic assumptions of existing groundwater models.
Self-organized criticality in river basins: Challenging sedimentary records of environmental change
Marco J. Van De Wiel and Tom J. Coulthard, Dept. of Geography, Univ. of Western Ontario, London, Ontario N6A 3K7, Canada. Pages 87-90.
The historical record of sediment outputs from river systems has been used to reconstruct past environmental conditions, including climate changes. Previous research has suggested that larger floods may yield more sediment, in turn leading to thicker deposits of sediment. However, the research of Van De Wiel and Coulthard, using a two-dimensional numerical model of a simple river system, shows that the sediment output from rivers is highly irregular and may not directly reflect the climate input. The pattern of sediment output is characteristic of "self-organized criticality," a condition whereby identical inputs of rainfall give wildly fluctuating sediment outputs. Importantly, their results show that environmental conditions are not the only cause of variations in sediment yield. This means that it may not be possible to reconstruct past environmental conditions from records of sediment yield alone.
Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structure
C.H. Tong et al., Dept. of Earth and Planetary Sciences, Birkbeck, Univ. of London, Malet Street, London WC1E 7HX, UK. Pages 91-94.
Tong et al. present new geophysical results showing the internal structure of the largest impact crater in South America (Araguainha in Brazil). The formation of impact craters is a dramatic, high-energy geological event and involves rapid ejection and movement of Earth materials. Computer models help to reconstruct these impact events, but how the crustal materials settled to result in the final form of the craters has been poorly supported by geological evidence. The research of Tong et al. shows, for the first time, the geological evidence for the resurgence of Earth materials to the center of an impact crater after the initial ejection. Their observation is made possible by the novel application of geophysical methods involving pumping electricity to the ground for detecting the shape and location of the different rock types inside the impact crater. They conclude that the form of an impact crater is determined by the pre-impact rock formations. This conclusion has major implications in the understanding of the interior of planetary bodies. Tong et al.'s results suggest that the form of impact craters on different rocky planetary bodies is a surface manifestation of their internal structures.
GSA Today Science Article
Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies
Jacques Malavieille, Universite Montpellier 2, CNRS UMR 5243, Geosciences Montpellier, 34095 Montpellier cedex 5, France; and International Laboratory, (LIA) "ADEPT," CNRS-NSC, France-Taiwan. Pages 4-10.
Interaction between surface and tectonic processes plays a key role in the structural evolution, kinematics, and exhumation of rocks in orogenic wedges. The deformation patterns observed in analog models show that strain partitioning has a strong impact on the vertical component of displacement of tectonic units, which in return favors erosion in domains of important uplift. Partitioning is controlled by tectonic processes and by climate-dependent surface processes, including erosion and sedimentation. The effects of partitioning include localization of deformed domains, exhumation above areas of deep underplating, and steady-state maintenance of wedges for long time periods. Simple models illustrate well how the morphostructural evolution of mountain belts is determined by these complex interactions.
Journal
Geology