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LiDAR studies of the Sept. 2013 Colorado Front Range flooding and debris flows

New Geology articles published online ahead of print March 27, 2015

Geological Society of America

Boulder, Colo., USA - Scott W. Anderson and colleagues use repeat aerial LiDAR to quantify the erosional impact of the heavy rains that inundated the Colorado Front Range in September 2013. The five-day storm triggered more than 1,100 landslides and debris flows in a 3,430-square-kilometer area due to 200-450 mm of heavy, steady rainfall. This number of hillslope failures in a single event represents unprecedented activity for the region in its ~150 years of written history.

This study for Geology addresses the role of such large, rare events in shaping landscapes by documenting the location and size of landslides and debris flows. Anderson and colleagues use before-and-after high-resolution topographic data from airborne laser mapping (LiDAR) to quantify landslide erosion.

The "before" LiDAR mapping of Boulder Creek was completed in 2010. With a few weeks of the storm, the authors repeated the aerial survey. They then subtracted the 2013 topographic data from the 2010 topographic data where the datasets overlapped -- west of Boulder, Colorado -- to produce a digital elevation model (DEM) of difference.

They located 120 landslides and debris flows ranging in size from 10 to 21,000 cubic meters, all on slopes greater than 20 degrees. On average, about 15 mm of lowering occurred in basins in which landslides occurred.

Other methods have shown that it takes hundreds to thousands of years to loosen this much sediment from rock. These results therefore show that it is these rare debris flows that transport the sediment off the steep hillslopes along the eastern edge of the Front Range.

This study both highlights the importance of rare events in long-term erosion of this landscape and helps to explain why modern sediment yields may greatly underestimate long-term denudation rates in such settings. Debris flows dominate sediment export from storage on the steep hillslopes that bound the canyons draining the Colorado Front Range. Landscapes evolve over time scales that greatly exceed the period of historical records. It is therefore important to understand the degree to which modern observations capture the full range of geologically formative processes and process rates.


FEATURED ARTICLE

Exhumation by debris flows in the 2013 Colorado Front Range storm
Scott W. Anderson et al., Dept. of Geography and Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado, USA; currently at U.S. Geological Survey, Tacoma, Washington 98402, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36507.1.


Other recently posted GEOLOGY articles (see below) cover such topics as

    1. How the presence of cohesive, "sticky" muds within sandy sediment has a profound effect on modern ecosystems;
    2. New, independent evidence that the Hawaiian-Emperor bend is largely due to a change in the motion of the Hawaiian hotspot;
    3. Soil memory: Environmental processes recorded at the nanoscale; and
    4. How human impact (deforestation, cultivation, mining) gradually surpasses climate as a control of weathering and erosion in SW China.

GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.


Sticky stuff: Redefining bedform prediction in modern and ancient environments
Robert J. Schindler et al., University of Hull, Hull, UK; and Plymouth University, Drake Circus, Plymouth, UK. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36262.1. This paper is Open Access.

Sand and mud banks form important barriers around our estuaries and coastlines but climate change means that the way in which such sediments are moved around by water currents could change. Understanding how muds and sands are eroded, transported and deposited around such environments is important for understanding how to manage these modern ecosystems. It is equally important for interpreting the ancient rock record. Robert J. Schindler and colleagues show, from a series of carefully controlled laboratory flume experiments, how the presence of cohesive, "sticky" muds within sandy sediment has a profound effect on these processes. Just small additions of mud into the mixture can dramatically reduce the size of sandwaves and dunes on the river or seabed, radically altering flow pathways and the patterns of sandbars and deposits. They also show that addition of greater volumes of mud into the mixture can completely stop larger dune bedforms forming altogether, which is counter to present predictive models for such features.


Sequestration of inorganic carbon in soil and groundwater
H. Curtis Monger et al., New Mexico State University, Las Cruces, New Mexico, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36449.1.

Increasing amounts of carbon dioxide from fossil fuel combustion have prompted researchers to look for ways to sequester atmospheric carbon in oceans, geologic formations, land plants, and soils. The largest pool on Earth's landmasses is soil organic matter, which contains about three times the carbon in global forests and grasslands combined. Another carbon pool larger than vegetation is soil inorganic carbon, which forms in desert soils as calcium carbonate. Together with bicarbonate in groundwater, these forms of inorganic carbon comprise the largest terrestrial pools of carbon. However, are these pools active sinks that can be managed to sequester carbon or are they inert geological reservoirs? These questions have been debated for about 25 years. The debate, according to the authors of this new study for Geology, stems from the complexity of the system and the lack of a lexicon to describe it. In an attempt to increase order, they developed a classification system and the concept of carbonate-generations based on calcium source. This lexicon can not only enhance communication about how much carbon can realistically be sequestered, but can also enhance communication about the role of terrestrial inorganic carbon in the global carbon cycle in the geologic past.


Revision of Paleogene plate motions in the Pacific and implications for the Hawaiian-Emperor bend
Nicky M. Wright et al., EarthByte Group, University of Sydney, Sydney, New South Wales, Australia. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36303.1.

Deciphering the past movements of tectonic plates on Earth is important for understanding how the surface of the Earth evolved through time. One particular mystery is the bend in the Hawaiian-Emperor seamount chain, as this chain represents a trace of the Pacific plate moving over the Hawaiian hotspot. There are two alternative causes proposed for this 47 million-year-old bend: a large change in the Pacific plate's motion, due to a reorganization of the tectonic plates in the Pacific basin; or a change in the motion of the Hawaiian hotspot. To explore the plate reorganization scenario, we use data preserved on the Pacific seafloor to revise its spreading history between 66 and 33 million years ago. Our approach allows us to better determine the timing of any changes in the seafloor spreading history, with 95% confidence estimates. We find that only a small change in Pacific seafloor spreading occurred around 47 million years ago, implying that there was no major change in the motion of the Pacific plate. This provides independent evidence that the Hawaiian-Emperor bend is largely due to a change in the motion of the Hawaiian hotspot. These observations highlight the very active role of mantle convection in driving the formation of volcanic islands.


Abiotic methane from ultraslow-spreading ridges can charge Arctic gas hydrates
Joel E. Johnson et al., CAGE-Centre for Arctic Gas Hydrate, Environment, and Climate, The Arctic University of Norway, Tromsø, Norway; and University of New Hampshire, Durham, New Hampshire, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36440.1.

Biotic gas generation from the degradation of organic carbon in marine sediments supplies and maintains gas hydrates throughout the world's oceans. In ultraslow-spreading ocean basins, methane generation can also be abiotic, occurring during high-temperature reactions of seawater with exhumed ultramafic rocks. Joel E. Johnson and colleagues report on the evolution of a growing Arctic gas- and gas hydrate-charged sediment drift on oceanic crust in eastern Fram Strait, a tectonically controlled, deep-water gateway between the subpolar North Atlantic and Arctic Oceans. Ultraslow spreading ridges between northwest Svalbard and northeast Greenland permit the sustained interaction of a mid-ocean ridge transform fault and developing sediment drift since the late Miocene. Using geophysical data, we describe the buildup of a 2-million-year long-lived gas hydrate -- and free gas -- charged drift system on young oceanic crust that may be fed and maintained by a dominantly abiotic methane source. These results identify ultraslow-spreading, sedimented ridge flanks as a previously unrecognized carbon reservoir for abiotic methane that could supply and maintain deep-water methane hydrate systems throughout the Arctic.


Soil memory in mineral surface coatings: Environmental processes recorded at the nanoscale
Michael Schindler, Laurentian University, Sudbury, Ontario, Canada; and Michael F. Hochella, Jr., Virginia Tech, Blacksburg, Virginia, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36577.1.

Soils that contain information about past environmental conditions are commonly referred to as paleosols. Contrary to these soils, information about the environment in the relatively recent past are rarely found in modern surface soil layers as their chemical and mineral components are rapidly removed or changed through their strong interaction with weather (e.g. rain) and life (e.g. bacteria). Mineral particles in soils slowly break down by these interactions, and the remnant particles become surrounded by a coating that is composed of components from other mineral particles and various chemical components of the surrounding soil. These coatings are very thin (one thousandth of a millimeter or so) and contain incredibly small pores that become isolated from one another during coating formation. These pores contain even smaller mineral grains (down to a millionth of a millimeter in size) as well as petrified bacteria. These small objects give information about past environmental conditions as they cannot interact anymore with the current soil, weather, or living things. Our study shows that electron microscopes can be used to characterize these incredibly small objects to tell us important details of past environmental conditions, including contamination events caused by mining operations in the past.


Structural cause of a missed eruption in the Harrat Lunayyir basaltic field (Saudi Arabia) in 2009
Ivan Koulakov et al., Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia; and Novosibirsk State University, Novosibirsk, Russia. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36271.1.

Harrat Lunayyir, one of the youngest basaltic fields in western Saudi Arabia, is located close to the Holy Cities of Makkah and Al Madinah. In April-June 2009, substantial seismic unrest occurred in Harrat Lunayyir, featuring more than 30,000 earthquakes, some of which reached magnitude 5 or higher. Although there were indications of magmatic nature of this seismic crisis, it ended without a subaerial volcanic eruption. Based on a new seismic tomography study, we explain the causes of the missed 2009 eruption in Harrat Lunayyir. A large seismic anomaly below 7 km depth coincides with the locations of recent cinder cones (with ages of less than 100,000 years) and is interpreted to be a steady-state magma reservoir. We propose that most of the time, it remained passive, but episodically was activated due to injection of fluids and melts from deeper sources. In the tomography model, we identify another seismic anomaly at depths below 15 km, which may represent a conduit becoming active in 2009. Because of the location of this conduit slightly outside, the activation of the main crustal reservoir was not sufficiently strong to pierce the rigid basaltic cover and cause an actual eruption during the crisis in 2009.


Human impact overwhelms long-term climate control of weathering and erosion in southwest China
Shiming Wan, Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36570.1.

Discussion of the human impact on the environment has tended to focus on greenhouse gas emissions and the period since the industrial revolution. However, the older history when there were no instrument observations remains unclear. Shiming Wan and colleagues use a high-resolution sediment record from the western South China Sea to reconstruct the 6400-years of weathering and erosion history in the basin of the Red River that drains southeast Tibet. This study reveals how human impact (deforestation, cultivation, mining) gradually surpasses climate as a control of weathering and erosion in SW China, especially since 1800 years ago. Finding the balance for sustainable development between human and environment in the future is crucial for all of us. Another implication of this study is that silicate weathering in the regions with high denudation rate may be particularly important in influencing the carbon cycle on the millennial time-scale because of both high weathering flux and high sensitivity to long-term temperature-weathering feedbacks.


No place to retreat: Heavy extinction and delayed recovery on a Pacific guyot during the Paleocene-Eocene Thermal Maximum
Tatsuhiko Yamaguchi, Kochi University, Nankoku, Kochi, Japan; and Richard D. Norris, Scripps Institution of Oceanography, University of California-San Diego, La Jolla, California, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36379.1.

Global change is expected to make some mountain tops too warm to support alpine ecosystems. An analogous process may extinguish species from submarine mountains. We found that global change can cause heavier extinction in isolated submarine mountains than on continental margins and the effects of local extinction on these isolated "seamounts" or "gyuots" can last for more than 1 million years. At Allison Gyuot in the central Pacific, ostracodes (also called seed shrimps) experienced heavy extinction during the severe global changes associated with the Paleocene-Eocene Thermal Maximum (PETM) 56 Million years ago. About 64% of ostracode species became extinct during the PETM on the gyuot compared to only 25%-38% extinction on the North Atlantic continental margins. To our surprise, Pacific ostracodes took 1.1 million years to fully recover from the extinction, probably because of the difficulty of dispersing across the thousands of kilometers that separate Allison Guyot from other submarine mountain chains in the vast reaches of the Pacific. The susceptibility to extinction and difficulty in re-establishing populations may be expected for modern seamount faunas in an era of future global change.


Preserved near ultrahigh-pressure melt from continental crust subducted to mantle depths
Silvio Ferrero et al., Universität Potsdam, Potsdam, Germany; and Museum für Naturkunde (MfN), Berlin, Germany. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36534.1.

Partial melting of crustal rocks is the main process responsible for the chemical differentiation of the Earth's outer layers. It promotes the re-mobilization and re-concentration of elements, as well as releasing water and CO2; from the deepest regions of the collisional belts. Characterization of the melt produced in natural rocks during partial melting is difficult due to modification during exhumation and cooling; the deeper the melting process took place the more likely and greater the modification will be. Ferrero et al. (2015) reports the discovery of chemically unchanged remnants of melt, crystallized as aggregates called nanogranites, in crustal rocks which partially melted during deep subduction (~100 km). Sheltered as inclusions in garnet, they are natural "experimental charges" of melt, and offer an unrivalled opportunity to directly determine the original melt composition. Using a combined petrological and experimental approach, Ferrero and co-workers present the first chemical data measured in situ, including the water content, of natural melt produced at mantle depths. This study provides a bridge between natural rock investigation and re-melting experiments, shedding new light on the influence of partial melting on geodynamic evolution in orogens as well as mantle re-fertilization.


Simultaneous mountain building in the Taiwan orogenic belt
Yuan-Hsi Lee et al., National Chung Cheng University, Chiayi, Taiwan, Republic of China. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36373.1.

The Taiwan mountain belt represents one of the few active arc-collisions on Earth. Previous studies of the tectonic setting assumed an oblique collision between the northeast-trending continental margin of Eurasia and a north-trending Luzon arc. This tectonic setting predicts southward propagation of the collision at rates ranging from 60 to 90 km/m.y. We combine 62 new zircon fission-track ages with timing of rapid subsidence in the foreland basin to identify the timing of the onset of exhumation and orogenesis. The oldest completely reset zircon fission-track ages cluster around 5 Ma along the length of the belt, suggesting that the initial collision was synchronous along its length. We propose that north-south rifting in the South China Sea created a north-trending continental margin before the collision rather than the commonly assumed northeast-trending margin. Consequently, both this north-trending continental margin and the Luzon arc of the Philippine Sea plate were subparallel, resulting in an initial collision that was simultaneous along strike rather than oblique and propagating.


Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations
Alan D. Rooney et al., Harvard University, Cambridge, Massachusetts, USA. Published online ahead of print on 27 Mar. 2015; http://dx.doi.org/10.1130/G36511.1.

The snowball Earth hypothesis suggests that during the Neoproterozoic Era (1000-541 million years ago [Ma]) our planet experienced global glaciations that were among the most extreme episodes of climatic change seen in the geological record. However, a lack of reliable geochronological constraints bounding the glacial deposits has fueled disagreements about the nature and timing of these glaciations. In this contribution, we present four new rhenium-osmium (Re-Os) geochronological constraints on Neoproterozoic glacial deposits from NW Canada, Zambia and Mongolia. These ages confirm the age and duration of the Sturtian glaciation, which began ~717 Ma and lasted for more than 55 million years. This updated Neoproterozoic chronology also provides new age constraints from NW Canada on the termination of the younger Marinoan glaciation, which ended synchronously around 635 Ma. This new Neoproterozoic chronology provides further constraints to test and refine climate models of the nature of a long-duration glacial epoch. Together, these data unite age constraints sourced from multiple radiometric techniques and provide a refined temporal framework for this fascinating interval in Earth history.

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