Boulder, Colo., USA – Two Geology studies focus on debris flows and landslides, one from the point of view of alpine denudation and the other studying and quantifying hazards to human populations. Subjects of other studies include fossil microatolls and sea level; the potential rupture area for an earthquake offshore of the U.S. Pacific Northwest and British Columbia; paleoclimate; and the relationship between the formation of ore deposits and the growth cycle of microbial communities.
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Debris-flow-dependent variation of cosmogenically derived catchment-wide denudation rates
F. Kober et al., Institute of Geology, ETH Zürich, 8092 Zürich, Switzerland. Posted online 1 August 2012; doi: 10.1130/G33406.1.
Mass movements, and specifically debris flows, are episodic events that can redistribute large amounts of material in alpine catchments are thus a major agent of mountain erosion. If large debris flows close to populated areas occur, they place a considerable natural hazard to infrastructure. Recent focus on debris flows results from climate changes and related degrading permafrost areas -- a zone where debris flows are quite frequently initiated. This study by F. Kober and colleagues investigates the impact of episodic debris flows on catchment-wide denudation rates in alpine catchments obtained with cosmogenic nuclides. This method is convenient in establishing alpine denudation rates over larger areas and offers the opportunity of quantifying and charactering mountain range erosion and its driving forces (e.g., by climate, glacial history, or tectonics). This method also assumes that episodic events are balanced spatially and over the time the erosion rate is established. The result of this study shows for the Aare catchment (Switzerland), localized debris flows events can significantly perturb cosmogenic denudation rates by a factor of two. Kober and colleagues also suggest that only events with a large magnitude (and commonly with a low frequency) events perturb the system, if they are additionally coupled with the trunk stream.
Mid-Pacific microatolls record sea-level stability over the past 5000 yr
Colin D. Woodroffe et al., School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia. Posted online 1 August 2012; doi: 10.1130/G33344.1.
Radiometric dating of fossil corals and accurate survey of their elevations on Christmas Island in the central Pacific provides a record of sea level over the past 5000 years. Corals that grow into the intertidal zone adopt a "microatoll" form that is dead on top but can grow laterally to several meters in diameter. These are one of the more accurate indicators of sea-level change because their upper surface is constrained by low tide and growth around their perimeters responds quickly to rising or falling sea levels. Comparison of the height of many fossil microatolls with the elevation to which living counterparts grow implies that sea level has remained close to its present level (plus or minus 20 to 25 cm) over past millennia at this tectonically stable mid-ocean island. The continuity of growth within and between microatolls indicates that sea level has not oscillated by up to a meter on time scales of centuries as has been widely claimed by researchers using less precise evidence from other sites. The observational record presented by Colin D. Woodroffe and colleagues accords with geophysical modeling, implying that, at this location, the "eustatic" ice-melt component balances the "isostatic" adjustment of the ocean floor in response to loading.
A wider seismogenic zone at Cascadia due to fluid circulation in subducting oceanic crust
Brian D. Cozzens and Glenn A. Spinelli, Earth & Environmental Science Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Posted online 1 August 2012; doi: 10.1130/G33019.1.
Brian D. Cozzens and Glenn A. Spinelli demonstrate that the potential rupture area for an earthquake offshore of the U.S. Pacific Northwest and British Columbia extends farther landward than previously estimated. In this region, known as the Cascadia subduction zone, magnitude-9 earthquakes (similar to recent events offshore of Japan and Sumatra) have occurred in the past. However, the possible rupture extent for these Cascadia earthquakes is poorly known because there are very few small earthquakes highlighting the portions of the fault capable of nucleating earthquakes. Fault zone temperature is an important proxy, because rock friction experiments suggest that faults must be cooler than 350 degrees Celsius for earthquake nucleation. Previous estimates of the location of 350 degrees Celsius on the plate interface in Cascadia suggested that the possible earthquake rupture zone would be primarily offshore. In contrast, ongoing deformation of the land surface has been interpreted to indicate possible rupture extending landward of the coastline. Cozzens and Spinelli show that fluid circulation in the oceanic crust cools the subduction zone and widens the thermally defined potential rupture area by shifting to 350 degrees Celsius on the fault ~30-55 km landward. This places the landward edge of possible rupture under the Washington state coastline, closer to large cities and in better agreement with the land deformation studies.
Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle
Marco Scambelluri, Dipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, Corso Europa 26, 16132 Genoa, Italy; and Sonia Tonarini. Posted online 1 August 2012; doi: 10.1130/G33233.1.
The rock reservoirs enabling deep transport of ocean waters at subduction zones, together with fluid-transfer mechanisms to the mantle sources of arc magmas, are highly debated issues. Does fluid transfer occur by subducting-plate dehydration beneath arc fronts or by hydration of fore-arc mantle and subsequent subduction of the hydrated mantle? Historically, the deep slab dehydration hypothesis had strong support, but the hydrated mantle wedge idea is gaining ground. Defining subduction-fluid sources and dehydration mechanisms using appropriate isotopic tracers is therefore timely. Marco Scambelluri and Sonia Tonarini analyzed serpentinized (hydrated) mantle rocks recording deep subduction dehydration. Serpentinites are volatile and fluid-mobile element reservoirs for arc magmatism, though direct proof of their dominance in the subduction-zone volatile cycles has been elusive. Boron (B) isotopes are markers of fluid-mediated mass transfer during subduction. Altered oceanic crust and sediments release 11B-depleted fluids in the sub-arc mantle, which cannot explain 11B enrichment of many arcs. In contrast, Scambelluri and Tonarini document high 11B values retained in serpentinites. No 11B fractionation occurs in these rocks with subduction metamorphism: the released 11B-rich fluids uniquely explain the elevated 11B of arcs. B, O-H, and Sr isotopes indicate that serpentinization was driven by slab fluids that infiltrated the slab-mantle interface early in the subduction history.
Enhanced seasonality of precipitation in the Mediterranean during the early part of the Last Interglacial
Alice M. Milner et al., Department of Geography, University College London, London WC1E 6BT, UK. Posted online 1 August 2012; doi: 10.1130/G33204.1.
The Mediterranean is characterized by a strongly seasonal climate with hot dry summers and mild wet winters. With the rise of global temperatures and predicted warmer Mediterranean summers, understanding seasonality in this region is increasingly important. However, paleo-archives typically focus on mean annual conditions and are unable to resolve seasonal signals, meaning that trends in seasonality are rarely considered. Using new data from the Tenaghi Philippon peatland in Greece, a site renowned for its impressive record of Quaternary vegetation change, Alice M. Milner and colleagues apply a multi-proxy approach to investigate seasonality of precipitation during the early Last Interglacial (~130,000 to 119,000 years ago), an interval characterized by temperatures slightly warmer than today. Specifically, they address previous claims of abundant year-round rainfall during this interval, linked to deposition of organic-rich layers (sapropels) in the Mediterranean Sea. Detailed data from sediments, pollen, and macrofossils reveals evidence for both an increase in precipitation and summer aridity from the same sequence, which Milner and colleagues argue reflects an increase in seasonal contrast, with wetter winters and drier summers than the Holocene. Their finding may also indicate that winter precipitation in the northern borderlands of the Mediterranean influenced hydrographic conditions in the Mediterranean Sea that were crucial for the deposition of sapropels.
Holocene glacier culminations in the Western Alps and their hemispheric relevance
I. Schimmelpfennig et al., Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA. Posted online 1 August 2012; doi: 10.1130/G33169.1.
Human civilization developed during the balmy climate of the Holocene, over the last ~11,700 years. Emerging research on Holocene climate is showing that temperature swings were more common than previously thought and that climate changes happened on a broad, hemispheric scale. In this study, I. Schimmelpfennig and colleagues use high-precision beryllium-10 dating of moraines to show that Switzerland's Tsidjiore Nouve Glacier (Western Alps) advanced to its largest Holocene extents during at least three cold spells: (1) during the earliest Holocene around 11,400 years ago; (2) during the period between 3,800 to 3,200 years ago; and (3) during the well-documented Little Ice Age between AD 1300 and AD 1850. The glacier advance of the earliest Holocene is coeval with a cold-event recorded in Greenland ice cores, dubbed the "Preboreal Oscillation." The two latter cold spells in the Alps were coeval to dry spells in the tropical Atlantic, as the rain belt that circles Earth's equator pushed south. These findings suggest a Holocene climate link between temperature in the Alps and the polar North Atlantic and tropical precipitation patterns, adding new insights in far-field teleconnections of the climate system during our ongoing warm period.
Global patterns of loss of life from landslides
David Petley, International Landslide Centre, Durham University, Durham DH1 3LE, UK. Posted online 1 August 2012; doi: 10.1130.G33217.1.
Landslides are a threat in many environments around the world, causing loss of life, damage to property, and disruption to communications networks. In the recent (2008) Wenchuan earthquake in China, more than 100,000 landslides were triggered, leading to more than 20,000 deaths. However, until now, there has been no proper quantification of loss of life from landslides. This paper by David Petley of Durham University presents the results of a study to record the number of people killed by landslide worldwide for a seven year period between 2004 and 2010. The results show that more than 80,000 people were killed in this period, which is a rate that is about ten times more than previous studies had suggested. The data suggests that more people are killed each year by landslides than die from river floods. The majority of deaths occurred in Asia, especially in China, the Philippines, Indonesia, India, Nepal, Pakistan, and Colombia. Petley also shows that there is a relationship between the number of recorded fatality-inducing landslides and the population density in mountainous countries, which suggests that unless action is taken to mitigate dangerous slopes, loss of life from landslides will increase as the global population continues to grow.
Microbial action formed Jurassic Mn-carbonate ore deposit in only a few hundred years (Úrkút, Hungary)
Márta Polgári et al., Institute for Geochemical Research, Hungarian Academy of Sciences, Budaörsi str. 45, 1112 Budapest, Hungary. Posted online 1 August 2012; doi: 10.1130/G33304.1.
Márta Polgári and colleagues studied the The Úrkút, Hungary, manganese (Mn) ore, hosted by Jurassic black shale, using high-resolution mineralogical, microtextural, and chemical methods. The identified two independent superimposed biostructures consisting of rhythmic laminations that provide important proxies for paleoenvironments and duration of ore formation. Millimeter-scale laminae reflect a depositional series of Fe-rich biomats, mineralized microbially produced sedimentary structures. These biomats formed at the sediment-water interface under dysoxic and neutral pH conditions by enzymatic Fe2+ oxidizing processes that may have developed on a daily to weekly growth cycle. Microlamination here may reflect a 10-hour to daily rhythmicity produced by the growth of microbial communities. If true, then the giant Úrkút ore deposit may have formed over hundreds of years, rather than hundreds of thousands of years as previously thought.
Journal
Geology