Boulder, Colo., USA - Whether extreme river floods are becoming more frequent and/or severe in a warming world remains under debate, partly because instrumental measurements of river discharge are too restricted in length to detect shifts from natural variability. In this open access article for Geology, Daniel Schillereff and colleagues demonstrate for the first time the recovery in a systematic manner of flood frequency and magnitude data from temperate lakes that accumulate homogeneous (visually similar) sediments.
Characterizing contemporary sediment dynamics and material accumulated during recent floods of known-magnitude has established a relationship to river discharge and quantified a threshold of deposit preservation. Lakes of this type are widely distributed globally but largely unexploited for the purposes of paleoflood research; implementation of our approach will yield new sources of paleohydrological information to help model and mitigate future flood risk.
Hydrological thresholds and basin control over paleoflood records in lakes
Daniel N. Schillereff et al., School of Environmental Sciences, Roxby Building, University of Liverpool, L69 7ZT Liverpool, UK. This paper is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2015/11/20/G37261.1.
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Other recently posted GEOLOGY articles are highlighted below:
Decadal carbon discharge by a mountain stream is dominated by coarse organic matter
Jens M. Turowski et al., Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland. This paper is OPEN ACCESS online at http://dx.doi.org/10.1130/G37192.1.
Rivers redistribute nutrients throughout the landscape. Biospheric organic carbon derived from dead organic matter can be delivered by rivers to depositional environments such as lakes and the oceans, buried there, and thereby transferred into geological storage. This riverine carbon transfer is thought to be an important part of the global organic carbon cycle. However, current estimates of its magnitude generally ignore the carbon transported in coarse organic matter including leaves, twigs, wood fragments, branches, logs, or even entire trees. Measurements in the Erlenbach, a steep mountain stream in Switzerland draining a small headwater catchment, suggest that carbon in coarse organic matter is an important part of the total carbon transported by the stream, and may even dominate it. In addition, most coarse organic matter is water-logged such that it is denser than water, suggesting that is has the potential to contribute to long-term sedimentary burial. Although the Erlenbach is small, the observed importance of coarse organic matter for carbon transport in this catchment shows that coarse material cannot be generally ignored and needs to be better studied.
Constraints from fault roughness on the scale dependent strength of rocks
Emily E. Brodsky et al., Dept. of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA. This paper is OPEN ACCESS online at http://dx.doi.org/10.1130/G37206.1.
As faults slide, the bumps on opposite sides collide and deform. High-precision measurements on a wide range of faults have revealed that the principal slip surfaces have surprisingly similar roughness. However, faults are generally rougher at small scales than at large ones. This paper by Emily Brodsky and colleagues explains the observations by proposing that as the bumps slide past each other, they yield and break to create surface roughness that is limited by the rock strength. The increasing roughness at small scales is therefore a result of increasing strength at small scales. The new interpretation suggests that scale-dependent rock strength can be inferred directly from the roughness observations.
The role of discharge variability in determining alluvial stratigraphy
Andrew P. Nicholas et al., Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK. This paper is OPEN ACCESS online at http://dx.doi.org/10.1130/G37215.1.
River deposits have properties that reflect the flow conditions under which they form. Consequently, numerous studies have sought to use such deposits as an archive of past environmental conditions, for example, by linking deposit thickness to river or floodwater flow depths. Unfortunately, current understanding of the processes that control this link is incomplete. This situation reflects the difficulty of obtaining information describing both formative flow conditions and preserved deposits, for a range of contrasting environments. We demonstrate the potential for using numerical modeling to address this knowledge gap, by simulating the characteristics of deposits associated with contrasting rivers evolving under a range of flood regimes. We compare our model with geophysics data collected from a large sand-bed river, the Río Paraná, Argentina. Our model results illustrate a strong relationship between deposit characteristics and river depth, which applies across a wide range of river types. However, our results also demonstrate that flood duration and variability in flood discharge exert an important influence on deposits. This realization represents an important limitation on the application of established theory, and suggests that using river deposits to reconstruct past river dimensions or flood conditions may be even more difficult than thought previously.
Release of uranium from highly radiogenic zircon through metamictization: The source of orogenic uranium ores
Matthew V. McGloin et al., School of Earth, Atmosphere & Environment, Monash University, Melbourne, Victoria 3800, Australia. This paper is online at http://dx.doi.org/10.1130/G37238.1.
Uranium and rare earth elements are expected to become increasingly important as the world moves away from relying on fossil fuels to generate electricity. In this paper we provide an explanation for how mineral deposits rich in these elements can form during the plate tectonic processes responsible for forming mountain belts. This is important because it improves our understanding of where these deposits formed, and therefore improves our ability to find them. By understanding the geochemical requirements for formation of these deposits, explorationists can focus their efforts on particular regions that are most likely to host mineral deposits. We have identified the sequence of geochemical and tectonic processes that allowed a number of these deposits to form in the Mount Isa region, in northwest Queensland, Australia, and the knowledge gained here can be applied around the world.
Links between arc volcanoes and porphyry-epithermal ore deposits
Olivier Nadeau et al., Dept. of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada. This paper is online at http://dx.doi.org/10.1130/G37262.1.
Hydrothermal ore deposits are intimately associated with arc volcanoes above subduction zones, but the exact nature of this relationship has remained enigmatic. Although metal deposition is usually thought to occur during the waning stages of volcanism, numerous ore deposits have been demonstrated to be syn-volcanic. Here we show how the formation of these deposits is tied to volcanic cycles. We relate the chemical variations in vapors from Merapi volcano, Indonesia, to different stages of its eruptive cycle. The chemical compositions of volcanic vapors from subduction zone volcanoes are then compared globally to those of fluid inclusions from porphyry-epithermal deposits. These data show that adiabatic decompression is the principal control on mineralization. They also suggest that volcanic and sub-volcanic magmatic-hydrothermal systems are under lithostatic pressure during quiescence but decompress rapidly during injections of mafic magma and explosive eruptions. During quiescence, the magma evolves through fractional crystallization and devolatilization, gradually becoming oxidized and enriched in gold and other incompatible metals. Upon the injection of sulfur-rich mafic magmas, subvolcanic intrusions brecciate the overlying rocks, the systems are depressurized, the volcanoes erupt explosively, supercritical fluids unmix into vapor and brine, and base metal sulfides precipitate.
Antarctic marine ice-sheet retreat in the Ross Sea during the early Holocene
R. McKay et al., Antarctic Research Centre, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand. This paper is online at http://dx.doi.org/10.1130/G37315.1.
The retreating margin of the Antarctic ice sheet in the central regions of the Ross Sea following the last ice age (~20,000 years ago) passed Ross Island no later than 8,600 years ago. This is at least one thousand years earlier than previous studies from nearby coastal sites in the Western Ross Sea, which are influenced by local East Antarctic Ice Sheet outlet glaciers. The implication of these results, supported by numerical computer model outputs, is that substantial ice loss could have occurred in the Ross Sea prior to the Holocene and may have contributed to rapid global sea level rises (e.g. Meltwater Pulses) of up to 4 m per century. We conclude that marine-based ice sheet retreat was initiated synchronously by oceanic forcings along most of the Pacific Ocean sector of Antarctica. Marine-based ice sheet retreat was largely complete by the start of the Holocene in the Amundsen Sea and Wilkes Land regions, but retreat in the Ross Sea continued into the early Holocene due to the overdeepened, backward sloping continental shelf, which leads to rapid and ongoing retreat. This implies that once triggered, a marine-based ice sheet retreat event could persist for millennia in the absence of further forcings.
Basin self-similarity, Hack's law, and the evolution of experimental rill networks
Sean J. Bennett and Renjie Liu, Dept. of Geography, University at Buffalo, Buffalo, New York 14261-0055, USA. This paper is online at http://dx.doi.org/10.1130/G37214.1.
River basin drainage area and the length of river reaches exhibit a hierarchical organization. That is, as the river basin increases in size, key length dimensions remain constant or they vary systematically. While this organization has been well described, such analysis has been applied only to static landscapes. Little physical evidence exists on how such signatures change in time as a function of drainage network evolution. In this paper, we used an experimental flume 7 m long, 2.4 m wide, and 0.3 m deep filled with soil. We then subjected this landscape to simulated rainfall and baselevel lowering. This external forcing caused a 4th-order rill network to form and evolve over time. At discrete times during this network evolution, we determined drainage basin area and tributary length scales for rills using photogrammetry. The results show that rill basin area exhibits self-similarity across rill order (space) and as the network grew in time. Also, the coefficient of Hack's law, once the network began to grow, also was invariant with time. This experimental evidence strongly supports the use of scaling arguments for drainage basin evolution and for the prediction of rill network development and organization on hillslopes and agricultural fields.
Convection in a partially molten metasedimentary crust? Insights from the El Oro complex (Ecuador)
Nicolas Riel et al., Dept. of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, UK. This paper is online at http://dx.doi.org/10.1130/G37208.1.
Both the largest Himalayan-Tibet and Altiplano-Puna orogenic systems exhibit high surface heat flux with multiple evidence of partially molten middle-to-lower continental crust (750-900 °C at 20-30 km depth). Due to the presence of distributed magma (< 10% of the total rock), these weak layers are thought to play a major role in the formation of such high-elevation plateaux, by facilitating crustal flow (channel flow model). How magma can stay trapped during the long period of time needed for their formation (tens of millions of years) remain however a critical unknown. Based on a natural example of a fully preserved partially molten crust, we show that at 780-900 °C, the magmas produced during melting of mica-rich metasediments are unable to escape the system and stay pervasively distributed, which strongly diminishes the strength of the rock. As a consequence the rock-magma mix is able to convect (in the manner of the mantle but on shorter time and length scale) which strongly increases the surface heat flux. Our results show that episodic/continuous convection is a process that can explain both the high surface heat fluxes and the long lived partially molten crusts of the Earth's largest orogenic systems.
Seismically enhanced solute fluxes in the Yangtze River headwaters following the A.D. 2008 Wenchuan earthquake
Zhangdong Jin et al., SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China. This paper is online at http://dx.doi.org/10.1130/G37246.1.
From the abstract: Large earthquakes alter physical and chemical processes at Earth's surface, triggering landslides, fracturing rock, changing large-scale permeability, and influencing hydrologic pathways. The resulting effects on global chemical cycles are not fully known. Here we show changes in the dissolved chemistry of the Min Jiang, a river in the Yangtze River (China) headwaters, following the A.D. 2008 Mw 7.9 Wenchuan earthquake.
Grenville basement structure associated with the Eastern Tennessee seismic zone, southeastern USA
Christine A. Powell and William A. Thomas, Center for Earthquake Research and Information, The University of Memphis, Memphis, Tennessee 38152, USA and Geological Survey of Alabama, Tuscaloosa, Alabama 35486, USA. This paper is online at http://dx.doi.org/10.1130/G37269.1.
The Eastern Tennessee seismic zone extends more than 300 km from as far north as southeastern Kentucky southward into Alabama, southeastern United States. We propose that a large-scale shear zone, which originated as a continental transform fault during the Grenville orogeny and assembly of supercontinent Rodinia, constitutes the framework for earthquake activity in the Eastern Tennessee seismic zone. This new seismotectonic model is based on a diverse set of geophysical and geological observations, including paleomagnetic and isotopic constraints on the growth of southeastern Laurentia during the Grenville orogeny.
Magma reservoir response to transient recharge events: The case of Santorini volcano (Greece)
Wim Degruyter et al., School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, USA. This paper is online at http://dx.doi.org/10.1130/G37333.1.
The eruption frequency of a volcano and the volumes of magma that it will transport to Earth's surface are controlled by processes occurring in the magma reservoir that directly underlies the volcano. Predicting this behavior is crucial for assessing the hazards that are associated with volcanoes. However, since scientists cannot directly access these reservoirs, they have to rely on a number of circumstantial evidences coming from observations of already erupted products and ground deformation and seismic signals detected at the surface. In this study we model the dynamics of the magma reservoir at Santorini and are able to link the evolution of this reservoir to a range of petrological and geodetic observations. We suggest that the magmatic system at Santorini is currently in a growing phase that leads to the frequent, but small eruptions that have formed the Kameni islands. With our results we put constraints on the current state of the magma reservoir, which will help interpretation of unrest signals that might occur in the future.