Boulder, Colo., USA - Catastrophic floods in 2011 in Vermont and 2013 in Colorado devastated many communities. While flood waters were the highest in recorded history, much of the damage done by these floods was not related to inundation by flood water, but instead caused by abundant erosion and sedimentation. These floods provided a rare opportunity to better understand controls on the locations of these different hazards.
In their study for Geology, John D. Gartner and colleagues explore the effects of downstream increases and decreases in stream power, which are linked in part to variations in river slope constrained by underlying geology. A physics-based relationship indicates that river reaches are susceptible to erosion, such as landslides and bank failures, where stream power increases in the downstream direction. Conversely, river reaches are prone to floodplain sedimentation where stream power decreases in the downstream direction, because the river cannot carry the load delivered from upstream.
These predictions are compared with observed locations of erosion and sedimentation along four rivers severely affected by these floods. Gartner and colleagues' analysis successfully predicts river channel and floodplain responses in almost 90% of cases studied. This direct field evidence highlights the potential role of downstream changes in stream power in connections between river channels and laterally-adjacent banks, slopes, and floodplains.
Gradients in stream power influence lateral and down-stream sediment flux in floods
John D. Gartner et al., Department of Earth Science, Dartmouth College, Hanover, New Hampshire 03755, USA. This paper is online at http://dx.
GEOLOGY articles are online http://geology.
Other recently published GEOLOGY articles are highlighted below:
Sediment flux from the morphodynamics of elongating linear dunes
Antoine Lucas et al., Laboratoire Astrophysique, Instrumentation et Modélisation, UMR 7158 CNRS, Université Paris-Diderot, CEA Saclay, Gif-sur-Yvette, France. This paper is online at http://dx.
Linear dunes are characterized by straight ridges extending over considerable distance, often more than 10 km. They are the most common bedform in terrestrial sand seas but many questions remain open and are still under active investigation regarding their growth, either by extension or lateral accretion. Here, we use more than 50 years of high-resolution aerial and satellite imagery of the Ténéré desert (Niger), the world's largest source of mineral aerosols, to demonstrate that linear dunes can elongate in the direction of the resultant sand flux with no lateral migration. Thus, we can derive the local sand flux and assess the local wind regime. This study shows that the evolution of linear dunes under complex wind regimes can now be used to discuss climatic conditions on Earth but also on Mars and Titan where no direct meteorological data is available.
Antarctic streams as a potential source of iron for the Southern Ocean
W.B. Lyons et al., School of Earth Sciences, Byrd Polar and Climate Research Center, The Ohio State University, Columbus, Ohio 43210, USA. This paper is online at http://dx.
Iron is thought to be a major driver of primary production in the Southern Ocean surrounding Antarctica, especially in regions where macronutrients such as nitrate and phosphate are available. Although ice-free areas in Antarctica only make up a small percentage of the total continental area, streams draining these areas can potentially contribute iron into coastal environments around Antarctica. We have measured the filterable iron concentrations in streams from the largest ice-free region in Antarctica, the McMurdo Dry Valleys (~78 degrees south), and have extrapolated the iron fluxes to the Antarctic Peninsula region where ice is currently being lost. This source should continue to increase in magnitude as the climate warms and ice melt and ice-free areas increase.
Mid-latitude glacial erosion hotspot related to equatorial shifts in southern Westerlies
Frédéric Herman, Institute of Earth Surface Dynamics, University of Lausanne, CH-1015 Lausanne, Switzerland; and Mark Brandon, Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. This paper is online at http://dx.
Glaciation had a strong impact on erosion rates globally during the past two million years, which was particularly pronounced at mid-latitudes. The processes responsible for such latitudinal distribution of erosion are unknown. In this study, we have exploited the meridional extent of the Patagonian Andes to show that shifts in Westerlies to mid-latitudes during glacial periods in part explain the global variations in erosion rates. These results go beyond the common belief that temperature gradients solely control the patterns of glacial erosion. These results reveal a new mechanism in which climate shapes the Earth's surface. The impact of the westerly winds belts on global climate is known, but the idea it controls global erosion rates and topography is new. Erosion of the Patagonian Andes also plays a major role in the production of sediments and dust transported to the Southern ocean. This is the region where variations in iron availability by dust can have the largest effect on Earth's carbon cycle through its fertilizing effect on marine ecosystems, in turn promoting the feedbacks between global climate and erosion.
Bottoms up: Sedimentary control of the deep North Pacific Ocean's εNd signature
April N. Abbott et al., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CE-OAS Administration Building, Corvallis, Oregon 97331-5503, USA. This paper is online at http://dx.
Neodymium isotopes are commonly used as a tracer for past ocean circulation. However, recent work indicates that there is an important source of neodymium from the bottom sediments and this source could well impact the distribution of neodymium isotopes. April N. Abbott and colleagues provide neodymium isotope data from marine sedimentary pore fluids and evaluate the potential influence of this source on ocean neodymium signatures. They find that the bottom water neodymium isotope signature in the Northeast Pacific is between the expected signature and the measured sedimentary pore fluid signature. These data suggest that the neodymium from pore fluids exerts an important control over the neodymium isotope signature of deep water in the North Pacific Ocean.
Role of sills in the development of volcanic fields: Insights from LiDAR mapping surveys of the San Rafael Swell, Utah
J.A. Richardson et al., School of Geosciences, University of South Florida, 4202 E. Fowler Ave., Tampa, Florida 33617, USA. This paper is online at http://dx.
Magmatic sills are common features that are formed below volcanic fields while magma ascends to the surface. Terrestrial and aerial LiDAR surveys conducted between 2010 and 2013 have enabled precision mapping of several sills that have been exposed in the San Rafael Swell of Utah, USA. From these surveys, it has been found that over 90% of all igneous rock that was permanently stored in the study area was stored in sills, which were approximately 1 km deep during the volcanic activity 3-5 million years ago. At least one of the sills was likely formed during or soon before a volcanic eruption, which would have had the ability to modify the eruption style at the surface. Because sills are formed under all volcanic landforms, this study provides geometric constraints of sills that can improve sill emplacement models used for volcanic hazards assessment.
Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand
Robert Zinke et al., Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. This paper is online at http://dx.
The geomorphic expression of strike-slip faults in recent deposits is influenced by both the structural maturity of the underlying fault, and the cumulative displacement of the deposit. We combine high-resolution LiDAR topographic data and field observations to characterize surface faulting patterns along the structurally mature Wairau fault, and structurally less mature Awatere fault in South Island, New Zealand. At the well-known Branch River and Saxton River sites, the Wairau and Awatere faults (respectively) offset geologically identical, similar-aged flights of fluvial terrace deposits. Differences in surface faulting patterns between these sites reflect the different structural maturities of the two faults. We further compare the distribution and geomorphic expression of secondary faults in the terrace deposits along the structurally less-mature Awatere fault. Our results show that, whereas the processes that accommodate deformation both on and off the main fault plane are active during each earthquake, such processes may not be immediately evident in the landscape. Instead, off-fault deformation will become progressively more manifest in the geomorphology over the course of several earthquakes. These findings have numerous implications for the proper interpretation of surface faulting in the landscape and the seismic hazard associated with faults.
Bioturbating animals control the mobility of redox-sensitive trace elements in organic-rich mudstone
Dario Harazim et al., Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada. This paper is online at http://dx.
Bioturbating animals modify the original mineralogy, porosity, organic content, and fabric of mud, thus affecting the burial diagenetic pathways of potential hydrocarbon source, seal, and reservoir rocks. High-sensitivity, synchrotron rapid scanning X-ray fluorescence elemental mapping reveals that producers of phycosiphoniform burrows systematically partition redox-sensitive trace elements (i.e., Fe, V, Cr, Mn, Co, Ni, Cu, and As) in fine-grained siliciclastic rocks. Systematic differences in organic carbon content (total organic carbon >1.5 wt%) and quality (delta13Corg~0.6 parts per mil) are measured between the burrow core and host sediment. The relative enrichment of redox-sensitive elements in the burrow core does not correlate with significant neo-formation of early diagenetic pyrite (via trace metal pyritization), but is best explained by physical concentration of clay- and silt-sized components. A measured loss (~-15%) of the large-ionic-radius elements Sr and Ba from both burrow halo and core is most likely associated with the release of Sr and Ba to pore waters during biological (in vivo) weathering of silt- to clay-sized lithic components and feldspar. This newly documented effect has significant potential to inform the interpretation of geochemical proxy and rock property data, particularly from shales, where elemental analyses are commonly employed to predict reservoir quality and support paleoenvironmental analysis.
Slab flattening, magmatism, and surface uplift in the Cordillera Occidental (northern Peru)
Audrey Margirier et al., Université Grenoble Alpes, ISTerre, F-38041 Grenoble, France. This paper is online at http://dx.
The impact of subduction processes on surface uplift and relief building in the Andes is not well understood. In northern Peru we have access to a modern flat subduction zone (3 to 15 degrees south) where both the geometry and timing of the flattening of the slab are constrained. Some of the highest Andean peaks, the Cordillera Blanca (6768 m) and the Cordillera Negra (5187 m), are located just above the Peruvian flat-slab. This region is a perfect target to explore the impact of slab flattening and associated magmatism on the Andean topography and uplift. We present new thermochronologic data, that record thermal evolution of rocks under 110 degrees Celsius. Time-temperature paths obtained from inverse modeling of these data suggest that regional exhumation in the Cordillera Occidental started about 15 million years ago (15 Ma), synchronous with the onset of the subduction of the Nazca Ridge and eastward movement of regional magmatism. We propose that the Nazca Ridge subduction at 15 Ma and onset of slab flattening drove regional surface uplift, with an important contribution of magmatism to relief building in the Cordillera Occidental. This study provides evidence for the implications of both subduction processes and magmatism on paleogeography and surface uplift in the Andes.