Boulder, Colo., USA – Topics in this new batch of Geology papers posted online 29 June include ecospace utilization; Little Bahama Bank; climatic asynchrony; oceanic crust; sand budgets; the Alpine fault's seismic hazard to New Zealand; volcano behavior; gravity oscillations; chemical weathering in the Critical Zone; giant wave ripples; the location of high peaks as a function of drainage network; and soils as ledgers recording transactions of energy and material between Earth's plants, rocks, water, and atmosphere.
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Long-term origination rates are reset only at mass extinctions
Andrew Z. Krug and David Jablonski, Dept. of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, USA. Posted online 15 June 2012; doi: 10.1130/G33091.1.
Diversification during recovery intervals is rapid relative to background rates, but the impact of recovery dynamics on long-term evolutionary patterns is poorly understood. Age distributions for cohorts of marine bivalves show that intrinsic origination rates tend to remain constant, shifting only during intervals of high biotic turnover, particularly mass extinction events. Genera originating in high-turnover intervals have longer stratigraphic durations than genera arising at other intervals, and drive the magnitude of the shift following the Cretaceous-Paleogene extinction. Species richness and geographic range promote survivorship and potentially control rates through ecospace utilization, and both richness and range have been observed to expand more rapidly in recovery versus background states. Post-Paleozoic origination rates, then, are directly tied to recovery dynamics following each mass extinction event.
Canyon morphology on a modern carbonate slope of the Bahamas: Evidence of regional tectonic tilting
T. Mulder et al., Université de Bordeaux, UMR 5805 EPOC, 33405 Talence cedex, France. Posted online 29 June 2012; doi: 10.1130/G33327.1.
New high-quality bathymetry data collected in November 2010 during an international cruise led by the University of Bordeaux on the French Oceanographic research vessel Le Suroît depict the morphology of the northern slope of the Little Bahama Bank (North of Gran Bahama). The survey reveals the details of large and small-scale morphologies, including slope failure scars and canyons. The slope shows true turbidite systems extending over approx. 40 km and built by mass-flow events and turbidity currents. Slope failures show sinuous head scarps, most of them being filled with recent sediment. Canyons have amphitheater-shaped heads resulting from these coalescing slump scars. Canyons rapidly open on a short channel and a depositional lobe. Detailed analyses of bathymetric data show that the canyon and failure-scar morphology and geometry vary following a W-E trend along the bank slope, consistently with a westward tectonic tilt of the bank during the Cenozoic. The study of such carbonate turbidite systems could lead of the discovery of new types of rock-bearing hydrocarbon reservoirs in similar systems located in ancient carbonate-dominated depositional systems.
New Zealand and UK Holocene flooding demonstrates interhemispheric climate asynchrony
Mark G. Macklin et al., Institute of Geography and Earth Sciences, Aberystwyth University, Ceredigion SY23 3DB, UK. Posted online 29 June 2012; doi: 10.1130/G33364.1.
More than 1,000 radiocarbon ages sourced from floodplains in the UK and New Zealand have been used to detect the timing of large-scale flooding during the past ~10,000 years in these temperate maritime regions of the Northern and Southern Hemispheres. Probability and statistical analyses of these datasets shows that flood-rich centuries in New Zealand and the UK were largely asynchronous during this time period. Since these major periods of flooding, or "wet centuries," are controlled by large-scale shifts in atmospheric circulation, these findings demonstrate that short-term climate change was out of phase in the temperate maritime regions of the Northern and Southern Hemispheres. This supports recent research that has suggested that Holocene climate changes may have been antiphased between the polar regions, possibly related to variation in the strength of deep water formation. Reconstructing and interrogating flood histories in the UK and New Zealand provides a novel means of addressing one of the grand challenges in Holocene paleoclimate research, namely establishing whether centennial-scale climate change between the Hemispheres has been synchronous. These data strongly suggest climatic asynchrony.
Oxidation state of subarc mantle
K.A. Evans et al., School of Applied Geology, Curtin University, GPO Box 1987, WA 6845, Australia. Posted online 29 June 2012; doi: 10.1130/G33037.1.
Ocean crust forms from mantle-derived magmas at mid-ocean ridges, travels across the ocean floor as Earth's tectonic plates move, and descends down subduction zones to be recycled into the Earth. This cycling of material from Earth's interior to exterior and back again forms one of the planet's primary geochemical cycles. Oceanic crust reacts chemically with the ocean, and this process changes the composition of that crust. Thus, when oceanic crust is recycled back into the interior of the Earth, it adds components that were not present when it was first formed. One of the most interesting and least well known changes in the composition of ocean crust is its oxidation by the ocean. It has been proposed that subduction of this oxidized ocean crust, and release of the oxidized components within the Earth, cause portions of Earth's mantle to oxidize, and that this oxidation is a necessary prerequisite to the formation of some types of ore deposits that include copper, gold, and molybdenum. However, this conclusion is controversial. In this paper, K.A. Evans and colleagues used new analyses of mantle-derived material in oceanic crust and model calculations to explore the plausibility of different proposed fates for subducted oxidized material.
The offshore export of sand during exceptional discharge from California rivers
Jonathan A. Warrick and Patrick L. Barnard, U.S. Geological Survey, Pacific Coastal and Marine Science Center, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA. Posted online 29 June 2012; doi: 10.1130/G33115.1.
Having an accurate accounting of how much sand is available to build beaches is important for understanding and dealing with coastal erosion. Coastal rivers on the U.S. west coast are the primary sources of sand to this region's popular beaches, and the majority of this sand is discharged during floods on these rivers. Warrick and Barnard found that rivers with high rates of sediment discharge, like the Santa Clara River of southern California, discharge sand far offshore of the littoral cell -- the nearshore area that provides sand for beaches. Thus, a significant amount of the sand discharged by rivers such as these is deposited on the inner continental shelf, beyond the reach of waves that would otherwise move it up onto beaches. This suggests that a portion of the sand supplied to the southern California coastal regions by rivers will not resupply sand budgets of the coastal littoral cells.
New on-fault evidence for a great earthquake in A.D. 1717, central Alpine fault, New Zealand
G.P. De Pascale and R.M. Langridge, Dept. of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Posted online 29 June 2012; doi: 10.1130/G33363.1.
The Alpine fault is the major onshore plate-boundary structure between the Australian and Pacific plates on the South Island of New Zealand. No previous study of the central portion of the high-uplift central segment has provided on-fault evidence for the most recent earthquake (MRE). Using LiDAR (light detection and ranging) data and field mapping, G.P. De Pascale and R.M. Landridge discovered the main trace of the fault in the rainforest north of Gaunt Creek (Westland) as a north-trending fault -- a major deviation from the generally northeast-trending fault. They enhanced a natural exposure that revealed evidence for repeated thrust fault movement with a thick clay fault gouge layer juxtaposing hanging-wall (Pacific Plate) bedrock thrust over young footwall (Australian Plate) stream deposits and dated a buried peat on the fault scarp that correlates with the postulated A.D. 1717 earthquake. By comparing other records for the 1717 earthquake with recent research along the fault, De Pascale and Landridge calculate a moment magnitude (Mw) of between Mw 8.0 to 8.2 in 1717, a great earthquake that is larger than any previous estimate. Because the Alpine fault has not ruptured for ~300 years, it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to New Zealand.
Disequilibrium dihedral angles in dolerite sills: A new proxy for cooling rate
Marian B. Holness et al., Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Posted online 29 June 2012; doi: 10.1130/G33119.1.
Working out how long bodies of magma take to solidify is important if we want to understand how volcanoes behave. Until now this has been done using theoretical cooling models or by looking at how big the individual crystals are (the bigger the crystals the longer time they had to grow). Here we show that the way the grain fit together gives us an accurate picture of how long magma took to solidify: the geometry of three-grain junctions is a highly sensitive speedometer for crystallization times between 10 and 1000 years.
Gravity fluctuations induced by magma convection at Kilauea volcano, Hawai'i
Daniele Carbone and Michael P. Poland, Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Catania 95125, Italy. Posted online 29 June 2012; doi: 10.1130/G33060.1.
This paper reports on the first continuous gravity measurements accomplished at Kilauea volcano in Hawai'i. Daniele Carbone and Michael Poland recorded the shortest-period gravity oscillations related to volcanic activity that have ever been detected, which they associate with convection in a shallow magma chamber. Rapid magma convection has long been hypothesized at persistently active volcanoes based on numerical simulations, but supporting data are largely absent from the literature. Carbone and Poland interpret their gravity data to be the first geophysical fingerprint of convection occurring over a time scale on the order of minutes in a shallow magma reservoir. The implications of the paper are twofold: (1) the results provide a new perspective on how magma circulates at Kilauea and other persistently active volcanoes, which can impact the understanding of gas emissions, seismic activity, and eruptions; and (2) Carbone and Poland highlight the great potential of a continuous gravity, a relatively new technique, as a tool to study (and monitor) active volcanoes.
Thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon-cycle feedbacks
A. Joshua West, Dept. of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA. Posted online 29 June 2012; doi: 10.1130/G33041.1.
The thin veneer at Earth's surface, known as the Critical Zone, plays host to a range of chemical reactions that are critical for generating and sustaining the resources that support life and shape the natural environment. The chemical breakdown of minerals, known as chemical weathering, is one of the most important of these chemical reactions. Until now, little has been known about where chemical weathering takes place, with soils typically thought to define the most important interface across which rocks break down chemically and physically. The analysis presented here challenges this assumption, indicating that chemical weathering takes place across a distributed zone that is often at least partly hosted in soils, but may also include bedrock, for example along fractures at depth. As a result, even landscapes with minimal soil may generate significant weathering fluxes. Moreover, because the total flux of material associated with erosion is high in these settings, they may play disproportionately important roles in global geochemical cycles. The ultimate message is that just because landscapes don't have soils doesn't mean that they aren't significant players in the global exchange between the solid rock that makes up the bulk of the Earth and the life-sustaining surface.
Origin of giant wave ripples in snowball Earth cap carbonate
Michael P. Lamb et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Posted online 29 June 2012; doi: 10.1130/G33093.1.
The most extreme climate transitions in Earth history are recorded by Neoproterozoic glacial deposits with overlying "cap" carbonate rocks indicating a potential snowball, or ice-entombed, Earth. Giant symmetrical wave ripples preserved within these rocks are often interpreted to form under extreme wave conditions. Using a new model for ripple formation, we show that the first-order control on forming this type of ripple is sediment size, not wave climate. New measurements of ripple wavelengths and particle sizes from the approx. 635 million-year-old Nuccaleena Formation, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand, indicating that they likely formed under normal wave conditions. Numerical simulations of flow over ripples suggest that the ripples may have formed over long time periods with variable wave climates in conjunction with rapid seabed cementation. Together our analysis indicates that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during this unusual period of Earth history.
Influence of drainage divide structure on the distribution of mountain peaks
James A. Spotila, Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA. Posted online 29 June 2012; doi: 10.1130/G33338.1.
People have long been fascinated by mountainous topography, yet peaks and ridges have been left at the periphery of scientific investigation of landscape evolution, which traditionally has focused on valley-shaping processes. Author James Spotila shows that the distribution of mountain peaks conforms closely with drainage divide structure. Specifically, peaks occur along the intersection of drainage divides, and are not randomly distributed throughout the landscape. This is likely because drainage divide intersections are more stable and more difficult to erode, thus leading to the formation of pyramid-shaped peaks. This observation implies that the location of high peaks is a function of drainage network. For example, Mount Everest occurs where it does because major divides intersect there; there may be nothing else special about the rocks or tectonics of that location. The association of peaks and drainage divide intersections also implies that mountain peaks are anchor points in the landscape. Peaks may exert a previously unrecognized feedback on long-term landscape evolution, which limits the migration of drainage divides and valleys and thus stabilizes the topography. As a result, the drainage divide structure and the distribution of elevation along ridges are relevant characteristics that both influence and record how mountain landscapes evolve.
Coupling meteoric 10Be with pedogenic losses of 9Be to improve soil residence time estimates on an ancient North American interfluve
Allan R. Bacon et al., University Program in Ecology, and Nicholas School of the Environment, Duke University, Box 90328, Durham, North Carolina 27708, USA. Posted online 29 June 2012; doi: 10.1130/G33449.1.
Soils are like ledgers -- while residing on Earth's surface, they record transactions of energy and material between Earth's plants, rocks, water, and atmosphere. Deciphering these records and estimating how far back in time they go is not only a challenge, it is also important because understanding how plants, rocks, water, and our atmosphere have interacted in the past (under different climatic conditions) may help us predict how they will interact in the future. To take a step toward deciphering the records stored in the soils of the Southeastern United States, Allan R. Bacon and colleagues asked a simple question: How long have these soils been residing at Earth's surface? To answer this question, they combined analyses and interpretations from soil science and geology. What they found was that soils in this region are much older than previously thought. In fact, they estimate that, at minimum, the soils of the Southeastern United States may have been residing at Earth's surface for nearly 4 million years. www.geosociety.org
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