Boulder, Colo., USA Geology postings on 4 and 16 April 2013 cover volcanoes, lithospheric phenomena, convergence between north and south China, a kill mechanism for a mass extinction, fossil soils, active faults, explosivity, marine sulfate levels, base metal ore deposits, garnets and diamonds, polar reversals, earthquakes, and archeology. Each article is highlighted below.
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The architecture of the European-Mediterranean lithosphere: A synthesis of the Re-Os evidence
Jos M. Gonzlez-Jimnez et al., ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), and GEMOC, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G34003.1.
Rhenium-depletion model ages (TRD) of sulfides in peridotite xenoliths from the subcontinental mantle beneath central Spain (the Calatrava volcanic field) reveal that episodes of mantle magmatism and/or metasomatism in the Iberia microplate were linked to crustal growth events, mainly during supercontinent assembly and/or breakup about 1.8, 1.1, 0.9, 0.6, and 0.3 billion years ago. A synthesis of available in situ and whole-rock Os-isotope data on mantle derived peridotites shows that this type of mantle (maximum TRD of ca. 1.8 Ga) is widespread in the subcontinental mantle of Europe and Africa outboard from the Betics-Maghrebides-Appenines front. In contrast, the mantle enclosed within the Alpine domain records TRD as old as 2.6 Ga, revealing a previously unrecognized Archean domain or domains in the central and western Mediterranean. Observations presented here indicate that ancient fragments of subcontinental lithospheric mantle have played an important role in the development of the present architecture of the Mediterranean lithosphere.
A simple mechanism for mid-crustal shear zones to record surface-derived fluid signatures
Tom Raimondo et al., School of Natural and Built Environments, University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G34043.1.
This paper by Tom Raimondo and colleagues challenges a widely accepted notion about how surface-derived fluids can penetrate to deep levels (deeper than 15 km) in Earth's crust. Most models for deep fluid transport argue that infiltration occurs over very long distances through permeable pathways such as fractures and fault zones. However, these models must grapple with very tight mechanical restrictions that should prevent this from happening. They also have difficulty explaining how such fluids are able to retain a chemical signature diagnostic of surficial reservoirs - chemical interactions with the surrounding rock during downward migration should result in its eradication. Raimondo and colleagues use high precision and high spatial resolution analysis of oxygen isotopes in garnet to reconstruct the fluid-rock interaction history of deeply buried rocks that have interacted with a surface-derived fluid. Using this evidence, they propose a new model whereby such fluid is implanted at the surface during the exposure and alteration of preexisting fault panels. These altered packages of rock are subsequently buried by thick sedimentary deposits in a rift basin and then undergo structural reactivation during mountain building. Surface-derived fluids are thus imposed on the deep crust as a result of the exposure, weathering and reburial of preexisting fault structures.
What drove continued continent-continent convergence after ocean closure? Insights from high-resolution seismic-reflection profiling across the Daba Shan in central China
Shuwen Dong et al. (An Yin, corresponding), Institute of Geomechanics, Chinese Academy of Geological Sciences, 100081 Beijing, China; Department of Earth and Space Sciences, University of CaliforniaLos Angeles, Los Angeles, California 90095, USA. Published online ahead of print on 4 April 2013; http://dx.doi.org/10.1130/G34161.1.
Shuwen Dong and colleagues conducted deep seismic-reflection surveying across the Jurassic Daba Shan thrust belt of central China to investigate how and why the continued convergence between north and south China lasted ~50 million years after the Triassic closure of their intervening oceans. This study, together with surface geology, gravity surveying, and magnetic observations, indicates widespread occurrence of mafic plutons below the Daba Shan thrust belt. Dong and colleagues propose that subduction of dominantly eclogitized mafic crust of northern south China provided the driving force for continued convergence between north and south China after ocean closure.
Biogeographical and ecological patterns in bryozoans across the Cretaceous-Paleogene boundary: Implications for the phytoplankton collapse hypothesis
Caroline E. Sogot et al., Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK; and Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G34020.1.
The Cretaceous-Paleogene mass extinction had a significant impact on Earth's biota. The kill mechanism at this event is commonly hypothesized to be a global collapse in primary productivity. This would have had a particular impact on organisms directly dependent on primary producers as their main food source, such as suspension-feeding bryozoans. Cheilostome bryozoans are thought to be more nutrient-demanding than cyclostome bryozoans and, consequently, it is expected that cheilostomes would suffer the most from a reduced food supply. It has previously been suggested that cyclostome bryozoans thrived following the mass extinction event. However, data in this study by Caroline Sogot and colleagues show no such trend across the Cretaceous-Paleogene boundary. The dominant growth form of bryozoans was seen to change after the mass extinction event but not in the way predicted by a primary productivity crash. These results are consistent between Denmark and the Southeastern USA, which suggests that this is a global response. This study therefore questions the primary productivity crash hypothesis, or at least the extent to which this kill mechanism influenced suspension feeders at the Cretaceous-Paleogene mass extinction event.
Intermediate-depth earthquakes facilitated by eclogitization-related stresses
Junichi Nakajima et al., Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G33796.1.
This article by Junichi Nakajima and colleagues answers an untested hypothesis that the large volume reduction during eclogite formation should produce a paired stress regime along the eclogitization reaction front in which tension is underlain by compression. Nakajima and colleagues reveal that in an isolated, tiny seismic cluster tensional earthquakes lie 1 km above compressional earthquakes. This observation provides seismic evidence for the crucial roles of volume-change related stresses as viable processes for nucleating earthquakes in downgoing oceanic lithosphere.
Carbon isotopic analyses of ca. 3.0 Ga microstructures imply planktonic autotrophs inhabited Earth's early oceans
C.H. House et al., Department of Geosciences and Penn State University Astrobiology Research Center, The Pennsylvania State University, 220 Deike Building, University Park, Pennsylvania 16802, USA. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G34055.1.
Unusual and surprisingly complex, organic spindle-shaped structures in 3-billion-year-old sediments from Australia are likely the remains of an ancient planktonic life form. Interpretation of these microfossils has been debated due to their relatively large size and great antiquity. The new research, using state-of-the-art instruments, determined the amount of different stable carbon isotopes in fifteen of these microfossils. The results show isotopic compositions that are quite consistent with a biogenic origin and, at the same time, distinct from background carbon in the same rock. These new results also provide some metabolic constraints that imply that the preserved microorganisms fixed carbon from the atmosphere, meaning the cells produced complex organic compounds from carbon dioxide. The existence of similar fossils in even older rocks from South Africa and other localities in Australia suggests that this unusual life form was part of a biological experiment that appears to have been widespread on the early Earth and may have lasted for several hundred million years.
A new paleothermometer for forest paleosols and its implications for Cenozoic climate
Timothy M. Gallagher and Nathan D. Sheldon, Department of Earth and Environmental Sciences, University of Michigan, 2534 CC Little, 1100 N. University Avenue, Ann Arbor, Michigan 48109, USA. Published online ahead of print on 4 April; http://dx.doi.org/10.1130/G34074.1.
This article by Timothy Gallagher and Nathan Sheldon presents a new approach for reconstructing terrestrial temperatures using fossil soils. The approach is based on the principle that, as temperature increases, certain elements are more easily weathered and subsequently lost from a soil. Gallagher and Sheldon compiled modern forest soil data to derive a specific relationship between bulk soil chemistry and mean annual temperature. The relationship was then applied to previously published chemistry data from fossil soils in Oregon that span the past forty million years. In addition to validating the relationship with modern soils in the same region, Gallagher and Sheldon were able to produce a terrestrial temperature record that was in general agreement with the regional fossil plant record. A temperature drop of almost 3 degrees C was recorded for the Eocene‐Oligocene greenhouse‐icehouse transition, and the maximum temperature of the Neogene was recorded during the Mid‐Miocene Climatic Optimum, a global warm climate event. The new terrestrial temperature record was also compared to general warming and cooling trends originally identified from marine sediments. This comparison demonstrated that over the past forty million years, marine and terrestrial temperature trends in Oregon were both similar and synchronous, suggesting strong climate coupling between the marine and terrestrial realms.
Holocene scarp on the Sawtooth fault, central Idaho, USA, documented through LiDAR topographic analysis
Glenn D. Thackray et al., Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34095.1.
The Sawtooth Fault near Stanley, Idaho, has been identified as an active fault. Detailed topographic data from airborne laser scanning reveals a fault scarp cutting young sediments deposited along the range front by glaciers and rivers. The fault scarp is 4 to 9 meters high and marks the range front from Stanley Lake to Pettit Lake, a distance of at least 50 km. The fault appears to be capable of generating large earthquakes, and has ruptured 2 to 3 times in the last 14,000 years. The fault presents significant seismic hazards to the area.
Pushing the Volcanic Explosivity Index to its limit and beyond: Constraints from exceptionally weak explosive eruptions at Kilauea in 2008
B.F. Houghton et al., Geology and Geophysics, University of Hawai'i, Honolulu, Hawaii 96822, USA. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34146.1.
Very large and very small events are extremely useful to constrain processes of volcanic eruption. This study documents products of the smallest well-constrained eruptions from Kilauea volcano in 2008 and uses them to extend the scale for the commonly cited Volcanic Explosivity Index by 4 orders of magnitude. Explosive eruptions even of this size pose serious threats at frequently visited volcanoes such as Kilauea.
Sulfur isotope systematics of a euxinic, low-sulfate lake: Evaluating the importance of the reservoir effect in modern and ancient oceans
Maya L. Gomes and Matthew T. Hurtgen, Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34187.1.
The sulfur isotope composition of sedimentary rocks is widely used by geologists to track the cycling of elements through the ocean-atmosphere system throughout Earth history. Most notably, the rise of oxygen in Earth surface environments has been inferred from the sulfur isotope difference between seawater sulfate, preserved as carbonate-associated sulfate or evaporite minerals, and pyrite. In this manuscript, Maya Gomes and Matthew Hurtgen present sulfur isotope data from a permanently euxinic, low-sulfate lake and show that the sulfur isotope difference between sulfate and pyrite -- termed sulfur isotope fractionation -- is partially controlled by the size of the sulfate reservoir. The implication of this reservoir effect is that sulfate concentration places constraints on sulfur isotope values preserved in marine sediment: at low sulfate levels sulfate is rapidly consumed by sulfate reducing microorganisms and fractionation values are low, whereas in high sulfate systems, a smaller portion of the sulfate reservoir is reduced and fractionation values are high. Sulfur isotope fractionation values are particularly sensitive to sulfate levels at low sulfate concentrations. Thus, sulfur isotope fractionation values can be used to evaluate marine sulfate levels and biogeochemical cycling during periods of low marine sulfate in the Precambrian and Phanerozoic.
Fluid mixing forms basement-hosted Pb-Zn deposits: Insight from metal and halogen geochemistry of individual fluid inclusions
Tobias Fusswinkel et al., Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, CH-8092 Zrich, Switzerland. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34092.1.
The formation of hydrothermal base metal ore deposits is commonly assumed to result from fluid mixing processes. These involve hot, saline fluids transporting high metal concentrations from crystalline basement rocks to shallower crustal levels. Subsequent mixing with formation waters rich in reduced sulfur originating from sedimentary rocks then facilitates efficient sulfide ore mineral precipitation. Many studies inferred the importance of mixing processes at the scale of entire ore districts from such geochemical considerations. However, hydrological models questioned the efficiency of fluid mixing processes at the scale of individual ore veins. This study by Tobias Fusswinkel and colleagues presents compositional data of ore fluids that formed a hydrothermal Pb-Zn vein mineralization in southwest Germany. Remnants of ore fluids trapped in closed cavities (fluid inclusions) on successively formed growth zones inside individual quartz crystals were analyzed for their chemical composition by laser ablation inductively coupled plasma mass spectrometry. Correlations between elements and element ratios (Pb, Zn, Cl/Br) indicative of fluids that interacted with either crystalline or sedimentary rocks provide the first direct evidence for fluid mixing processes to occur even at the scale of individual fluid pulses. These new findings will have to be taken into account in future studies on the genesis of hydrothermal base metal ore deposits.
Quantitative mapping of the oxidative effects of mantle metasomatism
Andrew J. Berry et al., Department of Earth Science and Engineering, Imperial College London, South Kensington SW7 2AZ, UK. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34119.1.
The main rock type associated with diamond is garnet peridotite. The garnet records the oxygen fugacity of the mantle due to its ability to accommodate both Fe(II) and Fe(III). At the depths needed to produce diamond, the oxygen fugacity is sufficiently low for diamond rather than carbonate to be the stable phase of carbon. However, influx of melts or fluids (metasomatism) into the lithospheric mantle may impose higher oxygen fugacities. In this study by Andrew J. Berry and colleagues, the oxidation state of Fe in garnets from the Wesselton kimberlite (South Africa) was quantitatively mapped using XANES spectroscopy. The garnets were shown to have a rim and a core. The oxygen fugacity of the rim is more oxidized than that of the core and would have caused diamond to breakdown. The rim was only preserved because the time between metasomatism and transport to the surface must have been short. More commonly, the effects of metasomatism would be smoothed out at mantle temperatures by diffusive re-equilibration. As a result, homogeneous garnets that exhibit metasomatic signatures probably represent a re-equilibrated average of the original (core) and metasomatic (rim) oxygen fugacities. Metasomatism of the lithospheric mantle may thus be more oxidizing, and hence have a greater impact on diamond stability, than previously thought.
Cooling of the Bushveld Complex, South Africa: Implications for paleomagnetic reversals
R. Grant Cawthorn and Susan J. Webb, School of Geosciences, University of the Witwatersrand, Private Bag 3, 2050 Wits, South Africa. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34033.1.
There are magnetic minerals in nearly all igneous rocks. When these rocks cool below their Curie temperature (600 degrees C), they permanently preserve their magnetic orientation at that time. Earth reverses its magnetic polarity (i.e., the north pole becomes the south pole) with variable frequency, between 200,000 and over a million years. By taking a short drill core from igneous rocks it is possible to determine whether the Earth had a normal polarity (i.e. as of now) or a reverse polarity at the time of cooling. This property has proved very useful in studying the time intervals between lava flows in ancient volcanic sequences. The oldest flows are obviously at the bottom. The Bushveld Complex, South Africa, is an extremely thick (up to 8 km) igneous intrusion, emplaced at over 1200 degrees C, and cooled to 600 degrees C very slowly. Heat was lost mainly through the roof and less through the floor of the intrusion, so the upper rocks cooled below 600 degrees C before the lower and middle parts. Hence, the order of preservation of magnetic polarity is not from the bottom up as in volcanic rocks, but from the top (and slightly later from the bottom) inward toward the middle. Four magnetic reversals are inferred to have occurred during a period of 500,000 years in this intrusion.
Continental delamination and mantle dynamics drive topography, extension and fluid discharge in the Apennines
Claudio Chiarabba and Giovanni Chiodini, Istituto Nazionale di Geofisica Vulcanologia, Sezione CNT, Rome 00143, Italy. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G33992.1.
In this study, Claudio Chiarabba and Giovanni Chiodini show that the evolution of the Apennines mountain belt is sustained by a low velocity anomaly in the upper mantle associated with sub-lithospheric mantle replacement after removal of rigid continental lithosphere. This process is accompanied by the discharge of deep fluids that favor the generation of large normal faulting earthquakes, like the 2009 L'Aquila event. Chiarabba and Chiodini propose that this process named "delamination" is responsible for the past million years of lithospheric evolution in the area.
Reach-scale river dynamics moderate the impact of rapid Holocene climate change on floodwater farming in the desert Nile
Mark G. Macklin et al., Institute of Geography and Earth Sciences, Aberystwyth University, Aberystwyth SY23 3DB, UK; and School of Environment and Development, University of Manchester, Manchester M13 9PL, UK. Published online ahead of print on 16 April; http://dx.doi.org/10.1130/G34037.1.
The relationship between climate change and the development of Old World riverine civilizations is poorly understood because inadequate dating control has hindered effective integration of archaeological, fluvial, and climate records. This paper by Mark G. Macklin and colleagues presents the most comprehensive and robustly dated archaeological and paleoenvironmental data sets yet compiled for the desert Nile. It focuses on the valley floor hinterland of the Kingdom of Kerma (2400-1450 B.C.) in northern Sudan. Kerma emerged as a rival to Egypt during Africa's first "Dark Age" drought. In contrast to other irrigation-based agriculturists in Egypt and Asia, Kerma flourished during the environmental crisis ca. 2200 B.C. Macklin and colleagues studied the stratigraphy and archaeological records of paleochannels across an 80 km reach of the Nile upstream of Kerma using optically stimulated luminescence to date when channels flowed and when they dried up. The dynamics of the local alluvial environment were critical in determining whether climatic fluctuations and changes in river flow represented an opportunity for floodwater farmers (5000-3500 B.C.), a hazard that could be managed (2400-1300 B.C.), or an environmental catastrophe that resulted in settlement abandonment (after 1300 B.C.).
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