|23 August 2012
GSA Release No. 12-63
Modified image of Greece, with Corinth at center, courtesy Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC; see related article by M.R. Leeder et al., doi: 10.1130/G33360.1.
Geology's "Mystery Interval," the "Great Deepening," and the Largest Kill-Off in Earth History
New Geology articles posted online ahead of print between 9 and 22 August 2012
Boulder, Colo., USA – New Geology postings include understanding the "Mystery Interval" during the last deglaciation in the Northern Hemisphere; examining topographic change and recovery after the 2011 Tohoku-oki tsunami; asking whether self-mutilation or self-amputation in sea lilies was an adaptive response during the Paleozoic; discovering that powerfully erosive behavior can occur even on the lee side of a topographic barrier; and demonstrating for the first time that the PTB biotic crisis was probably triggered by enormous Plinian eruptions.
Highlights are provided below. Geology articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary Geology articles by contacting Kea Giles at the address above.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geology in articles published. Contact Kea Giles for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Abrupt landscape change post-6 Ma on the central Great Plains, USA
R.A. Duller et al., Dept. of Earth Science and Engineering, Imperial College, London SW7 2AZ, UK, and Dept. of Earth & Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G32919.1.
An increase in energy is required to deeply dissect and erode mountainous landscapes, work that is accomplished primarily by sediment-charged water. This energy increase can be provided by either increasing the slope of the river system, for instance by tectonic uplift, or by increasing the water discharge into the river system, for instance by a climatic shift. The sedimentary and geomorphic impact of these two competing roles should, in principal, be distinguishable in the sedimentary record, however this still remains challenging. A grasp of this is important because Earth scientists rely on landscape features and the sedimentary record, in combination with analytical datasets, to make inferences about past geological events and future landscape scenarios. Nowhere else has the issue of "tectonics" versus "climate" been as contentious as in the Rocky Mountains-Great Plains region of the United States. Building upon the plethora of data and concepts in the region, we contribute new data from key sedimentary units and paleo-surfaces, and add a different perspective on landscape evolution in the Rocky Mountains-Great Plains region. Crucially we constrain the timing of potential phases of surface uplift and landscape development and offer a more integrative model of the competing roles of that "tectonics" and "climate" might play in the region.
Early diagenesis of sulfur in a tropical upwelling system, Cabo Frio, southeastern Brazil
R. Diaz et al., Projeto Ressurgencia Team, Programa de Geoquímica Ambiental, Universidade Federal Fluminense, Outeiro São João Baptista s/n., Niterói RJ 24020-150, Brazil. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33111.1.
R. Diaz and colleagues investigated the behavior of inorganic of sulfur species, including sulfate, dissolve sulfide, iron monosulfide and pyrite (FeS2) to evaluate the sulfur redox cycling in sediments sampled across the upwelling area of Cabo Frio, southeastern Brazil. The pyrite formation was limited by availability of reduced sulfur species, since there were oxidizing conditions, as indicated by a low degree of iron pyritization. Textural analysis of pyrite was carried out by electronic microscopy, revealing degradation of pyrite crystal surfaces and processes of crystal overgrowth. These textural characteristics also indicate oxidant conditions in the sediments and sulfide reoxidation cycling. These findings are probably explained by sediment disturbance by faunal activity (bioturbation) and by physical disturbance by overlaying water currents (hydrodynamic features). Sulfate sulfur isotopes and sulfate concentration distribution in sediment pore-water demonstrated that there was low sulfate reduction in the study sites, while pyrite sulfur isotopes indicated a strong importance of sulfur re-oxidation cycling in all sites.
Belemnites originated in the Triassic -- A new look at an old group
Yasuhiro Iba et al., Hokkaido University of Education, Kushiro 085-8580, Japan. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33402.1.
Belemnites (order Belemnitida), a very successful group of Mesozoic cephalopods, provide an important clue for understanding Mesozoic marine ecosystems and the origin of modern cephalopods. Following current hypotheses, belemnites originated in the earliest Jurassic (Hettangian, 201.6 to 197 million years ago) with very small forms. According to this view, their paleobiogeographic distribution was restricted to northern Europe until the Pliensbachian (190 to 183 million years ago). Yasuhiro Iba and colleagues report two belemnite taxa from the Hettangian of Japan: a new species of the Sinobelemnitidae and a large taxon of the suborder Belemnitina. The Sinobelemnitidae, which may be included in the future in a new suborder, have also been recorded from specimens from the Triassic of China, and so far are poorly understood. New findings presented by Iba and colleagues therefore (1) extend the origin of the belemnites back by ~33 million years into the Triassic; (2) suggest that this group did not necessarily originate in northern Europe, and (3) imply that belemnites survived the Triassic-Jurassic extinction, one of the five big mass extinctions in the Phanerozoic. Because belemnites provided a considerable amount of food as prey, the origination of belemnites is probably an important event also for the evolution of their predators, such as marine reptiles and sharks.
Rapid reequilibration of H2O and oxygen fugacity in olivine-hosted melt inclusions
Glenn A. Gaetani et al., Dept. of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G32992.1.
Globally, ~500 million people live in close proximity to active volcanoes; there is an average of 50 to 70 volcanic eruptions each year. As magmas ascend toward the surface, the amount of water that they can hold decreases dramatically. Most of the water initially in the magma degasses, or "boils off," leading to violently explosive eruptions. Understanding these eruptions requires knowing how much water was originally in the magmas. As magmas cool and begin to crystallize deep below volcanoes, growing crystals trap tiny pockets of magma, known as melt inclusions. Melt inclusions are analyzed to determine pre-eruptive water contents of the magma from which they formed, because the strength of the host crystal allows them to retain their water. Glenn A. Gaetani and colleagues carried out experiments to investigate how much water is lost from melt inclusions via diffusion, or movement of individual atoms, through the structure of the host crystal. Their results demonstrate that this process is so efficient that pre-eruptive water contents can only be reliably determined from melt inclusions that erupted and cooled very quickly. Further, if the parent magma lost water while it was sitting deep below the volcano, the concentration of water in the melt inclusion will adjust to the new, lower concentration.
Large-scale active slump of the southeastern flank of Pico Island, Azores
A. Hildenbrand et al., Univ Paris-Sud, Laboratoire IDES, UMR8148, Orsay, F-91405, France. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33303.1.
Volcanic ocean islands are particularly prone to flank instability. Their growth is generally punctuated by large lateral failures, which can cause considerable damage, including the triggering of destructive tsunamis. A. Hildenbrand and colleagues report evidence for ongoing lateral movement of part of the SE flank of the Pico volcanic ridge in the Azores. Their new data show that the unstable sector is several cubic kilometers in size and involves the slow motion and rotation of large individual blocks. The central outer part of the island flank, especially, is displacing faster than the inner parts, which suggests recent individualization of a steep seaward-dipping fault in the moving mass. This unstable portion of Pico volcanic ridge therefore constitutes a priority target for further monitoring and hazards assessment.
Seasonal Laurentide Ice Sheet melting during the "Mystery Interval" (17.5-14.5 ka)
Carlie Williams et al., College of Marine Science, University of South Florida, 140 7th Avenue S, St. Petersburg, Florida 33701, USA. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33279.1.
The last deglaciation in the Northern Hemisphere was interrupted by two major stadials, the so-called "Mystery Interval" (17.5 to 14.5 thousand years ago) and the Younger Dryas (12.9 to 11.7 thousand years ago). During these events, the North Atlantic region was marked by cold surface conditions, yet simultaneous glacier and snowline retreat. Rerouting of Laurentide Ice Sheet meltwater from the Gulf of Mexico to an eastern or northern spillway may have reduced meridional overturning circulation at the onset of the Younger Dryas. However, this hypothesis has not been tested for the Mystery Interval. Paired Mg/Ca and delta-18O measurements on foraminifera from laminated Orca Basin sediments in the Gulf of Mexico, constrained by 35 carbon-14 dates, document the timing of meltwater input with subcentennial resolution. Isolating the delta-18O of seawater reveals three major melting episodes from about 17.5 thousand years ago until 12.9 thousand years ago, followed by a rapid cessation, consistent with meltwater rerouting at the onset of the Younger Dryas. Conversely, inferred meltwater flow to the Gulf of Mexico during the Mystery Interval does not support a simple routing event, but it is consistent with glacier and snowline retreat. Carlie Williams and colleagues suggest that summer melting of Northern Hemisphere ice sheets during this stadial may have been an important mechanism for enhanced winter sea-ice formation, hypercold winter conditions, and enhanced seasonality in the North Atlantic region.
Liquefaction as an important source of the A.D. 2011 Tohoku-oki tsunami deposits at Sendai Plain, Japan
Kazuhisa Goto et al., Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Narashino, Tsudanuma, Chiba 275-0016, Japan; and Disaster Control Research Center, Tohoku University, 6-6-11-1106 Aoba, Sendai, Miyagi 980-8579, Japan. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33123.1.
This paper by Kazuhisa Goto and colleagues describes the topographic change and the recovery process as well as the sediment sources for tsunami deposits based on field surveys and analysis of DEM data before and after the 2011 Tohoku-oki tsunami at the Sendai Plain, Japan. Goto and colleagues found that the amount of sediment deposited on land was approximately four times greater than the eroded volume of sediment at the beach. Large amounts of the sediments deposited at the studied transect probably originated from liquefaction. This result suggests that the vented sediments might have been an important source of the tsunami deposits, if liquefaction is generated by the strong ground motion of a near-field earthquake. In contrast, minor erosion was observed at the beach, and the beach berm was rebuilt within three months after the tsunami. Moreover, the erosional channel that had cut into the beach had been filled by sand within 13 days after the tsunami. Therefore, it is not expected that a sedimentary record of the tsunami will be preserved in the nearshore zone along the Sendai coast, although remnants of small scours on land might remain long after the tsunami.
Plagioclase compositions give evidence for in situ crystallization under horizontal flow conditions in mafic sills
Rais Latypov and Vera Egorova, Dept. of Geosciences, University of Oulu, Linnanmaa 90014, Finland. Posted online ahead of print 9 Aug. 2012; doi: 10.1130/G33173.1.
The relative roles of crystal settling and in situ crystallization in the formation of igneous bodies are still a subject of debate in igneous petrology. Here, Rais Latypov and Vera Egorova of the University of Oulu, Finland, show that plagioclase composition in dolerite sills can be best explained by in situ crystallization involving two distinct stages. During the initial stage, the sills evolved as an open system that was continuously filled by magmas, becoming more primitive with time. This gave rise to marginal zones, with minerals showing reverse compositional trends. During the subsequent stage, the sill evolved as a closed system by fractional crystallization. This resulted in minerals showing normal compositional trends. Several other mafic sills and layered intrusions show similar variations in plagioclase compositions, indicating that open-system, in situ crystallization may be a common process in the origin of igneous bodies.
Detrital zircon record and tectonic setting
P.A. Cawood et al., Dept. of Earth Sciences, University of St Andrews, North Street, St Andrews KY16 9AL, UK; and School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G32945.1.
Detrital zircon spectra reflect the tectonic setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional, and intracratonic settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.
Was autotomy a pervasive adaptation of the crinoid stalk during the Paleozoic?
Stephen K. Donovan, Netherlands Centre for Biodiversity-Naturalis, Postbus 9517, NL-2300 RA Leiden, Netherlands. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33262.1.
The stalked crinoids, or sea lilies, that are alive at the present day but living in the deep sea are relations of seastars and sea urchins. They were diverse and numerically common in the Paleozoic from about 480 to 250 million years ago, as well as being one of the most important groups of sessile invertebrates. Autotomy (self-mutilation or self-amputation) in the stem -- the casting off of the more distal part -- is only known to occur in one extant group of crinoids. Unlike these crinoids, Paleozoic species lacked specialized articulations adapted to autotomy but had other adaptations that may have singly or collectively facilitated autotomy. These include the linking of adjacent short lengths of stalk by short ligaments that the organism could dissolve; the evolutionary trend to narrow the axial canal; occlusion of the axial canal in some species; and the ability to seal the axial canal. The ability to autotomize the distal stem in many Paleozoic crinoid clades was, at the very least, latent; if overt, it would have facilitated escape from environmental disturbance. Authors Stephen K. Donovan and colleagues note that his may, in part, explain the great abundance of disarticulated crinoid stem fragments in many Paleozoic sedimentary deposits.
Large, deepwater slope failures: Implications for landslide-generated tsunamis
Claudio Lo Iacono et al., Unidad de Tecnología Marina, CSIC, 08003 Barcelona, Spain. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33446.1.
This paper yields fresh insights on the potential tsunami hazard related to deep-water landslides (deeper than 3000 m). Deep-water landslides are often underestimated as tsunami triggers, mainly due to the lack of high-resolution data required to assess this potential hazard and because only large-volume landslides will generate dangerous tsunami waves at these water depths. The North Gorringe debris avalanche (NGA) is a large (about 80 cubic kilometers), deep-water (2900 m to 5100 m depth) mass failure located at the northern flank of the Gorringe Bank on the SW Iberian Margin. Tsunami simulations show that a mass failure similar to the NGA could generate a wave higher than 15 meters that would hit the south Portuguese coasts in about 30 minutes. Steep slopes and pervasive fracturing are suggested as the main preconditioning factors for the NGA, while an earthquake is the most likely trigger mechanism. Geological factors that favored the NGA failure still persist in the area, suggesting that deep-water landslides require more attention in geo-hazard assessments models of Southern Europe, as well as at a global scale, in seismically active margins.
Mechanism of leached layer formation during chemical weathering of silicate minerals
Encarnación Ruiz-Agudo et al., Institut für Mineralogie, University of Münster, Corrensstrasse 24, 48149, Münster, Germany; and Dept. of Mineralogy and Petrology, University of Granada, Fuentenueva s/n, 18071 Granada, Spain. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33339.1.
The dissolution of many silicate minerals and glasses is typically incongruent, as shown by the nonstoichiometric release of the solid phase elements. This results in the formation of so-called surface leached layers, depleted in some elements with respect to the bulk mineral. Due to the important effects these layers may have on mineral dissolution rates and secondary mineral formation, they have attracted a great deal of research. However, the mechanism of leached layer formation is a matter of vigorous debate. Encarnación Ruiz-Agudo and colleagues report on an in situ Atomic Force Microscopy study of the dissolution of wollastonite, CaSiO3, as an example of leached layer formation during dissolution. Their in situ AFM results provide, for the first time, clear direct experimental evidence that leached layers are formed in a two-step process: stoichiometric dissolution of the original mineral surfaces and subsequent precipitation of a secondary phase (most likely amorphous silica) from a thin layer of fluid in contact with the mineral surface. The results reported here differ significantly from the concept of preferential release of specific elements from the mineral, as postulated by most currently accepted models. The confirmation of this model has important implications in understanding and evaluating dissolution kinetics of major rock-forming minerals.
Nature of alkali-carbonate fluids in the sub-continental lithospheric mantle
A. Giuliani et al., School of Earth Sciences, University of Melbourne, Parkville, 3010 Victoria, Australia. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33221.1.
The Earth mantle underlying continents has been largely modified by fluids of various origin and composition throughout Earth's history. A great deal of effort has been dedicated in the last 30 to 40 years to unravel the nature of mantle fluids by studying the effects of these fluids on mantle rocks. Pristine fluids are sometimes preserved as inclusions in mantle minerals, which provide direct evidence for the fluids once percolating the Earth mantle. In this study, A. Giuliani and colleagues describe the occurrence of unique inclusions of alkali carbonate fluids in large grains of ilmenite from a mantle polymict breccia sampled by the Bultfontein kimberlite (South Africa). Alkali carbonate fluids are often inferred to modify the composition of rocks from the sub-continental mantle, but have been only observed before as inclusions in diamonds. Therefore, Giuliani and colleagues propose that the alkali carbonate fluids documented here represent one of the major modifying agents of the Earth mantle.
A "Great Deepening": Chronology of rift climax, Corinth rift, Greece
M.R. Leeder et al., University of East Anglia, School of Environmental Sciences, Norwich NR4 7TJ, UK. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33360.1.
Continental rifts are linear depressions where Earth's crust is stretched and pulled apart by forces generated by plate motions with characteristic normal faulting earthquakes. Examples are the East Africa, Rio Grande, Rhine, Baikal, and Corinth rifts. Ancient examples like the North Sea rift contain abundant hydrocarbon reserves whose successful exploitation demands knowledge of subsurface rift architecture, chiefly the spatial disposition of faulting and sedimentary detritus swept into the developing rift. Onset of rifting is often marked by distributed extension across many parallel faults, an arrangement that evolves into more focused and therefore more rapid extension. However, in most cases the timing of this change in rifting style is obscure. In the fastest extending rift, at Corinth, central Greece, discovery of a prominent volcanic ash amongst early deep-water rift deposits and its accurate and precise dating by argon isotopes to 2.55 million years enables estimates of the onset of focused rapid extension. By comparison with a previous estimate made elsewhere in the rift it seems that the rift propagated in the manner of an eastward-opening zipper, with the southern rift margin rotating anticlockwise and migrating northwards through time. Such rotations were probably controlled by tractional gradients in the plastic deep crust.
Erosional characteristics and behavior of large pyroclastic density currents
Claudio Scarpati and Annamaria Perrotta, Dipartimento di Scienze della Terra, Università di Napoli, Largo San Marcellino 10, 80138 Naples, Italy. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33380.1.
Large pyroclastic density currents represent the most lethal and destructive product of explosive eruptions. These mixtures of hot volcanic particles and gas sweep across the ground, burying the landscape under thick pyroclastic sequences. Significant erosion occurs during the emplacement of pyroclastic currents. This paper by Claudio Scarpati and Annamaria Perrotta documents the depth and distribution of erosion at the base of pyroclastic current deposits in Italy to give some insight into how pyroclastic currents erode. These eruptions emplaced widespread ignimbrites that rest on basal fall deposits with sharp or erosive contacts. The loose packed basal fall deposits show a wide range of depth of erosion. Their field observations show how strongly erosive pyroclastic currents can be. They find that two factors have had impact on the erosive pattern of the ignimbrites studied: In a low-relief landscape, the depth of erosion is strongly controlled by the distance from the vent, decreasing steadily outward. When pyroclastic currents traveled across very rugged topography, the strongest erosion occurs at low elevations. A consequence of this behavior for risk assessment of inhabited areas is that powerfully erosive behavior can occur even on the lee side of a topographic barrier.
Lateral fold growth and fold linkage
Bernhard Grasemann and Stefan M. Schmalholz, Dept. for Geodynamics and Sedimentology, University of Vienna, A-1090 Vienna, Austria. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33613.1.
Bernard Grasemann and Stefan M. Schmalholz use a three-dimensional numerical computer model to investigate the growth and linkage of initially isolated fold segments during shortening of rocks. Depending on the initial geometry of the model setup, four different linkage modes, which result in either simple cylindrical folds or folds with complex curved shapes, could be observed. All model results have in common that the linked folds are separated by a saddle point. Grasemann and Schmalholz compare their model results with natural examples of lateral fold linkage from the Zagros mountains (Kurdistan Region of Iraq) and point out the economic interest, especially for hydrocarbon exploration, of saddle points linking the initially isolated fold segments.
Negative C-isotope excursions at the Permian-Triassic boundary linked to volcanism
Jun Shen et al., State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, P.R. China; and Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33329.1.
The mass extinction at the Permian-Triassic boundary (PTB), the largest in Earth's history, killed off ~90% of marine species. It is thought to have been caused by the Siberian Traps, a large flood basalt province that erupted ~252 million years ago, but this connection has been inferred on the basis of similarities in age and, thus, is circumstantial. This study by Jun Shen and colleagues documents a strong relationship between marine environmental changes and the deposition of volcanic ash layers in south China, demonstrating for the first time that the PTB biotic crisis was probably triggered by enormous Plinian (explosive) volcanic eruptions. Previous studies of ash layers in south China PTB sections have invoked a source in regional subduction-zone volcanism in the eastern Tethys, but the present study infers an origin as distal deposits of the Siberian Traps. Each ash layer in the Chinese sections shows a negative excursion in the ratio of carbonate or organic carbon isotopes, implying large releases of isotopically light, organic-derived CO2 or methane in conjunction with each eruptive event, consistent with large-scale magma injection into organic-rich sediments of the West Siberian coal province. If confirmed by further investigation, these results will have important implications both for kill mechanisms during the PTB crisis as well as for refinement of the eruption history of the Siberian Traps.
Estimating magnitudes of relative sea-level change in a coarse-grained fan delta system: Implications for Pennsylvanian glacioeustasy
Dustin E. Sweet, Dept. of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA; and Gerilyn S. Soreghan. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33225.1.
Estimating magnitudes of relative sea-level fluctuations from the sedimentary record is challenging but illuminates possible mechanisms of sea-level change. Dustin E. Sweet and Gerilyn S. Soreghan provide a new approach to calculate magnitudes of relative sea-level change using reconstructed depositional slopes and down-dip-to-up-dip extents of transgression across those slopes. Magnitudes of rise on transgressive surfaces within lower to middle Atokan (upper Bashkirian to middle Moscovian) fan-delta strata of the Fountain Formation (Colorado) range from minimums of ~8 to 15 m to maximums of ~13 to 20 m. These estimates are low relative to other intervals of the Permian-Pennsylvanian, but approach estimates from coeval deposits elsewhere. Each cycle shows evidence for the rise and fall of sea level that, coupled with the magnitude, and coincidence with inferred glacioeustatic cyclothems elsewhere, suggests a glacioeustatic origin.
How travertine veins grow from top to bottom and lift the rocks above them: The effect of crystallization force
Jean-Pierre Gratier et al., ISTerre, University Grenoble 1 & CNRS, BP 53, Grenoble 38041, France. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33286.1.
Travertine mounds form at the mouth of springs where CO2 degassing drives carbonate precipitation from water flowing from depth. Building of such mounds commonly involves the successive "stratigraphic" (bottom-to-top) deposition of carbonate layers that precipitate from waters rising from depth along vertical to horizontal open fissures that are episodically sealed. Much more intriguing structures can also be observed, such as widespread horizontal white veins of carbonate with vertical aragonite fibers, parallel or oblique to the "stratigraphic" travertines, which extend laterally over distances of several tens of meters and could represent up to 50% of the total volume of the travertine mound. Using highly precise U-Th dating, the growth direction of these horizontal veins is shown to be from top to bottom and this fact clearly indicates that they developed within the mound over a period of ~1000 years for the vein analyzed. A vein growth mechanism is proposed that is able to uplift the rock above the vein thanks to the force of crystallization. Development of such structures may occur in other places. Great care must be taken when interpreting travertine data from samples removed from drill hole since the ages of successive layers are not necessarily in stratigraphic order.
How tillite weathering during the snowball Earth aftermath induced cap carbonate deposition
Sébastien Fabre and Gilles Berger, IRAP (Institut de Recherche en Astrophysique et Planétologie), CNRS Université Paul Sabatier-IRD, 14 avenue Edouard Belin, 31400 Toulouse, France. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33340.1.
The first part of Earth's history, namely Precambrian (before 570 million years) saw the development of several global glaciations the extent of which is totally unique so far. The aftermath of this phenomenon was accompanied by the rapid deposition of worldwide intriguing thick carbonates sediments: the "cap carbonates." Sébastien Fabre and Gilles Berger discuss a numerical scenario about the origin and the timing of these depositions. The succession of events is as follows: the elevated carbon dioxide (CO2) atmospheric concentration (probably 300 times the present-day value) emitted by volcanoes lead to the deglaciation. Then, aggressive, acidic rains promoted an intense weathering of the glacial clastic sediments, forming the essential part of the continental surface (the emerged surfaces were totally devoid of plants). As a result, the dissolved CO2 and the leached cations were carried toward the ocean by the rivers where they could have conducted to the cap carbonate deposition. The final consequence was the diminution of the CO2 atmospheric concentration, trapped in the carbonates. This is an example of feed-back in the carbon cycle at the geological scale.
Investigating the Mid-Brunhes Event in the Spanish terrestrial sequence
Hugues-Alexandre Blain et al., IPHES (Institut Català de Paleoecologia Humana i Evolució Social), c/Escorxador s/n, 43003 Tarragona, Spain, and Àrea de Prehistòria, Universitat Rovira i Virgili (URV), Avinguda de Catalunya 35, 43002 Tarragona, Spain. Posted online ahead of print 22 Aug. 2012; doi: 10.1130/G33427.1.
Investigating how climate has changed in the past (in particular how hot were the warm periods of the Quaternary) may help us to understand how it may change in the future and its consequences on temperature and water resources. In the Mediterranean area, which is climatically stressed by limited water resources and extremes of heat, climate variations are known to play a crucial role in the ecosystems and environment. The Gran Dolina sequence (Sierra de Atapuerca, north-central Spain) provides a unique, long paleontological and archaeological record that documents the evolution of north Iberian ecosystems during the last million years. A fossil amphibian and reptile-based reconstruction of temperature and precipitation shows marked peaks that have been related to various interglacial peaks in accordance with numeric dates, and paleomagnetic and biochronological data. An analysis of the successive interglacial periods of the last million years has revealed that interglacials become warmer and drier after about 450 thousand years ago, and most of them were warmer than present day. As a result, climate trend in the Mediterranean area is found to be congruous with global climate changes as reconstructed from ice and sea surface temperature records over the last million years.