http://www.solid-earth-discuss.net/Solid Earth Discussions Latest Articlesenhttp://www.solid-earth-discuss.net/4/241/2012/<b>High resolution reflection seismic profiling over the Tjellefonna fault in the Møre-Trøndelag Fault Complex, Norway</b><br /><br />Solid Earth Discussions, 4, 241-278, 2012<br /><br />Author(s): E. Lundberg, C. Juhlin, and A. Nasuti<br /><br />The Møre-Trøndelag Fault Complex (MTFC) is one of the most prominent fault zones of Norway, both onshore and offshore. In spite of its importance, very little is known of the deeper structure of the individual fault segments comprising the fault complex. Most seismic lines have been recorded offshore or focused on deeper structures. This paper presents results from two reflection seismic profiles, located on each side of the Tingvollfjord, acquired over the Tjellefonna fault in the south-eastern part of the MTFC. Possible kilometer scale vertical offsets reflecting, large scale north-west dipping normal faulting separating the high topography to the south-east from lower topography to the north-west have been proposed for the Tjellefonna fault. In this study, however, the Tjellefonna fault is interpreted to dip approximately 50–60° towards the south-east to depths of at least 1.4 km. Travel-time modeling of reflections associated with the fault was used to establish the geometry of the fault structure at depth and detailed analysis of first P-wave arrivals in shot-gathers together with resistivity profiles were used to define the near surface geometry of the fault zone. A continuation of the structure on the north-eastern side of the Tingvollfjord is suggested by correlation of an in strike direction P-S converted reflection (generated by a fracture zone) seen on the reflection data from that side of the Tingvollfjord. The reflection seismic data correlate well with resistivity profiles and recently published near surface geophysical data. A highly reflective package forming a gentle antiform structure was also identified on both seismic profiles. The structure may be an important boundary within the gneissic basement rocks of the Western Gneiss Region. The Fold Hinge Line is parallel with the Tjellefonna fault trace while the topographic lineament diverges, following secondary fracture zones towards north-east.Fri, 03 Feb 2012 00:00:00 +0100http://www.solid-earth-discuss.net/4/203/2012/<b>Possibility of titanium transportation within a mantle wedge: formation process of titanoclinohumite in Fujiwara dunite in Sanbagawa belt, Japan</b><br /><br />Solid Earth Discussions, 4, 203-239, 2012<br /><br />Author(s): S. Ishimaru and S. Arai<br /><br />Titinoclinohumite-bearing dunites from Fujiwara, the Sanbagawa metamorphic belt of high-pressure type, Japan, were described to examine the possibility of Ti mobility during metasomatism within the mantle wedge. The Fujiwara dunite body and surrounding high-pressure Sanbagawa schists possibly form a subduction complex, and the dunites are a good analogue to the mantle wedge overlying the slab. The Fujiwara dunites are of deserpentinization origin; the deserpentinized olivine is high in Fo (up to 96) and low in NiO (0.2 to 0.3 wt %), and contains magnetite inclusions. Titanoclinohumites are associated with the deserpentinized olivine, as lamellar intergrowth or veinlets, up to 1 cm in width. Other metamorphic minerals include antigorite, brucite, chlorite, ilmenite, perovskite, Ti-rich ludwigite, and carbonates. The protolith of the Fujiwara dunite was partially serpentinized cumulative dunites from intra-plate magma, containing relatively low-Fo (85 to 86) olivines and TiO₂-rich (up to 3 wt %) chromian spinels. The metamorphic olivines and titanoclinohumites contain micro-inclusions of methane (CH₄) with or without serpentine and brucite. The source of Ti for titanoclinohumite was possibly the Ti-rich chromian spinel, but Ti was mobile through hydrocarbon-rich fluids, which were activated during the metamorphism. The hydrocarbons, of which remnants are carbonates and methane micro-inclusions, were derived from carbonaceous materials or bitumen, possibly incorporated in the precursory serpentinized and brecciated peridotite (= the protolith for the Fujiwara dunites) before subduction. Ti can be mobile in the mantle wedge if hydrocarbons are available from the subducted slab.Fri, 27 Jan 2012 00:00:00 +0100http://www.solid-earth-discuss.net/4/173/2012/<b>New developments in the analysis of volcanic pyroclastic density currents through numerical simulations of multiphase flows</b><br /><br />Solid Earth Discussions, 4, 173-202, 2012<br /><br />Author(s): S. Lepore and C. Scarpati<br /><br />A granular multiphase model has been used to evaluate the action of differently sized particles on the dynamics of fountains and associated pyroclastic density currents. The model takes into account the overall disequilibrium conditions between a gas phase and several solid phases, each characterized by its own physical properties. The dynamics of the granular flows has been simulated by adopting a Reynolds Average Navier-Stokes model for describing the turbulence effects. Numerical simulations have been carried out by using different values for the eruptive column temperature at the vent, solid particles frictional concentration, turbulent kinetic energy, and dissipation. The results obtained underline the importance of the multiphase nature of the model and characterize several disequilibrium effects. The low concentration (&le; 5 &middot; 10^&ndash;4) sectors lie in the upper part of the granular flow, above the fountain, and above the pyroclastic current tail and body as thermal plumes. The high concentration sectors, on the contrary, form the fountain and remain along the ground of the granular flow. Hence, pyroclastic density currents are assimilated to granular flows constituted by a low concentration suspension flowing above a high concentration basal layer (boundary layer), from the proximal regions to the distal ones. Interactions among solid, differently sized particles in the boundary layer of the granular flow are controlled by collisions between particles, whereas particles dispersal in the suspension is determined by the dragging of the gas phase. The simulations describe well the dynamics of a tractive boundary layer leading to the formation of stratified facies during eruptions having a different magnitude.Thu, 26 Jan 2012 00:00:00 +0100http://www.solid-earth-discuss.net/4/131/2012/<b>Geomagnetic jerks characterization via spectral analysis</b><br /><br />Solid Earth Discussions, 4, 131-172, 2012<br /><br />Author(s): B. Duka, A. De Santis, M. Mandea, A. Isac, and E. Qamili<br /><br />In this study we have applied spectral techniques to analyze geomagnetic field time-series provided by observatories, and compared the results with those obtained from analogous analyses of synthetic data estimated from models. Then, an algorithm is here proposed to detect the geomagnetic jerks in time-series, mainly occurring in the Eastern component of the geomagnetic field. Applying such analysis to time-series generated from global models has allowed us to depict the most important space-time features of the geomagnetic jerks all over the globe, since the beginning of XXth century. Finally, the spherical harmonic power spectra of the third derivative of the main geomagnetic field has been computed from 1960 to 2002.5, bringing new insights to understanding the spatial evolution of these rapid changes of the geomagnetic field.Fri, 20 Jan 2012 00:00:00 +0100http://www.solid-earth-discuss.net/4/33/2012/<b>Earth's rotation variations and earthquakes 2010&ndash;2011</b><br /><br />Solid Earth Discussions, 4, 33-130, 2012<br /><br />Author(s): L. Ostřihanský<br /><br />In contrast to unsuccessful searching (lasting over 150 years) for correlation of earthquakes with biweekly tides, the author found correlation of earthquakes with sidereal 13.66 days Earth's rotation variations expressed as length of a day (LOD) measured daily by International Earth's Rotation Service. After short mention about earthquakes M 8.8 Denali Fault Alaska 3 November 2002 triggered on LOD maximum and M 9.1 Great Sumatra earthquake 26 December 2004 triggered on LOD minimum and the full Moon, the main object of this paper are earthquakes of period 2010–June 2011: M 7.0 Haiti (12 January 2010 on LOD minimum, M 8.8 Maule Chile 12 February 2010 on LOD maximum, map constructed on the Indian plate revealing 6 earthquakes from 7 on LOD minimum in Sumatra and Andaman Sea region, M 7.1 New Zealand Christchurch 9 September 2010 on LOD minimum and M 6.3 Christchurch 21 February 2011 on LOD maximum, and M 9.1 Japan near coast of Honshu 11 March 2011 on LOD minimum. It was found that LOD minimums coincide with full or new Moon only twice in a year in solstices. To prove that determined coincidences of earthquakes and LOD extremes stated above are not accidental events, histograms were constructed of earthquake occurrences and their position on LOD graph deeply in the past, in some cases from the time the IERS (International Earth's Rotation Service) started to measure the Earth's rotation variations in 1962. Evaluations of histograms and the Schuster's test have proven that majority of earthquakes are triggered in both Earth's rotation deceleration and acceleration. Because during these coincidences evident movements of lithosphere occur, among others measured by GPS, it is concluded that Earth's rotation variations effectively contribute to the lithospheric plates movement. Retrospective overview of past earthquakes revealed that the Great Sumatra earthquake 26 December 2004 had its equivalent in the shape of LOD graph, full Moon position, and character of aftershocks 19 years earlier in difference only one day to 27 December 1985 earthquake, proving that not only sidereal 13.66 days variations but also that the 19 years Metons cycle is the period of the earthquakes occurrence. Histograms show the regular change of earthquake positions on branches of LOD graph and also the shape of histogram and number of earthquakes on LOD branches from the mid-ocean ridge can show which side of the ridge moves quicker.Thu, 19 Jan 2012 00:00:00 +0100http://www.solid-earth-discuss.net/4/1/2012/<b>The lithosphere-asthenosphere boundary observed with USArray receiver functions</b><br /><br />Solid Earth Discussions, 4, 1-31, 2012<br /><br />Author(s): P. Kumar, X. Yuan, R. Kind, and J. Mechie<br /><br />The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities and has no problems with multiples like P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary which we correlate with the lithosphere-asthenosphere boundary (LAB) since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas over the Colorado Plateau to the Columbia Basalts. These observations agree well with tomography results in the westernmost USA and at the east coast. However, in the central cratonic part of the USA the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) or Lekic and Romanowicz (2011) as a recently discovered mid lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).Fri, 06 Jan 2012 00:00:00 +0100http://www.solid-earth-discuss.net/3/1021/2011/<b>Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area</b><br /><br />Solid Earth Discussions, 3, 1021-1036, 2011<br /><br />Author(s): P. Tong, D. Zhao, and D. Yang<br /><br />High resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent low-velocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future.Thu, 22 Dec 2011 00:00:00 +0100http://www.solid-earth-discuss.net/3/1001/2011/<b>Effect of glacial-interglacial sea-level changes on the displacement and stress field in the forearc and along the plate interface of subduction zones</b><br /><br />Solid Earth Discussions, 3, 1001-1019, 2011<br /><br />Author(s): T. Li and A. Hampel<br /><br />Combined seismological, space-geodetic and numerical studies have shown that the seismicity at subduction zones may be modulated by tides and glacier fluctuations on timescales of 1–100 a, because these changes in loads on Earth's surface are able to alter the stress field in the upper plate and along the plate interface. Here we use a two-dimensional finite-element model of a subduction zone to investigate how glacial-interglacial sea-level changes affect the forearc region and the plate interface. The model results show that a sea-level fall by 125 m over 100 ka causes up to 0.7 m of vertical displacement, with the maximum uplift occurring between the trench and the coast. The uplift signal induced by the sea-level fall decreases to zero ~20 km landward of the coastline. A subsequent sea-level rise by 125 m over 20 ka causes subsidence, which is again most pronounced in the submarine part of the forearc. The sea-level changes cause horizontal displacements of up to 0.12 m, which are directed seaward during sea-level fall and landward during sea-level rise. With respect to the stress field, the sea-level changes lead to variations in the vertical stress and the shear stress of up to 1.23 MPa and 0.4 MPa, respectively. The shear stress variations are highest beneath the coast, i.e. in the area where the sea-level changes cause the strongest flexure. The resulting Coulomb stress changes on the plate interface are of the order of 0.2–0.5 MPa and indicate that earthquakes are promoted during sea-level fall and delayed during sea-level rise. Our findings imply that eustatic sea-level changes during glacial-interglacial periods may have induced displacements and stress changes that were large enough to affect the seismic cycle of subduction thrusts.Wed, 21 Dec 2011 00:00:00 +0100http://www.solid-earth-discuss.net/3/975/2011/<b>Floating sandstones off El Hierro (Canary Islands, Spain): the peculiar case of the October 2011 eruption</b><br /><br />Solid Earth Discussions, 3, 975-999, 2011<br /><br />Author(s): V. R. Troll, A. Klügel, M.-A. Longpré, S. Burchardt, F. M. Deegan, J. C. Carracedo, S. Wiesmaier, U. Kueppers, B. Dahren, L. S. Blythe, T. Hansteen, C. Freda, D. A. Budd, E. M. Jolis, E. Jonsson, F. Meade, S. Berg, L. Mancini, and M. Polacci<br /><br />The eruption that started off the south coast of El Hierro, Canary Islands, in October 2011 has emitted intriguing eruption products found floating in the sea. These specimens appeared as floating volcanic "bombs" that have in the meantime been termed "restingolites" (after the close-by village of La Restinga) and exhibit cores of white and porous pumice-like material. Currently the nature and origin of these "floating stones" is vigorously debated among researchers, with important implications for the interpretation of the hazard potential of the ongoing eruption. The "restingolites" have been proposed to be either (i) juvenile high-silica magma (e.g. rhyolite), (ii) remelted magmatic material (trachyte), (iii) altered volcanic rock, or (iv) reheated hyaloclastites or zeolite from the submarine slopes of El Hierro. Here, we provide evidence that supports yet a different conclusion. We have collected and analysed the structure and composition of samples and compared the results to previous work on similar rocks found in the archipelago. Based on their high silica content, the lack of igneous trace element signatures, and the presence of remnant quartz crystals, jasper fragments and carbonate relicts, we conclude that "restingolites" are in fact xenoliths from pre-island sedimentary rocks that were picked up and heated by the ascending magma causing them to partially melt and vesiculate. They hence represent messengers from depth that help us to understand the interaction between ascending magma and crustal lithologies in the Canary Islands as well as in similar Atlantic islands that rest on sediment/covered ocean crust (e.g. Cape Verdes, Azores). The occurrence of these "restingolites" does therefore not indicate the presence of an explosive high-silica magma that is involved in the ongoing eruption.Thu, 01 Dec 2011 00:00:00 +0100http://www.solid-earth-discuss.net/3/963/2011/<b>DInSAR coseismic deformation of the May 2011 M_w 5.1 Lorca earthquake, (Southern Spain)</b><br /><br />Solid Earth Discussions, 3, 963-974, 2011<br /><br />Author(s): T. Frontera, A. Concha, P. Blanco, A. Echeverria, X. Goula, R. Arbiol, G. Khazaradze, F. Pérez, and E. Suriñach<br /><br />The coseismic superficial deformation at the region of Lorca (Murcia, southeastern Spain) due to the M_w 5.1 earthquake occurred on 11 May 2011 was studied by a multidisciplinary team, integrating information from DInSAR, GPS and numerical modeling techniques. Despite the moderate magnitude of the event, quantitative information was obtained from the interferometric study of a pair of SAR images. Coseismic vertical deformation was differentiated from subsidence related to groundwater extraction at the footwall block through a numerical modeling deformation estimation based on elastic rupture dislocations. On the other hand, horizontal crustal deformation rates obtained from the analysis of a GPS network existent in the area are also coherent with the mechanism calculated for the earthquake.Wed, 09 Nov 2011 00:00:00 +0100http://www.solid-earth-discuss.net/3/943/2011/<b>The fate of fluids released from subducting slab in northern Cascadia</b><br /><br />Solid Earth Discussions, 3, 943-962, 2011<br /><br />Author(s): K. Ramachandran and R. D. Hyndman<br /><br />Large amounts of water carried down in subduction zones are driven upward into the overlying forearc upper mantle and crust as increasing temperature and pressure dehydrate the subducting crust. Through seismic tomography velocities we show that, (a) the overlying forearc mantle in Northern Cascadia is hydrated to serpentinite, and (b) the low Poisson's ratio at the base of the forearc lower crust that may represent silica deposited from the rising fluids. From the velocities observed in the forearc mantle, the volume of serpentinite estimated is ~30 %. This mechanically weak hydrated forearc region has important consequences in limits to great earthquakes and to collision tectonics. An approximately 10 km thick lower crustal layer of low Poisson's ratio (&sigma; = 0.22) in the forearc is estimated to represent a maximum addition of ~14 % by volume of quartz (&sigma; = 0.09). If this quartz is removed from rising silica-saturated fluids over long times it represents a significant addition of silica to the continental crust and an important contributor to its average composition.Fri, 21 Oct 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/901/2011/<b>Shallow water carbonate platforms (Late Aptian, Southern Apennines) in the context of supraregional to global changes</b><br /><br />Solid Earth Discussions, 3, 901-942, 2011<br /><br />Author(s): A. Raspini<br /><br />A preliminary study based on the comparison of recently published &delta;¹³C record of the Late Aptian Monte Tobenna and Monte Faito sections (Southern Italy) with reference carbon isotope curves reveals how sea-level fluctuations played a fundamental role in regolating the carbonate sedimentation in the inner lagoonal environments of the Apenninic platform and the occurrence of some peculiar facies during a time of increasing volcano-tectonic activity and trophic levels of the water. <br><br> During the lowering of the sea level, microbial carbonates were a common product of the shallow marine ecosystem in a general context of deterioration of the inner lagoon environmental conditions. When trophic levels were too high, due to the decisive contribution of a supraregional dictator (e.g. increase of the precipitation rate), and the environmental conditions were unsuitable for the main carbonate producers of the inner lagoonal settings, the Orbitolina (<i>Mesorbitolina parva</i> and <i>Mesorbitolina texana</i>) level formed, just before the minimum accommodation space on the platform was reached and fresh/brackish water environments spread. In deposits underlying the orbitolinid-rich facies of the carbonates studied, <i>Salpingoporella dinarica</i> alga is widespread, possibly due to the seawater's chemical composition that could have encouraged the development of its low-Mg calcite skeleton. On the contrary, during periods of sea level rise (and early highstand) no or minor microbial carbonates formed in the shallow lagoonal settings that were not influenced by the paleoenvironmental changes mostly induced by the mid-Cretaceous volcanism, and therefore easily remained in a healthy state.Fri, 21 Oct 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/857/2011/<b>Pore formation during dehydration of polycrystalline gypsum observed and quantified in a time-series synchrotron radiation based X-ray micro-tomography experiment</b><br /><br />Solid Earth Discussions, 3, 857-900, 2011<br /><br />Author(s): F. Fusseis, C. Schrank, J. Liu, A. Karrech, S. Llana-Fúnez, X. Xiao, and K. Regenauer-Lieb<br /><br />We conducted an in-situ X-ray micro-computed tomography heating experiment at the Advanced Photon Source (USA) to dehydrate an unconfined 2.3 mm diameter cylinder of Volterra Gypsum. We used a purpose-built X-ray transparent furnace to heat the sample to 388 K for a total of 310 min to acquire a three-dimensional time-series tomography dataset comprising nine time steps. The voxel size of 2.2 μm³ proved sufficient to pinpoint reaction initiation and the organization of drainage architecture in space and time. <br><br> We observed that dehydration commences across a narrow front, which propagates from the margins to the centre of the sample in more than four hours. The advance of this front can be fitted with a square-root function, implying that the initiation of the reaction in the sample can be described as a diffusion process. <br><br> Novel parallelized computer codes allow quantifying the geometry of the porosity and the drainage architecture from the very large tomographic datasets (6.4 &times; 10⁹ voxel each) in unprecedented detail. We determined position, volume, shape and orientation of each resolvable pore and tracked these properties over the duration of the experiment. We found that the pore-size distribution follows a power law. Pores tend to be anisotropic but rarely crack-shaped and have a preferred orientation, likely controlled by a pre-existing fabric in the sample. With on-going dehydration, pores coalesce into a single interconnected pore cluster that is connected to the surface of the sample cylinder and provides an effective drainage pathway. <br><br> Our observations can be summarized in a model in which gypsum is stabilized by thermal expansion stresses and locally increased pore fluid pressures until the dehydration front approaches to within about 100 μm. Then, the internal stresses are released and dehydration happens efficiently, resulting in new pore space. Pressure release, the production of pores and the advance of the front are coupled in a feedback loop. We discuss our findings in the context of previous studies.Fri, 21 Oct 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/839/2011/<b>Constraining fault interpretation through tomographic velocity gradients: application to Northern Cascadia</b><br /><br />Solid Earth Discussions, 3, 839-856, 2011<br /><br />Author(s): K. Ramachandran<br /><br />Spatial gradients of tomographic velocities are seldom used in interpretation of subsurface fault structures. This study shows that spatial velocity gradients can be used effectively in identifying subsurface discontinuities in the horizontal and vertical directions. Three-dimensional velocity models constructed through tomographic inversion of active source and/or earthquake traveltime data are generally built from an initial 1-D velocity model that varies only with depth. Regularized tomographic inversion algorithms impose constraints on the roughness of the model that help to stabilize the inversion process. Final velocity models obtained from regularized tomographic inversions have smooth three-dimensional structures that are required by the data. Final velocity models are usually analyzed and interpreted either as a perturbation velocity model or as an absolute velocity model. Compared to perturbation velocity model, absolute velocity model has an advantage of providing constraints on lithology. Both velocity models lack the ability to provide sharp constraints on subsurface faults. An interpretational approach utilizing spatial velocity gradients applied to northern Cascadia shows that subsurface faults that are not clearly interpretable from velocity model plots can be identified by sharp contrasts in velocity gradient plots. This interpretation resulted in inferring the locations of Tacoma Fault, Seattle Fault, Southern Whidbey Island Fault, and Darrington Devils Mountain fault much clearly. The Coast Range Boundary Fault, previously hypothesized on the basis of sedimentological and tectonic observations, is inferred clearly from the gradient plots. Many of the fault locations imaged from gradient data correlate with earthquake hypocenters indicating their seismogenic nature.Wed, 28 Sep 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/789/2011/<b>Bio-chemostratigraphy of the Barremian–Aptian shallow-water carbonates of the southern Apennines (Italy): pinpointing the OAE1a in a Tethyan carbonate platform</b><br /><br />Solid Earth Discussions, 3, 789-838, 2011<br /><br />Author(s): M. Di Lucia, M. Mutti, and M. Parente<br /><br />Low resolution and lack of chronostratigraphic calibration of carbonate platform biostratigraphy hinder precise correlation with coeval deep-water successions. This is the main obstacle when studying the record of Mesozoic oceanic anoxic events in carbonate platforms. In this paper we use carbon isotope stratigraphy to produce the first chronostratigraphic calibration of the Barremian–Aptian biostratigraphy of the Apenninic carbonate platform of southern Italy. According to our calibration, the "Selli level" black shales of epicontinental and oceanic basins corresponds in the southern Apenninic carbonate platform to the interval between the "Orbitolina level", characterized by the association of <i>Mesorbitolina parva</i> and <i>Mesorbitolina texana</i>, and the second acme of <i>Salpingoporella dinarica</i>. The biocalcification crisis of nannoconids corresponds to the interval going from the first acme of <i>S. dinarica</i> to just above the top of the "Orbitolina level". Since these bioevents have been widely recognized beyond the Apenninic platform, our calibration can be used to pinpoint the interval corresponding to the Early Aptian oceanic anoxic event in other carbonate platforms of central and southern Tethys.Wed, 14 Sep 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/769/2011/<b>The regulation of the air: a hypothesis</b><br /><br />Solid Earth Discussions, 3, 769-788, 2011<br /><br />Author(s): E. G. Nisbet, C. M. R. Fowler, and R. E. R. Nisbet<br /><br />We propose the hypothesis that natural selection, acting on the specificity of rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) for carbon dioxide over oxygen, has controlled the CO₂:O₂ ratio of the atmosphere since the evolution of photosynthesis and has also sustained the Earth's greenhouse-set surface temperature. Rubisco works in partnership with the nitrogen-fixing enzyme nitrogenase to control atmospheric pressure. Together, these two enzymes control global surface temperature and indirectly the pH and oxygenation of the ocean. Thus, the co-evolution of these two enzymes may have produced clement conditions on the Earth's surface, allowing life to be sustained.Wed, 14 Sep 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/743/2011/<b>Phanerozoic black shales and the Wilson Cycle</b><br /><br />Solid Earth Discussions, 3, 743-768, 2011<br /><br />Author(s): J. Trabucho-Alexandre, W. W. Hay, and P. L. de Boer<br /><br />The spatial and temporal distribution of black shales is related to the development of the environments in which they accumulate and to a propitious combination of environmental variables. Whereas much has been done in recent years to improve our understanding of the mechanisms behind the temporal distribution of black shales in the Phanerozoic, the interpretation of the palaeogeographical distribution of black shales is still dominated by an oversimplistic set of three uniformitarian depositional models that do not capture the complexity and dynamics of environments of black shale accumulation. These three models, the restricted circulation, the (open) ocean oxygen minimum and the continental shelf models, are in fact a uniformitarian simplification of the variety of depositional environments that arise and coexist throughout the course of a basin's Wilson Cycle, i.e. the dynamic sequence of events and stages that characterise the evolution of an ocean basin, from the opening continental rift to the closing orogeny. We examine the spatial distribution of black shales in the context of the Wilson Cycle using examples from the Phanerozoic. It is shown that the geographical distribution of black shales, their position in the basin infill sequence and their nature (e.g. type of organic matter, lithology) depend on basin evolution because the latter controls the development of sedimentary environments where black shales may be deposited.Thu, 08 Sep 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/713/2011/<b>Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number</b><br /><br />Solid Earth Discussions, 3, 713-741, 2011<br /><br />Author(s): M. Wolstencroft and J. H. Davies<br /><br />We investigate the influence on mantle convection of the negative Clapeyron slope ringwoodite to perovskite and ferro-periclase mantle phase transition, which is correlated with the seismic discontinuity at 660 km depth. In particular, we focus on understanding the influence of the magnitude of the Clapeyron slope (as measured by the Phase Buoyancy parameter, <i>P</i>) and the vigour of convection (as measured by the Rayleigh number, <i>Ra</i>) on mantle convection. We have undertaken 76 simulations of isoviscous mantle convection in spherical geometry varying <i>Ra</i> and <i>P</i>. Three domains of behaviour were found: layered convection for high <i>Ra</i> and more negative <i>P</i>, whole mantle convection for low <i>Ra</i> and less negative <i>P</i> and transitional behaviour in an intervening domain. The boundary between the layered and transitional domain was fit by a curve <i>P</i> = &alpha;<i>Ra</i>^&beta; where α = −1.05, and β = −0.1, and the fit for the boundary between the transitional and whole mantle convection domain was α = −4.8, and β = −0.25. These two curves converge at <i>Ra</i>≈2.5×10⁴ and <i>P</i>&approx;&minus;0.38. Extrapolating to high <i>Ra</i>, which is likely earlier in Earth history, this work suggests a large transitional domain. It is therefore likely that convection in the Archean would have been influenced by this phase change, with Earth being at least in the transitional domain, if not the layered domain.Thu, 04 Aug 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/679/2011/<b>Paleointensities on 8 ka obsidian from Mayor Island, New Zealand</b><br /><br />Solid Earth Discussions, 3, 679-711, 2011<br /><br />Author(s): A. Ferk, R. Leonhardt, K.-U. Hess, and D. B. Dingwell<br /><br />The 8 ka B.P. (6050 BCE) pantelleritic obsidian flow on Mayor Island, Bay of Plenty, New Zealand, has been investigated using 30 samples from two sites. Due to a very high paramagnetic/ferromagnetic ratio it was not possible to determine the remanence carriers. This is despite the fact that the samples were studied intensively at low, room and high temperatures. We infer that a stable remanence within the samples is carried by single- or close to single-domain particles. Experiments to determine the anisotropy of thermoremanence tensor and the dependency on cooling rate were hampered due to alteration resulting from the repeated heating of the samples to temperatures just below the glass transition. Nonetheless, a well-defined mean paleointensity of 57.0 ± 1.0 μT, based on individual high quality paleointensity determinations, was obtained. This field value compares very well to a paleointensity of 58.1 ± 2.9 μT which Tanaka et al. (2009) obtained for 5500 BCE at a site 100 km distant. Agreement with geomagnetic field models, however, is poor. Thus, gathering more high-quality paleointensity data for the Pacific region and for the Southern Hemisphere in general to better constrain global field models is very important.Thu, 28 Jul 2011 00:00:00 +0200http://www.solid-earth-discuss.net/3/655/2011/<b>Domains of Archean mantle lithosphere deciphered by seismic anisotropy &ndash; initial results from the LAPNET array in northern Fennoscandia</b><br /><br />Solid Earth Discussions, 3, 655-678, 2011<br /><br />Author(s): J. Plomerová, L. Vecsey, V. Babuška, and LAPNET Working Group<br /><br />An international LAPNET array (2007–2009, <a href="http://www.oulu.fi/sgo-oty/lapnet" target="_blank">http://www.oulu.fi/sgo-oty/lapnet</a>) of the POLENET/LAPNET sub-project of the POLENET-IPY consortium, related to POLENET seismic and geodetic studies in the Arctic regions, consisted of about 60 broadband seismic stations located on the territory of northern Finland and adjacent parts of Sweden, Norway and Russia. We analyze relative P-wave travel-time deviations evaluated for a subset of 90 teleseismic events recorded by the LAPNET array and show examples of lateral variations of shear-wave splitting to demonstrate variability of fabrics of the Archean mantle lithosphere. The initial results clearly demonstrate the Archean mantle lithosphere consists of domains with consistent fabrics reflecting fossil anisotropic structures. 3-D self-consistent anisotropic models with inclined symmetry axes accommodate two independent sets of body-wave anisotropic observations. Individual domains are delimited by boundaries (sutures), where the anisotropic parameters change. The results obtained from the LAPNET array fill a gap in structural studies of the upper mantle beneath northern Fennoscandia.Wed, 20 Jul 2011 00:00:00 +0200