Description: Publication date: 15 February 2015 Source: Earth and Planetary Science Letters, Volume 412 Author(s): Rory Dalman , Gert Jan Weltje , Pantelis Karamitopoulos A basin-scale numerical model with a sub-grid parameterization of fluvio–deltaic processes and stratigraphy was used to study the relation between alluvial sedimentation and marine deltaic deposition under conditions of time-invariant forcing. The experiments show that delta evolution is governed by a robust morphodynamic feedback loop, which provides a link between major avulsions, delta-lobe switches, and sequestration of sediments on the delta plain. Major avulsions, driven by local superelevation, result in abandonment of delta lobes and initiation of new lobes. Progradation of the delta front lengthens the fluvial profile and reduces its gradient, which induces aggradation upstream. The aggradation, in turn, causes local superelevation of the channel belt. Each major avulsion causes a wave of incision to migrate upstream, whereas downstream of the avulsion point, the rate of aggradation temporarily increases until a new equilibrium situation has been established. The feedback loop explains storage and release of fluvial sediments without the need to invoke changes in upstream or downstream controls and provides a plausible mechanism for the generation of high-frequency incision–aggradation cycles as the sole result of compensational stacking. The stratigraphic expression of a depocentre shift is an essentially isochronous surface. Hence, the stratigraphic record of fluvio–deltaic systems may be subdivided into a series of units representing intervals during which a channel belt and delta lobe were forming at a fixed location in the basin, so-called chronosomes. Fluvio–deltaic chronosomes are bounded by abandonment surfaces, which are clearly expressed in the marine as well as the fluvial domain. The surface marking the abandonment of a particular channel belt and delta lobe correlates with the surface at the base of a new delta lobe. Landward, this surface forms the base of an aggradational package of fluvial sediments downstream of an avulsion site associated with a lobe switch. Upstream of this site, the conformable surface passes into a surface of fluvial incision and terrace formation onlapped by aggradational channel-belt deposits. Identification of the isochronous bounding surfaces of chronosomes in the field offers the attractive prospect of high-resolution chronostratigraphic correlation throughout fluvio–deltaic systems.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Thomas Guidat , Stéphane Pochat , Olivier Bourgeois , Ondřej Souček The floor of Isidis Planitia, a giant impact basin located close to the martian equator, exhibits a landform assemblage, nicknamed Thumbprint Terrain, made of Arcuate Ridges, Aligned Cones, Isolated Cones, Cone Fields, associated with a peripheral network of Sinuous Ridges, Linear Depressions, and Mounds. From a new comprehensive mapping initiative of these landforms and from comparisons with terrestrial analogues (ribbed moraines, dirt cones, kettle holes, eskers, tunnel valleys and moraine plateaux), we demonstrate that this distinctive assemblage is a glacial landsystem inherited from the presence of a massive polythermal ice sheet over the basin during the Hesperian. The flow of the ice sheet was controlled by its basal thermal regime. Wet-based conditions led to the formation of Arcuate Ridges and Aligned Cones in most parts of the basin, while a negative geothermal anomaly due to impact-related crustal thinning was responsible for cold-based conditions in its central part, where only Isolated Cones and Cones Fields are present. Sinuous Ridges, Linear Depressions and Mounds at the basin margins are interpreted as relicts of a radial network of subglacial channels, which drained the glacial meltwater produced within the interior of the ice sheet across its cold-based periphery. Graphical abstract

Description: Publication date: 15 February 2015 Source: Earth and Planetary Science Letters, Volume 412 Author(s): Xinzhuan Guo , Takashi Yoshino , Akira Shimojuku The electrical conductivities of albite–water, albite–quartz–water and albite–water–NaCl systems have been measured in terms of impedance spectroscopy at 1 GPa and 400–1000 K. The relationship between electrical conductivity and temperature in the albite–(quartz)–water system cannot be expressed by the Arrhenian formula, whereas that in the brine-bearing system follow the Arrhenian law showing small temperature dependence. The electrical conductivity of the albite–(quartz)–water samples decreased with decreasing temperature from 1000 to 800 K, then increased rapidly upon further cooling from 800 to around 550 K. The bulk conductivities of the albite–(quartz)–water system are consistent with variation of the total concentration of the dissolved electric charge carriers of H + , OH − , Na + , AlO 2 − and HSiO 3 − in aqueous fluid with temperature based on the thermal dynamic calibration. There is a small negative dependence of bulk conductivity on aqueous fluid fraction. Electrical conductivity of the albite–water–NaCl samples is higher than that of the albite–(quartz)–water samples, which shows the following features: (1) small dependence of conductivity on the temperature; (2) increase of electrical conductivity with the fluid fraction and the salinity. Our results suggest that the high conductivity anomalies of 10 − 1 S/m typically observed in the continental crust can be explained by the presence of albite and quartz with fluid fraction as low as 0.014 at temperatures lower than 650 K. In the case that the geotherm is higher than 650 K, the observed value of 10 − 1 S/m can be explained by the brine-bearing albite with a fluid fraction of 1 vol% if the salinity is similar to the seawater.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Jean-Michel Brazier , Guillaume Suan , Théo Tacail , Laurent Simon , Jeremy E. Martin , Emanuela Mattioli , Vincent Balter The early Toarcian was punctuated by pulses of massive carbon injection that are thought to have triggered, through increased greenhouse conditions, elevated continental discharge and nutrient input, marine anoxia, seawater acidification and species extinctions. Nevertheless, the mode and tempo of changes in continental weathering across this interval remains highly debated, leading to considerable uncertainty about the main causes of these perturbations. In this study we present calcium isotope measurements ( δ 44/40 Ca) of well-preserved brachiopods and bulk rock samples from the hemipelagic strata of Pliensbachian–Toarcian age of Peniche in Portugal in order to constrain changes in the calcium cycle and hence changes in continental weathering during the early Toarcian. The data reveal a similar trend as carbon isotope data from the same section and show negative excursions of about 0.5‰ at the Pliensbachian–Toarcian transition (Pl–To) and at the base of the Toarcian Oceanic Anoxic Event (T–OAE) interval. The comparison of δ 44/40 Ca ratios recorded in brachiopods and bulk rock corrected for variable dolomite contribution indicates that these excursions reflect changes in the global isotopic composition of seawater rather than changes in the dominant mineralogy of calcifying organisms or in hydrological budget of the considered basin. Box modeling results suggest that the Pl–To and T–OAE δ 44/40 Ca excursions can be explained by a transient 90% decrease of carbonate accumulation due to seawater acidification followed by a 500% increase in continental weathering rates. The sharp increases in continental weathering inferred from the δ 44/40 Ca ratios seem overall consistent with lower Toarcian sedimentological and biotic records that document rapid crises in carbonate production followed by episodes of increased calcium carbonate burial. Nevertheless, the maximum of carbonate burial recorded by most NW European basinal successions occurs several hundreds of kyrs after that predicted by box modeling results. This mismatch either implies that the European records of carbonate accumulation do not reflect global trends or that the fundamental processes related to the removal of excess alkalinity caused by increased continental weathering are more complex than previously appreciated. Based on the amount of Ca input simulated by box modeling, the injection of tens of thousands of gigatons of carbon with an isotopic composition ( δ 13 C) comprised between − 6 ‰ and − 14 ‰ appears as the most likely causes of the δ 13 C excursions characterizing these two events. These results indicate that environmental and biotic changes of the Pl–To and T–OAE were mainly caused a cascade of environmental changes triggered by the massive carbon emissions from the Karoo–Ferrar volcanism.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): R. Vassallo , J.-L. Mugnier , V. Vignon , M.A. Malik , R. Jayangondaperumal , P. Srivastava , F. Jouanne , J. Carcaillet Three main Cenozoic thrusts at the front of Northwestern Himalaya have accommodated most of the India–Eurasia convergence across the belt over the last million years and produced the present relief. Their recent tectonic activity is poorly known because of the long period of inaccessibility of the Jammu and Kashmir state, and because the latest and only large earthquake recorded in the region occurred in 1555 AD. We show where the deformation is localized during the Late-Quaternary, and determine shortening rates across the structures by analyzing the geometry and chronology of geomorphic markers. The Main Boundary Thrust in this region ceased moving at least ∼30 ka ago. On the contrary, the more external Medlicott–Wadia Thrust and Main Frontal Thrust, both merging at depth on the sub-flat detachment of the Main Himalayan Thrust, exhibit hectometric-scale deformations accumulated during the last thousands of years. The total shortening rate absorbed by these faults over the last 14–24 ka is between 13.2 and 27.2 mm/yr ( 11.2 ± 3.8 and 9.0 ± 3.2 mm / yr , respectively). Part of this deformation may be associated to the geometry of the Chenab reentrant, which could generate an extra oblique component. However, the lower bound of our shortening rates is consistent with previously determined geodetic rates. Active deformation on these structures follows an in-sequence/out-of-sequence pattern, with breaking of both ramps being possible for earthquakes triggered on the main detachment.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Anne-Line Auzende , Javier Escartin , Nicolas P. Walte , Stéphane Guillot , Greg Hirth , Daniel J. Frost We performed deformation-DIA experiments on antigorite serpentinite at pressures of 1–3.5 GPa and temperatures of between 400 and 650 °C, bracketing the stability of antigorite under subduction zone conditions. For each set of pressure–temperature ( P – T ) conditions, we conducted two runs at strain rates of 5 × 10 − 5 and 1 × 10 − 4   s − 1 . We complemented our study with a sample deformed in a Griggs-type apparatus at 1 GPa and 400 °C ( Chernak and Hirth, 2010 ), and with natural samples from Cuba and the Alps deformed under blueschist/eclogitic conditions. Optical and transmission electron microscopies were used for microstructural characterization and determination of deformation mechanisms. Our observations on experimentally deformed antigorite prior to breakdown show that deformation is dominated by cataclastic flow with observable but minor contribution of plastic deformation (microkinking and (001) gliding mainly expressed by stacking disorder mainly). In contrast, in naturally deformed samples, plastic deformation structures are dominant (stacking disorder, kinking, pressure solution), with minor but also perceptible contribution of brittle deformation. When dehydration occurs in experiments, plasticity increases and is coupled to local embrittlement that we attribute to antigorite dehydration. In dehydrating samples collected in the Alps, embrittlement is also observed suggesting that dehydration may contribute to intermediate-depth seismicity. Our results thus show that semibrittle deformation operates within and above the stability field of antigorite. However, the plastic deformation recorded by naturally deformed samples was likely acquired at low strain rates. We also document that the corrugated structure of antigorite controls the strain accommodation mechanisms under subduction conditions, with preferred inter- and intra-grain cracking along (001) and gliding along both a and b . We also show that antigorite rheology in subduction zones is partly controlled by the presence of fluids, which can percolate within the exhumation channel via deformation-induced interconnected porosity.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): D. Güttler , F. Adolphi , J. Beer , N. Bleicher , G. Boswijk , M. Christl , A. Hogg , J. Palmer , C. Vockenhuber , L. Wacker , J. Wunder In 2012, Miyake et al. reported a sudden and strong increase of the atmospheric radiocarbon ( 14 C) content in Japanese cedar trees of 1.2% between AD 774 and 775. While their findings were quickly confirmed by a German oak chronology for the Northern Hemisphere (NH), the question remained if the effect was seen in both hemispheres. Here we present the first annually resolved Southern Hemisphere (SH) 14 C record spanning the interval AD 760–787, using New Zealand kauri ( Agathis australis ) chronology wood. An almost identical distinct increase compared to Northern Hemisphere data was observed, suggesting a cosmic event with globally uniform impact as a potential cause for the increase. Deploying a carbon cycle box model a worldwide averaged net 14 C production of 2.2 × 10 8   C 14   atoms   cm − 2 was estimated, which is 3.7 times higher than the average annual 14 C production. The immediate appearance of the event in tree rings on both hemispheres suggests a short duration event of significantly less than 1 yr.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Antonio Caracausi , Michele Paternoster , Pasquale Mario Nuccio Mantle volatiles are mainly lost from the Earth to the atmosphere through subaerial and submarine volcanism. Recent studies have shown that degassing of mantle volatiles also occurs from inactive volcanic areas and in tectonically active areas. A new challenge in Earth science is to quantify the mantle-derived flux of volatiles (e.g., CO 2 ) which is important for understanding such diverse issues as the evolution of the atmosphere, the relationships between magma degassing and volcanic activity, gas pressure and seismogenic processes, and the hazards posed by volcanic lakes. Here we present a detailed study of mantle-derived CO 2 budget from Mt. Vulture volcano in the Apennines, Italy, whose latest eruption occurred 141 ± 11 kyr ago. The relationship between δ 13 C CO2 and total dissolved carbon at Mt. Vulture volcano indicates that the emitted CO 2 is a mixture of a biogenic end-member with an average δ 13 C CO2 of about − 17 ‰ and a mantle-derived CO 2 end-member with δ 13 C CO2 values from − 3 ‰ to + 2 ‰ . These values of mantle-derived δ 13 C CO2 are in the range of those for gas emitted from active volcanoes in the Mediterranean. We calculated the contribution of individual components (CO 2 in groundwater, in lakes and from main pools) to the total CO 2 budget in the area. We used new measurements of water flow, combined with literature data, to calculate the CO 2 flux associated with groundwater, and measured the gas flux from the main pools on the volcanic edifice. Finally, we calculated the CO 2 flow in the lakes based on the gradient concentration and eddy diffusivity. The total mantle-derived CO 2 budget in the area is 4.85 × 10 8   mol yr − 1 , which is more than double previous estimates. This is higher than those observed in younger volcanic systems elsewhere, thereby supporting the existence of actively degassing mantle melts below Mt. Vulture volcano. A structural map highlights the tectonic control on CO 2 flow across the Mt. Vulture volcanic edifice. Indeed, the tectonic discontinuities that controlled the magma upwelling during the most recent volcanic activity are still the main active degassing structures. The new estimate of CO 2 budget in the Mt. Vulture area, together with literature data on CO 2 budget from historically active and inactive Italian volcanoes, suggests a power-law functional relationship between the age of the most recent volcanic eruption and both total discharged CO 2 ( R 2 = 0.73 ) and volcano size-normalized CO 2 flux ( R 2 = 0.66 ). This relation is also valid by using data from worldwide volcanoes highlighting that deep degassing can occur over very long time too. In turn, the highlighted relation provides also an important tool to better evaluate the state of activity of a volcano, whose last activity occurred far in time. Finally, our study highlights that in the southern Apennines, an active degassing of mantle-derived volatiles (i.e., He, CO 2 ) occurs indiscriminately from west to east. This is in contrast to the central–northern Apennine, which is characterized by a crustal radiogenic volatile contribution, which increases eastward, coupled to a decrease in deep CO 2 flux. This difference between the two regions is probably due to lithospheric tears which control the upwelling of mantle melts, their degassing and the transport of volatiles through the crust.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Guillaume Carazzo , Edouard Kaminski , Stephen Tait Volcanic columns produced by explosive eruptions commonly reach, at some stage, a collapse regime with associated pyroclastic density currents propagating on the ground. The threshold conditions for the entrance into this regime are mainly controlled by the mass flux and exsolved gas content at the source. However, column collapse is often partial and the controls on the fraction of total mass flux that feeds the pyroclastic density currents, defined here as the intensity of collapse, are unknown. To better understand this regime, we use a new experimental apparatus reproducing at laboratory scale the convecting and collapsing behavior of hot particle-laden air jets. We validate the predictions of a 1D theoretical model for the entrance into the regime of partial collapse. Furthermore, we show that where a buoyant plume and a collapsing fountain coexist, the intensity of collapse can be predicted by a universal scaling relationship. We find that the intensity of collapse in the partial collapse regime is controlled by magma gas content and temperature, and always exceeds 40%, independent of peak mass flux and total erupted volume. The comparison between our theoretical predictions and a set of geological data on historic and pre-historic explosive eruptions shows that the model can be used to predict both the onset and intensity of column collapse, hence it can be used for rapid assessment of volcanic hazards notably ash dispersal during eruptive crises.

Description: Publication date: 1 February 2015 Source: Earth and Planetary Science Letters, Volume 411 Author(s): Yohei Yukutake , Tetsuya Takeda , Akio Yoshida To investigate the applicability of frictional reactivation theory to active faults, we evaluated the slip tendency of active faults in Japan. Slip tendency is defined as the ratio of shear stress to the frictional resistance acting on a fault plane. To estimate the stress field near active faults, we determined focal mechanisms for numerous shallow earthquakes in the intraplate region of Japan, using data from dense seismic observation networks. The stress fields were estimated by applying the stress inversion method to the focal mechanisms. We found that most active faults are well oriented with respect to the stress field, having slip tendencies of ≥0.6, which indicates that fault reactivation theory is applicable to active faults, and that the present day tectonic stress field has contributed substantially to the development of active faults. Conversely, several steeply dipping active faults in northeast Japan, as well as the source faults of the 1995 Hyogoken–Nanbu earthquake, are mis-oriented, with slip tendencies of 〈0.6, which may indicate that highly pressurized fluids have contributed to earthquake triggering on these mis-oriented faults. We also discuss the applicability of slip tendency analysis to earthquake hazard assessment; i.e. estimating the probability of a future rupture on active faults. Our results indicate that slip tendency does not correlate with elapsed time since the most recent earthquake event. This observation shows that slip tendency may not be an efficient parameter with which to assess the risk of an earthquake occurring in the near future on a specific fault.