Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Francesco S.R. Pausata, Gabriele Messori, Qiong Zhang The West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000–5000 yr BP, when increased summer rainfall led to the so-called “Green Sahara” and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation–albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): F. Corfu, H. Svensen, A. Mazzini

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Bryan Sell, Maria Ovtcharova, Jean Guex, Annachiara Bartolini, Fred Jourdan, Jorge E. Spangenberg, Jean-Claude Vicente, Urs Schaltegger

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): V. Riboulot, N. Sultan, P. Imbert, S. Ker A review of recent literature shows that two geomorphologically different types of pockmarks, contribute to gas seepage at the seafloor. Type-1 pockmarks are defined as seafloor craters associated to fluid seepage and are the most classical type referred to as “pockmarks” in the literature. In contrast, Type-2 pockmarks reveal a complex seafloor morphology that may result from the formation/decomposition of gas hydrates in underlying sedimentary layers. Interpretation of very-high-resolution seismic data, sedimentological analyses and geotechnical measurements acquired from the Eastern Niger Submarine Delta reveal that Type-2 pockmarks are associated to the presence at depth of a conical body of massive gas hydrates. Based on acquired data, theoretical analysis and numerical modelling, it was possible to propose a novel geo-mechanical mechanism controlling the irregular seafloor deformations associated to Type-2 pockmark and to show that pockmark shapes and sizes are directly linked to the initial growth and distribution of sub-seafloor gas hydrates. The study illustrates the role of gas hydrates formation in the fracturation, deformation of the subsurface sediment and the formation of Type-2 pockmarks.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Shuai Zhang, Sanne Cottaar, Tao Liu, Stephen Stackhouse, Burkhard Militzer Fe and Al are two of the most important rock-forming elements other than Mg, Si, and O. Their presence in the lower mantle's most abundant minerals, MgSiO 3 bridgmanite, MgSiO 3 post-perovskite and MgO periclase, alters their elastic properties. However, knowledge on the thermoelasticity of Fe- and Al-bearing MgSiO 3 bridgmanite, and post-perovskite is scarce. In this study, we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure, Fe 3+ - and Fe 2+ -, and Al 3+ -bearing MgSiO 3 perovskite and post-perovskite, over a wide range of pressures, temperatures, and Fe/Al compositions. Our results show that a mineral assemblage resembling pyrolite fits a 1D seismological model well, down to, at least, a few hundred kilometers above the core–mantle boundary, i.e. the top of the D ″ region. In D ″ , a similar composition is still an excellent fit to the average velocities and fairly approximate to the density. We also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy, and find post-perovskite with predominant (001) slip across all compositions agrees best with seismic observations in the D ″ .

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Rotem Golan, Ittai Gavrieli, Jiwchar Ganor, Boaz Lazar The pH of aqueous environments is determined by the dominant buffer systems of the water, defined operationally as total alkalinity (TA). The major buffer systems in the modern ocean are carbonic and boric acids of which the species bicarbonate, carbonate and borate make up about 77%, 19% and 4% of the TA, respectively. During the course of seawater evaporation (e.g. lagoons) the residual brine loses considerable portion of the dissolved inorganic carbon (DIC) and carbonate alkalinity (CA) already at the early stages of evaporation. DIC and CA decrease due to massive precipitation of CaCO 3 , while total boron (TB) increases conservatively, turning borate to the dominant alkalinity species in marine derived brines. In the present work we assess the apparent dissociation constant value of boric acid ( K B ′ ) in saline and hypersaline waters, using the Dead Sea (DS) as a case study. We explain the DS low pH (∼6.3) and the effect of the boric and carbonic acid p K ′ -s on the behavior of the brine's buffer system, including the pH increase that results from brine dilution. The K B ′ in DS was estimated from TB, TA, DIC and pH data measured in this study and early empirical data on artificial DS brines containing just carbonic acid. The K B ′ value was corroborated by Pitzer ion interaction model calculations using PHREEQC thermodynamic code applied to the chemical composition of the DS. Our results show that K B ′ increases considerably with the brine's ionic strength, reaching in the DS to a factor of 100 higher than in “mean” seawater. Based on theoretical calculations and analyses of other natural brines it is suggested that brines' composition is a major factor in determining the K B ′ value and in turn the pH of such brines. We show that the higher the proportion of divalent cations in the brine the higher the dissociation constants of the weak acids (presumably due to formation of complexes). The low pH of the Dead Sea is accordingly explained by its extremely high ionic strength (TDS = 348 g/L) and the dominance of the divalent cation, Mg 2+ . Other natural hyper-saline brines with high concentration of divalent cations such as Kunteyi Lake in China and Don-Juan Pond in Antarctica follow the same general pattern. In contrast, the high pH of soda lakes results not only from their high TA but also by the dominance of the monovalent cation, Na + . Our study emphasizes the strong control of brine composition on p K B ′ and pH. These factors should be taken into consideration when reconstructing past and present environmental evaporitic environments.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Cristian Montanaro, Bettina Scheu, Magnus Tumi Gudmundsson, Kristín Vogfjörd, Hannah Iona Reynolds, Tobias Dürig, Karen Strehlow, Stefanie Rott, Thierry Reuschlé, Donald Bruce Dingwell Hydrothermal explosions frequently occur in geothermal areas showing various mechanisms and energies of explosivity. Their deposits, though generally hardly recognised or badly preserved, provide important insights to quantify the dynamics and energy of these poorly understood explosive events. Furthermore the host rock lithology of the geothermal system adds a control on the efficiency in the energy release during an explosion. We present results from a detailed study of recent hydrothermal explosion deposits within an active geothermal area at Kverkfjöll, a central volcano at the northern edge of Vatnajökull. On August 15th 2013, a small jökulhlaup occurred when the Gengissig ice-dammed lake drained at Kverkfjöll. The lake level dropped by approximately 30 m, decreasing pressure on the lake bed and triggering several hydrothermal explosions on the 16th. Here, a multidisciplinary approach combining detailed field work, laboratory studies, and models of the energetics of explosions with information on duration and amplitudes of seismic signals, has been used to analyse the mechanisms and characteristics of these hydrothermal explosions. Field and laboratory studies were also carried out to help constrain the sedimentary sequence involved in the event. The explosions lasted for 40–50 s and involved the surficial part of an unconsolidated and hydrothermally altered glacio-lacustrine deposit composed of pyroclasts, lavas, scoriaceous fragments, and fine-grained welded or loosely consolidated aggregates, interbedded with clay-rich levels. Several small fans of ejecta were formed, reaching a distance of 1 km north of the lake and covering an area of approximately 0.3 km 2 , with a maximum thickness of 40 cm at the crater walls. The material (volume of approximately 10 4 m 3 ) has been ejected by the expanding boiling fluid, generated by a pressure failure affecting the surficial geothermal reservoir. The maximum thermal, craterisation and ejection energies, calculated for the explosion areas, are on the order of 10 11 , 10 10 and 10 9 J, respectively. Comparison of these with those estimated by the volume of the ejecta and the crater sizes, yields good agreement. We estimate that approximately 30% of the available thermal energy was converted into mechanical energy during this event. The residual energy was largely dissipated as heat, while only a small portion was converted into seismic energy. Estimation of the amount of freshly-fragmented clasts in the ejected material obtained from SEM morphological analyses, reveals that a low but significant energy consumption by fragmentation occurred. Decompression experiments were performed in the laboratory mimicking the conditions due to the drainage of the lake. Experimental results confirm that only a minor amount of energy is consumed by the creation of new surfaces in fragmentation, whereas most of the fresh fragments derive from the disaggregation of aggregates. Furthermore, ejection velocities of the particles (40–50 m/s), measured via high-speed videos, are consistent with those estimated from the field. The multidisciplinary approach used here to investigate hydrothermal explosions has proven to be a valuable tool which can provide robust constraints on energy release and partitioning for such small-size yet hazardous, steam-explosion events.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Dimitri Saint-Carlier, Julien Charreau, Jérôme Lavé, Pierre-Henri Blard, Stéphane Dominguez, Jean-Philippe Avouac, Shengli Wang The investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range. Here, we estimate the shortening rate absorbed by a Quaternary emergent detachment fold on the southeastern piedmont of the Tianshan (China). Our work is primarily based on new 10 Be cosmogenic exposure dating of deformed alluvial surfaces. The method we have developed combines depth profiling with sampling of surface cobbles, thereby allowing exposure time, erosion rate and inheritance to be simultaneously constrained. The exposure ages of the uppermost uplifted alluvial surfaces are around 140 ± 17   ka , 130 ± 9   ka and 47 ± 9   ka , from west to east. A terrace lying below the 140 ka surface is dated at 65 ± 5   ka . The ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods: (1) the excess area method, where sedimentation rates, extracted from magnetostratigraphic studies, are used to determine the amount of sedimentation after the abandonment of the river; and (2) a folding model derived from sandbox experiments. The late Pleistocene shortening rates are shown to be between 0.4 ± 0.1   mm / yr and 0.8 ± 0.5   mm / yr on the western part of the fold and 2.1 ± 0.4   mm / yr along its central part. The central part of the frontal Yakeng anticline therefore accommodates up to 25% of the total shortening currently absorbed across the whole Eastern Tianshan range (8 mm/yr). However, this situation seems to have prevailed for only the last 150 ka, as the shortening rate absorbed by this nascent fold was previously ten times slower. While the initiation of folding of the Yakeng anticline can be traced back to 5.5 Ma ago, the basinward migration of the active deformation front onto the Yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east, probably in relation to the tectonic activity of the folds in the hinterland.

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): Laura J. Crossey, Karl E. Karlstrom, Brandon Schmandt, Ryan R. Crow, Daniel R. Colman, Brandi Cron, Cristina D. Takacs-Vesbach, Clifford N. Dahm, Diana E. Northup, David R. Hilton, Jason W. Ricketts, Anthony R. Lowry The discovery of oceanic black (and white) smokers revolutionized our understanding of mid-ocean ridges and led to the recognition of new organisms and ecosystems. Continental smokers, defined here to include a broad range of carbonic springs, hot springs, and fumaroles that vent mantle-derived fluids in continental settings, exhibit many of the same processes of heat and mass transfer and ecosystem niche differentiation. Helium isotope ( 3 He/ 4 He) analyses indicate that widespread mantle degassing is taking place in the western U.S.A., and that variations in mantle helium values correlate best with low seismic-velocity domains in the mantle and lateral contrasts in mantle velocity rather than crustal parameters such as GPS, proximity to volcanoes, crustal velocity, or composition. Microbial community analyses indicate that these springs can host novel microorganisms. A targeted analysis of four springs in New Mexico yield the first published occurrence of chemolithoautotrophic Zetaproteobacteria in a continental setting. These observations lead to two linked hypotheses: that mantle-derived volatiles transit through conduits in extending continental lithosphere preferentially above and at the edges of mantle low velocity domains. High CO 2 and other constituents ultimately derived from mantle volatiles drive water–rock interactions and heterogeneous fluid mixing that help structure diverse and distinctive microbial communities.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Zhimian Cao, Christopher Siebert, Ed C. Hathorne, Minhan Dai, Martin Frank The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope ( δ 137 Ba DBa ) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang (Yangtze River). We find pronounced and systematic horizontal and depth dependent δ 137 Ba DBa gradients. Beyond the river influence characterized by generally light signatures (0.0 to + 0.3 ‰ ), the δ 137 Ba DBa values in the upper water column are significantly higher ( + 0.9 ‰ ) than those in the deep waters ( + 0.5 ‰ ). Moreover, δ 137 Ba DBa signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and δ 137 Ba DBa from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The subsurface δ 137 Ba DBa distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land–sea interactions and ocean mixing processes.