Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): N. Cagney, F. Crameri, W.H. Newsome, C. Lithgow-Bertelloni, A. Cotel, S.R. Hart, J.A. Whitehead In order to link the geochemical signature of hot spot basalts to Earth's deep interior, it is first necessary to understand how plumes sample different regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the ‘vortex ring bubble’ is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from different depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coefficient, s c . High- s c plumes are composed of material which originated from very deep in the fluid domain, while low- s c plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coefficient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260–750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this unsampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle.

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): Peter W. Crockford, Benjamin R. Cowie, David T. Johnston, Paul F. Hoffman, Ichiko Sugiyama, Andre Pellerin, Thi Hao Bui, Justin Hayles, Galen P. Halverson, Francis A. Macdonald, Boswell A. Wing Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous Δ O 17 values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the Δ O 17 anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum Δ O 17 value of − 0.75 ‰ . For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the Δ O 17 anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir.

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): Johan C. Faust, Karl Fabian, Gesa Milzer, Jacques Giraudeau, Jochen Knies The North Atlantic Oscillation (NAO) is the leading mode of atmospheric circulation variability in the North Atlantic region. Associated shifts of storm tracks, precipitation and temperature patterns affect energy supply and demand, fisheries and agricultural, as well as marine and terrestrial ecological dynamics. Long-term NAO records are crucial to better understand its response to climate forcing factors, and assess predictability and shifts associated with ongoing climate change. A recent study of instrumental time series revealed NAO as main factor for a strong relation between winter temperature, precipitation and river discharge in central Norway over the past 50 years. Here we compare geochemical measurements with instrumental data and show that primary productivity recorded in central Norwegian fjord sediments is sensitive to NAO variability. This observation is used to calibrate paleoproductivity changes to a 500-year reconstruction of winter NAO ( Luterbacher et al., 2001 ). Conditioned on a stationary relation between our climate proxy and the NAO we establish a first high resolution NAO proxy record ( NAO TFJ ) from marine sediments covering the past 2800 years. The NAO TFJ shows distinct co-variability with climate changes over Greenland, solar activity and Northern Hemisphere glacier dynamics as well as climatically associated paleo-demographic trends. The here presented climate record shows that fjord sediments provide crucial information for an improved understanding of the linkages between atmospheric circulation, solar and oceanic forcing factors.

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): Huixia Ding, Zeming Zhang, Xin Dong, Zuolin Tian, Hua Xiang, Hongchen Mu, Zhengbin Gou, Xinfang Shui, Wangchao Li, Lingjuan Mao Despite several decades of investigations, the nature and timing of the India–Asia collision remain debated. In the western Himalaya, the leading edge of the Indian continent was deeply subducted to mantle depths and experienced ultrahigh-pressure metamorphism in the Eocene at c . 50 Ma. In this paper, however, we demonstrate that the North Himalayan metamorphic rocks in the eastern Himalaya underwent Early Eocene (48–45 Ma) medium-pressure (MP) metamorphism due to shallow subduction of the Indian continent beneath southeastern Tibet. The studied garnet–kyanite–staurolite schists occur in the core of the Yardoi gneiss dome, the easternmost North Himalayan Gneiss Dome, and represent the upper structural level of the Higher Himalayan Crystallines (HHC). Petrology and phase equilibria modeling show that these rocks have mineral assemblages of Grt + Pl + Bt + Qz ± Ky ± St ± Ms that were formed under conditions of 7–8 kbar and 630–660 °C. Zircon U–Pb chronology shows that these rocks have peak-metamorphic ages of 48–45 Ma and protracted zircon growth, indicating that the collision between Indian and Asian continents must have occurred at c . 50 Ma in southeastern Tibet. Combining with available data, we suggest that the HHC represents a crustal section of the subducted and subsequently exhumed Indian continent. Due to shallow subduction of the continent during the Eocene, the middle to lower crust of the continent was subducted into depths of 40–60 km and underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and intense anatexis, whereas the upper crust was buried to shallower depths of 20–30 km and witnessed MP metamorphism and intrusion of leucogranites derived from the lower structural level of the HHC. Graphical abstract

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): Dan MKenzie, Keith Priestley Surface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of similar thickness to that beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P , T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that it has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins.

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): M. Bruno, M. Rubbo, D. Aquilano, F.R. Massaro, F. Nestola The study of diamond and its solid inclusions is of paramount importance to acquire direct information on the deepest regions of the Earth. However, although diamond is one of the most studied materials in geology, the diamond-inclusion relationships are not yet understood: do they form simultaneously (syngenesis) or are inclusions pre-existing objects on which diamond nucleated (protogenesis)? Here we report, for the first time, adhesion energies between diamond (D) and forsterite (Fo) to provide a crucial contribution to the syngenesis/protogenesis debate. The following interfaces were investigated at quantum-mechanical level: (i) (001) D /(001) Fo , (ii) (001) D /(021) Fo , and (iii) (111) D /(001) Fo . Our data, along with the ones recently obtained on the (110) D /(101) Fo interface, revealed an unexpected thermodynamic behaviour, all interfaces showing almost equal and low adhesion energies: accordingly, diamond and olivine have an extremely low chemical affinity and cannot develop preferential orientations, even during an eventual epitaxial growth. Combining these results with those of our previous work concerning the morphology constraints of diamond on its inclusions, we can state that the two main arguments used so far in favour of diamond/inclusions syngenesis cannot be longer considered valid, at least for olivine. Graphical abstract

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): L. Remusat, L. Piani, S. Bernard Carbonaceous and ordinary chondrites (CCs and OCs) contain insoluble organic matter (IOM) with large D-excess compared to other objects in the solar system. The higher the temperature experienced by CCs, the lower the D/H ratio of their IOM. It seems to be the opposite for OCs. Here, we report NanoSIMS H- (and N-) isotopic imaging of IOM of three OCs that experienced thermal metamorphism in the sequence Semarkona, Bishunpur and GRO 95502. In addition, we performed flash heating experiments on the IOM of GRO 95502 at 600 °C and characterized the residues using NanoSIMS, Raman and XANES spectroscopy. The present study shows that, in contrast to IOM of CI, CM and CR, IOM of OCs exhibits very few D-rich (or 15 N-rich) hotspots. Furthermore, although the evolution of the molecular structure of OC and CC IOM is similar upon heating, their D/H ratios do not follow the same trend: the D/H of OC IOM drastically increases while the D/H of CC IOM decreases. In contrast to CC IOM, the D-rich component of which does not survive at high temperatures, the present results highlight the thermal recalcitrance of the D-rich component of OC IOM. This suggests that CCs and OCs did not accrete the same organic material, thereby challenging the hypothesis of a common precursor on chondritic parent bodies. The present results support the hypothesis that OC IOM contains an organic component that could originate from the interstellar medium. Graphical abstract

Description: Publication date: 1 February 2016 Source: Earth and Planetary Science Letters, Volume 435 Author(s): A. Lindoo, J.F. Larsen, K.V. Cashman, A.L. Dunn, O.K. Neill Permeability development in magmas controls gas escape and, as a consequence, modulates eruptive activity. To date, there are few experimental controls on bubble growth and permeability development, particularly in low viscosity melts. To address this knowledge gap, we have run controlled decompression experiments on crystal-free rhyolite (76 wt.% SiO 2 ), rhyodacite (70 wt.% SiO 2 ), K-phonolite (55 wt.% SiO 2 ) and basaltic andesite (54 wt.% SiO 2 ) melts. This suite of experiments allows us to examine controls on the critical porosity at which vesiculating melts become permeable. As starting materials we used both fine powders and solid slabs of pumice, obsidian and annealed starting materials with viscosities of ∼ 10 2 to ∼ 10 6  Pa s . We saturated the experiments with water at 900° (rhyolite, rhyodacite, and phonolite) and 1025 °C (basaltic andesite) at 150 MPa for 2–72 hrs and decompressed samples isothermally to final pressures of 125 to 10 MPa at rates of 0.25–4.11 MPa/s. Sample porosity was calculated from reflected light images of polished charges and permeability was measured using a bench-top gas permeameter and application of the Forchheimer equation to estimate both viscous ( k 1 ) and inertial ( k 2 ) permeabilities. Degassing conditions were assessed by measuring dissolved water contents using micro-Fourier-Transform Infrared (μ-FTIR) techniques. All experiment charges are impermeable below a critical porosity ( ϕ c ) that varies among melt compositions. For experiments decompressed at 0.25 MPa/s, we find the percolation threshold for rhyolite is 68.3 ± 2.2  vol.% ; for rhyodacite is 77.3 ± 3.8  vol.% ; and for K-phonolite is 75.6 ± 1.9  vol.% . Rhyolite decompressed at 3–4 MPa/s has a percolation threshold of 74 ± 1.8  vol.% . These results are similar to previous experiments on silicic melts and to high permeability thresholds inferred for silicic pumice. All basaltic andesite melts decompressed at 0.25 MPa/s, in contrast, have permeabilities below the detection limit ( ∼ 10 − 15  m 2 ), and a maximum porosity of 63 vol.%. Additionally, although the measured porosities of basaltic andesite experiments are ∼10–35 vol.% lower than calculated equilibrium porosities, μ-FTIR analyses confirm the basaltic andesite melts remained in equilibrium during degassing. We show that the low porosities and permeabilities are a consequence of short melt relaxation timescales during syn- and post-decompression degassing. Our results suggest that basaltic andesite melts reached ϕ c > 63  vol.% and subsequently degassed; loss of internal bubble pressure caused the bubbles to shrink and their connecting apertures to seal before quench, closing the connected pathways between bubbles. Our results challenge the hypothesis that low viscosity melts have a permeability threshold of ∼30 vol.%, and instead support the high permeability thresholds observed in analogue experiments on low viscosity materials. Importantly, however, these low viscosity melts are unable to maintain high porosities once the percolation threshold is exceeded because of rapid outgassing and collapse of the permeable network. We conclude, therefore, that melt viscosity has little effect on percolation threshold development, but does influence outgassing.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Daniel C. Reed, Bo G. Gustafsson, Caroline P. Slomp Coastal hypoxia is a growing and persistent problem largely attributable to enhanced terrestrial nutrient (i.e., nitrogen and phosphorus) loading. Recent studies suggest phosphorus removal through burial of iron (II) phosphates, putatively vivianite, plays an important role in nutrient cycling in the Baltic Sea — the world's largest anthropogenic dead zone — yet the dynamics of iron (II) phosphate formation are poorly constrained. To address this, a reactive-transport model was used to reconstruct the diagenetic and depositional history of sediments in the Fårö basin, a deep anoxic and sulphidic region of the Baltic Sea where iron (II) phosphates have been observed. Simulations demonstrate that transport of iron from shelf sediments to deep basins enhances vivianite formation while sulphide concentrations are low, but that pyrite forms preferentially over vivianite when sulphate reduction intensifies due to elevated organic loading. Episodic reoxygenation events, associated with major inflows of oxic waters, encourage the retention of iron oxyhydroxides and iron-bound phosphorus in sediments, increasing vivianite precipitation as a result. Results suggest that artificial reoxygenation of the Baltic Sea bottom waters could sequester up to 3% of the annual external phosphorus loads as iron (II) phosphates, but this is negligible when compared to potential internal phosphorus loads due to dissolution of iron oxyhydroxides when low oxygen conditions prevail. Thus, enhancing vivianite formation through artificial reoxygenation of deep waters is not a viable engineering solution to eutrophication in the Baltic Sea. Finally, simulations suggest that regions with limited sulphate reduction and hypoxic intervals, such as eutrophic estuaries, could act as important phosphorus sinks by sequestering vivianite. This could potentially alleviate eutrophication in shelf and slope environments.

Description: Publication date: 15 January 2016 Source: Earth and Planetary Science Letters, Volume 434 Author(s): Yukiho Tsuchiyama, Haider Zaman, Sieng Sotham, Yos Samuth, Eiichi Sato, Hyeon-Seon Ahn, Koji Uno, Kosuke Tsumura, Masako Miki, Yo-ichiro Otofuji Late Jurassic to Early Cretaceous red beds of the Phuquoc Formation were sampled at 33 sites from the Sihanoukville and Koah Kong areas of the Phuquoc–Kampot Som Basin, southwestern Cambodia. Two high-temperature remanent components with unblocking temperature ranging 650°–670 °C and 670–690 °C were identified. The magnetization direction for the former component ( D = 5.2 ° , I = 18.5 ° with α 95 = 3.1 ° in situ) reveals a negative fold test that indicates a post-folding secondary nature. However, the latter component, carried by specular hematite, is recognized as a primary remanent magnetization. A tilt-corrected mean direction of D = 43.4 ° , I = 31.9 ° ( α 95 = 3.6 ° ) was calculated for the primary component at 11 sites, corresponding to a paleopole of 47.7°N, 178.9°E ( A 95 = 3.6 ° ). When compared with the 130 Ma East Asian pole, a southward displacement of 6.0 ° ± 3.5 ° and a clockwise rotation of 33.1 ° ± 4.0 ° of the Phuquoc–Kampot Som Basin (as a part of the Indochina Block) with respect to East Asia were estimated. This estimate of the clockwise rotation is ∼15° larger than that of the Khorat Basin, which we attribute to dextral motion along the Wang Chao Fault since the mid-Oligocene. The comparison of the herein estimated clockwise rotation with the counter-clockwise rotation reported from the Da Lat area in Vietnam suggests the occurrence of a differential tectonic rotation in the southern tip of the Indochina Block. During the southward displacement of the Indochina Block, the non-rigid lithosphere under its southern tip moved heterogeneously, while the rigid lithosphere under the Khorat Basin moved homogeneously.