Impact of High Methane Flux on the Properties of Pore Fluid and Methane-Derived Authigenic Carbonate in the ARAON Mounds, Chukchi Sea

We investigated the pore fluid and methane-derived authigenic carbonate (MDAC) chemistry from the ARAON Mounds in the Chukchi Sea to reveal how methane (CH4) seepage impacts their compositional and isotopic properties. During the ARA07C and ARA09C Expeditions, many in situ gas hydrates (GHs) and MDACs were found near the seafloor. The fluid chemistry has been considerably modified in association with the high CH4 flux and its related byproducts (GHs and MDACs). Compared to Site ARA09C-St 08 (reference site), which displays a linear SO42- downcore profile, the other sites (e.g., ARA07C-St 13, ARA07C-St 14, ARA09C-St 04, ARA09C-St 07, and ARA09C-St 12) that are found byproducts exhibit concave-up and/or kink type SO42- profiles. The physical properties and fluid pathways in sediment columns have been altered by these byproducts, which prevents the steady state condition of the dissolved species through them. Consequently, chemical zones are separated between bearing and non-bearing byproducts intervals under non-steady state condition from the seafloor to the sulfate-methane transition (SMT). GH dissociation also significantly impacts pore fluid properties (e.g., low Cl-, enriched delta D and delta O-18). The upward CH4 with depleted delta C-13 from the thermogenic origin affects the chemical signatures of MDACs. The enriched delta O-18 fluid from GH dissociation also influences the properties of MDACs. Thus, in the ARAON Mounds, the chemistry of the fluid and MDAC has significantly changed, most likely responding to the CH4 flux and GH dissociation through geological time. Overall, our findings will improve the understanding and prediction of the pore fluid and MDAC chemistry in the Arctic Ocean related to CH4 seepage by global climate change

Blocking representation in the ERA-Interim driven EURO-CORDEX RCMs

While Regional Climate Models (RCMs) have been shown to yield improved simulations compared to General Circulation Model (GCM), their representation of large-scale phenomena like atmospheric blocking has been hardly addressed. Here, we evaluate the ability of RCMs to simulate blocking situations present in their reanalysis driving data and analyse the associated impacts on anomalies and biases of European 2-m air temperature (TAS) and precipitation rate (PR). Five RCM runs stem from the EURO-CORDEX ensemble while three RCMs are WRF models with different nudging realizations, all of them driven by ERA-Interim for the period 1981?2010. The detected blocking systems are allocated to three sectors of the Euro-Atlantic region, allowing for a characterization of distinctive blocking-related TAS and PR anomalies. Our results indicate some misrepresentation of atmospheric blocking over the EURO-CORDEX domain, as compared to the driving reanalysis. Most of the RCMs showed fewer blocks than the driving data, while the blocking misdetection was negligible for RCMs strongly conditioned to the driving data. A higher resolution of the RCMs did not improve the representation of atmospheric blocking. However, all RCMs are able to reproduce the basic anomaly structure of TAS and PR connected to blocking. Moreover, the associated anomalies do not change substantially after correcting for the misrepresentation of blocking in RCMs. The overall model bias is mainly determined by pattern biases in the representations of surface parameters during non-blocking situations. Biases in blocking detections tend to have a secondary influence in the overall bias due to compensatory effects of missed blockings and non-blockings. However, they can lead to measurable effects in the presence of a strong blocking underestimation.This work was funded by the Austrian Science Fund (FWF) under the project: Understanding Contrasts in high Mountain hydrology in Asia (UNCOMUN: I 1295-N29). This research was supported by the Faculty of Environmental, Regional and Educational Sciences (URBI), University of Graz, as well as the Federal Ministry of Science, Research and Economy (BMWFW) by funding the OeAD Grant Marietta Blau. This work was partially supported (JMG and SH) by the project MULTI-SDM (CGL2015-66583- R, MINECO/FEDER). DB was supported by the PALEOSTRAT (CGL2015-69699-R) project funded by the Spanish Ministry of Economy and Competitiveness (MINECO)

Low-Reynolds-Number Flow around an Impulsively Started Rotating and Translating Circular Cylinder

International audienceThis paper describes the two-dimensional unsteady low-Reynolds-number flow past an impulslvely started rotating and translating circular cylinder. Invoking the vorticity equation, we first derive a system of two coupled integral equations that govern the stream function and a modified vorticity function. This system, singular in the low-Reynolds-number, is then asymptotically solved by using a singular perturbation method and introducing five regions in the space-time domain. The first-order solutions are found to linearly depend on the translating and rotating motions within each region. Because of its importance for applications, a special attention is paid to the lift coefficient CL which results here from intricate interactions between rotation and translation. The obtained initial asymptotic behavior of CL actually exhibits a t-1/2 singularity and thereby differs from the prediction of Badr & Dennis(1) at moderate Reynolds numbers

Structure H (sH) Clathrate Hydrate with New Large Molecule Guest Substances

This study characterized new structure H (sH) clathrate hydrates with bromide large-molecule guest substances (LMGSs) bromocyclopentane (BrCP) and bromocyclohexane (BrCH), using powder X-ray diffraction (PXRD) and Raman spectroscopy. The lattice parameters of sH hydrates with (CH₄ + BrCP) and (CH₄ + BrCH) were determined from their PXRD profiles. On the basis of their Raman spectra, the M-cage to S-cage occupancy ratio (4³5⁶6³ and 5¹² cages, respectively), θ_M/θ_S, was estimated to be approximately 1.3, and the Raman shift of the symmetric C–H vibrational modes of CH₄ in S- and M-cages was 2911.1 and 2909.1 cm^–1, respectively. The phase-equilibrium conditions of sH hydrates with (CH₄ + BrCP) and (CH₄ + BrCH) were determined by an isochoric method. A comparison between the equilibria of sH hydrates with BrCP and BrCH and those with other typical nonpolar and polar LMGSs (methylcyclopentane, MCP; methylcyclohexane, MCH; neohexane, NH; and <i>tert</i>-butyl methyl ether, TBME) at the same temperature revealed that the equilibrium pressure increased in the order NH < MCH < BrCH < TBME ∼ MCP < BrCP. The phase stabilities of sH hydrates can be determined by not only molecular geometry but also their polar properties, which affect guest–host interactions

Phase Equilibrium Conditions for Clathrate Hydrates of Tetra-<i>n</i>-butylammonium Bromide (TBAB) and Xenon

Phase equilibrium pressure–temperature (<i>pT</i>) conditions for the xenon (Xe)–tetra-<i>n</i>-butylammonium bromide (TBAB)–water system were characterized by an isochoric method in the pressure range from (0.05 to 0.3) MPa using TBAB solutions with mole fractions ranging from (0.0029 to 0.0137). The phase equilibrium <i>pT</i> conditions in the system appeared at a lower pressure and higher temperature than in the pure Xe hydrate. Furthermore, under atmospheric pressure, the dissociation temperature in the Xe–TBAB–water system shifted to a higher region than in the pure TBAB hydrate. In the experimental TBAB concentration range, the powder X-ray diffraction patterns of the Xe–TBAB–water system revealed that the TBAB clathrate hydrate is TBAB·38H₂O

Structural Characterization of Structure H (sH) Clathrate Hydrates Enclosing Nitrogen and 2,2-Dimethylbutane

In this study, we characterized structure H (sH) clathrate hydrates (hydrates) containing nitrogen (N₂) and 2,2-dimethylbutane (neohexane, hereafter referred to as NH) molecules. On the basis of the powder X-ray diffraction profile, we estimated the unit cell dimensions of the sH hydrate of N₂ + NH to be <i>a</i> = 1.22342(15) nm and <i>c</i> = 0.99906(17) nm at 153 K. The <i>c</i> axis of this hydrate was slightly shorter (i.e., 0.00584 nm) than that of CH₄ + NH, whereas we observed no difference in the <i>a</i> axis between these two hydrates. We successfully observed a symmetric N–N stretching (N–N vibration) Raman peak with two bumps, and we determined that the N–N vibrational mode in the 5¹² and 4³5⁶6³ cages occurred at approximately 2323.8 and 2323.3 cm^–1, respectively. We found the cage occupancy ratio of the 4³5⁶6³/5¹² cages (θ_Mθ_S) of the sH hydrate of N₂ + NH to be approximately 1.30. From a comparison of the N–N vibrational modes in the 5¹², 4³5⁶6³, 5¹²6², and 5¹²6⁴ cages of the sI, sII, and sH hydrates, we determined that N₂ molecules in the distorted 4³5⁶6³ cages experience more <i>attractive</i> guest–host interaction than those in spherical 5¹²6⁴ cages, whereas the guest/cage diameter ratio of 4³5⁶6³ cages is larger than that of 5¹²6⁴ cages. We determined the L₁–L₂–H–V four-phase equilibrium pressure–temperature conditions in the N₂–NH–water system in the temperature range of 274.36–280.71 K. Using the Clausius–Clapeyron equation, we estimated the dissociation enthalpies of the sH hydrates of N₂ + NH to be 388.4 and 395.9 kJ·mol^–1 (per one molar of N₂ molecules) in the experimental temperature range

Phase Transition of Tetra‑<i>n</i>‑butylammonium Bromide Hydrates Enclosing Krypton

The phase equilibrium conditions for krypton (Kr)–tetra-<i>n</i>-butylammonium bromide (TBAB)–water systems were determined using an isochoric method. The pressure and temperature ranges were (0.06 to 1.0) MPa and (280 to 290) K, respectively, and TBAB solutions had TBAB molar fractions, <i>x</i>_TBAB, of 0.0062, 0.0138, 0.0234, and 0.0359. A second order transition of the TBAB hydrate was observed in all the Kr–TBAB–water systems. In the region at lower pressure than the phase transition point, the Kr–TBAB–water systems with low concentration (<i>x</i>_TBAB = 0.0062 and 0.0138) and high concentration (<i>x</i>_TBAB = 0.0234 and 0.0359) prefer to form TBAB·38H₂O and TBAB·26H₂O hydrates, respectively. However, a <i>new</i> TBAB hydrate was observed as a stable crystal structure in the higher pressure regions. Raman spectrum of the new TBAB hydrate shows band shapes remarkably similar to that of <i>pure</i> TBAB·38H₂O with the crystalline space group <i>Pmma</i> in the frequency ranges of the lattice for C–C stretching, C–H bending, the C–H stretching bands of the −CH₂ groups of TBA⁺ molecules, and the O–H stretching modes of water molecules, excluding the C–H stretching bands of the CH₃ groups of TBA⁺ molecules