Evaluation of a high-resolution regional climate simulation over Greenland

A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model (RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than 60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations.     

Construction of three-phase data to model multiphase flow in porous media: Comparing an optimization approach to the finite element approach

Multiphase flow modelling is a major issue in the assessment of groundwater pollution. Three-phase flows are commonly governed by mathematical models that associate a pressure equation with two saturation equations. These equations involve a number of secondary variables that reflect the fluid behaviour in a porous medium. To improve the computational efficiency of multiphase flow simulators, several simplified reformulations of three-phase flow equations have been proposed. However, they require the construction of new secondary variables adapted to the reformulated flow equations. In this article, two different approaches are compared to quantify these variables. A numerical example is given for a typical fine sand.     

Structural controls on the formation of BSR over a diapiric anticline from a dense MCS survey offshore southwestern Taiwan

A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.     

Record of enhanced late Pliocene tectono‐volcanic activity in the Lesser Antilles forearc basin: Insights into the long‐term behaviour of the Lesser Antilles subduction zone

In this study, we investigate the Lesser Antilles forearc basin, focusing on the late Pliocene to Pleistocene sedimentary archives in order to track the occurrence of extreme events triggered by enhanced subduction‐related tectono‐volcanic activity. We identify late Piacenzian deposits covering a major regional erosional surface, displaying sedimentary dykes and large marine boulders embedded in a mixed continental–marine matrix, characteristic of tsunamites. We interpret this episode of platform emersion and the successive cataclysmic deposits as resulting from enhanced tectonic activity at the interface of the subduction zone, synchronous with the initiation of the Lesser Antilles volcanic arc. We then discuss the implications in terms of the mechanical behaviour of the Lesser Antilles subduction zone.     

中国北方地区春季气溶胶光学特性地基遥感研究

基于中国北方地区地基太阳直接/散射辐射计观测,分析了2001年春季气溶胶光学厚度、尺度和吸收特性。沙尘期间,气溶胶光学厚度显著升高,而尺度上升,粗粒子浓度显著增加,大气气溶胶表现出中性或异常消光特征。除沙尘活动之外,中国华北地区人为排放也是导致气溶胶光学厚度偏高的一个重要原因。中国沙尘源区气溶胶单次散射反照率小于以往观测和模式结果。北京和香河两地气溶胶光学厚度、Angstrom指数和尺度谱基本相似,说明华北地区人为排放与沙尘活动类似,也表现出区域尺度的特点。由于沙尘气溶胶与人为排放气溶胶相比,折射指数实部偏大、虚部偏小,从而北京地区沙尘期间单次散射反照率明显大于非沙尘期间,特别是在近红外波段,说明沙尘活动不仅可以通过增加气溶胶光学厚度,同时也通过改变气溶胶物理、辐射特性来影响下游地区的气溶胶辐射强迫。     

Correlations and sequence stratigraphic model for Messinian carbonate platforms of the western and central Mediterranean

Distance correlations of Late Tortonian–Messinian littoral carbonate complexes are proposed from the study of eight platforms in the western and central Mediterranean. Correlations are based on the identification of two major biological sedimentary cycles and of two index surfaces. Surface A is a maximum flooding surface during cycle 1 at around 6.7 Ma. Surface B is a regional marine planation surface at around 5.95 Ma, at the base of cycle 2 (Terminal Carbonate Complex). A general sedimentary model is proposed for the 7–5.6-Ma time-span. The boundary between cycles 1 and 2 is coincident with the onset of the Messinian Salinity Crisis, and appears to be related to major environmental–paleo-oceanographic changes in the Mediterranean, rather than to a major sea-level drop or to climatic change.     

Carbon geochemistry of serpentinites in the Lost City Hydrothermal System (30°N, MAR)

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ¹³C_TC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ¹³C_TOC: −28.9‰ to −21.5‰) and variations in δ¹³C_TC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C₁₅ to C₃₀. Longer-chain hydrocarbons (up to C₄₀) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C₁₆ to n-C₂₀ alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.     

Processes controlling solubility of biogenic silica and pore water build-up of silicic acid in marine sediments

Dissolution experiments in batch and flow-through reactors were combined with data on sediment composition and pore water silicic acid profiles to identify processes controlling the solubility of biogenic silica and the build-up of silicic acid in marine sediments. The variability of experimentally determined biogenic silica solubilities is due, in part, to variations in specific surface area and Al content of biosiliceous materials. Preferential dissolution of delicate skeletal structures and frustules with high surface areas leads to a progressive decrease of the specific surface area. This may cause a reduction of the solubility of deposited biosiliceous debris by 10–15%, relative to fresh planktonic assemblages. Dissolution of lithogenic (detrital) minerals in sediments releases dissolved aluminum to the pore waters. This aluminum becomes structurally incorporated into deposited biogenic silica, further decreasing its solubility. Compared to Al-free biogenic silica, the solubility of diatom frustules is lowered by as much as 25% when one out of every 70 Si atoms is substituted by an Al(III) ion.The build-up of silicic acid in pore waters of sediments with variable proportions of detrital matter and biogenic silica was simulated in batch experiments using kaolinite and basalt as model detrital constituents. The steady-state silicic acid concentrations measured in the experiments decreased with increasing detrital-to-opal ratios of the mixtures. This trend is similar to the observed inverse relationship between asymptotic pore water silicic acid concentrations and detrital-to-opal ratios in Southern Ocean sediments. Flow-through reactor experiments further showed that in detrital-rich sediments, precipitation of authigenic alumino-silicates may prevent the pore waters from reaching equilibrium with the dissolving biogenic silica. This agrees with data from Southern Ocean sediments where, at sites containing more than 30 wt.% detrital material, the pore waters remain undersaturated with respect to the experimentally determined in situ solubility of biogenic silica.The results of the study show that interactions between deposited biogenic silica and detrital material cause large variations in the asymptotic silicic acid concentration of marine sediments. The production of Al(III) by the dissolution of detrital minerals affects the build-up of silicic acid by reducing the apparent silica solubility and dissolution kinetics of biosiliceous materials, and by inducing precipitation of authigenic alumino-silicate minerals.