CO_2 Density-Raman Shift Relation Derived from Synthetic Inclusions in Fused Silica Capillaries and Its Application

Abstract: The densities of CO2 inclusions in minerals are commonly used to determine the crystallizing conditions of the host minerals. However, conventional microthermometry is difficult to apply for inclusions of small size (< 5–10 μm) or low density. Raman analysis is an alternative method for determining CO2 density, provided that the CO2 density–Raman shift relation is known. This study aims to establish this CO2 density–Raman shift relation by using CO2 inclusions synthesized in fused silica capillaries. By using this newly-developed synthetic technique, we formed pure CO2 inclusions, and their densities were determined by microthermometry. The Raman analysis showed that the relation between CO2 density (D in g/cm3) and the separations (Δ in cm?1) between the two main bands (i.e. Fermi diad bands) in CO2 Raman spectra can be represented by a cubic equation: D (g/cm3)=0.74203(?0.019Δ3+5.90332Δ2?610.79472Δ+21050.30165)?3.54278 (r2=0.99920). Our calculated D value for a given Δ is between those obtained from two previously-reported equations, which were derived from different experimental methods. An example was given in this study to demonstrate that the densities of natural CO2 inclusions that could not be derived from microthermometry could be determined by using our method.     

High-resolution shallow seismic identification of gas escape features in the sediments of Loch Tay, Scotland: tectonic and microbiological associations

High-resolution seismic reflection profiles (3·5 kHz) have revealed the presence of extensive interstitial gas accumulation within the sedimentary sequences of Loch Tay, Scotland, as identified by acoustic turbidity masking the seismic stratigraphy. Within the central section of the loch, in the deepest water area directly above the zone of the seismically active Loch Tay Fault, focused flows of gas through the sediment pile to the loch bed via chimneys and pockmarks, together with gas seeps within the water column, have been identified. Microbiological observations indicate that the gas is biogenic CH₄, produced by both chemoautotrophic (which use CO₂ as a source of carbon and H₂ as a source of energy) and aceticlastic species (which use acetate as a source of carbon and energy) of methanogens in the fine-grained, organic rich deposits that have been focused into the zone of accumulation in the deep central part of the loch. The spatial distribution of the gas escape features suggests that earthquake movements along the Loch Tay Fault are responsible for facilitating focused gas escape in this part of the loch, by the creation of new pathways and conduits through the sediment pile, along which gas can migrate upwards and exit into the water column. Relict pockmarks and associated chimneys identified in the seismic records indicate that gas escape has been taking place since Pleistocene times though the precise timings cannot be ascertained. This is the first time that such features have been reported from a lake in the UK.     

Flow dynamics and geomorphology of a trough blowout

The dynamics and geomorphological development of a trough blowout located at Fiona Beach in the Myall Lakes National Park in NSW, Australia, are examined. Wind speeds, velocities and flow structure were measured utilizing an array of miniature Rimco cup anemometers, Gill bivane and UVW instruments, and wind vanes. Flow measurements indicate that when the wind approaches the trough blowout parallel to the throat orientation, jets occur both in the deflation basin and along the erosional walls, relative flow deceleration and expansion occurs up the depositional lobe, jets are formed over the depositional lobe crest accompanied by downwind flow separation on the leeward side of the lobe, and flow separation and the formation of corkscrew vortices occur over the crests of the erosional walls. Maximum erosion and transport occurs up the deflation basin and onto the depositional lobe. Trough blowout morphologies are explained as a function of these flow patterns. When the wind approaches the blowout obliquely, the flow is steered considerably within the blowout, and the degree and complexity of topographic steering is dependent on the blowout topography. The flow is usually extremely turbulent and large corkscrew vortices are common. The local topography of a blowout can be very important in determining overall sand transport and blowout evolutionary conditions and paths. Estimates of potential sand transport within the blowout may be up to two orders of magnitude lower than actual rates if remotely sensed wind data are used.     

Solid-Fluid Equilibria in the System KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HCl

Activity diagrams in the system KAlSi₃O₈-NaAlSi₃O₈-Al₂SiO₅-SiO₂-H₂O-HClhave been calculated in terms of a_K+/a_H+ and a_N+/a_H+ from existingexperimental data. They show the effect of temperature, pressure,and a_H2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, a_H2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al₂SiO₅+H₂O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO₂+2 K⁺=3 K-feldspar+2 H⁺ muscovite+6 SiO₂+3 Na⁺=3 Albite+K⁺+2 H⁺ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H⁺=3 Al₂SiO₅+3 SiO₂+3 H₂O+2 K⁺ is favored at high temperature and pressure and low a_H2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria.     

The importance of maintaining horizontal sieve screens when using a Ro-Tap

Sieve size analysis, using a Ro-Tap shaking machine, for horizontal and non-horizontal sieve screen nests are compared. The non-horizontal sieve condition results in more sediment being retained on the topmost sieve, with less sediment reaching certain sieves within the nest. It is thus essential to maintain the nest of sieves perfectly horizontal.     

Interplay between river discharge and topography of the basin floor in a hyperpycnal lacustrine delta

Basin‐floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the Red River delta in Lake Texoma, USA. The Red River discharge is typically a hyperpycnal plume due to elevated total dissolved solids. Because the river plume is a bottom‐hugging hyperpycnal flow, lake bathymetry and topography strongly influence deposition and subsequent delta morphology. In addition to elevated total dissolved solid concentrations compared with Lake Texoma water, the density contrast of the Red River outflow is increased by high suspended‐sediment concentrations during high‐discharge events. Steep lateral slopes in the Lake Texoma basin deflect hyperpycnal river plumes and, subsequently, change the delta progradation direction before the delta reaches the opposite bank of the lake. Analysis of multi‐temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake‐bottom gradients, corresponding to the pre‐impoundment river thalweg (i.e. bypassing shallow parts of the lake). An analytical model for the hyperpycnal‐plume trajectory indicates plume deflection during low‐discharge or high‐discharge events, towards the deepest part of the basin. The magnitude of plume deflection is a function of river discharge and basin‐margin gradients. Plume deflection can vary between 10° and 80° from the channel axis towards the old river thalweg. The high deflection appears in the case of maximum basin side gradients of 12·8° and in conditions of low river discharge. During low‐discharge periods, the Red River delta builds a lobate shape with multiple terminal distributary channels whereas, during high‐discharge periods the Red River delta builds an elongate shape with a single large distributary channel. The elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries. Therefore, the lobate shaped delta is expected to be preserved in the rock record.     

Glacial drainage towards the Mediterranean during the Middle and Late Pleistocene

To a varying degree the Middle and Late Pleistocene ice sheets in northern Eurasia redirected the drainage of major catchments in Europe and western Siberia from the North Sea and Arctic Ocean south to the Caspian, Black Sea, and ultimately the Mediterranean. During the Late Weichselian, glacial meltwater reached the Mediterranean through the Dniepr and Don catchments and to a minor extent through the Danube. During the Warthe Substage of the Saalian, meltwater from the Volga was most likely added. During the Drenthe Substagc of the Saalian the watershed shifted Par to the east, and meltwater reached the Mediterranean also from the Oh. Irtysh, Yenisei, and Tunguska catchments in Siberia. Depending on the extent of the ice sheets, the increase in freshwater supply during deglaciations resulted in reductions of Mediterranean overflow into the North Atlantic. Such overflow reductions may have reduced vapour transport to the ice sheets and thus accelerated wastage.     

Stable isotope signatures associated with palaeosols, Pennsylvanian Holder Formation, New Mexico

The Holder Formation (Pennsylvanian, Virgilian) of southern New Mexico, USA, consists of limestones interbedded with siliciclastics. It was deposited during times of glacio-eustatic sea-level change and was exposed subaerially during multiple sea-level lowstands. Microcomponents and whole-rock samples of limestones were analysed for δ¹³C and δ¹⁸O values to examine the method of whole-rock isotopic analysis for detecting subaerial exposure events and to determine the diagenetic processes acting during subaerial exposure. Whole-rock isotopic shifts are not consistently present across petrographically identified subaerial exposure surfaces. Apparently, whole-rock isotopic shifts do not result from wholesale replacement of the host sediment during soil formation. However, the isotopic shifts are present in calcareous, soil-precipitated microcomponents, such as rhizoliths, laminated crusts, and soil-precipitated cements. The components are heterogeneous in isotopic composition, but converge on a meteoric calcite line at about δ¹⁸O=?5.5‰. These microcomponents are heterogeneous in distribution and may either dominate or be a minor constituent of the whole rock at a single stratigraphic horizon. Without petrographic selection of palaeosol components, the detection of whole-rock isotopic shifts may depend on the selection or chance sampling of a rock containing abundant microcomponents precipitated in a soil environment. Only minor whole-rock isotopic shifts come from those rocks bearing no evidence of exposure and bearing lithological characteristics suggesting subaerial exposure was unlikely.     

Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina

Analysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation-controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud-rich central backbarrier sequence consisting of low marsh overlying fine-grained, tidal flat/lagoonal mud; (2) a sandy beach-ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island. Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine-grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell-rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded. Barrier progradation and sediment bar-bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach-ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining-upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization. This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand-rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea-level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.