Creep,stable sliding,and premonitory slip

Summary The current status of laboratory investigations into creep, stable sliding and premonitory slip is reviewed and some new material is presented. It is postulated that pre-cut rocks and those with simulated gouge layers undergo a transition with increasing confining pressure from (1) stable sliding to stick-slip, to (2) sliding along the pre-cut with deformation of the country rock, to (3) homogeneous flow of the specimen without slip along the pre-cut. Stick-slip behavior is not always present. Decreasing displacement rates are found to enhance stick-slip. Mixtures of gouge are found to be significant in controlling the behavior of sliding with 10–20 percent of anhydrite mixed with quartz or clays mixed with anhydrite shifting the sliding mode from stable stick-slip or stick-slip to stable sliding, respectively. Premonitory slip may be one of the most significant short term precursors of earthquakes. Although widely recognized in the laboratory, little systematic work has been completed. Variations in pore pressure, resistivity and seismic velocities have been investigated. Clearly much work needs to be done into these topics before a clear understanding is achieved.     

Radiative Effects on Temperature in the Stable Surface Layer

The interaction between longwave radiation and temperature fluctuations plays a role in the dissipation of temperature variance. This interaction is most easily described by spectral models of atmospheric turbulence and a spectral radiative dissipation function which gives the intensity of the damping at each radiative wavelength and wavenumber k. We have used a Corrsin–Pao closure for the spectral budgets of turbulent kinetic energy and temperature to study the coupling of radiation to turbulence. The spectral radiative dissipation function and a related integral have been fitted by analytical approximations with the correct asymptotic behavior. This resulted in a simple analytical formula for the dimensionless temperature spectrum as a function of Monin-Obukhov stability, and a new dimensionless parameter describing the relative importance of radiation in the temperature spectral budget. The radiative effects both on the temperature spectrum and on the dimensionless temperature variance can then be calculated. Based on typical values of the radiative dimensionless parameters for the surface layer, we conclude that radiative dissipation is probably negligible there.     

Nature and role of CO2 in some hot and cold HCO3/Na/CO2-rich Portuguese mineral waters: a review and reinterpretation

 At the northern part of the Portuguese mainland, the upflow zone of several hot and cold HCO₃/Na/CO₂-rich mineral waters is mainly associated with important NNE–SSW faults. Several geochemical studies have been carried out on thermal and non-thermal hydromineral manifestations that occur along or near these long tectonic alignments. The slight chemical differences that exist between these meteoric hot and cold HCO₃/Na/CO₂-rich mineral waters seem to be mainly caused by CO₂. δ¹³C_(TIDC) values observed in these groundwaters range between –6.00 and –1.00‰ versus V-PDB (V denotes Vienna, the site of the International Atomic Energy Agency; PDB originates from the CaCO₃ of the rostrum of a Cretaceous belemnite, Belemnitella americana, collected in the Peedee formation of South Carolina, USA) indicating a deep-seated (mantle) origin for most of the CO₂. Nevertheless, in the case of the heavier δ¹³C_(TIDC) values, the contribution of metamorphic CO₂ or the dissolution of carbonate rock levels at depth cannot be excluded. Concerning the hot waters, the lack of a positive ¹⁸O-shift should be attributed to water-rock interaction in a low temperature environment, rather than to the isotopic influence of CO₂ on the δ¹⁸O-value of the waters. Received: 9 August 1999 · Accepted: 8 March 2000     

Paragenesis of fluids under evaporates in the Volga-Ural Basin

To unravel the mystery of the relationship between evaporates, Ca–Cl brines and accumulations of oil and N₂ in the basins of ancient cratons, their N₂, CH₄ and He concentration ratios, as well as the isotopic composition (δ¹⁵N, δ¹³C and ³He/⁴He) were compared within the Volga-Ural basin. The study allowed subsalt fluids from Volga-Ural Basin to divide into two genetic groups. The first one is found within the basin's platform area. It includes Ca–Cl brines, high-viscosity heavy oil, bitumen and N₂, which has concentrations higher than that of CH₄ and positive values of δ¹⁵N. The second one is tied to the edge of the platform, the Ural Foredeep and Peri-Caspian Depression. In this group, only the oil and gas reservoirs, which have more CH₄ than N₂, and possibly negative values of δ¹⁵N, were discovered. Interaction of gas components in compared fluids indicates great role of degassing in the formation of their composition. It is suggested that the fluids of the first group (N₂ > CH₄) is what remains, and the second group (N₂ < CH₄) is what is disappears from the rocks during their metamorphism and degassing.     

Persistent organic pollutants and stable isotopes in biopsy samples (2004/2006) from Southern Resident killer whales

“Southern Resident” killer whales include three “pods” (J, K and L) that reside primarily in Puget Sound/Georgia Basin during the spring, summer and fall. This population was listed as “endangered” in the US and Canada following a 20% decline between 1996 and 2001. The current study, using blubber/epidermis biopsy samples, contributes contemporary information about potential factors (i.e., levels of pollutants or changes in diet) that could adversely affect Southern Residents. Carbon and nitrogen stable isotopes indicated J- and L-pod consumed prey from similar trophic levels in 2004/2006 and also showed no evidence for a large shift in the trophic level of prey consumed by L-pod between 1996 and 2004/2006. ∑PCBs decreased for Southern Residents biopsied in 2004/2006 compared to 1993–1995. Surprisingly, however, a three-year-old male whale (J39) had the highest concentrations of ∑PBDEs, ∑HCHs and HCB. POP ratio differences between J- and L-pod suggested that they occupy different ranges in winter.     

Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and δ¹³C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of ¹³C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in ¹³C in relation to the δ¹³_CCH4${\delta }^{13}{\mathrm{C}}_{{\mathrm{CH}}_4}$ signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH₄ between 0.005 and 0.02 Tg yr⁻¹. This is not a large contribution but the fluxes from the polynyas are 20–200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change.     

Fluid geochemistry of the Methana Peninsula and Loutraki geothermal area,Greece

A geochemical survey on the thermal fluids released by the volcanic/geothermal system of Methana Peninsula and Loutraki area was undertaken.     

Last glacial emplacement of methane-derived authigenic carbonates in the Sea of Japan constrained by diatom assemblage,carbon-14, and carbonate content

We studied diatom assemblages and CaCO₃ contents of methane-derived authigenic carbonates from the eastern margin of the Sea of Japan and assessed the formation time of these samples. Radioactive ¹⁴C date was determined in selected samples to obtain the maximum age of the time. The results of our study suggest mass formation of carbonate nodules in a glacial period within ∼40 ky, consistent with a published U/Th dating result of carbonate nodules in the study area. Diatom assemblages and contents in the carbonate nodules (abundance of ∼10⁶/g, dominance of neritic-littoral species, warm/cold water species ratio lower than ∼25) differ from the near-seafloor sediments in the study area, which have characteristics of Holocene sediments in the Sea of Japan, and suggest cementation of glacial sediments. Laminated sediments in some nodule samples are glacial sediments because laminations are records of a low sea level period in the semi-enclosed ocean. Similarity of diatom assemblages and contents in all carbonate samples is another evidence of glacial sediments in nodules. Glacial sediments with oceanic cold water species as low as Holocene sediments restricts the sediment age to before 20 cal. ky BP. Carbonate contents higher than 78 wt% suggest the cementation of poorly compacted sediments near the seafloor, and the date of carbonate cementation is, therefore, close to that of the cemented sediments. Most carbonate nodule samples in this study were formed in a glacial period and detection of ¹⁴C restricts this period to within ∼40 ky.     

Structural and fluid evolution of Saraburi Group sedimentary carbonates,central Thailand: A tectonically driven fluid system

A stable isotopic study, focused on calcite cements, vein-fill calcite and various bioclasts was conducted on variably deformed and thrusted Lower and Middle Permian carbonates of the Saraburi Group. Samples were collected in quarry faces across 3 areas in the Saraburi–Lopburi region of central Thailand. Stable isotope crossplots (carbon and oxygen), using texture-aware isotope samples, defined variable, but related, fluid-cement histories, which are tied to regional burial and then orogenic overprints driven by the Indosinian (Triassic) orogeny. This was followed by telogenetic overprints, driven by late Cenozoic uplift. The studied carbonates were deposited along the western margin of the Indochina Block, where they were deposited as isolated calcareous algal, sponge and fusilinid-rimmed platforms on highs bound by extensional faults. The platform areas passed laterally and vertically into more siliciclastic dominated sequences, deposited in somewhat deeper waters within probable fault-bound lows. Regional post-depositional mesogenetic fluid-rock re-equilibration of the isotope values in ongoing calcite precipitates occurred until the matrix permeability was occluded via compaction and pressure solution. This regional burial regime was followed by collision of the Indochina and Sibumasu blocks during the Indosinian (Triassic) blocks, which drove a set of structurally focused (thrust-plane related) increasingly warmer set of fluids through the studied sequences. The final diagenetic overprint seen in the isotopic values of the latest calcite cements occurs in a telogenetic (uplift) setting driven by Cenozoic tectonics and isostatic uplift. Integration of isotope data with its structural setting establishes a clear separation in fluid events related to two time-separate tectonic episodes; its fluid chemistry defines the Permo-Triassic closure of the Paleotethys and its subsequent reactivation during the Tertiary collision of India and Asia. The C–O covariant plot fields in the Permian carbonates of central Thailand are so distinct that it is possible to use their signatures to separate burial from meteoric cements in drill cuttings and hence recognise equivalent subsurface unconformities and likely zone of porosity development in possible “buried hill plays in Thailand.     

Trophic ecology and vertical patterns of carbon and nitrogen stable isotopes in zooplankton from oxygen minimum zone regions

The unique physical and biogeochemical characteristics of oxygen minimum zones (OMZs) influence plankton ecology, including zooplankton trophic webs. Using carbon and nitrogen stable isotopes, this study examined zooplankton trophic webs in the Eastern Tropical North Pacific (ETNP) OMZ. δ¹³C values were used to indicate zooplankton food sources, and δ¹⁵N values were used to indicate zooplankton trophic position and nitrogen cycle pathways. Vertically stratified MOCNESS net tows collected zooplankton from 0 to 1000 m at two stations along a north-south transect in the ETNP during 2007 and 2008, the Tehuantepec Bowl and the Costa Rica Dome. Zooplankton samples were separated into four size fractions for stable isotope analyses. Particulate organic matter (POM), assumed to represent a primary food source for zooplankton, was collected with McLane large volume in situ pumps.The isotopic composition and trophic ecology of the ETNP zooplankton community had distinct spatial and vertical patterns influenced by OMZ structure. The most pronounced vertical isotope gradients occurred near the upper and lower OMZ oxyclines. Material with lower δ¹³C values was apparently produced in the upper oxycline, possibly by chemoautotrophic microbes, and was subsequently consumed by zooplankton. Between-station differences in δ¹⁵N values suggested that different nitrogen cycle processes were dominant at the two locations, which influenced the isotopic characteristics of the zooplankton community. A strong depth gradient in zooplankton δ¹⁵N values in the lower oxycline suggested an increase in trophic cycling just below the core of the OMZ. Shallow POM (0–110 m) was likely the most important food source for mixed layer, upper oxycline, and OMZ core zooplankton, while deep POM was an important food source for most lower oxycline zooplankton (except for samples dominated by the seasonally migrating copepod Eucalanus inermis). There was no consistent isotopic progression among the four zooplankton size classes for these bulk mixed assemblage samples, implying overlapping trophic webs within the total size range considered.