Significance of OH,F and Cl content in biotite during metamorphism of the Western Adamello contact aureole

The hydroxyl (O(4)) site composition of biotite can in principle be used to retrieve information about fluid composition during fluid–rock interaction; however, due to low F and Cl content, as well as difficulties involved with analyzing the H₂O content using in situ techniques, measuring these species in biotite has remained an elusive goal. Here we present high-precision secondary ion mass spectrometry (SIMS) OH–F–Cl measurements from biotite within metapelites from the Western Adamello Tonalite (WAT) contact aureole, Northern Italy. Fluorine, chlorine and hydrogen are analyzed on the SIMS sequentially by peak-hopping at the same biotite spot; H₂O, F and Cl content were measured with a precision (1σ) οφ 0.06 ωτ%, 50 ανδ 5 ππµ, ρεσπεχτι?ελψ. The compositions of isolated biotite crystals in andalusite are compared with that of biotite in the matrix, documenting that halogens and H₂O behave refractory in biotite during the time scale of contact metamorphism. The H₂O and halogen contents of biotite are mostly locked in during the prograde to peak formation of biotite, and are not reset during further heating or cooling, unless significant biotite recrystallization occurs. It also appears that both Ti content and X_Mg of the biotite from the Western Adamello contact aureole were not significantly reset during cooling. The concentration of F and Cl does not vary systematically with metamorphic grade, which indicates that these species reflect initial compositions. No significant Rayleigh fractionation behavior was observed for these elements. H₂O variations in the biotite from samples throughout the Western Adamello contact aureole suggest that Al-oxy substitution partially controls the variations in OH content through charge balance of the type R^2+,VI + OH^? = Al^3+,VI + O^2? + H₂, while the Ti-oxy substitution does not seem to influence the O(4) site occupation. The main titanium substitution appears to be the Ti-vacancy (\({\text{2}}{{\text{R}}^{{\text{2}}+}}~=~{\text{T}}{{\text{i}}^{{\text{4}}+}}~+\,{\square ^{{\text{VI}}}}\)) exchange. Variations in H₂O and halogen concentrations in biotite define sub mm-scale areas of localized equilibration, even for biotite recrystallized during dehydration reactions that produced large amounts of fluid (chlorite or muscovite breakdown). Similar systematics were observed for Ti⁴⁺ and Al³⁺. These findings further support the increasing number of observations that kinetics control much of the mineralogical reactions occurring in contact aureoles, and hence care is advised in using equilibrium thermodynamics in this environment.     

Volcanic eruptions and the forgotten pearls

With this paper we demonstrate, using the example of the Segara Anakan lagoon in south Java, Indonesia, that exploring the environmental history of an area may provide important information for the understanding of present day problems and their possible solutions. The lagoon is seriously affected by sedimentation. Management measures aiming at its preservation were rather ineffective. Until now, most scholars have treated sedimentation of the lagoon as a short-term phenomenon. We found evidence that sedimentation is a long-term driver of ecosystem changes in Segara Anakan. Ecosystem changes are also obvious in the disappearance of species from the lagoon, like pearl oysters.We recommend that preconditions for the development of adequate management decisions regarding the lagoon's sedimentation problem are (1) a distinction between natural and anthropogenic causes of sedimentation and (2) an understanding of changes in species composition.     

Five years’ central pacific sea level from in situ array,satellite altimeter and numerical model: Research note

Abstract

Temporal and spatial features of central equatorial Pacific Ocean sea‐level variation appear similar, in measurements from two very different systems (one in the ocean and one carried on a satellite), and in results from a numerical model of the region. In particular, there is an interannual cycle: during El Nino, Kelvin waves appear at the equator, and the sea‐surface ridge associated with the equatorial current system shifts southward; in non‐El Nino years, instability waves appear at 6°N (strongest around the end of each calendar year), and the ridge shifts to the north. This three‐way comparison gives support to both measurement systems and to the numerical model.     

Aspects of CO2 advection measurements

Observations of vegetation–atmosphere exchange of carbon dioxide (CO₂) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO₂ concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity ( $\emph{w}$ ). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean $\emph{w}$ . Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE.     

Effects of Bubbles and Sea Spray on Air–Sea Exchange in Hurricane Conditions

The lower limit on the drag coefficient under hurricane force winds is determined by the break-up of the air–sea interface due to Kelvin–Helmholtz instability and formation of the two-phase transition layer consisting of sea spray and air bubbles. As a consequence, a regime of marginal stability develops. In this regime, the air–sea drag coefficient is determined by the turbulence characteristics of the two-phase transition layer. The upper limit on the drag coefficient is determined by the Charnock-type wave resistance. Most of the observational estimates of the drag coefficient obtained in hurricane conditions and in laboratory experiments appear to lie between the two extreme regimes: wave resistance and marginal stability.     

Large calcite and bulk-rock volume loss in metacarbonate xenoliths from the Quérigut massif (French Pyrenees)

Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Quérigut massif (Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO₂ contents (up to −90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO₂, Al₂O₃ and Fe₂O₃ contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to −86%) resulted from a combination of decarbonation reactions and loss of CaO and CO₂. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the isotope alteration could have occured over greater distances than those observed today.     

I/I ratios in Scottish coastal surface sea water: Geographical and temporal responses to changing emissions

This work constitutes the first survey of I isotope ratios for Scottish sea water including the first data for the west of Scotland. These data are of importance because of the proximity to the world’s second largest emission source of ¹²⁹I to the sea, the Sellafield nuclear reprocessing plant, because of the increasing importance of the sea to land transfer of ¹²⁹I and also as input data for dose estimates based on this pathway of ¹²⁹I. ¹²⁹I/¹²⁷I ratios in SW Scotland reached 3 × 10⁻⁶ in 2004. No strong variation of I isotope ratios was found from 2003 to 2005 in Scottish sea waters. Iodine isotope ratios increased by about a factor of 6 from 1992 to 2003 in NE Scotland, in agreement with the increase of liquid ¹²⁹I emissions from Sellafield over that time period. It is demonstrated that ¹²⁹I/¹²⁷I ratios agree better than ¹²⁹I concentrations for samples from similar locations taken in very close temporal proximity, indicating that this ratio is more appropriate to interpret than the radionuclide concentration.     

A conceptual framework for time and space scale interactions in the climate system

 Interactions involving various time and space scales, both within the tropics and between the tropics and midlatitudes, are ubiquitous in the climate system. We propose a conceptual framework for understanding such interactions whereby longer time scales and larger space scales set the base state for processes on shorter time scales and smaller space scales, which in turn have an influence back on the longer time scales and larger space scales in a continuum of process-related interactions. Though not intended to be comprehensive, we do cite examples from the literature to provide evidence for the validity of this framework. Decadal time scale base states of the coupled climate system set the context for the manifestation of interannual time scales (El Nino/Southern Oscillation, ENSO and tropospheric biennial oscillation, TBO) which are influenced by and interact with the annual cycle and seasonal time scales. Those base states in turn influence the large-scale coupled processes involved with intraseasonal and submonthly time scales, tied to interactions within the tropics and extratropics, and tropical–midlatitude teleconnections. All of these set the base state for processes on the synoptic and mesoscale and regional/local space scales. Events at those relatively short time scales and small space scales may then affect the longer time scale and larger space scale processes in turn, reaching back out to submonthly, intraseasonal, seasonal, annual, TBO, ENSO and decadal. Global coupled models can capture some elements of the decadal, ENSO, TBO, annual and seasonal time scales with the associated global space scales. However, coupled models are less successful at simulating phenomena at subseasonal and shorter time scales with hemispheric and smaller space scales. In the context of the proposed conceptual framework, the synergistic interactions of the time and space scales suggest that a high priority must be placed on improved simulations of all of the time and space scales in the climate system. This is particularly important for the subseasonal time scales and hemispheric and smaller space scales, which are not well simulated at present, to improve the prospects of successfully forecasting phenomena beyond the synoptic scales. Received: 3 April 2000/ Accepted: 6 November 2000     

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s^?1 (equivalent to pure wind velocity of 6 to 8 m.s^?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s^?1.     

H2O Content Measurement in Phengite by Secondary Ion Mass Spectrometry: A New Set of Reference Materials

Five new natural white mica reference materials (RMs) were developed for in situ H₂O content analyses by secondary ion mass spectrometry at the SwissSIMS laboratory of Lausanne University, Switzerland. The white mica reference materials cover a large part of the natural muscovite–phengite compositional range and are therefore suitable as reference materials for the analysis of natural rocks as well as individual minerals. The independent H₂O content of the reference materials UNIL_WM1 to UNIL_WM5 was obtained by thermal conversion elemental analyser and corresponds to 4.35 ± 0.02, 4.33 ± 0.03, 4.30 ± 0.07, 4.50 ± 0.02 and 4.42 ± 0.11 (% m/m, ± 1s), respectively. SIMS determinations of H₂O content revealed a matrix effect correlated to the FeO content of white mica. The compositional range in FeO of the reference materials that were calibrated for H₂O determination is from 1.13% to 3.67% m/m. No crystallographic orientation dependency was observed at the level of homogeneity of these reference materials. An analytical precision of 0.02% to 0.08% m/m (1SE) is expected for the final uncertainty on measurements of unknown white micas in natural samples.