Correction of data from a salt-spray contaminated temperature sensor

Sea salt spray contamination of a temperature sensor induces erroneous fluctuations due to the latent heat of evaporating or condensing water vapour associated with changes in humidity. A method is derived to correct this humidity sensitivity using data from a fast response humidiometer. When applied to aircraft measurements of turbulence in the mixed layer over the Coral Sea, spurious high frequency temperature variance is significantly corrected. Spatial separation of the thermometer and the humidiometer and the sampling rate limit the effectiveness of the method. The vertical profile of the structure function parameter C _T ², determined from the corrected data, agrees well with results from other studies in a marine environment, but differences with measurements made over land remain.     

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Field, structural, and metamorphic petrology investigations of Mabja gneiss dome, southern Tibet, suggest that contractional, extensional, and diapiric processes contributed to the structural evolution and formation of the domal geometry. The dome is cored by migmatites overlain by sillimanite-zone metasedimentary rocks and orthogneiss; metamorphic grade diminishes upsection and is defined by a series of concentric isograds. Evidence for three major deformational events, two older penetrative contractional and extensional events and a younger doming event, is preserved. Metamorphism, migmitization, and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid-crustal levels. Metamorphic temperatures and pressures range from 500 °C and 150–450 MPa in chloritoid-zone rocks to 705±65 °C and 820±100 MPa in sillimanite-zone rocks. We suggest that adiabatic decompression during extensional collapse contributed to development of migmatites. Diapiric rise of low density migmatites was the driving force, at least in part, for the development of the domal geometry. The structural and metamorphic histories documented in Mabja Dome are similar to Kangmar Dome, suggesting widespread occurrence of these events throughout southern Tibet.     

The composite shape and structure of coherent eddies in the convective boundary layer

Conditional sampling is used to locate coherent structures in a large data set obtained from flights by an instrumented light aircraft in convective boundary layers over Eyre Peninsula, South Australia. The high resolution and excellent spatial coverage of the data enable a detailed study of the internal structure of surface-layer plumes and mixed-layer thermals. A compositing technique is used to construct averaged traverses through coherent structures located within aircraft data runs of different altitudes and directions. Groups of composites are combined to form horizontal and vertical cross-sections which describe the internal flow patterns and the distribution of physical variables associated with typical coherent structures and their environment. In addition to the well-known along-wind features of surface-layer plumes, a strong, consistent inflow/entrainment pattern is evident in the lateral direction. Air from the horizontal plane channels around the sides and then in behind the microfront present at the upstream edge. Forces set up by the driving instability in the along-wind and vertical directions are counter-balanced by organised flow in the across-wind direction. It is found that mixed-layer thermal towers have a relatively simple form, consisting primarily of large columns of warm, upward-moving turbulent air, which may occasionally be in a state of slow rotation. An analysis of possible geometrical distortions within the results is performed, leading to a comparison ofw/z estimated from the horizontal velocity convergence field inside plumes/thermals, with that computed from the slope of partitionedw profiles.     

Sea-breeze Front Variations in Space and Time

Summary  This paper is a contribution to experimental meteorology: A sea-breeze front was investigated by aircraft observations and thorough numerical analysis using an unprecedented number of runs crossing the same front within a timespan of . The 33 runs were flown in a situation of offshore geostrophic wind of 5 m/s in 1000 hPa and with the strategy of obtaining information on the four-dimensional field (t=time, x=cross-coastal coordinate, y=coast-parallel coordinate, z=height): 9 runs in x-direction (and reverse) at different heights to yield x,z-cross-sections of the observed meteorological quantities (specific humidity q, potential temperature Θ and the components u, v and w of the wind velocity), assuming a frozen structure in time; the next 7 runs again in x-direction but all at the same level and on the same track to yield x,t-diagrams of the same quantities in order to study the temporal changes compared to those with x and z; the next 10 runs as a zig-zagging flight track crossing the front but drifting in y-direction, all at the same height, in order to obtain the y-dependency; andfinally 7 runs for another x,z-cross-sectional analysis, which can be compared to that evaluated from the runs at the beginning of the mission. The paper describes the 4-dimensional dependencies in detail. Pure x-variations at constant z are expressed by VCM low-pass filtered space series (VCM=variance conserving multiresolution, according to Howell and Mahrt, 1994). The x,z-analyses are similar to those in Kraus et al. (1990) and Finkele et al. (1995) verifying these results. The comparison of the x,z-studies gained from the data at the beginning and at the end of the mission show how the sea-breeze frontal area changes its structure. The fluctuations (in time) revealed by the low-pass filtered x,t-runs (same track and same height) are smaller than the contour intervals chosen in the x,z-cross-sections. This shows, that the single runs, from which the x,z-cross-sections are constructed, reliably and significantly contribute to the interpolated structure. The paper also demonstrates the overall development of the front within the 3^1/2 h of continuous observation. The x,y-fields demonstrate that the y-dependency of the various quantities is generally one order of magnitude smaller than the x-dependency and that the assumption of negligible y-dependency holds in the first order of approximation for a fairly homogeneous coast. Convective disturbances of a horizontal scale of 1 to 4 km at the landward side of the front, embedded in the offshore flow and bouncing against the landward propagating sea-breeze front, considerably contribute to variations of the frontal propagation speed and of the frontal shape and also to changes of the parameters with the along-frontal coordinate y. Received April 24, 1998 Revised November 3, 1998     

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO₂ relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (<29 ± 8 Ma) suggests that a thin mantle lithosphere (<80 km) or advective heating may have also been important in the formation of this high‐T, low‐P terrane.     

Uncoupled U/Pb and REE response in zircon during the transformation of eclogite to mafic and intermediate granulite (Blanský les,Bohemian Massif)

An eclogite–mafic granulite occurs as a rare boudin within a felsic kyanite–K‐feldspar granulite in a low‐strain zone. Its boundary is marked by significant metasomatism–diffusional gain of potassium at the centimetre‐scale, and probable infiltration of felsic melt on a larger scale. This converted the eclogite–mafic granulite into an intermediate‐composition, ternary‐feldspar‐bearing granulite. Based on inclusions in garnet, the peak P–T conditions of the original eclogite are 18 kbar at 850–950 °C, with later matrix re‐equilibration at 12 kbar and 950 °C. Four samples from the transition of the eclogite–mafic granulite through to the intermediate granulite were studied. In the eclogite, REE patterns in the garnet core show no Eu anomaly, compatible with crystallization in the absence of plagioclase and consistent with eclogite facies conditions. Towards the rim of garnet, LREE decrease, and a weak negative Eu anomaly appears, reflecting passage into HP granulite facies conditions with plagioclase present. The rims of garnet next to ternary feldspar in the intermediate granulite show the lowest LREE and deepest Eu anomalies. Zircon from the four samples was analysed by LASS (laser ablation–split‐stream inductively coupled plasma–mass spectrometry). It shows U–Pb ages from 404 ± 4.0 to 331 ± 3.3 Ma, with a peak at 340 ± 4.0 Ma corresponding to the likely exhumation of the rocks to 12 kbar. Older ages from zircon with steep HREE patterns indicate the minimum age of the protolith, and ages <360 ± 4.0 Ma are interpreted to correspond to the eclogite facies metamorphism. Only some zircon grains ≤350 ± 4.0 Ma have flat HREE patterns, suggesting that these are primarily modified protolith grains, rather than new zircon crystallized in the eclogite‐ or granulite facies. The metasomatic processes that converted the eclogite–mafic granulite to an intermediate granulite may have facilitated zircon modification as zircon in the intermediate granulite has flat HREE and ages of 340 ± 4.0 Ma. The difference between the oldest and youngest ages with flat REE patterns indicates a 16 ± 5.6 Ma period of zircon modification in the presence of garnet.     

Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest,USA

Land-based eutrophication is often associated with blooms of green macroalgae, resulting in negative impacts on seagrasses. The generality of this interaction has not been studied in upwelling-influenced estuaries where oceanic nutrients dominate seasonally. We conducted an observational and experimental study with Zostera marina L. and ulvoid macroalgae across an estuarine gradient in Coos Bay, Oregon. We found a gradient in mean summer macroalgal biomass from 56.1 g dw 0.25 m⁻² at the marine site to 0.3 g dw 0.25 m⁻² at the riverine site. Despite large macroalgal blooms at the marine site, eelgrass biomass exhibited no seasonal or interannual declines. Through experimental manipulations, we found that pulsed additions of macroalgae biomass (+4,000 mL) did not affect eelgrass in marine areas, but it had negative effects in riverine areas. In upwelling-influenced estuaries, the negative effects of macroalgal blooms are context dependent, affecting the management of seagrass habitats subject to nutrient inputs from both land and sea.     

High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea

In the lower main unit of the Attic‐Cycladic crystalline belt (Greece), white mica geochronology (Rb–Sr, K–Ar, ⁴⁰Ar–³⁹Ar) has established the timing of at least two metamorphic events: well‐preserved high‐pressure/low‐temperature (HP/LT) rocks yielded Eocene ages (c. 53–40 Ma) and their greenschist facies counterparts provided Oligocene–Miocene dates (c. 25–18 Ma). Marbles from Tinos Island contain high‐Si phengite with Rb–Sr (phengite–calcite) and ⁴⁰Ar–³⁹Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and ⁴⁰Ar–³⁹Ar total fusion ages generally are 3–6 Ma older than corresponding Rb–Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older ⁴⁰Ar–³⁹Ar dates. This concept also suggests that the Rb–Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca‐rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system. On Tinos, the lowermost part of the metamorphic succession has experienced a pervasive greenschist facies overprint. Si‐rich phengite from marbles representing this lithostratigraphic level yielded Rb–Sr ages of c. 24 Ma. If the earlier metamorphic history is not taken into account, such data sets may lead to the erroneous conclusion of Miocene HP metamorphism. This study indicates that this phengite experienced pervasive rejuvenation of the Rb–Sr isotope system during overprinting, without significant changes in Si content, due to bulk‐compositional constraints. This leads to the conclusion that in the absence of critical mineral assemblages the Si value of phengite is not a reliable indicator for metamorphic pressures in impure marbles. Recent studies have reported large displacements (>100 km) for detachment faults in the Aegean Sea. A critical parameter for such models is the age of HP metamorphism as deduced from white mica dating in the basal units of the Cyclades. We question the underlying idea of Miocene HP metamorphism and suggest, instead, that this age constrains the timing of the greenschist facies overprint and that the existence of mega‐detachments in the study area requires further investigation.     

Lidar atmospheric measurements on Mars and Earth

The LIDAR instrument operating from the surface of Mars on the Phoenix Mission measured vertical profiles of atmospheric dust and water ice clouds at temperatures around −65 °C. An equivalent lidar system was utilized for measurements in the atmosphere of Earth where dust and cloud conditions are similar to Mars. Coordinated aircraft in situ sampling provided a verification of lidar measurement and analysis methods and also insight for interpretation of lidar derived optical parameters in terms of the dust and cloud microphysical properties. It was found that the vertical distribution of airborne dust above the Australian desert is quite similar to what is observed in the planetary boundary layer above Mars. Comparison with the in situ sampling is used to demonstrate how the lidar derived optical extinction coefficient is related to the dust particle size distribution. The lidar measurement placed a constraint on the model size distribution that has been used for Mars. Airborne lidar measurements were also conducted to study cirrus clouds that form in the Earth’s atmosphere at a similar temperature and humidity as the clouds observed with the lidar on Mars. Comparison with the in situ sampling provides a method to derive the cloud ice water content (IWC) from the Mars lidar measurements.