Fractional melting and smelting on the ureilite parent body

We investigate petrologic and physical aspects of melt extraction on the parent asteroid of the ureilite meteorites (UPB). We first develop a petrologic model for simultaneous melting and smelting (reduction of FeO by C) at various depths. For a model starting composition, determined from petrologic constraints to have been CV-like except for elevated Ca/Al (2.5 × CI), we determine (1) degree of melting, (2) the evolution of mg, (3) production of CO + CO₂ gas and (4) the evolution of mineralogy in the residue as a function of temperature and pressure. We then use these relationships to examine implications of fractional vs. batch melt extraction.In the shallowest source regions (∼30 bars), melting and smelting begin simultaneously at ∼1050 °C, so that mg and the abundance of low-Ca pyroxene (initially pigeonite, ultimately pigeonite + orthopyroxene) begin to increase immediately. However, in the deepest source regions (∼100 bars), smelting does not begin until ∼1200 °C, so that mg begins to increase and low-Ca pyroxene (pigeonite) appears only after ∼21% melting. The final residues in these two cases, obtained just after the demise of augite, match the end-members of the ureilite mg range (∼94-76) in pyroxene abundance and type. In all source regions, production of CO + CO₂ by smelting varies over the course of melting. The onset of smelting results in a burst of gas production and very high incremental gas/melt ratios (up to ∼2.5 by mass); after a few % (s)melting, however, these values drastically decline (to <0.05 in the final increments).Physical modelling based on these relationships indicates that melts would begin to migrate upwards after only ∼1-2% melting, and thereafter would migrate continuously (fractionally) and rapidly (reaching the surface in < a year) in a network of veins/dikes. All melts produced during the smelting stage in each source region have gas contents sufficient to cause them to erupt explosively and be lost. However, since in all but the shallowest source regions part of the melting sequence occurs without smelting, fractional melting implies that a significant fraction of UPB melts may have erupted more placidly to form a thin crust (∼3.3 km thick for a 100 km radius body).Our calculations suggest that melt extraction was so rapid that equilibrium trace element partitioning may not have been attained. We present a model for disequilibrium fractional melting (in which REE partitioning is limited by diffusion) on the UPB, and demonstrate that it produces a good match to the ureilite data. The disequilibrium model may also apply to trace siderophile elements, and might help explain the “overabundance” of these elements in ureilites relative to predictions from the smelting model.Our results suggest that melt extraction on the UPB was a rapid, fractional process, which can explain the preservation of a primitive oxygen isotopic signature on the UPB.     

A large metabolic carbon contribution to the δC record in marine aragonitic bivalve shells

It is well known that the incorporation of isotopically light metabolic carbon (C_M) significantly affects the stable carbon isotope (δ¹³C) signal recorded in biogenic carbonates. This can obscure the record of δ¹³C of seawater dissolved inorganic carbon (δ¹³C_DIC) potentially archived in the shell carbonate. To assess the C_M contribution to Mercenaria mercenaria shells collected in North Carolina, USA, we sampled seawater δ¹³C_DIC, tissue, hemolymph and shell δ¹³C. All shells showed an ontogenic decrease in shell δ¹³C, with as much as a 4‰ decrease over the lifespan of the clam. There was no apparent ontogenic change in food source indicated by soft tissue δ¹³C values, therefore a change in the respired δ¹³C value cannot be the cause of this decrease. Hemolymph δ¹³C, on the other hand, did exhibit a negative relationship with shell height indicating that respired CO₂ does influence the δ¹³C value of internal fluids and that the amount of respired CO₂ is related to the size or age of the bivalve. The percent metabolic C incorporated into the shell (%C_M) was significantly higher (up to 37%, with a range from 5% to 37%) than has been found in other bivalve shells, which usually contain less than 10%C_M. Interestingly, the hemolymph did contain less than 10%C_M, suggesting that complex fractionation might occur between hemolymph and calcifying fluids. Simple shell biometrics explained nearly 60% of the observed variability in %C_M, however, this is not robust enough to predict %C_M for fossil shells. Thus, the metabolic effect on shell δ¹³C cannot easily be accounted for to allow reliable δ¹³C_DIC reconstructions. However, there does seem to be a common effect of size, as all sites had indistinguishable slopes between the %C_M and shell height (+0.19% per mm of shell height).     

Sea‐surface conditions in northernmost Baffin Bay during the Holocene: palynological evidence

The analysis of cores collected in northernmost Baffin Bay, from within the area of the North Water Polynya, permits definition of a composite sedimentary sequence ca. 12 m thick spanning the last 10 000 ¹⁴C yr, with only a few discontinuities. Palynological analyses were performed in order to reconstruct changes in surface water conditions and biogenic production. Transfer functions, using dinocyst assemblages, were applied to estimate sea‐surface temperature (SST) and salinity, as well as the seasonal duration of sea ice cover. At the base of the record, prior to 9300 ¹⁴C yr BP, dinocysts and organic linings of benthic foraminifers are sparse, indicating harsh conditions and low productivity. After ca. 9300 ¹⁴C yr BP, the increased concentration of benthic foraminifers (up to 10³ linings cm^?3) and dinocyst fluxes (10²–10³ cysts cm^?2 yr^?1) reveals high biological productivity related to open‐water conditions. The early to middle Holocene, from ca. 9000 to ca. 3600 ¹⁴C yr BP, is marked by relatively high species diversity in dinocyst assemblages and the significant occurrence of autotrophic taxa such as Spiniferites elongatus, Pentapharsodinium dalei and Impagidinium pallidum. This assemblage suggests conditions at least as warm as at present. From ca. 6400 to ca. 3600 ¹⁴C yr BP, transfer functions indicate warmer conditions than at present, with SST in August fluctuating up to 5.5°C. After 3600 ¹⁴C yr BP, the dinocyst record suggests a trend of decreasing temperature toward modern values, marked by recurrent cooling events. Copyright © 2001 John Wiley & Sons, Ltd.     

Object Models with Vector Steering

Object models are widely used to model the distribution of facies in a reservoir. Several computer programs exist for modelling fluvial channels or more general facies objects. This paper focuses on a marked point model with objects that are able to orient locally according to a vector field. In this way, objects with locally varying curvature are created. With this kind of objects it is possible to model complex depositional basins, that are not easily modelled with conventional methods. The new object type is called Backbone objects. The objects have a piecewise linear centerline and are able to follow the direction of a three-dimensional vector field locally in lateral and vertical direction. How well the objects follow the vector field is determined by three parameters. Use of different coordinate systems and mapping between the systems make it possible to generate Gaussian random fields that follow the shape and direction of the objects. The Gaussian fields can be used to model petrophysical variables, which is important for fluid flow modelling.     

Variability in habitat use by young-of-the-year winter flounder,<Emphasis Type="Italic">Pseudopleuronectes americanus</Emphasis>, in three northeastern U.S. estuaries

We compared distribution and abundance by habitat for age-0, young-of-the-year (YOY) winter flounder,Pseudopleuronectes americanus, in three estuaries (Hammonasset River, Navesink River, and Great Bay-Little Egg Harbor) in the northeastern United States to better define essential fish habitat (EFH). Two replicates of five representative habitats were sampled in most estuaries: eelgrass (Zostera marina), unvegetated areas adjacent to eelgrass, macroalgae, (primarilyUlva lactuca), unvegetated areas adjacent to macroalgae, and tidal marsh creeks. Fish were sampled every two weeks, May through October 1995 and 1996, with a beam-trawl (1-m width, 3-mm mesh net). Abundance of YOY winter flounder was highest in the Navesink River estuary and similar between years, but was significantly lower and differed between years in the Great Bay-Little Egg Harbor and Hammonasset River estuaries. Annual temperature differences appear to influence estuary use by YOY. In the years and estuaries studied, where habitat-related differences in abundance were significant, YOY were found in higher densities in unvegetated areas adjacent to eelgrass. The exception was in the Hammonasset River in 1995 when densities were higher in eelgrass. We conclude that the type of habitat most important to YOY winter flounder varies among estuaries and as a result, care should be taken in defining EFH, based only on limited spatial and temporal sampling.     

Apport des isotopes stables dans l'estimation des altitudes de recharge de sources thermales du MarocRecharge altitude estimation of thermal springs using stable isotopes in Morocco

The recharge altitude estimation of thermal springs from northern and eastern Morocco using ¹⁸O and ²H contents requires the definition of regional isotopic altitudinal gradients (?0.25‰ for 100 m for the Rif and ?0.30‰ for the East) and the calculation of residence time using ¹⁴C. The altitudes of emergence vary widely between 170 and 1040 m under the altitude of the recharge areas. The ¹⁸O and ²H compositions of palaeowaters ($\text{>10}\text{000}$ yr BP) indicate two effects, altitude and palaeoclimate. To cite this article: A. Winckel et al., C. R. Geoscience 334 (2002) 469–474.     

A site effect study in the Verchiano valley during the 1997 Umbria-Marche (Central Italy) earthquakes

Strong site effects were observed during the two M _W 5.7 and M _W 6.0 main shocks of the Colfiorito seismic crisis which occured on September 26, 1997 in Umbria-Marche (Central Italy).The most obvious indications of these effects are the dramatic differences in damage shown by buildings of similar construction in neighboring villages.Such observations were specifically made in the Verchiano valley in the fault area, 15 km south of Colfiorito where the Verchiano village and the Colle and Camino hamlets were heavily damaged (MCS intensity IX-X) since the first main shock of 1997/09/26,while in contrast, the Curasci village located 2 km eastwards remains almost intact.In order to study the anomalous ground motion amplifications in this area, an array of 11, 3-components seismo- and accelero-meters was set up during the 1997/10/20-24 period, extending from the western side of the valley, up to the top of Mount San Salvatore, going accross the Colle and Curasci hamlets.During the experiment, 67 aftershocks enlightened the valley from the Colfiorito (10 km north) and the Sellano (6 km south) active swarms.Seismic refraction experiments were conducted at the same time in the 500 m wide, 1500 m long Verchiano valley in order to determine the thickness and main characteristics of the alluvial infilling.The main results are: (i) compared to the valley side ground motion, and for all the events, recordings in the central part of the valley (piana di Verchiano) show relative amplification of 10 with a clear lengthening of the seismogram duration by a factor of 2 – (ii) broad band relative amplification of 6–8 is also clearly identified at the top of the Mount San Salvatore overhanging the valley – (iii) any of the site effect measurements done explains by itself the strongly contrasted damage observed at Colle and Curasci: i.e. the modification of the near-field radiation pattern by interaction with the free heterogeneous surface may have induced local shadow zones that saved Curasci.     

Indicators of Great Basin rangeland health

Early-warning indicators of rangeland health can be used to estimate the functional integrity of a site and may allow sustainable management of desert rangelands. The utility of several vegetation canopy-based indicators of rangeland health at 32 Great Basin rangeland locations was investigated. The indicators were originally developed in rangelands of the Chihuahuan Desert. Soil resources are lost through wind and water-driven erosion mainly from areas unprotected by plant canopies (i.e. bare soil). Study sites in Idaho had the smallest bare patches, followed by sites in Oregon. The more arid Great Basin Sagebrush Zone sites in Utah had the largest bare patches. Several vegetational indicators including percent cover by vegetation, percent cover by life-form, percent cover by sagebrush, and percent cover by resilient species were negatively related to mean bare patch size and are potential indicators of Great Basin rangeland condition. Plant community composition and the range of bare patch sizes were different at sites in the three locations in Idaho, Oregon and Utah. Therefore, expected indicator values are location specific and should not be extrapolated to other locations. The condition of study sites were often ranked differently by different indicators. Therefore, the condition of rangeland sites should be evaluated using several indicators.