Re–Os isotopic constraints on magma mixing in the Peridotite Zone of the Stillwater Complex, Montana, USA

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low ¹⁸⁷Re/¹⁸⁸Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (%_Os of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of %_Os and values of %_Os decrease upsection. These variations in %_Os were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial %_Os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in %_Os between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.     

Persistent millennial-scale climatic variability over the past 25,000 years in Southern Africa

Data from stalagmites in the Makapansgat Valley, South Africa, document regional climatic change in southern Africa in the Late Pleistocene and Holocene. A new TIMS U-series dated stalagmite indicates speleothem growth from 24.4 to 12.7 ka and from 10.2 to 0 ka, interrupted by a 2.5 ka hiatus. High-resolution oxygen and carbon stable isotope data suggest that postglacial warming was first initiated 17 ka, was interrupted by cooling, probably associated with the Antarctic Cold Reversal, and was followed by strong warming after 13.5 ka. The Early Holocene experienced warm, evaporative conditions with fewer C₄ grasses. Cooling is evident from 6 to 2.5 ka, followed by warming between 1.5 and 2.5 ka and briefly at AD 1200. Maximum Holocene cooling occurred at AD 1700. The new stalagmite largely confirms results from shorter Holocene stalagmites reported earlier. The strongest variability superimposed on more general trends has a quasi-periodicity between 2.5 and 4.0 ka. Also present are weaker 1.0 ka and 100-year oscillations, the latter probably solar induced. Given similarities to the Antarctic records, the proximate driving force producing millennial- and centennial-scale changes in the Makapansgat record is postulated to be atmospheric circulation changes associated with change in the Southern Hemisphere circumpolar westerly wind vortex.     

The chemistry of Offatts Bayou,Texas: A seasonally highly sulfidic basin

Offatts Bayou basin was created by use of this are as a borrow pit for landfill by the city of Galveston, Texas, in the first half of this century. Restricted exchange of water with the adjacent West Bay results in this basin changing, on a seasonal time scale, between oxic winter and highly sulfidic summer (greater than 500 μM ΣH₂S) bottom water conditions. It is, therefore, a “natural laboratory” for the investigation of the behavior of toxic metals in an estuarine environment where redox conditions undergo major variations. Here we report the first study of the chemistry of Offatts Bayou’s waters and sediment-associated trace metals. The high concentrations of dissolved sulfide in bottom water during summer cause a loss of macrofauna from most of the water deeper than about 4 m and the bottom of the basin. The potential exists for major mortality of organisms living in the oxic surface waters if rapid mixing of waters were to occur during the summer. Reactions of toxic metals with sulfides are probably the dominant influence on their potential bioavailability in this type of environment, as evidenced by large seasonal changes in concentrations of sedimentary sulfide minerals and associated trace metals. The trace metals As, Cu, and Hg are dominantly found in the pyrite phase (greater than 75% pyritization), Ag and Mn are moderately pyritized (40% to 60%), and Zn is not strongly associated with pyrite (less than 20% pyritization).     

Parasites and cyclopoid predators of age-0 fish in the Roanoke River,North Carolina

Altered river flow has been suggested as a cause for the low recruitment of striped bass,Morone saxatilis, in the Roanoke River (North Carolina) because of its effect on the proximity of zooplankton and larval striped bass. This results in unsuccessful feeding and subsequent starvation, which was considered to be a major mortality factor. Other mortality factors, such as parasitism and copepod predation on age-0 fish, may also be regulated to some extent by changes in river flow. The relationship of cestode plerocercoids, trematode metacercaria, mussel glochidia, and cyclopoid copepod predators with age-0 fish was evaluated in the lower Roanoke River and western Albemarle Sound from plankton net collections made in 1984 to 1986 and 1988. Plerocercoid prevalence was higher under low river flow conditions than under high flow conditions in darters (Percidae; 16.7% vs. 9.2%), minnows (Cyprinidae; 28.8% vs. 4.7%), andMorone (1.9% vs. 0%). Gut analysis of the age-0 fish revealed that copepods (source of the plerocercoids) were a major diet component ofMorone and darters but not of minnows or herring (Clupeidae). Decreases in river flow were associated with increases in copepod density (Pearson r=?0.62; p=0.0001) and plerocercoid prevalence inMorone (Pearson r=?0.29; p=0.03). The low correlation value forMorone may be quite strong considering the complexity of the variables associated with prevalence. Metacercaria were found only inMorone and minnows, and prevalence and mean intensity were less than that found for plerocercoids. Mussel glochidia prevalence was less than 0.5% for all affected taxa, an order of magnitude less that that found in other studies. The low value may indicate that the mussel population in the Roanoke River is declining. Prevalence of attacks by the predatory copepodMesocyclops edax on age-0 fish was similar to that in Chesapeake Bay, and striped bass was the primary prey. Spatial and temporal proximity of copepods and fish prey may be the key factors in regulating copepod attacks. The low prevalence of parasites and copepod predators seen in this tudy would suggest that mortality from these sources may not be a major factor in age-0 recruitment in this system. Confirmation of these conclusions would require a more controlled experimental approach.     

239+240Pu and 137Cs concentrations for zooplankton and nekton in the Northwest Pacific and Antarctic Oceans (1993-1996)

The concentrations of 239+240Pu and 137Cs in zooplankton and nekton in the Northwest Pacific and Southern Oceans were measured during the period 1993-1996. The object of the sampling was to assess the potential impacts of existing submerged anthropogenic-radioactive materials in the western North Pacific as well as the East China Sea. Samples from the Bransfield Strait of the Antarctic Ocean provided a control source impacted by only atmospheric bomb fallout. No particularly elevated levels of 239+240Pu were found in zooplankton samples from the Northwest Pacific, although significantly lower levels of 239+240Pu were found in three mixed zooplankton samples from the Bransfield Strait. The body burden of 239+240Pu in zooplankton appears to reflect concentrations in ambient seawater with some variation. Some additional measurements of 137Cs in fish are also reported here to complement existing databases and for future reference in the regional marine environmental radioactivity monitoring effort.     

Appinitic intrusions in the English Lake District

Summary Within the Skiddaw Group, which underlies the mid-Ordovician volcanic arc of the English Lake District, are minor intrusions of calc-alkaline, hornblende-augite rich, dioritic to picritic bodies reminiscent of the appinite suite of the Scottish Caledonides. Representative of these is the Scawgill Bridge microdiorite, a variable, hornblendic body which includes augite-phyric meladiorite and pyroxenite; others include the picrtte (augite-meladiorite or cortlandtite) plugs at Dash and Barkbeth. They are distinct from tholeiitic intrusions of similar age represented by the Embleton Diorite, and also from late Silurian biotite-lamprophyres. Despite alteration, primary variation trends indicated by immobile trace elements are also expressed by major elements including Mg, Fe and Ca. Differentiation at Scawgill Bridge is modelled as fractionation of olivine, augite, hornblende and chromite, and a similar process is inferred for the meladiorite intrusions. It is concluded these bodies represent primitive, calc-alkaline magmas related to the Lower Borrowdale Volcanic Group, which fractionated under confined conditions of high P(inH2o). In contrast, the Lower Borrowdale Volcanic Group itself is dominated by plagiophyric rocks which resulted from fractionation at lower P(inH2O).

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Appinitische Intrusionen im englischen Lake District

Zusammenfassung Die Skiddaw-Gruppe im Liegenden des mittelordovizischen Vulkanbogens des englischen Lake District enthält kleinere Intrusionen von kalkalkalischen, HornblendeAugit-reichen, dioritischen bis pikritischen Magmen, die an das Appinit-Vorkommen in den schottischen Kaledoniden erinnern. Für diese Intrusionen ist der Scawgill Bridge-Mikrodiorit repräsentativ. Er stellt einen variablen, Hornblende-reichen Körper mit Einschlüssen von Augit-porphyrischem Meladiorit und Pyroxenit dar; zu den anderen gehören die pikritischen (Augit-Meladiorit oder Cortlandit) Stöcke bei Dash und Barkbeth. Sie unterscheiden sich von tholeiitischen Intrusionen gleichen Alters, die durch den Embleton-Diorit vertreten sind, und auch von den spätsilurischen Biotit-Lamprophyren. Trotz Alteration werden primäre Korrelationen, die durch immobile Spurenelemente angezeigt werden, auch von Hauptelementen einschließlich Mg, Fe und Ca nachgezeichnet. Die Differentiation bei Scawgill Bridge wird als eine Fraktionierung von Olivin, Augit, Hornblende und Chromit modelliert, und ein ähnlicher Prozeß wird für die Meladiorit-Intrusion angenommen. Daraus wird geschlossen, daß diese Körper primitive, kalkalkalische Magmen repräsentieren, die zur Unteren Borrowdale Volcanic Group gehören und die unter hohem P_H2O fraktionierten. Im Gegensatz dazu wird die Untere Borrowdale Volcanic Group selbst von plagiophyrischen Gesteinen dominiert, die durch Fraktionierung unter niedrigem P(inH2O) entstanden sind.

     

X-ray emission in wind instability simulations

We estimate X-ray emission by shock-heated regions in hot star winds, using temperature and density profiles calculated by time-dependent dynamical models; the shocks result from the instability of the line scattering force that drives the wind. For main sequence late O and early B stars, the model X-ray flux is generally well below the observed flux, though the shape of the model spectrum is approximately consistent with observations. For the early O supergiant Pup, the model spectral shape again agrees with observation; the total flux predicted by models is well above the observed flux, though significant uncertainties remain.     

The development of a physiologically-based pharmacokinetic model using the distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the tissues of the eastern oyster (Crassostrea virginica)

A physiologically-based pharmacokinetic model (PBPK) was developed to describe the kinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the eastern oyster (Crassostrea virginica). The estimated t(1/2) of elimination for a bolus dose of TCDD in C. virginica is approximately 14-24 days based on both the experimental data and the PBPK model. The highest dioxin concentration reached during 28-days was in the digestive gland followed by the mantle, gonad, hemolymph, gill, adductor muscle, and the kidney/heart. A binding protein for 2,3,7,8-TCDD had been reported in the literature for both the digestive gland and gonad. Incorporating a binding component in the model resulted in a better fit for the data. The PBPK model predicted the distribution and the elimination concentrations for 2,3,7,8-TCDD within each of the tissue compartments. This model will serve as a useful tool for predicting the kinetics of other persistent organic pollutants as well as, allow for a more refined ecological risk assessment by estimating dioxin concentrations in sensitive tissues such as the gonad.     

Esterification of vertebrate-like steroids in the eastern oyster (Crassostrea virginica)

The esterification of two model vertebrate steroid hormones – estradiol (E2) and dehidroepiandrosterone (DHEA) – was studied in the oyster Crassostrea virginica. The activity of acyl-CoA:steroid acyltransferase was characterized in microsomal fractions isolated from oyster digestive glands. The apparent K_m and V_max values changed with the fatty acid acyl-CoA used (C20:4, C18:2, C18:1, C16:1, C18:0 or C16:0), and were in the range of 9–17 μM, and 35–74 pmol/min/mg protein for E2, and in the range of 45–120 μM, and 30–182 pmol/min/mg protein for DHEA. Kinetic parameters were also assessed in gonadal tissue. The enzyme saturated at similar concentrations, although conjugation rates were lower than in digestive gland. Preliminary data shows that tributyltin (TBT) in the low μM range (1–50) strongly inhibits E2 and DHEA esterification, the esterification of E2 being more sensitive to inhibition than that of DHEA. Overall, results indicate that apolar conjugation occurs in oysters, in both digestive gland and gonads, at a very similar rate to mammals, suggesting that this is a well conserved conjugation pathway during evolution. Esterification, together with other mechanisms, can modulate endogenous steroid levels in C. virginica, and might be a target for endocrine disrupters, such as TBT.