Comparison of continuous records of near-bottom dissolved oxygen from the hypoxia zone along the Louisiana coast

Oxygen depletion is a seasonally dominant feature of the lower water column on the highly-stratified, riverine-influenced continental shelf of Louisiana. The areal extent of hypoxia (bottom waters ≤2 mg l^?1 dissolved oxygen) in mid-summer may encompass up to 9,500 km², from the Mississippi River delta to the upper Texas coast, with the spatial configuration of the zone varying interannually. We placed two continuously recording oxygen meters (Endeco 1184) within 1 m of the seabed in 20-m water depth at two locations 77 km apart where we previously documented midsummer bottom water hypoxia. The oxygen meters recorded considerably different oxygen conditions for a 4-mo deployment from mid-June through mid-October. At the station off Terrebonne Bay (C6A), bottom waters were severely depleted in dissolved oxygen and often anoxic for most of the record from mid-June through mid-August, and there were no strong diurnal or diel patterns. At the station 77 km to the east and closer to the Mississippi River delta (WD32E), hypoxia occurred for only 50% of the record, and there was a strong diurnal pattern in the oxygen time-series data. There was no statistically significant coherence between the oxygen time-series at the two stations. Coherence of the oxygen records with wind records was weak. The dominant coherence identified was between the diurnal peaks in the WD32E oxygen record and the bottom pressure record from a gauge located at the mouth of Terrebonne Bay, suggesting that the dissolved oxygen signal at WD32E was due principally to advection by tidal currents. Although the oxygen time-series were considerably different, they were consistent with the physical and biological processes that affect hypoxia on the Louisiana shelf. Differences in the time-series were most intimately tied to the topographic cross-shelf gradients in the two locations, that is, station C6A off Terrebonne Bay was in the middle of a broad, gradually sloping shelf and station WD32E in the Mississippi River Delta Bight was in an area with a steeper cross-shelf depth gradient and likely situated near the edge of a hypoxic water mass that was tidally advected across the study site.     

Nuclear tracks in the Angra dos Reis and Moore County meteorites

Charged particle tracks were studied in the Angra dos Reis and Moore County meteorites, both of which contain an unexplained excess of He⁴. A selective annealing method was used to resolve cosmic-ray tracks from fission tracks. It gave the following cosmic-ray and fission-track densities, in units of 10⁶cm^?2: Angra dos Reis 1.3–4.4 and 7.8; Moore Co. feldspar 1.9–3.0 and 0.51; Moore Co. pigeonite 2.0–2.9 and 0.078–0.35. The fission-track densities are 10–100 times higher than expected from U²³⁸; the excess is probably due to extinct Pu²³⁸. The Pu²⁴⁴/U²³⁸ ratios at the start of track retention were 0.003 for Angra dos Reis and 0.002–0.03 for Moore Co. No evidence was found for fission tracks attributable to the unknown progenitor of excess He⁴ in these meteorites; the fission branch of this progenitor comprises less than 10^?5 the α-branch. A search for pleochroic halos also gave negative results.The preatmospheric radii of the two meteorites are ≥13 and ≥7cm. According to meteor theory, this implies geocentric velocities of ≥ 19 and ≥ 6 km/sec.The etching behavior of Angra dos Reis augite is highly anomalous, giving rise to spurious angular anisotropies and skewed length distributions. This confirms similar observation by Fleischer et al. (1970) on lunar augite.     

Biotite breakdown and the formation of gahnite in metapelitic rocks from Kemiö, southwest Finland

The formation of ferroan gahnite by breakdown of zinc-bearing biotite has been studied in metapelitic gneisses from the Kemiö region in southwestern Finland. Spinel compositions range from herc₅₀ gahn₄₀spin₁₀ to herc₁₅gahn₈₂spin₃; biotite contains up to 0.24 wt.% ZnO. Individual spinel grains show a gradual compositional zoning owing to increasing zinc contents towards the grain margins.Chemical and textural evidence indicate that spinel has been formed by breakdown of zinc-bearing biotite. During prograde metamorphism ferroan gahnite formed according to the reaction: biotite+sillimanite +quartz=cordierite+spinel+fluid, and during retrogressive metamorphism ferroan gahnite was formed by chloritization of biotite. It is proposed that the zinc-saturation limit of biotite depends on the metamorphic conditions. Decrease of this limit during retrogression to values below the zinc content of biotite initiated release of zinc and formation of ferroan gahnite. The activity of the Fe and Mg end members of retrogressively formed spinel are fixed by the coexisting biotite.     

Petrogenesis of complex veins in the Chantonnay (L6f) chondrite

Two cross-cutting veins in the Chantonnay (L6f) chondrite illustrate different patterns of fractionation of total chondritic shock melts. The earlier vein, which is dark-colored and bears abundant host rock xenoliths, is strongly reduced and sodium-poor relative to the bulk meteorite. It resembles and may be cogenetic with melt pockets in Chantonnay. The later vein, which is lighter-colored and somewhat vesicular, lacks evidence of either Na loss or reduction but shows modest internal differentiation. Its metal and total iron contents (26.5 wt.%) are higher than normal for L-group chondrites.The trend of chemical fractionation recorded in the earlier Chantonnay vein resembles that reported for chondrules in ordinary chondrites, suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts.     

Dependence of fishery species on salt marshes: The role of food and refuge

Salt marshes are widely believed to serve as nurseries for many fishes and crustaceans of fishery value as a result of the high production of vascular plant detritus and the protection from predation offered by shallow, spatially complex habitats. Comparisons of the yields of species which reside in salt marsh habitats during critical life history stages (such as penaeid shrimp) with the area of such habitats and their greater densities in flooded marshes and associated tidal creeks support the premise that marshes enhance the yield of such species. A range of evidence, including the amount of detrital export from marshes, the poor nutritive value of vascular plant detritus, and natural diets, casts doubt on the notion that production of fishery species is based on the direct consumption of marsh grass detritus or predominantly on food chains based on this detritus. Vascular plants and associated algae may, however, contribute to the production of prey species. The limited observations available support the hypothesis that salt marshes offer significant escape from mortality due to predation, but there have been yet few experimental tests of this hypothesis. Knowledge of relative importance of the food and refuge functions in support of living resources is of practical value in marsh and fisheries management. Better understanding of these roles is important to the effective evaluation of the effects of coastal habitat modifications on fisheries resources and design of alterations to minimize the losses of these values.     

Particulate dispersion into and within a forest

Particulate dispersion into and within a 10- to 13-m tall pine forest was studied experimentally at Brookhaven National Laboratory using stained ragweed pollen and other tracers ranging from 14 to 54 m in diam. Seventy-two continuous point source releases lasting 20 to 40 min were made at various distances from within the forest edge to 60 m upwind and at heights of 1.75 to 14.0 m. In most experiments, differently colored ragweed pollen was released simultaneously from three locations. Thirty-six longer tests were made using pollen from area sources of ragweed and three with pollen from distant sources. All tests were made during the day with steady winds and unstable lapse rates outside the forest. The sampling network consisted of 119 rotoslide samplers mounted at heights from 0.5 to 21.0 m at 57 positions extending 100 m into the forest. Deposition was sampled by greased microscope slides at each sampling position. Meteorological measurements were taken in and near the forest.Data were classified by particle characteristics; by source type, distance and height; and by meteorological parameters. Isopleths were drawn on scale diagrams of the sampling grid to illustrate concentration patterns. Changes in centerline concentration, crosswind integrated concentration, mass flux, plume width, plume height, deposition, and deposition velocity were related to distance within the forest and other variables. Results were compared to those of similar releases over open terrain and those of previous forest dispersion studies elsewhere.The plume approaching the forest is broadened both vertically and horizontally by increased turbulence at the forest edge and flows mainly into the trunk space and above the forest. Lateral spread is slow within the forest, but vertical spreading beyond the entrance region is greater than in the open. Particles become mixed uniformly below the canopy while appreciable interchange takes place through this layer. Concentration within the forest decreases at a faster rate than in the open, but change in total mass flux within and above the forest is not significantly different. Loss of material takes place by impaction near the forest edge and in the tree tops and by deposition within the forest. Most loss takes place to the foliage rather than the ground, and larger particles are lost faster than smaller ones.This research was carried out under the auspices of the New York State Museum and Science Service and the U.S. Atomic Energy Commission and was partially supported by Research Grant No. R-800677 from the Division of Meteorology, U.S. Environmental Protection Agency.     

Petrography and geochemistry of target rocks and impactites from the Ilyinets Crater,Ukraine

Abstract— The ～400 Ma old Ilyinets impact structure was formed in the Precambrian basement of the Ukrainian Shield and is now mostly covered by Quaternary sediments. Various impact breccias and melts are exposed in its southern section. The crater is a complex structure with a central uplift that is surrounded by an annular deposit of breccias and melt rocks. In the annulus, brecciated basement rocks are overlain by up to 80 m of glass-poor suevitic breccia, which is overlain (and partly intercalated) by glass-rich suevite with a thickness of up to 130 m. Impact-melt rocks occur within and on top of the suevites—in some cases in the form of devitrified bomb-shaped impact-glass fragments. We have studied the petrographic and geochemical characteristics of 31, mostly shocked, target rock samples (granites, gneisses, and one amphibolite) obtained from drill cores within the structure, and impact breccias and melt rock samples from drill cores and surface exposures. Multiple sets of planar deformation features (PDFs) are common in quartz, potassium feldspar, and plagioclase of the shocked target rocks. The breccias comprise more or less devitrified impact melt with shocked clasts. The impact-melt rocks (“bombs”) show abundant vesicles and, in some cases, glass is still present as brownish patches and schlieren. All impact breccias (including the melt rocks) are strongly altered and have significantly elevated K contents and lower Na contents than the target rocks. The alteration could have occurred in an impact-induced hydrothermal system. The bomb-shaped melt rocks have lower Mg and Ca contents than other rock types at the crater. Compared to target rocks, only minor enrichments of siderophile element contents (e.g., Ni, Co, Ir) in impact-melt rocks were found.