The Dynamics of Benthic Respiration at a Mid-Shelf Station Off Oregon

Mid-shelf sediments off the Oregon coast are characterized as fine sands that trap and remineralize phytodetritus leading to the consumption of significant quantities of dissolved oxygen. Sediment oxygen consumption (SOC) can be delayed from seasonal organic matter inputs because of a transient buildup of reduced constituents during periods of quiescent physical processes. Between 2009 and 2013, benthic oxygen exchange rates were measured using the noninvasive eddy covariance (EC) method five separate times at a single 80-m station. Ancillary measurements included in situ microprofiles of oxygen at the sediment–water interface, and concentration profiles of pore water nutrients and trace metals, and solid-phase organic C and sulfide minerals from cores. Sediment cores were also incubated to derive anaerobic respiration rates. The EC measurements were made during spring, summer, and fall conditions, and they produced average benthic oxygen flux estimates that varied between ?2 and ?15 mmol m^?2 d^?1. The EC oxygen fluxes were most highly correlated with bottom-sensed, significant wave heights (H _s). The relationship with H _s was used with an annual record of deepwater swell heights to predict an integrated oxygen consumption rate for the mid-shelf of 1.5 mol m^?2 for the upwelling season (May–September) and 6.8 mol m^?2 y^?1. The annual prediction requires that SOC rates are enhanced in the winter because of sand filtering and pore water advection under large waves, and it counters budgets that assume a dominance of organic matter export from the shelf. Refined budgets will require winter flux measurements and observations from cross-shelf transects over multiple years.     

Silurian collisional suturing onto the southern margin of the North China craton: Detrital zircon geochronology constraints from the Qilian Orogen

The Qilian Orogen of north western China records mid-Paleozoic collisional suturing of arc and continental blocks onto the south western margin of the North China craton. Silurian strata from the retroarc foreland basin mark the transition from ocean closure and northward subduction to the initiation of collision suturing. Detrital zircons were analysed from the western and eastern parts of the basin and show a spectrum of ages from Archean to Paleozoic with major age concentrations at around 2.5 Ga, 1.6 Ga, 1.2 Ga, 0.98 Ga, 0.7 Ga and 0.45 Ga. Archean age grains are derived from the North China craton, whereas the Central Qilian Bloc, which lies to the south provides the likely source for the bulk of the Proterozoic detritus. Paleozoic grains are restricted to Early Silurian samples from the western part of the basin and are considered to have been derived from the magmatic arc related to ocean closure and ultimate collision of the Central Qilian Belt with the North China craton.     

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes.Two east–west structural transects were mapped at Puyuhuapi and Aysén, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysén (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it.New ⁴⁰Ar–³⁹Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysén transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation.The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.     

Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone

The stable isotope composition (δ¹⁵N and δ¹⁸O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ¹⁵N and δ¹⁸O values for residual nitrate. Maximum δ¹⁵N and δ¹⁸O values amounted to 27.2 and 15.7‰, respectively.¹⁵N and ¹⁸O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ¹⁸O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ¹⁵N.     

Recent Precipitation Trends in Hungary in the Context of Larger Scale Climatic Changes

The time series of monthly precipitation totals from 14 Hungarian observing stations (1901–1998) were analysed to reveal the long term changes in precipitation characteristics occurred in the 20th century. A particular attention was given to the changes in the recent decades and their links with the larger scale climatic and circulation changes over Europe and the Atlantic.The statistical significancesof systematic changes are controlled by linear trend analysis and the Mann–Kendall test. The long term fluctuations are illustrated applying a 15-point Gaussian filter on the time series. The Standardised Precipitation Index is used to evaluate the changes in the drought event frequency. The relationships with larger scale changes are mostly discussed relying on contemporary papers, and the Grosswetterlagen Catalogue is used as well.The annual precipitation total decreased by 15–20% in Hungary during the 20th century. The decline is substantial in both halves of the century, but the precipitation sums in the transition seasons declined in the first 50 years, and the winter precipitation decreased in the latest decades. The precipitation total of the period November–February declined significantly in the last 50 years. In the same time the mean winter value of the North Atlantic Oscillation Index (NAOI) increased, the positions of the main pressure patterns over the Atlantic are shifted northeastward, and lot of other coherent changes detected in the winter climate of the European–Atlantic region. The mean summer precipitation total has hardly changed, but the frequency of summer drought events increased. There are some signs of a shift of the Hungarian summer climate towards a Mediterranean like climate.     

Examining Fluvial Stratigraphic Architecture Using Ground‐Penetrating Radar at the Fanta Stream Fossil and Archaeological Site,Central Ethiopia

The Fanta Stream site is an archaeological and paleontological locality in Addis Ababa, Ethiopia. The site contains a rich assemblage of fossil mammals and Acheulean artifacts of approximately 600 ka located in a rare high‐altitude context. A ground‐penetrating radar (GPR) survey was conducted in order to provide three‐dimensional imaging of the subsurface, which the authors use to interpret the geometry and distribution of fossil‐containing stratigraphic units. Utilizing the stream's natural cut bank exposure, we calibrate GPR data to known geologic units through radar facies analysis. Shallow, high‐amplitude coherent reflection geometries are attributed to volcanic tuff deposits, as these units exhibit subparallel continuous reflections consistent with planar stratified sedimentary deposition. Deeper, discontinuous reflection packages are interpreted as conglomeritic, fossil‐containing deposits. The results of the GPR survey outline the location of the Fanta Stream's paleodepositional features as well as suggest the extent of fossiliferous stratigraphic units for use in future excavations.     

<Emphasis Type="Bold">Paleoproterozoic Boninite-Like Rocks in an Intracratonic Setting from Northern Bastar Craton,Central India</Emphasis> by D.V. Subba Rao,V. Balaram,K. Naga Raju and D.N. Sridhar. Jour. Geol. Soc. India,v.72, 2008, pp.373–380

Discussion

Paleoproterozoic Boninite-Like Rocks in an Intracratonic Setting from Northern Bastar Craton, Central India by D.V. Subba Rao, V. Balaram, K. Naga Raju and D.N. Sridhar. Jour. Geol. Soc. India, v.72, 2008, pp.373–380     

Chesapeake Bay Plume Morphology and the Effects on Nutrient Dynamics and Primary Productivity in the Coastal Zone

To determine the effects of the Chesapeake Bay outflow plume on the coastal ocean, nutrient concentrations and climatology were evaluated in conjunction with nitrogen (N) and carbon (C) uptake rates during a 3-year field study. Sixteen cruises included all seasons and captured high- and low-flow freshwater input scenarios. Event-scale disturbances in freshwater flow and wind speed and direction strongly influenced the location and type of plume present and thus the biological uptake of N and C. As expected, volumetric primary productivity rates did not always correlate with chlorophyll a concentrations, suggesting that high freshwater flow does not translate into high productivity in the coastal zone; rather, high productivity was observed during periods where recycling processes may have dominated. Results suggest that timing of meteorological events, with respect to upwelling or downwelling favorable conditions, plays a crucial role in determining the impact of the estuarine plume on the coastal ocean.     

Observations of pulsating Ap stars in South Africa

The rapidly oscillating Ap (roAp) stars currently represent the only main sequence stars other than the Sun which exhibit non-radial acoustic pulsations of high overtone. This makes them excellent subjects for asteroseismology, an approach which promises to yield accurate knowledge of the interior structures of stars. Of the 27 known roAp stars, 24 were discovered in Sutherland despite extensive searches conducted elsewhere. This paper reviews the discovery of the roAp phenomenon and describes the factors that contribute to the high discovery rate for these stars at Sutherland. Two long-term observational projects in progress at Sutherland are discussed,viz. the Cape roAp Star Survey and long-term monitoring of frequency variations in roAp stars.     

The presolar grain inventory of fine‐grained chondrule rims in the Mighei‐type (CM) chondrites

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter's Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in ¹⁷O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in ¹⁷O and significantly depleted in ¹⁸O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced ¹⁸O/¹⁶O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca₂Mg[Si₂O₇]) or a an åkermanite‐diopside (MgCaSi₂O₆) intergrowth.