‘Pressure shadow’ structures in foliated rocks from Bermagui,New South Wales

‘Pressure shadow’ type structures, referred to here as ‘mica beards’ (Ransom, 1965), are described from a sequence of deformed Ordovician greywackes. The beards are very numerous and constitute a major fabric element in definition of an axial‐plane foliation. They consist of fine, polygonal grains of quartz and of layer silicates which are aligned approximately perpendicular to the surface of the host grain with which they are associated. Three possible explanations are offered for the formation of the structures and their relative merits are discussed briefly.     

Ediacaran ice-rafting and coeval asteroid impact,South Australia: insights into the terminal Proterozoic environment

Isolated quartzose pebbles, clusters of quartz granules, orthogonal aggregates of poorly sorted quartzose coarse sand, and ovoid pellets (≤2 mm long) of quartz silt occur in hemipelagic marine mudstone of the mid-Ediacaran Bunyeroo Formation exposed in the Adelaide Geosyncline (Adelaide Rift Complex), and ovoid pellets of quartz silt in cores of the correlative marine Dey Dey Mudstone from deep drillholes in the Officer Basin, South Australia. This detritus is interpreted respectively as dropstones, dumps, and frozen aggregates dispersed by sea ice possibly of seasonal origin, and till pellets transported by glacial ice. The ice-rafted material in the Bunyeroo Formation only has been found <10 m stratigraphically below and above a horizon of dacitic ejecta related to the 90 km diameter Acraman impact structure in the Mesoproterozoic Gawler Range Volcanics 300 km to the west. Furthermore, till pellets have been identified 4.4 to 68 m below distal Acraman ejecta in the Dey Dey Mudstone >500 km northwest of the impact site. The Acraman impact took place at a low paleolatitude (～12.5°) and would have adversely affected the global environment. The stratigraphic observations imply, however, that the impact occurred during, but did not trigger, a cold interval marked by sea ice and glacial ice, although the temporal relationship with Ediacaran glaciations elsewhere in Australia and on other continents is unclear. Release from the combined environmental stresses of a frigid, glacial climate near sea-level and a major impact in low latitudes may have been a factor influencing subsequent Ediacaran biotic evolution.     

Hydrologic Versus Biogeochemical Controls of Denitrification in Tidal Freshwater Wetlands

Tidal freshwater wetlands (TFW) alter nitrogen concentrations in river water, but the role of these processes on a river’s downstream nitrogen delivery is poorly understood. We examined spatial and temporal patterns in denitrification in TFW of four rivers in North Carolina, USA and evaluated the relative importance of denitrification rate and inundation on ecosystem-scale N₂ efflux. An empirical model of TFW denitrification was developed to predict N₂ efflux using a digital topographic model of the TFW, a time series of water level measurements, and a range of denitrification rates. Additionally, a magnitude-frequency analysis was performed to investigate the relative importance of storm events on decadal patterns in N₂ efflux. Spatially, inundation patterns exerted more influence on N₂ efflux than did the range of denitrification rate used. Temporal variability in N₂ efflux was greatest in the lower half of the tidal rivers (near the saline estuary) where inundation dynamics exerted more influence on N₂ efflux than denitrification rate. N₂ efflux was highest in the upper half of the rivers following storm runoff, and under these conditions variation in denitrification rate had a larger effect on N₂ efflux than variability in inundation. The frequency-magnitude analysis predicted that most N₂ efflux occurred during low flow periods when tidal dynamics, not storm events, affected TFW inundation. Tidal hydrology and riparian topography are as important as denitrification rate in calculating nitrogen loss in TFW; we present a simple empirical model that links nitrogen transport in rivers with loss due to denitrification in TFW.     

Monitoring US agriculture: the US Department of Agriculture,National Agricultural Statistics Service,Cropland Data Layer Program

The National Agricultural Statistics Service (NASS) of the US Department of Agriculture (USDA) produces the Cropland Data Layer (CDL) product, which is a raster-formatted, geo-referenced, crop-specific, land cover map. CDL program inputs include medium resolution satellite imagery, USDA collected ground truth and other ancillary data, such as the National Land Cover Data set. A decision tree-supervised classification method is used to generate the freely available state-level crop cover classifications and provide crop acreage estimates based upon the CDL and NASS June Agricultural Survey ground truth to the NASS Agricultural Statistics Board. This paper provides an overview of the NASS CDL program. It describes various input data, processing procedures, classification and validation, accuracy assessment, CDL product specifications, dissemination venues and the crop acreage estimation methodology. In general, total crop mapping accuracies for the 2009 CDLs ranged from 85% to 95% for the major crop categories.     

The Dynamics of Meteoroid streams

Meteors are streaks of light seen in the upper atmosphere when particles from the inter-planetary dust complex collide with the Earth. Meteor showers originate from the impact of a coherent stream of such dust particles, generally assumed to have been recently ejected from a parent comet. The parent comets of these dust particles, or meteoroids, fortunately, for us tend not to collide with the Earth. Hence there has been orbital changes from one to the other so as to cause a relative movement of the nodes of the meteor orbits and that of the comet, implying changes in the energy and/or angular momentum. In this review, we will discuss these changes and their causes and through this place limits on the ejection process. Other forces also come into play in the longer term, for example perturbations from the planets, and the effects of radiation pressure and Poynting–Robertson drag. The effect of these will also be discussed with a view to understanding both the observed evolution in some meteor streams. Finally we will consider the final fate of meteor streams as contributors to the interplanetary dust complex.     

Rotating garnets

The origin of snowball and sigmoidal inclusion patterns in porphyroblasts is discussed. Snowball garnets are peculiar to shear zones whereas sigmoidal patterns occur in porphyroblasts both in shear zones and on the limbs of folds. There are currently two models for the development of snowball garnets and these have been discussed extensively in the literature. We show that although the typical two-dimensional snowball pattern can be produced by either model, the three-dimensional inclusion patterns are model-specific thus providing a distinguishing criterion. We have applied this criterion to all the available data and find that the classical model, which is dependent on the rotation of garnet relative to a single foliation, is applicable in all cases. Syn-kinematic porphyroblasts on the limbs of horizontal normal folds generally show little rotation relative to geographical coordinates. What rotation they do show generally has the same sense as that of the host limb, but is less in magnitude. This has been used as evidence that the porphyroblasts have remained irrotational while the rocks deformed around them; the implication being that they were unaffected by vorticity associated with folding. This has been explained by claiming that the porphyroblasts are restricted in distribution to small domains of coaxial deformation path. We show that for reasonable deformation models of horizontal normal folds, porphyroblasts affected by vorticity will rotate little with respect to geographical coordinates and our results predict the commonly observed natural patterns. We conclude therefore that lack of rotation relative to geographical coordinates cannot be used to demonstrate that porphyroblasts have grown only in coaxially deforming domains; much less restrictive and more reasonable interpretations are possible. Consequently, the lack of rotation relative to geographical coordinates is more significant for fold modelling than it is for the garnet controversy.     

Past and present glaciological responses to climate in eastern Baffin Island

Much of Baffin Island is close to the modern glaciation limit and climatic changes within the last decade are already being reflected in snow cover extent. Statistical analysis of glacierized and ice-free corries indicates that changes in direct solar radiation due to astronomical factors are inadequate to account for glacierization of those at present ice-free. These and other sources of evidence demonstrate the need for augmented winter snowfall in order to increase the extent of glacierization. The pattern of glacial history in this area is for maximum ice extent during the early glacial phase (>68,000, <137,000 BP), folfxlowed by a reduction in ice volume during the cold pleniglacial (>24,000, < 68,000 BP) and then a limited late glacial advance (the Cockburn Stade, ca. 8,000 BP) due to increased precipitation. The Barnes Ice Cap did not disappear in the Holocene as it did in the last interglacial. The area is highly suitable for long-term monitoring of climatic change and glacial response.