The interaction between armouring and particle weathering for eroding landscapes

The interaction between particle weathering and surface armouring and its effect on erosion has been investigated. The effect of soil armouring is to decrease sediment transport with time by preferentially stripping away fine particles. On the other hand the effect of weathering, which breaks down the particles in the armour, is generally believed to increase erosion. By extending an existing armouring model, ARMOUR, and using a variety of published weathering mechanisms this interaction has been explored. The model predicts that while this is generally true, in some cases erosion can be decreased by weathering. When the particles generated by weathering were approximately of equal diameter, erosion increased while armouring decreased. When weathering produced very fine particles by spalling, erosion increased and armouring also increased. When weathering produced a range of particles from fine to coarse, the armour layer broke down and erosion decreased relative to the no‐weathering case. This latter decrease in erosion was due to the high entrainment of coarser transportable materials from the bed decreasing the sediment transport capacity of the flow. In these studies clear regimes could be identified where erosion was limited by either the energy of the flow alone (i.e. ‘transport‐limited’), or the rate of weathering (‘weathering‐limited’); however, for some mechanisms there was an interaction between the two, which we called ‘weathering/transport limited’. Copyright © 2006 John Wiley & Sons, Ltd.     

Estimating evapotranspiration for a temperate salt marsh,Newcastle, Australia

Evapotranspiration was studied at a salt marsh site in the Hunter River estuary, NSW, Australia, during 1996–8. Estimates of actual evapotranspiration (E_a) were obtained for three sites using the eddy correlation method. These values were compared with results obtained with the Penman and Penman–Monteith equations, and with pan evaporation. The Penman–Monteith method was found to be most reliable in estimating daily and hourly evapotranspiration. Surface resistance values averaging 12 s m^?1 were derived from the eddy correlation estimates. Recent tidal flooding and rainfall were found to decrease surface resistance and increase E_a/E_p ratios. Estimates of evapotranspiration obtained using the Penman–Monteith method were shown to be sensitive to changes in surface resistance, canopy height and the method used to estimate net radiation from incoming solar radiation. These results underline the importance of accurately estimating such parameters based on site‐specific data rather than relying on empirical equations, which are derived primarily for crops and forests. Copyright © 2001 John Wiley & Sons, Ltd.     

An Overview of the 28 October 2012 M<Subscript>w</Subscript> 7.7 Earthquake in Haida Gwaii,Canada: A Tsunamigenic Thrust Event Along a Predominantly Strike-Slip Margin

The boundary between the Pacific and North America plates along Canada’s west coast is one of the most seismically active regions of Canada, and is where Canada’s two largest instrumentally recorded earthquakes have occurred. Although this is a predominantly strike-slip transform fault boundary, there is a component of oblique convergence between the Pacific and North America plates off Haida Gwaii. The 2012 M_w 7.7 Haida Gwaii earthquake was a thrust event that generated a tsunami with significant run up of over 7 m in several inlets on the west coast of Moresby Island (several over 6 m, with a maximum of 13 m). Damage from this earthquake and tsunami was minor due to the lack of population and vulnerable structures on this coast.     

Recovery of benthic macroinvertebrate and epilithic communities following a large flood,in an unstable,braided, New Zealand river

Benthic macroinvertebrates and small stones were collected from a riffle in the Ashley River, North Canterbury, New Zealand, on 12 occasions within a 132‐day period following a particularly large flood in 1986. Despite the occurrence of smaller floods during this period, benthic macroinvertebrate communities and stone surface organic layers recovered rapidly. Mean concentration of stone surface organic carbon increased from 0.23 g/m² to 1.01 g/m² stone surface in the first 23 days after the flood and chlorophyll a concentration increased from 0.13 mg/m² (day 3) to 9.2 mg/m² by day 132. Minor floods during the recovery period had little effect on organic layer biomass. Mean faunal density increased from 230/m² to 7920/m² during this time and taxon richness from 7 to 21 per 5 benthic samples. Immediately after the flood and throughout the study period the fauna was dominated by larvae of Deleatidium (Ephemeroptera: Leptophlebiidae), Hydora (Coleoptera: Elmidae), and Chironomidae. Re‐establishment of Deleatidium populations in previously denuded brands was effected by oviposition, egg‐hatching, and larval immigration. Minor braids are likely to represent important epicentres from which recolonisation proceeds.     

Scaling Ecological Dynamics: Self-Organization, Hierarchical Structure, and Ecological Resilience

Assessing impacts of global change is complicated by the problems associated with translating models and data across spatial and temporal scales. One of the major problems of ecological scaling is the dynamic, self-organized nature of ecosystems. Ecological organization emerges from the interaction of structures and processes operating at different scales. The resilience of ecological organization to changes in key cross-scale processes can be used to assess the contexts within which scaling methods function well, need adjustment, and break down.     

Influence of regional structure on the development and quality of the Upper Elkhorn no. 2 coal bed, eastern Kentucky

The Upper Elkhorn No. 2 coal bed (Middle Pennsylvanian Breathitt Formation) in eastern Kentucky exhibits thickness and coal quality trends which suggest that peat deposition was contemporaneous with tectonism. The coal thickens and has high vitrinite and low ash and sulfur content the Belfry anticline in Pike Country. T in the vicinity of the Floyd Country channel, a basement trough. In both cases other Pennsylvanian coals exhibit similar trends, suggesting that the penecontemporaneous tectonism persisted at least through a portion of the Carboniferous.     

Tectonic significance of syn-rift sediment packages across the Gabon-Cabinda continental margin

The tectonic development of a continental margin is recorded in the stratigraphic successions preserved along and across the margin in terms of stratal relationships (e.g., onlap, downlap, truncation), lithofacies, biostratigraphy, and paleo-water depths. By using these observations coupled to a kinematic and flexural model for the deformation of the lithosphere, we have elucidated the tectonic significance of the preserved stratigraphy that comprises the Gabon-Cabinda margin of west Africa. Two hinge zones, an Eastern and Atlantic, formed along the Gabon-Cabinda margin in response to three discrete extensional events occuring from Berriasian to Aptian time. The Eastern hinge zone demarcates the eastern limit of a broadly distributed Berriasian extension that resulted in the formation of deep anoxic, lacustrine systems as evidenced by the silts and shales of the Sialivakou and lower Djeno Formations and the regressive packages of the upper Djeno Formation. Approximately 1.5 to 2 km of asymmetric footwall uplift was induced across the Eastern hinge zone in response to the mechanical unloading of the lithosphere during this first phase of rifting. In contrast, the Atlantic hinge, located approximately 90 km west of the Eastern hinge, marks the eastern limit of a second phase of extension that began in the Hauterivian. Footwall uplift and rotation exposed earlier syn-rift and pre-rift sediments to at least wavebase causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon-Cabinda margins. We interpret the thickness variations of reworked clastic sediment of this age (e.g. the Melania Formation) between the hinge zones as indicative of variations in the degree of uplift and erosional truncation of the Atlantic hinge. For example, the absence of Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Cabinda and Gabon margins, the latter being characterized by significant thicknesses of Melania Formation (or equivalent). Material eroded from the Cabinda and Gabon Atlantic hinge zone may in part account for the thick wedge of sediment deposited seaward of the Gabon-Cabinda Atlantic hinge (the Erva Formation). Our modelling suggests that this wedge of reworked elastics represents deposition by along-axis gravity flows within a deep water (≈2 km) environment. A third and final phase of extension in the late Barremian-early Aptian was responsible for breaching the continental lithosphere to form the ocean/continent boundary and thus the installation of open marine conditions. Elsewhere, the environments will tend to be marginal marine to brackish, depending on the efficiency of the Atlantic hinge zone to act as a barrier to marine enchroachment. This third rift phase reactivated both the Eastern and Atlantic hinge0zones thereby creating accomodation for the Marnes Noires Formation (and equivalent) source rock deposition between the hinges and the Falcão source rock equivalent seaward of the Atlantic hinge. Two possible scenarios exist for the lateral distribution of the Marnes Noires Formation. If the reactivated rift flank topography across the Atlantic hinge was significant, then sedimentation would be restricted between the hinge zones within discrete lacustrine settings (e.g., Congo margin). Alternatively, if hinge zone uplift was relatively minor, then a coral-rimmed archipelago may have developed parallel to the margin with restricted communication across the Atlantic hinge zone (e.g., Cabinda margin). In this latter scenario, dilution of the Marnes Noires source rocks by terrigenous input from the eroding Atlantic hinge zone should be relatively minor thereby enhancing source rock quality. Furthermore, potential marine upwelling outboard of the Atlantic hinge zone is likely the cause for the production and accumulation of organic-rich material associated with the Falcão source rock of the Kwanza basin. By late Aptian time, the remaining accomodation between the hinge zones was partially filled by across- and along-axis prograding deltaic systems of the Argilles Vertes and Tchibota Formations. The progradation and interaction of the Argilles Vertes depositional lobes resulted in the formation of residual paleo-relief. Subsequent marine incursions and flooding of this paleo-relief led to the development of basal conglomerates (the Chela ‘lag’ unconformity) grading upward into fine-grained sands and evaporites. Consequently, an inverse relationship should exist betweeb evaporite thickness (in particular, the lower members) and the thickness of the underlying Argilles Vertes and Tchibota Formations. Variations in Loeme evaporite thickness is a consequence of stratigraphic and structural control with salt instability influencing local variability.Our modelling suggests the occurrence of two distinct evaporite sequences on the Congo margin, an earlier evaporite deposited seaward (west) of the Atlantic hinge during the second and third rift phases and the late Aptian Loeme Formation deposited between the hinge zones. An evaporite sequence seaward of the Atlantic hinge is inferred on the basis of extensive diapirs and salt tectonic structures observed in seismic data. In order to match the distribution and thickness of the observed post-salt stratigraphy across the basin, however, we require large paleowater depths west of the Atlantic hinge during the later Aptian. The existence of large paleowater depths precludes the formation of thick evaporite sequences within the outer basin. Consequently, we propose that the evaporites seaward of the Atlantic hinge were formed during the syn-rift development of the margin and are not contemporaneous with the post-rift Loeme salts deposited between the hinge zones. This double salt hypothesis is consistent with observations from the conjugate Brazilian margin.     

The role of topography in controlling throughflow generation: A discussion

The distributions of specific catchment area and specific dispersal area values over the hillside studied by Anderson and Burt (1978) relate much more closely to the observed distributions of soil water matric potential than do the occurrences of contour concavity on which the authors rely. Highest potential always occurred in the zone of large specific catchment area except immediately after rainfall, when it occurred in the zone of small dispersal area. Isolines of low potential persistently conformed to those of specific dispersal area.