Upstream and downstream controls of recent avulsions on the Taquari megafan,Pantanal, south‐western Brazil

Avulsion, the natural relocation of a river, is a key process in the evolution of subaerial fans, river floodplains and deltas. The causes of avulsion are poorly understood, which is partly due to the scarcity of field studies of present avulsions. At present, two avulsions are occurring on the middle and lower Taquari megafan, Pantanal basin, south‐western Brazil. Here we present an analysis of the causes of these avulsions based on field and remote sensing data and show that avulsions on megafans can be controlled by both upstream and downstream processes. The middle fan avulsion (started in 1997–1998) is a result of upstream control: overbank aggradation was caused by the (variable) input of sandy sediment into the system, which caused channel‐belt superelevation and also created an easily erodible subsurface favouring bank retreat, crevassing, and scour of deep floodplain channels. The sandy subsurface in this area is inferred to have been a major factor in the causation of this avulsion under conditions of little gradient advantage. The lower fan avulsion (started c. 1990) results from interplay of upstream and downstream controls, the latter being related to the local base level (the Paraguay River floodplain) at the toe of the fan. Channel and overbank aggradation on the lower fan was influenced by fan sub‐lobe progradation and channel backfilling. Fan sub‐lobe progradation caused a significant gradient advantage of the avulsion channel over the parent channel. Avulsions are commonly supposed to be preferentially triggered by high‐magnitude floods, when there is considerable channel‐belt superelevation. However, both avulsions studied by us were triggered by small to average floods, with modest channel‐belt superelevation. We conclude that flood magnitude and channel‐belt superelevation have been overrated as causes of avulsion, and demonstrate additional causes that influence the growth of crevasses into avulsions. Copyright © 2012 John Wiley & Sons, Ltd.     

I/I ratios in Scottish coastal surface sea water: Geographical and temporal responses to changing emissions

This work constitutes the first survey of I isotope ratios for Scottish sea water including the first data for the west of Scotland. These data are of importance because of the proximity to the world’s second largest emission source of ¹²⁹I to the sea, the Sellafield nuclear reprocessing plant, because of the increasing importance of the sea to land transfer of ¹²⁹I and also as input data for dose estimates based on this pathway of ¹²⁹I. ¹²⁹I/¹²⁷I ratios in SW Scotland reached 3 × 10⁻⁶ in 2004. No strong variation of I isotope ratios was found from 2003 to 2005 in Scottish sea waters. Iodine isotope ratios increased by about a factor of 6 from 1992 to 2003 in NE Scotland, in agreement with the increase of liquid ¹²⁹I emissions from Sellafield over that time period. It is demonstrated that ¹²⁹I/¹²⁷I ratios agree better than ¹²⁹I concentrations for samples from similar locations taken in very close temporal proximity, indicating that this ratio is more appropriate to interpret than the radionuclide concentration.     

Evolution and strike variability of early post-rift deep-marine depositional systems: Lower to Mid-Cretaceous, North Viking Graben, Norwegian North Sea

The controls and development of early-post-rift, deep-water depositional systems are poorly understood due to their commonly deeply-buried nature. As a consequence, in the subsurface there is usually a lack of well penetrations and/or weak seismic imaging. At outcrop, early post-rift strata have commonly been deformed beyond reasonable recognition by later inversion tectonics. In contrast to these systems, the North Viking Graben shows a well-imaged Cretaceous early post-rift package with good well control and little effect from inversion. Therefore, this paper examines the early post-rift, deep-water depositional systems of the North Viking Graben to determine the controls on their stratigraphic position, geometry and evolution, and thus provide an analogue for comparable systems. Greater understanding of such systems will allow for the enhanced prediction of reservoir units in the subsurface and development of new play models since post-rift intervals are generally under-explored.The basin configuration inherited by the Cretaceous early post-rift in the northern North Sea was set up by Permo-Triassic and Late Jurassic rifting. In the North Viking Graben this established considerable along-strike variability, resulting in a northern basin segment surrounded by steep slopes and faulted-bounded structural highs and a southern basin segment margined by slopes with noticeably gentler gradients. Associated with the Cretaceous post-rift is an overall transgressional trend, which drowned local source areas, resulting in prevalent carbonate and hemipelagic mudstone deposition in the basins. In the North Viking Graben, the uplifted Oseberg fault-block provided the sub-aerial clastic source area until it was submerged in the early Upper Cretaceous.The early post-rift infill of the North Viking Graben was divided into four key seismic stratigraphic units (K1, K2, K3 and K4) using an integration of seismic and well data. Inside this stratigraphic framework, the depositional systems within each K-unit were resolved from characteristic seismic facies, amplitude anomalies, relationship with adjacent reflections, and geomorphologies. In the northern basin segment, the early post-rift stratigraphy contains basin-floor fans, a channel complex and a shoreline-like geometry, whereas the southern basin segment is solely characterised by hemipelagic and carbonate deposition. This spatial variability indicates that one of the dominant controls on the development of the early post-rift depositional systems in the North Viking Graben was the inherited syn-rift fault-controlled topography. The steep slopes bounding the northern basin segment aided the delivery of sediment from the sub-aerial Oseberg source area to the graben whereas the submerged, gentle slopes in the southern basin segment were relatively sediment-starved.Long- and short-term changes in relative sea-level also heavily influenced the evolution of the early post-rift basin stratigraphy. Short-term relative sea-level fall allowed basin-floor fan emplacement whereas short-term relative sea-level stand-still favoured deposition of a channel complex. Deposition of the shoreface-like geometry is associated with a short-term relative sea-level rise. This temporal difference in the style and scale of the depositional systems is also interpreted to reflect the gradual denudation and drowning of the Oseberg source area. Regional short-term trangressive and anoxic events in the northern North Sea further influenced the early post-rift strata, resulting in the deposition of stratigraphic units that can be correlated across the North Sea.     

Space Physics Interactive Data Resource—SPIDR

SPIDR (Space Physics Interactive Data Resource) is a standard data source for solar-terrestrial physics, functioning within the framework of the ICSU World Data Centers. It is a distributed database and application server network, built to select, visualize and model historical space weather data distributed across the Internet. SPIDR can work as a fully-functional web-application (portal) or as a grid of web-services, providing functions for other applications to access its data holdings.

Architecture and emplacement of the Nebo–Babel gabbronorite-hosted magmatic Ni–Cu–PGE sulphide deposit,West Musgrave,Western Australia

The Nebo–Babel Ni–Cu–platinum-group element (PGE) sulphide deposit in the West Musgrave Block, Western Australia, is the largest nickel sulphide discovery in the last 10 years. The deposit is hosted within a concentrically zoned, olivine-free, tube-like (chonolithic), gabbronorite intrusion associated with the, approximately, 1,078-Ma Giles Complex-layered intrusions in the Warakurna large igneous province. Emplaced into sulphide-free amphibolite facies orthogneiss, the fault-offset Nebo–Babel chonolith extends for 5 km and has a cross-section of 1 × 0.5 km. Igneous mineralogy, fabrics, and textures are well preserved. The lithostratigraphy includes variably textured leucogabbronorites (VLGN) that form an outer shell around mineralised gabbronorite (MGN), with barren gabbronorite (BGN) and oxide–apatite gabbronorite (OAGN) in the middle and lower parts of the chonolith. Mineral and whole-rock geochemistry indicate that the units become progressively evolved in the order: VLGN, MGN, BGN, and OAGN, and that incompatible trace-element concentrations increase downwards within the MGN and BGN. The mineralisation, which is confined to the early, more primitive units (VLGN and MGN), occurs as massive sulphide breccias and stringers and as disseminated gabbronorite-hosted sulphides. The massive sulphides were emplaced late in the intrusive sequence, have different PGE chemistry and Cu tenor to the disseminated sulphides, and have undergone sulphide fractionation. The distribution of disseminated sulphides, which are primary magmatic in origin, is related to chonolith geometry and magma flow regimes, rather than to gravitational settling. Sulfur-bearing country rocks are absent in the Nebo–Babel deposit area, and thus, local crustal S addition was unlikely to have been the major mechanism in achieving sulphide immiscibility. The Nebo–Babel intrusion is part of an originally continuous magma chonolith with multiple and related magma pulses. The parental magma was medium- to low-K tholeiite with 8–9 wt% MgO. The initial magma pulse (VLGN), the most primitive and sulphide saturated, was probably emplaced along a linear weakness in the country rock. After crystallisation of VLGN, marginally more fractionated, sulphide-saturated magma was injected through the thermally insulated core of the conduit, forming the MGN. Successive pulse(s) of more fractionated magma (BGN) were emplaced in the core of the intrusion. After magma flow ceased, closed system crystal fractionation produced consistent mineral and chemical fractionation trends within BGN and OAGN. After crystallisation, the intrusion was overturned and then offset by the Jameson Fault resulting in the apparent ‘reverse’ chemical and mineral trends in Nebo–Babel.     

Syn-folding remagnetization of Cambro-Ordovician carbonates from the Pennsylvania Salient post-dates oroclinal rotation

Paleomagnetic directions of 35 sites of Cambro-Ordovician carbonates from 10 anticlines were analyzed to test models of curvature along the Pennsylvania Salient of the Appalachians and to constrain the relative timing of magnetization acquisition. The sites yield directions of magnetization that are almost all reversed with near-horizontal inclinations upon appropriate structural correction. The common, Late Paleozoic (Kiaman-aged) direction and incremental fold tests show that these directions represent remagnetizations carried by authigenic magnetite, acquired just before or during the earlier phases of folding. No convincing indications were found of primary magnetizations. Mean declinations from the northeastern and southwestern limbs of the salient differ by a few degrees, indicating negligible, if any, rotation between the limbs. The results are similar to prior studies of overlying Siluro-Devonian carbonates, showing coherent behavior of the entire Paleozoic cratonic cover. We conclude that the statistically negligible difference in declination indicates that (previously demonstrated) oroclinal bending occurred before carbonates of the Paleozoic stratigraphic cover were remagnetized during the Permian and before regional folding was completed.     

Deterring effects of 8-45 kHz tone pulses on harbour seals (Phoca vitulina) in a large pool

The marine aquaculture industry suffers losses due to pinniped attacks which damage net enclosures and fish stocks. Acoustic harassment devices (AHDs) emit loud sounds which are intended to deter pinnipeds from approaching aquaculture enclosures. At present, many AHDs emit sounds in the 8-20 kHz frequency range. It is not known whether sounds of higher frequencies have a deterrent effect on seals. Therefore five captive harbour seals (Phoca vitulina) were subjected to four series of tone pulses together spanning a broad frequency range (8, 16, 32 and 45 kHz). Pulse duration was 250 ms and pulse interval was 5s. Each of the four sounds was made deterrent by increasing the amplitude. The seals reacted by swimming away from the sounds. The displacement effect of each sound was judged by comparing the animals' surface positions, and number of surfacings, during ten 45 min baseline periods with ten 45 min test periods per frequency (one frequency per day in rotation, 40 sessions in total). The seals were displaced by all four frequencies throughout the 40 trial days. The seals came to the surface more often when the test tones were produced than in the baseline periods. The initial displacement distances did not change over the 40 test days. This suggests that operating AHDs for only short periods will be more effective and less likely to result in habituation by the seals than operating them continuously. The discomfort threshold sound pressure level (SPL) was established for each of the four pulse frequencies. The acoustic discomfort threshold SPL is defined as the boundary SPL between the area that the animals generally occupied during the transmission of the sounds and the area that they generally did not enter during sound transmission. The discomfort threshold SPL may depend on the context.     

Dating Quaternary volcanism and incision by the River Tigris at Diyarbakır,southeast Turkey

New high‐precision Ar‐Ar dating of basalt underlying the city of Diyarbak?r, southeast Turkey, constrains the Quaternary incision history of the River Tigris, strengthening the pre‐existing chronology based on magnetostratigraphy and K‐Ar dates. The basalt, which overlies Tigris terrace gravel 70 m above the river, is part of the Karacada? shield volcano complex centred ca. 60 km to the southwest. The reverse‐magnetised Diyarbak?r basalt, one of relatively few flow units to reach the Tigris valley axis, has been dated to 1196 ± 19 ka (±2σ). Two lower terraces have been recognised in the Diyarbak?r area, 46 and 32 m above the present river, with at least one other further downstream. From this evidence the rate of Middle‐Late Pleistocene incision by the Tigris can be estimated; it probably reached ca. 0.07 mm a^?1, reflecting the characteristic global increase in uplift rates observed at that time. Copyright © 2007 John Wiley & Sons, Ltd.     

Pedosedimentary development of part of a Late Quaternary loess‐palaeosol sequence in northwest Argentina

The field properties, magnetic susceptibility, particle size, calcium carbonate content, soil micromorphology and optical luminescence ages of the upper 6.1 m and lowermost 4.7 m of the 45 m loess–palaeosol sequence at El Lambedero in the Tafí del Valle region of Tucumán Province (Sierras Pampeanas, northwest Argentina) have been used to set up a partial stratigraphy and chronology, as well as a basic pedosedimentary model of loess accumulation, palaeosol development, reworking and erosion for the site. The minimum ages derived from the basal part of the section suggest that loess began to accumulate some time before 165 ka. A thick and well‐developed pedocomplex in the upper profile is correlated with at least the latter part of marine isotope stage (MIS) 5, whereas the overlying palaeosol may be attributable to pedogenic activity during MIS 3. The absence of material younger than 33 ka close to the surface of this rounded spur landform is probably the result of either non‐deposition or erosional stripping in response to climatic change, or episodic uplift in this seismically active region. Copyright © 2004 John Wiley & Sons, Ltd.