Regional river flow,water quality,aquatic ecological impacts and recovery from drought

Abstract

During severe drought and low flows in the Otago Region of the South Island of New Zealand, increased bacterial contamination of rivers occurred in agricultural/pastoral catchments due to greater stock use of waterways and the lack of dilution and flushing flows. Concentrations and loadings of nitrogen and phosphorus generally decreased due to the lack of rainfall, runoff and transport from diffuse sources. Other analytes, including dissolved solids and pH, did not change to any considerable degree and were quite spatially variable. Although the health of benthic macroinvertebrate communities can decrease with a lower diversity and fewer sensitive taxa, effects were not significant in most rivers. Algal blooms occurred in some locations, particularly in lowland agricultural catchments with low water velocities and little shade. The prolonged lack of flushing flows can be a significant problem. Fish kills occurred where they became stranded and isolated from the main flow and where there was a lack of deeper pool refuges. Temperatures in excess of 25°C can also result in isolated fish kills, but the duration of these high temperatures appears to be the most critical factor. Adverse effects from drought can be widespread and severe in some locations, but appear to be relatively short-lived with recovery occurring rapidly with increasing flows at most locations. This indicates a degree of resilience of rivers to drought and low flows with long-term impacts that are not severe.     

Grenvillian-aged orogenesis in the Palaeoproterozoic Gascoyne Complex, Western Australia: 1030–950 Ma reworking of the Proterozoic Capricorn Orogen

In situ SHRIMP U–Pb geochronology of monazite and xenotime in pelitic schists from the central Gascoyne Complex, Western Australia, shows that greenschist to amphibolite facies metamorphism occurred between c. 1030 and c. 990 Ma. Monazite from an undeformed rare‐element pegmatite from the same belt gives a ²⁰⁷Pb/²⁰⁶Pb age of c. 950 Ma, suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism. Metamorphism in the central Gascoyne Complex was previously interpreted as Barrovian, largely based on the identification of kyanite in peak metamorphic assemblages, and has been attributed to intense crustal shortening and substantial tectonic thickening during Palaeoproterozoic continent–continent collision. However, the stable Al₂SiO₅ polymorph has been identified in this study as andalusite rather than kyanite, and the prograde assemblages of staurolite–garnet–andalusite–biotite–muscovite–quartz indicate temperatures of 500–550 °C and pressures of 3–4 kbar. These data show that the Palaeoproterozoic Gascoyne Complex underwent an episode of Grenvillian‐aged intracontinental reworking concentrated in a NW–SE striking corridor, during the Edmundian Orogeny. Until now, the Edmundian Orogeny was thought to have involved only reactivation of structures in the Gascoyne Complex, along with deformation and very low‐ to low‐grade metamorphism of Mesoproterozoic cover rocks some time between 1070 and 755 Ma. However, we suggest that it involved regional amphibolite facies metamorphism and deformation, granite magmatism and pegmatite intrusion between c. 1030 and c. 950 Ma. Therefore, the Capricorn Orogen experienced a major phase of tectonic reworking c. 600 Myr later than previously recognized. Our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events.     

High Field Strength Element Fractionation in the Upper Mantle: Evidence from Amphibole-Rich Composite Mantle Xenoliths from the Kerguelen Islands (Indian Ocean)

A basanite dyke in the Kerguelen Archipelago contains abundantcomposite mantle xenoliths consisting of spinel-bearing dunitescross-cut by amphibole-rich veins. Two types of veins (thickand thin) have been distinguished: the thick veins representalmost complete crystallization products of highly alkalinemelts similar to the host basanites, whereas thin veins areprecipitates from fractionates of the parental melts to thethick veins. These fractionated fluids are enriched in H₂O relativeto the parental melts. The amphiboles in the thin veins arelower in Ti and higher in Nb, Ta, Zr and Hf than amphibolesin the thick veins. This fractionation of high field strengthelements (HFSE) is consistent with a combination of the changingcomposition of the fractionated fluids and the change in intrinsicamphibole–fluid partition coefficients for HFSE in fluidswith higher a_H2O and lower a_TiO2. The trace element contentof amphiboles disseminated in dunitic wall-rocks is closelyrelated to the composition of adjacent veins and thus theseamphiboles are precipitates from fluids percolating into thedunite from the veins. Disseminated amphibole reflects the compositionof the percolating melt, which is similar to that of the associatedveins. KEY WORDS: mantle amphibole; Kerguelen; HFSE fractionation; mantle HFSE; mantle xenoliths     

Braidplain deposition of the Upper Triassic Molteno Formation in the main Karoo (Gondwana) Basin, South Africa

ABSTRACT The Upper Triassic (Carnian?) Molteno Formation in the main Karoo (Gondwana) Basin, South Africa forms a northerly thinning, intracratonic clastic wedge comprising sandstones, shales and coals occurring within thick (up to 140 m) laterally persistent fining-upward sequences. These sequences were deposited by braided streams draining an alluvial plain which may have been built on to the distal slopes of alluvial fan complexes of glacial outwash type. Geometric relations between sequences indicate three phases of tectonic activity. The lowermost fining-upward sequence in the south accumulated against a rising mountain front; cessation of movement and an eastward shift in the main locus of tectonism and sedimentation was followed by renewed uplift and basinwide progradation of the second fining-upward sequence adjacent to a fault-block granite terrain located close to the present south-east coastline of South Africa. This is believed to be the granite at the eastern end of the Falkland Island Plateau, an interpretation consistent with its position on most continental reconstructions and the fracture zone marking its northern scarp face. Faulting is attributed to the first phase of extension prior to continental breakup. The sourceward recession and lack of gross fining-upward trends shown by the uppermost fining-upward sequences is accounted for by limited back-faulting of the still active basin margin. Cessation of activity and further basin margin recession occurred with deposition of the overlying floodplain deposits (Elliot Formation) which were distal equivalents of the braided alluvial plain.     

PLACING KANGWANE IN THE NEW SOUTH AFRICA*

ABSTRACT. The South African homelands were central to the apartheid ideology of racial segregation and separate development and as a result became the location for large segments of the African population. Apartheid‐era theorizations of the homelands tended to emphasize their importance to the state, with less attention directed to the divergent and unique social formations that often existed within them. Recent geographical research has been intent on evaluating the spatial imprint of these geographies for resident populations, as well as the varied class, gendered, and institutional formations that accompanied the democratic transition. Using a case study from the former KaNgwane homeland, this article examines the diverse ways in which rural households access environmental and economic resources to produce livelihoods. It is argued that a focus on community variation is needed to interrogate the differential encounters of these places with the local politics and development processes that are emerging in the new South Africa.     

Palaeochannel reconstructions from point bar deposits: a three-dimensional perspective

Quantitative determination of palaeochannel geometry and hydraulics from point bar deposits requires an understanding of the interaction between channel-bend migration, temporal and spatial variation of point bar geometry and facies, and outcrop orientation. This interaction is modelled with the aid of a computer program which predicts three-dimensional (3-D) geometry and grain size variation of point bars. Synthetic deposits are produced for the cases of down-valley bend migration and/or increase in channel-bend sinuosity. Two-dimensional (2-D) cross-sections in varying orientations across these simulated deposits display lateral-accretion bedset surface geometry, and variation in mean bedset grain size and local palaeocurrent orientation. Most cross-sections show point bar deposits thickening away from the meander-belt axis due to a lateral progression from thinner bend-exit deposits to thicker bend-apex deposits (caused by down-valley channel translation), and/or due to a progression from thinner low sinuosity deposits to thicker high sinuosity deposits caused by channel bend expansion. In association with this lateral thickening, bedset surfaces become steeper and more convex upwards while the variation in mean grain size up bedsets commonly increases. Down-valley point bar translation allows preservation only of deposits formed downstream of the band apex, and produces characteristic fining upwards sequences. Marked lateral and vertical variations in palaeocurrent directions due to varying channel orientation relative to a given cross-section are also predicted. These results indicate a need in palaeochannel reconstructions, for a more detailed examination of 3-D variations in bedset surface geometry, palaeocurrent orientation and grain size distribution within and between bedsets of laterally accreted sediment.     

Submarine sedimentation on a developing Holocene fan delta

The submarine morphology, sediments, and three-dimensional geometry of a developing fan delta are described using data from acoustic surveys, bottom sampling, and observations from a manned submersible. The fan system is being built in a British Columbian fjord (water depth 410 m) supplied with coarse-grained sediments from a fjord-side river. Construction of the subaqueous fan began about 10–12,000 yr BP and is ongoing. The system is analogous to part of one fault-uplift sedimentation cycle in ancient fan deltas. Initially, when offshore relief was at a maximum, acoustically chaotic sediment wedges were emplaced over fjord-bottom glaciomarine deposits. Subsequent aggradation/progradation resulted in moderately dipping sequences interrupted by local chaotic units. The present fan surface (average slope 13°) is divided into six zones arranged concentrically from the fan apex, on the basis of form, sediment and process interpretations. Continued subaqueous fan growth results from settling of river-derived sediments from suspension and downslope sediment dispersal by episodic gravity flows, apparently fed by underflows from the river.