Framing realistic river rehabilitation targets in light of altered sediment supply and transport relationships: lessons from East Gippsland, Australia

Since European settlement, the Cann River in East Gippsland, Victoria has experienced a 700% increase in channel capacity, a 150-fold increase in the rate of lateral channel migration, a 45-fold increase in bankfull discharge and a 860-fold increase in annual sediment load. Over the last century, and primarily the last 40 years, channel incision has removed the equivalent of around 1500 years of floodplain deposition. A numerical floodplain evolution model is presented which suggests that under a best case scenario, infilling the incised channel trench will take 31,000 years and this is predicated on the full recovery of the immediate riparian vegetation and the in-channel loading of woody debris. The asymmetry in the recovery time following rapid channel change, compared with the original deposition of the material, is explained by a combination of the sediment-starved character of the catchment and the altered hydraulic conditions within the channel, principally associated with the role of woody debris. These factors have major implications for geomorphic recovery potential, constraining what can be realistically achieved in river rehabilitation.     

Interpreting the hydrological history of a temporary pond from chemical and microscopic characterization of siliceous microfossils

The hydrological history of a temporary pond in South Carolina was inferred from a 5500-year record of siliceous microfossils, including diatoms, freshwater sponge spicules, chrysophyte cysts, plates of testate amoebae and plant phytoliths. Microfossil abundance was estimated by microscopic quantification of siliceous particles and by chemical extractions of silica. Diatom, sponge and mineral particle volumes were correlated with silica concentrations attributable to these fractions. Both techniques suggested a sequence of four distinct community types. Basal sediments (4630–5520 ¹⁴C YBP) containing phytoliths and sponge spicules indicative of a wetland community were covered by sediments dominated by the remains of planktonic protists (3750–4630 ¹⁴C YBP) suggesting a transition from a vegetated marsh to an open-water, permanently flooded pond. Microfossil assemblages above this zone indicate the return of a wetland community ca. 3750 YBP that persisted until recently, when pond water levels stabilized as a result of seepage from a reservoir constructed nearby in 1985. This study suggests that the suite of siliceous microfossils commonly found in pond sediments can be used to infer historical alternations between macrophyte and plankton-dominated states in shallow basins. Regional climate inferences from this record include a mid-Holocene hydrological maximum and the onset of the modern climate ca. 3500 YBP.     

The Dynamics of Chironomidae (Insecta: Diptera) Assemblages in Response to Environmental Change during the past 700 years on Svalbard

The impact of recent natural and human-induced environmental change on chironomid faunas on Svalbard has been investigated. The modern chironomid fauna was studied from surface-sediment samples collected from 23 lakes in western Svalbard. A total of 18 taxa was found, of which three had not been recorded previously from Svalbard. The influence of water chemistry and physical variables on the distribution and abundance of the modern chironomid assemblages was investigated using correspondence analysis and multiple regression. The chironomid assemblages fall into four groups, which are primarily influenced by pH, nutrient concentrations, water temperature, and water depth. Sediment cores were taken from three lakes to investigate changes in chironomid assemblages over the last 700 years. At two of the sites there is evidence for a response to regional climatic change occurring about 200 years ago and may have been associated with the ‘Little Ice Age’. At the third site there is a response to local catchment changes, probably brought about, initially, by the establishment of a human settlement close to the lake 70 years ago, and subsequently, as a result of the abandonment of this settlement in 1988.     

Early stages in the differentiation of the Skaergaard magma as revealed by a closely related suite of dike rocks

Certain dikes in the Kangerdlugssuaq area of east Greenland correspond rather closely with the presumed bulk composition of the nearby Skaergaard intrusion and therefore allow a better characterization of the chemistry of the Skaergaard initial magma than has so far been possible. They indicate that Wager's chilled margin composition needs only minor revision, except in the case of Al₂O₃. Other dikes belonging to the same suite show a range of compositions extending up to the differentiation stage represented by the lower part of the Skaergaard middle zone and these allow an investigation, using computer least squares and Rayleigh fractionation modelling, of the early part of the Skaergaard differentiation trend which is otherwise unexposed. We conclude that the only important cumulus phases at this stage were olivine and plagioclase but clmopyroxene grew by heteradcumulus growth and effectively eliminated all intercumulus liquid. The liquids described here are similar to many ocean-floor basalts.     

Antarctic bottom water fluctuations in the Vema Channel: Effects of velocity changes on particle alignment and size

The anisotropy of magnetic susceptibility (AMS) and mean particle size in the silt fraction have been measured in surface sediments and down selected cores from the Vema Channel. Results show that a function (F_s) which represents magnetic grain long axis alignment is highly correlated with variation in mean size of the silt fraction. An increase in alignment is caused by an increase in bottom water velocity which corresponds to an increase in mean particle size. Best alignments and coarsest particle sizes are found in the axis of the Vema Channel. In contrast, poor particle alignment and fine silt mean characteristics the shallower region east of the channel axis, consistent with lower current velocities.Core CH 115-61 was collected near the Level of Least Motion at the transition between northward-flowing Antarctic Bottom Water (AABW) and southward-flowing North Atlantic Deep Water. A positive relationship exists between silt size and AMS in the core. Fluctuations in particle alignment and mean silt size indicate three or four periods of increased velocity during the past 150,000 years.Geographic orientation of the maximum AMS axes in cores CH 115-60 and -61 was accomplished using the declination of remanent magnetism and an axial dipole as the reference azimuth. The data show that the larger magnetic grains in these sediments are aligned normal to the bathymetric trend of the channel which suggests that these grains are aligned by traction transport. The result indicates that AABW in the Vema Channel has flowed along an azimuth of approximately N30°E for the past 150,000 years.     

Hyperaccumulation of nickel by some plants of Southeast Asia

A herbarium collection of plants from Obi Island (North Moluccas) was analyzed for nickel and other elements in order to identify new hyperaccumulators (> 1000 μg/g dry weight) which are indicative of ultrabasic substrates. Ultrabasic substrates are usually hosts for important economic minerals. Planchonella oxyedra Dubard and Trichospermum kjellbergii Burret were found to be hyperaccumulators of nickel and the work was therefore followed up by analysis of further specimens of these two species from localities throughout their range in Southeast Asia and Oceania. The data pinpointed previously known ultrabasic areas in Sulawesi (Celebes) and West Irian (Indonesian New Guinea) as well as one in Ambon (South Moluccas) which is not recorded on geological maps.     

Strontium and its isotopes in Canadian rivers: Fluxes and global implications

Systematic sampling of the 39 largest Canadian rivers shows that the weighted average ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio in the dissolved load is 0.7111, similar to previous measurements on such large rivers as the Amazon and Mississippi. Consequently, we believe that the above estimate is likely representative of the global average. This imposes a limit of 6.5 × 10¹¹g yr^?1 on the amount of Sr exchanged with basalts in hydrothermal cells on mid-oceanic ridges. Evaluation of geological information suggests that Sr from silicate sources is of considerable importance for all but the largest Canadian rivers. The latter have chemical and isotopic composition consistent with ~4:1 carbonate vs. silicate derivation of Sr, but such interpretation is not unique. In terms of their water discharge, the 39 Canadian rivers studied account for 4.2% of the world total and their weighted average concentrations for other dissolved solutes are: TDS 176 ppm, Ca 18 ppm, Cl 6.8 ppm and Sr 84 ppb.     

Natural gas seepage in the Gulf of Mexico

Hydrocarbon compositions and δ¹³C values for methane of fourteen natural seep gases and four underwater vents in the northwestern Gulf of Mexico are reported. The C₁/(C₂ + C₃) ratios of the seep gas samples ranged from 68 to greater than 1000, whereas δ_PDB¹³C values varied from ?39.9 to ?65.5‰. Compositions suggest that eleven of the natural gas seeps are produced by microbial degradation whereas the remaining three have a significant thermocatalytically produced component. Contradictions in the inferences drawn from molecular and isotopic compositions make strict interpretation of the origins of a few of the samples impossible.     

Regolith stripping and the control of shallow translational hillslope failure: application of a two-dimensional coupled soil hydrology-slope stability model, Hawke's Bay, New Zealand

Rainfall-triggered regolith landslides constitute a dominant erosional process in New Zealand hill country. Conversion of forest to pasture about 150 years ago decreased the size of event required to trigger slope failure resulting in greater landslide activity. Recent research indicates that shallow landslides then cause progressive regolith stripping with redeposition at the slope base. The exposed bedrock is less permeable than the pre-existing regolith cover and the redeposited soil has a higher unit weight. Hence, alterations in both hydrological and geotechnical conditions result, changing the triggering thresholds for further failure. In this paper three phases of regolith stripping are investigated using a process-based model, to define variations in both the triggering storms as well as the failure mechanisms under these altered hydrological and geotechnical conditions. Over time, as the regolith is stripped progressively upslope, the threshold for slope failure also changes such that the landscape becomes more stable. However, there is a varying sensitivity to storm type for each of the three phases, which adds complexity to the relationship between regolith stripping and slope failure. Model results suggest different hydrological conditions for slope instability, depending on the degree of stripping and redeposition, adding further complexity. Future advances in process-based modelling are required for detailed investigation of temporal changes in landscape susceptibility to slope failure, and uncertainty in both model parameterisation as well as process representation should be emphasised when applying such models to the long term.     

Modelling the role of climate,vegetation and pedogenesis in shallow translational hillslope failure

Recent research in geomorphology has considered the significance of progressive pedogenesis and climatic change to slope failure initiation for the Holocene, using physically based models. To date, the significance of vegetation change to slope stability has been largely unexplored through modelling, since available physically based models cannot consider vegetation effects directly. To address the existing deficiency this paper adapts, parameterizes and applies a physically based model of slope hydrology and stability to the combined effect of vegetation change and progessive pedogenesis on slope failure initiation. There is considerable debate in the literature concerning the relative significance of climatic change and vegetation modification to slope failure initiation in the Holocene. This paper uses the model to provide additional evidence for situations in which either climatic or vegetation change is significant to slope failure, depending on the prevailing degree of soil development. The results indicate that young podsols appear to be stable under all the climatic and vegetation conditions considered, but mature podsols may be susceptible to failure. Both climate and vegetation influence slope stability, but their relative significance depends on the stage of soil development. In particular, the stability of young soils is influenced considerably by vegetation, while climate assumes greater significance in mature soils. It is recognized that this conclusion is limited to freely draining podsol profiles, and that more research is needed to consider other soil type and vegetation combinations.