Canterbury Plains Research Articles

Restoring the amenity and nature conservation values of gravel pits: an ecological restoration and community engagement approach

In lowland areas of New Zealand, there are thousands of ‘gravel pits’ located on public and private land. These are the result of gravel (aggregate) extraction, largely for constructing roads during the 19th century. However, many of these gravel pits have since been used as rubbish dumps. This research concerns a gravel pit on public land adjacent to the small township of Prebbleton on the Canterbury Plains. A proposal was presented to the formally recognised community group and the District Council to convert the gravel pit into a nature reserve and amenity area. The completion of the project depended on several key factors, including the vision of one person, the early preparation and project planning, the locality of the gravel pit, support from local government, management of the project, support from the local community, and the legal status of the site. These key factors, together with the sequence of steps described here, could provide helpful background and guidance for restoration and management of other gravel pits and environmentally degraded areas.

Does farm-scale habitat composition predict pest-bird numbers and distribution?

Context Bird damage to horticultural crops causes significant economic losses for growers around the globe. However, bird damage is unpredictable and pest-bird movements and abundance patterns are poorly understood. Aims To assess whether habitat management is likely to be an effective approach for controlling two pest-bird species (house sparrow, Passer domesticus, and greenfinch, Carduelis chloris) in New Zealand’s arable landscape. Methods Breeding- and winter-bird and habitat surveys were carried out over a 3-year period (2003–05) on 19 1-km × 1-km squares with arable crops on the Canterbury Plains, New Zealand. Bird abundance and/or distribution were analysed with respect to both temporal and spatial (crop- and boundary-habitat composition) variables. Results Estimated breeding-season densities for house sparrow were higher but more stable than those for greenfinch (which increased by 70% over a 3-year period), and for individual farms were more predictable across years than were winter densities. Boundary habitat was the best predictor of bird densities and distribution in the breeding season; features associated with enhanced seed abundance or access were important in the winter. However, habitat composition alone could not account for temporal and spatial variation in bird densities on farms. Conclusions Either habitat management or the reduction of key seed resources could potentially control pest-bird numbers. However, habitat management is likely to have adverse consequences for other important functions (such as the shelter and biodiversity benefits of shelterbelts), and any form of bird control applied at the farm scale is unlikely to be effective. Implications To make control actions both effective and economically viable, a coordinated program involving multiple farms is likely to be needed. We recommend investigating the effectiveness of an experimental manipulation of key food resources at the landscape scale.

Overview of strong-motion data from the Darfield earthquake

The Darfield earthquake of 3rd September 2010 UT and its aftershocks have yielded New Zealand’s richest set of strong-motion data since recording began in the early 1960s. Main-shock accelerograms were returned by 130 sites, ten of which had peak horizontal accelerations in the range 0.3 to 0.82g. One near-fault record, from Greendale, had a peak vertical acceleration of 1.26g. Eighteen records showed peak ground velocities exceeding 0.5 m/s, with three of them exceeding 1 m/s. The records included some with strong long-period directivity pulses, some with other long-period components that were related to a mixture of source and site effects, and some that exhibited the effects of liquefaction at their sites. There were marked differences between records on the deep alluvium of Christchurch City and the Canterbury Plains, and those on shallow stiff soil sites. The strong-motion records provide the opportunity to assess the effects of the earthquake in terms of the ground motions and their relationship to design motions. They also provide an invaluable set of near-source motions for seismological studies. Our report presents an overview of the records and some preliminary findings derived from them.

Previously Unknown Fault Shakes New Zealand's South Island

At 4:35 A.M. local time on 4 September (1635 UTC, 3 September), a previously unrecognized fault system ruptured in the Canterbury region of New Zealand's South Island, producing a moment magnitude (Mw) 7.1 earthquake that caused widespread damage throughout the area. In stark contrast to the 2010 Mw 7.0 Haiti earthquake, no deaths occurred and only two injuries were reported despite the epicenter's location about 40 kilometers west of Christchurch (population ˜386,000). The Canterbury region now faces a rebuilding estimated to cost more than NZ$4 billion (US$2.95 billion).On the positive side, this earthquake has provided an opportunity to document the dynamics and effects of a major strike‐slip fault rupture in the absence of death or serious injury. The low‐relief and well‐maintained agricultural landscape of the Canterbury Plains helped scientists characterize very subtle earthquake‐related ground deformation at high resolution, helping to classify the earthquake's basic geological features [Quigley et al., 2010]. The prompt mobilization of collaborating scientific teams allowed for rapid data capture immediately after the earthquake, and new scientific programs directed at developing a greater understanding of this event are under way.

High‐resolution seismic images of potentially seismogenic structures beneath the northwest Canterbury Plains, New Zealand

The transpressional boundary between the Australian and Pacific plates in the central South Island of New Zealand comprises the Alpine Fault and a broad region of distributed strain concentrated in the Southern Alps but encompassing regions further to the east, including the northwest Canterbury Plains. Low to moderate levels of seismicity (e.g., 2 > M 5 events since 1974 and 2 > M 4.0 in 2009) and Holocene sediments offset or disrupted along rare exposed active fault segments are evidence for ongoing tectonism in the northwest plains, the surface topography of which is remarkably flat and even. Because the geology underlying the late Quaternary alluvial fan deposits that carpet most of the plains is not established, the detailed tectonic evolution of this region and the potential for larger earthquakes is only poorly understood. To address these issues, we have processed and interpreted high‐resolution (2.5 m subsurface sampling interval) seismic data acquired along lines strategically located relative to extensive rock exposures to the north, west, and southwest and rare exposures to the east. Geological information provided by these rock exposures offer important constraints on the interpretation of the seismic data. The processed seismic reflection sections image a variably thick layer of generally undisturbed younger (i.e., < 24 ka) Quaternary alluvial sediments unconformably overlying an older (>59 ka) Quaternary sedimentary sequence that shows evidence of moderate faulting and folding during and subsequent to deposition. These Quaternary units are in unconformable contact with Late Cretaceous–Tertiary interbedded sedimentary and volcanic rocks that are highly faulted, folded, and tilted. The lowest imaged unit is largely reflection‐free Permian ‐ Triassic basement rocks. Quaternary‐age deformation has affected all the rocks underlying the younger alluvial sediments, and there is evidence for ongoing deformation. Eight primary and numerous secondary faults as well as a major anticlinal fold are revealed on the seismic sections. Folded sedimentary and volcanic units are observed in the hanging walls and footwalls of most faults. Five of the primary faults represent plausible extensions of mapped faults, three of which are active. The major anticlinal fold is the probable continuation of known active structure. A magnitude 7.1 earthquake occurred on 4 September 2010 near the southeastern edge of our study area. This predominantly right‐lateral strike‐slip event and numerous aftershocks (ten with magnitudes ≥5 within one week of the main event) highlight the primary message of our paper: that the generally flat and topographically featureless Canterbury Plains is underlain by a network of active faults that have the potential to generate significant earthquakes.

Modeling of Nonequilibrium Bromide Transport through Alluvial Gravel Vadose Zones

Understanding solute transport and flux through the vadose zone is important for predicting potential contaminant loading to groundwater systems. Two dual‐permeability models: mixing cell (MC) and HYDRUS 1D (H1DDP) were used to simulate long‐term nonequilibrium Br− leaching using data derived from suction cup samples at two field sites (sampled to 7‐ and 4‐m depths) on the Canterbury Plains, New Zealand. Effective model parameters were derived by inverse methods to represent average transport processes through the heterogeneous profiles. Suction cup samples indicated rapid initial movement of solute through the profiles followed by a long tail. The “management type” MC model results were comparable to the more complex HYDRUS model, providing similar overall fits to the observed data (similar RMSE ∼ 0.05–0.1). Modeling indicated that macropore flux was significant in transporting solute through the profile (MC range 5–59%; H1DDP range 15–54% of total fluxes) and simulations of cumulative fluxes from both models were similar. The MC model results indicated macropore transporting water contents of ∼0.003 (v/v) and matrix domain transporting water contents of around 0.1 to 0.15 (v/v) for the two field sites. These estimates suggest that only 3 mm of bulk drainage is required to transport solutes 1 m through the macropore domain, whereas approximately 100 to 150 mm of drainage is required to transport solutes the same distance through the matrix domain. More accurate representation of boundary conditions, texture spatial distributions, and hydraulic interactions is important for obtaining a better understanding of flux dynamics in future studies.

Nitrous oxide emissions from animal urine as affected by season and a nitrification inhibitor dicyandiamide

Nitrous oxide emissions from pasture soils account for one third of total agricultural greenhouse gas emissions in New Zealand. The aim of this study was to determine nitrous oxide (N2O) emissions from animal urine patches under summer (with irrigation) and winter conditions as affected by dicyandiamide (DCD) in grazed grassland in New Zealand. Sixteen monolith lysimeters were collected from an established perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture on a Templeton fine sandy loam (Udic Ustochrept) located near Lincoln University on the Canterbury plains of New Zealand to study the N2O emissions from animal urine as affected by season and a nitrification inhibitor dicyandiamide. The experiment was conducted in the summer of 2008–2009 and the winter of 2009 with four treatments (control, control + DCD (10 kg ha−1), urine (1,000 kg N ha−1), urine + DCD) for both seasons. N2O fluxes from the pasture were measured in summer and winter. Results showed that the application of urine significantly increased N2O emissions and the application of nitrification inhibitor, DCD, to pasture soil was very effective in reducing N2O emissions both in summer and winter. Total N emissions from urine-N applied at 1,000 kg N ha−1 were 7.8 kg N2O–N ha−1 in the summer season and this was much lower than the 12.7 kg N2O–N ha−1 emitted during the winter season. The application of DCD reduced summer N2O–N emissions to 4.8 kg N2O–N ha−1 and winter N2O–N emissions to 3.9 kg N2O–N ha−1, representing 40% and 69% reductions in the summer and winter seasons, respectively. The results demonstrate that N2O emissions from animal urine-N in grazed pastures were much higher in the winter season than in the summer season with irrigation and that DCD was more effective in reducing N2O emissions in the high-emitting winter season.

First seismic imaging results of tectonically complex structures at shallow depths beneath the northwest Canterbury Plains, New Zealand

Seismogenic structures underlie many regions of the vast Canterbury Plains on the South Island of New Zealand. Most of these structures are hidden beneath a layer of rapidly deposited Late Pleistocene sediments, the youth and thickness of which make the general application of conventional paleoseismological studies impractical. In an attempt to improve our understanding of potentially active structures in this region, we have acquired, processed and interpreted shallow seismic reflection data across the northwest Canterbury Plains. To separate the useful reflected signals from unusually high amplitude ambient and source-generated noise, we subjected the data to a specially tailored processing scheme that included time- and space-variant spectral balancing, custom static corrections and mutes, F-X deconvolution, DMO corrections and finite-difference migration. The final stacked and migrated seismic sections supply high-resolution images of the basement and overlying layered Cretaceous- to Quaternary-age supracrustal rocks that have been complexly faulted and folded. At one location, the uppermost Late Pleistocene layers appear to have been gently buckled.

Effect of pH on Zinc Sorption–Desorption by Soils

The objective of this study was to examine the effect of soil pH on zinc (Zn) sorption and desorption for four surface soils from the Canterbury Plains region of New Zealand. Zinc sorption by the soils, adjusted to different pH values, was measured from various initial solution Zn concentrations in the presence of 0.01 M calcium nitrate [Ca(NO3)2]. Zinc desorption isotherms were derived from the cumulative Zn desorbed (µg g−1 soil) after each of 10 desorption periods by sequentially suspending the same soil samples in fresh Zn‐free 0.01 M Ca(NO3)2. Zinc sorption and desorption varied widely with soil pH. Desorption of both native and added Zn decreased continuously with rising pH and became very low at pH values greater than 6.5. The proportion of sorbed Zn that could be desorbed back into solution decreased substantially as pH increased to more than 5.5. However, there were differences between soils regarding the extent of the hysteresis effect.

Australian black beetle expansion through Christchurch city highlights a risk to pastoral agriculture on the Canterbury plains

The Australian black beetle Adoryphorus couloni (also known as the redheaded cockchafer) was found in large numbers in Barrington Park in Christchurch in May 2008 Severely damaged areas comprised 1020 of the park with populations reaching over 300 larvae/m2 The thatch layer produced on sports fields appears ideal for this insect which favours grasslands with high organic matter levels and could explain the very high grub numbers To reach a population of this size A couloni had probably colonised and remained undetected in the park for several years In Australia the insect is a serious pest of pastures Severe damage occurred in Tasmania in the 1980s and 1990s when the insect spread through the central Midlands stripping bare thousands of hectares of pasture The beetle was accidentally introduced to Canterbury through Lyttelton harbour in the 1960s and has slowly spread around the Port Hills in lowquality hill pastures The outbreak in Christchurch marks the first move of this pest into habitat favourable for development of populations large enough to pose a serious risk of further westward expansion onto the Canterbury plains AgResearch is assisting the Christchurch City Council with identification and control recommendations for the pest and is evaluating potential biocontrol measures

Seismic hazard of the Canterbury region, New Zealand

We present a new probabilistic seismic hazard model for the Canterbury region, the model superseding the earlier model of Stirling et al. (1999, 2001). The updated model incorporates new onshore and offshore fault data, new seismicity data, new methods for the earthquake source parameterisation of both datasets, and new methods for estimation of the expected levels of Modified Mercalli Intensity (MMI) across the region. While the overall regional pattern of estimated hazard has not changed since the earlier seismic hazard model, there have been slight reductions in hazard in some areas (western Canterbury Plains and eastern Southern Alps), coupled with significant increases in hazard in one area (immediately northeast of Kaikoura). The changes to estimated acceleration for the new versus older model serve to show the extent that major changes to a multidisciplinary source model may impact the final estimates of hazard, while the new MMI estimates show the added impact of a new methodology for calculating MMI hazard.

Reconstruction of a daily flow record along a hydrologically complex alluvial river

Summary Complex spatial and temporal flow patterns in alluvial plain rivers can be caused by variation in groundwater surface-water exchange, channel planform, and climatic variation. Such complexity poses a challenge for developing relationships between runoff, groundwater recharge, and river flow, for predicting effects of water resource developments, and for understanding hydrologic effects on ecological processes. The Selwyn River of New Zealand is one such hydrologically complex river. To begin to understand the spatio-temporal flow patterns of the Selwyn River, we used linear and logistic models to reconstruct a 22-yr record of river flow across the alluvial Canterbury Plains. The 60 km ‐ long model domain encompasses perennial, ephemeral, and intermittent reaches embedded within larger effluent and influent sections. Flow at 18 cross-sections distributed along the river mainstem was modelled as a function of flow at stage recorders located at each end of the domain with flow interpolated between cross-sections to generate a flow record continuous in space. Data from 38-months of flow-gauging at the crosssections was used to calibrate model parameters. The reconstructed record indicates that while the central section of the river was dry during most of the 38-month observation period, it was not uncommon during the preceding two decades for the river to flow along its entire course for several months consecutively. The recent dry period may be part of a longer trend: the mean annual simulated length of dry river channel has increased by 0.6 km yr � 1 over the last two decades. Output metrics from the model can increase our understanding of hydrologic control of ecological processes in alluvial rivers. We provide several examples of potential model applications to ecological studies. a 2008 Elsevier B.V. All rights reserved.

Relevance of exotic pine plantations as a surrogate habitat for ground beetles (Carabidae) where native forest is rare

Plantation forests are of increasing importance worldwide for wood and fibre production, and in some areas they are the only forest cover. Here we investigate the potential role of exotic plantations in supporting native forest-dwelling carabid beetles in regions that have experienced extensive deforestation. On the Canterbury Plains of New Zealand, more than 99% of the previous native forest cover has been lost, and today exotic pine (Pinus radiata) plantations are the only forest habitat of substantial area. Carabids were caught with pitfall traps in native kanuka (Kunzea ericoides) forest remnants and in a neighbouring pine plantation, grassland and gorse (Ulex europaeus) shrubland. A total of 2,700 individuals were caught, with significantly greater abundance in traps in young pine, grassland and gorse habitats than in kanuka and older pine. Rarefied species richness was greatest in kanuka, a habitat that supported two forest specialist species not present in other habitat types. A critically endangered species was found only in the exotic plantation forest, which also acts as a surrogate habitat for most carabids associated with kanuka forest. The few remaining native forest patches are of critical importance to conservation on the Canterbury Plains, but in the absence of larger native forest areas plantation forests are more valuable for carabid conservation than the exotic grassland that dominates the region.

Patterns of host damage by the cabbage tree monophage Epiphryne verriculata Feld (Lepidoptera: Geometridae) across urban, rural and native forest habitats

Little is known about the persistence of host-specialised insect herbivores on their host plants in highly modifi ed urban and rural landscapes, and the ecological processes driving their abundance and distribution in these landscapes. We surveyed cabbage trees (Cordyline australis) at 13 sites encompassing a variety of habitat types located across a highly modifi ed landscape, the Canterbury Plains, for damage by the larvae of the endemic monophagous geometrid moth, Epiphryne verriculata. We recorded the relative amount of E. verriculata herbivory on cabbage tree crowns and measured a number of environmental variables we expected would explain variation in this herbivory. While E. verriculata occurred at all sampled sites and on 244 of the 257 sampled trees, there was substantial variation among trees in the amount of herbivory. Epiphryne verriculata herbivory per tree was higher on adult trees than seedlings and was highest, unexpectedly, in areas of low con specific density within 25 m. We found no effects on herbivory of substrate nor the presence of a skirt of dead leaves, which has been suggested to be an important refuge for adult E. verriculata. The amount of herbivory differed among habitat types, with trees in native forest sites having signifi cantly higher amounts of damage than those in urban and rural sites. This pattern has been found in other New Zealand insect herbivore systems, and may represent a general pattern in the New Zealand insect herbivore fauna. We predict that this pattern is not a result of dispersal limitation in E. verriculata, but rather ecological processes increasing larval mortality, and perhaps decreasing adult fecundity and oviposition, within urban and rural habitats.

Sustained Alluvial Aggradation and Autogenic Detachment of the Alluvial River from the Shoreline in Response to Steady Fall of Relative Sea Level

Abstract Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant conditions of relative sea-level fall, sediment discharge, and water discharge, demonstrates that sustained alluvial aggradation is the inherent stratigraphic response of alluvial rivers that are steeper than the receiving basin. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (low rate of terrestrial diffusion). During the course of the experiments, fluvial grade was never reached, fluvial incision was never observed, and aggradation occurred independently of shoreline position. The experiments revealed detachment of the shoreline from the alluvial river as an autogenic response of certain fluviodeltaic systems to steady relative sea-level fall, creating a non-deltaic alluvial-river system that continues to aggrade beyond shoreline detachment. We term this behavior autodetachment. Autodetachment is a consequence of sustained, extensive alluvial aggradation during relative sea-level fall. Sediment supply was increasingly partitioned into the ever-enlarging alluvial profile to the detriment of the delta front prior to shoreline detachment. Thus, as relative sea level fell, the length of the delta front decreased to zero. Simultaneously, alluvial aggradation also led to decreasing rates of delta progradation. Therefore, as the delta front disappeared, the rate of shoreline regression exceeded the rate of progradation, producing a bedrock river between the detached alluvial river and the shoreline. The Canterbury Plains of New Zealand is a natural-world system with alluvial gradient greater than shelf gradient. Geometric modeling suggests that autodetachment could have occurred at the Canterbury Plains within the timescale of Quaternary glacio-eustatic sea-level falls, and therefore, is a realistic stratigraphic response at the scale of natural-world systems. Our model demonstrates that deltas with these geometric parameters become poorly supplied during regression, and large lowstand deltas should not be expected for these types of systems. The results of this study complement existing autostratigraphic theory, and provide an alternative to allogenic drivers for geomorphic and architectural evolution of fluviodeltaic wedges.

Back from extinction: Rediscovery of the Canterbury knobbled weevil Hadramphus tuberculatus (Pascoe 1877) (Coleoptera: Curculionidae), with a review of its historical distribution

The Canterbury knobbled weevil, Hadramphus tuberculatus (Coleoptera: Curculionidae: Molytini), was once widespread in the Canterbury plains, foothills and fringing ranges, ranging from Oxford in the north to Waimate in the south. Habitat degradation, reduction in host plant numbers through grazing and predation by rodents are thought to be responsible for the decline in its numbers and distribution. In December 2004, more than 80 years since the last sighting of H. tuberculatus in 1922, a single individual was found at Burkes Pass Scenic Reserve. This paper records the rediscovery of H. tuberculatus, reviews the historical distribution records for H. tuberculatus, discusses them with respect to changes in host plant habitat in the eastern South Island, and suggests conservation strategies to protect this and other relictual populations that might be found in future.

Predicting weed distribution at the landscape scale: using naturalized Brassica as a model system

Summary Quantifying and predicting the distribution of naturalized plant species is a major concern for weed risk assessment and understanding the potential impacts of gene‐flow between crop species and feral or naturalized populations. We developed a rapid field assessment to quantify the distribution of naturalized Brassica populations at the landscape scale, using the Canterbury Plains region of New Zealand as a model system. Internationally, brassicas are one of the most widely cultivated crops, and have well‐documented problems of crop escapes, gene‐flow and hybridization with wild relatives. Brassicas are cultivated intensively in the study system for both seed production and forage, are widely naturalized and have raised concerns for gene‐flow from feral populations to cultivated varieties. Generalized linear models (GLM) were constructed in order to predict the presence–absence of brassicas using survey data of naturalized populations located in 50 3 × 3 km plots. This model was validated subsequently both by ground‐truthing predicted presence–absence in a second field survey, and by comparing cumulative probability distributions from a presence‐only botanical survey. The best model produced from the field survey predicted naturalized Brassica presence–absence robustly (r2 = 0·52) and was well validated against results from the presence‐only survey carried out in the same year. Results from the second field survey during a different growing season did not match predictions from the original model, suggesting that strong between‐year differences influence large‐scale Brassica distribution. The differences between years were driven by an overall increase in the prevalence of brassicas over the course of the study that was not associated with any predictive variable or climate. Synthesis and applications. These results have important implications both for developing models of species distribution and for predicting gene‐flow risk at the landscape scale. Despite constructing a model that was subsequently validated from presence‐only data in one year, it did not predict Brassica distribution in a subsequent year; this suggests that between‐year or stochastic factors operate strongly and are a major consideration for risk assessment. Models used to predict species distributions, in general, need to account for these between‐year effects either by incorporating mechanistic processes or long‐term data.

A good servant but a tyrannous master: Gorse in New Zealand

The hedge plant gorse was introduced to New Zealand as a living fence and became a prominent feature of the landscape, particularly on the Canterbury Plains. Escaping from cultivation, gorse commenced its second life in New Zealand, that of a noxious weed troublesome to pastoral agriculture. In the twentieth century it came to be regarded as an invasive, exotic species that threatened indigenous plants and landscapes. Throughout its history in New Zealand, gorse, a highly significant element in the New Zealand environment, has been the object of ambivalent attitudes—a challenge to simplified conceptions of “native,” “exotic,” and “nature.”

Zinc sorption–desorption by soils: Effect of concentration and length of contact period

Zinc sorption/desorption phenomena in soils are important in determining the bioavailability of soil Zn to plants. Some soils of the Canterbury Plains, New Zealand are regarded as potentially Zn deficient, however there is little information available for these soils, or in fact for soils in general, on the factors that influence the sorption and desorption of soil Zn. In order to increase our understanding of soil Zn sorption/desorption phenomena, and hence improve our understanding of Zn bioavailability, the effect of Zn concentration and length of contact time on Zn sorption by, and desorption from, four surface soils from the Canterbury Plains has been examined. Zinc sorption isotherms were derived by sorption of Zn from solutions containing a range of Zn concentrations equivalent to 6, 12, 18, 24 and 30 μg g − 1 soil in the presence of a background electrolyte, 0.01 M Ca(NO 3) 2. Zinc desorption isotherms were derived from the Zn desorbed (μg g − 1 soil) after each of ten desorption periods, carried out by sequentially equilibrating single soil samples in Zn-free 0.01 M Ca(NO 3) 2. To establish changes in the desorbability of added Zn as a result of its reaction with soil, an aerobic incubation technique was used. The amount and pattern of release of Zn was markedly different between the experimental soils, indicating their differing capacities for the continuous supply of Zn to the soil solution. The cumulative desorption of Zn occurred more rapidly with the Lismore and Summit soils (lower Zn retention capacity) than with the Temuka and Waimakariri soils. Added Zn was found to desorb back readily from the Lismore and Summit soils, and Zn desorption appeared to closely follow the original sorption isotherms irrespective of the concentration of Zn added to these soils. This suggests that in these soils, desorption reactions could be described by the original sorption isotherm. The Temuka and to a lesser extent the Waimakariri soil, which have a higher capacity to retain added Zn showed poor reversibility of Zn sorption, there being a hysteresis effect in the desorption of recently sorbed Zn. For three of the four soils studied, the longer the contact period that elapsed before starting desorption, the less complete the Zn desorption that was achieved.

An unusual fluvial to tidal transition in the mesoarchean Moodies Group, South Africa: A response to high tidal range and active tectonics

The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels. The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.