4051 research outputs found
Sort by
Hydrological connectivity and ecological functional processes in inland floodplain wetlands: nutrient and carbon cycling
No full textFloodplain wetlands have intricate multi-channeled networks and unpredictable wet and dry phases related to variable hydrological regimes and geomorphic processes such as sedimentation and erosion. Hydrological reconnection of river channels with outer floodplain and wetland habitats initiates mobilisation and transformation of nutrients and carbon in inland floodplain wetlands. In this study, we aim to show habitat-dependent patterns of mobilisation and transformation of nutrients (total and dissolved nitrogen and phosphorus) and dissolved organic carbon (DOC) following environmental water releases, based on the available data from the Murrumbidgee Wetlands, Macquarie Marshes and Gwydir Wetlands. In general, concentrations of nutrients and DOC are lower within channels and higher on the floodplain and in wetlands where shallow inundation and mixing of topsoil with water occurs. Higher concentrations of nutrients and DOC on the floodplain represent a combination of supply from channels coupled with in situ releases from the water-soil interface. The volume, timing, depth, rate of rise and fall, and spatial distribution of water being introduced to floodplain wetlands influence the amount and distribution of nutrients and carbon in these systems. Rates of ecological functional processes such as primary productivity and respiration (or decomposition) are closely related to concentrations of nutrients and DOC. We propose a nutrient-DOC framework, combined with hydrological regimes and geomorphic processes, to better predict and understand the relationship between hydrological connectivity and ecological responses of inland floodplain wetlands. © The Author
Independent Bayesian age modelling in subtropical wetlands to assess the influence of global climate drivers across Australia
No full textRigorously dated, continuous sedimentological records capturing multiple glacial/interglacial cycles
are important for evaluating the magnitude and range of drivers influencing ecosystem change in
Australia. Multi-proxy palaeoenvironmental reconstructions are commonly used to identify changes
in long-term environmental conditions, particularly when exploring the climatic backdrop to
Australia’s large scale faunal extinctions. However, interpretations of these records may not be
straightforward as local and regional climate signals are often mixed in proxy records. In order to
evaluate whether improved temporal constraint can help with differentiating such convoluted
signals, thereby increasing the confidence placed in the role of teleconnections across the Southern
Hemisphere, we present a comprehensively dated 12.7 m (basal age ~130 ka) wetland core
consisting of 21 optically stimulated luminescence and seven radiocarbon ages from North
Stradbroke Island. The amalgamation of stratigraphic information and independent age constraints
within a Bayesian framework, highlights the complex depositional history of Welsby Lagoon between
late MIS 5 and MIS 2. ITRAX core scanning data reveals fluctuations in elemental abundance through
time, in particular the decrease in the amount of aeolian sediment following MIS 3. Variability is
attributed to regional environmental regime changes controlled by global drivers, including Heinrich
events, and the influence of moisture across mainland Australia. The comprehensive dating
approach undertaken at Welsby Lagoon highlights the role that the terrestrial palaeoenvironmental
records of North Stradbroke Island can play in assessing long-term climate drivers across continental
Australia, without relying exclusively on isotopic tuning of remote (ice core or marine) records. © The Author
Understanding human-environment interactions in space and time: 3 case studies from the Holocene in Australia
No full textUnderstanding human-environment interactions has emerged as a prominent research agenda
within both archaeology and palaeoecology. Palaeoenvironmental research has demonstrated the
important role that climate has in determining the development and distribution of ecosystems,
however the impact of humans on ecosystems and of environmental change on humans remains
contentious, particularly when studies of environmental change are conducted in isolation from
archaeology. Here we present three separate analyses of human-environment interactions through
time in Australia that are focussed at range of spatial scales: local, regional and continental. We
highlight a tight coupling between palaeoenvironmental change and human population dynamics at
all of these scales in the case studies and attempt to disentangle the reciprocal relationships
between environmental change and human activity. The governing question underpinning this
presentation is: how does the information provided by palaeoecology and archaeology about
human-environment interactions through time vary with the spatial and temporal scale of the
primary data? © The Author
Integrated age modelling of numerical, correlative and relative dating of a long lake sediment sequence from Orakei maar paleolake, Auckland, New Zealand
No full textAccurate and precise chronologies are fundamental for any successful Quaternary paleo-climate/-
environment reconstruction. Aside from varved lake sequences, all records depend on sediment
core age models developed from a limited number of dated horizons, often with large errors. Hence,
it is crucial to combine every piece of available information on the depositional history of the basin
with modelling tools used for chronology development such as Bacon, a Bayesian age modelling
package. Whilst sediment core chronology development has progressed from linear interpolation
between dated horizons to considering uncertainties and Bayesian accumulation models, these age
models rarely reach the resolution and precision desired for reliable paleo-climatic interpretations,
especially in pre-Holocene sequences as errors increase and radiocarbon dating is not suitable
anymore. We address this issue here in the context of Orakei maar paleolake sequence, Auckland,
New Zealand. This record spans ca. 120 to 10 cal ka BP and is underpinned by tephrochronology,
radiocarbon dating and relative changes in paleointensity of the earth magnetic field. Pre-40 cal ka
BP, the age model relies on comparison with and correlation to the global PISO-1500 paleointensity
stack through dynamic time warping (DTW in R). The chronology for the time interval ca. 50 to 10 cal
ka BP has been estimated in Bacon (rBacon in R) with non-normal error distribution of un-calibrated
ages, variable mean accumulation rates and accounting for many “slumps” (horizons of
instantaneous deposition).The Orakei maar paleo-lake age model is a work in progress but serves as
an example for extended age modelling from lake sediment sequences, particularly beyond the
radiocarbon age limit. Furthermore, the Orakei maar sequence offers a high-resolution and
continuous record of climatic variations that span much of the last glacial cycle and is one of the few
from the mid-latitudes of the Southern Hemisphere. The rarity of records of this type makes Orakei
maar a crucial record for development of an improved understanding of the global climate system
because of its potential to be tied directly to the polar ice core and tropical lake and speleothem
records
Wetland development nutrient accumulation and runoff in New Zealand dairy pastures with very high annual rainfall
No full textAgricultural activity in regions with high annual rainfall raises particular
challenges for managing nutrient runoff. Here we report on a four-year
study of patterns of nutrient accumulation and loss in dairy pastures from a
catchment of the West Coast of the South Island of New Zealand, receiving
up to 3500 mm annual rainfall. Due to poor drainage in the gley soils, farming
practice involves contouring the pasture in broad corrugations, termed
‘humps and hollows’, to improve drainage; grazing is restricted to the
humps, whilst wetland vegetation (predominantly a marsh of Juncus and
Carex species) naturally colonizes the wetter drainage hollows. We aimed to
quantify nutrient accumulation and runoff at this site, paying particular attention
to the function of the wetland hollows in nutrient cycling. Subsurface
flow was found to be negligible in the poorly draining soil, with water loss
and nutrient yields restricted to surface flows. No fertiliser was applied the
first year at one site. Specific nitrogen and phosphorus yields ranged from
11-56 kg N ha-1 yr-1 and 2.0-13.4 kg P ha-1 yr-1. The highest P yield (mostly in
particulate form) was recorded even when no P fertiliser was applied. Wetland
vegetation and soils showed prolonged accumulation of both N and P
throughout their development, but failed to intercept peak events that occurred
when cattle were allowed to graze the pastures following heavy rainfall
events. Potential soil denitrification, measured from denitrification enzyme
activity experiments, was very low, with median values < 50 ng N g-1
h-1 throughout the study, predominantly due to low nitrate concentrations.
We conclude that (i) significant nutrient discharge is a feature of this system;
(ii) the natural wetland formation in drainage hollows is an important nutrient
sink that mitigates discharge; and (iii) much of the remaining discharge
could be ameliorated by improved farming practices. © University, DCE – Danish Centre for Environment and Energ
Quantifiying seasonal-scale changes in El Niño Southern Oscillation for the past millennia
No full textThe El Niño-Southern Oscillation (ENSO) is the greatest source of interannual climate variability, yet model
forecasts of the response of this system to global warming are inconsistent. The brevity of the instrumental
record and lack of detailed knowledge of ENSO under different background states contribute to the
uncertainty.
Here we present a sequence of Porites coral microatoll !18O records from Kiritimati (Christmas) Island in
the central equatorial Pacific showing ENSO variability during discrete “windows” spaced between 1500
and 6000 years ago (mid- to late Holocene), when background climate conditions were different due to
changes in the Earth’s orbit around the sun.
Our sequence includes a 175-year monthly-resolved microatoll !18O record showing ENSO variability 4,300
thousand years ago. The record shows a 60% reduction in the ENSO variance, a stronger annual cycle that
persisted for the full 175 years of the record, and limited low frequency (multi-decadal) modulation of the
ENSO signal. El Niño events were ‘damped’ during their June-December growth phase, but still phaselocked
to the seasonal cycle. La Niña events were reduced and together ENSO seasonal phasing was likely
similar to that observed during the weak ENSO period of the 1920-1950s.Further, results from corals aged between 1,500 and 2,000 years ago also show reduced ENSO, as well as
changes in the contribution of the annual cycle, El Niño and La Niña events to the overall coral !18O signal.
Our results show fundamental metrics on the seasonal characteristics of ENSO during the altered background
conditions of the mid- to late Holocene. The results suggest that Holocene ENSO responded to changes in
orbital forcing and that there was limited, unforced variability. This may have implications for ENSO under
future global warming conditions
Constraining groundwater flow, residence ties, interaquifer mixing and aquifer properties using environmental isotopes in the southeast Murray Basin, Australia
No full textEnvironmental isotopes (particularly δ18O, δ2H, and δ13C values, 87Sr/86Sr ratios, and a14C) constrain geochemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province of the southern Murray Basin. Due to methanogenesis and the variable δ13C values of matrix calcite, δ13C values are highly variable and it is difficult to correct 14C ages using δ13C values alone. In catchments where δ13C values, 87Sr/86Sr ratios, and major ion geochemistry yield similar a14C corrections, ∼15% of the C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be used to correct ages in other catchments. Most groundwater has a14C above background (∼2 pMC) implying that residence times are 30 ka probably resulting from episodic recharge during infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in 14C ages being poorly correlated with distance from the basin margins in many catchments; however, groundwater flow in palaeovalleys where the deeper Calivil–Renmark Formation is coarser grained and has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing. Despite the range of ages, δ18O and δ2H values of groundwater in the Riverine Province do not preserve a record of changing climate; this is probably due to the absence of extreme climatic variations, such as glaciations, and the fact that the area is not significantly impacted by monsoonal systems. © 2020 Elsevier B.
Incorporation and activation of arsenic in MBE-grown HgCdTe
No full textResearch into p-type doping of HgCdTe with arsenic has concentrated on the use of a conventional effusion cell and optimization of growth conditions to achieve an increase in incorporation efficiency. This study investigates the use of a cracker cell, which is now the preferred method of doping HgCdTe due to its higher arsenic incorporation efficiency under optimum growth conditions. A detailed investigation of a number of arsenic doped HgCdTe layers grown on CdZnTe substrates by molecular beam epitaxy using a cracker cell as a source of arsenic is presented. Growth parameters influencing the amount of arsenic incorporated, such as the cracker-cell bulk temperature and substrate temperature, were investigated. Arsenic depth profiles were obtained via detailed secondary ion mass spectrometry where all major constituents in the epilayers were analysed. Magneto-transport Hall measurements were performed on as-grown material and those that underwent high-temperature anneals typical for arsenic activation. Using the quantitative mobility spectrum analysis technique, contributions to total conductivity arising from various carriers present in the samples have been separated. As-grown samples were found to exhibit n-type behaviour consistent with arsenic incorporating on cation sublattice, while samples that underwent high-temperature annealing show partial activation of arsenic with electron compensation. © Copyright 2007 IOP Publishin
Effect of grain size on Hertzian contact damage in 9 mol% Ce-TZP ceramics
No full textThe Hertzian contact damage in 9 mol% Ce-TZP ceramics with different grain sizes has been investigated. Single-cycle tests were conducted on materials of four grain sizes, 1.1, 1.6, 2.2 and 3 μm. The indentation stress–strain curves for all materials show striking nonlinearity and deviation from the Hertzian elastic response, illustrating a significant quasi-plastic component in the contact damage response. Subsurface damage patterns for these four materials are compared and contrasted using a bonded-interface sectioning technique. The transformation and deformation behaviour, characterised using optical and scanning electron microscopy, of the surface and subsurface regions revealed extensive deformation and compression-driven subsurface damage in the materials. Acoustic emission was used as a complementary technique in order to identify the damage processes during a load–unload cycle. Contact deformation and radial bands extending from the indent impressions due to autocatalytic tetragonal–monoclinic transformation are evident in all except the finest grained (1.1 μm) material. Irrespective of grain size there is no evidence of ring or cone cracking with all material showing hemispherical subsurface damage or yield zones resulting from the stress-induced tetragonal–monoclinic (t–m) transformation with extensive distributed microcracking within these areas for the 1.6, 2.2 and 3 μm grain-size materials. © 2002 Elsevier Science Lt
A tribute to Eric Raymond (Lou) Vance: Ceramic materials physicist and nuclear wasteform expert – 15th November, 1942‐7th March, 2019
No full textThis special issue of the Journal of the American Ceramic Society is a tribute to the life and work of Eric Raymond Vance. He was known to most of his friends and colleagues from his youth not as “Eric” or “Raymond” but “Lou” (taken from the name of his father “Albert Louis Vance”). © 1999-2020 John Wiley & Sons, Inc