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Constraining the sources of the CH4 increase during the Oldest Dryas-Bølling abrupt warming event using 14CH4 measurements from Taylor Glacier, Antarctica
No full textMethane (CH4) is an important greenhouse gas with both natural and anthropogenic
sources. Understanding how the natural CH4 budget has changed in response to changing
climate in the past can provide insights on the sensitivity of the natural CH4 emissions to the
current anthropogenic warming. Low latitude wetlands are the largest natural source of
CH¬4 to the atmosphere. It has been proposed, however, that in the future warming world
emissions from marine CH4 clathrates and Arctic permafrost might increase significantly.
CH4 isotopes from ice cores in Greenland and Antarctica have been used to constrain the
past CH¬4 budget. 14CH4 is unique in its ability to unambiguously distinguish between
“old” CH4 sources (e.g. marine clathrate, geologic sources, old permafrost) and “modern”
CH4 sources (e.g. tropical and boreal wetlands). We have successfully collected six large
volume (~1000 kg) samples of ancient ice from Taylor Glacier, Antarctica that span the
Oldest Dryas – Bølling (OD-BO) CH4 transition (~14.5ka). The OD-BO is the first large
abrupt CH4 increase following the Last Glacial Maximum, with atmospheric CH4 increasing
by ≈30% in the span of ≈ 200 years. All samples have recently been successfully measured
for 14CH4, δ13C-CH4, and δD-CH4. 14CH4 measurements of accompanying procedural
blanks show that effects from extraneous carbon addition during processing are small.
Results are currently undergoing corrections for in-situ cosmogenic 14C based on 14CO
measurements in the same samples. We will present the corrected 14CH4 results and
preliminary interpretation with regard to causes of the OD-BO CH4 increase
Dating Antarctic ice cores using high-temporal resolution black carbon records
No full textBlack carbon aerosols (BC) emitted by fires in the Southern Hemisphere (SH) are transported to
Antarctica and preserved in the Antarctic ice sheet. Recent efforts to develop ice core records of
BC deposition to Antarctica show variability in BC over a broad range of time scales. The ~
monthly-resolution BC record from the WAIS divide deep ice core displayed strong seasonal
variability in modern sections of the record consistent with the timing of SH biomass burning. The
record was subsequently used as an annual layer dating proxy in conjunction with other chemical
species. If the emissions and transport of BC to Antarctica are stable over long periods of time it
may be useful as an annual layer proxy at sites other than WAIS. To date, a rigorous comparison of
Antarctic ice core BC seasonality from different locations have not been conducted. Here we
present a comparison of BC ice core data from the top sections of the WAIS divide deep core, the
Roosevelt Island RICE core, and the Law Dome DSS1213 core. The RICE and Law Dome sites are
separated from WAIS by large distances and experience different atmospheric circulation and
climate regimes. A detailed description of the data uncertainties and its use in annual layer
counting will be discussed
A Monte Carlo software for the 1-dimensional simulation of IBIC experiments
No full textThe ion beam induced charge (IBIC) microscopy is a valuable tool for the analysis of the electronic properties of semiconductors. In this work, a recently developed Monte Carlo approach for the simulation of IBIC experiments is presented along with a self-standing software equipped with graphical user interface. The method is based on the probabilistic interpretation of the excess charge carrier continuity equations and it offers to the end-user the full control not only of the physical properties ruling the induced charge formation mechanism (i.e., mobility, lifetime, electrostatics, device’s geometry), but also of the relevant experimental conditions (ionization profiles, beam dispersion, electronic noise) affecting the measurement of the IBIC pulses. Moreover, the software implements a novel model for the quantitative evaluation of the radiation damage effects on the charge collection efficiency degradation of ion-beam-irradiated devices. The reliability of the model implementation is then validated against a benchmark IBIC experiment. © 2014 Elsevier B.V
Global distribution and controls on cave drip water oxygen isotope composition
No full textThe oxygen isotope composition of speleothems is a widely utilised paleoclimate proxy that is
responsible for the current state-of-knowledge of past Asian monsoon dynamics, the timing of
glacial-interglacial cycles, and the insolation control on inter-tropical convergence zone position,
among other climate processes. Because speleothems are deposited by cave drip water, and this is
derived from meteoric precipitation, it is critical to understand the empirical relationship between
precipitation and cave drip water d18O. Here, we present the first global analysis, based on data
from 148 drip sites, 38 caves, and five continents. Globally, drip water d18O is most similar to the
amount-weighted precipitation d18O where mean annual temperature (MAT) is < 10 °C. For
seasonal climates with MAT > 10 °C and < 16 °C, we demonstrate that drip water d18O records
the recharge-weighted d18O. Our analysis implies that speleothems (formed in near isotopic
equilibrium) are most likely to have d18O that directly reflect meteoric precipitation only in cool
climates. In warmer and drier environments, speleothems will have a seasonal bias toward the
precipitation d18O of recharge periods and, in some cases, the extent of evaporative fractionation of
stored karst water. We highlight the implications of our analysis for the interpretation of oxygen
isotope records in Australasian speleothems. © The Author
The evolution of Thirlmere lakes: a long-term sedimentary record of climate and fire dynamics in the Sydney Basin
No full textThe Thirlmere lakes are located 40 km from the coast and are at ~300 m elevation and fall within the
Greater Blue Mountains World Heritage area. The series of five lakes sit within a narrow and sinuous
former river valley within the Hawkesbury sandstone with surrounding dry sclerophyll forest. Recent
declines in water levels have prompted the NSW Office of Environment and Heritage to fund
research about the history of Thirlmere lakes, the sub-surface characteristics and the potential
frequency of past drying. This research builds on some existing work and has highlighted the
extraordinary potential for the region for a long-term archive for palaeoenvironmental research.To
date we have taken multiple vibracores across three lakes to depths of 7 m and we have
supplemented this with some preliminary deep drilling to depths of 14 m. Our initial chronology is
based on radiocarbon and OSL and we have employed a raft of geochemical and palaeoecological
techniques to investigate changes through time. The lakes contain excellent organic preservation
with deposition of the ‘modern’ peat environments commencing ~11 ka across two of the lakes
investigated. This phase is represented by the upper 2 -3 m of organic rich peat (50% TOC). The
underlying sediments are a mix of weakly bedded organic clays and oxidised clay facies that
represent lake-wide drying intervals, a sequence that is repeated down profile. All five lakes are
separated by alluvial sills that are comprised of medium to well-sorted sands, interbedded with
organic ‘marker’ horizons that indicate these separate lakes were once joined, prior to the Last
Glacial Maximum. The sandy sills that separate the lakes are derived from tributary alluvial fans
accumulating progressively over the Holocene and effectively blocking and separating the lakes into
their current configuration. This paper provides a preliminary overview of the chrono-stratigraphic
history of Thirlmere lakes. © The Author
Estimating diffusion in heterogeneous groundwater systems using short-lived radioisotopes and stable isotopes of iodine or bromine
No full textDiffusion is an important and ubiquitous phenomenon in nature, but too often neglected or unmeasured in water
resource hydrogeology or solute transport. Diffusion may, in fact, be the dominant process that dictates
hydrogeochemistry and affects tracers. Conservative and age tracers are commonly used for water resource or
contaminant plume transport estimations, but will give misleading results, if diffusion is ignored. Diffusion of tracers
into aquitards, matrix pores of fractured rocks, blind fractures or other low conductivity zones lead to retardation
and possible adsorption, exchange, precipitation or decay. This becomes increasingly important as heterogeneity of
flow domains increase, for example, in fractured rock aquifers, interlayered sediments or aquifers associated with
aquitards. Traditional methods of measuring diffusion coefficients in small slices of heterogeneous rock are
unreliable for upscaling, so this study presents an alternative method based on lab-scale drill-core tests and suggests
field-scale borehole tracer tests.
Fick’s first law shows that diffusion rates are driven by concentration gradients. Short-lived radiotracers soon reach a
steady-state concentration gradient with enhanced flux where diffusion rate equals decay, while stable tracers trend
towards saturation and ever-decreasing fluxes. We compare diffusion of short-lived radiotracers 131I (half-life 8 days)
or 82Br (half-life 1.5 days) to their stable equivalent (I or Br) into 45–50 cm lengths of ~60 mm diameter drill core.
Five cores were selected from three fractured rock environments: sandstone, limestone and metavolcanics. By
regularly sampling and refilling the annulus with tracers around the enclosed core, we are able to discern differential
in-diffusion between stable- and radio-tracers.
For example, the annulus was sampled and refilled weekly with an (equivalent decayed) 131I activity of 22 Bq/g and
within three weeks (2.6 half-lives) each core had reached a characteristic steady state flux. The net fluxes were
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around 1.0 Bq/cm2
/week in the sandstone cores, 0.2 Bq/cm2
/week in the metavolcanics, and 0.05 Bq/cm2
/week in
the limestone. This was compared to stable iodine weekly refills at 2.6 mg/L, which gave ever-diminishing diffusion
results. The net fluxes of stable iodine diminished steadily, e.g. over three weeks from 98 to 26 ng/cm2
/week for the
most porous (medium sandstone ~15%), and from 11 to 7 ng/cm2
/week for the least porous (limestone ~2%).
Experiments were also performed using 82Br and stable bromine, with sampling and refills performed on a daily (0.68
half-lives) basis. Similar trends were apparent, though the data was noisier due to more frequent refills and less time
for diffusion to generate significant changes in the annulus reservoir solutions.
This method enables analysis of drill cores for comparative effective diffusion coefficients of different systems.
Quantitative interpretation is currently being refined. In principle, the method should be transferrable to single
boreholes or tracer tests between multiple boreholes to gain larger scale representation of effective diffusion within
a groundwater system. The normally confounding factors, such as dilution, advection, exchange, adsorption and
precipitation, are negated by comparing the stable with radio-tracer results, as all isotopes of these elements are
identically affected by such processes and losses. © Geological Society of Australia Inc
Groundwater modernisation and associated chemical changes in a Hawkesbury sandstone acquifer (Kulnura–Mangrove Mountain, NSW, Australia)
No full textLand and groundwater usage has the potential to influence the groundwater chemistry of an aquifer. Progressive
modernisation of groundwater, variation in pH and associated water/rock reactions have been identified in areas of
the Kulnura–Mangrove Mountain aquifer (KMMA).
Detailed temporal and spatial sampling of groundwater (general hydrogeochemistry, H2O stable isotopes,
13CDIC,
3H,
14C and 87Sr/86Sr) revealed important inter-annual variations driven by groundwater extraction showing a progressive
influx of modern groundwater at >100 m depth in some areas. In the Peats Ridge plateau, shallow groundwater
samples show high 14C bomb pulse signatures, indicating modern recharged groundwater, while deeper
groundwater shows a yearly increase in modern 14C inputs, instead of lower a14C values, as observed in other wells
and generally expected. Values evolved from 36.1 pMC (5.2 ka BP) in 2007, to modern values of 103 pMC in 2010
with the latest sample in 2012 failing to graphitise, probably due to the high CO2 generally linked in the study area
with modern groundwater. The 3H activities have also evolved from values below the quantification limit in 2007 and
2008 to values of ~1.1 TU in 2012.
The minimal buffering capacity of the quartzose sandstone aquifer, at least in its upper zone where dispersed
carbonates have long been dissolved, means that shallow groundwater generally has a low pH. Limited historical
data (1998) shows higher pH for all samples compared to the same wells analysed for this work. However, it is in the
central area where pH changes are most evident. During 2007, groundwater pH was similar to that expected for
samples at similar depths with consistent groundwater residence times; however, successive samples show a shift to
lower pH similar to those found in much shallower samples, as well as modern groundwater ages. Groundwater
extraction is therefore causing an inflow of modern waters at depth with associated acidification.
An important consequence of acidification is the capacity to mobilise trace metals. Of particular interest is
aluminium as it has been linked to enhanced risks of cognitive decline for subjects with a high daily intake from
drinking water (≥ 3.7 µM·day−1
). Shallow samples in the Mangrove Mountain area and some of the deeper samples
with Al concentrations of ~3.45 µM are a risk for average drinking water intakes. The movement of low pH shallow
groundwater is causing an increase in Al concentrations, particularly in the central area of the KMMA, and this may
be affecting groundwater for local consumption or that recovered in bottling plants. © Geological Society of Australia In
Investigating the origin of salinity and aquifier interaction in a seasonally pumped confined aquifer system in Southeast Australia (Western Port Basin)
No full textThis ongoing study aims to identify the different sources of fresh/saline water, pathways, mechanisms of
groundwater salinisation, and factors controlling salt-water intrusion in the Western Port Basin aquifer (Victoria,
Australia). The multi-layered aquifer is characterised by variable distributions of fresh and saline groundwater at the
coastline, which are speculated to result due to geological heterogeneity, and the influence of past sea levels. The
spatial salinity distribution observed indicates that the typical situation of a dense saltwater wedge underlying fresh
groundwater does not apply. Further to natural drivers, an understanding of the potential short and long term
effects of seasonal pumping for irrigation in the basin is desired, particularly whether it induces migration of water
from other areas in the basin or causes leakage from low-permeability horizons.
The impact of seasonal pumping on the variation and distribution of salinity was examined at a nested bore site 500
m from the coast. Analyses of chloride concentration and 18O in groundwater indicate various degrees of mixing
between seawater and fresh meteoric groundwater. Multi-depth sampling in the upper aquifer, identified high
salinity water with ~19% seawater mixture above slightly less saline water – with ~17% seawater; while samples
from the lower aquifer yielded significantly fresher groundwater (~4 to 7% seawater). This is likely to be due to the
relatively good connection between the upper aquifer and the ocean, and low-permeability material separating the
shallow and deeper aquifers. Seasonal salinity changes were observed as a response to pumping, with salinity in the
upper aquifer decreased by ~15.5% at the onset of pumping, and salinisation gradually increased once pumping
ceased by ~22%.
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Recent 14C dating of groundwater in the proximity of the pumping area revealed low radiocarbon activities (<25
pMC) and a lack of tritium, which rules out the possibility of modern recharge reaching the pumping area. A thick
Holocene clay cap (ranging from 10 to 30 m) covering significant areas and extending beneath the bay, limits rainfall
recharge to the basin margins where the aquifers outcrop. Beneath the shallow bay a paleoriver channel incised
deep into the bay floor sediments expose the aquifer to potential vertical and horizontal input from modern
seawater. However, the lack of long-term salinisation despite decades of pumping, and the relatively old
groundwater ages, imply buffering against saline intrusion by stores of relict fresh water. Incorporation of these
results into a conceptual model will contribute to an improved understanding of the groundwater flow system and
the long-term implications of pumping on future groundwater sustainability in the Western Port Basin. © Geological Society of Australia In
Three-dimensional stress distribution of surplus assembled 7050Al alloy ring and plug determined by neutron diffraction
No full textThe hoop, radial and axial stress and strain distribution of surplus assembled 7050Al alloy ring and plus were investigated by neutron diffraction method. The measured results from crystal planes (200) and (311) are coincided with the data obtained by analytical calculation and international standard sample experiment. The discontinuous stress and strain at the interference of ring and plug can be clearly shown by the neutron diffraction method. Monte Carlo method was used to simulate the neutron diffraction process to reduce the deviation of the stress at the surface. The results show that the measured accuracy can be improved when the slits are substituted by the radial collimation. Compared the results obtained from (311) and (200) with analytical calculation, it reveals that the stress distribution among crystal planes will cause system deviation. Therefore the experiment accuracy may be further improved if the inner plane stress could be evaluated. The results also indicate that the strain accuracy will be affected by microstress and texture, and the gauge volume should be reduced as small as possible at the region where the stress distribution is steep. Copyright 2020 Science China Press Co., Ltd
Continuous monitoring of mixing depth with radon-222 and lidar
No full textWe report on the development of a combined approach using both lidar and radon-222 measurements to
obtain a near-continuous record of mixing depth which can, in turn, be used to assist in the interpretation of
simultaneous trace gas measurements.
Interactions between the land surface and the atmosphere above are moderated by the strength and depth of
mixing in the lower atmosphere which ranges diurnally between several meters at night to over one
kilometer during the day. Elastic backscatter lidar can be used to measure the depth of mixing during the
day, i.e. the height of the planetary boundary layer (PBL), by employing the change in aerosol concentration,
and hence lidar signal, at the boundary between the PBL and the free atmosphere. These measurements are
only possible when the mixing depth is large.
A complimentary approach, based on radon measurements, works well from the time turbulence decays in
the afternoon through till mid morning when mixing depths are too small to be observed using lidar. Radon-
222 is chemically inert and is released from the surface at a relatively constant rate and as such is a natural
passive tracer. Since it is radioactive, with a half-life of 3.8 days, it does not accumulate in the atmosphere.
At horizontally homogeneous inland sites, vertical mixing is the main process affecting near-surface
concentration. An estimate can therefore be obtained of an “equivalent mixing depth” from time-series of
radon concentration measurements, which can themselves be obtained with robust and low-maintenance
instrumentation.
Using two measurement techniques sidesteps the limitations of each to make a combined dataset a useful
component of field studies which seek to understand the exchanges of trace gases between the land surface
and atmosphere