Vladimir Levchenko

Modeling air movement and bubble trapping in firn

A finite difference model for gas diffusion and bubble trapping in firn is described. The model uses prescribed profiles of density, open and closed porosity, and diffusivity to determine the diffusion and trapping processes. The model is calibrated and tested by using measured air composition in the firn at the DE08-2 site on Law Dome, Antarctica. In particular, we focus on carbon dioxide (CO2), methane (CH4), and sulfur hexafluoride (SF6), which have well-determined atmospheric records (CO2 since 1958, CH4 since 1983, and SF6 since 1978). These trace gases are used to tune the diffusivity-porosity relationship, which is the most uncertain of the model inputs. Modeled trace gas profiles in the DE08-2 firn are improved if allowance is made for reduced diffusion through the most prominent DE08-2 melt layer from the summer of 1989/1990. The relatively rapid growth rate of SF6 in the atmosphere permits good definition of the diffusion reduction due to the melt layer (about 80%). The model quantifies the smoothing effect of the firn diffusion and bubble trapping on atmospheric signals. Gravitational separation in the firn is investigated by comparison of modeled δ15N2 with observations. The model is used to calculate the isotopic diffusion correction for δ13C02 and δ13CH4. This corrects for the fractionating effects of the firn diffusion process on the different isotopes. The diffusion and gravitational corrections are critical at the measurement precision currently being obtained; for δ13CO2 the diffusion correction is up to about 10 times the current measurement precision. The diffusion correction is even more significant for δ13CH4; at over 1‰ at the bottom of the firn it is more than double the change over the last decade. The fully corrected δ13C02 record from the DE08-2 firn is compared with the history of Cape Grim direct atmospheric measurements with excellent agreement.

Review of Radiocarbon Data from Atmospheric and Tree Ring Samples for the Period 1945-1997 Ad

A summary of 14C data from atmospheric sampling and measurements on wood from annual tree rings for the period 1945-1997 AD is presented and evaluated. Atmospheric records are characterized by different distributions of bomb-test 14C between the Northem and Southem Hemispheres, latitude dependence, and seasonal fluctuations. Radiocarbon data from tree rings are summarised and plotted against atmospheric records from similar latitudes. In some cases, discrepancies are found. Possible reasons for this include: 1) the use of stored carbohydrate from the previous year, 2) different 14C levels in the air around subcanopy trees due to respiration of CO2, 3) regional and local effects of anthropogenic CO2 and 14C sources, 4) sampling of wood material too close to ring boundaries, and 5) insufficient pretreatment of tree ring sampies for dating. But in cases where trees were carefully selected and the sampies adequately pretreated, radiocarbon data from tree rings show excellent agreement with direct atmospheric sampling records.

Cultural innovation and megafauna interaction in the early settlement of arid Australia

Elucidating the material culture of early people in arid Australia and the nature of their environmental interactions is essential for understanding the adaptability of populations and the potential causes of megafaunal extinctions 50–40 thousand years ago (ka). Humans colonized the continent by 50 ka, but an apparent lack of cultural innovations compared to people in Europe and Africa has been deemed a barrier to early settlement in the extensive arid zone. Here we present evidence from Warratyi rock shelter in the southern interior that shows that humans occupied arid Australia by around 49 ka, 10 thousand years (kyr) earlier than previously reported. The site preserves the only reliably dated, stratified evidence of extinct Australian megafauna, including the giant marsupial Diprotodon optatum, alongside artefacts more than 46 kyr old. We also report on the earliest-known use of ochre in Australia and Southeast Asia (at or before 49–46 ka), gypsum pigment (40–33 ka), bone tools (40–38 ka), hafted tools (38–35 ka), and backed artefacts (30–24 ka), each up to 10 kyr older than any other known occurrence. Thus, our evidence shows that people not only settled in the arid interior within a few millennia of entering the continent, but also developed key technologies much earlier than previously recorded for Australia and Southeast Asia.

Beeswax as dental filling on a Neolithic human tooth

Evidence of prehistoric dentistry has been limited to a few cases, the most ancient dating back to the Neolithic. Here we report a 6500-year-old human mandible from Slovenia whose left canine crown bears the traces of a filling with beeswax. The use of different analytical techniques, including synchrotron radiation computed micro-tomography (micro-CT), Accelerator Mass Spectrometry (AMS) radiocarbon dating, Infrared (IR) Spectroscopy and Scanning Electron Microscopy (SEM), has shown that the exposed area of dentine resulting from occlusal wear and the upper part of a vertical crack affecting enamel and dentin tissues were filled with beeswax shortly before or after the individual’s death. If the filling was done when the person was still alive, the intervention was likely aimed to relieve tooth sensitivity derived from either exposed dentine and/or the pain resulting from chewing on a cracked tooth: this would provide the earliest known direct evidence of therapeutic-palliative dental filling.

In search of in-situ radiocarbon in Law Dome ice and firn

Abstract Results of AMS radiocarbon measurements on CO and CO2 separated from firn air directly pumped from the ice sheet, and on CO2 separated from air extracted from ice cores by a dry grating technique, are presented. The firn air samples and ice cores used in this study were collected from the region of Law Dome, Antarctica. No evidence of in-situ 14CO2 was found in the firn air samples or the ice core air samples from one site although a slight enhancement of 14CO above expected polar atmospheric concentrations was observed for some firn air samples. A clear in-situ 14CO2 signal for ice pre-dating the radiocarbon bomb pulse was found, however, in air samples extracted from an ice core from a second site. We compare these results and propose an hypothesis to explain this apparent contradiction. The degree to which in-situ 14C is released from the ice crystals during trapping and bubble formation is considered and discussed. The selectivity of the dry grating technique for the extraction of trapped atmospheric gases from ice cores is also discussed and compared with other methods.

7Be and 10Be concentrations in recent firn and ice at Law Dome, Antarctica

Abstract Over the past three years, the Australian National Tandem for Applied Research (ANTARES) AMS facility at ANSTO has been expanding its sample preparation and measurement capability, particularly for 10 Be, 26 Al and 36 Cl. During this time, ANSTO has continued its collaboration with the AAD and CSIRO Atmospheric Research on the measurement of cosmogenic isotopes from Law Dome, Antarctica. This research program has been supported by the construction of a dedicated geochemistry laboratory for the processing of ice and rock samples for the preparation of AMS targets. Here we present our first results for 10 Be concentrations measured in ice cores from three sites at Law Dome and describe the sample processing protocol and aspects of the AMS measurement procedure. These sites are characterised by an eightfold difference in accumulation rate with a common precipitation source. In combination with an established ice chronology, this has enabled some preliminary findings concerning the relationship between the snow accumulation rate and the measured 10 Be concentration for Law Dome during recent times. Additionally, we present 7 Be and 10 Be/ 7 Be measurements made for a few surface snow samples from Law Dome and Australia.

Measurements of the 14CO2 bomb pulse in firn and ice at Law Dome, Antarctica

Abstract 14CO2 produced in the atmosphere by nuclear weapons testing in the 1960s is now incorporated in the air bubbles of Antarctic ice. The high atmospheric radiocarbon growth rates through the period of tests and subsequent decline provide a unique and independent test for the smoothing of atmospheric CO2 signals due to firn diffusion and bubble close off. The level of smoothing quantifies the time resolution with which atmospheric trace gas histories can be reconstructed from ice cores. In this paper, the methodologies for the preparation and AMS measurements of ice core and firn 14CO2 from high accumulation sites at Law Dome are detailed. The results are compared with predictions of a numerical model incorporating firn air diffusion and bubble close-off. The sample sizes, precision of measurements and sources of contamination are discussed for both firn and ice samples.

A New Method for Analyzing 14C of Methane in Ancient Air Extracted from Glacial Ice

We present a new method developed for measuring radiocarbon of methane (14CH4) in ancient air samples extracted from glacial ice and dating 11,000-15,000 calendar years before present. The small size (~20 µg CH4 carbon), low CH4 concentrations ((CH4), 400-800 parts per billion (ppb)), high carbon monoxide concentrations ((CO)), and low 14C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air sam- ples was quantitatively removed using the Sofnocat reagent. 14C procedural blanks were greatly reduced through the construc- tion of a new CH4 conversion line utilizing platinized quartz wool for CH4 combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and 14C activity of extraneous carbon added in the new CH4 conversion line were determined to be 0.23 ± 0.16 µg and 23.57 ± 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 µg. The overall procedural blank for all stages of sample handling was 0.75 ± 0.38 pMC for ~20-µg, 14C-free air samples with (CH4) of 500 ppb. Duration of the graphitization reactions for small (<25 µg C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. 14C corrections for each step of sample handling have been determined. The resulting overall 14CH4 uncertainties for the ancient air samples are ~1.0 pMC.

New 14C ages on cellulose from diprotodon gut contents: explorations in oxidation chemistry and combustion

We report radiocarbon ages on cellulose isolated from the gut contents of a Diprotodon found at Lake Callabonna, South Australia. The maximum age obtained corresponds to a minimum age of >53,400 BP for this extinct giant marsupial. This is older than, and hence consistent with, the generally accepted Australian megafauna extinction window. We argue that dichromate and other strong oxidants are less selective than chlorite for lignin destruction in wood, and our results suggest that ages approaching laboratory background can be obtained using a repeated pretreatment sequence of chlorite-alkali-acid and measurement of the sometimes discarded 330 C combustion fraction.

No turnover in lens lipids for the entire human lifespan

Lipids are critical to cellular function and it is generally accepted that lipid turnover is rapid and dysregulation in turnover results in disease (Dawidowicz 1987; Phillips et al., 2009; Liu et al., 2013). In this study, we present an intriguing counter-example by demonstrating that in the center of the human ocular lens, there is no lipid turnover in fiber cells during the entire human lifespan. This discovery, combined with prior demonstration of pronounced changes in the lens lipid composition over a lifetime (Hughes et al., 2012), suggests that some lipid classes break down in the body over several decades, whereas others are stable. Such substantial changes in lens cell membranes may play a role in the genesis of age-related eye disorders. Whether long-lived lipids are present in other tissues is not yet known, but this may prove to be important in understanding the development of age-related diseases. DOI: http://dx.doi.org/10.7554/eLife.06003.001