Antarctic ice core dating

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Edited by Thure E. Ice outcrops provide accessible archives of old ice but are difficult to date reliably. Here we demonstrate 81 Kr radiometric dating of ice, allowing accurate dating of up to 1. The technique successfully identifies valuable ice from the interglacial period at Taylor Glacier, Antarctica. Our method will enhance the scientific value of outcropping sites as archives of old ice needed for paleoclimatic reconstructions and can aid efforts to extend the ice core record further back in time.

We present successful 81 Kr-Kr radiometric dating of ancient polar ice. Our experimental methods and sampling strategy Antarctic ice core dating validated by i 85 Kr and 39 Ar analyses that show the samples to be free of modern air contamination and ii air content measurements that show the ice did not experience gas loss.

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We estimate the error in the 81 Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the interglacial period Marine Isotope Stage 5e, — ka before present can be found in abundance near the surface of Taylor Glacier.

Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA 81 Kr analysis requires a 40—kg ice sample; as sample requirements continue to decrease, 81 Kr dating of ice cores is a future possibility. Antarctic BIAs have attracted much attention for their high concentration of meteorites, which accumulate at the surface over time 7.

More recently, BIAs have also been used for paleoclimate studies, as large quantities of old ice are available at the surface where it can be sampled with relative ease 89. Because the ice stratigraphy is exposed laterally along the BIA surface, such ice records are often referred to as horizontal ice cores. Determining the age of the ablating ice is the main difficulty in using BIAs for climate reconstructions 4.

The most reliable method is stratigraphic matching, where dust, atmospheric composition, or water-stable isotopes of the horizontal core are compared with well-dated, regular ice core records to construct a chronology 10 This technique, however, requires extensive sampling along the ice surface and relatively undisturbed stratigraphy, cannot be used past ka B. Several radiometric methods have been applied to ice dating, all of which have distinct limitations. Radiocarbon dating of trapped CO 2 suffers from in situ cosmogenic 14 C production in the ice Other methods rely on the incidental inclusion of datable material, such as sufficiently thick Tephra layers 13 or meteorites 7.

The terrestrial age of meteorites is not likely to be Antarctic ice core dating of the surrounding ice, however, because they accumulate near the BIA surface as the ice ablates. A promising new technique uses the accumulated recoil U in the ice matrix from U decay in dust grains as an age marker 14 Antarctic ice core dating currently, the method still has a fairly large age uncertainty 16— ka.

There is ificant scientific interest in obtaining glacial ice dating beyond ka, as such an archive would extend the ice core record further back in time, providing valuable constraints on the evolution Antarctic ice core dating past climate, atmospheric composition, and the Antarctic ice sheet Of particular interest is the middle Pleistocene transition — ka B.

Such old ice can potentially be found in Antarctic BIAs such as the Allan Hills site 23providing a strong impetus to developing reliable absolute dating tools for glacial ice. Kr is produced in nuclear fission and released into the atmosphere primarily by nuclear fuel reprocessing plants. Kr is naturally produced Antarctic ice core dating the upper atmosphere by cosmic ray interactions with the stable isotopes of Kr, primarily through spallation and thermal neutron capture The long half-life of 81 Kr allows for radiometric dating in the 50—1,ka age range 28well past the reach of radiocarbon dating.

First, krypton is not chemically reactive. Second, due to its long residence time, 81 Kr is well-mixed in the atmosphere. Third, the method does not rely on sporadically occurring tephra, meteorite, or organic inclusions in the ice but is widely applicable, as all glacial ice contains trapped air. Fourth, it does not require a continuous or undisturbed ice stratigraphy. Finally, in contrast to 14 C, 81 Kr does not suffer from in situ cosmogenic production in the ice Here we describe the successful 81 Kr radiometric dating of polar ice using air extracted from four ice samples from the Taylor Glacier blue ice area in Antarctica.

Using 85 Kr we demonstrate that our samples are uncontaminated by modern air. We independently date our samples using stratigraphic matching techniques and show an excellent agreement with the 81 Kr radiometric ages. Stratigraphic matching of water-stable isotopes to the nearby Taylor Dome ice core 38 ly identified ice in the B Satellite imagery of Taylor Glacier.

Antarctic ice core dating sampling locations are indicated as blue dots. C Comparison of 81 Kr radiometric ages to independently derived stratigraphic ages, in thousands of years before C. All ice sampling was below 5-m depth to avoid gas loss and exchange due to near-surface fractures.

There are three main contributions to the stratigraphic age uncertainty; for our samples we will list the root-sum-square of these. First, there is some ambiguity in linking Taylor Glacier samples to ice core records due to analytical uncertainties and the possible nonuniqueness of the synchronization. Second, the ice core chronologies themselves are subject to uncertainties.

Third, the Kr samples contain a spread in ages due to their finite size. We estimate this last effect is only important for the oldest sample where the layers are very strongly compressed. The first sample Kr-1 was obtained along the main transect.

The sample is from the Younger Dryas period, which is clearly identified by its characteristic CH 4 sequence. The top axis shows the distance along the transect in meters; note that the position—age relationship is nonlinear.

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We as a stratigraphic age of Going down-glacier the ice gets progressively older; ice with ages between 10 and 55 ka is found in stratigraphic order 0—15 km downstream of the first measurement of the profile. Past 55 ka the age interpretation is more ambiguous, and ice from MIS 4 appears to be absent from the sampling profile. It must be noted that the ice stratigraphy in this lower part of the glacier is strongly disturbed by ice flow, and the sequence shown in Fig.

Apart from the four ice samples we took an additional atmospheric sample upwind from the field camp, which was processed identically to the air samples extracted from the ice. Stratigraphic dating of Kr samples. Measurements along the stratigraphically dated profiles white dots with Kr samples black with age uncertainty.

A Sample Kr-1, located on the main transect Fig. B Samples Kr-2 and 4, located on the along-flow profile. C Sample Kr-3, Antarctic ice core dating on the downstream transect. Taylor Glacier transect positions have been corrected for the isochrone dip angle Dataset S1. The from our analyses are given in Table 1. In this work, the isotope ratio has a statistical uncertainty of 3. ATTA analyses in the future will benefit from a newly demonstrated technique that has reduced the systematic uncertainty in the 83 Kr measurement down to 0.

For all four ice samples we find that both ages agree within the analytical uncertainty. The t Kr we obtain for sample Kr-3 ka clearly identifies this ice as originating from the MIS 5e interglacial period, eliminating any remaining age ambiguity in the stratigraphic dating. Our analyses show that the integrity of our samples has not been compromised. Dataset S1. Taylor Glacier ice originates on the slopes of Taylor Dome and is expected to have slightly higher air content because of the lower elevation of the deposition site. By comparing to our atmospheric sample we estimate a 1.

It must be noted that the sample size is too small for precise 39 Ar analysis. With the exception of sample Kr-1, the 39 Ar activity of the samples is below the detection limit Table 1. The combination of a negligible 85 Kr activity and measureable 39 Ar activity in sample Kr-1 is puzzling. Another possibility is a modern contamination of the Ar sample fraction after Ar-Kr separation in the laboratory. For all samples we observe Antarctic ice core dating 2.

Summarizing, we contend that within the precision of our analyses the samples are free of gas loss and gas exchange due to surface fracturing, sampling, or processing; isotopic fractionation during sample processing introduces errors that are well within the stated 81 Kr dating uncertainty.

Changes in ocean temperature on the timescale of glacial cycles can modify the atmospheric Kr inventory through the dependence of gas solubility on temperature; this effect is on the order of 0. The cosmogenic 81 Kr production rate in the upper atmosphere is expected to vary in response to changing solar activity and geomagnetic field strength 52 Consequently, for all practical purposes the 81 Kr abundance is insensitive to production variability on annual to millennial timescales related to solar cycles 54 and geomagnetic excursions such as the Laschamp event Long-term reconstructions of geomagnetic dipole strength show pronounced variations on multimillennial timescales 56as plotted in Fig.

To estimate the impact on atmospheric 81 Kr we converted the geomagnetic variations to cosmogenic nuclide production rates using the method of Wagner et al. Since the Brunhes—Matuyama reversal ka ago the geomagnetic field has been relatively strong, leading to increased cosmic ray shielding and reduced radionuclide production. Our estimate suggests that during the last 1. For the last ka which includes this study this error is below 3 ka and well within analytical uncertainty. No correction was therefore applied to the 81 Kr ages.

In principle, if independently dated old ice is available, such as at the Mount Moulton BIA 1381 Kr can be used as a tracer of past cosmic ray variability. Such a 81 Kr-based reconstruction would be insensitive to past changes in atmospheric transport or biogeochemistry, Antarctic ice core dating is not the case for 10 Be and 14 C, respectively Stability of atmospheric 81 Kr.

A Relative paleointensity of the geomagnetic field Magnetic reversals are indicated by vertical lines. B Relative spallogenic production rate orange with relative 81 Kr abundance black. Our result shows that 81 Kr radiometric dating of ancient ice is both feasible and accurate within the specified analytical uncertainty. In the 50 ka to 1.

Kr dating therefore has great potential for the dating of BIAs, thereby enhancing their scientific value as archives of easily accessible old ice. For ice older than 1. Kr sample-size requirements for the ATTA method have decreased by almost four orders of magnitude since the first experimental realization 33and currently a minimum of 40 kg of ice is required for a single analysis. If technological advances can further reduce sample requirements in the future, 81 Kr dating can be applied to regular ice core samples as well.

It will be particularly helpful with traditionally difficult dating problems, such as basal ice. Our study reveals that the Taylor Glacier BIA contains large quantities of ice from both the penultimate deglaciation and the interglacial period MIS 5e, — ka B. This is of interest to paleoclimatic reconstructions that require large ice samples, such as isotopic measurements of atmospheric trace gases 58 The deglaciation is accompanied by a global reorganization of biogeochemical cycles, as evidenced by abrupt changes in the atmospheric abundance of trace gases such as Antarctic ice core dating 4CO 2and N 2 O.

Detailed records of the isotopic composition of these gases can help better constrain changes in their global budgets. The valuable MIS 5e ice can be sampled at Taylor Glacier with a much smaller logistical footprint than would be required for a deep drilling campaign, as the ice outcrops at the surface and the site is within easy helicopter reach of McMurdo station. The presence of MIS 5e ice at Taylor Glacier furthermore has implications for attempts to reconstruct past ice sheet stability and ice flow in the larger Taylor Dome region Ice was sampled using a cm-diameter electromechanical ice drill without drilling fluid.

All ice sampling was done below 5-m depth, either as two 5-m cores or a single m core. No fractures were observed in any of the samples. Except for the Samples were Antarctic ice core dating with an electropolished chisel and placed in an onsite L aluminum Antarctic ice core dating tank The tank was evacuated for at least 1.

Using a bubbler manifold the air was recirculated for 0.

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Next, the hepace air was transferred into L electropolished stainless steel sample flasks transfer time 0. Air extraction was completed within 12 h of drilling. To prevent the ice cores from warming up, drilling and sample handling were done during the coldest hours of the night when the sun dips below the Kukri Hills. The Kr and Ar were separated from sample air at the University of Bern using molecular sieve absorption and Gas Chromatography In the apparatus, atoms of a targeted isotope 81 Kr, 85 Kr, or the control isotope 83 Kr are captured by resonant laser light into an atom trap and counted by observing the fluorescence of the trapped atoms.

For quality control in the analysis of environmental samples, the instrument is calibrated with a standard modern sample both before and Antarctic ice core dating the analysis of every group of two to three environmental samples. Ar activity was measured in Bern using low-level decay counting CH 4 was measured at Oregon State University using a melt—refreeze air extraction followed by gas chromatography with a flame ionization detector We want to thank X. Mitchell, H. Schaefer, A.

Antarctic ice core dating

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Dating annual layers of a shallow Antarctic ice core with an optical scanner