Radiocarbon dating ice cores
The apparent agreement between seemingly independent dating methods is seen as a powerful argument for millions of years. But closer inspection reveals that these methods are not truly independent, and the agreement between them is the result of circular reasoning. Since they also think some organisms lived only during certain periods of Earth history, they conclude that these fossils can be used to date different rock layers. For instance, suppose one particular organism has so far been found only in rocks thought to be between and million years old. In other words, the fossils found in rocks are used to date other rocks.
Krypton used to accurately date ancient Antarctic ice
Author contributions: 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 previous 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 are 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. 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 previous 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.
Ice cores from the Greenland and Antarctic ice sheets provide highly resolved, well-dated climate records of past polar temperatures, atmospheric composition, and aerosol loading up to ka before present 1 — 3. In addition to deep ice cores, old ice can also be found at ice margin sites and blue ice areas BIAs where it is exposed due to local ice dynamics and ablation 4 — 6.
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 8 , 9. 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 representative 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 ; currently, the method still has a fairly large age uncertainty 16— ka. Flow modeling can provide useful constraints on the age of BIAs 15 — 18 , but large errors are introduced by the limited availability of field data and necessary assumptions about past flow and ice thickness.
There is significant 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 of 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 23 , providing a strong impetus to developing reliable absolute dating tools for glacial ice.
Krypton is a noble gas present in the atmosphere at a mixing ratio of around 1 ppm 24 and has two long-lived radioisotopes of interest to earth sciences 25 , Kr is produced in nuclear fission and released into the atmosphere primarily by nuclear fuel reprocessing plants. Kr is naturally produced in 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 28 , well past the reach of radiocarbon dating.
Kr—Kr dating has already been used to determine the residence time of groundwater in old aquifers 29 — 31 and for several reasons has great potential for applications in dating polar ice as well. 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 previously identified ice in the B Satellite imagery of Taylor Glacier. Kr 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.
For the last 60 ka, an annual layer-counted age scale is available for Greenland, to which Antarctic records can be tied using globally well-mixed CH 4 ; beyond this age, ice flow modeling is commonly used to reconstruct the chronology 39 — The uncertainty in the ice core chronologies can be evaluated by comparing them to independently dated speleothem records showing concomitant events 41 — 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.
We assign 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, located on the downstream transect. Taylor Glacier transect positions have been corrected for the isochrone dip angle Dataset S1. The results from our analyses are given in Table 1. Kr measurements are normalized to modern atmospheric Kr and reported in percent of modern Kr pMKr: 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 a 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 56 , as 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 13 , 81 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, which 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.
I know Carbon 14 is one method, but some ice cores go back hundreds of Other ways of dating ice cores include geochemisty, wiggle matching of ice core . PDF | A recently developed dating method for glacier ice, based on the analysis of radiocarbon in carbonaceous aerosol particles, is thoroughly.
A team of scientists has successfully identified the age of ,year-old Antarctic ice using radiometric krypton dating — a new technique that may allow them to locate and date ice that is more than a million years old. The ability to discover ancient ice is critical, the researchers say, because it will allow them to reconstruct the climate much farther back into Earth's history and potentially understand the mechanisms that have triggered the planet to shift into and out of ice ages. Results of the discovery are being published this week in the Proceedings of the National Academy of Sciences. Krypton dating is much like the more-heralded carbon dating technique that measures the decay of a radioactive isotope — which has constant and well-known decay rates — and compares it to a stable isotope. Unlike carbon, however, krypton is a noble gas that does not interact chemically and is much more stable with a half-life of around , years.
An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier.
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How are ice cores dated?
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Ice cores from Greenland contain information about climate changes in the past over the Greenland ice sheet and the surrounding areas. Likewise, cores drilled from the sea bed or from the sediments in a lake tell about past temperatures and other climate indicators in the oceans and over continents. The different cores all give their story of the climate of the past from their respective geographical locations, and they agree on one thing: Climate has always been changing. On millennial time scales, the climate oscillates between cold and warm periods. However, one question which is still under investigation is to what degree the climate oscillates in phase all over the Earth or whether there are regional differences during climate shifts. Solving this question is hampered by the uncertainty on the dating of the different records. Because of the uncertainty on the dating of the cores it is difficult to compare the climate profiles and investigate whether climate change happens synchronously or transgresses from one region to the other.
I was wondering how ice cores are dated accurately.
An analysis has been completed of the global carbon cycle and climate for a 70, year period in the most recent Ice Age, showing a remarkable correlation between carbon dioxide levels and surprisingly abrupt changes in climate. The findings, just published in the online edition of the journal Science, shed further light on the fluctuations in greenhouse gases and climate in Earth's past, and appear to confirm the validity of the types of computer models that are used to project a warmer climate in the future, researchers said. The analysis was made by studying the levels of carbon dioxide and other trace gases trapped as bubbles in ancient ice cores from Antarctica. In the last Ice Age, as during most of Earth's history, levels of carbon dioxide and climate change are intimately linked.
Deep Core Dating and Circular Reasoning
Author contributions: 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 previous 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 are 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. 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 previous 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. Ice cores from the Greenland and Antarctic ice sheets provide highly resolved, well-dated climate records of past polar temperatures, atmospheric composition, and aerosol loading up to ka before present 1 — 3.
Guest commentary from Jonny McAneney. You heard it here first …. Back in February, we wrote a post suggesting that Greenland ice cores may have been incorrectly dated in prior to AD This was based on research by Baillie and McAneney which compared the spacing between frost ring events physical scarring of living growth rings by prolonged sub-zero temperatures in the bristlecone pine tree ring chronology, and spacing between prominent acids in a suite of ice cores from both Greenland and Antarctica. Last month, in an excellent piece of research Sigl et al. The clinching evidence was provided by linking tree-ring chronologies to ice cores through two extraterrestrial events…. In , Miyaki et al.
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