Methods of dating ice cores

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Research article 26 Mar Correspondence : Theo M. Jenk theo. High-alpine glaciers are valuable archives of past climatic and environmental conditions. The interpretation of the preserved al requires a precise chronology. Radiocarbon 14 C dating of the water-insoluble organic carbon WIOC fraction has become an important dating tool to constrain the age of ice cores from mid-latitude and low-latitude glaciers. However, in some cases this method is restricted by the low WIOC concentration in the ice. In this work, we report first 14 C dating using the dissolved organic carbon DOC fraction, which is present at concentrations of at least a factor of 2 higher than the WIOC fraction.

We attribute this offset to two effects of about equal size but opposite in direction: i in-situ-produced 14 C contributing to the DOC resulting in a bias towards younger ages and ii incompletely removed carbonates from particulate mineral dust 14 C-depleted contributing to the WIOC fraction with a bias towards older ages. The estimated amount of in-situ-produced 14 C in the DOC fraction is smaller than the analytical uncertainty for most samples.

While during DOC extraction the removal of inorganic carbon is monitored for completeness, the removal for WIOC samples was so far only assumed to be quantitative, at least for ice samples containing average levels of mineral dust. Future optimization of the removal procedure has the potential to improve the accuracy and precision of WIO 14 C dating. This approach thus has the potential of pushing radiocarbon dating of ice forward even to remote regions where the carbon content in the ice is particularly low.

Fang, L. For a meaningful interpretation of the recorded palaeoclimate als in ice cores from glacier archives, an accurate chronology is essential. Annual layer counting, supported by and tied to independent time markers such as the nuclear fallout horizon evident by a peak maximum in tritium or other radioisotopes or distinct als from known volcanic eruptions in the past, is the fundamental and most accurate technique used for ice core dating. However, for ice cores from high-alpine glaciers this approach is limited to a few centuries only because of the exceptionally strong thinning of annual layers in the vicinity of the bedrock.

Most of the current analytical techniques do not allow high enough sampling resolution for resolving seasonal fluctuations or detecting distinct single events in this depth range. Ice flow models, which are widely used to retrieve full-depth age scales e. Nye, ; Bolzan, ; Thompson et al. Even with 3D models, which require extensive geometrical data, it is highly challenging to simulate a reasonable bottom age e.

Licciulli et al. This emphasizes the need for an absolute dating tool applicable to the oldest bottom parts of cores from these sites. Radioactive isotopes contained in the ice offer the opportunity to obtain absolute ages of an ice sample.

For millennial-scale ice cores, 14 C dating is the technique of choice. The 14 C dating approach using water-insoluble organic carbon WIOC from glacier ice has become a well-established technique for ice core dating, and its accuracy was recently validated Uglietti et al.

Accordingly, the low WIOC concentration in some glaciers and in polar regions and the related large demands of ice mass put a limit to this application. Using the DOC fraction for 14 C dating could therefore reduce the required amount of ice or, for sample sizes similar to what would be needed for 14 C dating by WIOC, improve the achievable analytical dating precision, which strongly depends on the absolute carbon mass even for state-of-the-art micro-radiocarbon dating.

DOC in ice is composed of atmospheric water-soluble organic carbon WSOC contained in carbonaceous aerosol particles and organic gases taken up during precipitation Legrand et al. WSOC is formed in the atmosphere by oxidation of gases emitted from the biosphere or from anthropogenic sources Legrand et al. Carbonaceous aerosols transported in the atmosphere can be deposited on a glacier by wet and dry deposition. Before the industrial revolution, these organic carbon species, then entirely of non-fossil origin, contain the contemporary atmospheric 14 C al of the time when the snow deposited on the glacier Jenk et al.

The mixed age of trees in Swiss forests today is estimated to be slightly less than 40 years Mohn et al. Back in time, prior to extensive human forest management, the mixed age of trees in Europe was likely older, and the mean age of old-growth-forest wood ranged from around 70 to years depending on the region, i. For biogenic DOC, May et al. Such an inbuilt age is negligible considering the analytical uncertainty, which is similarly the case for a bias from oceanic sources since concentrations of marine organic tracers are more than 1 order of magnitude lower than terrestrial tracers for the vast majority of glacier sites.

This conclusion is supported by the fact that Uglietti et al. However, these first suggested a potential in situ production of 14 C in the DOC fraction based on the obtained super-modern F 14 C values i. F 14 C values higher than ever observed in the recent or past ambient atmosphere.

Building on these initial findings, May questioned the applicability of the DOC fraction for radiocarbon dating. Although the in situ 14 C production of 14 CO and 14 CO 2 in air bubbles contained in polar ice has been studied thoroughly and is well understood Van de Wal et al. In order to minimize potential contamination, the entire system is protected from ambient air by inert gas helium flow or vacuum. Pooling samples from several subsequent extractions would be feasible, allowing dating of samples with lower DOC concentration.

Here, we succeeded in analysing such parallel samples from four different Eurasian glaciers. The bedrock was reached, and the total length of the ice core is The Chongce ice cap is located in the western Kunlun Mountains on the north-western Tibetan Plateau, covering an area of Two of those cores reached bedrock with lengths of The annual net accumulation rate is about 0. A summary of the metadata for the study sites and ice cores can be found in the Supplement Table S1 and details about sampling depths and sample sizes in Tables 1 and 2.

No from any of the cores analysed in this study have been published ly. In order to remove potential contamination in the outer layer of the ice core, pre-cut samples from the inner part of the core were additionally rinsed with ultra-pure water Sartorius, These initial steps were performed in a laminar-flow box to ensure clean conditions. For DO 14 C analysis, sample preparation follows the procedure described in Fang et al. Simultaneously, a pre-cleaning step was applied to remove potential contamination in the system. After the ice melted, the meltwater was filtrated under a helium atmosphere using a pre-baked in-line quartz fibre filter.

The sample volume was determined by measuring the reactor fill level. The filtrate was acidified by mixing with the pre-treated base water. All steps were carried out under a constant flow of helium. The sample CO 2 was further cleaned from residual water vapour and quantified manometrically before being sealed into a glass vial for offline 14 C analyses.

Procedural blanks were determined and continuously monitored by processing and analysing frozen ultra-pure water Sartorius, They were prepared every time when cutting ice and then processed and analysed along with the samples at least twice a week. Procedural blanks are 1. All AMS F 14 C values presented here are finally corrected for the system and method characteristic contributions as reported ly e.

Uglietti et al. For the cracking system applied for DOC samples the constant contamination is 0. Further corrections applied for the AMS cross-contamination 0. All uncertainties were propagated throughout data processing until final 14 C calibration. These corrections have a larger effect on low-carbon-mass samples higher noise-to-sample ratio , resulting in a larger dating uncertainty. Radiocarbon ages are given in years before present BP , with the year of reference being Stuiver and Polach, To obtain calibrated 14 C ages, the online program OxCal v4.

This is likely explained by temporal variability because most samples in this study are several thousand years old, whereas the literature data only cover the last few centuries, including values from the industrial period, in which additional anthropogenic sources exist e. Because the Belukha glacier is surround by extensive Siberian forests, the higher ratio may be explained by particularly high emissions of biogenic volatile organic compounds. This is particularly obvious for the samples from Chongce, characterized by high mineral dust load and from a site of very high elevation with low net accumulation.

In terms of F 14 C a and calibrated ages b. For the linear fit in both panels, the data from Chongce open symbols were excluded. In addition, calibrated ages derived when applying the OxCal sequence deposition model for further constraint are shown.

For all sites, the calibrated 14 C ages from both fractions show an increase in age with depth Table 3. In both fractions, the oldest age was derived for the sample from the deepest part of the Belukha ice core. In Table 3, we present the first radiocarbon dating of ice using the DOC fraction.

The DOC calibrated 14 C age of ice increases with depth for all four sites, as expected for undisturbed glacier archives from the accumulation zone. For samples from three out of the four sites, our Sect. The analytical uncertainty mainly arises from correction for the procedure blank introduced during sample treatment prior to AMS analysis see Sect. The contribution thereby depends on carbon mass larger for small samples and sample age larger the bigger the difference between sample and blank F 14 C. How the overall analytical uncertainty in F 14 C decreases with higher carbon mass is shown in Fig.

S1 in the Supplement. An additional benefit is that the DOC extraction procedure allows the removal of inorganic carbon to be monitored for completeness see Sect. studies have suggested that 14 C of the DOC fraction may be influenced by in situ production of 14 C in the ice matrix May ; Hoffman Induced by cosmic radiation, the production of 14 C atoms within the ice matrix, i. The mechanism of incorporation of in-situ-produced 14 C incorporation into organic molecules is not well understood Woon, ; Hoffman, The natural neutron flux, relevant for the 14 C production rate, strongly depends on altitude and latitude with a generally uniform energy distribution of the incoming neutrons Gordon et al.

The 14 C in situ production in natural ice further depends on the depth in the glacier and the snow accumulation rate of the site Lal et al. Following Lal et al. The shift is largest for the Chongce samples 0. The samples from Belukha and Colle Gnifetti are least affected 0.

Note that for any site, the size of this effect is reduced with higher DOC concentration of the sample. In terms of F 14 C top and calibrated ages bottom. Error bars in a and b reflect the propagated uncertainty in analysis and correction. In b and c , measured values are shown as grey crosses. It was speculated that this was at least partly caused by the visible, exceptionally high loading of mineral dust on the WIOC filters Hou et al.

Such high mineral dust loading was also observed during filtration of the Chongce Core 1 samples presented here. High mineral dust content in the ice can influence 14 C dating with WIOC in two ways: by affecting filtration through clogging of the filter and by potentially contributing 14 C-depleted carbon from carbonate, as has been discussed in most studies. They all concluded that, although for dust levels typically observed in ice cores from high-elevation glaciers no ificant bias is detectable for 14 C of WIOC, it is of concern for the elemental carbon EC fraction combusted at higher temperatures during OC—EC separation.

In any case, the carbonate removal efficiency during WIOC sample preparation was never quantified. Applying an isotopic-mass-balance-based model to our dataset, the carbonate removal efficiency in WIO 14 C samples was estimated.

On the one hand, this is in agreement with the findings of studies, confirming that the potential carbonate-related bias for 14 C dating using WIOC is hardly detectable for ice samples with normal dust loading effect masked by the analytical uncertainty; see Fig. S2 in the Supplement. For example, Uglietti et al.

The likely bigger particle size in such samples will affect their solubility, i. In the current procedure, this time is not adjusted accordingly Sect. Instead of a correction, which does not seem feasible for this effect because of large uncertainties and likely substantial site-to-site sample-to-sample variations, we suggest future improvement in the analytical procedure of the carbonate removal step e.

In the following we discuss our new DO 14 C in the context of ages from studies. For final calibration of 14 C ages, most of those earlier studies took advantage of the assumption of sequential deposition in the archive, i. Particularly in case of relatively large analytical uncertainties compared to the age difference in the samples, the sequential deposition model can moderately constrain the probability distribution of the calibrated age range in each sample of the dataset. For consistency we applied the same calibration approach here by using the inbuilt OxCal sequence model Ramsey, While the underlying assumption may not generally be valid for all sites and individually needs to be carefully assessed, we find no difference in the calibrated ages using the sequence model and the ages from the conventional calibration approach for all DO 14 C data presented in this study Table 3.

Note that no correction for a potential in situ 14 C bias was applied to the DO 14 C data used here Sect. Table 5 DO 14 C dating for near-bedrock ice compared to from studies visualized in Fig. The latter age was obtained for an ice core from the nearby Belukha West Plateau glacier extracted in B03 Aizen et al.

Also, the according sample from B03 was from a slightly shallower depth 0. The age range modelled for B03 for the same depth above bedrock 0. Nevertheless, our new age is still in the range of the ly estimated bottom age Table 5, Fig.

Methods of dating ice cores

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How are ice cores dated?