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Next generation analysis of the oldest ice core layers

Periodic Reporting for period 1 - GOLD-ICE (Next generation analysis of the oldest ice core layers)

Periodo di rendicontazione: 2019-01-15 al 2021-01-14

"Polar ice cores are an indispensable key to understanding the natural climate dynamics and have become one of the cornerstones in today’s paleoclimate research. The next generation of international polar drilling projects aims to conduct ground-breaking research investigating the cause of the so-called “mid-Pleistocene transition” around 1.2 million years ago. A prominent example in this regard is the prestigious European project ""Beyond EPICA Oldest Ice Core"", funded by the EU, which has already taken off to retrieve and exploit a 1.5 million year ice core record from central Antarctica. Since the “oldest ice” is also inevitably most highly thinned, the introduction of novel technology for high-resolution ice core measurements becomes of special interest.
The MSCA-IF project “GOLD-ICE” successfully accomplished taking a novel approach to establish laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for ice core analysis at the Università Ca' Foscari Venezia (UNIVE). The novelty of the GOLD-ICE approach lies in the careful consideration of the potential and limitations to a paleo-climatic interpretation of LA-ICP-MS signals, in view of the ice core micro stratigraphy. Only so, misinterpretation of the novel micron-scale LA-ICP-MS signals is reliably avoided.
Recent advances in the laser ablation community have turned LA-ICP-MS into a particularly powerful technique for producing high-resolution, artifact-free images of the chemical sample composition. Using this imaging technique for carefully mapping the spatial impurity distribution in ice core samples can provide important insights into the connection between the LA-ICP-MS signals and the ice crystal features. Such state-of-the-art imaging remained to be fully exploited for ice cores, thus far. GOLD-ICE has not only established a new LA-ICP-MS setup for refined glacio-chemical ice core analysis at UNIVE. The project has also, for the first time, demonstrated how the application of LA-ICP-MS for imaging the impurity distribution in ice cores can be refined and strongly improved. This next generation of LA-ICP-MS ice core analysis now offers scan speeds increased by one order of magnitude for single line profiles and the ability to map the localization of impurities with LA-ICP-MS at unsurpassed high spatial resolution (tens of microns). As a result, LA-ICP-MS ice core analysis has taken the step from 1D into 2D, making the close relationship between high-resolution LA-ICP-MS signals and the ice crystal matrix fully evident."
The project has fully achieved its objectives and milestones. The following work was carried out in order to achieve the three specific objectives of GOLD-ICE:

1. The first specific objective (SO1) was to characterize a LA-ICP-MS setup at UNIVE specifically dedicated to ice core analysis

GOLD-ICE has designed a cryogenic sample holder specifically dedicated to holding strips of ice cores, artificial ice as well as glass standards.

Special focus was the integration of a “aerosol rapid introduction system” – allowing to reduce the washout time from seconds to milli-second, i.e. the time needed for transferring the ablated sample aerosol plume to the ICP-MS. With washout times in the millisecond range, the recording of baseline-separated single pulses at high repetition rates becomes possible – the GOLD-ICE method uses almost 300 shots per second (previous ice core LA-ICP-MS studies: 10 – 20 shots per second).
As a result, the scan speed employed in the GOLD-ICE method is roughly 20 times higher with respect to previous studies on ice cores – now reaching around 1 millimeter per second. This is the basis to employ state-of-the-art 2D LA-ICP-MS imaging, allowing to move much faster with the laser, and to generate artifact-free images in realistic time with higher spatial resolution: 35µm.
Various elements have been tested for the new imaging application. Due to constraints by the imaging method, four elements are measured at a time. Usually these elements are Na, Mg, Mn and Sr – all of interest with respect to ice core applications.

2. The second specific objective (SO2) was to explore the potential and limitations to a paleo-climatic interpretation of LA-ICP-MS data in view of the ice core micro stratigraphy

With the integrated camera mosaic of visual images of the ice sample surface were obtained. The comparison of the impurity images generated by LA-ICP-MS with these mosaics allowed to assess clearly the localization of impurities in the ice crystal matrix. GOLD-ICE revealed an unequivocal correlation between high impurity intensities and the locations of grain boundaries, most pronounced for Na. This affinity of Na to be located at grain boundaries is consistent with previous investigations with other, non-imaging methods showing how LA-ICP-MS can provide a complementary tool for investigating the localization of impurities in glacier ice.
Consequences of the impurity localization with respect to the interpretation of individual LAICP-MS lines measured along the main core axis were assessed: Due to the localization at boundaries, the grain network strongly affects the high-frequency variability of the LA-ICP-MS signal: Individual peaks occur when the laser beam intersects a grain boundary. Accordingly, GOLD-ICE demonstrated that, in such cases, the significance of single peaks (the high-frequency signal components) is tied closely to the ice crystal network, rather than to individual past climate-related events.

3. The third specific objective (SO3) was to fully exploit the high resolution of LA-ICP-MS for the analysis of the oldest layers in polar ice cores

The new imaging approach of GOLD-ICE delivered an improved understanding of the LA-ICP-MS signal origin in ice cores. GOLD-ICE showed that the influence of the grain boundary network depends on the type of impurity (e.g. strong for Na, weaker for Mg) and on spatial resolution. At coarser spatial resolution the imprint of individual crystal features weakens – making the choice of resolution and scale a cornerstone of the experimental design in LA-ICP-MS ice core analysis.
Several large images of deep ice in Antarctic ice cores were obtained (EPICA Dome C and Talos Dome). Samples were chosen to represent chemical and physical properties characteristic for deep ice and both glacial and interglacial periods, e.g. with comparatively large and small grain sizes, respectively.
Advanced statistical methods have been employed for data analysis. It is shown how an experimental design can be adapted targeting to disentangle climate- and crystal-related signal components in the LA-ICP-MS analysis.
Achieving an improved signal understanding opens the way to exploit LA-ICP-MS for high-resolution analysis of the oldest layers of ice cores from Antarctica and elsewhere. By this means, GOLD-ICE has opened new avenues of ice core research with LA-ICP-MS, not least for making an important contribution to “Beyond EPICA: Oldest Ice”.
In conclusion, GOLD-ICE has fulfilled all its originally foreseen tasks and has even gone a step beyond since it developed novel chemical imaging tools for ice core characterization. This may impact also the investigation of the dielectric and deformational properties of glacier ice, e.g. for remote sensing and ice flow modeling.
GOLD-ICE generates 2D impurity images of ice cores with LA-ICP-MS.