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Deciphering the greenhouse gas record in deepest ice using continuous sublimation extraction / laser spectrometry

Periodic Reporting for period 4 - deepSLice (Deciphering the greenhouse gas record in deepest ice using continuous sublimation extraction / laser spectrometry)

Reporting period: 2020-04-01 to 2021-03-31

Past climate changes and their causes and in particular climate conditions during past warmer climates than today represent important benchmarks for climate models which are used for predictions of future warming due to increased atmospheric CO2 concentrations. Past climate conditions can be reconstructed using a variety of natural climate archives, however, polar ice cores represent the only direct archive of the past atmosphere and, as such, allow the unambiguous reconstruction of greenhouse gas concentrations back in time. In fact, everything we know about variations in greenhouse gas concentrations before the advent of direct atmospheric measurements in the second half of the 20th century comes from ice core research.

However, the use of the paleo atmospheric archive in polar ice cores is currently limited in terms of temporal coverage, resolution and sample availability in the bottom-most part of the Antarctic and Greenland ice sheets where extreme thinning of the ice by glacier flow occurs. The overarching goal of deepSLice is, therefore, to develop a new analytical method that overcomes these limitations, opening the window for high quality gas records, for example for a future Oldest Ice ice core from Antarctica covering the Mid Pleistocene Transition 0.9-1.5 million years ago.

Accordingly, deepSLice is developing
1) a Quantum Cascade Laser Absorption Spectrometer (QCLAS) to measure CO2, CH4 and N2O concentrations as well as the carbon isotopic signature of CO2 (δ13CO2) in small ice core samples from highly thinned ice.
2) a quantitative, contamination free, and non-fractionating extraction system for the air enclosed in such small ice samples using Continuous Sublimation Extraction (CSE).
Development of the QCLAS and the CSE components is carried out in parallel within the deepSLice project. A complete prototype QCLAS system was designed, developed, and tested in continuous flow through mode. Additional electronics and software components for laser control and spectrum evaluation have been developed and optimized for this low volume/low pressure application. In this mode the system achieved or surpassed the required precision targets. However, the commercial multi-pass absorption cell showed significant adsorption/desorption effects, limiting the precision and accuracy for discrete samples as provided by the CSE. Currently a new custom-made absorption cell is built to overcome this problem. The final QCLAS system will be completed by the end of 2018 and will then be coupled to the CSE unit.

A complete CSE system for ice core samples was built, comprising near infrared (NIR) irradiation of ice rods under vacuum conditions, efficient removal of water vapor to allow for high sublimation speed and cryogenic sampling of the complete air for further analysis. At the beginning of the project a IR light bulb (black-body emitter) was used as infrared light source, which is now being replaced by a high power, monochromatic NIR laser source. Currently adsorption/desorption effects within the extraction device and the sample fingers are tested. Completion of the system is planned in 2019 after coupling of the CSE and the QCLAS system.

First ice core measurements are envisaged in the last year of the project including publication of first ice core results.
The new QCLAS system measuring CO2, CH4 and N2O concentrations as well as δ13CO2 is the first of its kind and provides high precision measurements of all these parameters simultaneously at low pressure, i.e. for very small samples. Also the CSE unit is unique and continuous sublimation extraction has not been attempted before. Together they provide the necessary analytical foundation to measure all above parameters in the bottom-most ice of polar ice sheets, glaciers, and outcropping ice, where vertical resolution becomes a crucial issue and where limitations of ice availability require novel analytical approaches with higher depth resolution and minimum sample consumption.
An inside view into continous laser sublimation of ice cores