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Task 3: Ice core content analysis

by Vincent Favier - 23 February 2016 ( maj : 15 November 2016 )

Analysis of moisture sources will be carried out on ice cores through the combined analysis of water (isotopes) and aerosol characteristics (chemical content). The low elevation regions are mainly affected by marine sources, while the regions at intermediate elevation receive a stronger contribution from larger scale meteorological systems and the high elevation plateaus concentrations are impacted by clear sky precipitation (“diamond dust”).

Water stable isotope measurements will be performed at LSCE at high resolution (typically 10 samples per year) for 18O and deuterium for the period 1975-present, giving access to deuterium excess data. It is planned to perform discrete measurements from shallow ice core successive segments using LSCE mass spectrometers and laser instruments. In parallel, LSCE and LGGE are developing a continuous flow analysis system coupled with the introduction of water for continuous laser measurements. If this system is validated in time for ASUMA with an analytical accuracy comparable to that obtained from discrete measurements (at least 0.07 per mille for 18O and at least 0.7 per mille for D), it will be applied to produce high resolution (1 cm) records. Longer ice cores will be measured with a resolution of 2 samples per year for the past 100 or 200 years, respectively, and will also include measurements of 17O using the dedicated LSCE mass spectrometer with an accuracy of 5 ppm. If a specific signal is detected from this low resolution profile, further analyses will be performed in order to refine the record. For instance, we are interested to detect if there is a specific response of Antarctic climate to large volcanic events (e.g. the unknown 1809 eruption followed by the 1815 Tambora eruption). So far, no signal emerges from the background variability (or noise) in individual ice core records and even in multi-ice core synthesis (Goose et al., 2012; PAGES 2k, 2013).

High-resolution chemical (at LGGE) measurements including cations (Na+, Mg2+, Ca2+, and NH4+) and anions (Cl-, NO3-, SO42- as well as methanesulfonate (CH3SO3-)) will be performed for the period 1965-present (10 samples per year). The reason to extend the high-resolution record back to 1965 (instead of 1975 for water isotope) is related to our aim to search the Mt Agung reference horizon there. Longer records will be measured with a resolution of 2 to 5 samples per year for the past 100 or 200 years. Again the planned higher resolution to be applied for the chemistry than for water isotopes is mandatory by the search of volcanic layers that would be more easily detected with such a time resolution. Given the availability of high-resolution aerosol records at DDU and Concordia (from daily to weekly sampling), we will attempt to relate these coastal and inland records together with very high-resolution snow pit studies (1 cm) in view to identify potential fingerprints of specific changes of air masses such as sudden intrusion of warm marine air masses. If successful, it will allow to examine the evolution of the corresponding chemical signature along the transect coast/high plateau along the air mass trajectory.

Statistical analyses of the water isotope records will be performed in order to characterize the signal to noise ratio using replicate ice cores for a reference site. Time series analyses will be applied in order to identify the power spectrum of variability. Results will be confronted with calculations of moisture transportation using atmospheric reanalyses using back-trajectory tools, including the use of the MCIM isotopic model applied along trajectories (e.g., Helsen et al., 2006). We expect to provide new understanding of the processes controlling the variability of deuterium excess, and, vice versa, combine deuterium excess and chemistry records to detect spatial and temporal changes in moisture and aerosol origins. Particularly critical is the detection of signals related to changes in sea ice extent. Obtaining recent ice cores will allow to investigate the ice core fingerprint associated with the increased intrusion of maritime, moist air masses, and the ice core fingerprint of moisture formed at the sea ice margin. Recent studies performed in the Arctic have suggested that moisture formed at the sea ice margin could be associated with high kinetic fractionation due to evaporation associated with very dry air masses reaching the ocean margin (Kurita et al, 2011; Steen-Larsen et al, 2013). The mapping of the spatial and temporal variability using accurate deuterium excess data, combined with atmospheric moisture calculation, should allow to explore the validity of this hypothesis for Antarctica.