Total air content in the NGRIP and isotopic signatures of N2O in the Talos Dome ice core

Eicher, Olivier (2016). Total air content in the NGRIP and isotopic signatures of N2O in the Talos Dome ice core (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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This thesis contains two thematic key aspects. The main focus is total air content (TAC) in the North Greenland Ice Core Project (NGRIP) ice core. We present the first TAC record over an entire Greenland ice core, consisting of 1688 datapoints in total. The TAC record shows a local insolation signature, an effect known for Antarctic TAC records, now independently confirmed for Greenland by our study. We are also the first showing direct imprint of fast warmings, the Dansgaard-Oeschger (DO) events, in TAC. This TAC response is larger than expected considering only changes in air density by local temperature and atmospheric pressure changes as a driver, pointing to a transient firnification response in pore volume. We hypothesize that the transient firnification response is caused by the accumulation-induced increase in the load on the firn at bubble close-off, while at close-off temperature changes are still small. Using a firn model, we provide an upper limit of this effect. Our results contribute to the understanding of firnification processes and show that TAC is influenced by processes on both ice age and gas age scale. The study on TAC is published as a discussion version in Climate of the Past and has been revised since. The revised version represents chapter 2 of this thesis.
The second thematic focus of this thesis is N2O and its isotopic signature. We present 99 δ15N(N2O) and δ18O(N2O) samples form the Antarctic Talos Dome ice core in the time span 90,000 to 30,000 years before Present. N2O is the third most important greenhouse gas, after water vapour, and the least well understood of the big three anthropogenic climate change drivers CO2, CH4 and N2O. We confirm previous studies stating that both the marine and the terrestrial source of N2O contributed about equally to N2O increases in the past. Additionally we identify a long term trend towards heavier values in δ15N(N2O), after around 50,000 years before present. We argue that the long term trend could be caused by increased nitrogen supply to the biosphere, leading to enriched δ15N in the ecosystem and/or precipitation-induced changes in nitrogen loss from soils that also lead to δ15N changes in soils, which are imprinted in the nitrogen isotopic signature of N2O. Over DO event 21, we observe a parallel increase of δ18O(N2O) with N2O that we think was caused by a northward shift of the Intertropical Convergence Zone (ITCZ), causing higher δ18O values in precipitation over the southern tropics. The more northward shift of the ITCZ leads to more precipitation in the northern tropics, causing presumably also less N2O emission with a lighter δ18O signature in the northern tropics. Together both effects could be responsible for the increased δ18O(N2O) values observed over DO 21. Our results on N2O and its isotopic signatures are presented in chapter 3.
Two appendices are part of this thesis. Appendix A starts with a very short section on CH4 measurements on Silvretta glacier ice. We initially intended to use the drastic CH4 concentration rise since the industrialisation for the dating of this alpine ice core. Unfortunately, the data from three differentdepths only shows approximately current atmospheric CH4 concentration in all the samples. The results are presented in Appendix A.1.
A third scientific output during this thesis is a publication by M. Baumgartner on NGRIP CH4 in the time span 120,000-10,000 years before present and its relation to δ15N temperature reconstructions from the same ice core (Appendix A). This study was performed with CH4 data from the same NGRIP samples used in the TAC publication (chapter 2). The study shows a lag of CH4 of a few decades compared to reconstructed temperature from δ15N and provides further constraints for modelling studies by the results in interpolar CH4 difference between Dansgaard-Oeschger events (DO events) 18 and 20. Section A.2 contains the aforementioned CH4 publication "NGRIP CH4 concentration from 120 to 10 kyr before Present and its relation to a δ15N temperature reconstruction from the same ice core" by M. Baumgartner.
Section A.3 is a publication in Geophysical Research Letters, focussing on the nitrous oxide record from the NGRIP ice core. N2O concentration is measured in unprecedented temporal resolution of ∼75 years. The N2O concentration record shows pronounced relative minima during stadials with Heinrich events, massive iceberg discharges from the Laurentide ice sheet. N2O minima and the recovery in N2O concentration after Heinrich events are consistent with modelling studies on varying ceanic nitrous oxide emissions.
Section A.4 is a publication of the North Greenland Eemian Ice Drilling (NEEM) community members, an international joint project. The deepest part of the NEEM ice core was affected by folding, complicating interpretation. In this publication, the Eemian interglacial is reconstructed from the folded part. The University of Bern contributed by measuring CH4 and N2O in the deepest part of the NEEM ice core. This part, originating from the last interglacial, showed huge concentration spikes that were accompanied by low TAC values, pointing to possible melt layer formation at the last interglacial. By measuring a Holocene sample of the Dye 3 ice core with a visible melt layer we confirmed melt layers indeed result in concentration spikes in CH4 and N2O with parallel low TAC values.
In appendix B, all data measured during this thesis are listed.

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