Noble gas ratios in polar ice cores: a new proxy to infer the mean ocean temperature over the last 700 ka

Häberli, Marcel (2019). Noble gas ratios in polar ice cores: a new proxy to infer the mean ocean temperature over the last 700 ka. (Dissertation, Klima- und Umweltphysik, Physikalisches Institut, Universität Bern, Philosophisch-Naturwissenschaftliche Fakultät)

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Here, we present a proxy for the mean ocean temperature (MOT) based on measurements of atmospheric inert and noble gases trapped in ice core samples. Changes in the relative atmospheric concentrations of krypton, xenon and nitrogen trace total ocean heat content as they are caused by temperature-driven changes in gas solubilities in ocean water. In this dissertation, we report measurements of δKr/N2, δXe/N2, and δXe/Kr from the European Project for Ice Coring in Antarctica (EPICA) Dome C ice core. The aim of our studies is threefold: to further improve the novel technique of ice core noble gas thermometry, to analyse temporal changes in the gas transport in the firn column, and to reconstruct the MOT in peak glacials and interglacials during the past 700’000 years.
The motivation for the technical improvements was the need to measure δXe/Kr and δXe/N2 to derive the MOT based on three proxies in addition to the results of Headly and Severinghaus (2007), which were based on δKr/N2 only. Therefore, we improved our mass spectrometer technique to carry out high-precision measurements of elemental ratios δXe/Kr, δKr/Ar δXe/Ar and δAr/N2 and the isotopic ratios of Xe, Kr, Ar, and N2 with a high precision of 0.002-0.008‰ per unit mass difference, representing the major analytical prerequisite for precise MOT reconstruction using noble gases in ice cores. The total analytical uncertainty in the MOT (including the measurement precision, the uncertainty introduced through the error propagation of the gravitational correction and the influence of thermodiffusion) is ± 0.4 °C (1 σ) for all three proxies (δXe/Kr, δKr/N2 and δXe/N2). Moreover, the sample throughput of this time demanding analysis was substantially improved, which allows us now to extract and measure four samples per week.
The second focus of this dissertation are temporal changes in the gas transport in the firn. Using our extended suite of multi-isotopic gas parameters, we are able to quantify the glacial-interglacial changes in the isotopic composition of N2, Ar, Kr and Xe, which show a coherent depletion of heavier isotopes during glacial periods compared to interglacials for EDC, pointing to a glacial shortening in the diffusive column length of the firn. Despite the overall coherence of the isotopic evolution over the last 700 ka, the individual isotopic ratios of different noble gases show systematic offsets by 0 to +50 per meg relative to Ar isotopes. These offsets cannot be explained by thermodiffusion alone as these would be opposite in sign for a negative temperature gradient in the firn column towards the surface as expected for Dome C. Moreover, we see the same offsets in other cores where no temperature gradient is observed. Such offsets, however, can be explained by a disequilibrium fractionation between diffusive and advective processes caused by layering combined with barometric pumping and results in depleted heavy noble gas isotope ratios.
Third, we present the dynamics of ocean heat uptake over the last glacial- interglacial transition and show the long-term change in glacial and interglacial ocean heat content over the last 700 ka. We reconstructed MOT based on noble gas from the EPICA Dome C ice core covering the last 40 ka. In combination with recent results from the West Antarctic Ice Sheet (WAIS) Divide ice core and the sea-level record, they allow us to quantitatively reconstruct the history of the climate system energy budget. The temporal derivative of this quantity is equal to the planetary radiative imbalance. The results reveal a positive imbalance of typically +0.2 Wm−2 maintained for about 10 ka during the last deglaciation. Two distinct radiative imbalance peaks were found during times of substantially reduced Atlantic Meridional Overturning Circulation (AMOC). These show that variations in AMOC are not only connected with an energy redistribution within the Earth climate system, but also to a transient net change in the global energy balance.
Using the noble gas ratios trapped in the EDC ice core we present the first application of this method during peak glacials and interglacials from Marine Isotope Stage (MIS) 5 to MIS 17. The reconstructed MOT in peak glacials over the last 700 ka is consistently about 3 to 4 °C lower compared to the Holocene, while lukewarm interglacials before the Mid Brunhes event 450 ka ago are characterized by about 1.5 °C lower temperatures in the ocean than the Holocene in close correspondence to the reduced greenhouse gas forcing. It is also in line with the recently published findings that pointed out the similarity between MOT and Antarctic temperature. One remarkable feature of the record are the significantly increased MOTs at the onset of MIS 9.3. This is coeval with the CO2 and CH4 overshoot and might be directly connected to an AMOC resumption.

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