Evaluating the potential of I-type cosmic spherules as a proxy for the Δ’17O of atmospheric O2 for reconstructing paleo-CO2 levels and GPP throughout the Phanerozoic
by Fabian Zahnow
Date of Examination:2024-04-30
Date of issue:2024-12-09
Advisor:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Luigi Folco
Sponsor:Deutsche Forschungsgemeinschaft [Project number PA909/25-1]
Persistent Address:
http://dx.doi.org/10.53846/goediss-10920
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Abstract
English
Entirely molten micrometeorites, termed I-type cosmic spherules, are micrometer to millimeter-sized remnants of extraterrestrial Fe, Ni-rich metal that can be recovered on Earth. I-type cosmic spherules acquire all oxygen during their passage through the Earth’s atmosphere, forming Fe, Ni oxides. Therefore, the oxygen isotope variations in modern Antarctic I-type cosmic spherules are used to calibrate the reconstruction of the oxygen isotope composition of atmospheric O2. Fossil I-type cosmic spherules can be preserved in sedimentary rocks and collected for geochemical and extraterrestrial influx studies. As the Δ’17O of atmospheric O2 is a proxy for atmospheric CO2 levels and gross primary production, the oxygen isotope study of fossil I-type cosmic spherules can provide information on the composition of the Earth’s atmosphere from the past. However, up to now, no studies on the oxygen isotope composition of fossil I-type cosmic spherules have been conducted. This dissertation aims to evaluate the potential of fossil I-type cosmic spherules preserved in sedimentary rocks as archives for the Δ'17O composition of ancient atmospheric O2. From that, quantitative constraints on paleo-CO2 levels and gross primary production can be obtained. In this framework, four studies were carried out. They include: (I) The setup of a triple oxygen isotope microanalytical technique and its validation using urban rooftop micrometeorites. (II) The acquisition and isotopic (oxygen and iron isotopes) characterization of fossil I-type cosmic spherules from different Phanerozoic sedimentary rocks. (III) The geochemical investigation of the acquired spherules for their major and minor elemental compositions. (IV) The isotopic exploration of one diagenetic environment from which some of the I-type cosmic spherules were collected. The first study reveals that laser fluorination coupled with continuous flow isotope ratio gas source mass spectrometry enables high-precision (i.e. ± 0.04‰, 1SD) Δ'17O analysis for silicate and oxide samples as small as 1 – 27 µg. Studied as a proof of concept, the oxygen isotope signatures of urban micrometeorites align with those of modern Antarctic collections, indicating a shared parent body population predominantly composed of carbonaceous chondrites. The second study includes I-type cosmic spherules from ten different localities of Quaternary to Silurian ages and of varying host sedimentary rock lithologies. The study shows that post-depositional alteration is widespread in fossil I-type cosmic spherules, irrespective of the host rock or residence time. Altered I-type cosmic spherules retain no atmospheric iron isotope signal, but 10 – 35% of primary atmospheric O2 is still preserved. The triple oxygen isotope composition of altered I-type cosmic spherules allows modelling the degrees of alteration and mixing with diagenetic fluid. Rare unaltered spherules indicate constant atmospheric entry conditions at least over the past 87 million years. The Δ'17O of ancient atmospheric O2 and paleo-CO2 levels can be reconstructed from pristine I-type cosmic spherules, aligning with existing paleo-CO2 proxies for the Miocene (~8.5 Ma) and late Cretaceous (~87 Ma). In the third study, a subset of fossil I-type cosmic spherules was polished for textural and geochemical analysis of their interiors. The spherule sections show that pseudomorph Mn-bearing magnetite formation is characteristic during diagenetic alteration. Manganese incorporation serves as a robust geochemical tracer for spherule alteration. Large quantities of altered fossil I-type cosmic spherules can still retain information about the atmospheric oxidation state based on their oxidation rates. The fourth study examines the crystallization conditions of authigenic quartz in a Zechstein halite deposit, which serves as the host rock for some of the studied cosmic spherules. Analyzing the Δ’17O in minerals derived from fluids, such as authigenic quartz, enables the inference of the fluid's isotopic composition and the crystallization temperature of the quartz. The triple oxygen isotope composition of authigenic quartz from the Zechstein basin suggests crystallization from highly evaporated seawater-derived pore fluid. Pore fluid is a major driver of terrestrial cosmic spherule alteration. Overall, it is demonstrated that Δ'17O variations of fossil I-type cosmic spherules serve as a proxy for paleo-atmospheric CO2 levels, despite abundant diagenetic alteration. This thesis presents the first reconstruction of paleo-atmospheric CO2 levels based on triple oxygen isotope signatures of pristine fossil I-type cosmic spherules. It is favorable to apply this approach to a larger set of pristine samples per time interval studied to improve the precision of reconstructed paleo-atmospheric CO2 levels. Fossil I-type cosmic spherules qualify as a unique record of paleo-atmospheric conditions dating back billions of years.
Keywords: Triple oxygen isotopes; Cosmic spherules; Micrometeorites; Paleo-CO2 levels; Paleo atmosphere; Paleo environment
