Open Access

Philipp Rammensee

• The Earth’s surface condition we find today is a result of long exposure to metabolism of life forms. Particularly, molecular oxygen in the atmosphere is a feature which developed over time. The first substantial and lasting rise of atmospheric oxygen level happened ≈ 2.5 Ga ago, but localities are reported where transiently elevated oxygen levels appeared before this time-point. To trace the timing and circumstances of the earliest availability of free oxygen in the atmosphere is important to understand the habitats of early microbial life forms on Earth. This thesis focuses to obtain information of oxygen levels and the related atmospheric cycling of metals in sediments of the 3.5 to 3.2 Ga Barberton Greenstone Belt. First, as iron was a ubiquitous constituent of Archean seawater, I investigated its isotopic composition in minerals of chemical sediments. Hereby, I tried to resolve the changes within the water basin on small scale sedimentary sequence cycles. Second, I focused on the minor constituents of Archean seawater. The Re-Os geochronologic system and the abundance patterns of the platinum-group elements were chosen to integrate information of oxygen promoted weathering of a large source area. To integrate information of a large time interval, the isotopes of uranium were investigated over a large stratigraphic section. The two key findings of this thesis are: • Quantitative oxidation of ferrous iron in surface layers of Paleoarchean seawater occurred during the onset and termination of hydrothermal FeIIaq delivery into shallow waters. • Paleoarchean sedimentary successions of the Barberton Greenstone Belt lack any evidence of transient basin-scale oxygenation. The Manzimnyama Iron Formation (IF, Fig Tree Group, Barberton Greenstone Belt, South Africa) has been deciphered to exist of cyclic stacks of lithostratigraphic units with varying amounts of iron oxide and carbonate minerals. In-situ femtosecond-Laser-Ablation ICP-MS iron isotope measurements showed that the majority of siderite (γ56Fe ≈ −0.5 ‰) precipitated directly from seawater of γ56Fe ≈ 0 ‰. Ferric iron from the surface layers is preserved in ≤ 1μ m hematite and in magnetite that has been grown within the consolidated sediment. During FeIIaq events, fine-grained hematite (γ56Fe ≈ 2.2 ‰) and magnetite (γ56Fe 0.5 to 0.8 ‰) indicate oxygen levels in surface waters of lower than 0.0002 μM. Upon onset and termination of iron oxide abundance, magnetite with γ56Fe ≈ 0 ‰ indicates that low concentrations of FeIIaq in surface waters were oxidized quantitatively. These observations demonstrate the existence of iron oxidation in Paleoarchean surface waters independent of FeIIaq concentration. This is the first investigation of Paleoarchean IF showing that lithostratigraphic cyclicity can be traced in iron isotopic composition of oxide minerals. ID-ICP-MS measurement of Re, Ir, Ru, Pt and Pd, trace element (SF-ICP-MS) and ID-MCICP- MS uranium isotope determination have been applied to carbonaceous shale of the Mapepe Fm. (Fig Tree Group) after inverse Aqua Regia leaching and bulk digestion. The sediments reveal a silicified fraction which exhibits a seawater REE signature and a mixture of detrital and meteoritic PGE. Neither enrichment of the redox-sensitive elements Re or Mo nor fractionated uranium isotopes have been found on a stratigraphic interval of several hundred meters. The non-silica fraction shows no depletion of Re which indicates that the detrital material had no contact to oxidizing fluids. ID-TIMS measurements of Re and Os after the CrO3-SO4 Carius Tube method of two sample intervals showed that the Re-Os isotopic systems of the non-silica fractions are identical to two komatiite occurrences. Weltevreden Fm. and Komati Fm. rocks were uplifted, eroded and transported to the deep part of the sedimentary basin without any change to the Re-Os system. Negative fractionated uranium isotopes (γ238U = −0.41 ± 0.01 ‰) associated with detrital Ba-Cr-U occurrences suggest the existence of distal redox-processes that involve uranium species. This study demonstrates that over the time of exposure and deposition of the Mapepe Fm. sedimentation, free oxygen was not available for weathering in the catchment area.
• Die geochemischen Bedingungen an der Erdoberfläche sind ein Ergebnis eines langen Entwicklungsprozesses deren wesentliche Dynamik als Resultat von Stoffwechselprodukten verschiedener Lebensformen gesehen werden kann. Insbesondere entwickelte sich die Konzentration von molekularem Sauerstoff in der Athmosphäre über die Erdgeschichte. Erstes nachhaltiges und deutliches Auftreten von Sauerstoff in der Athmosphäre erfolgte vor ≈ 2.5 Ga (Lyons et al., 2014). Verschiedene Funde deuten darauf hin dass schon lange vorher, regional und zeitlich beschränkt, freier Sauerstoff auftrat (Anbar et al., 2007). Diese frühe Verfügbarkeiten zu finden und deren Bedingungen herauszuarbeiten ist essentiell um die Lebensräume des frühen mikrobiellen Lebens zu verstehen...