On 9 January at 12:15 Timmu Kreitsmann will defend his doctoral thesis „Application of carbon isotope and rare earth elements as recorders of environmental conditions in the aftermath of the Paleoproterozoic Lomagundi-Jatuli Event“.
Professor Kalle Kirsimäe
Professor Aivo Lepland
Professor Juha Karhu (University of Helsinki, Finland)
The first appearance of free atmospheric oxygen ~2.4 billion years ago brought about fundamental changes in the atmospheric-ocean system and biogeochemical elemental cycles. The increase in oxygen concentration was accompanied by the largest perturbation in global carbon cycle called the Lomagundi-Jatuli Event (LJE), during which marine carbonates of anomalously enriched in heavy carbon isotope were precipitated. According to the most widely held notion, the O2 levels dropped to no more than 0.1% from present atmospheric levels after the termination of LJE 2.06 billion years ago and only increased again at the end of the Neoproterozoic. This thesis focuses on two post-LJE successions in north-western Russia – the 1.98 Ga Zaonega Formation and the 1.92 Ga Pilgujärvi Sedimentary Formation. These successions have previously been used to describe the global post-LJE environmental conditions, however, with contradicting results. This thesis characterises the petrography, mineralogy, carbon isotope composition and behaviour of rare earth elements of carbonates and apatites in these Formations. The thesis concludes that the negative carbon isotope excursion in the Zaonega Formation is primarily due to basin-specific processes (e.g., carbonates enriched in light carbon isotope were influenced by methane oxidation) that cannot be directly used to describe the global ocean. In addition, the behaviour of rare earth elements, particularly redox-sensitive Ce, showed that during the formation of the post-LJE Zaonega and Pilgujärvi sedimentary rocks, sufficient oxygen concentration was required in the atmosphere-ocean system in order to decouple Ce from other rare earth elements (negative Ce-anomaly). This calls into question the global decline in oxygen levels following the Lomagundi-Jatul Isotope Event and suggests prolonged elevated oxygen levels at the time.