IAP-24-005

Unravelling nutrient feedbacks across the Great Oxidation Event

The Great Oxidation Event (GOE) in the early Paleoproterozoic, ca. 2.4-2.2 billion years ago (Ga), marks arguably the most significant environmental transition in the history of our planet. Atmospheric O2 levels increased from < 1ppm to perhaps 1% modern levels, sufficient to oxygenate biogeochemical cycles and make Earth habitable for complex life (reviewed in Lyons et al., 2024). Crucial in this transition was the supply of nutrients to maintain biological O2 production. This includes in particular nitrogen and phosphorus, which often limit productivity in terrestrial and marine environments to this day. While the supply of phosphorus depends on weathering fluxes (Cox et al., 2018) and sedimentary recycling (Kipp & Stüeken, 2017), nitrogen can be acquired by biological N2 fixation albeit at a high energetic cost to microbial ecosystems (reviewed in Stüeken et al. 2024). It is therefore generally thought that phosphorus is bio-limiting in geological timescales while nitrogen may be bio-limiting in the short-term (Tyrrell, 1999). Both nutrient-limitations must have been lifted in the lead-up to and during the GOE to enable widespread O2 production; however, the drivers and internal feedbacks of that transition are not well understood. The aim of this project is to shed new light on spatial distributions in nutrient availability during the GOE to better characterize and quantify nutrient sources and sink during this crucial time interval.
Some models have postulated an enhanced phosphate influx into the ocean caused by the onset of oxidative weathering on Paleoproterozoic land surfaces (Bekker & Holland, 2012). In contrast, others found limited evidence of changing phosphate availability through the Precambrian (Reinhard et al., 2017) or possibly even a decline from the Archean into the Proterozoic (Boden et al., 2024). Reconstructions of nitrogen availability find evidence of nitrate accumulation in oxygenated surface waters (Kipp et al., 2018), but spatial and temporal heterogeneities are not well resolved. The aim of this project is to undertake a detailed analysis of nitrogen and phosphorus cycling along environmental gradients during the GOE to (a) explore which environmental settings were most nutrient rich and supportive of microbial activity, and (b) infer processes that placed limits on the supply of bioavailable phosphorus and nitrogen.
Samples for this project will be obtained from the Onega Basin (Russia) and Francevillian succession (Gabon). Drill cores from the Onega Basin are archived in the Geological Survey in Norway and accessible through external collaborator Dr Aivo Lepland. These include evaporite beds and associated carbonate facies, as well as deeper-marine phosphatic horizons (Blättler et al., 2018; Lepland et al., 2014) that will enable a comparison of nutrient supplies from shallow to deep water. Complementary core material will be obtained from the recently funded ICDP drilling program in Gabon that will start in summer 2025 and provide cores through several sedimentary facies across the Francevillian basin. Dr Aivo Lepland is the lead PI of that program and will enable core access.
By comparing the nutrient inventory between different sedimentary facies, we will gain new insights into the major nutrient sources and sink, and we will be able to address questions about the limits of biological activity and O2 production during this crucial time in Earth’s history. The study will include novel applications of phosphorus speciation analyses that will reveal abundances of reduced and oxidized phosphorus in the rock record. Over the last few years, it has become increasingly clear that phosphorus can exist in multiple oxidation states, where the presence of reduced forms may carry important information about biogeochemical phosphorus limitations (Boden et al., 2024; Pasek, 2019). This project will explore that hypothesis. The phosphorus analyses will be combined with established isotopic measurements of carbon, nitrogen and sulfur to reconstruct redox conditions and metabolic pathways. All combined, we anticipate that this research project will advance understanding of how Earth’s surface transitioned from an anoxic to an oxic world.

The project will involve travel to core repositories in Norway to log core sections and select samples.
In the laboratory, isotopic analyses will be carried out with an elemental analyzer coupled to a gas-source mass spectrometer. Different substrates for analyses may be extracted physically or chemically prior to analysis. These analyses will be carried out at the Universities of Durham and St Andrews.
Phosphorus speciation analyses will be performed on on leachates and conducted by ion chromatography coupled to an ICP-MS. This work will be done at the University of St Andrews.

Year 1

Reading background literature; core sampling trip to Norway; first batch of sample preparation; introduction to analytical techniques

Year 2

Second sampling trip if required; focus on data generation for isotopic analyses and phosphorus speciation; present first dataset at a local conference.

Year 3

Complete geochemical analyses; draft publications on nitrogen and phosphorus cycling; present findings at an international conference

Year 3.5

Write thesis and finalize publications

The student will learn a number of practical and transferrable skills including:
– Geological sample characterization
– Laboratory techniques for sample preparation
– State-of-the-art analytical techniques (IRMS, IC, ICPMS)
– Critical evaluation of the scientific literature
– Statistical data analysis and data management
– Scientific writing
– Project management
– Teamwork
– Public speaking