PhD: Nutrient controls on climate and life

In this project you will use new and exciting experimental, analytical and numerical techniques to explore how the availability of iron and manganese in the oceans controls the global carbon and oxygen cycles, and hence the evolution of Earth’s climate and surface conditions over past, present and future timescales.

The availability of macronutrients like phosphorus and nitrogen, and micronutrients like iron and manganese, has played a fundamental role in regulating climate and surface conditions over Earth’s history, and will play a critical role in sustaining life on our planet into the future.

Traditionally nutrients are understood to affect climate and surface temperature through their impact on primary productivity. Nutrients are weathered from the continents and transported to the oceans by rivers, where they fuel photosynthetic primary productivity, which draws down carbon dioxide. When primary producers die most of their sequestered carbon is decomposed and returned to the atmosphere, but a small amount sinks to the sediments and is buried for millions of years. This locks carbon away from the atmosphere and so helps regulate climate and surface temperatures. This also frees up oxygen that would otherwise be used during decomposition, and so helps regulate oxygen levels in the ocean and atmosphere.

Recently however, research is revealing a more intricate interplay between nutrient availability and the carbon and oxygen cycles that control climate and life. For example, we suggest that the rise of oxygen to breathable levels during the Great Oxidation Event (some 2.4 billion years ago) was driven by feedbacks between the phosphorus and oxygen cycles (Alcott et al., 2019). We also suggest that the subsequent regulation of oxygen and carbon dioxide levels within habitable bounds has been modulated by iron and manganese (Moore et al., 2023). Similar key roles in past environmental change are proposed for other nutrients, yet how human perturbations to global nutrient cycling will impact future habitability is largely unknown.

We think that iron and manganese in particular play a dual role in controlling climate and life. This is because not only do they fuel photosynthetic primary productivity and thus regulate carbon burial, but they also form iron and manganese minerals in marine sediments. These minerals protect carbon from decomposition and so they can enhance carbon burial independently of primary productivity (Xiao et al., 2024). The availability of iron and manganese may therefore play a double role in controlling the carbon and oxygen cycles, and ultimately the evolution of climate and life on Earth (Zhao et al., 2023), but we are only just beginning to understand this double role and its impact on the Earth system.