The role of phosphorus cycling in Early Earth oxygenation

Stepwise increases in oxygen abundance have been recognised as some of the most significant events throughout Earth history, but the mechanisms that drive these increases are still elusive. Atmospheric oxygen increased above 10-5 PAL (present atmospheric level) at approximately 2.3 Billion years ago (Ga). This rise, now termed the Great Oxidation Event (GOE), is the first of 3 canonical increases. The second is termed the Neoproterozoic Oxygenation Event (~0.8 - 0.5 Ga), and the third is termed the Palaeozoic Oxygenation Event (~0.4 Ga). Throughout the majority of the Precambrian (prior to ~0.5 Ga), the dominant redox state of the deep oceans has also been debated, with the common consensus being a ferruginous state with progressively more frequent restricted euxinic conditions on the continental shelves, which may have been better established following the GOE. Many studies have invoked changes in the input of the key limiting nutrient phosphorus as a driver of these changes, but the behaviour of phosphorus under these differing redox conditions has not been adequately explored. Through the use of biogeochemical modelling, palaeoredox proxies and phosphorus speciation, this thesis aims to shine a light on the behaviour of phosphorus throughout Earth history and its relative importance in creating a more habitable Earth for complex life. Biogeochemical models of the oxygen and phosphorus cycles have typically either considered an incomplete oxygen cycle or a simplified view of the redox dependant phosphorus cycle. A biogeochemical model was built to better constrain oxygen and the behaviour of phosphorus, and shows that stepwise changes in atmospheric oxygen may have been a consequence of the feedbacks between the carbon, oxygen and phosphorus cycles. This model is then used in conjunction with geochemical data from before the GOE (~2.6 Ga). The geochemical data show that the onset of oxidative weathering of sulfides on the continents drove euxinia and promoted phosphorus recycling from sediments to the water column. This benthic phosphorus flux would then have allowed for increased productivity and oxygenation on a planetary scale, facilitating the GOE. Other authors have begun to propose oscillatory rise of atmospheric oxygen following deglaciation of a Snowball Earth interval associated with the GOE. The use of phosphorus speciation across this interval highlights elevated phosphorus availability following deglaciation which stimulated oxygen production and resulted in the loss of MIF-S as atmospheric oxygen rose. However, the behaviour of phosphorus provided a self-limiting feedback on the rise of oxygen, as oxygenated conditions led to phosphorus sequestration and a return to predominantly anoxic conditions in the midst of the GOE.