Phosphorus Cycling under different redox conditions

Phosphorus is a basic but essential nutrient element for life, as a major limiting nutrient, is also important for controlling primary productivity on geological timescales. However, P cycling is highly redox dependent through Earth history. The fate of P cycling in many environments is still not clear. This thesis is to investigate mineralogical controls on P cycling under different redox conditions. New geochemical data is reported in the low sulfate, euxinic Lake Cadagno, Switzerland, to investigate the behaviour of the phosphorus cycle. Sulfide-driven release of phosphorus from organic matter and Fe (oxyhydr)oxide minerals is re-trapped by Fe(II) phosphate to constrain the extent of P recycling, and this process is highly sulfate dependent. New experiments are designed to investigate what minerals control P cycling in the water column under different conditions. With the increase of P, Green rust is transformed to vivianite which shows that P concentrations exert first order controls on the mineralogy of Fe minerals, with strong implications for the availability of dissolved P in the water column. In order to quantify the extent of P cycling versus fixation in the sediments under different redox conditions, mass balance models are made based on 4 key redox scenarios. Analogous to those which were prevalent at various intervals in Earth history, under ancient episodes of ferruginous conditions, low flux of recycled P back to water column is controlled by both P re-uptake by Fe(III) (oxyhydr)oxide and the formation of Fe(II) phosphate. During the mid-Proterozoic and during Phanerozoic, under low sulfate euxinia, the flux of recycled P is constrained by Fe(II) phosphate formation. This study provides supporting evidence to the suggestion that the positive primary productivity feedback is constrained by these processes.