The oxygen isotope composition of dissolved inorganic phosphate and the marine phosphorus cycle

Phosphorus is a key nutrient often present in concentrations sufficiently low to limit or co limit primary production in natural aquatic environments. In eutrophic settings, anthropogenic phosphorus loadings can contribute to hypoxia and toxic algal blooms. On a global scale over millennial time scales and longer, phosphorus availability in the oceans has been invoked as a control on export production and organic carbon burial with feedbacks on atmospheric CO₂ and O₂ levels.; This dissertation encompasses two different approaches to the study of the marine phosphorus cycle. The first half of the dissertation focuses on the oxygen isotope composition of dissolved inorganic phosphate (P i) in natural waters. A technique is developed that permits the measurement of the δ18O of Pi with a high degree of accuracy and an analytical reproducibility of 0.2–0.3‰ (1 s.d.). Measured δ18O values for Pi span a 9‰ range from mid-latitude river water to coastal waters from Long Island Sound to deep water from the Pacific and Atlantic Oceans. P i was found to be near to a temperature dependent isotopic equilibrium with water oxygen in all parts of the ocean. Both equilibrium conditions and small deviations from equilibrium values provided useful information regarding the kinetics of phosphate transport and biological turnover in aquatic ecosystems.; The second half of the dissertation focuses on a critical but previously overlooked global flux of phosphate into the oceans. This flux is the benthic regenerative flux of Pi from reactive particulate phosphorus phases deposited to sediments, which was determined to be significantly larger than riverine inputs of dissolved phosphorus to the oceans. Small variations in the benthic regeneration efficiency of phosphate can translate to large changes in the oceanic phosphorus inventory on a time scale of 10,000 years. Furthermore, this return flux of phosphorus from sediments to ocean water was found to be sensitive to sediment redox conditions, supporting a hypothesis that phosphate may provide negative feedback stabilization of atmospheric oxygen levels on geologic time scales.