Oxygen Isotope Geochemistry of Phosphate from Igneous Rock Weathering Profiles

Abstract

Biological phosphorous (P) -cycling in marine, terrestrial and atmospheric realm is key to evolutionary and climatic changes in Earth history. Oxygen isotope composition of phosphate reveals mechanisms of bond breaking and reforming during P-cycling as well as ambient water oxygen isotope compositions. I first review the state-of-art knowledge of phosphate P-O bond breaking and reforming induced by enzymatic processes (Chapter 1). Literatures in both the geochemistry and biochemistry have shown that on the cellular level, equilibrium exchange between phosphate and water happens within the cell and a non-equilibrium kinetic isotope effect occurs in the bond breaking and forming catalyzed by extracellular enzymes. Triple oxygen isotope could potentially separate the mixed signals between the two dominant processed which are otherwise hard to interpret. After we have concluded that P-O bond cannot be readily altered without extensive biological activities, we move to explore the history of land colonization by biota. I obtained a set of drill core samples from a paleoweathering profile of Middle Cambrian age (~500 Ma). The extracted phosphate oxygen isotope signature shows typical igneous ä18O and ∆17O values with no change between pristine and weathered igneous rocks. This is in contrary to the 13.2 ‰ change in the modern profile as demonstrated by Dustin Boyd’s thesis, suggesting a lack of any significant P-cycling in Middle Cambrian land surface (Chapter 2). The same triple oxygen isotope approach was applied to explore the weathering nature of weathering rinds, which shows a 0.8 ‰ excursion from pristine to weathered rinds, implying that biological activities are playing a role in the formation of the weathering rinds (Chapter 3). Finally, a set of phosphate samples extracted from a paleoweathering profile from the Late Permian Emeishan Large igneous provinces (ca. 260 Ma) reveals not only a modern-like land biological P-cycling but also local meteoric water’s triple oxygen isotope composition at that time, opening up a promising venue for studying paleo-precipitation and paleoaltimitry (Chapter 4). Much of this study is explorative and has revealed many new applications of triple oxygen isotopes of phosphate. It paves the way for a more systematic study of geological materials with extensive sampling in space and time.