Oxygen Isotope Composition of Phosphate Along Vertical Weathering Profiles that Developed on Igneous Rocks: Testing Modern Analogues for Identifying Terrestrial Life in Ancient Paleosols

Abstract

Life on Earth began in its early oceans before moving on to land. Determining the exact timing of an established biosphere on land, however, has been elusive, due largely to the lack of either fossil or chemical evidence on land deposits. Here I test a hypothesis that the activity of land biota can be recorded in the oxygen isotope composition of phosphate in weathering profiles developed on igneous parent rocks. Igneous rock contains phosphate (PO43-), which is readily utilized for biological activities during soil development. The oxygen in PO43- does not readily exchange its oxygen with ambient water. However, during intracellular pyrophosphate hydrolysis, the P-O bonds in PO43- break and reform repeatedly, resulting in exchange of oxygen with that of ambient water. Since the exchange occurs at low temperature and approaches equilibrium, the biologically processed PO43- will have much higher δ18O values than the ones from igneous sources. Thus, if an igneous weathering profile had been colonized by land biota, we shall expect to see an increase in the oxygen isotope composition of the bulk phosphate (δ18OPO4) from pristine un-weathered igneous rocks to the upper more weathered ones. To test this hypothesis, I targeted two recent weathering profiles: 1) a basalt weathering profile with continuous weathering since ~2 Ma near the town of Karamu, New Zealand, and 2) a basalt weathering profile with continuous weathering since ~4 Ma on Hainan Island in China. Samples were collected vertically across the profiles and then phosphate was extracted from them and precipitated as silver phosphate. Phosphate analyzed from the New Zealand profile shows a dramatic increase in its δ18O across the entire weathered section relative to fresh basalt. The δ18OPO4 of weathered samples range from ~19.4-24.9‰ while fresh unweathered parent basalt samples are between 7.4-8.6‰. The transition between the two is abrupt. The data from the Hainan profile are also consistent with the presence of biological P-recycling. The result of this study has an important implication for chronicling the early colonization of land by life. As long as there were biological activities on land the phosphate oxygen isotope composition will display a dramatic increase in δ18OPO4 up soil profiles developed on igneous parent rocks. Even if the upper section of an ancient paleosol is not preserved, which is often the case for paleosols, the dramatic increase in δ18OPO4 at the very beginning of signs of weathering, as shown by the Karamu section, increases