PALEOSOL EVIDENCE FOR THE PHOSPHORUS BIOGEOCHEMICAL CYCLE IN THE PRECAMBRIAN

Recent studies have linked the termination of Middle Proterozoic banded iron formations (BIFs) to changes in nutrient and trace metal availability that enhanced oxygenic photosynthesis. Phosphorus was specifically implicated because phosphorus may adsorb onto oxides of redox sensitive metals such as iron and molybdenum and be sequestered in the BIFs. If the supply of oxides were reduced or phosphorus increased, photosynthetic production would have increased, which in turn would have increased the flux of oxygen to the atmosphere leading to the observed rise in pO₂. The primary flux of phosphorus to the oceans is the weathering of rocks and soils. New results from Precambrian paleosols indicate a significant change to the global phosphorus cycle about 1.9 Ga ago. Paleosols older than 1.9 Ga lost phosphorus extensively, while younger paleosols, including Phanerozoic analogues, accumulated or lost very little phosphorus, as in modern soils. Further, while the bulk abundance of phosphorus in source rocks is virtually unchanged over Earth’s history, depth profiles of paleosols indicate that soils have become considerable richer in phosphorus after 1.9 Ga. Phosphorus is bioaccumulated in modern soils by plants and soil microbiota. The observed shift in paleosols may indicate an emergent or changing terrestrial biota, with a greater capacity for bioaccumulation than before. Precambrian paleosols that had bioaccumulated phosphorus in association with microbiota would have delivered a more concentrated dose of both carbon and phosphorus to the oceans, stimulating primary productivity.