EARLY DIAGENETIC FERROUS-PHOSPHATE IN THE EDIACARAN DOUSHANTUO FORMATION PHOSPHORITE IN SOUTH CHINA AND ITS PALEOENVIRONMENTAL IMPLICATIONS

The termination of Cryogenian Snowball Earth glaciations was followed by an increase in atmospheric oxygen levels and the rise of complex life. An oceanographic record of these environmental and evolutionary transitions is hosted in the Ediacaran Doushantuo Formation of South China. In western Hubei Province, the Doushantuo Formation contains phosphorites deposited within ~20 Myr of the Marinoan cap carbonate. Modern phosphorite formation is often facilitated by microbes near the interface of deep oxygenated seawater and anoxic sediments, precipitating as inorganic calcium-phosphate, a major sink for phosphorus. However, deep ocean anoxia, inferred from geochemical evidence in the wake of Snowball Earth, likely attenuated this process. Reduction of this sink would have stimulated primary productivity, affecting oceanic and atmospheric redox conditions in both short and long terms. Here we present evidence that the formation of early diagenetic reduced-iron phosphate minerals present in Doushantuo phosphorites represents a previously undocumented phosphorus sink in Ediacaran oceans.

Based on petrographic analyses, we identified ferrous-phosphate (e.g. phospho-siderite) phases that precipitated during early diagenesis (pre-compaction) as botryoidal, fibrous, pore-filling cements in Doushantuo phosphorite. X-ray diffractometry, SEM/EDS and Raman analyses support the composition of these cements occurring between reworked phosphatic grains. Although granular phosphorite and interbedded pristine phosphorite were the dominant forms of phosphorus burial, ferrous-phosphate formation constitutes a previously undocumented sink. Increased phosphorus sequestration in ferruginous pore-water conditions would have limited primary production and sustained relatively low atmospheric O₂ levels in the long run. Pristine and reworked intervals also preserved microscopic fossils, implicating microbially mediated phosphorite formation. Ferrous-phosphate formation as a sink for phosphorus may have been elemental in governing the nature of the Neoproterozoic oxygenation event.