Iron phosphate in the Ediacaran Doushantuo Formation of South China: A previously undocumented marine phosphate sink

Abstract

Ediacaran phosphorites capture the dynamics of the ultimate biolimiting nutrient, phosphorus, during perhaps the most critical transition of Earth's climatic and ecological history. Concomitant with the Neoproterozoic Oxygenation Event was the deposition of the first extensive phosphorites across marine shelves, typically interpreted as a basinward shift in the locus of phosphogenesis facilitated by deep-ocean oxygenation. Petrographic and spectroscopic analyses of proximal phosphorites from the Ediacaran Doushantuo Formation near the Yangtze Gorges area of South China reveal the presence of interlaminated pristine, muddy, and granular phosphorites that were cemented by early diagenetic iron phosphate, an additional, previously undocumented seafloor P sink. Transmitted- and reflected-light microscopy, cathodoluminescence microscopy, and back-scattered electron scanning electron microscopy were used to reconstruct the paragenesis of phosphorites at the Wanjiagou section near Zhangcunping (Hubei Province) and demonstrate that pristine phosphorite precipitated authigenically as francolite via microbially-mediated phosphogenesis as evidenced by preserved microbial laminations along with filamentous and coccoidal microfossils. Subsequent tidal reworking of pristine hardgrounds and phosphatic mudstones produced minimally-coated intraclasts and peloidal phosphatic grains that were cemented by iron phosphate soon after deposition but before compaction. Raman spectroscopy, bulk-sample and micro-X-ray diffractometry indicate the iron phosphate cement is composed of phosphosiderite/strengite (Fe+3PO4•2H2O). The presence of phosphosiderite can be attributed to either syndepositional oxidation of phosphorites or thermal stabilization of vivianite (Fe2+3(PO4)2•8H2O). In both cases, the occurrence of phosphosiderite represents the first direct evidence for iron phosphate minerals as a P sink during the Ediacaran and suggests phosphogenesis may have partially proceeded in ferruginous porewaters in contrast to the redox-independent process of francolite precipitation. Integration of petrographic and geochemical techniques, such as those employed in this study, offers the ability to corroborate P speciation analyses and provides an independent determination of P phase partitioning. Ultimately, this approach is paramount in testing hypotheses that suggest ferrous iron phosphate minerals played a regulatory role in global Proterozoic oxygenation.