『Abstract
　Authigenic phosphorite crusts from the shelf off Peru (9゜40' S to 13゜30' S) consist of a facies with phosphatic coated grains covered by younger phosphatic laminite. The crusts are composed of carbonate fluorapatite, which probably formed via an amorphous precursor close to the sediment water interface as indicated by low F/P2O5 ratios, Sr and Ca isotopes, as well as rare earth element patterns agreeing with seawater-dominated fluids. Small negative Ce anomalies and U enrichment in the laminite suggest suboxic conditions close to the sediment-water interface during its formation. Increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and, consequently, point to episodical development of anoxic conditions during phosphogenesis. The Peruvian phosphorites formed episodically over an extended period of time lasting from Middle Miocene to Pleistocene. Individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide-rich phosphatic layers. Coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions. Episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer. Phosphatic coated grains were later transported to the place of crust formation, where subsequent laminite formation was favored under lower energy conditions. A similar succession of phosphatic layers with varying contents of organic matter and sulfide-rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains.

Keywords: phosphogenesis; phosphorites; coated grains; suboxic conditions; anoxic conditions; sulfate reduction; Peru』

1. Introduction
2. Regional setting
3. Materials and methods
　3.1. Material
　3.2. Methods
　　3.2.1. Petrography and element geochemistry
　　3.2.2. Isotope geochemistry
4. Results
　4.1. Petrography
　　4.1.1. Facies with phosphatic coated grains
　　4.1.2. Phosphatic laminite
　4.2. Element distribution patterns
　　4.2.1. Major, minor, and trace elements of the facies with phosphatic coated grains and the phosphatic laminite
　　4.2.2. Rare earth elements
　4.3. Strontium and calcium isotopes
5. Discussion
　5.1. Phosphogenesis close to the sediment-water interface in the suboxic zone
　　5.1.1. Implications on the environmental setting of phosphorite formation
　　5.1.2. Redox conditions during phosphorite formation
　5.2. Longterm episodic formation of phosphorite crusts
　　5.2.1. Age and growth history
　　5.2.2. Formation of phosphatic coated grains
　　5.2.3. Formation of phosphorite crusts
6. Conclusions
Acknowledgements
References