『Abstract
Manganese carbonate deposits in Japanese Jurassic sedimentary rocks were studied petrogeochemically. The deposits are characteristically composed of spheroidal micronodules, up to 1 mm in diameter, and always contain well-preserved radiolarian shells. Chemical elemental composition and mineralogical characteristics indicate that the micronodules contain rhodochrosite in a mixed carbonate phase composition (Mn86.7-92.2Ca2.2-2.9Mg2.6-6.7Fe2.6-5.6)CO3. Carbon and oxygen isotope values, which range from -7.99 to -4.78‰ and -4.05 to 0.28‰ relative to PDB, respectively, suggest that the manganese carbonate was precipitated in a suboxic zone. The micronodules closely resemble agglutinated benthic foraminifera in shape. We suggest that agglutinated foraminiferal tests composed of radiolarian shells accumulated selectively on the sediment surface during redeposition of bottom sediments and were replaced by manganese carbonate in suboxic diagenetic conditions of manganese reduction.』

Introduction
Geological setting
Experimental methods and results
Texture
Mineralogy
Electron microprobe analysis
Chemical analysis
Oxygen and carbon isotope composition
Discussion
Petrology and chemistry of manganese carbonate
Depositional environment of manganese carbonate
Origin of the micronodules
Conclusions
Acknowledgments
References