Pennsylvanian paleoclimatic record in rhythmites developed on an oceanic island of low-latitude Panthalassa

Abstract

Terrestrial carbonate deposits associated with exposed limestone are potential recorders of paleoclimates and karst paleohydrology. Here we examine the depositional process of the Omi rhythmite layers, which filled karstic cavities in the Pennsylvanian (middle Kasimovian) section of the Omi Limestone, a seamount-type limestone in central Japan. The laminated rhythmite layers accompanied signatures of subaerial exposure, including Microcodium, which indicates a seasonally dry climate in the late Kasimovian. The rhythmite layers are calcareous, low in siliciclastic content, and composed of millimeter-scale laminae. In each lamina, the grain size decreases and the δ18O value increases upwards. We inferred that each lamina corresponds to a single cave flood event followed by a dry and evaporative interval. 87Sr/86Sr ratios of the rhythmites are almost identical to the ratio of the surrounding limestone, but clearly lower than the Kasimovian seawater ratio. This indicates that the rhythmites and the surrounding limestone were subjected to the same post-depositional alteration, before the subduction of the seamount in the Guadalupian (middle Permian). The sedimentary and isotopic features of the Omi rhythmite and occurrence of Microcodium support repetition of wetting and drying on an equatorial Panthalassan island during the Pennsylvanian (late Carboniferous).