カンブリア紀最前期胚化石の化石化過程と古海洋的背景

概要

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Paleontological, petrological and geochemical studies from the Kuanchuanpu Formation in southern Shaanxi Province, strata known for the earliest Cambrian animal embryos, revealed the paleoceanographic settings and processes permineraling an exceptionally rich fauna. Lack of highenergy sedimentary structures and presence of fossilized photosynthetic microbes indicated that a calm and shallow depositional environment was developed on the northern marginal Yangtze platform. The upward-decreasing trend of reductive elements (Mn and Fe) in carbonate fraction indicated a gradual transition to oxic conditions, which allowed animals to inhabit during the deposition of the upper Kuanchuanpu Formation. The animal embryos and small shelly fossils were permineralized by apatite due to intensified upwelling that rose the concentration of phosphates in the bottom water. The intensified upwelling was probably responsible for the upward-decreasing trend of the carbonate δ^<13>C values. The permineralization selectively proceeded in the organic lipid membranes that were resistant against decomposition. The lowered pH, locally developed by bacterial activities inside the embryos was another potential factor for the precipitation of apatite.
2008-10-01