| Widespread biomineralization originated in the latest Proterozoic, but the major radiation of skeletal organisms, which mediated changes in ocean chemistry, did not occur until the Early Cambrian. The earliest known skeletal metazoan Cloudina constructed a shell of numerous, thin layers in the form of slightly flaring tubes with one end open and the other closed. The layers were organic-rich and impregnated with high-magnesian calcite. Growth was episodic and each new layer was elevated from and to one side of the previous layer. Abundant pore-space between these layers made the whole structure susceptible to post-mortem breakdown into fragments characteristic of the genus. In situ Cloudina biosparites and reworked biomicrites are common throughout the Nama Basin, and this study has facilitated the identification of similar remains in uppermost Proterozoic deposits around the worlds. Thus, metazoan biomineralization was certainly global in extent by the latest Proterozoic.; A probable calcified metaphyte also occurs in the Huns Limestone Member, Nama Group. Originally aragonite-encrusted seaweeds are preserved as inclusion-rich zones in diagenetic calcite. The arched sheet-like to foliose morphology, aragonite mineralogy, relic ghosts after subcrustal, botryoidal aragonite (= hypobasal aragonite), and growth on a carbonate mud substrate have analogues in extant peyssonnelid red algae and late Paleozoic phylloid algae.; Petrographic and geochemical analyses of coated grains indicate that aragonite was the dominant nonskeletal precipitate forming in the Nama Basin. Thus, biological control over calcite nucleation can be inferred for Cloudina. Trace element and stable isotopic trends demonstrate that diagenetic textures containing increasingly small crystals were exposed to increasing water:rock ratios. Large neomorphic crystals containing ghosts after precursor fabric represent the geochemically best preserved texture.; {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm carb{rcub}{dollar} stratigraphic variation in the Lower Cambrian Forteau Formation, Newfoundland records organic diagenesis and differences in lithology. {dollar}sp{lcub}13{rcub}{dollar}C-depleted skeletons versus {dollar}sp{lcub}13{rcub}{dollar}C-enriched nonskeletal carbonates demonstrates that early skeletal organisms were exerting a 'vital effect' on the carbon-isotopic composition of their shells. Thus, the explosive radiation of biomineralizing organisms had an effect on the biogeochemical cycling of carbon-isotopes such that Proterozoic and Phanerozoic {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm carb{rcub}{dollar} records are not directly comparable. |
