NUTRIENT-ENRICHED WATERS AND THE EARLY SKELETAL FOSSIL RECORD

This paper examines the role of stratified, oxygen-depleted and nutrient-enriched water masses (NEW) in transforming the fossil record over the Precambrian-Cambrian boundary interval. The following may serve as markers for the encroachment of NEW over low-latitude cratons: trace-element enriched black shales, 'stone coals' and hydrocarbon source rocks, phosphorites, phosphatized shells and organic matter, sponge spicule cherts, early appearance of silica and phosphatic skeletons, appearance of Anabarites and Coleoloides biofacies, peak diversity of phosphatic species, occurrence of black shale lagerstatten and light delta-C-13 values of carbonates. Nutrient-depleted waters (NDW) are inferred over inner cratonic areas, and nutrient-starved waters (NSW) may well have developed over deeper waters at times through the later Cambrian, related to the effects of sea-level rise and salinity stratification. Carbon isotopic fluctuations are suggested to record the history of these water masses, with major changes in carbon burial and palaeoproductivity. The latter may well imply episodes of 'greenhouse' climate, with globally raised levels of carbon dioxide and relatively high ocean temperatures.