NEOPROTEROZOIC BIOGEOCHEMISTRY AND THE TERMINAL PROTEROZOIC REORGANIZATION

Isotopic relationships between individual organic compounds and bulk organic carbon in Neoproterozoic sediments differ profoundly from those observed in the Phanerozoic. Few algal biomarkers have survived. Lipid carbon skeletons apparently produced by consumers are abundant. In contrast, Phanerozoic sedimentary molecules are dominated by primary products. Neoproterozoic records of the abundances of carbon-13 in carbonate minerals and total organic carbon and of sulfur-34 in sulfates and sulfides also differ significantly from their Phanerozoic counterparts. The former have been interpreted in terms of surging flows of carbon, either between the atmosphere and the ocean or between chemically distinct pools of carbon in the ocean. For sulfur, it’s been suggested that the widespread occurrence of sulfides enriched in sulfur-34 relative to marine sulfate reveals the existence of a sulfate-minimum zone analogous to the oxygen-minimum zone in modern oceanic waters.

The global replacement of the strange isotopic relationships and extreme variations characteristic of the Neoproterozoic by normal (Phanerozoic) signals plausibly resulted from a microscopic change, namely the development of rapidly sinking organic material. This was triggered by some combination of the packaging of primary materials in fecal pellets and the advent of biomineralization. It greatly reduced concentrations of dissolved and suspended-particulate organic matter and reorganized global biogeochemical cycles. Reconstruction of these events will benefit from attention to the isotopic records of biogeochemical products (organic material, sulfides) as well as precursors (carbonates and sulfates).