SEA-LEVEL-DRIVEN CHANGES IN OCEAN CHEMISTRY AT AN UPPER CAMBRIAN EXTINCTION HORIZON

A combined stratigraphic and isotopic study across a Late Cambrian mas s-extinction horizon (Pterocephaliid-Ptychaspid biomere boundary) in c ratonal and miogeoclinal sections of the western United States reveals evidence for a pattern of sea-level rise, decreasing Sr-87/Sr-86 rati os, and increasing delta(13)C. The positive shift in delta(13)C, reach ing maximum values a few metres above the extinction interval, is inte rpreted to reflect elevated primary productivity and an increase in th e rate of burial of organic matter during expansion of the oxygen-mini mum zone. The Sr-87/Sr-86 decrease is interpreted to reflect some comb ination of increased sea-floor spreading rates and decreased continent al weathering rates. The delta(13)C and Sr-87/Sr-86 trends independent ly support the field-based hypothesis that a sea-level rise led to cat astrophic oceanic overturn, possibly culminating in anoxic conditions, that ultimately proved lethal to a large fraction of the existing she lf fauna.