Sea-level-driven changes in ocean chemistry at an Upper Cambrian extinction horizon

A combined stratigraphic and isotopic study across a Late Cambrian mass-extinction horizon (Pterocephaliid-Ptychaspid biomere boundary) in cratonal and miogeoclinal sections of the western United States reveals evidence for a pattern of sea-level rise, decreasing ⁸⁷/Sr⁸⁶Sr ratios, and increasing δ¹³C. The positive shift in δ¹³C, reaching maximum values a few metres above the extinction interval, is interpreted to reflect elevated primary productivity and an increase in the rate of burial of organic matter during expansion of the oxygen-minimum zone. The ⁸⁷/Sr⁸⁶Sr decrease is interpreted to reflect some combination of increased sea-floor spreading rates and decreased continental weathering rates. The δ¹³C and ⁸⁷/Sr⁸⁶Sr trends independently support the field-based hypothesis that a sea-level rise led to catastrophic oceanic overturn, possibly culminating in anoxic conditions, that ultimately proved lethal to a large fraction of the existing shelf fauna.