Carbon isotope evidence for deep-water invasion at the Marjumiid-Pterocephaliid biomere boundary, Black Hills, USA: A common origin for biotic crises on Late Cambrian shelves

An integrated carbon isotope, sedimentologic, and biostratigraphic study across a Late Cambrian mass-extinction boundary (Marjumiid-Pterocephaliid biomere boundary) in the Deadwood Formation, Black Hills, South Dakota, indicates invasion of the carbonate shelf by deep, cold, marine waters. Carbonate samples stratigraphically below the extinction boundary show an apparent trend of upwardly increasing δ¹³C values, from −0.4‰ to +0.1‰ and relate well to secular δ¹³C trends of Late Cambrian limestones. Coincident with the extinction horizon is a negative shift in δ¹³C values of 0.4‰ to 0.7‰. This shift can be attributed to introduction of cold, deep, ¹²C-enriched seawater accompanying invasion of opportunistic, cold-water trilobites onto depopulated shallow-water continental shelves. A similar abrupt negative shift in δ¹³C values has been observed at the suprajacent Pterocephaliid-Ptychaspid biomere boundary. Others have found that decreases in ⁸⁷Sr/⁸⁶Sr values synchronous with increases in δ¹³C values accompany renewed sea-level rise in this part of the Late Cambrian. Abrupt negative shifts in δ¹³C values at biomere boundaries require a companion process responsible for the extinction events. This process may be impingement of a shallow thermocline on the shelf or overturning of a stratified Cambrian ocean. Coupling of sea-level rise with location of the thermocline and shelf geometry may create conditions responsible for recurrent extinctions of shallow-shelf communities throughout the Late Cambrian.