Silurian δ13C stratigraphy: A view from North America

Three positive shifts in δ 13 C are recognized in well-dated carbonate successions in North America and can be confidently correlated with shifts in the Baltic region by using conodont and graptolite zonations. The δ 13 C excursions reach distinct peaks of +3‰ to +4‰ at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone. The three events are associated with the most significant extinction horizons in the Silurian and occur near clastic-carbonate transitions that mark eustatic or oceanographic changes. The magnitudes of the two oldest δ 13 C peaks compare well with the Baltic region; however, the end- siluricus excursion reaches far greater values of as much as +11‰ in Sweden, likely reflecting local modification of the global seawater signature.