LATE ORDOVICIAN d13CCARB, δ13CORG, AND 87/86SR STRATIGRAPHY ON OPPOSITE SIDES OF LAURENTIA: TECTONIC AND CLIMATIC IMPLICATIONS FOR A GREENHOUSE-ICEHOUSE TRANSITION

Sr and C (organic and carbonate) isotope curves from well-dated sections of Late Ordovician strata in Nevada and Anticosti Island, Quebec are used to investigate the timing and causes of the greenhouse-icehouse transition in the Late Ordovician. A Sr drop of 0.0008 in ~5 myr (late Whiterockian-early Mohawkian) spans the contact between the Antelope Valley Limestone and Copenhagen Formation in NV and equivalents in eastern Laurentia. Any regional input of radiogenic Sr from old cratonal rocks during sea level drop at this time was overwhelmed by input from young volcanic rocks. The rapid erosion of juvenile arc volcanics associated with the beginning stages of the Taconic orogeny in eastern Laurentia provide a possible source. Sr isotope values change little for the remaining ~ 10 myr of the Late Ordovician. Ccarb isotope values are steady through the interval of the Sr drop, but shift to heavier values (+ 3.5) in the Chatfieldian Stage (mid-Caradoc). This C isotopic shift was closely followed by relative sea level drop during deposition of the Eureka Quartzite in NV. Ccarb values are steady during the Cincinnatian until the Hirnantian excursion, which is known from both NV and Anticosti. Peak Ccarb and Corg isotope values are not coincident in the Anticosti Hirnantian section. Heaviest Corg values occur prior to the carbonate peak. Paired Corg-Ccarb analyses are consistent with Late Ordovician pCO2 decline that reached lowest levels just prior to the Hirnantian Ccarb peak, and higher pCO2 coincident with the Ccarb excursion. The Ccarb excursion on Anticosti is recorded in clean carbonates of the LaFramboise reef interpreted to reflect an interglacial rise in sea level. Continental shelf/margin chert and black shale deposition in NV took place prior to the excursion (ornatus/pacificus zones), and signals strong thermohaline circulation during glaciation. The Hirnantian carbonate carbon isotope excursion may thus be attributed to enhanced preservation of organic carbon in the deep sea during the switch over to warm saline bottom water formation.