New conodont δ18O records of Silurian climate change: Implications for environmental and biological events
Abstract A near-continuous, highly dynamic record of Silurian climate has been determined from conodont δ 18 O compositions (δ 18 O phos ), representing stratigraphic sections from four palaeocontinents. Following the late Ordovician–early Silurian glaciation, numerous small-scale climate cycles show an overall warming trend until the mid Llandovery (early-mid Telychian), then gradual cooling through the late Llandovery. Two major, multi-step cooling transitions of similar magnitude and duration (+ 2‰ within ~ 1.5 Myrs from onset to peak) occurred during the late Llandovery–early Wenlock and late Wenlock, the former culminating in a prolonged cool interval (~ 1 Myr). These Wenlock δ 18 O phos excursions coincided closely with the Ireviken and Mulde biotic and carbon isotope (δ 13 C) events. Rapid, large, but short-lived shifts reflect climate instability within the mid Ludlow, prior to and during the Lau bio-event. The prime δ 18 O phos records are from Laurentia and Baltica, which show mostly consistent and synchronous cycles, albeit slightly offset for much of the Llandovery and warmer intervals of the Wenlock. These offsets most likely reflect differences in the relative palaeogeographic positions of these continents, and changes in ocean dynamics driven by major shifts in glacial–interglacial climate modes. Many of the positive δ 18 O phos maxima correlate with sea level lowstands, as well as graptolite faunal turnover throughout the Silurian. Strong correlations between the large, first-order, positive δ 18 O phos shifts, increases in global δ 13 C carb , and the major Silurian bio-events (Ireviken, Mulde, Lau) imply that global climate change played an important role in producing these phenomena.