Late Paleozoic Climate History Based on Oxygen Isotopes of Biogenic Apatite

The climatic evolution of the Late Palaeozoic is characterized by a prominent change from the Devonian greenhouse to the Permocarboniferous icehouse and the Late Permian warm mode. The Devonian, especially the Middle Devonian, is generally interpreted as a supergreenhouse, characterized by the maximum extent of Phanerozoic metazoan reef development. The oxygen isotope record of conodont apatite (δ¹⁸O_apatite) challenges this view. The isotope ratios suggest that the Early and Late Devonian (Famennian) were warm climatic periods whereas the Middle Devonian was characterized by temperate climatic conditions. Interestingly, the warm climatic periods are characterized by sparse metazoan reefs (Early Devonian) or by the extinction of the metazoan reef ecosystem (Late Devonian). Instead, metazoan reefs flourished during the cooler Middle Devonian time interval.

The Carboniferous is marked by the transition from the Late Devonian greenhouse to the icehouse of the Permocarboniferous glaciation. The onset of the glaciation is generally dated around the Mississippian-Pennsylvanian boundary. However, the oxygen isotope record of conodont apatite suggests that the initial onset of the glaciation occurred much earlier in the Tournaisian. We observe two positive shifts in δ¹⁸O_apatite of 2‰ and 1.5‰ in the Tournaisian and Serpukhovian, respectively. Both positive shifts in δ¹⁸O_apatite are interpreted to reflect climatic cooling and ice build-up at high latitudes. A first major glaciation event is interpreted to have occurred in the Tournaisian with ice masses persisting during the Visean. The second ice buildup event occurred in the Serpukhovian and early Bashkirian and culminated in the first glacial maximum of the Permocarboniferous glaciation. δ¹⁸O_apatite values decrease in the Bashkirian and show cyclic variations during the Pennsylvanian as consequences of the waning and waxing of high latitude ice caps.