A mid-Caradocian (453 Ma) drawdown in atmospheric pCO2 without ice sheet development?

Previous workers have recognized a positive δ13C excursion during the mid-Caradocian in Laurentian and Baltic platform carbonates. This study documents the mid-Caradocian Guttenberg excursion (GICE) in the Kullsberg Limestone in central Sweden. Significantly, we observed increases in both δ13C and δ18O unlike most previous studies of GICE which only observed increased δ13C marine values. Brachiopods from the lower Kullsberg Limestone reflect pre-excursion conditions having more negative δ13C (0–1‰) and δ18O (− 4‰ to − 6‰) values compared with least-altered marine cement from the upper Kullsberg Limestone, which have more positive δ13C (2.1–3.6‰) and δ18O (− 1.8‰ to − 3.6‰) values. Least-altered marine cements from the Kullsberg Limestone include: columnar calcite, first-generation equant calcite, and translucent fibrous calcite (from earliest to latest in timing of formation). Kullsberg seawater during the δ13C excursion event likely decreased in temperature by up to 15 °C with an approximate ≈1‰ increase in δ18OH2O attributable to increased salinity during the mid-Caradocian. The results of this study are consistent with multiple independent approaches to characterize the composition of the Ordovician atmosphere/hydrosphere systems. Specifically, results of this study are consistent with formation of abiotic calcite at moderately low paleotemperatures during the GICE excursion of 10–17 °C and Ordovician atmospheric pCO2 values of < 10 present atmospheric level.