A MID-CARADOCIAN (453 M.Y.) DRAWDOWN IN PCO2: EVIDENCE FOR A DECREASE IN SEAWATER TEMPERATURE AND ICESHEET DEVELOPMENT?

Previous workers have recognized a positive d¹³C excursion during the mid-Caradocian from Laurentian and Baltic platform carbonates. This study documents the mid-Caradocian excursion in Kullsberg Limestone (central Sweden). Significantly, we observe increases in both d¹³C and d¹⁸O unlike previous studies which only observed increased d¹³C marine values from the Baltic platform during this event. Brachiopods from the lower Kullsberg Limestone reflect pre-excursion conditions having more negative d¹³C (0 to 1‰) and d¹⁸O (-4 to –6‰) values compared with least-altered marine cement from the upper Kullsberg Limestone, which have more positive d¹³C (2.1 to 3.6‰) and d¹⁸O (-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 excursion event likely decreased in temperature by @ 5 ^oC with an approximate @ 1‰ increase in d¹³O_H2O attributable to increased salinity and/or formation of a small to moderate-sized transient ice sheet during the mid-Caradocian event. The results of this study are consistent with multiple independent approaches that characterize the composition of the Ordovician atmosphere/hydrosphere systems. Specifically, marine abiotic calcite formed at moderately low paleotemperatures (15 to 21 ^oC) coincident with a transient drawdown in mid-Caradocian atmospheric pCO₂ (<10 times present atmospheric level, PAL). Ambient, pre-excursion paleotemperatures on the Baltic platform were 20 to 25 ^oC with atmospheric pCO₂ values significantly greater than 10 PAL.