A mid-Caradocian (453 Ma) drawdown in atmospheric pCO(2) without ice sheet development?

摘要

Previous workers have recognized a positive delta(13)C 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 delta(13)C and delta(18)O unlike most previous studies of GICE which only observed increased delta(13)C marine values. Brachiopods from the lower Kullsberg Limestone reflect pre-excursion conditions having more negative delta(13)C (0-1 parts per thousand) and delta(18)O (-4 parts per thousand to -6 parts per thousand) values compared with least-altered marine cement from the upper Kullsberg Limestone, which have more positive delta(13)C (2.1-3.6 parts per thousand) and delta(13)C (-1.8 parts per thousand to -3.6 parts per thousand) 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 delta(13)C excursion event likely decreased in temperature by up to 15 degrees C with an approximate approximate to 1 parts per thousand increase in delta(18)O(H2O) 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 fori-nation of abiotic calcite at moderately low paleotemperatures during the GICE excursion of 10-17 degrees C and Ordovician atmospheric pCO(2) values of < 10 present atmospheric level. (c) 2005 Elsevier B.V. All rights reserved.