C-O isotopic composition of Devono-Carboniferous carbonates of Belgium and Ireland: evidence of basinal anoxia and global change
Geochemical studies of Phanerozoic marine sediments have portrayed a dynamic ocean subject to episodes of rapid chemical change. The Devono-Carboniferous transition is the most dramatic of such episodes. It is marked by perturbations in delta/sup 13/C, delta/sup 18/O, delta/sup 34/S, /sup 87/Sr//sup 86/Sr trends and global disruption of biostratigraphic continuity with major extinctions of shallow-water benthos and widespread deposition of organic-rich black shales. In order to clarify the temporal relationships of marine isotopic variations to sedimentologic and faunal breaks, this study documents the fine scale secular isotopic variation of Devono-Carboniferous marine carbonates. Detailed conodont stratigraphy of the Devono-Carboniferous section of the Dinant Basin of south-central Belgium and the lower Carboniferous section of west-central Ireland provided the stratigraphic base for this carbon and oxygen isotopic study. Marine components, cements, micrites and biogenic grains were analyzed along with diagenetic phases to estimate primary marine carbonate isotopic compositions. Results indicate that delta/sup 13/C of upper Devonian (Frasnian) carbonates decreased from +5.5 to +2.0 per thousand PDB before increasing sharply on the verge of the Famennian to +4.0 per thousand PDB, while the delta/sup 18/O composition decreased from -4.0 to -6.0 per thousand PDB. An enrichment in /sup 13/C and /sup 18/O in marine carbonates occurred in the early Carboniferous amounting to a delta/sup 13/C change of +2.5 to 4.0 per thousand and a delta/sup 18/O shift from -6.0 to 2 per thousand PDB. Importantly, data from Canadian Devonian reefs and from the lower Carboniferous of North America indicate that this change occurred on a global scale. The Frasnian-Famennian carbon kick corresponds to the maximum Devonian transgression in western Europe, and a black shale event, which probably reflects the establishment of anoxic conditions.
- Research Organization:
- Univ. of Michigan, Ann Arbor (USA)
- OSTI ID:
- 6610729
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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