Multi-carbonate component reconstruction of mid-carboniferous (Chesterian) seawater 13C
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This study presents the first stratigraphically continuous 13 C carb record for the Chesterian interval developed from Antler foreland basin platform successions. The bulk carbonate-derived 13 C curve was constructed using 700 biostratigraphically constrained samples providing a 10 5 -yr resolution. A multi-component study of 13 C and 18 O variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and 13 C heterogeneity in these ancient epicontinental carbonates. Furthermore, it constrains the influence of kinetic and vital effects, and local-to-regional-scale C cycling on their 13 C carb values. The 13 C and 18 O values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability. Overall, biogenic calcite 13 C and 18 O values are higher than the average isotope compositions of bulk carbonates and micrites, although 13 C values in contemporaneous samples are only minimally (0.3; 2std err of 0.3) different relative to much larger differences in 18 O values. Although brachiopod 13 C and 18 O values are higher than average bulk carbonate and micrite values, their 13 C values exhibit considerable isotopic variability by taxa and ultrastructure. Notably, the 13 C values of brachiopod secondary shell layers overlap with bulk carbonate, micrite and marine cement 13 C values. Conversely, 13 C values of brachiopod tertiary prismatic layers, including Composita, are on average 2 to 5 higher than the 13 C values of contemporaneous bulk carbonates, micrites and brachiopod secondary shell layers. An observed average 2 decrease in the 13 C values of secondary cross-bladed laminar layers in productids relative to contemporaneous bulk carbonate, micrite and marine cement values suggests a previously unrecognized kinetic and/or vital effect on Carboniferous productid 13 C values given that they have the highest 18 O values of all analyzed components. The observed higher 13 C values in these tertiary prismatic layers are comparable to greater than the 13 C-enrichment ( 1) previously documented in Pennsylvanian Composita. The overlap in 13 C values of bulk carbonates with diagenetically screened, microdrilled matrix, marine cements and brachiopod secondary shell layers indicates that the Chesterian bulk carbonate-derived 13 C values may reliably record, to within 1, the evolution of ambient seawater 13 C in the outer carbonate ramp of the Antler basin. Comparison of the Chesterian 13 C record to the bulk-carbonate C isotope curve derived from the Arrow Canyon, NV carbonate succession, further suggests that these bulk carbonate-derived 13 C records are reliable proxies of the composition and trends in seawater 13 C regionally along the equatorial eastern Panthalassan margin. The newly resolved Chesterian stratigraphy documents multiple 13 C excursions, including previously unrecognized significant C isotope shifts. A shift to higher frequency and amplitude shorter-term 13 C fluctuations in the upper third of the Chesterian Stage occurred simultaneously with a change in stratigraphic cyclicity from dominantly 3rd-order to 4th-order carbonate cycles suggesting a mechanistic linkage between ambient seawater 13 C and sea-level fluctuations in the Antler basin. This change in the character of the shorter-term 13 C fluctuations coupled with a subsequent significant ( 4) positive C isotope excursion across the mid-Carboniferous boundary, previously recognized in all Carboniferous curves, is interpreted to record the onset of late Paleozoic glaciation and glacio-eustasy. 2007 Elsevier B.V. All rights reserved.
