Multi-carbonate component reconstruction of mid-carboniferous (Chesterian) seawater δ13C

Abstract

This study presents the first stratigraphically continuous δ 13C carb record for the Chesterian interval developed from Antler foreland basin platform successions. The bulk carbonate-derived δ 13C curve was constructed using ∼ 700 biostratigraphically constrained samples providing a 10 5-yr resolution. A multi-component study of δ 13C and δ 18O variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and δ 13C 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 δ 13C carb values. The δ 13C and δ 18O values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability. Overall, biogenic calcite δ 13C and δ 18O values are higher than the average isotope compositions of bulk carbonates and micrites, although δ 13C values in contemporaneous samples are only minimally (0.3‰; 2 std err of ± 0.3‰) different relative to much larger differences in δ 18O values. Although brachiopod δ 13C and δ 18O values are higher than average bulk carbonate and micrite values, their δ 13C values exhibit considerable isotopic variability by taxa and ultrastructure. Notably, the δ 13C values of brachiopod secondary shell layers overlap with bulk carbonate, micrite and marine cement δ 13C values. Conversely, δ 13C values of brachiopod tertiary prismatic layers, including Composita, are on average 2 to 5‰ higher than the δ 13C values of contemporaneous bulk carbonates, micrites and brachiopod secondary shell layers. An observed average ∼ 2‰ decrease in the δ 13C 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 δ 13C values given that they have the highest δ 18O values of all analyzed components. The observed higher δ 13C values in these tertiary prismatic layers are comparable to greater than the 13C-enrichment (∼ 1‰) previously documented in Pennsylvanian Composita. The overlap in δ 13C values of bulk carbonates with diagenetically screened, microdrilled matrix, marine cements and brachiopod secondary shell layers indicates that the Chesterian bulk carbonate-derived δ 13C values may reliably record, to within 1‰, the evolution of ambient seawater δ 13C in the outer carbonate ramp of the Antler basin. Comparison of the Chesterian δ 13C record to the bulk-carbonate C isotope curve derived from the Arrow Canyon, NV carbonate succession, further suggests that these bulk carbonate-derived δ 13C records are reliable proxies of the composition and trends in seawater δ 13C regionally along the equatorial eastern Panthalassan margin. The newly resolved Chesterian stratigraphy documents multiple δ 13C excursions, including previously unrecognized significant C isotope shifts. A shift to higher frequency and amplitude shorter-term δ 13C 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 δ 13C and sea-level fluctuations in the Antler basin. This change in the character of the shorter-term δ 13C 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.