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

Abstract This study presents the first stratigraphically continuous δ13Ccarb record for the Chesterian interval developed from Antler foreland basin platform successions. The bulk carbonate-derived δ13C curve was constructed using ∼ 700 biostratigraphically constrained samples providing a 105-yr resolution. A multi-component study of δ13C and δ18O variability in petrographically and geochemically screened abiotic and biogenic calcite components delineates the degree of preservation and δ13C 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 δ13Ccarb values. The δ13C and δ18O values of micrites and bulk carbonates overlap regardless of lithofacies and show minimal intrasample variability. Overall, biogenic calcite δ13C and δ18O values are higher than the average isotope compositions of bulk carbonates and micrites, although δ13C values in contemporaneous samples are only minimally (0.3‰; 2 std err of ± 0.3‰) different relative to much larger differences in δ18O values. Although brachiopod δ13C and δ18O values are higher than average bulk carbonate and micrite values, their δ13C values exhibit considerable isotopic variability by taxa and ultrastructure. Notably, the δ13C values of brachiopod secondary shell layers overlap with bulk carbonate, micrite and marine cement δ13C values. Conversely, δ13C values of brachiopod tertiary prismatic layers, including Composita, are on average 2 to 5‰ higher than the δ13C values of contemporaneous bulk carbonates, micrites and brachiopod secondary shell layers. An observed average ∼ 2‰ decrease in the δ13C 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 δ13C values given that they have the highest δ18O values of all analyzed components. The observed higher δ13C values in these tertiary prismatic layers are comparable to greater than the 13C-enrichment (∼ 1‰) previously documented in Pennsylvanian Composita. The overlap in δ13C values of bulk carbonates with diagenetically screened, microdrilled matrix, marine cements and brachiopod secondary shell layers indicates that the Chesterian bulk carbonate-derived δ13C values may reliably record, to within 1‰, the evolution of ambient seawater δ13C in the outer carbonate ramp of the Antler basin. Comparison of the Chesterian δ13C record to the bulk-carbonate C isotope curve derived from the Arrow Canyon, NV carbonate succession, further suggests that these bulk carbonate-derived δ13C records are reliable proxies of the composition and trends in seawater δ13C regionally along the equatorial eastern Panthalassan margin. The newly resolved Chesterian stratigraphy documents multiple δ13C excursions, including previously unrecognized significant C isotope shifts. A shift to higher frequency and amplitude shorter-term δ13C 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 δ13C and sea-level fluctuations in the Antler basin. This change in the character of the shorter-term δ13C 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.