δ18O and δ13C of Permian brachiopods: A record of seawater evolution and continental glaciation
Abstract Two hundred sixty-four δ 18 O and δ 13 C values of Permian articulate brachiopod shells were analyzed and 81 of these values were characterized as well preserved and biostratigraphically well defined. These were then utilized for construction of baseline oxygen and carbon isotope curves for the Permian interval. In addition, 21 δ 13 C whole rock values are reported for the Wordian and Capitanian. The early Permian, Asselian to Artinskian, times are characterized by ∼ 2.5‰ decrease in oxygen isotope values, from ∼ − 0.7‰ to − 3.3‰ (V-PDB). This is attributed to a ∼ 4–7 °C increase in temperature in the Southern Urals, concomitant with the retreat of the Permo-Carboniferous ice sheets and return of the 18 O-depleted melt water into the oceans. The Late Permian samples from Iran (Jolfa at Kuh-e-Ali-Bashi) and China (Meishan) yield δ 18 O values, and presumably temperatures, similar to those that followed the termination of the large-scale glaciation in the Lower Permian. In between, the upper Kungurian to Capitanian samples from the Delaware Basin (Guadalupe Mountains) are enriched in 18 O, at − 1.5‰ to − 3‰. We have no definitive explanation for these data, but tentatively suggest that the “anomaly” can potentially be a result of evaporative enrichment of seawater in 18 O, due to intracratonic arid setting of the basin. The 18 O-enriched nature of the Zechstein samples (− 1.2‰ to + 2.5‰), on the other hand, is in all probability a reflection of the high evaporation rates in the Zechstein sea. The Permian interval is characterized by a relatively constant δ 13 C, at about 4‰. The exceptions are again the brachiopods from the Delaware Basin (Guadalupe Mountains), which show ∼ 1.6‰ increase in the Guadalupian, to values of up to 5.9‰ in the Wordian. A tentative explanation, as in the case of oxygen, is based on the proposition that the semi-enclosed Delaware Basin was likely stratified, with sequestration of the 13 C-depleted carbon to the deeper water layers and a complementary 13 C enrichment in the upper oxygenated layer. The coeval open ocean water DIC may have been similar to that of the remainder of the Permian interval, at ∼ 4‰, as indicated by whole rock carbonate samples from Oman, Sicily, and Iran. In the latest Permian, the trend mimics the well-known δ 13 C drop at the Permian/Triassic boundary.