The upper Cambrian SPICE carbon isotope excursion from the Alborz Ranges, northeastern Iran

Abstract

Seawater chemistry has been highly variable during Earth's history. Whereas variations in the ratio of organic relative to inorganic carbon burial in oceanic basins are considered to trigger positive shifts in global seawater δ13C values, those in δ18O are related to Earth's climate fluctuations. Positive shifts in the carbon isotope record known from Cambrian marine carbonates on a global scale have provided invaluable stratigraphic insights. Carbon and oxygen isotope data from Middle-Upper Cambrian shallow marine carbonates of the Mila Formation, eastern Alborz Ranges, Iran are presented herein. Accordingly, the Furongian SPICE (Steptoean Positive Carbon Isotope Excursion) event is documented from the Qol-Qol Member of the Mila Formation at two stratigraphic exposures where this prominent and globally recorded isotopic event is detected in micrites/skeletal limestone with a maximum δ13C value of +4‰ at both localities. The positive shift initiates at the base, reaches the climax in the middle, and then declines in upper part of the Qol-Qol Member. Therefore, the base of the latter denotes the base of Furongian Series/Paibian Stage in the Alborz Ranges. The event occurred in the Palaeadotes–Torifera and Prochuangia trilobite Zones but detailed biostratigraphic studies are yet to be undertaken. The occurrence of glauconite and abundant benthic fauna in some intervals within the rock unit can be interpreted as recurring slightly-reduced and oxygenated sea floor circumstances during deposition of the Qol-Qol Member of the Mila Formation.