Evidence for recurrent changes in Lower Triassic oceanic circulation of the Tethys: The δ13C record from marine sections in Iran

Abstract

Stable carbon isotope curves derived from Lower Triassic carbonate rocks from three Iranian sections are established to investigate changes in the carbon cycle during the Early Triassic in this area. The sections are located in the south-center (Abadeh), north (Amol), and northwest (Zal) of Iran. All three curves show a similar pattern starting out with high δ 13C values in the uppermost Permian decreasing across the Permian–Triassic boundary, an increase toward more positive values during the Griesbachian that slowly increase further up during the Dienerian, followed by a positive excursion to values as high as + 8‰ near the Dienerian/Smithian boundary. During the Smithian values return to below 0‰, whereas second positive excursion to values higher than + 3‰ is recorded at the Smithian/Spathian boundary, again followed by a drop in δ 13C into the Spathian and a final excursion to positive values at the Spathian/Anisian boundary. The results from these Iranian sections are consistent with previous studies from Italy and China, thus strongly suggesting that the recorded δ 13C variability represents at least Tethys-wide geochemical signals. Moreover, the new curves reveal evidence of high-amplitude, frequent oscillations pointing toward rapid and profound changes in the global carbon cycle during the Lower Triassic. Stratification of the ocean interrupted by episodic overturning transporting deep water to the ocean surface is a viable mechanism to account for the recorded isotope variations. Provided that the δ 13C curve is representative of the global Lower Triassic ocean, it has high potential for accurately dating sedimentary successions via chemostratigraphy.