MAJOR ENVIRONMENTAL CHANGES IN THE TETHYS OCEAN AT THE EARLY TRIASSIC INDUAN-OLENEKIAN BOUNDARY

The Induan-Olenekian boundary (IOB), an important stratigraphic horizon in the mid-Lower Triassic, is characterized by a large (to +8‰) positive excursion in carbonate δ¹³C profiles. The Lower Triassic section at Mud in the Spiti Valley of northern India was deposited in a deep-shelf setting on the southern margin of the Paleotethys Ocean. The Mud section, which was a candidate for the IOB global stratotype section and point, has been exceptionally well-studied biostratigraphically (Krystyn et al., 2007, Albertiana 35:5; Brühwiler et al., 2010, JAES 39:724), providing a basis for detailed chemostratigraphic investigations. In this study, we analyzed d¹³C_carb, d¹³C_org, and d¹⁵N as well as major, trace, and REE concentrations for a 16-m-thick interval spanning the mid-Griesbachian to early Spathian substages of the Early Triassic at Mud. Our results show parallel excursions in the d¹³C_carb and d¹³C_org profiles, consistent with a productivity control and indicative of a major increase in organic carbon burial at the IOB. A concurrent positive excursion in d¹⁵N may reflect enhanced water-column denitrification in globally or regionally expanded oxygen-minimum zones, as a consequence of intensified marine productivity and organic carbon sinking fluxes. The Mud section exhibits a major change in lithology at the IOB, marked by a substantial reduction in the concentrations of clay minerals, quartz, and dolomite content and a large increase in calcite. Since calcareous plankton are unknown at this time, these changes may reflect shifts in seawater and atmospheric chemistry, e.g., an increase in carbonate saturation and lower pCO₂. Decreases in Mo and U concentrations suggest a shift toward locally less reducing conditions above the IOB, possibly due to better oceanic ventilation. Understanding the full significance of this oceanographic event will require a global analysis of IOB sections.