Integrated bio-chemostratigraphy of Lower and Middle Triassic marine successions at Spiti in the Indian Himalaya: Implications for the Early Triassic nutrient crisis

摘要

In this paper we study the Lilang Super Group in the Spiti area, Indian Himalaya to understand environmental changes in the aftermath of the end-Permian mass extinction. We focus on the Mikin and Kaga Formations, which span the Induan to Ladinian stages of the Lower and Middle Triassic. These strata formed on the southern mid-palaeolatitude margin of East Gondwana and are interpreted as condensed, mixed carbonate - siliciclastic ramp deposits that deepened distally. Carbon isotope ratios of carbonate (delta C-13(carb)) show an increase from -2.6 to 0 parts per thousand from the Griesbachian substage to the Dienerian-Smithian transition, followed by a negative shift to -3 parts per thousand in the Smithian substage and a large positive excursion from -3.0 to 3.5 parts per thousand across the Smithian-Spathian boundary. A short negative shift to -1.0 parts per thousand occurred in the early Spathian substage, and is followed by a positive trend from similar to-1.0 to 1.0 parts per thousand in the Middle Triassic, with several minor excursions occurring during the Aegean substage. Carbon isotope ratios of total organic carbon (delta C-13(org)) co-vary with delta C-13(carb), suggesting that both proxies represent the original isotopic signatures. Sedimentary and palaeontological evidence, as well as trace metal geochemistry (Mo/Al ratio), indicate anoxic conditions developed from the late Griesbachian to the Dienerian substages, and dysoxic to probably fully oxic conditions from the late Dienerian substage onward. Anoxic conditions only very briefly reoccurred in the late Smithian substage. Ti/Ca and Zr/Al ratios suggest a consistently high terrestrial input in the first three substages of the Early Triassic, followed by a decrease across the Smithian-Spathian boundary. Thus, in the Spiti area, the positive delta C-13(carb) excursion across the Smithian-Spathian boundary is recorded in generally well-oxygenated sediments and coincides with a decrease in terrestrial input. While evidence for enhanced weathering is lacking, observations are strongly at odds with studies postulating eutrophication as a universal kill mechanism for the Smithian-Spathian crisis. The strata in the Spiti area contain an Early Triassic gap in phosphorite deposition indicating a similar to 5 Myr waning of coastal upwelling in an otherwise persistent (100 Myr) upwelling zone of the East Gondwana margin. The phosphorite gap suggests low P availability, low rates of organic matter degradation, and a reduction in nutrient exchange between deep and surface water masses. Altogether, the phosphorite gap, the paucity of sessile and filtering fauna in oxygenated waters, and low organic carbon burial rates indicate a collapse in marine productivity in the aftermath of the end-Permian mass extinction.