STRONTIUM ISOTOPE CONSTRAINTS ON PERMIAN-TRIASSIC GLOBAL CHANGE

The most rapid increase in seawater ⁸⁷Sr/⁸⁶Sr during the Phanerozoic occurred from the latest Permian through the Early Triassic. Strontium isotope values jumped from 0.7070 to 0.7082 in 10 million years or less. However, the rate and timing of Early Triassic isotopic change are poorly known because of gaps in the data and poor age control. Previous work has suggested that enhanced continental weathering drove the increase in seawater ⁸⁷Sr/⁸⁶Sr ratio, but the underlying driver remains poorly understood because the timing of major Permian glacial and orogenic events is too early to account for the large radiogenic excursion. To address this problem, we collected new Sr isotope data from Early Triassic carbonates of south China and Turkey, and use this record to model the Sr cycle through the same time interval. Constrained by recent improvements in Early Triassic geochronology, the Sr data reveal a dramatic rate of change in seawater ⁸⁷Sr/⁸⁶Sr: most of the excursion occurred during the first two million years of Early Triassic time. We use a strontium cycle box model to investigate possible causes of the Sr excursion and to quantitatively assess the potential role of Siberian Traps volcanism through weathering feedbacks. We find that the proposed magnitude of CO₂ release (132,000 Gt) during Siberian Traps volcanism is sufficient to account for much of the observed increase in seawater ⁸⁷Sr/⁸⁶Sr through CO₂ enhancement of continental weathering rates. This interpretation of the Sr cycle is consistent with geological, geochemical, and paleontological evidence for end-Permian and Early Triassic global warming, enhanced continental weathering and nutrient delivery, high primary productivity, and marine anoxia.