The Early Triassic hothouse: what we know and what we don’t

The hothouse climate in the Early Triassic has been recognised for a decade. Yet it remains the most recently discovered hothouse and is poorly understood in many aspects. Initially triggered by the Siberian Traps in the latest Permian, the Early Triassic represents one of the most extreme and long-lasting greenhouses in the Phanerozoic. Although the outgassing of the Siberian Traps probably already decreased in the late Griesbachian, the Equatorial SSTs peaked at ~40 ℃ later in the late Smithian. The late Smithian thermal maximum coincided with resumed volcanic activities of a smaller scale. However, why lesser volcanism triggered Phanerozoic’s warmest hyperthermal is puzzling. The extreme warmth ameliorated in the latest Spathians, marking the termination of a ~5 Myr hothouse.

Many key questions about the Early Triassic climate remain unanswered. These include how warm the poles were, how flat the latitudinal SST gradient was, and how climate interacted with the global ocean circulation. However, the most fundamental question is how to maintain such an extreme hothouse climate for such a long time.

As most shelly fossils died out during the end-Permian mass extinction and the Early Triassic oceans were dominated by aragonite-shelled mollusks, reconstruction of Early Triassic seawater temperatures relies almost solely on oxygen isotope thermometer in conodont bioapatite. One of the key challenges is that Early Triassic conodonts are rare, small, and cannot be found everywhere due to the subduction of old ocean floors. These hinder the acquisition of proxy data in a broader palaeogeographic context. Future work combining proxy data with state-of-the-art Earth system modelling would be an ideal solution to better understand the hottest time in the Phanerozoic.