Algal reorganization in post-crisis early Triassic oceans revealed by biomarker evidence

The end-Permian mass extinction (EPME) fundamentally reshaped marine ecosystems. However, the long-term response of eukaryotic algae, a key foundation for marine primary production, is poorly understood. To address this limited knowledge, we determine the long-term change in algal communities using molecular fossil steranes. We use samples that span the uppermost Permian to the Lower Triassic from sections that were located in Boreal Sea (Sverdrup Basin, Arctic Canada) as well as the tropical Tethys (Xiakou, South China), and complement these new data with published datasets. Sterane to hopane ratios, reflecting the relative contribution of eukaryotic algal to bacterial sources, vary in absolute values between sites but show no significant decrease in the earliest Griesbachian compared to the pre-crisis Permian. However, Early Triassic ratios changed dramatically. In the Sverdrup Basin, they were stable during the Griesbachian and, following an interval where both hopane and sterane concentrations diminished, became much higher in the late Spathian. This confirms suggestions that there was a major decline in algal productivity after the EPME that may have delayed recovery. Sterane C28/C29 ratios, which monitor algal composition, increase at the EPME level in Meishan and are generally higher in the rest of the Early Triassic in the Sverdrup Basin and Chaohu. The increase shows that algae that preferentially produce C28 over C29 sterols were thriving, possibly including those predominant in modern oceans. It further implies a reorganized marine algal community–apparently in the tropics and in the post-crisis interval in the Boreal realm. Our findings suggest that instead of a simple collapse and recovery, the Early Triassic saw a complicated reorganisation for algae.