Land-ocean connections in organic carbon cycling amid the Early Triassic (Smithian-Spathian) revealed through compound specific isotope analysis

Following the largest mass extinction of the Phanerozoic, the Early Triassic was characterized by a series of carbon cycle perturbations as revealed through multiple global carbon isotope excursions (CIEs). The mechanistic drivers behind these perturbations are a subject of debate due to limited records that differentiate terrestrial and marine carbon cycling processes. In this study, we focus on the Smithian-Spathian boundary, which is characterized by a global positive CIE approximately 2 million years after the onset of the carbon cycle perturbations. We present the results of biomarker molecular distributions (i.e., n-alkanes) and compound-specific carbon isotope analyses (δ¹³C_alkane) for organic matter extracted from shales deposited at the Stensiöfjellet section in Spitsbergen, Norway. The measured middle Smithian δ¹³C_alkane values are among the lowest in the Phanerozoic and potentially indicate high atmospheric pCO₂ and a low δ¹³C value for CO₂ as a result of the oxidation of organic carbon. Marine and terrestrial δ¹³C_alkane records show parallel CIEs reflecting that both systems were equally affected by carbon cycle perturbations. Our data suggest the onset of the CIE started in the late middle Smithian, suggesting an earlier perturbation of the carbon cycle than previously recognized. Spathian δ¹³C_alkane values remain elevated and diverge from bulk δ¹³C trends, reflecting an intrinsic shift in both the marine and terrestrial carbon cycle. Considered together, our compound-specific carbon isotope analyses foster useful insights into the multiple carbon cycle perturbations during an interval of extreme environmental conditions marked by continuous biological radiation and extinction pulses, which might even be analogous to imminent future anthropogenic changes in climate. This study further shows that compound specific carbon isotope analyses can potentially also disentangle deep-time carbon cycle perturbations.