Variability in Early Jurassic background pCO2

• Early Jurassic background p CO 2 reconstructed over eight million years. • Orbital long-eccentricity pacing of p CO 2 . • Sinemurian–Pliensbachian transition marked by increase in p CO 2 . • Quantification of carbon release using LOSCAR. Quantifying past concentrations of atmospheric carbon dioxide ( p CO 2 ) is critical to understanding Earth’s carbon cycle dynamics and environmental responses. During the Mesozoic, large-scale volcanic episodes drove changes in p CO 2 that resulted in global biotic crises. The dynamics outside the realm of such upheavals remain largely unevaluated but are crucial to constraining background variability that contextualize disturbances. Using stable carbon-isotope fractionation as measured in molecular remains of chlorophyll on samples from the Prees 2C drill core and Robin Hood’s Bay section located within NW European shelf, we reconstruct p CO 2 over eight million years during Early Jurassic times (∼196–188 Ma). We identify fluctuations driven by long-eccentricity orbital cycles and higher average p CO 2 levels during the Early Pliensbachian (∼590 ppm) relative to the Late Sinemurian (∼400 ppm), separated by a sharp, short-lived rise at the Sinemurian–Pliensbachian transition reaching 1,080 ±680 ppm. Based on Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir (LOSCAR) model simulations, we quantify the carbon release associated with the Sinemurian–Pliensbachian transition to ∼7,000 Gt C, which is best explained by emission of thermogenic carbon, while net injection of carbon maintained through low-level solid Earth degassing may have driven elevated p CO 2 levels during the Early Pliensbachian. Our data provide new detailed insights into Early Jurassic background carbon cycle dynamics that support a view of the Jurassic, and potentially extending into younger Mesozoic times, in which the p CO 2 was shifting within a bounded envelope, punctuated by distinct p CO 2 pulses of ≥1000 ppm that mark episodes of global carbon-cycle and environmental perturbation.