The lower Bajocian (Middle Jurassic) deposits record two successive carbon isotopes excursions (CIE) with a negative ~0.5‰ CIE near the Aalenian-Bajocian boundary followed by a more protracted positive ~1.5‰ CIE ending at the base of upper Bajocian deposits. Both events are observed in both northern and southern Tethys and have been attributed to a global carbon cycle perturbation. In order to constrain the possible cause of the lower Bajocian CIEs, we present new geochemical and sedimentological data from Chaudon-Norante (France) and Murtinheira (Portugal) sections. We have measured δ13Corg values and phosphorus content and calculated CaCO3, siliciclastics, organic matter and phosphorus accumulation rates using published astronomical age models. In the sections studied, there is no carbonate crisis as opposed to south Tethyan localities, pointing to a strong geographical control of calcium carbonate accumulation in the western Tethys during the Bajocian. Both sites record an increase in siliciclastics and phosphorus AR but relatively constant organic matter AR. The δ13Cbulk carbonate vs δ13Corg stratigraphic variations suggest that both negative and positive CIEs were likely caused by an increase in phosphorus and weathered carbon riverine input. Together, our new results suggest that this event was triggered by an increase in productivity and organic carbon burial. However, there are no recurrent black shales nor organic-rich deposits identified for this time interval in western Tethys. In summary, the lower Bajocian carbon cycle perturbation bears many similarities with the Valanginian event and direct comparison between both events may help to refine current hypotheses about the causes of Mesozoic positive CIEs.