A common feature of records of the early Aptian oceanic anoxic event (OAE) 1a is the sharp negative δ 13C excursion displayed in both carbonate and organic matter at the onset of this event. A synchronous negative δ 13C excursion has also been noted for terrestrial organic matter. This negative excursion has been attributed to either an injection of 13C-depleted light carbon into the atmosphere or, in case of marine sediments, recycling of 13C-depleted CO2. However, most studies were done on separate cores, and no information on the relative timing of the negative spikes in terrestrial versus marine records has been obtained. Here we examine early Aptian core sections from two geographically distal sites (Italy and the mid-Pacific) to elucidate the causes and relative timing of this negative "spike." At both sites, increased organic carbon (Corg) and decreased bulk carbonate contents characterize the interval recording OAE 1a (variously referred to as the "Selli event"). The organic material within the "Selli level" is immature and of autochthonous origin. Measured δ 13C values of marine and terrestrial biomarkers largely covary with those of bulk organic carbon, with lowest values recorded at the base of the organic-rich section. By contrast, sediments enveloping the "Selli level" exhibit very low Corg contents, and their extractable Corg is predominantly of allochthonous origin. Hydrous pyrolysis techniques used to obtain an autochthonous, pre-Selli δ 13C value for algal-derived pristane from corresponding sample material yielded a negative δ 13C shift of up to 4?. A negative δ 13C shift of similar magnitude was also measured for the terrigenous n-alkanes. The results are collectively best explained by means of a massive, syndepositional, rapid input of 13C-depleted carbon into the atmosphere and surface oceans, likely delivered either via methane produced from the dissociation of sedimentary clathrates or perhaps by widespread thermal metamorphism of Corg-rich sedimentary rocks.