Among various paleoenvironmental proxy records across the lower Aptian Selli Level (Umbria-Marche basin, Italy) and its equivalents (i.e., sedimentary expressions of oceanic anoxic event [OAE] 1a), one very intriguing feature is a prominent negative δ¹³C excursion at the base of this sedimentary unit, generally by as much as a few per mil in marine carbonates. This early Aptian δ¹³C event has received special attention as an important clue to the genesis of OAE 1a, but the exact amplitude of this relatively short lived δ¹³C variation is not precisely constrained, and this fact has been an obstacle in paleoenvironmental modeling. Particularly enigmatic is the large amplitude, by −7‰, in pelagic limestones at the central Pacific Deep Sea Drilling Project (DSDP) Site 463; it may be a primary δ¹³C signal, considering its deposition in a fully open-ocean setting and a conservative burial diagenetic environment. Nevertheless, new Sr isotope data help identify a peculiar diagenetic overprint on the lower Aptian interval of this site. While the majority of examined samples represent slightly shifted, but acceptable, ⁸⁷Sr/⁸⁶Sr ratios for marine carbonates of this age, markedly unradiogenic ⁸⁷Sr/⁸⁶Sr ratios are recorded within and just below the Selli Level–equivalent interval. In particular, extremely unradiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.70623–0.70642) are detected at exactly the same interval as the most negative δ¹³C values and a smectite-rich lithology. It is therefore proposed that hitherto unknown diagenetic process, or processes, under peculiar interstitial-water geochemistry, resulting from volcanic ash alteration, played a role for the paired δ¹³C-⁸⁷Sr/⁸⁶Sr anomaly at DSDP Site 463. By removing the δ¹³C data from samples that possess the highly unradiogenic ⁸⁷Sr/⁸⁶Sr ratios, the amplitude of early Aptian negative δ¹³C excursion is reevaluated to be −2.7‰ or −3.3‰, facilitating comparison with published δ¹³C records from other sections. The refined Site 463 δ¹³C profile further implies that the early Aptian negative δ¹³C event is better described as a twin excursion.