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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.

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