Isotope studies of carbonate rocks of La Luna Formation (Venezuela) to constrain the oceanic anoxic event 3 (OAE3)

Abstract 87 Sr/ 86 Sr ratios, δ 13 C, and δ 18 O determinations performed on 30 bulk carbonate rocks are presented to constrain the isotope characteristics regarding Oceanic Anoxic Event 3 in the La Luna Formation, Maracaibo Basin. The samples were collected along a 22 m profile in San Miguel region (Merida State, Venezuela). The exhibited 87 Sr/ 86 Sr ratios are between 0.707659, at the base, and 0.707733 at the top of studied section, corresponding estimated younger numerical ages than the inferred ages dated as Santonian (85.8–83.5 Ma) based on occurrence of Dicarinella asymetrica . The obtained 87 Sr/ 86 Sr ratios are certainly higher than expected for Santonian sea Sr and they are attributed to regular riverine strontium fluxes in the restricted (gulf-like) Maracaibo Basin. The values of δ 18 O data between −3.76‰ and −11.76‰ are distributed in two distinct clusters in a same way and coherent as for δ 13 C data which are in a range between −1.75‰ and −13.87‰. One of the clusters δ 13 C = –2.5 ± 1.0‰ and δ 18 O = -10.0 ± 1.5‰), illustrate the homogeneous marine deposition under tropical conditions, while the other, more variable, probably could reflect the cyclicity of temporally changes in Pacific versus Atlantic Ocean circulation with cooling water or cooling trend climate. These δ 13 C values around −2.5 ± 1.0‰ do not fit with the significant carbon isotope excursion as expected for the oceanic anoxic event (OAE) and support the work hypothesis that Coniacian–Santonian anoxic event (denominate as OAE3) is not a global one.