Volcanism of the Ontong Java Plateau recorded in the Early Aptian pelagic sediments and its implication as a trigger of Aptian Oceanic Anoxic Event

It has been proposed that the emplacement of Ontong Java Plateau (OJP) was a trigger of the Early Aptian climatic perturbation, because the ages of basaltic rocks from OJP (125-119 Ma) are closed to the age of the global oceanic anoxic event termed OAE-1a (ca. 120 Ma). Although the causal linkage between LIP eruptions and OAEs has been proposed, no general consensus about the trigger of the OAEs has developed yet. We have investigated the possibility of the OJP volcanism instigating OAE-1a based on sedimentary isotopic records of lead (Pb) in bulk sediments throughout the Early Aptian pelagic sediments from central Pacific (ODP Site 1207 in Shatsky Rise and Yokonami Chert in Japan) and western Tethys (Livello Selli in Italy), by comparing the previously reported osmium (Os) isotopic record (Tejada et al., submitted) from the Tethyan section. Sedimentary Os isotopic record from Tethyan section illustrates two negative shifts (down to ~0.2) across the OAE-1a (Tejada et al., submitted), suggesting duplicate inputs of less radiogenic Os from the mantle source, which would be associated with the OJP emplacement. Lead isotopic compositions are totally different between the central Pacific and western Tethys, reflecting the regional differences in sediment provenances. The Pb isotopic ratios in the Pacific chert (Yokonami Chert) are much closer to those of OJP basalts than those from western Tethys section (Italian section), the latter of which are consistent with the Pb isotopic ratios of Eurasia-Africa continental crusts. Pb isotopic ratios of Shatsky Rise are plotted between Yokonami (close to OJP) and Tethys (close to continental crust) sections, suggesting that the contribution of mineral particles from OJP was less than Yokonami site. It is noteworthy that the Pb isotopic ratios of sediment show a slight shift toward those of OJP basalts (Kwaimbaita-Kroenke type). The shift was observed in all three sites, and corresponds to the first decreasing event of the two Os isotopic negative shift which is preceding the OAE-1a. The second negative shift of Os isotope is larger and longer than the first spike, but is not associated with Pb isotopic shift suggestive of OJP eruption. Because we consider that Pb would be supplied with fine mineral particle, the Pb isotopic shift toward the OJP values suggests subaerial or explosive eruption to transport fine mineral particles into the wide area of the ocean. We conclude that there were two OJP volcanic events during the Early Aptian; (1) the first volcanic event was more explosive, resulting in isotopic shifts of both Pb and Os, and (2) the second event corresponds to Livello Selli in Italy, was less explosive but much larger in volume and longer than the first one, resulting in the isotopic shift of Os alone.