[B-CG07] Decoding the history of Earth: From Hadean to the present

[BCG07-P09] The origin of the early Aptian (~120Ma) volcanic ash deposited at the Mid-Pacific Mountains: Implication for the volcanic history of Ontong Java Nui

★Invited Papers

*Hironao Matsumoto¹, Kosuke T Goto², Gen Shimoda², Yasuto Watanabe³, Kotaro Shirai⁴, Maria Luisa Tejada¹, Takashi Sano⁵, Junichiro Kuroda⁴, Katsuhiko Suzuki¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.National Institute of Advanced Industrial Science and Technology, 3.Department of Earth and Planetary Science, The University of Tokyo, 4.Atmosphere and Ocean Research Institute, The University of Tokyo, 5.Department of Geology and Paleontology, National Museum of Nature and Science)

Keywords:OAE1a, Ontong Java Plateau, Pb isotope ratio

The Ontong Java Plateau, formed in early Aptian (~120 Ma) in the western Pacific Ocean is the largest Large Igneous Province on Earth. Recent studies have suggested that Ontong Java Plateau (OJP), Manihiki Plateau (MP), and the Hikurangi Plateau used to form a single large oceanic plateau called the Ontong Java Nui (OJN). Since OJN volcanism could be related to the contemporaneous environmental perturbations (e.g., oceanic anoxic event (OAE) 1a and biotic crises of calcareous planktons). However, since previous ocean drilling has recovered only the shallowest part of the OJN samples, the lack of information on its earliest volcanic history hampers our understanding of its evolution and its impact on the earth's environment.
The Deep Sea Drilling Project (DSDP) Site 463 is a borehole core composed of pelagic carbonate succession deposited on the Mid-Pacific Mountains, northeast of the OJN. The lower Aptian of the DSDP Site 463 also contains the OAE1a interval with tuffaceous sediments. In this study, we reconstructed detailed carbon (δ¹³C_carb), and Os (¹⁸⁷Os/¹⁸⁸Os) isotope stratigraphies of this core to constrain the exact duration of hydrothermal activity of the OJN formation. In addition, we determined the origins of silicate minerals contained in the DSDP Site 463 by using the Pb isotopic ratios (i.e., ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb) of the silicate fractions.
As a result, our new Os and δ¹³C data were consistent with those of previous studies. Furthermore, the Pb isotopic compositions of the volcanic ash interval during OAE1a got close to the Singgalo type OJP basalt and high-Ti type MP basalt. Considering that the Singgalo-type basalt is located on top of OJP, the volcanic ash could be derived from the most explosive subaerial/shallow-marine volcanic eruptions during the later stage OJN volcanism. Since the Singgalo-type volcanic ash interval occurs ~300 kyr after the onset of the Os isotopic decline, the formation of OJN could have been completed within the early stage of OAE1a. Based on the box-model calculations, the carbon isotopic variations during OAE1a can be best explained by the ~300 kyr CO₂ input during the early stage of OAE1a, which is consistent with our Pb isotopic data. Since this explosive subaerial/shallow-marine eruption during the early stage of OAE1a corresponds to the negative carbon isotopic shift, high temperature, and biotic crises, extremely rapid construction of OJN and associated subaerial/shallow-marine eruptions could have injected a large amount of CO₂ into the atmosphere directly and caused those environmental perturbations.