14:45 〜 15:00
[BPT02-05] Prolonged reducing and ferruginous oceanic condition after OAE1a evidenced from the siderite occurrences in the Mid-Pacific Mountains
★Invited Papers
キーワード：海洋無酸素事変1a、オントンジャワ海台、菱鉄鉱
The volcanic eruption forming the Ontong Java Nui (OJN) at the Pacific Ocean is considered as a trigger of early Aptian oceanic anoxic events (e.g., OAE1a) (~120 Ma). However, precise transition of the oceanic redox condition and its influence on the marine environment after OAE1a has been poorly understood. Deep Sea Drilling Project (DSDP) Site 463 is a borehole core drilled on the Mid-Pacific Mountains, the proximal site to OJN. The early to mid-Aptian of this core contains abundant siderite/pyrite grains potentially recording palaeoceanographic information at Pacific Ocean. Here, we provide a detailed description of the morphological and chemical features of siderite grains and its stratigraphic distributions in DSDP Site 463. Based on these data, we investigated the detailed changes in the paleo-oceanic redox conditions after the OAE1a.
At DSDP Site 463, abundant pyrite occurred during early Aptian OAE1a, while siderite grains became dominant for ~2 million years after the OAE1a. The distribution of pyrite/siderite grains suggests that strongly reducing/ferruginous condition prevailed in Pacific during OAE1a whereas mildly reducing/ferruginous condition continued for ~2 myr after the OAE1a. Considering that pyrite/siderite bearing interval contains volcanic ash derived from the OJN, the primary source of iron, required for the formation of pyrite/siderite, could be the volcanic ash and/or hydrothermal activity associated with the formation of OJN. The siderite bearing interval is only reported from the Mid-Pacific Mountains and not confirmed in Tethyan sedimentary sequence, suggesting that hydrothermal plume and volcanic ash did not reach the far distant section. Following the disappearance of siderite, brownish limestone began to appear, indicating the presence of hematite under oxidizing oceanic condition. Similar occurrences of brownish sedimentary rocks are also documented in the contemporaneous Tethyan and Atlantic sedimentary records, suggesting that oxidation was the global event.
At DSDP Site 463, abundant pyrite occurred during early Aptian OAE1a, while siderite grains became dominant for ~2 million years after the OAE1a. The distribution of pyrite/siderite grains suggests that strongly reducing/ferruginous condition prevailed in Pacific during OAE1a whereas mildly reducing/ferruginous condition continued for ~2 myr after the OAE1a. Considering that pyrite/siderite bearing interval contains volcanic ash derived from the OJN, the primary source of iron, required for the formation of pyrite/siderite, could be the volcanic ash and/or hydrothermal activity associated with the formation of OJN. The siderite bearing interval is only reported from the Mid-Pacific Mountains and not confirmed in Tethyan sedimentary sequence, suggesting that hydrothermal plume and volcanic ash did not reach the far distant section. Following the disappearance of siderite, brownish limestone began to appear, indicating the presence of hematite under oxidizing oceanic condition. Similar occurrences of brownish sedimentary rocks are also documented in the contemporaneous Tethyan and Atlantic sedimentary records, suggesting that oxidation was the global event.