[M-IS14] 古気候・古海洋変動

[MIS14-P07] 南半球高緯度地域における海洋無酸素事変(OAE1a)の安定炭素同位体比層序による年代対比と火成活動をしめすシグナル

*上村 真優子¹、長谷川 卓²、Lee Young Eun³、Han Seunghee⁴、Jeong Hakwon⁴ (1.金沢大学大学院自然科学研究科自然システム学専攻、2.金沢大学地球社会基盤学系、3.Department of Geology, University of Vienna, Austria、4.School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju, Republic of Korea)

キーワード：安定炭素同位体比層序、海洋無酸素事変、OAE1a

The Oceanic Anoxic Event 1a (OAE1a) represents one of the key intervals of carbon cycle perturbation in Earth’s history. OAE1a occurred during the lower Aptian of the Lower Cretaceous, characterized by the event horizon recording notable sharp negative excursion of stable carbon isotope ratio followed by positive shift of the value. International Ocean Discovery Program (IODP) Site U1513, located on the western margin of the Mentelle Basin, provides an important southern high-latitude record of this event. This study analyzed stable carbon isotope ratio of organic matter (δ¹³C_org ), total organic carbon contents (TOC), calcium carbonate contents (CaCO₃ ), and mercury contents (Hg) in bulk sediments from 20 samples across the Aptian interval, collected from Hole U1513D. Our results reveal that the δ¹³C_org events can be correlated to the stratigraphic segments identified in the Cismon section (Menegatti et al., 1998), with the magnetostratigraphic constraints. C3 segments was recognized based on a notable negative excursion to −28‰ near the base of Chron C34n and was associated with a high TOC and low CaCO₃ values that certify the event of OAE1a. Following positive excursion can be correlated to C7 segment. C6 segment shows an increasing trend and a thin horizon in the Cismon section, so a positive excursion interval of U1513D could be correlated with at least a part of the C7 segment. Additionally, sample rich in glauconite should be interpreted to be correlated to the C4-C5 segment, based on its δ¹³C_org value being similar to the C2 segment and lower than the C7 segment. Below C3, we can recognize C2 segments with its values larger than −28‰. Hg data provide insight into the timing of volcanic activity relative to OAE1a. Our findings indicate that the volcanic activity started with substantial CO₂ release during the C2 interval. During the C3 period, notable organic carbon was buried and concentrated in association with the onset of OAE1a. Volcanism persisted through the C4-C5 segment and peaked in the C6-C7 interval, as evidenced by a significant increase in Hg concentrations. The Hg record was correlated with other sections based on carbon isotope stratigraphy, enhancing our scope of tight causal relationship between volcanic activity and OAE1a.