[B-PT02] 地球生命史

[BPT02-P03] Oceanic anoxic event 2 observed in Southwestern Newfoundland Ridge

*守屋 和佳¹、宮倉 寧子²、長谷川 卓³、後藤 晶子⁴ (1.早稲田大学 教育・総合科学学術院 地球科学専修、2.早稲田大学大学院 創造理工学研究科 地球・環境資源理工学専攻、3.金沢大学 理工学域 地球社会基盤学類 地球惑星科学コース、4.京都大学 生態学研究センター)

We have re-examined Oceanic Anoxic Event 2 (OAE2) at Southwestern Newfoundland Ridge. Mid-Cretaceous sequences including a major black shale approximately 40 cm thick and a few minor black shale layers were recovered by Integrated Ocean Drilling Program Expedition 342 at Site U1407. Shipboard and shore-based biostratigraphy indicates that the sections including these black shales are correlated to the Cenomanian through Turonian. Here, we report total organic carbon content (TOC), organic carbon/total nitrogen ratio (C/N ratio), and organic stable carbon isotope ratio (δ¹³C_org). Maximum TOC observed becomes more than 20 wt% at the major black shale, while background TOC is less than 1 wt%. δ¹³C_org stratigraphy suggests well-known isotope signature at OAE2, namely, first build-up (FB), trough interval (TI), second build-up (SB), plateau (PT), and recovery (RV), is preserved at this site. At the PT interval, δ¹³C_org shows an approximately 5‰ positive excursion from the pre-excursion background. The magnitude of this positive excursion is comparable to that observed in Oceanic Drilling Program Site 1276 at Newfoundland Basin. On the other hand, stratigraphic distribution of Black shales in Site U1407 is different from those in Site 1276. In Site 1276, the deposition of black shales started at the base of FB and continued up to PT. However, the major black shale in Site U1407 observed only at SB and PT. Estimated paleodepth at the mid-Cretaceous is deeper than 2000 m for both sites, indicating depths may not regulate the timing of the black shale deposition. Considering bathymetry of western north Atlantic and position of Grand Banks, we hypothesize that these two sites were under the different bottom water current system at the Cenomanian through Turonian.