Ostracode abundance and Simpson's index over the past 700,000 years at IODP Site 346-U1427

The Sea of Japan is a marginal sea, semi-enclosed by the Eurasian Continent, Korean Peninsula, Japanese archipelago, and connected to the Pacific Ocean and adjacent seas by three shallow straits (water depths < 130 m). Marginal seas are ideal natural laboratories to study biotic responses to large-scale environmental changes as they are typically sensitive to glacial-interglacial and stadial-interstadial climatic cycles. However, only limited number of studies covers time periods beyond 1 to 2 glacial-interglacial cycles. Here we present a 700,000-year record of benthic biotic response to past oceanographic changes in the southern Sea of Japan, covering the past seven glacial-interglacial cycles, based on ostracode assemblages at the IODP Site U1427. The results indicate that the long-term oxygen variability has been a major control of deep marginal sea biota. Five local extirpation events were recognized as barren zones during glacial maxima (i.e., sea-level minima) immediately before terminations I, II, IV, V, and VII in MISs 2, 6, 10, 12, and 16, which are probably caused by bottom-water oxygen depletion. The results of multivariate analyses indicated clear faunal cyclicity influenced by glacial-interglacial scale oxygen variability with the succession from opportunistic species dominance through tolerant infauna dominance to barren zone during the deoxygenation processes and the opposite succession during the recovery processes. The Sea of Japan ostracode abundance and faunal composition showed distinct difference between the post- and pre-MBE (Mid-Brunhes Event at around 400-350 ka) periods, indicating the MBE as a major disturbance event of deep-sea, especially marginal-sea ecosystems. The MBE shortened the duration of the extirpation events, fostered dominance of warmer-water species, and amplified the glacial-interglacial faunal cyclicity. Our long-term biotic response study clearly indicates that deep marginal sea ecosystems are dynamic and vulnerable to climate changes.

日本海是一个边缘海（marginal sea），由欧亚大陆、朝鲜半岛和日本群岛半包围，通过三条浅海峡（水深<130米）与太平洋及邻近海域相连。边缘海是研究生物响应（biotic responses）对大规模环境变化的理想天然实验室，因其通常对冰期-间冰期旋回（glacial-interglacial cycles）和冰段-间冰段气候旋回（stadial-interstadial climatic cycles）敏感。然而，仅有有限研究覆盖超过1-2个冰期-间冰期旋回的时间段。本文基于国际大洋发现计划（IODP）U1427站位的介形虫组合（ostracode assemblages），呈现日本海南部70万年以来底栖生物响应（benthic biotic response）对过去海洋学变化的记录，涵盖过去七个冰期-间冰期旋回。结果表明，长期氧含量变化是边缘海深层生物群的主要控制因素。在海洋同位素阶段（Marine Isotope Stages, MISs）2、6、10、12和16中，终止期I、II、IV、V和VII前夕的冰期极盛期（glacial maxima，即海平面最低期）期间，识别出五次局域灭绝事件（local extirpation events），表现为无生物区（barren zones），这可能由底层水缺氧（bottom-water oxygen depletion）引起。多变量分析（multivariate analyses）结果显示，冰期-间冰期尺度的氧含量变化驱动了明显的动物群周期性（faunal cyclicity）：脱氧过程中，生物群落从机会性物种（opportunistic species）占优，经耐受性底内生物（tolerant infauna）占优，演替至无生物区；恢复过程中则发生相反演替。日本海介形虫丰度（ostracode abundance）和动物群组成（faunal composition）在中布容事件（Mid-Brunhes Event, MBE，约400-350千年）前后时期存在显著差异，表明MBE是深海（尤其是边缘海生态系统）的重大扰动事件。MBE缩短了灭绝事件的持续时间，促进暖水物种的优势地位，并增强冰期-间冰期动物群周期性。我们的长期生物响应研究明确表明，边缘海深层生态系统具有动态性且易受气候变化影响。