Deep-sea ostracod faunal dynamics in a marginal sea: biotic response to oxygen variability and mid-Pleistocene global changes

Abstract

Deep-sea benthic ostracod assemblages covering the last 2 Myr were investigated in Integrated Ocean Drilling Program Site U1426 (at 903 m water depth) in the southern Sea of Japan. Results show that (1) orbital-scale faunal variability has been influenced by eustatic sea-level fluctuations and oxygen variability and (2) secular-scale faunal transitions are likely associated with the mid-Brunhes event (MBE, ~0.43 Ma) and the onset of the Tsushima Warm Current (TWC, ~1.7 Ma). Krithe, Robertsonites, and Acanthocythereis are the three most abundant genera throughout the core, accounting for 78.5% of total specimens. Multiple-regression tree analysis indicated that the TWC, the MBE, and oxygen content are the significant controlling factors of ostracod dominance. Changes in assemblages exhibit decline and recovery patterns corresponding to orbital-scale cyclicity of sea-level changes. In the Sea of Japan marginal ocean setting, this cyclicity shows a close relationship with bottom-water oxygen variability since the onset of the TWC influx. The MBE amplified the influence of the TWC and oxygen variability to the deep-sea ecosystem through larger sea-level fluctuations. Acanthocythereis dunelmensis, a circumpolar species, dominates before the TWC onset. After the TWC onset and during the mid-Pleistocene transition (MPT, ~1.2–0.7 Ma) Krithe spp., known for their low-oxygen tolerance, substantially increase under moderate oxygen depletion. At the end of the MPT, Krithe dominance diminishes and is replaced by Robertsonites hanaii and Propontocypris spp. after the MBE. The post-MBE assemblage, characterized by R. hanaii, suggests a slightly warmer environment under the development of the TWC. In addition, the post-MBE high-amplitude climate system may have caused the increased abundance of active-swimming Propontocypris spp. due to their superior migration ability. Benthic ecosystems in marginal seas are sensitive and vulnerable to both short- and long-term climatic changes, and the MBE is suggested to be a global biotic event affecting benthic ecosystems substantially.

System controls of coastal and open ocean oxygen depletion

“…In deeper waters (>900 m), the abundance and diversity of ostracods are lower than in the intermediate water depths. Nevertheless, statistical analyses also show that oxygen variability was important in controlling deeper-water faunal changes (Huang et al, 2018b;Yamaguchi et al, 2017). Other major taxonomic groups, such as the foraminifera and radiolarians, show consistent patterns in that the anoxic conditions resulted in their absence from the deep-sea fauna (Gallagher et al, 2018;Itaki et al, 2004).…”

Time Machine Biology: Cross-Timescale Integration of Ecology, Evolution, and Oceanography

“…However, after the MBE warm-adapted taxa went extinct regionally in the Arctic, with shrinking distributions to the south (DeNinno et al, 2015;Cronin et al, 2017). In the Sea of Japan, endemic cool water species replaced circumpolar species after the MBE, and many circumpolar species went extinct regionally (Ozawa and Kamiya, 2005;Cronin and Ikeya, 1987;Huang et al, 2018Huang et al, , 2019. In sum, when orbital cyclicity is consistent, microfossil species seem to show evidence of repeated community assembly that matches prevailing conditions, indicating that community assembly may be deterministic.…”

“…Temperature likely controls biodiversity because fewer species can physiologically tolerate conditions in colder places than in warmer places on these ecological timescales (Currie et al, 2004;. Changes in the dominant mode of orbital cyclicity can, by contrast, permanently disturb marine ecosystems (Hayward et al, 2007;DeNinno et al, 2015;Cronin et al, 2017;Huang et al, 2018Huang et al, , 2019, such as during the transition from 41,000-to 100,000-year cycles in the Mid-Pleistocene Transition (MPT) and the Mid-Brunhes Event (MBE). For example, deep-sea benthic foraminifera show a prominent global extinction event during the MPT (Hayward et al, 2007).…”

Biogeographic Atlas of the Deep NW Pacific Fauna

“…The Sea of Japan is geographically isolated and has an abnormally high level of dissolved oxygen in its deeper parts; however, its bottom-water oxygen level has declined up to 8-10% in the past three decades (Elsner et al 2013;Huang et al 2019;Huang et al 2018). The abyssal depths of the Sea of Japan were formed in the Cenozoic as a caldera of the mainland (Malyutina and Brandt 2013).…”