Hisayo Okahashi

Abrupt climate change and collapse of deep-sea ecosystems

We investigated the deep-sea fossil record of benthic ostracodes during periods of rapid climate and oceanographic change over the past 20,000 years in a core from intermediate depth in the northwestern Atlantic. Results show that deep-sea benthic community “collapses” occur with faunal turnover of up to 50% during major climatically driven oceanographic changes. Species diversity as measured by the Shannon–Wiener index falls from 3 to as low as 1.6 during these events. Major disruptions in the benthic communities commenced with Heinrich Event 1, the Inter-Allerød Cold Period (IACP: 13.1 ka), the Younger Dryas (YD: 12.9–11.5 ka), and several Holocene Bond events when changes in deep-water circulation occurred. The largest collapse is associated with the YD/IACP and is characterized by an abrupt two-step decrease in both the upper North Atlantic Deep Water assemblage and species diversity at 13.1 ka and at 12.2 ka. The ostracode fauna at this site did not fully recover until ≈8 ka, with the establishment of Labrador Sea Water ventilation. Ecologically opportunistic slope species prospered during this community collapse. Other abrupt community collapses during the past 20 ka generally correspond to millennial climate events. These results indicate that deep-sea ecosystems are not immune to the effects of rapid climate changes occurring over centuries or less.

Temporal latitudinal-gradient dynamics and tropical instability of deep-sea species diversity

A benthic microfaunal record from the equatorial Atlantic Ocean over the past four glacial-interglacial cycles was investigated to understand temporal dynamics of deep-sea latitudinal species diversity gradients (LSDGs). The results demonstrate unexpected instability and high amplitude fluctuations of species diversity in the tropical deep ocean that are correlated with orbital-scale oscillations in global climate: Species diversity is low during glacial and high during interglacial periods. This implies that climate severely influences deep-sea diversity, even at tropical latitudes, and that deep-sea LSDGs, while generally present for the last 36 million years, were weakened or absent during glacial periods. Temporally dynamic LSDGs and unstable tropical diversity require reconsideration of current ecological hypotheses about the generation and maintenance of biodiversity as they apply to the deep sea, and underscore the potential vulnerability and conservation importance of tropical deep-sea ecosystems.

Late Quaternary deep-sea ostracod taxonomy of the eastern North Atlantic Ocean

Taxonomic revision and re-evaluation of the eastern North Atlantic deep-sea ostracods are conducted based on late Quaternary sediments from Ocean Drilling Program (ODP) Hole 982A, Rockall Plateau, eastern North Atlantic. Twenty-one genera and 51 species were examined and (re-)illustrated with high-resolution scanning electron microscopy images. Six new species are described: Polycope lunaris, Argilloecia labri, Bythoceratina nuda, Cytheropteron colesoabyssorum, Cytheropteron colesopunctatum and Cytheropteron paramediotumidum. Excellent fossil ostracod preservation in this sediment core enabled us to provide a robust taxonomic baseline of the eastern North Atlantic deep-sea ostracods for application to palaeoceanographical, palaeoecological and biogeographical studies.

QUATERNARY DEEP-SEA OSTRACODE TAXONOMY OF OCEAN DRILLING PROGRAM SITE 980, EASTERN NORTH ATLANTIC OCEAN

Abstract Ocean Drilling Program (ODP) Holes 980 B and C, Feni Drift at the eastern slope of the Rockall Plateau, eastern North Atlantic, were examined for late Quaternary deep-sea ostracode taxonomy. Nineteen genera and 32 species were examined and (re-)illustrated with high-resolution scanning electron microscopy images. One new species Cytheropteron paramassoni n. sp. is described and one new name Eucytherura zehali is proposed for Eucytherura hazeli Yasuhara et al., 2009. This study provides updated taxonomic information for deep-sea ostracode genera and species from the eastern North Atlantic, which is an important baseline for application of deep-sea ostracodes to paleoceanographical reconstructions and paleoecological studies in this region.

Quaternary deep-sea ostracods from the north-western Pacific Ocean: global biogeography and Drake-Passage, Tethyan, Central American and Arctic pathways

Twelve genera and 19 species of deep-sea ostracods from the Shatsky Rise, north-western Pacific, were examined and illustrated for taxonomy. Three new species, Cytheropteron nasutum sp. nov., Poseidonamicus shatskyensis sp. nov. and Legitimocythere stellae sp. nov., are described. Based on these Shatsky Rise ostracods and a comprehensive literature survey of synonyms, we discuss global biogeography and possible migration pathways of deep-sea species. The four possible deep-water pathways connecting the Atlantic and Pacific Oceans are: the Drake Passage (Southern Ocean), established by 30 Ma; the Tethys Seaway, which had closed by 19–14 Ma; the Central American Seaway, which had closed by 3 Ma; and the Arctic Ocean Seaway via the Bering Strait, which opened about 4.8–7.4 Ma, and via the Fram Strait, which opened between 10 and 20 Ma. The Drake Passage is likely the major pathway. We argue that the Arctic was an important pathway for some deep-sea species before the mid-Pleistocene. Most deep-sea organisms have poor fossil records, and thus the rich fossil record of deep-sea ostracods is an ideal model system for the study of deep-time biogeography of deep-sea organisms. It may well be that other deep-sea organisms had similar palaeobiogeographical histories and patterns. http://zoobank.org/urn:lsid:zoobank.org:pub:3B70B7E5-04E0-4FF7-91EF-88CE1B199C8D

Biogeographic distributions of Cytheropteron species (Ostracoda) in Icelandic waters (sub-polar North Atlantic)

The ostracod genus Cytheropteron (Ostracoda: Crustacea) has been widely used for paleoenvironmental, paleoceanographic, and paleobiogeographic studies. But species-level data remain limited, especially for deepwater species. Here, we show the depth and spatial distribution patterns of 33 species of the ostracod genus Cytheropteron found in recent surface sediments of Icelandic waters. Species-specific depth ranges may serve as a useful tool in ostracod-based paleoenvironmental reconstruction. Biogeographic distributions of Cytheropteron species indicate the shallow Greenland-Iceland-Faroe Ridge as the major barrier for the distribution of deep-sea organisms.