Biotic response of benthic foraminifera to OMZ variations in the northwestern Pacific since the last deglaciation

Abstract

Recent declines in oxygen concentrations in the global oceans have been particularly acute in oxygen minimum zones (OMZ) at intermediate depths. Fossil benthic foraminifera have the potential to provide useful information on the long-term biotic responses of marine organisms to past variations in the OMZ. During the last deglaciation, the OMZ at 1 km water depth in the northwestern Pacific Ocean intensified. However, temporal variations in benthic foraminifera in the upper layer of the OMZ since the last deglaciation are poorly understood. Thus, we studied temporal variations in benthic foraminifera assemblages in a sediment core taken from a water depth of 777 m off of Hokkaido, North Japan, to determine their biotic responses to environmental changes in the upper layer of the OMZ from the last deglaciation to the late Holocene. The results suggested that dysoxic species such as Bolivina seminuda and Buliminella tenuata increased rapidly during the Bolling/Allerod warming event, which was associated with severe oxygen depletion and expansion of the OMZ. Similar dysoxic assemblages were recorded at 1 km water depth in this region. On the other hand, the dominance of the oxic species Nonionellina labradorica during the Younger Dryas cold period may indicate that oxygenation events were induced by intensification of intermediate water ventilation. Suboxic assemblages composed of Uvigerina akitaensis, Bolivina spissa, and Elphidium batialis appeared in the late Holocene. The increase in this late Holocene assemblage, which is similar to the modern assemblage in this region, may coincide with gradual cooling around the Japanese Islands.