ABUNDANCE BIOZONES AND COMMUNITY STRUCTURES IN THE UPPER QUATERNARY OF ODP HOLE 994C (BLAKE RIDGE, WESTERN ATLANTIC OCEAN) COMPARED WITH MARINE ISOTOPE STAGES, GLACIAL CYCLES, AND TERMINATIONS

摘要

Bathyal and abyssal benthonic foraminiferal assemblages have been thought to undergo community- and species-level regime shifts between glacials and interglacials. Comparisons between communities and oxygen isotope curves are, however, few. Comparisons between multiple aspects of the benthonic foraminiferal community and the oxygen isotope curve are made here for the abyssal Upper Quaternary of ODP Hole 994C (Blake Ridge, western Atlantic Ocean). Presumed allochthonous species were removed from the dataset to give an in situ abyssal assemblage dominated by Brizalina lowmani with subordinate Cibicides wuellerstorfi and other species. The total number N of benthonic foraminifera/20 cm3 sediment sample, the point diversity measured using the information function H, and max(p_i) (the proportional abundance of the most abundant species in each sample) all fluctuated between samples. These variables apparently mirrored the oxygen-isotope curve based on planktonic foraminifera, and so show potential for tracking marine isotope stages (MIS). However, neither abundance biozone boundaries nor species-level regime shifts (i.e., points of change in percentage carrying capacity, K_p) coincided or showed consistent associations with terminations (pronounced boundaries between isotopic maxima and minima). Cibicides wuellerstorfi was most abundant towards the base of the section, which might indicate stronger current action. Most abundance biozones (ABs) had a Type 0 community structure, but Type 1 community structure was recorded in ABs from the upper part of Glacial Cycle E (glacial MIS 10) and the lower part of Glacial Cycle D (interglacial MIS 9), for which this community structure may mark an extended period of stress. Peaks in percentage abundance of Uvigerina spp. occurred in glacial MIS 2, 6, and 8, but not in MIS 4, 10, and 12. It is, therefore, recommended that workers be cautious when using glacial-interglacial cycles to explain ecostratigraphic events in abyssal Pleistocene successions, especially in the absence of an oxygen-isotope curve.