Abstract

We examine the bathymetric distribution of benthic foraminifera in a depth transect of three Neogene sections ranging from 1,300 m to 3,200 m on the eastern flank of Lord Howe Rise to New Caledonia Basin, Southwest Pacific (DSDP Sites 590-1,299 m; 591-2,131 m; 206-3,196 m). Of 50 species that occur in moderate abundance (3% frequencies), eight have systematically increased in total water depth range of their occurrence during the Neogene. Forms that extended their range to deeper waters are Sigmoilopsis schlumbergeri, Stainforthia complanata and Gavelinopsis lobatulus. Forms that extended their range into shallower waters are Quinqueloculina venusta, Sigmoilina edwardsi, Cassidella bradyi and Eponides tumidulus. One form, Melonis pompilioides, extended its range into both shallower and deeper waters. In general, these forms are sporadic during the early part of their stratigraphic range and become both persistent and more abundant later in their histories at all water depths within their range. The increase in bathymetric range exhibited by these species was considerable during the Neogene (at least 2,000 m). Sampling resolution does not allow accurate calculations of the rate of depth migration, but rates are a maximum of 125m/My in some species. We could not determine whether the migrations were gradual or rapid between particular water depths. Melonis pompilioides is not an isobathyal form, as previously believed, and occurs at shallower depth (as shallow as 1,200 m) in the Southwest Pacific region than in other regions (usually reported deeper than 1,700 m). Extension of the species' bathymetric ranges occurred at various times during the Neogene, but the data suggest that greater numbers of migrational events occurred at about 10 Ma within the late Miocene, between about 6 to 4 Ma near the Miocene/Pliocene boundary and at 2 Ma in the late Pliocene. These three intervals encompass distinct paleoceanographic events, indicating that bathymetric migrational extensions may have been triggered by paleoenvironmental change in the deep sea. Nevertheless, we believe that the bathymetric extensions of the species ranges resulted from expansion of their ecological tolerance, an evolutionary process, rather than migration in association with specific water masses. Bathyal benthic species such as these are of limited use for stratigraphic correlation because their stratigraphic ranges are different at different paleodepths.

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