ECOSTRATIGAPHIC REGIME SHIFT DURING LATE QUATERNARY MARINE ISOTOPE STAGES 8-9 IN SANTAREN CHANNEL, WESTERN TROPICAL ATLANTIC OCEAN: BENTHONIC FORAMINIFERAL EVIDENCE FROM ODP HOLE 1006A

Abstract

The Late Quaternary, presumed bathyal foraminiferal fauna in ODP Hole 1006A (Santaren Channel, west of the Bahama Platform) does not show marked glacial-interglacial contrasts. Instead, a major regime shift occurred during Marine Isotope Stages (MIS) 8–9 from one steady community structure to another. ODP Hole 1006A was drilled through the Santaren Drift, a contourite containing an admixture of foraminifera of a) reefal and inner-neritic (0–20 m) and b) deeper-water aspect. The deeper-water assemblage, which includes species with habitats that encompass outer-neritic (100–200 m)–abyssal depths, is for statistical purposes here treated as being in situ. Of the benthonic foraminifera collected from 66 Late Quaternary (MIS 1–17) samples in this study, 60.9% comprise the presumed bathyal assemblage. Percentages of presumed bathyal specimens/sample are unrelated to most marine isotope stages, but are low in MIS 8–9. Sedimentology indicates that the fluctuations were not caused by turbidites and debris flows delivering shallow-water foraminifera from the Bahama Platform and Cay Sal Bank to the site, but possibly from a source off northern Cuba or Hispaniola. Some species abundance counts (e.g., Cassidulina laevigata , Lenticulina rotulata ) may have been augmented by downslope transport. Others (e.g., Globocassidulina subglobosa , Sigmoilopsis schlumbergeri ) are predominantly in situ. Only three species each ( Globocassidulina subglobosa , Planulina ariminensis , Cibicidoides pachyderma ) formed >5% of the total recovery of the presumed bathyal foraminiferal assemblage, but Cibicidoides spp. collectively formed 18.9%, while Uvigerina spp., Brizalina spp., and Bulimina spp. collectively formed 12.8%, indicating a low flux of organic carbon. The mean information function was higher in the subsection below MIS 9 (H = 3.56) than above MIS 8 (H = 3.40), but the mean percentages of Uvigerina spp., Brizalina spp., and Bulimina spp. did not differ, which suggests that there was little change in the organic flux. Instead, the regime shift may have been related to a change in the flux of shallow-water foraminifera, which was greater after MIS 8 than before MIS 9, or to long-term circulation changes. The subsections across the regime shift were separated by a Type-1β boundary, but a complementarity of 76.5 indicates that the interval MIS 8–9 did not act as a barrier to species continuance.