Recent deep-sea benthonic for aminiferal distributions in the southeast Indian Ocean: Inferred bottom-water routes and ecological implications
Deep-sea benthonic foraminifera have been quantitatively examined from a suite of 64 trigger-core tops in the southeast Indian Ocean between 25-50° S and 80-140° E to evaluate their possible relationships with deep-sea water masses. Factor analysis of the species-frequency data shows two faunal assemblages in the region. The first major faunal assemblage is dominated by Epistominella umbonifera (Cushman), Planulina wuellerstorfi (Schwager), Globocassidulina subglobosa (Brady), Pullenia bulloides (d'Orbigny), and Oridorsalis tener (Brady). This assemblage is associated with Antarctic Bottom Water (AABW). Two subgroups exist within this assemblage. E. umbonifera dominates the faunal assemblage where the coldest (-0.2-0.4° C) AABW is found; G. subglobosa dominates the assemblage where warmer (0.6-0.8° C) AABW is present. The second major faunal assemblage is marked by a strong dominance of Uvigerina spp. and Epistominella exigua (Brady) and is associated with Indian Bottom Water (IBW) on the Southeast Indian Ridge and water intermediate between AABW and IBW in the South Australian Basin. The distribution of the first faunal assemblage suggests that AABW flows northward through the Southeast Indian Ridge at about 105-120° E into the South Australian Basin, and turns westward, forming a narrow western boundary current along the northern flank of the ridge. The bottom water is inferred to flow north to the Broken Ridge, where it is diverted eastward, and finally flows northward into the Wharton Basin between Broken Ridge and Naturaliste Plateau. The presence of such a contour current is supported by the existence of hiatuses and/or very low sedimentation rates in cores in the region of the inferred current. The bottom-water masses are clearly reflected by the benthonic foraminiferal faunal assemblages, but a correlation analysis between the faunal data and in-situ temperature, potential temperature, salinity, dissolved-oxygen content, sigma-t, > 63-μm weight percent of sediment, depth, and a calcium carbonate dissolution indicator shows that none of these individual variables adequately explains the patterns of benthonic foraminifera. Furthermore, surface-water productivity does not appear to influence the distribution of the benthonic foraminiferal assemblages. Other environmental variables associated with distinct water masses must therefore influence the distribution of the faunas. Similar deep-sea benthonic foraminiferal faunal assemblages are found in the Atlantic, southwest Atlantic, and in the southeast Indian Ocean, but the physical variables associated with each assemblage are different. It is suggested that the availability of calcium carbonate in the bottom waters may play an important role as an environmental stress to the living organisms, and explain the distribution of deep-sea benthonic foraminifera. © 1979.
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