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This benthic foraminiferal study, mostly with paleoecological and paleoclimatical purposes, is a part of a wider interuniversity program whose main objective is to improve the knowledge of biotic responses to the major climate changes occurred as a consequence of the so called "Middle Pleistocene Transition". That period (1.25 My to 0.75 My) represents the temporal expression of the transition from 41 Ky and 23 Ky climate cycles to high amplitude cycles of 100 Ky, without any evident modification in the orbital forcing. As a consequence of this important orbital change, the ocean circulation system tries a re-organization of marine currents and also of both temporal and spatial distribution of calcareous marine microorganisms.
In the frame of this research program, this work aims to propose a paleoecological and paleoenvorinmental analysis of benthic foraminiferal assemblages of the IMAGES Site 97-2114, during the last million years. The site is located at a middle bathyal water depth on the north-eastern flank of Chatham Rise (New Zealand). The Chatham Rise is a major E-W trending submarine high, extended about 300 km eastward from the North Island. It plays the role of physiographic barrier for the major oceanic circulation cell because it separates the Sub-Tropical Water to the North from the Sub-Antarctic Water to the South, while the Sub-Tropical Front is located along the rise itself.
The stratigraphic succession analyzed is 30 meters length, with a substantial continuity of sedimentation, as confirmed by the integration of bio- and magnetostratigraphical data. The 242 samples studied were prepared through the washing with 150 ìm and a 63 ìm sieves and analyzed with two counting methods: the first count was made on the whole foraminiferal assemblage in order to calculate the Benthos percentage (B%), the Organic Matter % (OM%) and the Fragmentation Index (F.I.); the second count, made exclusively on the benthic assemblage, consists in the identification at specific level of 200 specimens. The data obtained were statistically processed using the most common and applied techniques of multivariate analysis such as: Cluster Analysis, Principal Component Analysis and SHE Analysis. Cluster Analysis has allowed to identify 8 benthic foraminiferal assemblages mostly dominated by deep infaunal (Bulimina marginata, Bulimina striata, Uvigerina peregrina) or by shallow infaunal (Oridorsalis umbonatus) taxa. The PCA Analysis highlights that the main forcing factors that drive species distribution, probably are the kind of organic flux (continuous high stand VS seasonal) and the level of primary productivity (high VS low). Otherwise we can highlight some major changes in biodiversity parameters, using the SHE Analysis. Those changes appear to be coevals with both the beginning and the end of the period of adjustment of the Deep Western Boundary Current (DBWC) as a consequence of the Middle Pleistocene Climate Transition.
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