Variations in deep-sea benthic foraminifera at ODP Hole 756B, southeastern Indian Ocean: Evidence for changes in deep ocean circulation
Abstract Seventy three core samples of 10 cc volume were analysed from Ocean Drilling Program (ODP) Site 756, Hole B with an average interval of 236 kyr per sample. Factor and cluster analyses of 37 high ranking species of benthic foraminifera enabled us to identify four clusters representing four biofacies. Biofacies Sl-Om is characterised by Stilostomella lepidula, Orthomorphina modesta, Robulus gibbus, Bulimina miolaevis and Cibicides cf. lucidus. This biofacies dominates the latest Oligocene to early Miocene interval (24–21 Ma and 18.75–16.5 Ma), indicating low to intermediate dissolved oxygen, organic carbon rich environment, warm and sluggish deep waters. During 21–18.75 Ma, the biofacies Gp-Nu (Globocassidulina pacifica, Nuttallides umbonifera, Trifarina angulosa, Quinqueloculina weaveri, Robulus gibbus and Pullenia bulloides) dominates the benthic assemblage at Hole 756B. This biofacies also has a short lived presence at ~ 16.5 Ma. This was an interval of intense circulation of cold and corrosive deep water similar to Antarctic Intermediate Water (AAIW), and intermediate to high flux of organic matter to Hole 756B. The Antarctic Circumpolar Current (ACC) initiated during this time. From 16.5 to 10.6 Ma, the benthic assemblage at Hole 756B is dominated by biofacies Ub-Gs (characteristic species Uvigerina buzasi, Globocassidulina subglobosa, Laticarina pauperata), indicating intermediate to high food supply and cold deep water. This interval coincides with the middle Miocene positive oxygen isotope shift and permanent build up of Antarctic ice sheets. The late Miocene interval (10.6–6.5) is characterised by biofacies Ec-Bs (dominant species Ehrenbergina carinata, Bolivina spathulata, Osangularia culter and Pullenia osloensis) indicating low oxygen condition with high food supply in the study area. This corresponds to an interval of global biogenic bloom.