(Table 2) Occurrence of the most abundant nannofossil taxa in middle Cretaceous sediments of ODP Leg 129 sites

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Middle Cretaceous calcareous nannofossils were quantitatively studied at Sites 800, 801, and 802 of Leg 129. Samples were selected after careful inspection of the total abundance and preservation of nannofloras in pelagic sediments in order to analyze only the best-preserved assemblages and exclude major secondary modifications of the nannofloras due to dissolution/diagenesis. Nannofossil data were compared with radiolarian distribution and paleolatitude values to trace the response of planktonic communities to plate motions toward the paleoequator. Aptian to Cenomanian calcareous nannofloras record changes in composition, with sharp increases in abundance of Biscutum Constans and Zygodiscus erectus. Both species were previously interpreted as high-fertility indicator characteristic of the paleoequatorial belt of the Pacific basin and of upwelling sites. At Sites 800 and 801 the increases in abundance of these indices correspond to increases of radiolarians. At both sites this change was recorded when paleolatitude values pass from 10°S to 5°S, and therefore seems to mark the southern edge of the paleoequatorial divergence. At paleolatitudes of approximately 2°S, in the core of the upwelling zone, calcareous nannofossils disappeared and were replaced by extremely abundant radiolarians. Site 800 reached the high-fertility belt during the late middle Albian and the core of the paleoequatorial divergence in the Cenomanian. Site 801 approached the upwelling belt during the late Albian and reached the inner part of the divergence in the Cenomanian. Data from Site 802 are less clear; here, calcareous nannofossils are abundant but poorly preserved in the upper Aptian-Cenomanian interval. The high-fertility indices do not show increases in abundance and indeed, paleolatitude values point to a location south of the paleoequatorial upwelling zone. However, nannofossil assemblages might be partially altered by dissolution because of the deeper paleoenvironment.

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