Site 646 ( ODP).

More than 100 m of double APC cores provides a nearly complete, high-resolution record from 1.2 Ma to present. Fluctuations in carbonate content (0%-40%), vague color variations, magnetic-susceptibility changes, microfossil abundance and diversity, and dropstone abundances record continuous glacial-interglacial cycles. A continuous record through isotope stage 21 is probably contained in APC cores. The onset of major ice rafting at about 2.5 Ma (236 mbsf) coincides with a major change in seismic character. A significant biotic change occurred at the early Pliocene/late Pliocene boundary (3.4 Ma; 314 mbsf). Biosiliceous material is common to abundant above, whereas calcareous nannofossils and planktonic foraminifers are more abundant below. This event probably represents significant cooling of surface water masses over the site accompanied by higher productivity of siliceous plankton. The changes in surface waters also closely correspond to a period of apparent intensification of deep circulation, beginning at about 4.0 Ma, as suggested by erosional modification of the drift sequence and construction of a pronounced ridge observed in the regional seismic lines. A generally warm early Pliocene is indicated by relatively high and constant carbonate contents and abundant calcareous nannofossils and dinocysts.-from Author