The dark horizons generally contain slightly higher abundances of pyrite and exhibit significant compositional variations in the relative abundances of diatoms, foraminifers, and nannofossils. The biogenic component of the dark layers is commonly dominated by diatom resting spores. Calcareous nannofossils are abundant within the top 280 mbsf and between 410 and 600 mbsf and provided 13 biohorizons that were constrained within an average depth interval of 3 m. Planktonic foraminifer zonation is difficult because of an absence of marker species and dissolution of planktonic foraminifers, particularly in the lower Pleistocene to upper Pliocene sediments. It is difficult to determine whether the absence of marker species is caused by ecological conditions or by selective dissolution. The benthic foraminifers are abundant and well preserved, and abundances correlate well with the different lithostratigraphic units.
Radiolarians are abundant and well preserved in almost all samples examined.
In contrast to the other sites, diatom abundances, although highly variable, remain moderately high throughout the Pleistocene sediments. No short polarity-reversal event during the Brunhes Chron was detected in spite of the high sedimentation rates. Sediments at Site 1084 are notably rich in marine organic matter. Higher concentrations of organic carbon from 0 to 392 mbsf indicate that productivity was higher during the last 2 m.y. than earlier in the history of this upwelling system. Sulfate is completely depleted within the uppermost 5 mbsf, which is much more rapidly than in any other Leg 175 site and speaks to the availability of oxidizable organic matter.