Quantitative analyses of benthic foraminifera from the Peru-Chile Trench area, off Central America, and the Gulf of California allow assessment of general faunal trends between 32°S and 32°N along the eastern margin of the North Pacific and South Pacific. Six widespread faunal divisions can be recognized in this broad region reflecting major variations in substrate and water-mass character across the neritic-to-abyssal gradient. The 50-m isobath represents the average line of wave touchdown on open shelves, in turn separating inner and outer neritic substrates, mixing, and biofacies.

Inner neritic biofacies (< 50 m) are characterized by Bulimina denudata, Hanzawaia nitidula, Nonionella basispinata, and N. stella. The 100 to 150-m depth interval encompasses the base of the mixed (surface) layer, the base of the effective photic zone, and average point of shelf-slope declivity in turn separating outer neritic and upper bathyal biofacies. Outer neritic biofacies (50 to 150 m) include Cancris panamensis, Cassidulina spp., Uvigerina juncea, and Valvulineria inflata. A well developed oxygen minimum layer intersects the upper and middle slope areas between depths of 150 and 1,500 m with the core of this feature commonly found at depths of 200 to 600 m. Upper bathyal biofacies (150 to 500 m) reflect this association and include high relative and absolute abundances of olivina interjuncta, B. rankini, B. seminuda, Buliminella exilis tenuata, Martinotiella communis, Suggrunda eckisi, and Trifarina carinata. Middle slope faunas are influenced by the deeper portions of the oxygen minimum, the base of the permanent thermocline, and the presence of Pacific deep water. Middle bathyal biofacies (500 to 2,000 m) include Bulimina striata mexicana, B. rostrata, Cassidulina cushmani, Cibicides mckannai, Uvigerina peregrina dirupta, and U. hispida. A lower-slope bathyal biofacies (2,000 to 4,000 m) includes Gyroidina neosoldani, Melonis pompilioides, and Uvigerina senticosa. Increasing relative and absolute abundances of agglutinated species between 3,000 and 4,000 m reflect the elevated calcium carbonate lysocline and compensation depth in this region with the ab ssal biofacies (> 4,000 m) dominated by various species of Alveolophragmium, Bathysiphon, Cystammina, Glomospira, Reophax, Rhabdammina, Spiroplectammina, and Trochammina. Deviations from these general trends occur in conjunction with substrate anomalies, complex water-mass associations (i.e., double oxygen minima), and in marginal basins where sill depth effectively controls water-mass character, substrate, and biofacies.

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