Several sites Of Ocean Drilling Program Leg 175 between 20 degrees and 30 d
egrees S (1081, 1082, 1083, 1084 and 1085) in the Benguela Current system s
how a distinct opal (and diatom) maximum within the late Pliocene and early
Quaternary, spanning the lower half of the Matuyama reversed polarity Chro
n (MDM, Matuyama Diatom Maximum). This maximum is centered around 2.0-2.6 M
a, and follows a rapid increase of diatom deposition near 3.1 Ma. Within th
e MDM, we recognized intervals dominated by the antarctic/subantarctic spec
ies Thalassiothrix antarctica which have the aspect of diatom mats, The MDM
(and the mats) developed during a time of sustained Southern Ocean influen
ce (3.2-1.6 Ma), probably reflecting a period of seasonally-pulsed continuo
us advection of subantarctic waters into the Benguela Oceanic Current, The
onset and cessation of the MDM raises questions about modes in the operatio
n of the silica cycle and the relation of these to major oceanographic even
ts. A depositional model for the late Pliocene is proposed which involves p
ositional changes in a longshore system of fronts, substantial equatorward
excursions of subantarctic waters during MDM time, and subsurface waters gr
eatly enriched in silicate. (C) 1999 Elsevier Science B.V. All rights reser
ved.