The early Matuyama Diatom Maximum off SW Africa, Benguela Current System (ODP Leg 175)

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.