Diatom abundance index and biogenic opal content for ODP Leg 175 holes

Several sites of Ocean Drilling Program Leg 175 between 20° and 30°S (1081, 1082, 1083, 1084 and 1085) in the Benguela Current system show a distinct opal (and diatom) maximum within the late Pliocene and early Quaternary, spanning the lower half of the Matuyama reversed polarity Chron (MDM, Matuyama Diatom Maximum). This maximum is centered around 2.0-2.6 Ma, and follows a rapid increase of diatom deposition near 3.1 Ma. Within the MDM, we recognized intervals dominated by the antarctic/subantarctic species Thalassiothrix antarctica which have the aspect of diatom mats. The MDM (and the mats) developed during a time of sustained Southern Ocean influence (3.2-1.6 Ma), probably reflecting a period of seasonally-pulsed continuous advection of subantarctic waters into the Benguela Oceanic Current. The onset and cessation of the MDM raises questions about modes in the operation of the silica cycle and the relation of these to major oceanographic events. A depositional model for the late Pliocene is proposed which involves positional changes in a longshore system of fronts, substantial equatorward excursions of subantarctic waters during MDM time, and subsurface waters greatly enriched in silicate.