Depth stratification of planktonic foraminifers in the Miocene ocean

Gerta Keller

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A depth stratification of planktonic foraminifers based on oxygen isotopic rankingis proposed for the Miocene. Species are grouped into surface, intermediate, and deepdwellers based upon oxygen isotopic composition of individual species. The depth stratificationis applied to planktonic foraminiferal populations in three Miocene time-slices(21 Ma, 16 Ma, and 8 Ma) in the equatorial, north, west, and east Pacific. The lateMiocene time-slice is compared with modern Pacific GEOSECS transect water-massprofiles of temperature and salinity in order to illustrate the similarities between thedepth ranking of planktonic foraminifers and temperature and salinity conditions. Thegeographic distribution of inferred surface, intermediate, and deep water dwellers wasfound to be very similar to modern temperature profiles: surface dwellers appear to beassociated with warmest temperatures (>20°C), upper intermediate water dwellers withtemperatures between 10 and 20°C, and lower intermediate and deep water dwellerswith temperatures below 10°C. Tropical high-salinity water appears to be associatedwith the upper intermediate Globorotalia menardii group in the modern ocean.Depth stratification applied to two Miocene time-series analyses in the equatorialPacific (Sites 77B and 289) indicates increased vertical and latitudinal provincialismbetween early, middle, and late Miocene time. The early and middle Miocene equatorialPacific was dominated by the warm surface water group, which shows distinct eastwestprovincialism. This provincialism is interpreted as the periodic strengthening of theequatorial surface circulation during polar cooling phases. During the late Miocene theupper intermediate group increased and the surface group declines. At the same time theeast-west provincialism disappeared. This faunal change may have been associated withthe major Antarctic glaciation and resultant strengthening of the general gyral circulationand the strengthening of the Equatorial Countercurrent due to the closing of theIndonesian Seaway at that time.

Original languageEnglish (US)
Pages (from-to)177-195
Number of pages19
JournalMemoir of the Geological Society of America
Issue number1
StatePublished - Jan 1 1985

All Science Journal Classification (ASJC) codes

  • Geology


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