Spatial heterogeneity of the Late Miocene Biogenic Bloom
Résumé
The late Miocene and early Pliocene is marked by a majoroceanographic and geological event called the Late Miocene BiogenicBloom (LMBB). This event is characterized by high accumulation rates ofopals from diatoms and high calcite accumulation rates from calcareousnannofossils and planktic foraminifera. The LMBB extends over severalmillion years and is present in the Pacific, Atlantic and Indian Oceans. Twohypotheses have emerged from the literature to explain this event: aglobal increase in the supply of nutrients to ocean basins through chemicalalteration of the continents and/or a major redistribution of nutrients in theoceans. The objective of this study is to provide a more comprehensivelook at the temporal and geographical aspects of the LMBB. We havecompiled ocean drilling data (ODP-IODP) covering the late Miocene andearly Pliocene. This compilation contains sedimentation rates as well asCaCO3, opal and terrigenous accumulation rates. After a careful screeningof the database, checking that all data are on the same time scale, we firstwork on global trends of sedimentation and biogenic production beforegoing into more details. For instance, we show that the magnitude of theBiogenic Bloom strongly varied between the three oceanic basins.Normalization to a post-LMBB state allows comparison of rates of increasein CaCO3 accumulation in different geographical areas (grouping severalsites). A very strong LMBB signature is present in oceanic area borderingthe western side of Australia. In the Atlantic Ocean, it is mainly presentnear the equator and over South Africa. The LMBB signature is lesspronounced in the Indian Ocean but remains trackable near the northerncoasts of the basin. Moreover, it is also heterogeneous in terms of themineralogy produced and deposited in the deep ocean between regions.For example, in the equatorial eastern Pacific, the LMBB signature ispresent in the silica accumulation term but not in carbonates accumulationone. Outputs from coupled ocean/atmosphere models (IPSL-CM5A2) usinglate Miocene paleogeography and integrating a marine biogeochemistrymodule (PISCES) have been gathered and will be discussed in regard toour database.