Spatial heterogeneity of the Late Miocene Biogenic Bloom
Résumé
The late Miocene and early Pliocene is marked by a major oceanographic and geological event called the Late Miocene Biogenic Bloom (LMBB). This event is characterized by high accumulation rates of opals from diatoms and high calcite accumulation rates from calcareous nannofossils and planktic foraminifera. The LMBB extends over several million years and is present in the Pacific, Atlantic and Indian Oceans. Two hypotheses have emerged from the literature to explain this event: a global increase in the supply of nutrients to ocean basins through chemical alteration of the continents and/or a major redistribution of nutrients in the oceans. The objective of this study is to provide a more comprehensive look at the temporal and geographical aspects of the LMBB. We have compiled ocean drilling data (ODP-IODP) covering the late Miocene and early Pliocene. This compilation contains sedimentation rates as well as CaCO3, opal and terrigenous accumulation rates. After a careful screening of the database, checking that all data are on the same time scale, we first work on global trends of sedimentation and biogenic production before going into more details. For instance, we show that the magnitude of the Biogenic Bloom strongly varied between the three oceanic basins. Normalization to a post-LMBB state allows comparison of rates of increase in CaCO3 accumulation in different geographical areas (grouping several sites). A very strong LMBB signature is present in oceanic area bordering the western side of Australia. In the Atlantic Ocean, it is mainly present near the equator and over South Africa. The LMBB signature is less pronounced in the Indian Ocean but remains trackable near the northern coasts of the basin. Moreover, it is also heterogeneous in terms of the mineralogy produced and deposited in the deep ocean between regions. For example, in the equatorial eastern Pacific, the LMBB signature is present in the silica accumulation term but not in carbonates accumulation one. Outputs from coupled ocean/atmosphere models (IPSL-CM5A2) using late Miocene paleogeography and integrating a marine biogeochemistry module (PISCES) have been gathered and will be discussed in regard to our database.
