The Miocene-Pliocene transition: emergence of the modern carbon cycle – MioCarb

The project is divided into 3 methodological axes. The first is a micropalaeontological and sedimentological approach with a quantification of the carbonate production of calcareous nannofossils and CaCO3. CaCO3 is quantified by calcimetry using CO2 degassing after the reaction of CaCO3 with HCl. The carbonate production of calcareous nannofossils is measured by optical microscopy. The amount of CaCO3 is measured by birefringence of the calcite under the effect of polarisation filters. Accumulation rates are calculated from age models from the literature. The second axis is a compilation of accumulation rate data on a large geographical scale. The data is purely compiled and verified using only that covering the whole study period. The third axis is a climate modelling using the IPSL-CM5A2 model coupled with the biogeochemistry module PISCES. The climate modelling plans to propose several boundary conditions (mainly CO2 and geography) and to see if it is possible to simulate the conditions observed in the literature.

Axis 2 of the project is the most advanced. This axis should focus on the quantification of sediment accumulation rates on a large geographical scale. The compilation produced by Quentin Pillot in his thesis shows that 1) a large amount of data are not adequate to be included in the database either due to lack of data (resolution) or reliability; 2) accumulation rates are very variable between different regions of the world and do not follow a single pattern. Indeed, the objective is to estimate the impact of the Biogenic Bloom event on global ocean sedimentation. We observe that this event - which is marked by an increase in accumulation rates - is not recorded in all boreholes. However, it is recorded in most oceanic regions. This event therefore appears to be global but heterogeneous. We therefore propose a new hypothesis for its origin, linked to surface and bottom current changes. The paper is currently being written with a submission target of end of September 2022.
A sub-axis of axis 2 focuses on the variation of the calcite compensation depth (CCD) at the global scale in a spatialized reference frame. A compilation of CCD reconstruction data was carried out as well as a review of the underlying concepts. The first point that emerges is the paucity of reconstructions (about 4 totally unpublished since 1975). Furthermore, there are glaring discrepancies in concepts and definitions behind similar words. The study is ongoing with the future reconstruction of the current CCD and the parameterisation of a reconstruction for the Neogene.
Axis 3 has been underway for a few weeks but no simulation outputs are available yet.

As it stands, no perspective is provided. There is a need to cover much more of the 3 axes to provide coherent perspectives.

Quentin Pillot, Baptiste Suchéras-Marx, Anta-Clarisse Sarr, Clara T. Bolton, Jean-Baptiste Ladant, Yannick Donnadieu. Spatial heterogeneity of the Late Miocene Biogenic Bloom. EGU General Assembly, May 2022, Vienne, Austria. ?hal-03594217?
2. Quentin Pillot, Baptiste Suchéras-Marx, Anta-Clarisse Sarr, Clara T. Bolton, Jean-Baptiste Ladant, Yannick Donnadieu. Spatial heterogeneity of the Late Miocene Biogenic Bloom. ICP, August 2022, Bergen, Norway.

MioCarb project aims to understand the modern carbon cycle emergence at the late Miocene-early Pliocene (10-4 Ma) transition. This time interval is marked by a global cooling and a steepening of sea surface latitudinal temperature gradient forced by a decrease in atmospheric CO2. Between 9 Ma and 4 Ma, there is an increase in deep oceans carbonate and opal sedimentary fluxes called the Biogenic Bloom which is synchronous with the climatic change. The pelagic carbonate production – an important driver of the carbon cycle – is sustained by the calcareous nannoplankton, photosynthetic algae producing micrometric calcite platelets amount them the coccoliths. During the Biogenic Bloom, there is an increase in the calcareous nannoplankton accumulation rates in the deep oceans. Nevertheless, there are also major macroevolutionary changes within the calcareous nannoplankton community with the decrease in species richness and size and mass per nannofossil. Thel Miocene-early Pliocene carbon cycle transition is then marked by a set of major macroevolutive, biogeochemical and climate changes. MioCarb project aims to understand the origin of the Biogenic Bloom and its impact on the carbon cycle. In order to answer those questions, several complementary tasks are planned: 1) Quantification of calcium carbonate and organic matter mass accumulation rates and calcareous nannofossils and their calcium carbonate mass accumulation rates based on absolute abundance and size quantification over 10 deep-sea drilling sites. This task will quantify CaCO3 by automatic calcimetry and TOC by elementary analysis. The quantification of calcareous nannofossils and their mass will be done with an automatic light microscope purchase with the ANR financial support and the coccolith automatic identification system by artificial intelligence (SYRACO) developed at CEREGE. This task will then quantify the impact of macroevolutionary and paleoceanographic changes on the pelagic sedimentary production during the Biogenic Bloom. 2) Quantification of carbonate mass accumulations at the global scale. This task will focus on data compilation from deep-sea drilling expeditions associated with the state-of-the-art oceanic basins reconstruction from the Australian scientific team EarthByte. This task will lead to a quantitative estimation of CCD evolution through time, the pelagic sedimentary budget and the mass of carbon stored during the Biogenic Bloom. 3) Climatic modeling study including the late Miocene climatic, biologic, sedimentary and geographic specificities in order to reconstruct the origin of the Biogenic Bloom and its impact on the carbon cycle. A team of micropaleontologists, paleoceanographers, sedimentologists, climate modeler, light microscopy engineer and micropaleontology lab technician from CEREGE, AMU and University of Sydney is gathered to carry out this project.