High-resolution climatic reconstruction in the tropical Pacific based on biocarbonate archives

In the current context of global change and the associated need to provide forecasts on the future evolution of the climate as precise as possible, it is crucial to have a deep understanding of the earth climate system. Pacific Ocean is an important compartment of this system. It is the place of strong ocean-atmosphere interactions and the birthplace of the El Niño Southern Oscillation (ENSO), the impacts of which are catastrophic for the environments and the populations. The current knowledge on ENSO partly relies on observations which are however limited in time. To increase this dataset and understand the natural climate variability, additional data are acquired for past periods, based on various type of archives. Because ENSO occurrences are connected with the seasonal variability and last several months, high temporal resolution information, are necessary. In this manuscript I shall describe how I tried to develop, and to apply, environmental proxies from biocarbonate archives in the tropical Pacific Ocean to reconstruct the monthly or infra-monthly variability of the sea surface temperatures (SSTs) and the evaporation/precipitation balance. To do that, I focused my works on the mollusk shells and Porites sp coral skeleton. These archives contain various tracers (growth and geochemical variations) of the sought environmental variables. I will show that the thicknesses of growth decreases of the Protothaca thaca bivalve shells (East Pacific) indicate occurrences of El Nino events. In the Western part of the Pacific basin, changes in daily increment thicknesses in two Tridacnidae species, Tridacna maxima and Hippopus hippopus are related to SST changes but also depend on physiological condition of the animals. While certain geochemical tracers (Sr/Ca, δ18O) within Porites sp. Skeleton can be used with a certain confidence, this is not always the case for the mollusk shells. I will show that the use of Mg/Ca as a SST proxy depends on the species and that physiology and very early diagenesis are obstacles in the use of this proxy. In T. maxima and H hippopus, δ18O can be used as a tracer of SST and of the evaporation/precipitation balance. To reconstruct variations of these parameters during the Mid-Holocene, I worked on fossil giant clams and corals, after diagenesis assessment. I will show how various types of diagenesis impact the microstructure, the geochemistry, and the reconstructed environmental parameter. I will also propose hypotheses on the diagenesis process itself. Based on results obtained from Mid-Holocene archives, I will show that the post-glacial SST rise was reached at around 6.5-6 ka BP in the Southwest Pacific, that the South Pacific Convergence Zone influence was more probably reduced, the Hadley circulation enhanced, and that ENSO variability was probably reduced. The work done on Lapita (3 ka BP) Tridacnidae confirmed that the Lapita migration has probably been favored by an increase in climatic variability.The last chapter deals with the future directions I would like to give to my research that include i/ better understand the incorporation of proxies in mollusk and ii/ try to propose solution for sustainable management of tropical ecosystems based on the use of biocarbonate as recorder of the environment and its disturbances.