Le projet MONOPOL vise à recueillir et analyser des séries sédimentaires déposées avec de très forts taux de sédimentation dans la Baie du Bengale et les Maldives, afin de reconstruire la variabilité passée de la Mousson Indienne et sa sensibilité à différents types de forçages (aux échelles multi-décadales à multi-milléniales), depuis l’Holocène supérieur (derniers milliers d’années) jusqu’à plusieurs cycles climatiques. L’aspect le plus important et le plus novateur des études envisagées porte sur l’utilisation du couple d18O-Mg/Ca pour reconstruire les changements du d18Osw de l’eau de surface et contraindre ainsi les changements de précipitations associés à la mousson indienne et les apports des deux systèmes fluviaux principaux (Ganges-Bramhapoutre et Irrawady) qui se déversent en Baie du Bengale. Les reconstructions paléocéanographiques et paléoclimatiques seront basées sur une approche multi-traceurs, comprenant des traceurs biologiques (ex. études des assemblages de nannofossiles et de pollens), sédimentaires et géochimiques (ex. Ti/Al, d18O, Mg/Ca). Un effort tout particulier sera fait pour assurer une excellente contrainte temporelle aux différentes échelles d’âges étudiées (datations 210Pb et AMS-14C, stratigraphie d18O et calage magnétique (inversions, excursions, intensités)). Le second objectif prioritaire concerne l’estimation des changements passés d’altération de l’Himalaya en relation avec les variations de la mousson. Cette étude sera basée sur l’analyse de traceurs sédimentaires (argiles) et géochimiques (isotopes du Nd, du Sr et du Pb).

CEREGE : Thibault de Garidel-Thoron (local coord.), Luc Beaufort, Laetitia Licari, Vincent Moron, Noëlle Buchet, Yves Gally, Jean-Charles Mazur, Magali Ermini (CDD). .

Coccolithophores and planktonic foraminifera produce more than 90% of pelagic carbonates, thus being major actors in the global carbon cycle. Calcification of these unicellular organisms is known to be influenced by carbonate ion concentration and calcite saturation state of seawater as well as by other physico-chemical parameters. Carbon dioxide produced by human activities since the industrial revolution has already induced a decrease of ocean pH by about 0.1 units. The objective of CALHIS is to undertake the first very high-resolution evaluation of the impact of this pH drop on pelagic carbonate production. To this end, we have selected 3 categories of oceanic zones covering a wide range of carbonate ion concentration, calcification types and trophic levels: (1) the Mediterranean and Caribbean seas with waters with high carbonate ion concentration, oligotrophy, and organisms with well calcified shells; (2) the north Papuan coast and Patagonian shelf with lower carbonate concentration, higher nutrient levels and lightly calcified shells; (3) the Eastern Pacific margin (Peru and Mexico) characterized by upwelled waters with high nutrient levels and such low carbonate ion concentration and pH that coccolithophores would be predicted to no longer calcify, although some in fact exhibit highly calcified shells. In each of these zones, we have privileged access to research vessels for sampling. We will lead or participate in mini-cruises in each of these coastal areas in order to retrieve surface sediment cores that record the history of sedimentation of the last centuries and to collect water samples from the photic zone to establish present-day relationships between calcification and carbonate chemistry, through genetic, morphological and physico-chemical analyses. A range of coccolithophore genotypes and morphotypes will be isolated into laboratory culture in order to quantify eco-physiological tolerances and calibrate specific biomarkers. From well-dated (14C/ 210Pb) sediments, we will establish records of the state of calcification of foraminifera and coccolithophores, temperature, d13C, concentrations of specific biomarkers, and relative abundance of coccolithophore morphotypes and, when possible, genotypes (i.e. in anoxic sediments). These data will enable accurate quantification of changes in pelagic carbonate production over the last 300 years and determination of whether changes are related to recent global ocean acidification. This project is the logical continuation of a study recently published in Nature by most of the proponents of the proposal.

Anthropogenic carbon dioxide (CO2) emissions lead not only to oceanic warming, but also to a decrease in the pH of surface waters. Such oceanic acidification is known to affect the calcifying marine biota, a key component of the biological pump. Fossilized shells of planktonic foraminifera constitute one of the largest geological archives of biodiversity and paleoclimates. Historical changes in planktonic foraminiferal assemblages have not been synthesized at on the global scale, although they likely are already responding to anthropogenic changes in the ocean. In order to assess these changes, the FORCIS synthesis aims at compiling a comprehensive database of all available foraminiferal census and accompanying hydrographic data based on plankton samples collected since the early 1950’s. This new synthesis will yield insights into (i) spatial (ii) vertical and (iii) seasonal patterns of planktonic foraminifera over the last decades. The knowledge on the ecology of planktonic foraminifera will be used to improve ecophysiological and population dynamics models of this taxon. The ultimate goal of the the FORCIS synthesis is to understand the significance of the main stressors (temperature and oceanic acidification) that govern calcification processes, and their potential effect on future changes in the ocean.