CALcification HIStory of the pelagic environment over the last 300 years – CalHis

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.