Nannoconus are a group of calcareous nannofossils (um size) produced by marine planktonic organisms that were important calcifiers during the Cretaceous. They represent an important group due to their successful development and the high calcification rate, which contributed to buffer the acidification of the oceans in the Early Cretaceous. Despite their abundance for a long time, nothing is known about both the mode of calcification and the chemical composition of Nannoconus exoskeleton. These informations are needed to explain their success as marine calcifiers in changing lower Cretaceous climate and oceans, and to decipher the paleochemistry of the oceans. In particular, determining metal (Sr, Mg) contents in their calcite plates can help solving these questions. Here, we propose to reconstruct the chemical composition of three families of nannofossils abundant in the Early Cretaceous by using X-ray micro-fluorescence spectroscopy (µ-XRF) with synchrotron radiation at ESRF (ID21).