Ragueneau¹, O., P. Claquin¹, L. Corrin¹, D.J. DeMaster², N. Dittert¹, M. Gallinari¹, C. Heinze³, A. Leynaert¹, V. Martin-Jézéquel¹, P. Pondaven¹ and P. van Cappellen⁴

¹UMR 6539, IUEM, Technopôle Brest-Iroise, 29280 Plouzané, France, E-mail: Olivier.Ragueneau@univ-brest.fr ²Department of MEAS, NCSU, Raleigh NC 27695, USA, ³National Environmental Research Institute, DK-4000 Roskilde, Denmark and ⁴Department of Geochemistry, Utrecht University, 3508 TA Utrecht, the Netherlands

Calibration of biogenic silica (BSi) as a paleo productivity proxy: Towards a mechanistic understanding of BSi dissolution/preservation and Si/C decoupling in the world ocean

During JGOFS, the importance of diatoms in the global C cycle has been demonstrated. Silicic acid is now well recognized as a key nutrient in many biogeochemical provinces; its availability in part controls the relative contribution of diatoms to the total primary production, with important consequences for the export of carbon, towards both higher trophic levels and the deep sea. Owing to this importance of diatoms, the Mass Accumulation Rate of biogenic silica (BSi or opal MAR) in deep-sea sediments has a high potential as a paleoproductivity proxy. The deciphering of this sedimentary record, however, is complicated by numerous processes, including (i) spatial and temporal variations in opal preservation and (ii) the decoupling between Si and C biogeochemical cycles. Recognizing both the potential of this tracer as a proxy for paleoproductivity and the difficulties in interpreting its sedimentary record, the OPALEO community has been working towards a better understanding, in the modern ocean, of these processes that control the Si cycle and its coupling with the C biogeochemical cycle, from production to burial. Results are presented on these two fronts. New insights have been provided on the controls of opal dissolution/preservation in deep-sea sediments, owing to the use of flow-through reactors. At the same time, the processes that control the decoupling between Si and C cycles, at the cellular, the ecosystem and the global scales, become better and better understood and modeled. These new results are presented, as important steps for the calibration of opal MAR as a paleoproductivity proxy.