UMR CNRS 5805 EPOC
Environnements et Paléoenvironnements
Océaniques et Continentaux

        

Consignes de Sécurité et Numéros d'Urgence
(à l'usage des personnels du laboratoire)

le 23-01-2009 à 10:45

Lieu Salle Stendhal, DGO, Bâtiment B18 Intervenant(s) Laetitia Pichevin, Université d'Edimbourg, UK

A shift towards increased relative opal production in the glacial ocean compared to today has been invoked to explain lower atmospheric PCO2 during the last glacial period through enhanced biological pump of carbon. However, sedimentary evidences of such a global increase in opal burial are lacking or equivocal. For instance, the Eastern Equatorial Pacific (EEP) shows a global decrease in opal accumulation rates whereas our new data from the tropics show a clear increase in opal burial.
The modern EEP is a major oceanic source of carbon to the atmosphere and increased deep-sea carbon export in this region during glacial periods could contribute towards lower CO2 levels. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments but the lower opal accumulation rates had cast doubts. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP resulted from a decrease in Si:C uptake ratio of diatoms under conditions of increased Fe availability from enhanced dust inputs. Consequently, our study provides support for an invigorated biological CO2 pump in this region during the glacial periods that could have contributed to glacial CO2 drawdown. Additionally, using evidences from silicon and nitrogen isotope changes we infer that in contrast to modern situation the biological productivity in this region during glacial periods is not constrained by the availability of Fe, Si and N but instead by P.
Based on these results, we argue that an invigorated biological CO2 pump constrained only by P limitation was a common occurrence in low-latitude areas of the glacial ocean. The additional silicic acid availability in the EEP occurs at a time when the demand by biota is already reduced due to Fe-fertilisation. This argues for drastic reduction in Si limitation over large areas of the ocean during glacial periods and explains the concomitant observed increase in opal accumulation in the eastern tropical pacific both north and south of the equator during the last glacial period.