Effect of diatom growth and dissolution on silicon isotope fractionationin an estuarine system
(English)Manuscript (preprint) (Other academic)
Si isotopes provide a powerful tool to reveal past and present patterns in diatom production. Most studies have focused on Si fractionation factors during diatom growth in open ocean systems and have found lower Si isotope values in diatom shells (biogenic silica). Recent findings indicate that even the fractionation of Si isotopes during the physicochemical dissolution of diatom shells in the opposite direction produces higher δ30Si values in the remaining biogenic silica (BSi), allowing for the interpretation of diatom production patterns over geological time scales. However, estuarine and coastal primary production represents approximately 30-50% of global marine production, and there are hardly any studies on Si isotope fractionation during either diatom growth or dissolution. In this study, Si isotope fractionation during diatom growth and the dissolution of the frustule were measured. Two species of diatoms from the Baltic Sea, one of the largest estuarine systems in the world, were selected for this study. The results show that both species of diatoms during growth yields an identical Si isotope fractionation factor of 0.99925 for 29Si and 0.9984 for 30Si. In contrast to findings from open ocean species, no Si isotope fractionation during dissolution was observed even after 90% of the diatoms dissolved. Whether there is isotope fractionation during dissolution or not will have profound implications for studies using Si isotopes to interpret the Si cycle in marine and estuarine systems. We propose that the small size of the diatoms living in estuarine systems with low salinity may explain the non-existence of Si isotope fractionation during dissolution. Therefore, we suggest that Si isotopes are an instrumental variable holding information about original environmental conditions of estuarine and even coastal systems. Finally, we tested the Si isotope fractionation patterns gained from the lab experiments on a sediment core, corroborating the observed dissolved silicates (DSi) uptake rates in the above water column during diatom growth.
Research subject Geochemistry
IdentifiersURN: urn:nbn:se:su:diva-79184OAI: oai:DiVA.org:su-79184DiVA: diva2:547892
FunderSwedish Research Council, 2007-4763