HR: 14:10h
AN: V53B-03    [Abstracts]
TI: Si Isotopic Signatures of Diatoms in the Spring Southern Ocean
AU: * Cardinal, D
EM: damien.cardinal@africamuseum.be
AF: Musee Royal de l'Afrique Centrale, Dpt. of Geology and Mineralogy Leuvensesteenweg, 13, Tervuren, B3080 Belgium
AU: Alleman, L Y
EM: laurent.alleman@africamuseum.be
AF: Musee Royal de l'Afrique Centrale, Dpt. of Geology and Mineralogy Leuvensesteenweg, 13, Tervuren, B3080 Belgium
AU: Savoye, N
EM: nsavoye@vub.ac.be
AF: Vrije Universiteit Brussel, Dpt. of Analytical and Environmental Chemistry Pleinlaan 2, Brussels, B1050 Belgium
AU: Dehairs, F
EM: fdehairs@vub.ac.be
AF: Vrije Universiteit Brussel, Dpt. of Analytical and Environmental Chemistry Pleinlaan 2, Brussels, B1050 Belgium
AU: Trull, T W
EM: Tom.Trull@utas.edu.au
AF: Antarctic Climate and Ecosystem Cooperative Research Centre, University of Tasmania Private Bag 80, Hobart, TAS 7001 Australia
AU: Andr\'{e}, L
EM: luc.andre@africamuseum.be
AF: Musee Royal de l'Afrique Centrale, Dpt. of Geology and Mineralogy Leuvensesteenweg, 13, Tervuren, B3080 Belgium
AB: Marine Si isotopic signatures have been shown to be of great relevance in quantifying the diatoms nutrient utilization efficiency, a key factor for many studies related to oceanic carbon sequestration (De La Rocha et al., 1998; Brzezinski et al., 2002). In order to better constrain and apply this new tool, spring diatoms and seawater have been sampled at five stations distributed in different biogeochemical provinces of the Southern Ocean's Australian sector: Polar Front Zone (PFZ), Antarctic Zone (AZ), Sea Ice Zone (SIZ). Total ($>$0.45um), medium-sized (20$<$ $<$70um), and large diatoms ($>$70um) have been sampled at 2-4 depths in the upper 150m. Biogenic silica (BSi) was first digested in a hot diluted NaOH solution. After purification, silicon isotopic compositions of diatoms and seawater were then measured by MC-ICP-MS, in dry plasma mode using external Mg doping, with an overall repeatability of 0.08 p.mil (Cardinal et al., 2003). Results are expressed as \delta$^{29}$Si relatively to NBS28 standard. The isotopic composition of diatoms is generally homogeneous in the mixed layer and does not seem to exhibit an isotopic fractionation linked to a size effect. Diatoms \delta$^{29}$Si are systematically lighter than the ambient seawater signature, reflecting their preferential uptake of light isotopes (De La Rocha et al., 1997). We observe a trend of lighter isotopic signatures southward, both in diatoms and seawater samples, but the BSi isotopic gradient is much steeper with a diatoms \delta$^{29}$Si signature as low as -0.26 p.mil in the southernmost SIZ station, which strongly contrasts with the +0.65 p.mil signature measured on PFZ diatoms. Such latitudinal variation of diatom isotopic signature is well in accordance with the one observed in summer by Varela et al. (2004). In contrast, our spring samples display a difference between the ambient seawater and diatom isotopic signatures that strongly increases southward: it goes from 0.45 in the PFZ up to 1.08 p. mil in the SIZ. This points toward different time scales, likely to be recorded in the mixed layer between diatoms and seawater as a function of the zonal physical and ecological characteristics. The observed latitudinal trends will be discussed in terms of seasonal nutrient utilization and sources, sea ice influence, diatom assemblages and proxy validation.
DE: 4845 Nutrients and nutrient cycling
DE: 4207 Arctic and Antarctic oceanography
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting