Growth rate and size effect on carbon isotopic fractionation in diatom-bound organic matter in recent Southern Ocean sediments

• Published in: Earth and Planetary Science Letters
• Main Authors: Stoll, Heather M., Mendez-Vicente, Ana, Abrevaya, Lorena, Anderson, Robert F., Rigual-Hernández, Andrés S., Gonzalez-Lemos, Saul
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: diatom; phytoplankton; carbon isotope; Southern Ocean; AESOPS; Polar Front; Antarc*; Antarctic; ice core
• Online Access: https://researchers.mq.edu.au/en/publications/c5de6fb5-9db4-419a-8a29-3c30e666c0ef; https://doi.org/10.1016/j.epsl.2016.09.028; http://www.scopus.com/inward/record.url?scp=84995794330&partnerID=8YFLogxK

Carbon isotopic fractionation during photosynthesis (ε p ) is used to reconstruct past CO 2 and phytoplankton growth rates, typically by measuring the δ 13 C of biomarkers produced by coccolithophorids. However, organic molecules bound within diatom frustules represent another phase for measurement of δ 13 C and offer the opportunity to obtain ε p for specific diatom sizes and geometries. Here, from core top sediments covering a strong productivity gradient in the Southern Ocean, we present determinations of δ 13 C and ε p from frustule-bound organic matter from a fine opal fraction dominated by pennate diatoms and a coarse opal fraction dominated by larger centric diatoms. The δ 13 C of the pennate diatom fraction is typically 2.8‰ more positive than that of the centric fraction. Both fractions show a comparable range of 9–10‰ over the core top transect. ε p is lowest (6.3‰ in pennate fraction) between the Polar Front (PF) and Southern Antarctic Circumpolar Current Front (SACCF) and increases both to the north and south, with maximum values at greatest distance from the PF (18‰ in the pennate fraction). These spatial changes in ε p are too large to arise from the rather modest variation in dissolved CO 2 in surface waters across the core top transect. We suggest instead that the maximum ε p reflects higher diatom growth rates, and in the case of pennate diatom F. kerguelensis also an increase in the frustule width and volume to surface area ratio. Both processes may result from enhanced Fe supply due to upwelling of circumpolar deep water between the PF and SACCF. Farther south, diatom growth is strongly Fe-limited and farther north it is Fe and Si co-limited. The optima of growth rates between the PF and SACCF appears to be a general feature in all sectors of the Southern Ocean. Such growth rate-induced changes in diatom ε p allow us to resolve a 5° northward displacement of the PF during glacial times compared to interglacial times. By estimating CO 2 aq in equilibrium with the Holocene ice core atmospheric ...