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

Abstract

Carbon isotopic fractionation during photosynthesis (εp) is used to reconstruct past CO2 and phytoplankton growth rates, typically by measuring the δ13C of biomarkers produced by coccolithophorids. However, organic molecules bound within diatom frustules represent another phase for measurement of δ13C and offer the opportunity to obtain εp for specific diatom sizes and geometries. Here, from core top sediments covering a strong productivity gradient in the Southern Ocean, we present determinations of δ13C and εp 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 δ13C 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. εp 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 εp are too large to arise from the rather modest variation in dissolved CO2 in surface waters across the core top transect. We suggest instead that the maximum εp 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 εp allow us to resolve a 5° northward displacement of the PF during glacial times compared to interglacial times. By estimating CO2 aq in equilibrium with the Holocene ice core atmospheric CO2 concentrations, we quantify this growth rate effect and document that it is strongly correlated with indicators of trace metal supply, such as frustule Zn content, as well as indicators of diatom productivity such as opal % and opal accumulation rates in sediments and sediment traps. These relationships may be applied to constrain the effect of growth rate variations on εp and more accurately derive CO2 variations from εp during periods prior to ice core CO2 proxy records.