Isotope Fractionation During Sorption of Different Thiomolybdate Species to Marine Phytoplankton Using IC-Separation & Detection by MC-ICP-MS
Esther Breuninger (09/2017-10/2018)
Support: Britta Planer-Friedrich, Samer Bachmaf, Carolin Kerl
In recent years Molybdenum (Mo) isotopes have been suggested as a paleo-ocean redox proxy. However, its potential is restricted by an incomplete understanding of isotope fractionation occurring during key biogeochemical processes. For the euxinic burial pathway an initial sulfidation reaction has been proposed, where in the presence of sufficient amounts of dissolved sulfide, Mo is converted to particle-reactive thiomolybdate species (MoO4-xSx2-). Previous studies of the Mo isotope paleo-proxy assumed quantitative scavenging of formed MoS42- when buried into euxinic sediments, giving a similar sedimentary isotope signature to that of seawater.
To address one possible burial pathway under sulfidic conditions, the isotope fractionation during the sorption of different thiomolybdate species on marine phytoplankton was investigated by species-selective isotope analysis (offline IC-separation & detection by MC-ICP-MS). This thesis compared the sorption behaviour of MoO42-, MoS42- and a Mo:S mixture (1:50) in regard to Mo isotope composition and speciation by assessing the influence of different sorbent concentrations and characterizing different reaction patterns during kinetic sorption. Thiomolybdates were readily adsorbed by marine phytoplankton. In contrary, MoO42- showed no sorption under sulfidic experimental conditions. Both kinetic sorption experiments with MoS42- and the Mo:S mixture showed isotope fractionation between dissolved and adsorbed Mo, with Δdiss-ads of 1.47+/0.28 ‰ and 1.31+/-0.16 ‰, respectively. Besides adsorption of higher sulfidized thiomolybdates (MoS42- and polythiomolybdates), the data indicated adsorption of intermediate thiomolybdates (MoO2S22- and MoOS32-), which was associated with larger isotope fractionation between both examined phases (Δdiss-ads ranging from 1.5-1.8 ‰). Furthermore, a strong catalysing effect on thiomolybdate (trans)formation by phytoplankton was observed. Light Mo isotopes are preferentially adsorbed. Comparison of the measured isotope data of both kinetic sorption experiments with predictions for closed system reversible and irreversible fractionation models indicated that isotope fractionation occurs as a result of closed system irreversible fractionation between dissolved and adsorbed Mo.
The observed experimental isotope fractionation does not reflect the assumed quantitative scavenging of thiomolybdates by organic matter. This suggests other prominent burial pathways or intermediate reactions before sedimentary fixation, which are associated with no fractionation between open ocean water and euxinic sediments. Nevertheless, the results demonstrate that the applied species-selective isotope analysis provides a unique tool in understanding biogeochemical processes.