Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)
We have investigated the transfer of oxygen isotope signals of diatomaceous silica (δ18Odiatom) from the epilimnion (0–7 m) through the hypolimnion to the lake bottom (∼20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The δ18Odiatom values of the 7 m trap varied from 29.4‰ in spring/autumn to 26.2‰ in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance δ18Odiatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September δ18Odiatom values of the 16 m trap were roughly ∼1.5‰ enriched in 18O compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the δ18Odiatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5‰ enriched in 18O compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate δ18Odiatom signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched δ18Odiatom value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline 18O at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high pH conditions do not affect the δ18Odiatom values of sub-fossil and even fossil opal. IR analyses show that the 18O enrichment of the sedimentary silica is associated with a decrease in Si–OH groups and the formation of Si–O–Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary δ18Odiatom values in terms of palaeothermometry.
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