Insights into the transfer of silicon isotopes into the sediment record

Abstract

Abstract. The first δ30Sidiatom data from lacustrine sediment traps are presented from Lake Baikal, Siberia. Data are compared with March surface water (upper 180 m) δ30SiDSi compositions for which a mean value of +2.28 ‰ ± 0.09 (95 % confidence) is derived. This value acts as the pre-diatom bloom baseline isotopic composition of waters (δ30SiDSi initial). Open traps were deployed along the depth of the Lake Baikal south basin water column between 2012–2013. Diatom assemblages display a dominance (>85 %) of the spring bloom species Synedra acus var radians, so that δ30Sidiatom compositions reflect spring bloom utilisation. Diatoms were isolated from open traps and in addition, from 3 monthly (sequencing) traps (May, June and July 2012) for δ30Sidiatom analyses. Mean δ30Sidiatom values for open traps are +1.23 ‰ ± 0.06 (at 95 % confidence and MSWD of 2.9, n = 10) and, when compared with mean upper water δ30SiDSi signatures, suggest a diatom fractionation factor (εuptake) of −1.05 ‰, which is in good agreement with published values from oceanic and other freshwater systems. The near constant δ30Sidiatom compositions in open traps demonstrates the full preservation of the signal through the water column and thereby justifies the use and application of the technique in biogeochemical and palaeoenvironmental research. Data are finally compared with lake sediment core samples, collected from the south basin. Values of +1.30 ‰ ± 0.08 (2σ) and +1.43 ‰ ± 0.13 (2σ) were derived for cores BAIK13-1C (0.6–0.8 cm core depth) and at BAIK13-4F (0.2–0.4 cm core depth), respectively. Trap data highlight the absence of a fractionation factor associated with diatom dissolution (εdissolution) down the water column and in the lake surface sediments, thus validating the application of δ30Sidiatom analyses in Lake Baikal and other freshwater systems, in palaeoreconstructions.