Insights into the transfer of silicon isotopes into the sediment record

Abstract. The first δ₃₀Si_diatom data from lacustrine sediment traps are presented from Lake Baikal, Siberia. Data are compared with March surface water (upper 180 m) δ₃₀Si_DSi compositions for which a mean value of +2.28‰ ±  0.09 (95 % confidence) is derived. This value acts as the pre-diatom bloom baseline silicic acid isotopic composition of waters (δ₃₀Si_{DSi initial}). Open traps were deployed along the depth of the Lake Baikal south basin water column between 2012 and 2013. Diatom assemblages display a dominance ( > 85 %) of the spring/summer bloom species Synedra acus var radians, so that δ₃₀Si_diatom compositions reflect predominantly spring/summer bloom utilisation. Diatoms were isolated from open traps and, in addition, from 3-monthly (sequencing) traps (May, July and August 2012) for δ₃₀Si_diatom analyses. Mean δ₃₀Si_diatom values for open traps are +1.23‰ ±  0.06 (at 95 % confidence and MSWD of 2.9, n = 10). Total dry mass sediment fluxes are highest in June 2012, which we attribute to the initial export of the dominant spring diatom bloom. We therefore argue that May δ₃₀Si_diatom signatures (+0.67‰ ±  0.06, 2σ) when compared with mean upper water δ₃₀Si_DSi initial (e.g. pre-bloom) signatures can be used to provide a snapshot estimation of diatom uptake fractionation factors (ϵ_uptake) in Lake Baikal. A ϵ_uptake estimation of −1.61 ‰ is therefore derived, although we emphasise that synchronous monthly δ₃₀Si_DSi and δ₃₀Si_diatom data would be needed to provide more robust estimations and therefore more rigorously test this, particularly when taking into consideration any progressive enrichment of the DSi pool as blooms persist. The near-constant δ₃₀Si_diatom composition 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 (ϵ_dissolution) (particularly as Synedra acus var radians, the dominant taxa in the traps, is very susceptible to dissolution) down the water column and in the lake surface sediments, thus validating the application of δ₃₀Si_diatom analyses in Lake Baikal and other freshwater systems, in palaeoreconstructions.