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Abstract. The first δ 30 Si diatom data from lacustrine sediment traps are presented from Lake Baikal, Siberia. Data are compared with March surface water (upper 180 m) δ 30 Si DSi compositions for which a mean value of +2.28 ‰ ± 0.09 (95 % confidence) is derived. This value acts as the prediatom bloom baseline silicic acid isotopic composition of waters (δ 30 Si 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 δ 30 Si 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 δ 30 Si diatom analyses. Mean δ 30 Si 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 δ 30 Si diatom signatures (+0.67 ‰ ± 0.06, 2σ ) when compared with mean upper water δ 30 Si DSi initial (e.g. prebloom) signatures can be used to provide a snapshot estimation of diatom uptake fractionation factors ( uptake ) in Lake Baikal. A uptake estimation of −1.61 ‰ is therefore derived, although we emphasise that synchronous monthly δ 30 Si DSi and δ 30 Si 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 nearconstant δ 30 Si 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 ( 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 δ 30 Si diatom analyses in Lake Baikal and other freshwater systems, in palaeoreconstructions.

Distinct layers of iron(III) and manganese(IV) (Fe/Mn) oxides are found buried within the reducing part of the sediments in Lake Baikal and cause considerable complexity and steep vertical gradients with respect to the redox sequence. For the on-site investigation of the responsible biogeochemical processes, we applied filter tube samplers for the extraction of sediment porewater combined with a portable capillary electrophoresis instrument for the analyses of inorganic cations and anions. On the basis of the new results, the sequence of diagenetic processes leading to the formation, transformation, and dissolution of the Fe/Mn layers was investigated. With two exemplary cores we demonstrate that the dissolution of particulate Fe and Mn is coupled to the anaerobic oxidation of CH₄ (AOM) either via the reduction of sulphate (SO₄(2-)) and the subsequent generation of Fe(II) by S(-II) oxidation, or directly coupled to Fe reduction. Dissolved Fe(II) diffuses upwards to reduce particulate Mn(IV) thus forming a sharp mineral boundary. An alternative dissolution pathway is indicated by the occurrence of anaerobic nitrification of NH₄(+) observed at locations with Mn(IV). Furthermore, the reasons and consequences of the non-steady-state sediment pattern and the resulting redox discontinuities are discussed and a suggestion for the burial of active Fe/Mn layers is presented.

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