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 silicic acid isotopic composition of waters (δ30SiDSi 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 δ30Sidiatom 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 δ30Sidiatom analyses. Mean δ30Sidiatom 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 δ30Sidiatom signatures (+0.67‰ ± 0.06, 2σ) when compared with mean upper water δ30SiDSi initial (e.g. pre-bloom) signatures can be used to provide a snapshot estimation of diatom uptake fractionation factors (ϵuptake) in Lake Baikal. A ϵuptake estimation of −1.61 ‰ is therefore derived, although we emphasise that synchronous monthly δ30SiDSi and δ30Sidiatom 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 δ30Sidiatom 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 (ϵdissolution) (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 δ30Sidiatom analyses in Lake Baikal and other freshwater systems, in palaeoreconstructions.
Highlights
Records of diatom silicon isotopes (δ30Sidiatom) provide a key means to investigate changes in the global silicon cycle (De La Rocha, 2006; Hendry and Brzezinski, 2014; Leng et al, 2009; Tréguer and De La Rocha, 2013)
Diatom concentrations show some variability, varying between ca. 3 × 104 and 7 × 104 valves g−1 wet weight (Fig. 4), lowest concentrations are seen in the open sediment trap at 1350 m depth (3 × 104 valves g−1 wet weight Fig. 4). This is coincident with lowest diatom (S. acus var radians) valve abundances (86 %; Table 2). δ30Sidiatom data from the open sediment traps show little variability down the water column profile in Lake Baikal (Table 2; Fig. 4) with values ranging from +1.11 to +1.38 ‰
The first δ30Sidiatom data from lacustrine sediment traps are presented from Lake Baikal, Siberia, and their use in interpreting the fate of δ30Sidiatom in the sediment record is shown
Summary
Records of diatom silicon isotopes (δ30Sidiatom) provide a key means to investigate changes in the global silicon cycle (De La Rocha, 2006; Hendry and Brzezinski, 2014; Leng et al, 2009; Tréguer and De La Rocha, 2013). Estimations of uptake (−1.1 ‰ ± 0.4 to −1.2 ‰ ± 0.2) have to date shown it to be independent of temperature, pCO2 (aq) and other vital effects (De La Rocha et al, 1997; Fripiat et al, 2011; Milligan et al, 2004; Varela et al, 2004), more recent work on marine diatoms in laboratory cultures has argued for a species-dependent fractionation effect (Sutton et al, 2013) In this case, uptake estimations were documented between −0.53 ‰ ± 0.11 and −0.56 ‰ ± 0.07 for the Fragilariopsis kerguelensis species (depending on the culturing strains used) and up to −2.09 ‰ ± 0.09 for the Chaetocerous brevis species (Sutton et al, 2013). Procedural blank compositions are difficult to accurately measure, but may have deviated from sample compositions by ca. 0.04 ‰, contributing up to a less than 0.01 ‰ shift in the sample compositions
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