Insights into the transfer of silicon isotopes into the sediment record

Abstract

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.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…These all show that the new secondary mineral phases have less of the heavier isotopes relative to river waters. The same discrimination is also well established for plants (Ding et al, 2005(Ding et al, , 2008aOpfergelt et al, 2006a,b), freshwater diatoms Panizzo et al, 2016) and sponges, which preferentially utilize 28 Si over 30 Si (and 29 Si), thus leading to an increase in δ 30 Si in the host water.…”

“…For example, the approach has been demonstrated as a means to identify biogeochemical responses to climate change (Street-Perrott et al, 2008;Swann et al, 2010) and its value over more recent timescales is also stressed here as a means to address current pressures. Comprehensive contemporary limnological monitoring is also emphasized (Opfergelt et al, 2011) to enable quantitative estimations of seasonal DSi and bSiO 2 cycling (via open or closed system modeling), by providing key constraints on deepsurface water δ 30 Si DSi exchange, Si residency times and Si uptake (Panizzo et al, 2016(Panizzo et al, , 2017. These data also serve to validate conventional palaeolimnological approaches, which can be applied to constrain continental bSiO 2 export from lake and reservoirs, particularly under a projected future of enhanced anthropogenic and climatic pressures on these systems.…”

“…−1.1‰ that is independent of temperature and species . Furthermore, contemporary data of δ 30 Si diatom from open sediment traps placed down the Lake Baikal water profile found a near constant δ 30 Si diatom composition, suggesting the full preservation of the signal through the water column (Panizzo et al, 2016). More importantly, their comparison with δ 30 Si diatom in surface sediments, where high diatom valve dissolution occurs, argued for the absence of a Si isotope fractionation associated with diatom dissolution (Panizzo et al, 2016).…”

“…Similar to marine work (Varela et al, 2004;Closset et al, 2015), lake sediment traps have shown the δ 30 Si signature of diatoms (δ 30 Si diatom ) to be resilient to dissolution and preserved through the water column into the sediment record (Panizzo et al, 2016). Accordingly, δ 30 Si diatom from lacustrine sediment cores can be used to reconstruct past changes in biogeochemical cycling at the catchment/drainage basin scale.…”

“…As outlined above, there have been a handful of studies to date that have examined the application of δ 30 Si DSi in limnological settings Opfergelt et al, 2011;Panizzo et al, 2016Panizzo et al, , 2017. Lakes, in addition to the soil-vegetation system, play an important role in buffering continental DSi export to the oceans.…”