Insights into the transfer of silicon isotopes into the sediment record

[摘要] The first δ₃₀Si_diatom data from lacustrine sedimenttraps are presented from Lake Baikal, Siberia. Data are compared withMarch surface water (upper 180 m) δ₃₀Si_DSicompositions for which a mean value of +2.28‰ ±  0.09(95 % confidence) is derived. This value acts as the pre-diatombloom baseline silicic acid isotopic composition of waters(δ₃₀Si_{DSi initial}). Open traps were deployed alongthe depth of the Lake Baikal south basin water column between2012 and 2013. Diatom assemblages display a dominance ( > 85 %) ofthe spring/summer bloom species Synedra acus var radians, so thatδ₃₀Si_diatom compositions reflect predominantly spring/summer bloomutilisation. Diatoms were isolated from open traps and, in addition,from 3-monthly (sequencing) traps (May, July and August 2012) forδ₃₀Si_diatom analyses. Meanδ₃₀Si_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 theinitial export of the dominant spring diatom bloom. We therefore argue that Mayδ₃₀Si_diatom signatures (+0.67‰ ±  0.06, 2σ) when comparedwith mean upper water δ₃₀Si_DSi initial (e.g. pre-bloom) signaturescan be used to provide a snapshot estimation of diatom uptake fractionation factors(ϵ_uptake) in Lake Baikal. A ϵ_uptake estimation of −1.61 ‰ istherefore derived, although we emphasise that synchronous monthly δ₃₀Si_DSi andδ₃₀Si_diatom data would be needed to provide more robust estimations and thereforemore rigorously test this, particularly when taking into consideration any progressive enrichment of the DSi pool as blooms persist.The near-constant δ₃₀Si_diatom compositionin open traps demonstrates the full preservation of the signal throughthe water column and thereby justifies the use and application of thetechnique in biogeochemical and palaeoenvironmental research. Data arefinally compared with lake sediment core samples, collected from thesouth basin. Values of +1.30‰ ±  0.08 (2σ) and+1.43‰ ±  0.13 (2σ) were derived for coresBAIK13-1C (0.6–0.8 cm core depth) and at BAIK13-4F(0.2–0.4 cm core depth) respectively. Trap data highlight theabsence of a fractionation factor associated with diatom dissolution(ϵ_dissolution) (particularly as Synedra acus var radians, thedominant taxa in the traps, is very susceptible to dissolution) down the water column and in the lakesurface sediments, thus validating the application ofδ₃₀Si_diatom analyses in Lake Baikal and otherfreshwater systems, in palaeoreconstructions.

[学科分类] 地球化学与岩石