situ silicon stable isotope analysis of phytoliths

11 Silicon is a beneficial element for many plants, and is deposited in plant tissue as amorphous bio‐opal 12 called phytoliths. The biochemical processes of silicon uptake and precipitation induce isotope 13 fractionation: the mass‐dependent shift in the relative abundances of the stable isotopes of silicon. At 14 the bulk scale, δ30Si ratios span from ‐2 to +6 ‰. To further constrain these variations in situ, at the 15 scale of individual phytolith fragments, we used femtosecond laser ablation multicollector inductively 16 coupled plasma mass spectrometry (fsLA‐MC‐ICP‐MS). 17 A variety of phytoliths from grasses, trees and ferns were prepared from plant tissue or extracted from 18 soil. Good agreement between phytolith δ30Si ratios obtained by bulk solution MC‐ICP‐MS analysis and 19 in situ isotope ratios from fsLA‐MC‐ICP‐MS validates the method. Bulk solution analyses result in at 20 least two‐fold better precision for δ30Si (2SD on reference materials ≤0.11 ‰) over that found for the 21 means of in situ analyses (2SD typically ≤0.24 ‰). 22 We find that bushgrass, common reed, and horsetail show large internal variations up to 2 ‰ in δ30Si, 23 reflecting the various pathways of silicon from soil to deposition. fsLA provides a means to identify the 24 underlying processes involved in the formation of phytoliths using silicon isotope ratios. 25

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