Extreme triple oxygen isotope fractionation in <i>Equisetum</i>.

Abstract

Triple oxygen isotope values of xylem water were measured along the length of smooth horsetail stems (<i>Equisetum laevigatum</i>). Extreme isotope enrichment is observed moving from base to stem tip. δ¹⁸O values range from -8.3‰ at the base to 82.6‰ at the tip. Δ'¹⁷O values range from 0 to -1,797 per meg. The δ¹⁸O and Δ'¹⁷O values are the most extreme measured for any terrestrial material and expand the known range of Δ'¹⁷O values by fivefold for mass-dependent fractionation on Earth. The extreme isotope enrichments are explained using a hybrid evaporation/chain-of-lakes model, allowing us to refine the leaf respiration coefficient to θ_k = 0.511 ± 0.001. This new value is required to explain the low Δ'¹⁷O values previously measured in desert plants and animals and is critical when using fossil samples for paleoclimate reconstruction. Coexisting phytoliths and stem water were also measured. The 1000<i>ln</i>¹⁸α_silica-water value at the plant base (35.89‰) appears to be in isotopic equilibrium, with far smaller fractionations of 10.3‰ near the tip. The smaller fractionations at higher levels are explained by continual silica deposition as the plant elongates and the δ¹⁸O values of each segment become higher. The overall integrated phytolith value is a combination of early and late silica growth. The Δ'¹⁷O_silica - Δ'¹⁷O_water values are not in equilibrium, explained by a kinetic isotope effect, with a λ value of 0.5205 vs. 0.5244 for equilibrium. Phytolith isotope values may lead to erroneous interpretations for paleoclimate reconstruction.