Extreme triple oxygen isotope fractionation in Equisetum.

Triple oxygen isotope values of xylem water were measured along the length of smoothhorsetail stems (Equisetum laevigatum). Extreme isotope enrichment is observed mov-ing from base to stem tip. δ18O values range from −8.3‰ at the base to 82.6‰ at thetip. Δ′17O values range from 0 to −1,797 per meg. The δ18O and Δ′17O values are themost extreme measured for any terrestrial material and expand the known range ofΔ′17O values by fivefold for mass-dependent fractionation on Earth. The extreme isotopeenrichments are explained using a hybrid evaporation/chain-of-lakes model, allowing usto refine the leaf respiration coefficient to θk = 0.511 ± 0.001. This new value is requiredto explain the low Δ′17O values previously measured in desert plants and animals andis critical when using fossil samples for paleoclimate reconstruction. Coexisting phy-toliths and stem water were also measured. The 1000ln18αsilica-water value at the plantbase (35.89‰) appears to be in isotopic equilibrium, with far smaller fractionationsof 10.3‰ near the tip. The smaller fractionations at higher levels are explained bycontinual silica deposition as the plant elongates and the δ18O values of each segmentbecome higher. The overall integrated phytolith value is a combination of early and latesilica growth. The Δ′17Osilica − Δ′17Owater values are not in equilibrium, explained by akinetic isotope effect, with a λ value of 0.5205 vs. 0.5244 for equilibrium. Phytolithisotope values may lead to erroneous interpretations for paleoclimate reconstruction.