141-8 Extraordinary, Out-Of-This-World Triple Oxygen Isotopic Enrichment in Equisetum

Session: New Advances in Geobiology

Presenting Author:

Zachary Sharp

Authors:

Sharp, Zachary¹, Banerjee, Payal², Knutson, Cloe³, Peshek, Catherine⁴, Johnson, Jessica Marie⁵, Gargano, Anthony⁶, Cerling, Thure E.⁷, Wostbrock, Jordan⁸

Abstract:

Evaporation leads to distinctive trends in triple oxygen isotope values. δ¹⁸O values increase and Δˈ¹⁷O values (where Δˈ¹⁷O = δˈ¹⁷O - 0.528*δ'¹⁸O)   strongly decrease. Greater enrichment is seen in highly evaporated water, and the trend is enhanced further in arid environments.  Triple oxygen isotope values of xylem water were measured along the length of smooth horsetail stems (Equisetum laevigatum). Extreme isotope enrichment is observed moving from the base to tip of the stem. δ¹⁸O values range from -8.3‰ at the base to 82.6‰ at the tip, and Δˈ¹⁷O values range from 0 to -1797 per meg. The ¹⁸O/¹⁶O data are explained using a hybrid leaf evaporation/chain-of-lakes model using published coefficients of fractionation. The Δˈ¹⁷O data can only be explained if the leaf respiration coefficient q_k=0.512±0.02, far lower than the commonly accepted value of 0.518. This new value explains the low Δˈ¹⁷O values previously measured in desert plants and animals, and is critical in the use of fossil samples for paleoclimate reconstruction.

               Coexisting phytoliths and stem water were also measured. The 1000ln¹⁸α_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 Δˈ¹⁷O_silica-Δˈ¹⁷O_water values are not in equilibrium, explained by a kinetic isotope effect, with a θ value of 0.5205 vs 0.5244 for equilibrium. Phytolith isotope values may lead to erroneous interpretations for paleoclimate reconstruction.

               The δ¹⁸O and especially Δˈ¹⁷O values are the most extreme measured for any terrestrial material and expand the known range of Δˈ¹⁷O values by five-fold for mass dependent fractionation on Earth. The extreme fractionation observed in equisetum leads to Δˈ¹⁷O values that were previously only found in extraterrestrial materials.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9391

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