141 New Advances in Geobiology

This session will focus on new research at the intersection between geologic and biologic processes with special emphasis on novel materials and methods, new field sites, and advances at the intersections of scientific fields.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 305

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

Paper number: 141-9
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

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 qk=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 1000ln18αsilica-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 Δˈ17Osilica-Δˈ17Owater 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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413755

© Copyright 2025 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.