METHOXY CLUMPED ISOTOPES IN WOOD AS A RECORD OF PHOTORESPIRATION: CASE STUDIES FROM GLACIAL-INTERGLACIAL PERIODS IN THE QUATERNARY

Carbon and hydrogen isotopes of wood are used to reconstruct ancient climate. Recently, “clumped” isotopologues of wood (methoxy groups from lignin with two rare isotopes, e.g., ¹³CH₂D) have been employed to reconstruct photorespiration – an important process in plant physiology that is often related to plant stress: the tendency of RuBisCO to bind to oxygen over CO₂. Photorespiration rates are controlled by several factors, including the proportion of O₂:CO₂ in the atmosphere (V_o/V_c), temperature, and water availability. Typical Δ¹³CH₂D values in wood lignin are ~8.0-10.0‰. Increased photorespiratory export is thought to result in more depleted Δ¹³CH₂D values (e.g., as low as 5.0 ‰), while more enriched Δ¹³CH₂D values indicate less photorespiration, or a more “closed” photorespiratory cycle (e.g., 10.0‰). We measured isotopologues for glacial woods from Alaska (USA) (ranging from 34 to 4 kiloyears before present) and rings from the Bristlecone Pines of the White Mountains (California, USA) from 7 to 0.01 kiloyears before present, during which time the White Mountains have become increasingly dry. Both sets of wood grew in a low CO₂-high O₂ atmosphere (i.e., high V_o/V_c ratio). Woods from Alaska grew in environments with ample water, whereas woods in California experienced drying over their lifetimes. Glacial woods from Alaska ranged in ¹³CH₂D values from between 7 and 8.0‰ (glacial, between 40-25ka) to 9.15‰ (~17ka) with Δ¹³CH₂D values becoming more enriched with concurrent time and increased pCO₂ (R² = 0.58). Woods from California ranged from 7.31‰ to 9.95‰ with no clear trends associated with pCO₂, demonstrating confounding factors of increased water stress on V_o/V_c controls on photorespiration.