The response of the¹⁸O/¹⁶O composition of atmospheric CO₂to changes in environmental conditions

Abstract

[1] This study investigates the response of the global mean and spatial variations of the δ 18 O value of atmospheric CO 2 (δC a ) to changes in soil CO 2 hydration rates, relative humidity, the δ 18 O value of precipitation and water vapor, visible radiation, temperature, and ecosystem flux partitioning. A three-dimensional global transport model was coupled to a mechanistic land surface model and was used to calculate isotopic fluxes of CO 2 and H 2 O and the resulting δC a . The model reproduced the observed global mean and north-south gradient in δC a . The simulated seasonal amplitude and phases of CO 2 and δC a agreed well at some but not all locations. Sensitivity tests with relative humidity increased by 3.2% from its original value decreased δC a by 0.21‰. Similarly, a global 3.3‰ decrease in the isotopic composition of both precipitation and water vapor (δW P and δW AV , respectively) caused a 2.6‰ decrease in δC a . A 1 K increase in atmospheric temperatures also affected δC a , but there was a very small δC a response to realistic changes in light levels. Experiments where leaf and soil CO 2 fluxes were repartitioned revealed a nontrivial change to δC a . The predicted north-south δC a gradient increased in response to an increase in soil CO 2 hydration rates. However, the δC a gradient also had a large response to global changes in δW P and δW AV . This result is particularly important since most models fail to deplete δW P enough at middle and high latitudes, where the influence of δW P and δW AV on the δC a gradient is strongest.

The role of soil processes in δ18O terrestrial climate proxies

“…Specifically, IsoLSM uses an advective transport model for soil water, nonfractionating root water uptake, and surface boundary layer resistance to predict soil water δ 18 O in each of six subsurface soil layers. These modules have been validated in previous studies [see Still et al ., ; Buenning et al ., , and references therein]. In the present study, the total depth of the six‐layer soil column is 1.5 m and individual layer thicknesses, dz , are 0.10, 0.20, 0.20, 0.25, 0.25, and 0.50 m. Each grid cell has a specific vegetation type and associated rooting depth and there are 12 different vegetation types for forested, wooded, nonwooded, and wetland areas (see the NCAR LSM Users Guide for additional details regarding plant types).…”

Evaluation of modeled land‐atmosphere exchanges with a comprehensive water isotope fractionation scheme in version 4 of the Community Land Model

“…In purely turbulent conditions, n = 0 is appropriate, because turbulent mixing treats all isotopic species equally, regardless of differing molecular properties [ Noone and Sturm , ]. Although the optimal choice for the exponent n for a given set of environmental conditions is an area of ongoing study, (M78) remains a popular choice [ Roden et al ., ; Wang and Yakir , ; Williams et al ., ; Xu et al ., ; Buenning et al ., ; Rothfuss et al ., ].…”

Hydrologic control of the oxygen isotope ratio of ecosystem respiration in a semi-arid woodland

“…The problem becomes even more acute when isotope-enabled land models are integrated into global models (e.g., Buenning et al, 2012;Wingate et al, 2009). Despite these complications, we contend these models can be helpful to investigate relationships between forcing and net isotope exchanges with the atmosphere, as long as an awareness of these uncertainties is maintained.…”