Abstract
Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO2 uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements (34S/32S; δ34S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ34S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of -1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.
Highlights
Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO2 uptake) are needed
This rise in CO2 is mitigated by plant uptake; it is important to estimate global and regional photosynthesis rates and trends [1]
The vast majority of our samples (90%) showed a Carbonyl sulfide (COS) concentration of 480 ± 60 ppt which agrees with the National Oceanic and Atmospheric Administration (NOAA) 20-y global COS monitoring program’s mean concentration of 480 ± 40ppt
Summary
Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO2 uptake) are needed. Sulfur isotope measurements (34S/32S; δ34S) have been suggested as a useful tool to constrain COS sources Such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. We present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete databased tropospheric COS isotopic mass balance, which allows improved partition of the sources. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination. In contrast to previous studies that used assessments for these isotopic values, our aim was to directly measure the isotopic values of these missing components, and to determine the tropospheric COS δ34S variability over space and time
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