Seasonal variations of stable, including clumped, isotopologues of CO2 in air: Initial observations from La Jolla, Ca

Abstract:

The budget of atmospheric CO₂ is commonly studied using records of CO₂ concentrations and isotopic abundances. δ¹³C and δ¹⁸O are the most frequently used isotopic tracers and can be used to constrain marine and terrestrial fluxes. However, the large number of sources and sinks prevents concentrations, d¹³C and d¹⁸O alone from uniquely constraining the budget. Δ¹⁷O as well as clumped isotopologues of CO₂ can provide independent constraints on CO₂ fluxes. Here we present a record covering multiple 1+ year periods between 2003 and 2012 documenting seasonal variations at La Jolla, CA, of δ¹³C, δ¹⁸O and clumped isotopologues of CO₂ as well as preliminary results for Δ¹⁷O. Samples were collected from the La Jolla pier only on days a seaward breeze was present, therefore local anthropogenic effects should be minimal. We report the clumped isotopologues of CO₂ using Δ₄₇ notation in the absolute reference frame.

 In the years studied, δ¹³C showed a seasonal cycle that is consistent with previous measurements and can be explained by the seasonal variation in photosynthetic activity of plants as well as a longer term trend of fossil fuel injection into the atmosphere. Similarly δ¹⁸O showed a seasonal and interannual cycle that is consistent with previous measurements and can be explained by the interaction of the terrestrial biosphere with the hydrologic cycle.

The La Jolla ∆₄₇ record shows a strong seasonal pattern for all years studied, and initial results suggest seasonality is also present in ∆¹⁷O. The La Jolla ∆₄₇ pattern is similar to the Pasadena ∆₄₇ record collected for 2003-2004. Both records exhibit amplitudes and phasing of ∆₄₇ variations that are not consistent with the equilibration of CO₂ with the local air or sea surface temperatures, nor can they be explained by fossil fuel burning, which can only weakly influence the seasonal cycles. The seasonal cycle may be explained in part by a competition between low ∆₄₇ from respiration sources and higher values from CO₂-water exchange during photosynthesis and marine air-sea exchange. We construct a box model to explain our stable isotope results and quantify the sizes of the CO₂ fluxes influencing our La Jolla samples.