Clumped isotopes in atmospheric CO2: Seasonal and diurnal variations in urban air

Hagit Affek

Abstract

The budget of atmospheric CO2 is widely studied using records of temporal and spatial variations of concentrations, d13C and d18O values. However, the number and diversity of sources and sinks prevents these alone from fully constraining the budget. Molecules containing two rare isotopes can serve as additional tracers and potentially provide additional, independent, constraints. The talk will present data documenting seasonal and diurnal variations of CO2 having a mass of 47 u (mostly 13C18O16O) in air from Pasadena, CA. Data is reported using the 'mass 47 anomaly' (D47), which is defined as the deviation of R47(=[47]/[44]) from that expected for a random distribution of isotopologues. As such it reflects a temperature dependent preference on 13C and 18O to 'clump' into one chemical bond.

The talk will discuss the observed seasonal variations in atmospheric CO2 in urban air in Pasadena, CA, which are very different than those expected based on the monthly mean local temperature and assuming thermodynamic equilibrium in CO2 fluxes that involve CO2-H2O isotopic exchange. Diurnal variations in D47 and D14C are then used to identify a deviation from equilibrium in ecosystem respiration fluxes. The seasonal cycle may then be interpreted as a competition between low D47 values in respiration and higher D47 values resulting from CO2- H2O exchange in photosynthesis.

The talk will also briefly describe the use of 'clumped isotopes' in carbonate minerals as a new temperature proxy, presenting data from carbonate cave deposits.