Updated Time-varying Maps of Surface Ocean δ¹³C of DIC and the ¹³CO₂ Isotopic Disequilibrium Flux with Uncertainty

D. Munro^1,2, J.B. Miller², A. Fay³ and P.D. Quay⁴

¹Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 303-497-5765, E-mail: david.munro@noaa.gov
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305
³Department of Earth and Environmental Sciences, Columbia University and Lamont Doherty Earth Observatory
⁴University of Washington, Seattle, WA 98105

Estimates of spatial and temporal patterns in the ¹³C:¹²C ratio (expressed as δ¹³C) of surface ocean dissolved inorganic carbon (DIC) are essential for interpreting atmospheric δ¹³C of CO₂. The work described here is part of a larger project focused on using atmospheric CO₂ and δ¹³C measurements to quantify terrestrial and oceanic net carbon fluxes as well as isotopic fractionation by terrestrial plants. Previously, we compiled a dataset of more than 8700 surface ocean δ¹³C of DIC measurements collected from research vessels and ships of opportunity from 1978 to 2018. In this analysis, we use a trend analysis over different regions of the global oceans and the approach of Takahashi et al. [2009] to create monthly maps of δ¹³C of DIC on a 4˚ (latitude) x 5˚ (longitude) spatial grid from 1990 to 2021. These values are then used to calculate the ¹³C isotopic disequilibrium flux at the same spatial and temporal resolution. The uncertainty of the δ¹³C of DIC is estimated using cross-validation whereby surface ocean δ¹³C of DIC observations are withheld and then compared to gridded predictions. We then use the cross-validation statistics to inform a Monte Carlo scheme to estimate the uncertainty of ¹³C isotopic disequilibrium flux for each grid cell. We use the same Monte Carlo scheme to assess the uncertainty of both the gross and net air-sea CO₂ flux from 1990 to 2021 and compare this uncertainty to the spread of estimates based on widely-available surface ocean partial pressure of CO₂ (pCO₂) products covering the same time interval. We find that uncertainties related to air-sea gas exchange can be substantial relative to the spread among estimates due only to differences in the pCO₂ product used in the flux estimate.

Figure 1. Fig. 1. Annual mean ¹³CO₂ isotopic disequilibrium in per mille for 1990-2021.