A Next Generation Ocean Carbon Isotope Model for Climate Studies

The ¹³C/¹²C of dissolved inorganic carbon (δ¹³C DIC ) carries valuable information on ocean
biological C-cycling, air-sea CO₂ exchange, and circulation. Paleo-reconstructions of oceanic ¹³C
from sediment cores provide key insights into past as changes in these three drivers. As a step
toward full inclusion of ¹³C in the next generation of Earth system models, we implemented ¹³C-
cycling in a 1° lateral resolution ocean-ice-biogeochemistry Geophysical Fluid Dynamics
Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year
forcing. The model improved the mean of modern δ¹³C DIC over coarser resolution GFDL-model
implementations, capturing the Southern Ocean decline in surface δ¹³C DIC that propagates to the
deep sea via deep water formation. The model is used here to quantify controls on modern and
anthropogenic δ¹³C DIC as well as to test their sensitivity to wind speed/gas exchange
parameterizations.
We found that reducing the coefficient for air-sea gas exchange following OMIP-CMIP6
protocols reduces deep sea modern δ¹³C DIC by 0.2 permil and improves the depth-integrated
anthropogenic δ¹³C DIC relative to previous gas exchange parameterizations. This is because the
δ¹³C DIC of the endmembers ventilating the deep sea and intermediate waters are highly sensitive to
the wind speed dependence of the air-sea CO₂ gas exchange. Additionally, meridional gradients
of surface modern δ¹³C DIC are better resolved with OMIP-CMIP6. While this model was initially
constructed to study the anthropogenic ¹³C response, it has promising applications toward longer
time scales. For example, BLING 13 C includes controls on the biological C-pump thought to be
important in the glacial ocean: light and iron limitation, and controls on ¹³C of organic matter
formation, and thus on ocean δ¹³C DIC and its vertical gradient, that depend on pCO₂ .