Poster presented at the 2018 Fall Meeting of the American Geophysical Union in San Francisco, California, USA.

1. Circulation
Fig. 7: As Fig. 6 but due to changes in circulation.
Sea Ice
Fig. 6: As Fig. 5 but due to changes in sea ice. Note the compensating effects on Cdis,phy
and Cdis,bio
. This leads to minor effects of
sea ice on the total disequilibrium (Fig. 3). Sea ice effects on Cdis,phy
and Corg
were not considered by Stephens and Keeling (2000).
Air-Sea Disequilibrium Enhances Glacial Ocean Carbon Storage
Samar Khatiwala1
, Andreas Schmittner2
& Juan Muglia2
1
University of Oxford, 2
Oregon State University
Inventories
Fig. 2: Carbon inventories of Ceq
(left axis) and other components (right axis) for the preindustrial control
(PIC) and LGM equilibrium simulations.
Export Production
Iron
Fig. 5: Change in export production (d) and carbon components (Corg
and Cdis,bio
) due to increased aeolian soluble
iron fluxes.
Comparison with Observations
Fig. 8: Comparison of zonally-averaged radiocarbon (top), δ13
C (bottom), export production (top right) and δ15
N (bottom right)
with LGM reconstructions from Skinner et al. (2017), Peterson et al. (2014), Kohfeld et al. (2005) and Galbraith et al. (2012).
Theory
Fig. 1 Carbon Decomposition Method. Total Dissolved Inorganic Carbon (DIC) can be decomposed
into preformed (Cpre
) and regenerated (Creg
) components. Due to the slow equilibration time surface
carbon is typically not in equilibrium with the atmosphere. Thus the preformed carbon can be separated
into equilibrium (Ceq
) and disequilibrium (Cdis
) components. Regenerated carbon is separated into
organic (Corg
) and calcium carbonate (Ccaco3
) components. The disequilibrium is further separated into
physical (Cdis,phy
) and biological (Cdis,bio
) components. At high latitudes and in the ocean interior Cdis,phy
is
negative because heat fluxes out faster than carbon fluxes in. On the other hand Cdis,bio
is positive there
because upwelling of regenerated carbon outgasses only incompletely.
Temperature
Fig. 4: Change in zonally-averaged (a) surface temperature, air-sea heat flux and (b,c) Cdis,phy
due to changes in
temperature.
Atlantic Pacific
Method
Use data-constrained model (Muglia et al. 2018) of the LGM and Transport
Matrix Method (TMM, Khatiwala 2007) to quantify carbon components
precisely (we do not use the AOU approximation, which doesn't work).
This model fits LGM δ13
C, δ15
N and radiocarbon data and features a weak
and shallow AMOC and enhanced iron fertilization in the Southern Ocean.
Funded by NSF's Marine Geology and Geophysics Program.
Preindustrial Control (PIC)
Fig. 3 Zonally-averaged distributions of carbon
components. Note the opposite signs of the physical
(d,e,f) and biological (g,h,i) disequilibrium
components.
Perturbation Experiments
Fig. 3: Change in ocean carbon storage and atmospheric CO2
(inset) in response to LGM perturbations to the PIC state.
Motivation
Which processes caused the ocean to store more carbon during the LGM?
Conclusions
• Biological Pump (Corg
) was weaker during LGM
• Disequilibrium was stronger
• Temperature (44-45 ppm) and iron (26-39 ppm) account for 3/4
of the total CO2
change
• Sea Ice and Circulation effects are minor due to opposing effects
on different carbon components
• Cooler temperatures and reduced air-sea heat fluxes reduce the
physical disequilibrium and increase carbon storage by 20 ppm
more than the equilibrium solubility effect alone
References
• Stephens, B. B. & Keeling, R. F. The influence of Antarctic sea ice on glacial-interglacial CO2
variations. Nature 404 (2000).
• Muglia, J., Skinner, L. C. & Schmittner, A. Weak overturning circulation and high Southern Ocean nutrient utilization maximized
glacial ocean carbon. Earth and Planet. Sci. Lett. 496 (2018).
• Kohfeld, K., Quere, C. L., Harrison, S. P. & Anderson, R. F. Role of marine biology in glacial-interglacial CO2 cycles. Science 308
(2005).
• Peterson CD, Lisiecki LE, Stern JV (2014) Deglacial whole-ocean δ13
C change estimated from 480 benthic foraminiferal records.
Paleoceanography 29(6):549–563.
• Skinner L, et al. (2017) Radiocarbon constraints on the glacial ocean circulation and its impact on atmospheric CO2
. Nature
Communications 8.
• Galbraith, E. D., et al. (2012) The acceleration of oceanic denitrification during deglacial warming, Nature Geosc., 6, 579-584,
doi:10.1038/ngeo1832.