Presentation at the General Assembly of the European Geophysical Union.

1. Weak Overturning Circulation and
Increased Iron Fertilization
Maximize Carbon Storage in the
Glacial Ocean
Juan Muglia1, Luke Skinner2 and Andreas Schmittner1
1College of Earth, Ocean and Atmospheric Sciences, Oregon State
University, Oregon, USA
2Godwin Laboratory for Palaeoclimate Research, Department of Earth
Sciences, Univ. of Cambridge, Cambridge, UK
EGU General Assembly 2018, April 12

2. Questions
• What was the Atlantic Meridional Overturning Circulation
(AMOC) during the Last Glacial Maximum (LGM, ~20,000
years ago)?
• How did the AMOC change affect ocean carbon storage?
• How did iron fertilization affect ocean carbon storage?

3. Method
• Use data-constrained model
• Model: University of Victoria (UVic) climate model (3D Ocean, 2D Energy
Moisture Balance Atmosphere, sea ice) + Model of Ocean
Biogeochemistry and Isotopes (MOBI)
• MOBI includes Δ14C, δ13C, δ15N
• Perturb AMOC and Southern Ocean iron fluxes
• Data: Δ14C (Skinner et al., 2017, n = 247), δ13C (Peterson et al. 2014 +
…, n = 434), δ15N (Galbraith, Kienast & NICOPP, 2013 + Francois et al.,
1997 + …, n = 105)

4. Perturb AMOC by reducing southern hemisphere
meridional moisture fluxes => AABW gets saltier => AMOC
gets weaker
PI
LGM_13
LGM_8
LGM_0

5. Perturb AMOC by reducing southern hemisphere
meridional moisture fluxes => AABW gets saltier => AMOC
gets weaker
PI
LGM_13
LGM_8
LGM_0

6. Test Method With Modern/Late Holocene Data
Subsample data
at locations of LGM
sediment cores
GLODAP
Core-Top
Vertical orange lines are
modern AMOC estimates
from RAPID
Maxima in R and minima in RMSE coincide with RAPID estimates.
=> Method passes test suggesting that it can be used to reconstruct AMOC
=> We proceed with LGM data
Correlation
Coefficient
Root-Mean-
Squared Error

7. Atlantic Pacific

8. RMSE normalized by combining data (620 14C-yr) and model (180 14C-yr) uncertainties
Radiocarbon Age
Root-Mean-Squared Error

9. Increased Southern Ocean soluble iron fluxes => increased export production
LGM
LGM_SOFe

10. RMSE = 0.85
RMSE = 0.86RMSE = 1.14
RMSE = 1.12
N=105

11. N=434
Atlantic Pacific

12. δ13C
Root-Mean-Squared Error
SOFe

13. δ15N provides strong
constraints on iron fluxes
δ13C provides strong
constraints on circulation
Ocean carbon storage has maximum
for weak AMOC states
Iron fertilization also increases DIC

14. Atlantic Pacific

15. Conclusions
• Weak shallow AMOC is viable LGM circulation (consistent
with δ13C and 𝚫14C data)
• Increased Souther Ocean soluble iron fluxes (or another
mechanism that removes surface nutrients) is required to
fit δ15N and δ13C data
• Both mechanisms increase deep ocean carbon storage
• Tomorrows talk at 15:45 by Samar Khatiwala (#5513)
quantifies effects on ocean carbon and atmospheric CO2
(spoiler 𝚫CO2 = -77±10 ppm)

16. Atlantic Pacific