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Inferred δ13C and δ18O distributions in the modern and Last Glacial Maximum deep Atlantic
Holly Dail
ECCO Meeting
November 1, 2012.
blah blah blah……
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Last Glacial Maximum
Artist’s renditionImbrie & Imbrie 1979
Last Glacial Maximum climate– Generally colder– Sea level lower by ~120 m– CO2 ~190 ppm
Standard approaches + limitations– Compilations of proxy records– Model intercomparison
projects
After Waelbroeck et al. (2002)
Sea level
450,000years ago
LGM
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Ocean circulation model+ model adjoint
δ18O171 records
δ13C171 records
Alkenones55 records
Forams181records
Mg/Ca32 records
Deep ocean isotope dataNear sea surfacetemperatures
Dinocysts53 records
Seek estimate of Atlantic Ocean state at the LGM that is consistent with
available data ocean general circulation model
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Key questions in this talk
Is it possible to find a model state close to the available LGM data?
Are LGM δ13C proxies consistent with an AMOC shallower than today’s?
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75°N
35°S
100°W 20°E
LGM bathymetry based on ICE-5G (Peltier, 2004)
MIT GCM + adjoint
Model– MIT GCM ocean / sea ice
model– Prescribed atmosphere– 1° degree, 50 vertical levels– Open southern boundary
Cost function– Misfit to MARGO SSTs
• uncertainties: as published by MARGO NGS’09
– Misfit to deep ocean isotope data
• uncertainties: 0.2 ‰
– Penalties on controls
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mean
State estimate fits the SST proxies better than PMIP2 coupled model simulations do
PMIP2: Braconnot et al., 2007
Sta
te e
stim
ate
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Incorporating a simple model for isotopes[draws on Marchal and Curry, JPO 2008]
δ13C– δ13Ccalcite = 0.13 + 0.90 δ13CDIC
– δ13CDIC modeled as a passive tracer + remineralization
– Uncertainty = 0.2‰
– Data excluded above 1000 m
– δ13CDIC model tested against modern GEOSECS δ13C
δ18O – not discussed here
New controls for initial and boundary conditions on tracers
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A gap in available modeling methods
Paleoclimate simulations w/ coupled models (e.g. PMIP): – run to equilbrium, but model state may be far from
data
Standard ocean state estimation (e.g. ECCO): – close to data, but deep ocean not in equilibrium
Our solution: long-running state estimates– goal: build connection between deep ocean and model
forcing– results: reach maximum 80 years -- still not long
enough
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Modern: GEOSECS δ13CDIC distribution at 2730 m
(‰)
10 year state estimate 80 year state estimate
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1000 m 1900 m 2500 m
3000 m 3900 m 4600 m
LGM: 93% of modeled δ13C values are within 2σ of the proxy data
(‰)
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Benthic δ13C records: shifts in water mass boundaries?
Observation:– weak gradients with
depth in modern ocean– strong gradients with
depth at LGM
Inference:– deep Atlantic dominated
by AABW?– changes in end-member
concentrations?Curry & Oppo (2005)
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Curry & Oppo (2005)
State estimate δ13C at 28°W
State estimate δ13C at 28°W
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Modern AMOC streamfunction (Sv)(OCCA, Forget JPO 2010)
LGM AMOC streamfunction (Sv)(Inferred with state estimation)
• Results are preliminary: - model partially equilibrated- tendency towards weak MOC in modern tests- other AMOC arrangements have not been excluded
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Key questions
Is it possible to find a model state close to the available LGM data?– Unconstrained coupled models: not yet– With state estimation: yes
Are LGM δ13C proxies consistent with an AMOC shallower than today’s?– Yes