Inferred δ 13 C and δ 18 O distributions in the modern and Last Glacial Maximum deep Atlantic Holly Dail ECCO Meeting November 1, blah blah blah……

Last Glacial Maximum Artist’s rendition Imbrie & Imbrie 1979 Last Glacial Maximum climate –Generally colder –Sea level lower by ~120 m –CO 2 ~190 ppm Standard approaches + limitations –Compilations of proxy records –Model intercomparison projects After Waelbroeck et al. (2002) Sea level -120 m Present 450,000 years ago LGM

Water mass distributions from benthic isotopes Ocean circulation model + model adjoint δ 18 O 171 records δ 13 C 171 records Alkenones 55 records Forams 181records Mg/Ca 32 records Deep ocean isotope data Near sea surface temperatures Dinocysts 53 records Seek estimate of Atlantic Ocean state at the LGM that is consistent with  available data  ocean general circulation model

Key questions in this talk Is it possible to find a model state close to the available LGM data? Are LGM δ 13 C proxies consistent with an AMOC shallower than today’s?

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

mean State estimate fits the SST proxies better than PMIP2 coupled model simulations do PMIP2: Braconnot et al., 2007 State estimate

Incorporating a simple model for isotopes [draws on Marchal and Curry, JPO 2008] δ 13 C –δ 13 C calcite = δ 13 C DIC –δ 13 C DIC modeled as a passive tracer + remineralization –Uncertainty = 0.2‰ –Data excluded above 1000 m –δ 13 C DIC model tested against modern GEOSECS δ 13 C δ 18 O – not discussed here New controls for initial and boundary conditions on tracers

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

Modern: GEOSECS δ 13 C DIC distribution at 2730 m (‰) 10 year state estimate80 year state estimate

1000 m1900 m2500 m 3000 m3900 m4600 m LGM: 93% of modeled δ 13 C values are within 2σ of the proxy data (‰)

Benthic δ 13 C 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)

Curry & Oppo (2005) State estimate δ 13 C at 28°W

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

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 δ 13 C proxies consistent with an AMOC shallower than today’s? –Yes