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Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea- surface temperature reconstructions Richard J. Telford Camille Li Michal Kucera MARUM & Fachbereich Geowissenschaften Universität Bremen [email protected] Geophysical Institute, University of Bergen & Bjerknes Centre for Climate Research [email protected] Department of Biology, University of Bergen & Bjerknes Centre for Climate Research [email protected]

Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

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Page 1: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions

Richard J. Telford

Camille Li

Michal Kucera MARUM & Fachbereich Geowissenschaften Universität Bremen [email protected]

Geophysical Institute, University of Bergen & Bjerknes Centre for Climate Research [email protected]

Department of Biology, University of Bergen & Bjerknes Centre for Climate Research

[email protected]

Page 2: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Motivation

• LGM tropics anomalies are a promising target for estimating climate sensitivity (Hargreaves et al 2012; GRL)

• Reconstructions of tropical temperatures are inconsistent

• MARGO estimates tropical cooling of 1-3°C

• Some other proxies estimate 5-6°C

• Cold LGM tropics would imply high climate sensitivity

• LGM SST reconstructions are dominated by estimates based on planktonic foram assemblages

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MARGO tropical LGM SST anomalies

Mean tropical SST anomaly: -1.5 1.2°C

Mean tropical Atlantic SST anomaly: -2.31.5°C

Several proxies ( Mg/Ca; alkenones; foram assemblages)

Dominated by foram assemblages-based estimates

MARGO 2009 Nat Geosci

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Other reconstructions of tropical LGM cooling

• Coral Sr/Ca ratios: 5-6°C ?LGM Sr/Cawater? • Clumped isotopes on planktonic forams & coccoliths: 4-5°C • Tropical snowline depression ?LGM lapse rates?

Marshall & McCulloch 2002 Geochim Cosmochim Acta Mark et al 2005 Quat Int Tripati et al 2014 Nat Geosci

Page 5: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

CMIP5 Model data comparison

Schmidt et al 2013; COP

MARGO estimates of tropical ocean cooling are smaller than most CMIP5 models

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Tropical Atlantic CMIP5 LGM-PI 10m anomalies

Lack east-west gradient in anomalies

Page 7: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Sum

mer

Se

a su

rfac

e te

mp

erat

ure

°C

Imbrie & Kipp 1971

Calibration set

1) Find modern relationship

between foram assemblages

and SST in a calibration set

2) Use this relationship to infer

past SST from fossil forams

• Usually calibrated against

10 m SST

Reconstructing SST from planktonic foraminifera

•Is this depth appropriate?

•Are the uncertainties fully

accounted?

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Depth distribution of planktonic foraminifera Western North Atlantic

Fairbanks et al. (1980)

Page 9: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Performance of transfer functions at different depths

r2 of predicted vs. measured SST for different depths and seasons

Performance of MARGO north Atlantic training set calibrated against different depths and seasons using Modern Analogue Technique with 5 analogues

• Summer SST performance is

highest 30-50 m depth rather than at 10 m

Page 10: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Reconstructions from different depths

Tropical cores spanning the deglaciation with reasonable resolution Combined MARGO North & South Atlantic training set

Page 11: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Reconstructions

Calibration depth matters Alternative (not joint) reconstructions

Page 12: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Which (if any) reconstruction to trust?

• Make a reconstruction with the transfer function calibrated to 0 m SST

• Use a constrained ordination to find the proportion of variance in the fossil data explained by the reconstruction

• Repeat at 10 m – 300 m depth

• Depth with the reconstruction that explains the most variance may be the most relevant

Telford et al, 2013; COP

Page 13: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Which reconstruction to use

Reconstructions and proportion of variance in the fossil data explained by the different reconstructions

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Depth that explains maximum variance

Not 10 m Hints of geographic patterns

Page 15: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

MARGO reanalysis

MARGO tropical LGM samples 30°S – 30°N Combined MARGO North & South Atlantic training set

Page 16: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Anomalies calibrated to 10 m

East-west gradient in anomalies

Page 17: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Anomalies calibrated to 50 m

East-west anomaly pattern reversed

Page 18: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Difference between 10 and 50 m anomalies

10 m reconstruction warmer anomalies than 50 m over most of the ocean except African coast Mean difference for sites showing warmer 10m anomalies ~0.5°C

Page 19: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Reconstruction uncertainties

• MARGO foram-SST uncertainty estimate ~1°C

• Almost certainly underestimated

– Uncertainty due to uncertain depth attribution (~0.5°C)

– Uncertainty due to spatial autocorrelation in the calibration set (~0.5°C)

– Uncertainty due to non-analogue assemblages (~???)

• Total uncertainty probably 50-100% higher

Page 20: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Non-analogue foram assemblages

Taxonomic distances from each observation to its nearest neighbour in the calibration set Tropical LGM assemblages have weak analogues in modern calibration set Uncertainty of reconstructions will be higher than advertised

Page 21: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Non-analogue ocean thermal structure

Euclidean distance

between LGM July

temperatures over the top

300m of the water column

and the nearest analogues

in the PI ocean for four

CMIP5 models: a) GISS, b)

MIROC, c) IPSL, d) MPI.

The modelled tropical LGM

oceans have no good

analogs in the PI runs with

respect to thermal structure

of the upper ocean.

Telford et al, 2013; COP

Page 22: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Revised LGM SST schematic

Schmidt et al 2013; COP

Assuming remainder of tropics show similar patterns to Atlantic New estimate of LGM tropics is colder, but more uncertain More models compatible with the data Difficult to get anomalies as large as the corals

Page 23: Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic foraminifera assemblage-based sea-surface temperature reconstructions Richard J. Telford

Conclusions

• 10 m SST is not the ideal calibration target for tropical Atlantic planktonic foraminifera SST reconstructions

• Ideal calibration target varies spatially

• Typically 30 or 50 m depth

• Calibrating against 50 m depth SST gives colder reconstructions over most of the tropical Atlantic

– Exception is African margin

• Should view reconstructions from different depths as alternative reconstructions, probably not all valid

• LGM Tropical Atlantic was likely colder than predicted by MARGO, and reconstructions are more uncertain