Re-evaluating tropical LGM planktonic foraminifera ... · Re-evaluating tropical LGM planktonic...
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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
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
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
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
CMIP5 Model data comparison
Schmidt et al 2013; COP
MARGO estimates of tropical ocean cooling are smaller than most CMIP5 models
Tropical Atlantic CMIP5 LGM-PI 10m anomalies
Lack east-west gradient in anomalies
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?
Depth distribution of planktonic foraminifera Western North Atlantic
Fairbanks et al. (1980)
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
Reconstructions from different depths
Tropical cores spanning the deglaciation with reasonable resolution Combined MARGO North & South Atlantic training set
Reconstructions
Calibration depth matters Alternative (not joint) reconstructions
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
Which reconstruction to use
Reconstructions and proportion of variance in the fossil data explained by the different reconstructions
Depth that explains maximum variance
Not 10 m Hints of geographic patterns
MARGO reanalysis
MARGO tropical LGM samples 30°S – 30°N Combined MARGO North & South Atlantic training set
Anomalies calibrated to 10 m
East-west gradient in anomalies
Anomalies calibrated to 50 m
East-west anomaly pattern reversed
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
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
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
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
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
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