what do glacial moraine chronologies really tell us about climate? martin p. kirkbride geography...
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What do glacial moraine chronologies really tell us What do glacial moraine chronologies really tell us about climate?about climate?
Martin P. Kirkbride Martin P. Kirkbride
GeographyGeography School of the EnvironmentSchool of the Environment
University of DundeeUniversity of DundeeDundee Dundee DD1 4HNDD1 4HN
Scotland UK.Scotland UK.
European Geophysical Union General Assembly 2011European Geophysical Union General Assembly 2011
Session GM9.1/9.2 Session GM9.1/9.2
1. Introduction: moraine dating, and the principles of correlation1. Introduction: moraine dating, and the principles of correlation
Principles of correlation
physical similarity (classic stratigraphic correlation)
texture, structure, provenance, fossil assemblage
spatial continuity
tephra layers, Heinrich layers etc.
statistical matching
measures of central tendency and dispersion
Moraine are physically similar and lack spatial continuity, so we depend on statistical properties of proxy age samples
Five steps to linking climate records and moraines
Climate variations (various amplitudes, frequencies, and phenomena)
Glacier responses(variable reaction and response times re. glacier size and form)
Moraine deposition(variable debris supply)
Erosion censoring
Moraine dating
Correlation
(local, regional. interhemispheric scales, “lumping/splitting” decisions)
Climate variations (various amplitudes, frequencies, and phenomena)
Glacier responses(variable reaction and response times re. glacier size and form)
Moraine deposition(variable debris supply)
Erosion censoring
Moraine dating
Correlation
(local, regional. interhemispheric scales, “lumping/splitting” decisions)
Five steps to linking climate records and moraines
Timescales of glacier reaction and response
Cirque glacier = annual variationPotentially 6 moraines
Small valley glacier = decadal variationPotentially 3 moraines
Compound valley glacier = century scale variation0 moraines
Different glaciers respond to different frequencies of climatic fluctuation
Larger glaciers filter and smooth complex high-frequency fluctuations
(Haeberli and Hoelzle)
2. Relations between spatial correlation and temporal resolution2. Relations between spatial correlation and temporal resolution
How do different ranges of uncertainty in dating affect out ability to correlate distant moraine sequences?
Mt Cook, NZ: Schaefer et al (2009) Science
Uncertainty is c. 20% of individual 10Be age (ie. ± 10%)
Chronological resolution (uncertainty) and correlation
Our ability to correlate depends on the range of uncertainty (“error bars”) of the dated moraines.
“Error” Correlation Interpretation
± 500 M1 = M2 M3 1 glacier advance “event”
± 250 M1 = M2 M3 2 glacier advance “events”
± 75 M1 M2 M3 3 glacier advance “events”
“Lumping” is the tendency to correlate across wide areas because the temporal resolution of individual chronologies is too coarse to distinguish separate events
“Splitting” is the tendency to distinguish between closely-spaced events which are dated to high resolution.
The “Lumping/Splitting” Rule:
The spatial resolution of correlation is in inverse relation to the temporal resolution of the component chronologies.
The “Lumping/Splitting” Rule:
The spatial resolution of correlation is in inverse relation to the temporal resolution of the component chronologies.
“Lumped” chronologies may be appropriate for interhemispheric correlation where millenial-scale glacial fluctuations (eg LIA) are due to global climate changes.
“Split” chronologies may be appropriate for local to regional correlation where decadal scale glacial fluctuations (eg LIA maximum) are due to regional atmospheric circulation changes.
Response scale, chronological resolution, and correlation error
Type I error
False rejection of null hypothesis
False correlation of different climatic events.
Null hypothesis:
“there is no temporal or climatic association between moraines in different places”
Type II error
False acceptance of null hypothesis
False differentiation of the same climatic event.
Kaplan et al. (2010) NatureKaplan et al (2010) Nature 467
Response scale, chronological resolution, and correlation error
Ages spread over 2,500 years produce a statistical uncertainty of only 100 years (weighted mean).
Which is the most realistic estimate of uncertainty?
This affects our ability to correlate with confidence and risks errors of false differentiation.
Risk of a Type 2 error = false acceptance of null hypothesis: more likely if chronologies are unrealistically resolved.
Kirkbride & Brazier (1998) Quaternary Proceedings.
Age structure of moraine sequences
Age-difference curves: Graphs of moraine age against age-difference from next youngest moraine.
An increase in age difference with age is common – but is it due to:
Shift in recorded climate signal?
Loss of evidence?
“Correlation shift”?
Ag
e d
iffe
ren
ce
Age
3. Completeness of the moraine record: erosion censoring3. Completeness of the moraine record: erosion censoring
How do we know how complete our moraine records are?
Kirkbride & Brazier (1998 ) Quaternary Proceedings 6
Climate varies with frequencies of 2000, 200 and 20 years.
Age-difference line shows both the climate frequency recorded by the moraine field, and the degree of erosion censoring.
Erosion censoring of moraine assemblages
Mt Cook LIA
y = 0.1051x + 107.02R2 = 0.8143
0
50
100
150
200
0 100 200 300 400 500 600
Moraine age (Cal yr BP)
Ag
e d
iffe
ren
ce (
yr)
Blabreen LIA
y = 0.069x + 13.494R2 = 0.103
0
50
100
150
200
0 50 100 150 200 250
Moraine age (Cal yr BP)
Ag
e d
iffe
ren
ce (
yr)
Erosion censoring of moraine assemblagesLittle Ice Age chronologies
Mt Cook: Schaefer et al (2009)
Blåbreen: Bickerton & Matthews (1992)
If the age differences between moraines increase with age, older deposits may have been censored by erosion or aggradation.
Mt Cook: possible erosion censoring
Blåbreen: no erosion censoring
Smorstabbtinden 14C chronology (Matthews & Dresser 2008)
y = 401.68e0.0003x
R2 = 0.8528
0
500
1000
1500
2000
2500
3000
3500
4000
0 1000 2000 3000 4000 5000 6000 7000 8000
Moraine age (Cal yr BP)
Ag
e d
iffe
ren
ce (
yr)
Mount Cook 10Be chronology (Schaefer et al. 2009)
y = 7E-05x2 + 0.0092x + 129.08R2 = 0.9707
0
500
1000
1500
2000
2500
3000
3500
0 1000 2000 3000 4000 5000 6000 7000
Moraine age (Cal yr BP)
Ag
e d
iffe
ren
ce (
yr)
.
Erosion censoring of moraine assemblagesHolocene chronologies
Dec
reas
ing
qual
ity o
f pro
xy e
vide
nce
Iceland
(Kirkbride & Dugmore 2008 Quatern. Res.)
4. Conclusions and recommendations.4. Conclusions and recommendations.
1. We require a theoretical and practical basis for correlation. We have no systematic criteria at the moment.
2. Correlation decisions need to be informed by chronological resolution, timescale of glacier response, and presumed frequency of climatic variability.
3. We need better integration of modelling of glacier sensitivity into chronological investigations.
4. We need some way of assessing the completeness of moraine records as part of correlation attempts.
Thank you