MULTI-PROXY STUDIES IN
QUATERNARY PALAEOECOLOGY
– ORIGINS, DEVELOPMENT,
AND POTENTIALS
John Birks and Hilary Birks
University of Bergen &
University College London
INTRODUCTION
WHAT ARE MULTI-PROXY STUDIES?
WHY DO WE NEED MULTI-PROXY
STUDIES?
HISTORICAL BACKGROUND
ADVANTAGES AND DISADVANTAGES
PROBLEMS
FUTURE DEVELOPMENTS
CONCLUDING WORDS
INTRODUCTION
What this talk is not:
1. It is not a review of the methods or research
techniques used in Quaternary multi-proxy
palaeoecological studies.
2. It is not a review of numerical methods that can
be useful in multi-proxy studies.
3. It is not a summary of our paper in Festschrift
Brigitta Ammann published in Vegetation History
and Archaeobotany.
So what is this talk?
1. A personal view of why we think multiproxy studies are useful.
2. Some historical background for our own
interests in multi-proxy studies.
3. A personal view of the advantages and
disadvantages, the problems, and the future
development of multi-proxy studies in
palaeoecology.
4. A tribute to Brigitta Ammann and her great
contributions to multi-proxy studies in
Quaternary palaeoecology.
Inevitably reflects our personal research interests,
biases, and collaborations.
WHAT ARE MULTI-PROXY STUDIES?
A multi-proxy study uses more than one palaeoecological proxy to reconstruct past ecosystems &
environmental conditions and to test hypotheses
about causes of ecological change.
Proxy (temperature)
Proxy (man)
Proxy (animals)
Proxy (plants)
Familiar proxies
• pollen
• plant macrofossils
• diatoms
• chrysophytes
• cladocera
•
•
•
•
•
• sediment magnetics
chironomids
mollusca
ostracods
beetles
trichoptera
•
•
•
•
•
oribatid mites
testate amoebae
phytoliths
charcoal
stomata
• sediment grain-size
• sediment inorganic
• sediment organic
chemistry
chemistry
• sediment pigments and biomarkers
• stable isotopes 14N, 13C, 18O
• radiometric chronologies
• C:N ratios
• laminations
All combinations are possible depending on the
research problem, site type, preservation, and
expertise of the research team
Palaeoecological Developments
1. New proxies, e.g. organic biomarkers, freshwater
alkenones (e.g. TEX86), XRF analyses, near infrared spectroscopy
2. Quantification – modern calibration data-sets,
numerical and statistical tools for data analysis
3. Fine-resolution studies
laminated sediments
and
the
study
of
4. Improved chronologies and slowly improving
age-depth models
5. Increased concern for careful and rigorous
project design and site selection and greater
emphasis on hypothesis testing and on studying
biotic responses to environmental change
Basic Requirements for a Multi-Proxy Study
1. Clear research questions. These are especially
important in multi-proxy studies that inevitably
involve many scientists. Multi-proxy studies are
very time-consuming and are therefore expensive
in time, effort, and resources.
2. A good leader is required, with effective
communication and co-ordination skills, a broad
knowledge, a flexible approach, and an enthusiasm
and determination to synthesise and publish the
results.
3. Careful site selection to provide potential
answers to the original research questions.
4. Careful coring.
5. Reliable chronology.
6. Careful data-base storage of the primary
and secondary data.
7. Clear presentation of the results on depth
and age scales.
8. Numerical techniques to help summarise
the major patterns ('signal') of variation in
different data-sets and to test specific
hypotheses about causes of observed
changes.
WHY DO WE NEED MULTI-PROXY
STUDIES?
Many factors can determine the composition and
abundances of living communities and hence fossil
assemblages.
• environment (climate, soils, moisture, other
abiotic variables, etc.)
• biotic factors (competition, population
dynamics, disease, predation, parasitism, etc.)
• history (biogeography, disturbance, land-use,
etc.)
• interactions between factors
• chance
Interpretation of palaeoecological data
Two major approaches
1. Descriptive – descriptive and narrative approaches.
Hypothesis generation.
2. Causal – causes of observed changes or observed
stability, analytical approach. Hypothesis testing.
Descriptive approaches to interpretation
Emphasis is on reconstruction
1. Past biota – what species were present in the past?
2. Past populations – what were the population sizes
in the past?
3. Past communities – what communities or 'life
assemblages' were present in the past?
4. Past landscapes – what was the past landscape
and how did it vary in space and time?
5. Past environment – what was the environment at
particular times in the past (e.g. climate, lakewater pH, etc.)?
Reconstructions can be:
• qualitative ('indicator species') or
• quantitative (representation factors, multivariate data analysis, transfer functions)
End results are descriptions of the basic data
and narratives or reconstructions that are, in
some way, derived from the basic data.
Interpretation in terms of underlying
causes
What caused the observed changes in our fossil
assemblages? Climate, land-use, biotic interactions,
soil, pathogens, etc?
Interested in causal underlying processes or 'forcing
functions' for observed stratigraphical patterns.
Usually there are two or more competing hypotheses
to explain the observed patterns. Need at least three
'independent' proxies to resolve or test two
competing hypotheses.
Hence the value of multi-proxy studies.
HISTORICAL BACKGROUND
Personal view:
All started with H.E. Wright, Jr. 1966 Stratigraphy of
lake sediment and the precision of the paleoclimatic
record. In: World Climate 8000 to 0 BC. Royal
Meteorological Society.
Discussed multi-proxy studies in Minnesota & South
Dakota (e.g. Kirchner Marsh, Jacobson Lake, Pickerel
Lake), in New England (e.g. Linsley Pond), Europe
(e.g. Esthwaite Water), & Iran (e.g. Lake Zeribar).
Features of these pioneering early studies are:
1. Tested, using the palaeoecological record, ideas
about lake ontogeny and biotic responses over
time to external perturbations & internal processes
2. Studied relatively long time scales (e.g. Holocene)
3. Used selected taxa & proxies
4. No quantification
5. Provided elegant & carefully argued narratives
6. Invariably selective & perhaps only reported what
'fitted' the current paradigm (but not always!)
We went to Minnesota in 1970 to work with Herb
Wright at the Limnological Research Center (LRC),
University of Minnesota.
Hilary took up plant macrofossils, studied modern macrofossil representation in lakes,
and did a multi-proxy study on
the impacts of recent cultural
eutrophication on 3 Minnesotan
lakes.
John did late-glacial pollen
stratigraphy and acquired an
interest in numerical methods
from Ed Cushing and John
Imbrie.
Lake Itasca
We also learnt how to use multi-items in coring with
Herb Wright and how to survive (just!)
In 1971 John went on a multi-proxy expedition to the
Klutlan Glacier, Yukon
Botany, glacial geology, geomorphology, soils, pollen,
limnology, diatoms, cladocera, and philosophy of
science
In 1977 we met Brigitta Ammann at the INQUA
Congress in Birmingham. We were introduced to
her by Dan Livingstone from Duke University.
During the Congress, we introduced Brigitta to
Herb Wright. He was instantly thinking of projects
in Minnesota that could bring Brigitta to the LRC!
In 1985 John went to
the International
Geological Correlation
Programme (IGCP)
158B meeting in
Switzerland led by
Gerhard Lang
There were many highlights, but the greatest was
the day at Lobsigensee on the Swiss Plateau, the
site of THE late-glacial multi-proxy study masterminded and led by Brigitta Ammann
It was the major highpoint.
Studied
• pollen stratigraphy
• algal & bacterial pigments
• plant macrofossils
• cladocera
• beetles & trichoptera
• chironomids
• molluscs
• stable isotopes
• radiocarbon dating
• diatoms & chrysophytes
Some of the Lobsigensee team and other Swiss colleagues 1985
Major international effort –
Switzerland, Germany, Poland, USA, UK, Canada
Many major publications
1985
1999
1983
M.C. Escher
"Sky and Water I"
Our collaboration with the Bern group started in 1985
when we were held up below the Simplon Pass.
Brigitta asked John if numerical methods could be
used to analyse Swiss late-glacial pollen data.
John was more interested in photographing Saxifraga
cotyledon on the cliffs at the time and John fears that
he rather rudely replied to Brigitta "of course" and
then ignored her in preference to Saxifraga cotyledon!
Saxifraga cotyledon
Gerhard Lang and Brigitta Ammann invited John to
Bern to give a course about numerical methods in
Quaternary palaeoecology in 1988. The person who
helped organise it was Andy Lotter, just about to go
to Umeå as a post-doctoral fellow.
From this, John's collaboration with Andy began but
that is another story.
Brigitta Ammann's Lobsigensee study inspired Hilary
to initiate a late-glacial multi-proxy study at Kråkenes
in western Norway
Plant macrofossils, Salix herbacea stomatal density &
CO2 concentrations, pollen, diatoms, chironomids,
trichoptera, oribatid mites, cladocera, beetles, radiocarbon dating, sediment magnetics & lithology, etc.
Birks, Hilary H. & Wright, Jr., H.E. 2000 Journal of
Paleolimnology 23(1) 114 pp.
Brigitta Ammann has always had a strong interest
in biotic responses to environmental change
This led to the so-called 'Rapid Warming Project'
(Biotic responses to rapid climatic change around the
Younger Dryas) where the oxygen isotope record was
used as a record of climate 'forcing' against which the
responses of various biotic proxies could be assessed.
2000
Gerzensee
Birks & Ammann 2000
Birks & Ammann 2000
ADVANTAGES & DISADVANTAGES
Advantages
1. More proxies provide more 'degrees of freedom' or
independent variables in data interpretation and
hypothesis testing.
2. More proxies, more detail. Can lead to a more
holistic view of the past ecosystem.
3. Least publishable unit (LPU) principle. Multi-proxy
studies can, if you wish, provide many LPUs!
4. Careful and critical synthesis avoids LPU and library
and information overload.
5. The synthesis of multi-proxy results in successful
studies exceeds the sum of the component parts.
Major challenge to provide a rigorous and critical
synthesis.
Disadvantages
1. Very time consuming. Many co-workers are
essential.
2. Too much data. Major challenges in data storage,
handling, display, analysis, interpretation, and
synthesis.
3. Different proxies reflect environmental factors at
a range of spatial scales and consequently show
different strengths, weaknesses, and sensitivities in
different ecological contexts.
4. Interpretation is a major challenge. Need to avoid
the 'reinforcement syndrome' or to adopt a
'confirmatory' approach where data interpretation is
forced to fit into a particular favoured paradigm or
stratigraphical sequence of environmental changes.
"It is infinitely more difficult, if not impossible, to
prove that a given magnetic field behaviour has
not taken place, than to 'show' it has occurred.
Superimposed on this is an important human
element: it is far more reasonable to generate
the energy and the belief (? faith) required for
publication of data confirming a discovery than
to publish more negative data of a pedestrian
nature. Thus the initial discovery is reinforced."
Watkins 1971
Comments on Earth Sciences: Geophysics 2: 3643
An invidious effect of the reinforcement syndrome is
so-called 'publication bias' where only confirmatory
results are published, especially in so-called 'highimpact journals' and non-confirmatory results are
published in other journals or, worst of all, are
never published.
Watkins (1971) proposed "it would be instructive to
compile examples of other applications of this
'reinforcement syndrome' to see if there are any
natural laws governing the blossoming or survival of
possibly spurious, or at least partially correct,
observations or ideas."
Cycles or periodicities in the Holocene and the
global extent of rapid 'events' in the early Holocene
might be an interesting topic to develop Watkins'
ideas.
5. Psychological beliefs that one type of proxy is, in
some way, more reliable or more informative than
another proxy record, lead to greater weight being
given to some proxies than to others. Challenge to
avoid the natural tendency to believe that one
favoured proxy is, in some way, more reliable or
more informative.
6. Major challenge to synthesise and write up a
multi-proxy study.
7. Major challenge to publish large-ish papers in
these times of LPUs.
8. To some, a disadvantage is no LPUs!
PROBLEMS
1. Very demanding on
material. Ideal to have one
continuous large-diameter
(10-11 cm) core with 2-6
m core tube length from
which all the samples are
done, ideally all at the
same levels.
Nesje (1992) 'hammer'
corer used in up to 80 m
water.
2. Coping with vast amounts of primary data. Need for
a relational data-base – PALICLAS (Palaeoenvironmental Analysis of Italian Crater Lakes and Adriatic
Sediments).
Juggins
1996
PALICLAS data
retrieval and
manipulation
Juggins
1996
3. Data display of different proxies analysed at
different resolutions.
Albano
Core 94-1E
Oldfield 1996
Albano Core 94-1E and Adriatic Core 92-43
Oldfield 1996
4. In data analysis, principle of the 'minimal adequate
model' or Ockham's Razor is essential in model
selection (principle of parsimony). Select the least
complex model with the fewest parameters and
hence with the largest number of degrees of freedom
and maximum robustness. Not always popular with
palaeoecologists who feel numerical results should be
complex and precise!
5. Interpretation 'let the data speak for themselves'.
Not easy with many proxies that no one person can
interpret. Essential therefore to have an effective
team of collaborators with a good, tolerant, wideranging, and efficient leader.
6. Multi-proxy studies are deceptively simple and highly
seductive. They are a huge amount of work – never
simple, full of surprises and shocks. Nature is rarely
neat, tidy, or consistent.
7. Important, if the potential of multi-proxy studies
is to be maximised, that the synthesis presents
points of similarity and points of difference. Vastly
more interesting to discuss apparent contradictions as these raise important questions about
the proxies.
Palaeoceanographers now recognise that different
proxies (diatoms, planktonic foraminifera, benthic
foraminifera, chemical ratios, stable isotopes,
sediment grain-size) reflect different aspects of
the ocean system in terms of stratification,
currents, and rates of over turn.
A similar approach could profitably be adopted in
the interpretation of terrestrial multi-proxy data.
8.
In Holocene palaeoclimatology, we may be at
the limits of resolution and predictive abilities
of our reconstruction methods.
9.
Multi-proxy studies are not really safe science.
10. It is a challenge to improve on the classical
pioneer studies or at least to argue as logically
and as rigorously as they did. What are the
research questions we are trying to answer, the
hypotheses we are trying to test, and the
limitations of our methods, tools and,
approaches?
FUTURE DEVELOPMENTS
Data-splitting as a means of testing palaeoecological
hypotheses is becoming increasingly possible as
studies are becoming more and more multi-proxy.
Use one proxy (e.g. chironomids) to reconstruct mean
July air temperature and then use this reconstruction
to interpret another independent stratigraphical proxy
(e.g. pollen).
Two recent examples to illustrate this 'data-splitting'
approach, both from palaeolimnology and looking at
lake development in relation to catchment changes.
Assessing potential 'drivers' on aquatic ecosystems.
What determines changes in lake organisms and
lake sediments?
1. External climate forcing functions
2. Catchment forcing functions
3. Lake as an isolated system that evolves through
time with its own internal dynamics
Birks et al.
2000
(a) Sägistalsee,
Bernese
Oberland,
Swiss Alps
Andy Lotter
A.F. Lotter et al. 2003
J. Paleolimnology 30: 253-342
Age-depth model
Lotter & Birks 2003
Wick et al. 2003
Wick et al. 2003
Heiri & Lotter 2003
Ideal study:
1. Critical ecological situation at tree-line today; sensitive to
change
2. One core. Many proxies (pollen, macros, chironomids,
cladocera, grain size, sediment magnetics, sediment
geochemistry)
3. Well dated; 18 AMS
14C
dates on terrestrial plant material
4. Well co-ordinated by A.F. Lotter
5. High quality data:
No. of
samples
No. of
taxa/variables
Pollen
212
203
Plant macros
372
53
Chironomids
82
30
Cladocera
112
7
Geochemistry
176
14
Grain-size
294
6
Magnetics
504
5
Data-set
6. Consistent numerical methodology on all proxies
7. Numerical methods used to test hypotheses about
the influence of climate and catchment processes
on the aquatic ecosystem in the perspective of the
Holocene time-scale. (Partial redundancy analysis
with restricted Monte Carlo permutation tests)
Of the catchment changes, the main ones appear to
be the spread of Picea abies at about 6300 cal BP and
Bronze Age and subsequent forest clearances and
conversion to grazing pastures.
8. Split proxy data into one predictor variable (plant
macrofossils as a reflection of catchment vegetation)
and several response variables (pollen, cladocera,
chironomids, sediment grain-size, geochemistry,
magnetics)
Predictor variables:
Lotter &
Birks
2003
Hypotheses tested:
1. Climate has had a significant control on lake ecosystem changes
2. Catchment vegetation has played significant role on lake changes
"Responses"
(proxies)
Terrestrial
Scale
Climate a significant predictor?
Catchment vegetation
a significant predictor?
Pollen
Catchment &
regional
Y
Y
Lake biotic
Chironomids
Lake
N
Cladocera
Lake
N
Lake abiotic
Grain size
Lake
Magnetics
Lake
Geochemistry
Lake
*
* Tested against insolation, central
European cold phases, & Atlantic IRD record
Y
Y
Y
Y
(Y)
#
# Veg phases: Betula-Pinus cembra; Alnus-Pinus cembra; Picea abies
~ 6300 cal BP; Pasture phases from Bronze Age to present
(b) Dalland Sø, Funen, Denmark
Dalland Sø, a small (15 ha), shallow (2.6 m) lowland
eutrophic lake on the island of Funen, Denmark.
Catchment (153 ha) today
agriculture
77 ha
built-up areas
41 ha
woodland
32 ha
wetlands
3 ha
Nutrient rich – total P 65-120 mg l-1
Emily Bradshaw
Bradshaw et al. 2005 The Holocene 15: 1152-1162
Map of
Dalland Sø
Multi-proxy study to assess role of potential external 'drivers'
or forcing functions on changes in the lake ecosystem over
the last 7000 yrs.
Data:
No. of
samples
560
560
Transformation
560
Log (x + 1)
280
Log (x + 1)
90
118
118
None
None
None
Biogenic silica
84
Not used
Pediastrum %
90
None
Zooplankton
31
Not used
Sediment loss-on-ignition %
Sediment dry mass
accumulation rate
Sediment minerogenic
matter accumulation rate
Plant macrofossil
concentrations
Pollen %
Diatoms %
Diatom inferred total P
None
Log (x + 1)
Terrestrial landscape or
catchment development
Bradshaw et
al. 2005
Aquatic ecosystem development
Bradshaw et al. 2005
Detrended Correspondence Analysis of pollen and
diatom data separately to summarise major
underlying trends in both data sets
Pollen – high scores for
trees, low scores for
light-demanding
herbs and crops
Bradshaw et al. 2005
Diatom - high scores mainly
planktonic and large
benthic types,
low scores for
Fragilaria spp. and
eutrophic spp. (e.g.
Cyclostephanos
dubius)
Major contrast between pollen and diatom
samples before and after Late Bronze Age
forest clearances on DCA axis 1
'Lake'
Prior to clearance,
lake experienced
few impacts.
After the
clearance, lake
heavily impacted.
'Catchment'
Bradshaw et al. 2005
Canonical correspondence analysis
Response variables
- diatom taxa
Predictor variables
- pollen taxa, LOI, dry mass
and minerogenic accumulation rates, plant macrofossils, Pediastrum
Covariable - age
69 matching samples
Partial CCA with age partialled out as a covariable.
Makes interpretation of effects of predictors easier
by removing temporal trends and temporal autocorrelation
Partial CCA all variables- 18.4% of variation in
diatom data explained by Poaceae pollen, Cannabistype pollen, and Daphnia ephippia.
As different external factors may be important at different times,
divided data into 50 overlapping data sets – sample 1-20, 2-21,
3-22, etc.
Bradshaw et al. 2005
CCA of 50 subsets from bottom to top and % variance explained
1. 4520-1840 BC Poaceae is sole predictor variable
(20-22% of diatom variance)
2. 3760-1310 BC LOI and Populus pollen (16-33%)
3. 3050-600 BC Betula, Ulmus, Populus, Fagus,
Plantago, etc. (17-40%)
i.e. in these early periods, diatom change
influenced to some degree by external catchment
processes and terrestrial vegetation change.
4. 2570 BC – 1260 AD Erosion indicators
(charcoal, dry mass accumulation), retting
indicator Linum capsules, Daphnia ephippia,
Secale and Hordeum pollen (11-52%)
i.e. changing water depth and catchment
agricultural factors
5. 160 BC – 1900 AD Hordeum, Fagus,
Cannabis pollen, Pediastrum boryanum,
Nymphaea seeds (22-47%)
i.e. nutrient enrichment as a result of retting
hemp, also changes in water depth and water
clarity
Bradshaw
et al. 2005
Strong link between inferred catchment change and within-lake
development. Timing and magnitude are not always perfectly
matched, e.g. transition to Medieval Period
This type of 'data-splitting'
provides a means of testing
hypotheses about causal
factors and of implementing
Ed Deevey's (1969) brilliant
idea of 'coaxing history to
conduct experiments'.
'Using the geological record
as an ecological laboratory'
Flessa & Jackson, NRC
2005
OUR CONCLUDING TRIBUTE TO
BRIGITTA AMMANN
The basic idea of exploring and interpreting the
palaeoecological record as a record of past ecological
dynamics and as a means of understanding the biotic
effects of past and future environmental change has,
we believe, been the major vision behind so much of
Brigitta Ammann's research and of her research
group in Bern.
Ecological palaeoecology par excellence
Brigitta Ammann pioneered the first fully
integrated detailed multi-proxy approach at
Lobsigensee. She set new and very high
analytical and intellectual standards in
Quaternary palaeoecology. She changed the
concept of a 'multi-proxy' study and the
meaning of the word 'detail' in Quaternary
palaeoecology by her Lobsigensee study.
Fantastic developments and contributions in her
own scientific career
2000
1989
1975
Brigitta Ammann has greatly influenced many
people by her research, by her approach to
science and to life, by her remarkable
generosity, and by her helpfulness, loyalty,
support, and encouragement.
Outstanding scientists create subconsciously
what Diane Crane (1972) calls an "invisible
college" around them.
What does the "invisible college" of Brigitta
Ammann look like?
Here is a very incomplete representation of
Brigitta's "invisible college".
Max Welten
Gerhard
Lang
Dan
Livingstone
Knut
Fægri
Herb Wright,
Jr
Hilary and John Birks
MarieJosé
Gaillard
Pim van der
Knaap &
Jacqueline
van Leeuwen
Willy
Tinner
Andy
Lotter
Lucia
Wick
and her very many colleagues, former students,
and friends in Switzerland, Germany, Austria,
Bulgaria, Italy, Czech Republic, Poland, Russia,
and elsewhere that she has helped & influenced.
Oliver
Heiri
Frank
Oldfield
WARMEST THANKS
• Willy Tinner and Pim van der Knaap
• Herb Wright and Andy Lotter
• Saxifraga cotyledon
• Rick Battarbee, John Smol, and John Anderson
• Steve Juggins and Richard Telford
• Cathy Jenks
and, of course,