MULTI-PROXY STUDIES IN QUATERNARY PALAEOECOLOGY ORIGINS, DEVELOPMENT, AND POTENTIALSJohn Birks and Hilary BirksUniversity of Bergen & University College London

INTRODUCTIONWHAT ARE MULTI-PROXY STUDIES?WHY DO WE NEED MULTI-PROXY STUDIES?HISTORICAL BACKGROUNDADVANTAGES AND DISADVANTAGESPROBLEMSFUTURE DEVELOPMENTSCONCLUDING WORDS

INTRODUCTIONWhat this talk is not:It is not a review of the methods or research techniques used in Quaternary multi-proxy palaeoecological studies.It is not a review of numerical methods that can be useful in multi-proxy studies.It is not a summary of our paper in Festschrift Brigitta Ammann published in Vegetation History and Archaeobotany.

So what is this talk?A personal view of why we think multi-proxy studies are useful.Some historical background for our own interests in multi-proxy studies.A personal view of the advantages and disadvantages, the problems, and the future development of multi-proxy studies in palaeoecology.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 palaeo-ecological proxy to reconstruct past ecosystems & environmental conditions and to test hypotheses about causes of ecological change.

Familiar proxiesAll combinations are possible depending on the research problem, site type, preservation, and expertise of the research team

pollen chironomids oribatid mites plant macrofossils mollusca testate amoebae diatoms ostracods phytoliths chrysophytes beetles charcoal cladocera trichoptera stomata

sediment magnetics sediment grain-sizesediment inorganic chemistrysediment organic chemistry sediment pigments and biomarkers

stable isotopes 14N, 13C, 18O C:N ratios radiometric chronologies laminations

Palaeoecological Developments1.New proxies, e.g. organic biomarkers, freshwater alkenones (e.g. TEX86), XRF analyses, near infra-red spectroscopy2.Quantification modern calibration data-sets, numerical and statistical tools for data analysis3.Fine-resolution studies and the study of laminated sediments4.Improved chronologies and slowly improving age-depth models5.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 Study1.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.

Careful coring.Reliable chronology.Careful data-base storage of the primary and secondary data.Clear presentation of the results on depth and age scales.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 factorschance

Interpretation of palaeoecological dataDescriptive descriptive and narrative approaches. Hypothesis generation.Causal causes of observed changes or observed stability, analytical approach. Hypothesis testing.Two major approaches

Descriptive approaches to interpretationEmphasis is on reconstructionPast biota what species were present in the past?Past populations what were the population sizes in the past?Past communities what communities or 'life assemblages' were present in the past?Past landscapes what was the past landscape and how did it vary in space and time?Past environment what was the environment at particular times in the past (e.g. climate, lake-water pH, etc.)?

Reconstructions can be:qualitative ('indicator species') orquantitative (representation factors, multi-variate 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 causesWhat 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 BACKGROUNDPersonal 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:Tested, using the palaeoecological record, ideas about lake ontogeny and biotic responses over time to external perturbations & internal processesStudied relatively long time scales (e.g. Holocene)Used selected taxa & proxiesNo quantificationProvided elegant & carefully argued narrativesInvariably 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 macro-fossils, studied modern macro-fossil 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, YukonBotany, 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 master-minded and led by Brigitta AmmannIt was the major highpoint.

Studied pollen stratigraphy algal & bacterial pigments plant macrofossils cladocera beetles & trichoptera chironomids molluscs stable isotopes radiocarbon dating diatoms & chrysophytes

Major international effort Switzerland, Germany, Poland, USA, UK, CanadaSome of the Lobsigensee team and other Swiss colleagues 1985

Many major publications198319851999

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 Ume 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 Krkenes in western Norway

Plant macrofossils, Salix herbacea stomatal density & CO2 concentrations, pollen, diatoms, chironomids, trichoptera, oribatid mites, cladocera, beetles, radio-carbon 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 & DISADVANTAGESAdvantagesMore proxies provide more 'degrees of freedom' or independent variables in data interpretation and hypothesis testing.More proxies, more detail. Can lead to a more holistic view of the past ecosystem.Least publishable unit (LPU) principle. Multi-proxy studies can, if you wish, provide many LPUs!Careful and critical synthesis avoids LPU and library and information overload.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.

Disadvantages1. 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 1971Comments on Earth Sciences: Geophysics 2: 36-43

An invidious effect of the reinforcement syndrome is so-called 'publication bias' where only confirmatory results are published, especially in so-called 'high-impact 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.

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.Major challenge to synthesise and write up a multi-proxy study.Major challenge to publish large-ish papers in these times of LPUs.To some, a disadvantage is no LPUs!

PROBLEMSVery 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 (Palaeoenviron-mental Analysis of Italian Crater Lakes and Adriatic Sediments).Juggins 1996

PALICLAS data retrieval and manipulationJuggins 1996

3.Data display of different proxies analysed at different resolutions.

Oldfield 1996Albano Core 94-1E

Oldfield 1996Albano Core 94-1E and Adriatic Core 92-43

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!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, wide-ranging, and efficient leader.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.

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 contra-dictions 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.

In Holocene palaeoclimatology, we may be at the limits of resolution and predictive abilities of our reconstruction methods.Multi-proxy studies are not really safe science.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 DEVELOPMENTSData-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.

External climate forcing functionsCatchment forcing functionsLake as an isolated system that evolves through time with its own internal dynamicsAssessing potential 'drivers' on aquatic ecosystems.What determines changes in lake organisms and lake sediments?

Birks et al. 2000

(a) Sgistalsee, Bernese Oberland, Swiss AlpsA.F. Lotter et al. 2003J. Paleolimnology 30: 253-342Andy Lotter

Lotter & Birks 2003

Wick et al. 2003

Wick et al. 2003

Heiri & Lotter 2003

Ideal study:Critical ecological situation at tree-line today; sensitive to changeOne core. Many proxies (pollen, macros, chironomids, cladocera, grain size, sediment magnetics, sediment geochemistry)Well dated; 18 AMS 14C dates on terrestrial plant materialWell co-ordinated by A.F. LotterHigh quality data:

Data-setNo. of samplesNo. of taxa/variablesPollen212203Plant macros37253Chironomids8230Cladocera1127Geochemistry17614Grain-size2946Magnetics5045

6.Consistent numerical methodology on all proxies7.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.

Predictor variables:Lotter & Birks 2003

Hypotheses tested:1.Climate has had a significant control on lake ecosystem changes2.Catchment vegetation has played significant role on lake changes* Tested against insolation, central European cold phases, & Atlantic IRD record# Veg phases: Betula-Pinus cembra; Alnus-Pinus cembra; Picea abies ~ 6300 cal BP; Pasture phases from Bronze Age to present

"Responses" (proxies)ScaleClimate a signif-icant predictor?Catchment vegetation a significant predictor?Terrestrial PollenCatchment & regionalYYLake biotic ChironomidsLakeNY CladoceraLakeNYLake abiotic Grain sizeLake-Y MagneticsLake-YGeochemistryLake-(Y)*#

Dalland S, a small (15 ha), shallow (2.6 m) lowland eutrophic lake on the island of Funen, Denmark.Catchment (153 ha) todayagriculture 77 habuilt-up areas41 hawoodland32 hawetlands 3 haNutrient rich total P 65-120 mg l-1Bradshaw et al. 2005 The Holocene 15: 1152-1162(b) Dalland S, Funen, DenmarkEmily Bradshaw

Map of Dalland S

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 samplesTransformationSediment loss-on-ignition %560NoneSediment dry mass accumulation rate560Log (x + 1)Sediment minerogenic matter accumulation rate560Log (x + 1)Plant macrofossil concentrations280Log (x + 1)Pollen %90NoneDiatoms %118NoneDiatom inferred total P118NoneBiogenic silica84Not usedPediastrum %90NoneZooplankton31Not used

Terrestrial landscape or catchment developmentBradshaw et al. 2005

Aquatic ecosystem developmentBradshaw et al. 2005

Detrended Correspondence Analysis of pollen and diatom data separately to summarise major underlying trends in both data setsPollen high scores for trees, low scores for light-demanding herbs and cropsDiatom - high scores mainly planktonic and large benthic types, low scores for Fragilaria spp. and eutrophic spp. (e.g. Cyclostephanos dubius)Bradshaw et al. 2005

Major contrast between pollen and diatom samples before and after Late Bronze Age forest clearances on DCA axis 1Prior to clearance, lake experienced few impacts.Bradshaw et al. 2005'Catchment''Lake'After the clearance, lake heavily impacted.

Canonical correspondence analysisResponse variables- diatom taxaPredictor variables- pollen taxa, LOI, dry mass and minerogenic accumulation rates, plant macro-fossils, PediastrumCovariable- age69 matching samplesPartial CCA with age partialled out as a covariable. Makes interpretation of effects of predictors easier by removing temporal trends and temporal auto-correlationPartial CCA all variables- 18.4% of variation in diatom data explained by Poaceae pollen, Cannabis-type 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.CCA of 50 subsets from bottom to top and % variance explainedBradshaw et al. 2005

4520-1840 BC Poaceae is sole predictor variable (20-22% of diatom variance)3760-1310 BC LOI and Populus pollen (16-33%)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.

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 factors160 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

Strong link between inferred catchment change and within-lake development. Timing and magnitude are not always perfectly matched, e.g. transition to Medieval PeriodBradshaw et al. 2005

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 AMMANNThe 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 career197519892000

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".

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.

WARMEST THANKSWilly Tinner and Pim van der KnaapHerb Wright and Andy LotterSaxifraga cotyledonRick Battarbee, John Smol, and John AndersonSteve Juggins and Richard TelfordCathy Jenksand, of course,