Abrupt climate changes recorded over the european land mass : multi-proxy records of late-holocene climate variability in europe.

Deliverables

The development of biomarkers from peat has been progressed as follows. High-resolution proxy-climate records based on biomarker distributions have been generated for four ombrotrophic bogs located on a latitudinal transect across Europe, namely: Ballyduff Bog (Ireland), Butterburn Flow (Britain), Bissendorfer Moor (Germany) and Kontolanrahka (Finland). All the biomarker records have been generated at a resolution of 2 cm, and focus on the time intervals of 2950 - 2450 BP, which spans the Sub Boreal/Sub Atlantic transition, and AD800-1800, which includes the transition from the Medieval Warm Period to the Little Ice Age, as well as the modern interval of AD 1800-2000. The biomarker distributions of modern bog-forming vegetation have also been analysed to aid further interpretation of the biomarker records. The suite of new and established biomarkers measured largely reflected changes in the macrofossil proxy data, but were particularly useful where macrofossils were scarce or highly decayed in the peat record. Biomarkers recorded changes in environmental conditions, as well as vegetation, as certain biomarker transformations appear indicative of changes in redox conditions. The method used was developed as follows. Peat samples were freeze-dried, ground and small aliquots of powdered peat submitted to elemental analysis for nitrogen, organic carbon, hydrogen, and sulphur determinations (percentage dry weight). Lipids were extracted from approximately 0.5 g peat using repeated ultrasonication with 5 ml of dichloromethane/methanol (1:1, v/v). Total lipid extracts were hydrolysed with a 0.5 M methanolic sodium hydroxide solution for 1 h at 70oC and the neutral lipids were removed using hexane. Neutral lipids were further separated into apolar and polar fractions using alumina columns and eluting with hexane/dichloromethane (9:1, v/v) for apolar fractions and dichloromethane/methanol (2:1, v/v) for polar fractions. All fractions were analysed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). Lipid extracted peat residues were also analysed by pyrolysis-GC/MS (pyrolysis-GC/MS). Compound-specific stable isotope work has been initiated and a small number of analyses have been performed. Gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) was used to provide d13C values on individual lipid biomarkers. The stable carbon isotope composition (d13C) of n-alkanes and hopanes were measured at Butterburn Flow, GB site, to confirm the origin of hopanoid biomarkers. Results show the two C31-hopanes lie in the range -25.9 0 to -26.8 0 and are enriched relative to the C23 and C31 n-alkanes (-31.9 0 and -31.5 0, respectively). This implies a heterotrophic bacterial origin for the hopanes at Butterburn Flow, consistent with observations for other peats. Work on the dD records of n-alkane biomarkers has been initiated. All the n-alkanes have been extracted and quantified from fossil and modern peat samples. dD values obtained are consistent with values obtained from other bogs but further analyses need to be performed to complete the records. This is part of on-going research at the University of Bristol, building on our published work which has shown dD values can yield a temperature signal from peat bogs. The above work involved the extraction, analysis and interpretation of biomarker records from ~400 individual samples which equates to 800 apolar and polar fractions each of which contain numerous biomarkers (10 to 100). This has generated an unprecedented volume of geochemical data and a huge biomarker archive, which will underpin important future research into understanding the biogeochemical and palaeoclimate signals preserved in ombrotrophic bogs, and is to be taken further involving collaboration between the University of Bristol and the ACCROTELM scientific co-ordinator.

This result is concerned with a quantitative and qualitative estimate of carbon balance in Baltic mires over the latter half of the Holocene. Peatlands are stores of carbon dioxide bound from the atmosphere by plants. They are also sites of anaerobic methanogenesis and aerobic methanotrophy. As hydrology has regulated both the rates of peat accumulation and net release of methane (CH4), peatlands have been sensitive to climatic excursions, and their climatic feedback may have favoured either cooling or warming of the atmosphere during the Holocene. The aim of this part of the ACCROTELM project was to reconstruct and evaluate the climatic feedbacks of a bog, as revealed by a high-resolution multi-proxy time series over the past four millennia. A core for multiproxy analysis was taken from the centre of Männikjärve Bog, Central Estonia. The 448 cm long core was analysed at every 4 cm for testate amoebae and at every 5 cm for plant macrofossils. The chronology of the peat layers was obtained using wiggle matching of 40 AMS-dates of macrofossils over the depth range. Fossil testate communities were used, together with modern samples over a range of microsites, in order to reconstruct the water tables during the past c. 4300 years. Aerial digital images of the present bog surface were classified in GIS to identify pools, hollows, lawns, and hummocks. Emission of CH4 through the bog surface was estimated using microsite-specific flux rates as controlled by the water table. The palaeo-CH4 emissions at each time step were simulated by letting the microsites be wetter or drier according to the reconstructed water tables. The microsite development history was constructed by running the GIS model backwards in time, starting from the present-time microsite snapshot. A set of rules was defined in the model to constrain how the pools, hollows, and lawns follow the water table dynamics. The initial results of these analyses were presented in a poster at the HOLIVAR 2006 meeting. Dynamics of WT do not always result in a similar response of the atmospheric intercation in different bogs, as viewed by means of GWP. Dry shifts leaded towards net cooling with lower CH4 emissions, but the two bogs showed different sensitivity to changes in water table. A warming shift in GWP at the Finnish site, Kontolanrahka bog ca. 2700 BP, is probably associated with evidence of a fire. Thus, GWP derived from a combination of palaeo-wetness index and peat bulk density profile seems a reasonable indicator of bog-atmosphere interaction. The average annual C accumulation rate is a result of C flows in new litter deposition minus the release of C in gaseous form as CO2 and CH4, and in dissolved organic matter DOC leaching out of the mire during a year. The ACCROTELM peat cores, dated with great resolution, show continuous net C accumulation with varying apparent annual accumulation rates throughout the cores, with an apparent increase in accumulation rate close below the peat surface. Recent gas exchange studies have indicated annual net C losses in mires over the northern hemisphere. In spite of these seemingly contrasting results, there is actually no disagreement between the two results, since they only deal with different temporal scales. While annual mass losses can be estimated by integrating the gaseous and dissolved matter fluxes showing a negative balance when losses exceed the input, geological estimates based on dated peat cores integrate peat accumulation over much longer periods of time. They also cannot show the matter that has been lost. However accurately the peat layers are dated, only positive present storage situations are visible in layers between two consecutive dates. According to the preliminary investigation, bogs have mostly acted in favour of atmospheric cooling. GWP balance has shown to be sensitive to both changes in hydrology or other allogenic factors affecting the bog. Further research and comparison with other proxies made available by the ACCROTELM collaboration lets us evaluate the detailed responses during climatic excursions. An animation of the peat accumulation of another ACCROTELM peat site - that of Kontolanrahka Bog, in Finland, has been produced, and part of this has been incorporated in a DVD of the ACCROTELM project (see Dissemination products from the project). The 3-D modelling of the Baltic bogs and the estimation of carbon balance and calculation of Global Warming Potential (GWP) is to be disseminated in peer-reviewed papers based on the work carried out in ACCROTELM. These overall results of the project may be of interest to and use by climate modellers.

We have cored and analyzed peat from eight Eropean ombrotrophic mire sites along two transects (E W from Estonia to Ireland; and N S from The Faroes to Spain) to produce plant macrofossil and peat humification datasets that serve as continuous proxy-climate records for up to the past 4500 years. Identification and quantification of plant macrofossils used an updated protocol of the Quadrat and Leaf Count Macrofossil Analysis technique (QLCMA). This experimental methodology assesses the volume percentages of botanical macrofossils in peat samples. The abundance of unidentified organic matter (UOM), identifiable Sphagnum, monocotyledons and ericaceous roots were determined using averaged quadrat counts, and all other identifiable plant remains were recorded. Core sampling. Coring teams were co-ordinated regionally: Partner 4 (UH.DES) led the Baltic mire coring teams; Partner 2 (UvA) the teams for the central group of mires; Partner 7 (TCD), for the Irish and Spanish mires and Partner 1 (UoG), for the Faeroese mire. After agreeing common protocols for sampling, we surveyed stratigraphically and cored all the mires, taking particular care over the sampling locations according to mire microtopography. We sub-sampled the cores contiguously, taking thin parallel samples for the two methods described here. Macrofossil analysis. Here we used a standardized method of QLCMA. We analyzed contiguous 1 cm subsamples for plant macrofossils to produce proxy-climate records for all named sites found suitable on the E W and N S mire transects. This allowed us to characterize the climate variability of terrestrial Europe over the past few thousand years and to focus attention on periods of abrupt climate change for further detailed multi-proxy analyses. Peat humification. We analysed sub-samples for determining peat humification by colorimetry using a standard protocol. These analyses were performed on contiguous thin peat samples at all sites. The E W transect of sites was extended to Newfoundland by agreement of a third party. Publications resulting from this work have been targeted at international peer-refereed journals. It is expected that each site will yield at least one publication on the proxy-climate data, and that papers combining the records from several sites will also be prepared. The data are expected to be archived in the Pangaea database.

ACCROTELM has targeted three focus periods for more detailed analysis of mire sites. These are the periods 4500 to 4000 cal. BP, 3000 to 2500 cal. BP and 1200 to 200 cal. BP. The first of these is only present wholly at three of the ACCROTELM mire sites. So far, the first mire sites to have a published overview of the proxy-climate records are those from Estonia and Newfoundland (the latter by collaboration with a third party). The targeting of the focus periods involves use of detailed pollen and non-pollen microfossil analysis at some of the mire sites. For example, the analysis of microfossils and macroremains of the ACCROTELM cores from Denmark and England has resulted in a series of papers (published in international refereed journals, submitted or in preparation) on a number of interrelated topics including: high precision chronology and radiocarbon dating (1, 2); palaeohydrology and climate change (3, 4, 5, 10); vegetation history and human impact (6, 7, 8); fungal hyperparasitism and climatic conditions (9). These numbers relate to the papers listed below, as this is a more complete list than can be included in the Documentation field. 1. Yeloff, D. E., Bennett, K. D., Blaauw, M., Mauquoy, D., Sillasoo, U., van der Plicht, J. & van Geel, B. 2006. High precision 14C dating of Holocene peat deposits: a comparison of Bayesian calibration and wiggle-matching approaches. Quaternary Geochronology 1: 222-235. 2. Yeloff, D. E., van der Linden, M., Chambers, F., Toms, P., van Geel, B. and van der Plicht, J., submitted. Dating recent peat accumulation in European ombrotrophic bogs. 3. Yeloff, D. E., Charman, D., van Geel, B. and Mauquoy, D., submitted. Reconstruction of hydrology, vegetation and past climate change on ombrotrophic bogs using fungal microfossils. 4. McClymont. E.L., Mauquoy, D., Yeloff. D. E., Broekens, P., van Geel, B., Pancost, R.D. and Evershed, R.P., submitted. Increased bog wetness, changing nutrient supplies, and the disappearance of Sphagnum imbricatum from Butterburn Flow, U.K. 5. Mauquoy, D., Yeloff, D. E., van Geel, B., Charman, D., and Blundell, A., in prep. Precisely dated mire surface wetness reconstructions using a c. 5250 year peat sequence from Northern England and an exceptional high resolution record from Denmark deposited since c. AD 690. 6. Yeloff, D. E., Broekens, P., Innes, J. and van Geel, B., submitted. Late Holocene vegetation and land-use history in Denmark: the record from Lille Vildmose, northeast Jutland. 7. Yeloff, D. E. and van Geel, B., in press. Abandonment of farmland and vegetation succession following the Eurasian plague pandemic of AD 1347-1352. Journal of Biogeography. 8. Yeloff, D. E., van Geel, B., Broekens, P., Bakker, J. and Mauquoy, D., in press. Mid late Holocene vegetation and land-use history in the Hadrian s Wall region of northern England: the record from Butterburn Flow. The Holocene. 9. van Geel, B., Aptroot, A. and Mauquoy, D., 2006. Sub-fossil evidence for fungal hyperparasitism (Isthmospora spinosa on Meliola ellisii, on Calluna vulgaris) in a Holocene intermediate ombrotrophic bog in northern-England. Review of Palaeobotany and Palynology 141: 121-126. 10. Yeloff, D. E., Mauquoy, D. 2006. The influence of vegetation composition on peat humification: implications for palaeoclimatic studies. Boreas, 35, 662-673.

A first synthesis of available lake-level data was established for west-central Europe. A data set of 180 radiocarbon, tree-ring and archaeological dates obtained from sediment sequences from 26 lakes in the Jura Mountains, the northern French Pre-Alps and the Swiss Plateau was used to construct a Holocene mid-European lake-level record. The dates do not indicate a random distribution over the Holocene, but form clusters suggesting an alternation of lower and higher, climatically driven lake-level phases. For the last 4500 years, they provide evidence of an unstable Holocene climate punctuated by phases of higher lake level at 4150-3950, 3500-3100, 2750-2350, 1800-1700, 1300-1100, 750-650 cal BP and after AD 1394. A comparison of this mid-European lake-level record with the GISP2-Polar Circulation Index record, the North Atlantic ice-rafting debris events, and with the 14C record suggests teleconnections in a complex cryosphere-ocean-atmosphere system. This also suggests that changes in solar activity played a major role in Holocene climate oscillations over the North Atlantic area. New investigations have been made in west-central Europe to document better the regional pattern of palaeohydrological variations over the last 4500 years. Recent archaeological investigations around Neolithic and proto-historic lake-shore settlements have provided the opportunity to obtain additional lake-level data. New sediment sequences have been observed and analysed from Lakes Pluvis and Bourget (sites of Tresserve and Chindrieux) in the southern Jura, and from Lakes Morat (site of Montilier) and Neuchâtel (site of Marin-les-Piécettes) on the Swiss Plateau. Additional lake-level data were also obtained from Lakes Morat and Neuchâtel on the Swiss Plateau. Sediment sequences marked by an alternation of peaty-organic and carbonate lake-marl layers give evidence of successive lake-level fluctuations during the last 7 millennia. The lake-level changes recognised at Lakes Pluvis and Bourget in the southern Jura, and at Lakes Morat and Neuchâtel on the Swiss Plateau fully agree with the regional pattern defined from previous data. On the basis of glacier records established in the Swiss Alps and lake-level records established in west-central Europe, a comparison has been attempted between high-resolution palaeohydrological data in west central Europe over the past 3300 years. A data set of tree-ring, radiocarbon and archaeological dates in addition to historical sources was used to reconstruct fluctuations of the Great Aletsch, the Gorner and the Lower Grindelwald Glaciers in the Swiss Alps. The three ice-streams experienced nearly synchronous advances at c. 1000-600 BC, 500-600, 800-900, 1100-1200 and 1300-1860 AD. These glacier fluctuations show strong correspondences with lake-level variations reconstructed in eastern France (Jura Mountains and Pre-Alps) and on the Swiss Plateau. Pollen and lake-level data provided by the sediment sequence from Tresserve (Lake Bourget, France) that documents the period 3800-2500 cal BP allow us to reconstruct climatic parameters. Results suggest that phases of higher lake level coincided with an increase in annual precipitation by ca 100mm, a decrease in summer temperature by 1°C and a shortening of the growing season. Conversely, periods of low lake level corresponded to a decrease in annual precipitation, an increase in summer temperature and a longer growing season. This general pattern appears to be consistent with correlations observed between high-level phases of regional lake levels and glacier advances in the Swiss Alps. It may have resulted from alternate southward-northward displacements of the Atlantic Westerly Jet Stream. Lake Joux in the Swiss Jura was investigated to establish a high-resolution record of environmental changes (lake-level and vegetation cover) over the last millennium. The Lake Joux sediment sequence offers a detailed lake-level record for the Little Ice Age (LIA) and more particularly for the transition between the Medieval Warm Period (MWP) and the LIA. Generally speaking, the late MWP (around AD 1250-1400) and the last century appear to be characterised by lower lake-level conditions than the LIA that experienced marked high-level phases. The comparison between the Lake Joux lake-level record and the radiocarbon record used as an indicator of past variations in solar activity gives evidence of a general coincidence between phases of decrease in solar irradiance (higher 14C values) and phases of higher lake level. In particular, the Wolf, Spörer, Maunder and Dalton minimums of solar activity corresponded to phases of higher lake level. With the support of the program ECLIPSE and the MSH-Besançon of the French CNRS, additional investigations have been undertaken in northern Italy to prolong towards southern Europe the palaeohydrological transect documented by Danish and German mire sites in northern Europe.

The ACCROTELM peat cores have been precisely dated by 14C AMS wiggle-match dating. In addition, recent peat accumulation has been dated by a variety of techniques including the 14C �bomb peak�, 210Pb, 137Cs, spheroidal carbonaceous particles (SCPs) and pollen horizons. The ACCROTELM dating strategy has resulted in papers on high-precision radiocarbon chronologies and the dating of recent (< 150 yr) peat deposits. The precise dating of the peat cores and construction of robust age-depth models is crucial for all aspects of the ACCROTELM project, as indicated by the publications on palaeohydrology and climate change; vegetation history, climate change and human impact; fungal hyperparasitism and climatic conditions. Articles about high-precision radiocarbon dating were produced. The 14C AMS wiggle-match data are important for all other aspects of ACCROTELM, and especially for the evaluation of the role of climate forcing factors. See documentation section for forthcoming papers.

A set of proxy-climate data based on testate amoebae A total of 1060 samples was analysed from nine sites. Some of the results are already published (Hughes et al, 2006; Sillasoo et al., in press) and publications are in preparation for the rest of the data. Generation of hydroclimatic curves for evaluation of proxy-climate data A series of hydroclimatic curves was generated from 6 out of 8 European sites, the other sites proving unsuitable for testate amoebae analysis. The hydroclimatic curves (expressed as depth to water table) were generated by applying a transfer function based on modern testate amoebae and hydrological data to the fossil testate amoebae assemblage data. Estimates were made using a weighted averaging model with tolerance downweighting and error estimates were calculated by bootstrapping using 1000 bootstrap cycles. A new technique for palaeohydrological reconstruction at a pan-European scale A new training set of 143 samples from the eight European sites was collected. This has resulted in the production of a new transfer function for reconstructing the depth to water table from fossil testate amoebae assemblages from ombrotrophic peatlands. The resulting palaeohydrological reconstructions can be regarded as proxy records of climate changes as the main influences on changing hydrology are precipitation and evapotranspiration. The results of this work have been published.

A public access website has been created giving details of the project, its aims, methods, the protocols developed for use and adopted in the project, the project partners, details of the mire sites, and a cumulative list of publications in the refereed, and in the non-refereed literature. The website is currently hosted at http://www2.glos.ac.uk/accrotelm. Details of posters presented at the HOLIVAR 2006 meeting are also given. The publications intended for scientific peer-reviewed journals have so far been targeted at specialist international journals in the fields of palaeoecology, Quaternary (incl. Holocene) environmental change, vegetation history, chronology and climate change. Site-based studies for lakes or for individual mires have been produced first. The lake-level change data have also been aggregated to produce regional syntheses of lake-level changes, and also comparisons with other data archives (such as glacier movements; ice core records, etc.). Inter-site syntheses of the mire data await drafting into manuscripts for peer review. For the HOLIVAR 2006 international science meeting ACCROTELM scientists produced eight posters, one of which was an overview of the project. The list of posters is given on the project website at http://www2.glos.ac.uk/accrotelm/holipost.html and Abstracts of the posters (and in some cases a full pdf of the poster) can be downloaded from the HOLIVAR2006 website at http://www.holivar2006.org/abstracts/ . One of these posters won a HOLIVAR prize. It is intended that major findings of the project would be disseminated via targeted Press Releases and through placement on the European science AlphaGalileo Website, for maximum exposure and dissemination to the print ansd broadcast media. It is intended that datasets from the project will be archived in the database Pangaea, for future scientific access (datasets are currently embargoed, pending publication of site-specific studies, to within the ACCROTELM consortium and to external collaborators). Remaining core samples are archived for potential future analyses, with provisional agreement for some samples from two of the mire sites to be used in follow-up work with an agency external to the ACCROTELM consortium. Potentially the most productive future scientific links would be with (a) climate modellers, to ascertain whether existing climate models can reproduce the magnitude, rate and spatial variability of the changes indicated in the ACCROTELM proxy-climate datasets, and (b) palaeoclimatologists specializing in data from climate archives other than mires or lakes, to compare proxy-climate datasets. A film DVD has been produced under specialist sub-contract, and this sets the ACCROTELM work within the context of current concern over recent and future climate change, and includes filmed interviews with leading climate scientists. This DVD will be available for use in Higher Educational establishments for undergraduates and in taught courses for postgraduates. The DVD is also suitable for policy-makers and planners to provide them with a long-term context for climate change, and to emphasize that there is a greater degree of natural variability in the Holocene climate of Europe over past millennia than has perhaps been appreciated, and that past climate change had either fateful or favourable effects on human societies, and this evidence has relevance for adaptation strategies to future climate change.

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