The Preboreal oscillation around the Nordic Seas: terrestrial and lacustrine responses

The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10000–9900 and 9600–9500 ¹⁴C plateaux (c. 11300–11150 calendar yr BP) it is also difficult to ¹⁴C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch–pine forest in Germany–Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased ²H and ¹³C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric ¹⁴C/¹²C ratio. A slow-down of the thermohaline circulation may temporarily have pushed the Polar Front further south. © 1997 John Wiley & Sons, Ltd.     

Late-Glacial climatic changes in Eastern France (Lake Lautrey) from pollen, lake-levels, and chironomids

High-temporal resolution analyses of pollen, chironomid, and lake-level records from Lake Lautrey provide multi-proxy, quantitative estimates of climatic change during the Late-Glacial period in eastern France. Past temperature and moisture parameters were estimated using modern analogues and ‘plant functional types’ transfer-function methods for three pollen records obtained from different localities within the paleolake basin. The comparison of these methods shows that they provide generally similar climate signals, with the exception of the Bölling. Comparison of pollen- and chironomid-based temperature of the warmest month reconstructions generally agree, except during the Bölling. Major abrupt changes associated with the Oldest Dryas/Bölling, Alleröd/Younger Dryas, and the Younger Dryas/Preboreal transitions were quantified as well as other minor fluctuations related to the cold events (e.g., Preboreal oscillation). The temperature of the warmest month increased by ∼5°C at the start of Bölling, and by 1.5°-3°C at the onset of the Holocene, while it fell by ca. 3° to 4°C at the beginning of Younger Dryas. The comparative analysis of the results based on the three Lautrey cores have highlighted significant differences in the climate reconstructions related to the location of each core, underlining the caution that is needed when studying single cores not taken from deepest part of lake basins.     

Climate and environment during the Younger Dryas (GS‐1) as reflected by composite stable isotope records of lacustrine carbonates at Torreberga,southern Sweden

Climatic and environmental changes during the Younger Dryas stadial (GS‐1) and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient Lake Torreberga, southern Sweden. Event GS‐1 is represented in the sediment sequence by 3.5 m of clay containing lacustrine carbonates of various origins. Comparison of isotopic records obtained on mollusc shells, ostracod valves, and Chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures. Variations in soil erosion rates can also be estimated from a simple isotope–mass‐balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments. The well‐known, rapid climatic shifts characterising the Last Termination in the North Atlantic region are clearly reflected in the isotopic data, as well as longer‐term changes within GS‐1. Following maximum cooling shortly after the Allerød–Younger Dryas (GI‐1–GS‐1) transition, a progressive warming and a slight increase in aquatic productivity is indicated. At the Younger Dryas–Preboreal (GS‐1–PB) transition mean annual air temperature rapidly increased by more than 5°C and summer lake‐water temperature increased by ca. 12°C. The subsequent Preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature. These results are in harmony with recent findings about large‐scale climate dynamics during the Last Termination. Copyright © 1999 John Wiley & Sons, Ltd.     

Reconstruction of Late Glacial summer temperatures from chironomid assemblages in Lac Lautrey (Jura,France)

A chironomid–July air temperature inference model based on chironomid assemblages in the surface sediments of 81 Swiss lakes was used to reconstruct Late Glacial July air temperatures at Lac Lautrey (Jura, Eastern France). The transfer‐function was based on weighted averaging–partial least squares (WA‐PLS) regression and featured a leave‐one‐out cross‐validated coefficient of determination (r²) of 0.80, a root mean square error of prediction (RMSEP) of 1.53 ° C, and was applied to a chironomid record consisting of 154 samples covering the Late Glacial period back to the Oldest Dryas. The model reconstructed July air temperatures of 11–12 ° C during the Oldest Dryas, increasing temperatures between 14 and 16.5 ° C during the Bølling, temperatures around 16.5–17.0 ° C for most of the Allerød, temperatures of 14–15 ° C during the Younger Dryas and temperatures of ca. 16.5 ° C during the Preboreal. The Lac Lautrey record features a two‐step July air temperature increase after the Oldest Dryas, with an abrupt temperature increase of ca. 3–3.5 ° C at the Oldest Dryas/Bølling transition followed by a more gradual warming between ca. 14 200 and 13 700 BP. The transfer‐function reconstructs a less rapid cooling at the Allerød/Younger Dryas transition than other published records, possibly an artefact caused by the poor analogue situation during the earliest Younger Dryas, and an abrupt warming at the Younger Dryas/Holocene transition. During the Allerød, two centennial‐scale 1.5–2.0 ° C coolings are apparent in the record. Although chronologically not well constrained, the first of these cold events may be synchronous with the beginning of the Gerzensee Oscillation. The second is inferred just before deposition of the Laachersee tephra at Lac Lautrey and is therefore coeval with the end of the Gerzensee Oscillation. In contrast to the Greenland oxygen isotope records, the Lac Lautrey palaeotemperature reconstruction lacks a clearly defined Greenland Interstadial (GI) event 1d and the decreasing temperature trend during the Bølling/Allerød Interstadial. Copyright © 2005 John Wiley & Sons, Ltd.     

The methodological basis for fine‐resolution,multi‐proxy reconstructions of ombrotrophic peat bog surface wetness

Amesbury, M. J., Barber, K. E. & Hughes, P. D. M. 2010: The methodological basis for fine‐resolution, multi‐proxy reconstructions of ombrotrophic peat bog surface wetness. Boreas, 10.1111/j.1502‐3885.2010.00152.x. ISSN 0300‐9483. The need for Holocene peat‐based palaeoclimatic records of increased temporal resolution has been widely identified in recent research. The often rapid growth rates of ombrotrophic bogs, when combined with fine‐resolution (i.e. millimetre‐scale) sampling, provide an as yet largely unexploited potential to derive sub‐decadal palaeoclimatic data from this proxy‐archive. However, multi‐proxy, fine‐resolution analyses require changes to standard methodologies, and the application of sampling techniques that are new to peat‐based palaeoclimate research. A peat sampler was custom‐built to allow precise and replicable millimetre‐scale subsampling. Subsequent methodological testing revealed that, irrespective of sample thickness (i.e. resolution), halving the standard sample volume used for plant macrofossil (from 4 cm³ to 2 cm³) and testate amoebae (from 2 cm³ to 1 cm³) analyses and the sample weight used for peat humification analysis (from 0.2 g to 0.1 g dried peat) did not affect the interpretation of the results. A contiguous 1‐mm sampling resolution for plant macrofossil analysis was also tested, but it was found that contiguous 5‐mm samples provided a more reliable background record to fine‐resolution testate amoebae and peat humification analyses. Based on these findings, a standardized and systematic methodological approach was developed, using the custom‐built peat slicer to take millimetre‐scale samples that provide enough sample material for both testate amoebae and peat humification analyses to be performed at 1‐mm resolution. This approach will facilitate the testing of the palaeoclimatic reliability of multi‐proxy, fine‐resolution peat‐based records.     

Lateglacial and early Holocene tephrostratigraphy and sedimentology of the Store Slotseng basin,SW Denmark: a multi‐proxy study

The history of the Lateglacial and Preboreal sedimentary succession from the Store Slotseng kettle hole basin, SW Denmark is presented. A tephrostratigraphical and multi‐proxy investigation of the sediments, including stable isotope geochemistry, reveals small‐ and large‐scale changes in the surrounding environment through time. Three distinct tephra horizons are observed. Two of them are identified as the Preboreal Hässeldalen Tephra and the Younger Dryas Vedde Ash. The third was deposited around the Pre‐Bølling/Bølling transition. The Preboreal sediments record two significant decreases in authigenic carbonate content. Using tephrostratigraphy the lower one is identified as occurring during the Preboreal Oscillation, while the upper one is contemporaneous with the Rammelbeek Phase, which by some is recognised above the Preboreal Oscillation. This period has not previously been observed in this region. The discovery of the Hässeldalen Tephra in the Store Slotseng basin expands the known southwestern limit of the ash cloud, and increases the area for potential future observations. The Hässeldalen Tephra (c. 11.3 cal. ka BP) was deposited just prior to the Preboreal Oscillation and as such has a large potential for precise correlation and characterization of this short climatic perturbation.     

Correlation of Swedish glacial varves with the Greenland (GRIP) oxygen isotope record

A mean varve thickness curve has been constructed for a part of the Swedish varve chronology from the northwestern Baltic proper. The mean varve thickness curve has been correlated with the δ¹⁸O record from the GRIP ice-core using the Younger Dryas–Preboreal climate shift. This climate shift was defined by pollen analyses. The Scandinavian ice-sheet responded to a warming at the end of the Younger Dryas, ca. 10995 to 10700 clay-varve yr BP. Warming is recorded as a sequence of increasing mean varve thickness and ice-rafted debris suggesting intense calving of the ice front. The Younger Dryas–Preboreal climatic shift is dated to ca. 10650 clay-varve yr BP, about 40 yr after the final drainage of the Baltic Ice Lake. Both the pollen spectra and a drastic increase in varve thickness reflect this climatic shift. A climate deterioration, correlated with the Preboreal oscillation, is dated to ca. 10440 to 10320 clay-varve yr BP and coincides with the brackish water phase of the Yoldia Sea stage. The ages of the climatic oscillations at the Younger Dryas–Preboreal transition show an 875 yr discrepancy compared with the GRIP record, suggesting a large error in the Swedish varve chronology in the part younger than ca. 10300 clay-varve yr BP. Copyright © 1999 John Wiley & Sons, Ltd.     

Can testate amoeba‐based palaeohydrology be extended to fens?

Numerous palaeoecological studies have used testate amoeba analysis to reconstruct Holocene hydrological change in peatlands, and thereby past climatic change. Current studies have been almost exclusively restricted to ombrotrophic bogs and the period since the fen–bog transition. Although the critical link between peatland surface wetness and climate is less direct in minerotrophic peatlands, such records may still be of value where there are few others, particularly if multiple records can be derived and inter‐compared. Expanding the temporal and spatial scope of testate amoeba‐based palaeohydrology to minerotrophic peatlands requires studies to establish the primacy of hydrology and the efficacy of transfer functions across a range of sites. This study analyses testate amoeba data from wetlands spanning the trophic gradient in the eastern Mediterranean region. Results demonstrate that different types of wetlands have distinctly different amoeba communities, but hydrology remains the most important environmental control (despite water table depth being measured at different times for different sites). Interestingly, Zn and Fe emerge as significant environmental variables in a subset of sites with geochemical data. Testate amoeba–hydrology transfer functions perform well in cross‐validation but frequently perform poorly when applied to other sites, particularly with sites of a different nutrient status. It may be valid to use testate amoebae to reconstruct hydrological change from minerotrophic peatlands with an applicable transfer function; however, it may not be appropriate to use testate amoebae to reconstruct hydrological change through periods of ecosystem evolution, particularly the fen–bog transition. In practice, the preservation of amoeba shells is likely to be a key problem for palaeoecological reconstruction from fens. Copyright © 2010 John Wiley & Sons, Ltd.     

Testate amoebae as estuarine water‐level indicators: modern distribution and the development of a transfer function from a freshwater tidal marsh (Scheldt estuary,Belgium)

Little is known about the century‐scale response of water levels in inland estuaries to sea‐level change and human modifications to estuarine morphology. This study explored the ability of using testate amoebae (Protozoa, Rhizopoda) from sediments of a freshwater tidal marsh as indicators of water level in an inland estuary. The hypothesis was that modern testate amoeba assemblages change with surface elevation (approximately the duration of tidal flooding) within a freshwater tidal marsh. Variation in testate amoeba assemblages in relation to multiple environmental variables and sediment characteristics was studied through redundancy analysis. This demonstrated that a significant part of the variation in modern testate amoeba assemblages could be explained by flooding frequency, surface elevation, organic content and particle size of the soil. Transfer functions, partial least squares and weighted average regressions were made to show that testate amoebae can be used for reconstruction of water level (with an accuracy of 0.05 Normalized Elevation). A preliminary test of application of the transfer function to palaeo testate amoeba assemblages showed promising results. Testate amoebae from a freshwater tidal marsh provide a potentially powerful new tool for estuarine water‐level reconstructions. Copyright © 2011 John Wiley & Sons, Ltd.     

Lateglacial and early Holocene summer temperatures in the southern Swiss Alps reconstructed using fossil chironomids

We present a Lateglacial and early Holocene chironomid‐based July air temperature reconstruction from Foppe (1470 m a.s.l.) in the Swiss Southern Alps. Our analysis suggests that chironomid assemblages have responded to major and minor climatic fluctuations during the past 17 000 years, such as the Oldest Dryas, the Younger Dryas and the Bølling/Allerød events in the Lateglacial and the Preboreal Oscillation at the beginning of the Holocene. Quantitative July air temperature estimates were produced by applying a combined Norwegian and Swiss temperature inference model consisting of 274 lakes to the fossil chironomid assemblages. The Foppe record infers average July air temperatures of ca. 9.9 °C during the Oldest Dryas, 12.2 °C during most of the Bølling/Allerød and 11.1 °C for the Younger Dryas. Mean July air temperatures during the Preboreal were 14 °C. Major temperature changes were observed at the Oldest Dryas/Bølling (+2.7 °C), the Allerød/Younger Dryas (?2 °C) and the Younger Dryas/Holocene transitions (+3.9 °C). The temperature reconstruction also shows centennial‐scale coolings of ca. 0.8–1.4 °C, which may be synchronous with the Aegelsee (Greenland Interstadial 1d) and the Preboreal Oscillations. A comparison of our results with other palaeoclimate records suggests noticeable temperature gradients across the Alps during the Lateglacial and early Holocene. Copyright © 2011 John Wiley & Sons, Ltd.     

Coccolithophores as palaeoecological indicators for shifts of the ITCZ in the Cariaco Basin during the late Quaternary

Coccoliths were studied from the ODP Hole 1002C and core PL07‐39PC in the Cariaco Basin. Increases in Emiliania huxleyi are synchronous with decreases of Gephyrocapsa oceanica and vice versa. A new index (GEX) based on the relative abundances of these two taxa is proposed, and correlates with various other proxies. It is shown that GEX can serve as upwelling proxy. This confirms that the Intertropical Convergence Zone shifted north during the Bølling/Allerød, south during the Younger Dryas and back north during the Preboreal. The upwelling proxy shows few discrepancies with the terrigenous record. Copyright © 2008 John Wiley & Sons, Ltd.     

Late-glacial climatic oscillations as recorded in Swiss lake sediments

Regional pollen assemblage zones for the late-glacial period of the Swiss Plateau are introduced and defined. They include four major zones (Artemisia, Juniperus—Hippophaë, Betula, Pinus PAZ) with several subzones. Pollen and oxygen-isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000-9500 yr BP. The first oscillation, termed the Aegelsee fluctuation, occurs shortly before 12 000 yr BP and varve counts suggest its duration was ca. 100 yr. It is characterised by a short decrease in the oxygen isotopes as well as a short increase in NAP associated with a depression in birch pollen values. The second oscillation, which occurs in the δ¹⁸O record shortly before the deposition of the Laacher See Tephra (ca. 11 000 yr BP), is termed the Gerzensee fluctuation. It occurs during a pine-dominated phase and its vegetational effects cannot be determined palynologically. The most prominent regressive phase is the Younger Dryas biozone (ca. 10 700-10 000 yr BP) characterised by an increase in heliophilous NAP and low δ¹⁸O values. The Younger Dryas biozone can often be subdivided palynologically into two parts: a first part rich in grasses and juniper and a second part with higher Filipendula and birch values. During the Preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios. Comparison of the Swiss oxygen isotope profiles with the Greenland Dye 3 record suggests that not only the three major shifts in the δ¹⁸O curves but also the minor ones are closely comparable, suggesting some common climatic control.     

Late‐glacial and early Holocene changes in vegetation and lake‐level at Hauterive/Rouges‐Terres,Lake Neuchâtel (Switzerland)

Palynological and sedimentological analyses of a sedimentary sequence sampled at Hauterive/Rouges‐Terres, Lake Neuchâtel (Switzerland) provide documentation of changes in vegetation and lake‐level during the Bølling, Younger Dryas and Preboreal pollen zones, and have allowed a comparison with sequences covering the same period from other sites located in the western part of the Swiss Plateau. The Juniperus–Hippophaë zone (regional pollen assemblage zone (RPAZ) CHb‐2, first part of the Bølling, ca. 14 650–14 450 cal. yr BP) was characterised by a generally low lake‐level. A weak rise occurred during this zone. The Juniperus–Hippophaë to Betula zone transition coincided with a lake‐level lowering, interrupted by a short‐lived but marked phase of higher lake‐level recorded at the neighbouring site of Hauterive‐Champréveyres, but not present at Hauterive/Rouges‐Terres owing to an erosion surface. Shortly after the beginning of the Betula zone (RPAZ CHb‐3, second part of the Bølling, ca 14 450–14 000 cal. yr BP), a marked rise in lake‐level occurred. It was composed of two successive periods of higher level, coinciding with high values of Betula, separated by a short episode of relatively lower lake‐level associated with raised values in Artemisia and other non‐arboreal pollen. The last part of RPAZ CHb‐3 saw a fall in lake‐level. The lower lake‐levels during RPAZ CHb‐2 to early RPAZ CHb‐3 can be correlated with the abrupt warming at the beginning of the Greenland Interstadial (GI) 1e thermal maximum. The successive episodes of higher lake‐level punctuating the GI 1e might be linked to the so‐called Intra‐Bølling Cold Oscillations identified from several palaeoclimatic records in the North Atlantic area, and also documented in oxygen‐isotope data sets from Swiss Plateau lakes. The Hauterive/Rouges‐Terres lake‐level record provides evidence for marked climatic drying through the second part of the Younger Dryas event (GS1), during the GS1–Preboreal (RPAZ CHb‐4b–4c) transition (except for a rise at ca. 11 450–11 400 cal. yr BP), and at the RPAZ CHb‐4c–5 (Preboreal–Boreal) transition, following the Preboreal Oscillation (after 11 150 cal. yr BP). The Preboreal Oscillation coincided with higher lake‐levels, its end being followed by a rapid expansion of Corylus, Quercus, Ulmus and Tilia. The Hauterive/Rouges‐Terres lake‐level record suggests that radiocarbon plateau at 12 600, 10 000 and 9500 ¹⁴C yr BP corresponded to periods of generally lower lake‐level. This suggests that an increase in solar activity may have contributed to both climatic dryness and a decrease in atmospheric radiocarbon content. Copyright © 2002 John Wiley & Sons, Ltd.     

Peat multi-proxy data from Männikjärve bog as indicators of late Holocene climate changes in Estonia

Sillasoo, Ü., Mauquoy, D., Blundell, A., Charman, D., Blaauw, M., Daniell, J. R. G., Toms, P., Newberry, J., Chambers, F M. & Karofeld, E. 2007 (January): Peat multi‐proxy data from Männikjärve bog as indicators of late Holocene climate changes in Estonia. Boreas, Vol. 36, pp. 20–37. Oslo. ISSN 0300–9483. As part of a wider project on European climate change over the past 4500 years, a 4.5‐m peat core was taken from a lawn microform on Männikjärve bog, Estonia. Several methods were used to yield proxy‐climate data: (i) a quadrat and leaf‐count method for plant macrofossil data, (ii) testate amoebae analysis, and (iii) colorimetric determination of peat humification. These data are provided with an exceptionally high resolution and precise chronology. Changes in bog surface wetness were inferred using Detrended Correspondence Analysis (DCA) and zonation of macrofossil data, particularly concerning the occurrence of Sphagnum balticum, and a transfer function for water‐table depth for testate amoebae data. Based on the results, periods of high bog surface wetness appear to have occurred at c. 3100,3010–2990,2300, 1750–1610, 1510, 1410, 1110, 540 and 310 cal. yr BP, during four longer periods between c. 3170 and 2850 cal. yr BP, 2450 and 2000 cal. yr BP, 1770 and 1530 cal. yr BP and in the period from 880 cal. yr BP until the present. In the period between 1770 and 1530 cal. yr BP, the extension or initiation of a hollow microtope occurred, which corresponds with other research results from Mannikjarve bog. This and other changes towards increasing bog surface wetness may be the responses to colder temperatures and the predominance of a more continental climate in the region, which favoured the development of bog micro‐depressions and a complex bog microtopography. Located in the border zone of oceanic and continental climatic sectors, in an area almost without land uplift, this study site may provide valuable information about changes in palaeohydrological and palaeoclimatological conditions in the northern parts of the eastern Baltic Sea region.     

Late glacial and holocene lake level fluctuations in Lake Biel,western Switzerland

Cores and outcrops from the southern shore of Lake Biel were studied to reconstruct the nearshore environment of the lake between ca. 12000 and 5000 yr BP. Core correlations were established by lithostratigraphical and pollen analytical correlations. From the Allerød to the Preboreal time quiet hydrodynamic conditions favoured the deposition of lake marl in the littoral zone and peat on the shore. Between the Preboreal(?) and the Atlantic the littoral zone shows a higher hydrodynamic environment with allochthonous material, whereas peat and clay layers are recorded from the shore. During the Older Atlantic severe erosional episodes caused the erosion of Boreal, Preboreal and Younger Atlantic layers. The previously described long hiatus between the Allerød and the Boreal time can now be connected with these erosional episodes. From Younger Atlantic to Subboreal time the littoral zone displays quiet conditions again with sedimentation of lake marl. On the basis of these results a lake level curve for Lake Biel is proposed: high lake level stands can be traced during the Allerød, Boreal, Older Atlantic and Younger Atlantic biozones; low lake level stands are found during the Allerød, Younger Dryas, Preboreal and Older Atlantic biozones.     

Reinvestigation of the classic late‐glacial Bølling Sø sequence,Denmark: chronology,macrofossils, Cladocera and chydorid ephippia

The late‐glacial Bølling period was first identified by Johs. Iversen on the basis of pollen results from Lake Bølling Sø in Denmark. Because there were no radiocarbon dates from the sequence the Bølling Chronozone (12 000–13 000 ¹⁴C yr BP) was later established on the basis of dates from other sites. A new project is reinvestigating the sediments from the Bølling Sø sequence with AMS radiocarbon dating and multiproxy analyses. Here we present results of AMS radiocarbon dating, macrofossil analyses, cladoceran analyses (Cladocera concentrations and chydorid ephippia) and Pediastrum analyses (concentrations). The AMS dates on land plant remains show that the lower part of the sequence is around 12 500 ¹⁴C yr BP, and thus clearly pre‐dates the Allerød chronozone. However, construction of a chronology for the sequence was problematic, partly because of reworking of macroscopic plant remains. The climate ameliorated after glacial conditions to such an extent that growth of plants could begin at ca. 12 500 ¹⁴C yr BP, but the results of multiproxy analyses show little evidence for a further warming period during the pre‐Allerød part of the sequence. Lake productivity was low, and tree birch rare or maybe absent. This may reflect widespread occurrence of dead ice, unstable soils, heavy in‐wash of minerogenic matter to the lake, resulting in turbid water and rapid sedimentation. The early pioneer vegetation was characterised by Salix polaris and Dryas octopetala, and by herbs. The Allerød Chronozone, and especially its initial part, appears to have been relatively warm but reduced cladoceran concentrations and increased proportion of chydorid ephippia suggest that climate cooled in the middle Allerød and that the late Allerød was colder than the early part. The early Younger Dryas was probably colder than the late Younger Dryas. Clear warming is apparent at the beginning of the Holocene, where the first macrofossil evidence of trees (Betula pubescens, Populus tremula) is found. Copyright © 2004 John Wiley & Sons, Ltd.     

A chironomid-based reconstruction of late glacial summer temperatures in the southern Carpathians (Romania)

Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ¹⁸O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.     

A near‐annual palaeohydrological study based on testate amoebae from a sub‐alpine mire: surface wetness and the role of climate during the instrumental period

We present the first testate amoeba‐based palaeohydrological reconstruction from the Swiss Alps, and the first depth to the water table (DWT) calibration dataset for this region. Compared to existing models, our new calibration dataset performs well (RMSEP = 4.88), despite the length of the water table gradient covered (53 cm). The present‐day topography and vegetation of the study mire Mauntschas suggest that it is partly ombrotrophic (large Sphagnum fuscum hummocks, one of which was the coring site) but mostly under the minerotrophic influence of springs in the mire and runoff from the surrounding area. Ombrotrophic Sphagnum fuscum hummocks developed at the sampling site only during the last 50 years, when testate amoebae indicate a shift towards dry and/or acid conditions. Prior to AD 1950 the water table was much higher, suggesting that the influence of the mineral‐rich water prevented the development of ombrotrophic hummocks. The reconstructed DWT correlated with Pinus cembra pollen accumulation rates, suggesting that testate amoebae living on the mire and P. cembra growing outside of it partly respond to the same factor(s). Finally, temperature trends from the nearby meteorological station paralleled trends in reconstructed DWT. However, contrary to other studies made on raised bogs of northwestern Europe, the highest correlation was observed for winter temperature, despite the fact that testate amoebae would more logically respond to moisture conditions during the growing season. The observed correlation with winter temperature might reflect a control of winter severity on surface moisture during at least the first part of the growing season, through snow melt and soil frost phenomena influencing run‐off. More ecohydrological work on sub‐alpine mires is needed to understand the relationships between climate, testate amoebae and peatland development. Copyright © 2009 John Wiley & Sons, Ltd.     

Climatic oscillations in central Italy during the Last Glacial–Holocene transition: the record from Lake Accesa

This paper presents an event stratigraphy based on data documenting the history of vegetation cover, lake‐level changes and fire frequency, as well as volcanic eruptions, over the Last Glacial–early Holocene transition from a terrestrial sediment sequence recovered at Lake Accesa in Tuscany (north‐central Italy). On the basis of an age–depth model inferred from 13 radiocarbon dates and six tephra horizons, the Oldest Dryas–Bølling warming event was dated to ca. 14 560 cal. yr BP and the Younger Dryas event to ca. 12 700–11 650 cal. yr BP. Four sub‐millennial scale cooling phases were recognised from pollen data at ca. 14 300–14 200, 13 900–13 700, 13 400–13 100 and 11 350–11 150 cal. yr BP. The last three may be Mediterranean equivalents to the Older Dryas (GI‐1d), Intra‐Allerød (GI‐1b) and Preboreal Oscillation (PBO) cooling events defined from the GRIP ice‐core and indicate strong climatic linkages between the North Atlantic and Mediterranean areas during the last Termination. The first may correspond to Intra‐Bølling cold oscillations registered by various palaeoclimatic records in the North Atlantic region. The lake‐level record shows that the sub‐millennial scale climatic oscillations which punctuated the last deglaciation were associated in central Italy with different successive patterns of hydrological changes from the Bølling warming to the 8.2 ka cold reversal. Copyright © 2006 John Wiley & Sons, Ltd.     

Can pollution bias peatland paleoclimate reconstruction?

Peatland testate amoebae are widely used to reconstruct paleohydrological/climatic changes, but many species are also known to respond to pollutants. Peatlands around the world have been exposed to anthropogenic and intermittent natural pollution through the late Holocene. This raises the question: can pollution lead to changes in the testate amoeba paleoecological record that could be erroneously interpreted as a climatic change? To address this issue we applied testate amoeba transfer functions to the results of experiments adding pollutants (N, P, S, Pb, O₃) to peatlands and similar ecosystems. We found a significant effect in only one case, an experiment in which N and P were added, suggesting that pollution-induced biases are limited. However, we caution researchers to be aware of this possibility when interpreting paleoecological records. Studies characterising the paleoecological response to pollution allow pollution impacts to be tracked and distinguished from climate change.