Environmental impact of the Laacher See eruption at a large distance from the volcano: Integrated palaeoecological studies from Vorpommern (NE Germany)

Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 196–214

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p a l a e o

Environmental impact of the Laacher See eruption at a large distance from the volcano: Integrated palaeoecological studies from Vorpommern (NE Germany) Pim de Klerk a,⁎, Wolfgang Janke a, Peter Kühn b, Martin Theuerkauf c a b c

Geographical Institute, Ernst-Moritz-Arndt-University, Friedrich-Ludwig-Jahnstraße 16, D-17487 Greifswald, Germany Institute of Geography, Chair of Physical Geography, Laboratory of Soil Science and Geoecology, Eberhard-Karls-University, Rümelinstraße 19-23, D-72070 Tübingen, Germany Institute of Botany and Landscape Ecology, Ernst-Moritz-Arndt-University, Grimmer Straße 88, D-17487 Greifswald, Germany

a r t i c l e

i n f o

Article history: Received 2 May 2008 Received in revised form 16 September 2008 Accepted 17 September 2008 Keywords: Diatom analysis Laacher See eruption/Laacher See tephra Micromorphology NE Germany Palynology Weichselian Lateglacial

a b s t r a c t In order to investigate the environmental impacts of the Laacher See eruption (12,900 calendar years B.P.) and the deposition of the Laacher See tephra at a large distance from the volcano, fine-resolution micromorphological, diatomological, and palynological investigations were carried out on closely spaced areas in the Reinberg basin (Vorpommern, NE Germany). The lower part of the LST originates from atmospheric input, whereas the upper part was washed-in from the surrounding basin slopes. Diatom populations expanded as a consequence of the input of silica, while simultaneously dissolution of dead diatoms decreased. The tephra provided a favourable habitat for many epilithic and epipelic diatom taxa. The diatoms show a slight acidification and an increase in specific conductivity of the water, whereas no eutrophication is indicated. These effects disappeared after lake sediments covered the tephra. The pollen diagrams from the Reinberg basin as well as many other pollen diagrams from NE Germany indicate that the vegetation reacted to rising temperatures after the cooler Gerzensee fluctuation, to a short-lived water-level rise connected with higher precipitation after the volcanic eruption, and to minor openings in the forest vegetation due to increased fire probably connected with severe thunderstorms. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The eruption of the Laacher See volcano (Eiffel, W Germany, cf. Fig. 1) spread a tephra over central Europe that at many localities functions as an important macroscopically visible chronostratigraphic marker dating from ca. 12,900 calendar years B.P. (cf. Van den Bogaard and Schmincke, 1985; Brauer et al., 1999; Schmincke et al., 1999; Baales et al., 2002). The Laacher See eruption occurred towards the end of the Lateglacial warm period generally known as “Allerød”, approximately 200 calendar years before the onset of the final Lateglacial cold phase generally referred to as the “Younger Dryas” (Brauer et al., 1999) (due to a widespread confusion on stratigraphic and geochronologic terminology of the Weichselian Lateglacial, the traditional terminology should be used with care, cf. De Klerk, 2004). Oxygen-isotope data show that the Laacher See eruption occurred towards the end of a minor cooler episode within the Allerød known as the “Gerzensee fluctuation” after temperatures had started to rise, but prior to the completion of this rise (cf. Lotter et al., 1992; Baales et al., 2002).

⁎ Corresponding author. Present address: Staatliches Museum für Naturkunde Karlsruhe, Erbprinzenstraße 13, D-76133 Karlsruhe, Germany. Tel.: +49 721 1752876; fax: +49 721 1752110. E-mail address: [email protected] (P. de Klerk). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.09.013

A volcanic eruption with the magnitude of the Laacher See eruption, one of the major Plinian eruptions of the Late Quaternary (cf. Schmincke et al., 1999), must have had an impact on the environment far beyond its immediate surroundings. Especially the large amount of emitted sulphur (at least 2 Mt, but possibly ranging up to 150 Mt) makes a major effect on climate very likely (Schmincke et al., 1999). Expected supra-regional effects include ash deposition, increased precipitation, acid rain resulting in a general acidification of habitats, an increased atmospheric albedo (i.e. reduced net solar radiation) and a consequent decrease in the average temperatures in the Northern Hemisphere (Schmincke et al., 1999; Graf and Timmreck, 2001; Baales et al., 2002). A small fluctuation in the oxygen-isotope curves of the Greenland ice-cores (that chronologically correlates with the Laacher See eruption and of which abundant sulphur connects it to a major volcanic eruption) indicates that temperatures had fallen for ca. 2 years (cf. Baales et al., 2002), which is in accordance with model simulations (Graf and Timmreck, 2001). Kaiser (1993) inferred a reduction of temperatures for a period of 6 years from tree rings in fossil pine trunks that date from around the Laacher See eruption, but such thinner tree rings might also reflect a reaction to the deposition of acid debris (Highwood and Stevenson, 2003). The most severe recent and historic eruptions caused a temperature decrease of less than 1 °C temperature for only 1 to 2 years (cf. Bertrand et al.,1999; Zielinski, 2000). Although several studies deal with the environmental impact of the Laacher See eruption at relatively close distances to the volcano (e.g. Lotter and Birks, 1993; Birks and Lotter, 1994; Schmincke et al.,

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Fig. 1. A: Location of the Reinberg basin and of the localities of Fig. 9; B: Contour lines of the Reinberg basin, location of cross-sections A–A' and B–B' and locations of the analysed cores; C: Cross-sections A–A' and B–B' through several sub-basins of the Reinberg basin.

1999; Baales et al., 2002), no investigations have been carried out to study the environmental effects at large distances. It remains, therefore, unclear to what extent the predicted effects had actually occurred. In NE Germany, the Laacher See tephra (LST) occurs in many basins as a light-grey band with a thickness ranging from few mm to ca. 1 cm (Müller, 1965; Kleissle and Müller, 1969; Theuerkauf, 2003; Theuerkauf and Joosten, in press). One of these localities is the Reinberg basin in Vorpommern (Fig.1) located at ca. 575 km distance from the Laacher See volcano. A small fluctuation in pollen types immediately above the LST led to the conclusion that the vegetation had reacted to the Laacher See eruption and/or on the deposition of the tephra (De Klerk et al., 2001, in press), which prompted more detailed palaeo-environmental studies. The present paper reconstructs the environmental impact of the Laacher See eruption in NE Germany from palaeoenvironmental studies of the Reinberg basin including micromorphological, diatom and pollen analyses. A comparison with other studies from NE Germany and beyond allows the interpretation of the data in a broad spatial context. 2. Study area and core locations The Reinberg basin (Fig. 1) contains no inlet or outlet and consists of several sub-basins originating from the thawing of buried dead-ice (cf. Klafs et al., 1973). The NW–SE orientated cross-section A–A' cuts the northern, the middle and the southern sub-basin, which are separated by mineral ridges. The SW–NE cross-section B–B' only crosses the middle sub-basin. The middle sub-basin has a size of ca. 40 × 50 m and is filled with a sequence of lake sediments and peat layers. The LST is embedded within a layer of algal gyttja. At most spots a thin layer of brown-moss peat covers the algal gyttja just a few cm over the tephra. Although the various sub-basins probably formed one large basin during periods

with high water-levels, the sediments attributed to the Allerød are not connected, indicating lower water-levels and the existence of separate basins. Core Reinberg C (REC) at the intersection of the cross-sections contains the LST at 191 cm depth. Core Reinberg I (REI) — located ca. 10 m further to the southeast — includes the LST at 176 cm below surface. Core Reinberg D (RED) originates from the marginal parts of the large northern sub-basin and includes the LST at 181 cm depth. A thin section from the Laacher See tephra was derived from a test-core at the same spot. 3. Research methods Cores REC and RED were retrieved with a “Usinger corer” (Livingstone corer modified by H. Usinger). Core REI and the testcore for RED were taken with a Russian chamber corer. The sample for micromorphological investigation was air dried, impregnated with Oldopal P80-21 and sliced into a 4.0 × 2.4 cm thin section following Beckmann (1997). It was examined at 25–400 times magnification under a petrological microscope and described using the terminology of Stoops (2003). Samples for diatom analysis were dried at 550 °C, boiled in 10% HCl and 10% H2O2, and mounted in Canada balsam. Counting was carried out with a light-microscope with magnifications up to 1200×. Identification, nomenclature, and palaeo-ecological interpretation follow Krammer (1992) and Krammer and Lange-Bertalot (1986, 1988, 1991a,b). Presentation of the diatom data involves the TGView 1.6.2-software (Grimm, 2004). The diatom frequencies represent percentages of all observed specimens; sponge spicules were calculated relative to the total number of diatoms. The curves (Figs. 3 and 4) present actual percentages (solid curves) and a five-time exaggeration (hollow curves with depth bars), and are ordered stratigraphically in order to facilitate a successional interpretation.

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Fig. 2. A: Thin section of the Laacher See tephra (LST) from a test-coring for core Reinberg D. Different layers are marked by numbers. The downward extension of layer 2 is artificial due to air drying of the sample. B: Vitric shard (“Faserbims”) in the LST (layer 2). C: Initial micro-layering in the sediments covering the LST (layer 3). Fig. 2C presents the upper left corner of Fig. 2A.

tephra-layer using a paper cutter (“Damocles”; cf. Joosten and De Klerk, 2007). The samples are referred to their depth (mm) below the top of the analysed trajectory. A known amount of spores of Lycopodium clavatum was added to the palynological samples for estimation of pollen concentrations (cf. Stockmarr, 1971). Sample preparation further included treatment with HCl, KOH, sieving (120 μm), acetolysis (7 min) and mounting in glycerine (REC) or silicone oil (REI) (cf. Fægri and Iversen, 1989). Counting was carried out with a light microscope at 400× magnification; larger magnifications were used for the identification of problematic grains. In order to differentiate clearly between inferred plant taxa and observed palynomorphological types, the latter are displayed in the text in SMALL CAPITALS (cf. Joosten and De Klerk, 2002). Pollen and spore types were identified and named after (f): Fægri and Iversen (1989); (m): Moore et al. (1991); (p): the Northwest European Pollen Flora (Punt, 1976; Punt and Clarke, 1980, 1981, 1984; Punt et al., 1988; Punt and Blackmore, 1991; Punt et al., 1995, 2003). Types not described in this literature are marked with an asterisk (⁎) and described by De Klerk (2002) and De Klerk et al. (2001). A distinction was made between JUNIPERUS TYPE pollen that contains conspicuous gemmae and can be unambiguously attributed to Juniperus, and JUNIPERUS-WITHOUT-GEMMAE that may also originate from similar-looking algal or bryophyte spores (cf. Moore, 1980). The curves ‘Total PINUS’ and ‘Total BETULA’ represent the sum of PINUS DIPLOXYLON TYPE, PINUS HAPLOXYLON TYPE and PINUS UNDIFF. TYPE, and the sum of BETULA PUBESCENS TYPE, BETULA NANA TYPE and BETULA UNDIFF. TYPE respectively. The curve ‘assumed exotic types’ is the sum of types produced by plants assumed to have been absent in NE Germany during the Lateglacial and to have resulted from long-distance transport (extraregional deposition sensu Janssen, 1973) or erosional redeposition (cf. Iversen, 1936). If reference is made to pollen types from other studies, in general the nomenclature of the original studies is used. Calculation and presentation of the palynological data are implemented by the software TILIA 1.12, TILIA GRAPH 1.18, and TGView 1.6.2 (Grimm, 1992, 2004). Pollen percentage values were calculated relative to a pollen sum including only types which, within the Lateglacial landscape, are attributable to dryland trees and shrubs (AP) and dryland herbs (NAP); the ratio AP/NAP indicates the relative openness of the landscape. Pollen types that might also be produced by wetland herbs (e.g. WILD GRASS GROUP and CYPERACEAE) were excluded from the pollen sum since these might falsely indicate an open dryland vegetation if that pollen stems from a wetland vegetation within the basin (cf. Janssen and IJzermansLutgerhorst, 1973; De Klerk, 2004). Pollen types are ordered stratigraphically in order to facilitate a successional interpretation. The pollen frequencies are displayed as percentage values or concentrations (solid curves) and a 5-times exaggeration (hollow curves with depth bars). The ‘sum’ numbers present the absolute figure of the pollen sum. Statistical analyses for studying the significance of fluctuations in diatom and pollen content were not carried out, because these changes are only small and likely to be statistically insignificant (cf. Lotter and Birks, 1993; Birks and Lotter, 1994, who also found some interesting fluctuations around the LST that were statistically insignificant). Instead a kind of indicator-species approach (cf. Birks and Birks, 1980) is used in which palaeoecological inferences derive from comparing the individual ecological requirements of single diatom taxa or inferred plant taxa. 4. Results and interpretation of the data from the Reinberg basin 4.1. Micromorphology

The palynological samples of core REC were taken volumetrically; they are referred to as the actual core depth (cm) below surface. Section REI was cut in slices of 0.5 mm thickness from a column with a length and width of ca. 1 cm with a more or less horizontal positioned

Three different layers can be distinguished in the thin section (Fig. 2A). Voids, with the exception of channels, are omitted from the following description since they are mainly artificial due to drying.

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Layer 1 (below the LST) contains a larger proportion of clastic material N5 μm than the LST (layer 2); grain sizes reach up to 300 μm. This layer contains glauconite. Abundant fine amorphic organic material is responsible for a brown discolouring. Larger plant remains, phytoliths, and diatoms are abundantly present. There is an incidental horizontal orientation of the coarser components and an initial microlayering. Roundish channels (up to 250 μm diameter) individually show as vertically smoothed ellipsoids. Volcanic material is hardly present. Layer 2 (LST, 0.5–1 cm thick) is clearly recognisable by its light colouring. A sharp boundary separates layers 1 and 2. The downward extension of the LST is an artefact due to air drying of the sample and is not a contradiction to the sharp boundary. The proportion of fine organic material and clastic material N5 μm is smaller than in layers 1 and 3. Identification of minerals is complicated due to the smaller grain sizes; glauconite, however, is absent. Typical is a flaky appearance of the groundmass (50–125 μm diameter of the roundish ‘flakes’) with in their centres a dark-brown discolouring (organic substance?). The individual components do not show any orientation. Large amounts of fine substances are isotropic with crossed polarizers. Plant remains, phytoliths, and diatoms are abundantly present. Channels occur only in the transitional range to layer 3 (with diameters 125– 250 μm). Vitric shards (‘faserbims’, Fig. 2B) have longitudinal axes of around 100 μm; they contain regularly mineral enclosures and alternate with elongated bubbles. These shards are similar to type b2 of Van den Bogaard and Schmincke (1985). Layer 2 grades diffusely into layer 3. Layer 3 (above the LST) contains more clastic material N5 μm (up to 150–300 μm) than the LST. The layer contains glauconite. Abundant fine organic particles again caused a brown discolouring. Plant remains, phytoliths, and diatoms are abundantly present. The local horizontal orientation of larger components shows initial microlayering (Fig. 2C). Vitric shards similar to type b2 of Van den Bogaard and Schmincke (1985) have longitudinal axes of around 100 μm; they contain regularly mineral enclosures and alternate with elongated bubbles. The sharp boundary between layer 1 and the LST (layer 2) in the thin section, as well as the virtual absence of volcanic material in layer 1, shows that these layers were hardly mixed after deposition of the tephra. This indicates the absence of biotic activity, which is also shown by the absence of channels in layer 2. The tephra is characterised by the flaky appearance of the ground mass and a large amount of fine particles b5 μm. The fine fabric b5 μm, appearing isotropic under crossed polarizers, consists probably mainly of ash particles; this also explains the small amount of clastic material N5 μm. The ash particles b5 μm are partly weathered into clay minerals. These are, however, largely hidden by the dispersed organic substance due to which the ground mass appears isotropic instead of stipple speckled b-fabric typical for newly formed clay minerals (cf. Stoops, 2003). Glauconite is a product of submarine weathering processes (Matthes, 2001) and should, therefore, be omnipresent in the glacial and glacifluvial sediments of NE Germany that were transported by the Weichselian glaciers from the Baltic basin (Duphorn et al., 1995). Glauconite, therefore, would have been mixed with the volcanic ashes if these had been washed-in from the surrounding basin slopes. Its absence in the pure tephra (layer 2) indicates that the bottom 0.5 cm of the LST was deposited from atmospheric input into the basin, i.e. was not washed-in. The thickness of the LST is 0.5 to 1 cm after compression by the overlying sediments and peat: the original thickness prior to compression will have been larger. No minerals were found in layer 2 that are unequivocally attributable to the LST (cf. Van den Bogaard and Schmincke, 1985). The low content of mafic minerals is typical for the north-eastern fans of the tephra layers LLST and MLST-B as described from NW Poland (Juvigné et al., 1995). The smaller size of the vitric shards of 100 μm in the Reinberg sample compared to the 180 μm diameter described from NW Poland (measured on grain samples) is mainly due to the fact that

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in thin sections minerals are predominantly not cut along their longest axes: grain sizes are, therefore, normally underestimated (Harrel, 1984). The gradual transition from layer 2 to 3 is mainly attributable to an increase in dispersed organic material. Since glauconite and vitric shards occur simultaneously in layer 3, a mixing of tephra and the covering sediments must have occurred, either by tephra and till material washed-in together from the surrounding slopes or by bioturbation. Since bioturbation did not play a major role at the base of the LST, it is unlikely that it was an important mechanism: washing-in, therefore, may have been the major cause for the observed phenomenon. 4.2. Diatom analysis The low concentration of dissolved SiO2 in oligotrophic water does not only limit the expansion of diatom populations, but also favours the dissolution of the SiO2-valves of dead diatoms (cf. Harwood, 1999). The lowest diatom zones REC-dia1 and RED-dia1 (Figs. 3 and 4) are scarce in diatoms as a result of dissolution: only the most robust taxa (e.g. some Pinnularia) and sponge spicules were preserved. Additional mechanical force caused the large Pinnularia species to break into many indeterminable small parts. The typical roundish shape of the fractures suggests either frost damage as result of the freezing of the lake floor in winter, or strong wave dynamics in a littoral zone. Since the Reinberg sub-basins were too small for generating substantial wave dynamics, the damage can be attributed to regular freezing of the shallow lake floor (the single freezing and thawing event due to the storage of the cores in a refrigerator is insufficient to have caused this damage). Most taxa observed in the lowest diatom zones are epipelic (i.e. living in and on sediments such as gyttja). Stauroneis anceps, Navicula cryptocephala, Tabellaria flocculosa, and Pinnularia viridis point to dystrophic–oligotrophic to mesotrophic, slightly acid to neutral water, with a low to moderate specific conductivity. A major change occurs in the samples 1 to 2 cm below the LST (diatom zones REC-dia2a and RED-dia2a): conspicuously more taxa occur and the percentages of indeterminable Pinnularia remains and sponge spicules decrease. Most observed taxa are currently epipelic. The occurrence of some epiphytic taxa (i.e. living on plants) in RED (e.g. Eunotia praerupta, Gomphonema acuminatum and G. hebridense) shows that a vegetation of either water plants or lake-shore plants inhabited the northern sub-basin. Most observed taxa occur in dystrophic–oligotrophic to mesotrophic slightly acid to neutral water with a low to moderate specific conductivity. In spite of the richer diatom flora, the environment seems not to have changed conspicuously. The larger diversity of the diatom flora in the 2 cm below the LST, therefore, may result from changes in preservation: also the more vulnerable taxa were preserved. The sudden input of silica by the tephra will have prevented dissolution of the diatoms buried in the upper 2 cm of the underlying sediments (cf. Harwood, 1999; Sancetta, 1999). Since no concentrations can be estimated for the diatom data of cores REC and RED, the data provide no information about the size of the populations before and after the deposition of the LST. Quantitative data of total diatoms (however without identification of the taxa) from core REI (cf. Figs. 5 and 6) show a marked increase in the diatom concentration between approximately 1.5 and 4.5 mm above the tephra. This shows that the nutrient input of silica caused diatom populations to expand (cf. Harper et al., 1986; Hickman and Reasoner, 1994). The reduction of solution of the silicate frustules — although resulting in the greater taxonomic variation below the tephra — was thus only a minor effect. The diatom content of the LST itself was studied only in core RED. Numerous taxa are restricted to the tephra, whereas the trajectory immediately above the tephra (zones REC-dia2b and RED-dia2b) shows a different spectrum. Epilithically living taxa (i.e. attached to stones and sand), e.g. Rhopalodia gibba, and many epipelic taxa that

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Fig. 3. Diatom diagram of core Reinberg C (REC), selected levels. Diatom frequencies are calculated relative to all observed taxa. The curves are displayed with real values (closed curves) and a 5-times exaggeration.

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Fig. 4. (A/B): Diatom diagram of core Reinberg D (RED).

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Fig. 4 (continued ).

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P. de Klerk et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 196–214 Fig. 5. Pollen percentage diagram of core Reinberg I (REI), selected types, calculated relative to a dryland pollen sum. The curves are displayed with real values (closed curves) and a 5-times exaggeration. For pollen type nomenclature: see text. 203

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Fig. 6. Pollen concentration diagram of core Reinberg I (REI).

peak within the LST found a more favourable habitat in the porous tephra than in the gyttja below and above the tephra. The REC-samples above the tephra (Figs. 3 and 4) contain high values of taxa that indicate water with a low to moderate specific conductivity (e.g. Pinnularia cardinalis, P. gibba, P. lata, P. major, P. nobilis, P. viridis). Whereas the dominant taxa in RED are also indicative for low to moderate specific conductivity (e.g. Cymbella minuta, Fragilaria capucina (amphicepala tribes), Navicula cryptocephala, Pinnularia brevicostata, P. subcapitata, P. viridis, Stauroneis anceps, and Tabellaria flocculosa), some taxa were found that occur in water with a moderate to high specific conductivity (e.g. Cocconeis pediculus, Cymbella helvetica, Diploneis domblittensis, Epithemia frickei, Fragilaria elliptica, and Neidum ampliatum). This indicates that specific conductivity was moderate, and points to a (minor) enrichment of the northern sub-basin with ions after deposition of the LST. Although various taxa occur that currently inhabit oligotrophic to eutrophic environments, the abundant presence of taxa that are

restricted to dystrophic–oligotrophic and mesotrophic conditions and the absence of taxa that only appear in eutrophic environments indicate that the deposition of the LST did not cause a major eutrophication in the Reinberg basin. Most taxa observed in and above the LST require neutral to slightly acid conditions. Only Neidum ampliatum (observed in both cores and in RED already rising within the tephra) and Pinnularia interrupta (observed in REC) have their optimum in low pH-ranges (Bigler et al., 2000). Since these taxa only occur in low amounts, acidification of the Reinberg basin after the Laacher see eruption can only have been very slight. Similarly, peaks of Eunotia sp. (in REC; Fig. 3) and Eunotia praerupta (in RED; Fig. 4) above the tephra may indicate that the basin had become slightly more acid. In RED, the diatom spectra change approximately 2 cm above the tephra. The increased amounts of sponge spicules and indeterminable Pinnularia remains in zone RED-dia3 show that again mainly the most robust taxa were preserved as a result of high dissolution of diatom frustules. Also in REC the amount of sponge spicules increases in the

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Fig. 7. Pollen percentage diagram of core Reinberg C (REC).

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Fig. 8. Pollen concentration diagram of core Reinberg C (REC).

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second sample above the LST. The increased dissolution results from the covering of the LST by a new gyttja layer which prevented extraction of silica from the tephra. Dominant taxa in RED are Cymbella minuta, Epithemia frickei, Eunotia praerupta, Pinnularia cardinalis, P. viridis, Stauroneis anceps, S. phoenicenteron, and especially E. praerupta (curta tribes), and Pinnularia nobilis. These taxa indicate dystrophic–oligotrophic to mesotrophic conditions in a neutral to slightly acid environment with low to moderate specific content. Most of these taxa are epipelic. High values of the epiphytic Eunotia praerupta indicate that aquatic or lake-shore plants grew abundantly in the basin. The upper zone of the REC-diagram (REC-dia3) shows the diatom population of the layer of brown-moss peat in the middle sub-basin (cf. De Klerk et al., in press). 4.3. Pollen analysis The general vegetation development around the Reinberg basin during the Allerød, i.e. the “Lateglacial Betula/Pinus forest phase” sensu De Klerk (2002, 2008) can be reconstructed from the pollen diagram of core REC (Figs. 7 and 8). The first expansion of birch forests is shown by an increase of BETULA PUBESCENS TYPE in sample 216. NAP values and values of HIPPOPHAË RHAMNOIDES and JUNIPERUS TYPE/JUNIPERUSWITHOUT-GEMMAE decrease in the higher samples 213 and 212 and demonstrate that the birch tree vegetation needed some time to form closed forests. Afterwards, Betula trees formed the dominant forest element whereas Pinus was only incidentally present in the surroundings of the Reinberg basin (Theuerkauf, 2003; Theuerkauf and Joosten, in press; De Klerk et al., in press). Prominent lower values of BETULA PUBESCENS TYPE and higher values of PINUS DIPLOXYLON TYPE between samples 200 and 192 are interpreted to represent the Gerzensee fluctuation: due to lower temperatures the Betula tree vegetation or Betula tree pollen production were reduced and consequently the relative values of PINUS DIPLOXYLON TYPE increased (cf. De Klerk, 2002, 2008; De Klerk et al., in press). Towards the end of the Lateglacial Betula/Pinus forest phase BETULA PUBESCENS TYPE proportions increase again and PINUS DIPLOXYLON TYPE values decrease. At the end of the vegetation phase a brown-moss peat was formed in the middle sub-basin as a consequence of a floating mat terrestrialisation (De Klerk et al., in press). This brown-moss peat contains the rise in NAP pollen types that marks the beginning of the Younger Dryas (“Open vegetation phase III” sensu De Klerk, 2002, 2008). At the same level as the decrease of PINUS pollen in diagram REC (i.e. above sample 194), pollen concentrations start to decrease (cf. Fig. 8). Their absolute minimum is in the sample immediately above the LST. Also in REI (Figs. 5 and 6) pollen concentrations decrease prominently between the lowest two samples and show a minimum in the lowest sample above the LST. Increases of pollen production with rising temperatures at the end of the Gerzensee fluctuation would normally have resulted in higher pollen concentrations: however, the concentrations decrease, indicating that increased sediment accumulation rates outweighed the effect of increased pollen production. This rise in sediment accumulation rates results from increased biomass production due to the higher temperatures. The presence of pollen-free tephra material is the reason for the extreme low pollen concentration in sample 190.5 in core REC. Also in the REI-diagram pollen concentrations directly above the tephra are lower than below the LST (Fig. 6), but to a smaller extent than in REC as consequence of the very precise sampling with the Damoclesdevice. In REC, the relative values of BETULA NANA TYPE pollen rise in the sample immediately below the tephra. In sample 190.5, i.e. the sample directly over the tephra, PINUS DIPLOXYLON TYPE and PINUS UNDIFF. TYPE decrease to almost zero whereas a peak of TOTAL BETULA occurs (mainly resulting from an increase of BETULA UNDIFF. TYPE pollen: relative values of BETULA PUBESCENS TYPE and BETULA NANA TYPE remain at a similar level as in the sample underlying the tephra). Since pollen

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produced by tree birches have a more protruding pore than pollen produced by Betula shrubs (cf. Punt et al., 2003), a minor corrosion of the pore area would still allow the recognition of pores of the BETULA PUBESCENS TYPE as protruding and, therefore, the identification of this pollen type. A similar degree of corrosion of the pore area of BETULA NANA TYPE pollen, on the other hand, would immediately result in pores of which the original protrusion cannot be accurately estimated, i.e. an unambiguous identification of the type is not possible. It seems, therefore, plausible that the BETULA UNDIFF. TYPE peak mainly includes pollen from shrub birches. Also JUNIPERUS TYPE and JUNIPERUS-WITHOUT-GEMMAE have higher relative values above than below the tephra. In the next sample 190, i.e. the second sample over the tephra, PINUS DIPLOXYLON TYPE and PINUS UNDIFF. TYPE reaches the values of the remaining part of the Allerød. This sample contains a peak of ARTEMISIA. Values of JUNIPERUS-WITHOUT-GEMMAE and BETULA NANA TYPE remain high and first decrease in sample 189, i.e. the third sample over the LST. This indicates that small-scale openings may have occurred in the forest vegetation around the basin following the volcanic eruption. Strangely, the pollen fluctuations that are so clearly visible in REC (Fig. 7) do not occur in pollen diagram REI (Fig. 5): almost all types remain at a similar level throughout the complete analysed section. Only a minor decrease of PINUS UNDIFF. TYPE and PINUS DIPLOXYLON TYPE and a small increase in BETULA PUBESCENS TYPE occur in sample 7 (i.e. 1.5 mm over the tephra). Also values of ARTEMISIA have increased over the LST. BETULA NANA TYPE and JUNIPERUS TYPE occur with very low values over the tephra and are absent below it. The absence of these fluctuations in REI is difficult to explain: generally in lake sediments seasonal water circulation causes regular redeposition of the sediment resulting in similar pollen frequencies within the complete basin (cf. Davis, 1968). Obviously, such a mixing had not occurred in the Reinberg middle sub-basin, probably because it was too small in the relevant time period (ca. 50 × 40 m) for redeposition processes similar as in large lake basins. Also the sharp base of the LST in the thin section (cf. Section 4.1.) shows no mixing of sediments. A possible explanation for the differences between diagrams REC and REI is that core REC is located within the extralocal pollen deposition trajectory (sensu Janssen, 1973) of Juniperus and Betula shrubs, whereas core REI lies outside such a trajectory, which is in accordance with the fact that core REI was derived at a more central position within the middle subbasin. It seems unlikely, however, that shrubs with fewer flowers than trees (resulting from their limited height) had a sufficiently high pollen production to suppress completely the regional pollen deposition signal of pine trees. A satisfactory explanation for the differences, thus, cannot be given. Prominent in diagram REI is an increase of charcoal above the LST, indicating an increase of fires in the surroundings of the Reinberg basin after the volcanic eruption. These fires will have created minor open areas in which a shrub vegetation of Juniperus and Betula nana could have expanded temporarily. This phenomenon is only visible in the REI-data because the pollen samples of core REC were not screened for charcoal and the particles were too small to be observed in the micromorphological study. 5. Discussion: comparison with other studies and regional palaeoenvironmental interpretation 5.1. Deposition of the tephra, specific conductivity, and acidification A similar sharp transition at the base of the LST as observed in the thin section from the Reinberg basin was macroscopically observed at several other sites in NE Germany (Theuerkauf, 2003). It proves that the predominant absence of bioturbatic mixing of the LST with underlying sediments is a widespread phenomenon: in general, thus, the tephra was deposited undisturbed on the basin floors. That washing-in of tephra particles from the surrounding basin slopes after

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the deposition of the LST was also a widespread phenomenon is confirmed by the occurrence of volcanic ash up to 11 mm above the actual LST at many localities (Theuerkauf, 2003). Heinrichs et al. (1999) found that ionic concentrations of water rose after the deposition of a tephra, which is in good accordance with the data from the Reinberg basin. This effect is enlarged since tephra itself also conducts electricity (cf. Kenedi et al., 2000). A slight acidification after, but not necessarily connected with the deposition of the LST, was found in lakes in S Germany and Switzerland (cf. Lotter and Birks, 1993; Birks and Lotter, 1994). Since acidification can be expected to have caused a decrease of CaCO3 precipitation, the lime content of sediments forms another proxy for acidification. Whereas the relevant sediments in the Reinberg basin are non-calcareous (cf. De Klerk et al., in press), a core from the nearby Endinger Bruch shows a conspicuous decrease in CaCO3-content at a level that palynostratigraphically corresponds with the position of the LST in the Reinberg basin (although the tephra was not macroscopically found here; cf. De Klerk, 2002). This can be an indication that acidification after the eruption was a regional phenomenon. Also in a basin in Brandenburg (NE Germany) the CaCO3-content of the sediments decreases above the LST (Brande et al., 1990), but since the lime content was already decreasing below the LST, it is not necessarily connected to the volcanic eruption: the decrease of the lime content may be connected to ongoing pedogenesis and decalcification of the soils in the catchment area (cf. Kühn, 2003; Kühn et al., 2006) leading to a water supply with a reduced carbonate content. 5.2. Short-lived changes in vegetation Small pollen fluctuations similar (but not identical) to that of the REC-pollen diagram occur in many pollen diagrams from NE Germany. Increasing values of BETULA and decreasing values of PINUS are a general phenomenon around the LST in NE Germany (e.g. Müller, 1961a,b, 1965, 1967; Lange et al., 1986; Böse et al., 1993; Gärtner, 1998; Wolters, 1999, 2002; Mathews, 2000; Peterss et al., 2002; Theuerkauf, 2002, 2003; De Klerk and Helbig, 2006; cf. Fig. 9). Most of these pollen diagrams, however, do not show changes in other pollen types. If other types occur with a peak, these are generally different pollen types at different locations, e.g. CYPERACEAE (Müller, 1961b, 1965; Peterss et al., 2002; Theuerkauf, 2003; cf. Fig. 9A), ARTEMISIA (Müller, 1961b; Lange et al., 1986; Theuerkauf, 2003; De Klerk and Helbig, 2006; cf. Fig. 9B/C), CHENOPODIACEAE and AMARANTHACEAE (Theuerkauf, 2003; cf. Fig. 9A), LYCOPODIUM ANNOTINUM (Lange et al., 1986; cf. Fig. 9C), SALIX (Müller, 1961b; Theuerkauf, 2003), JUNIPERUS (this study; Müller, 1961b) and RUMEX ACETOSELLA (De Klerk and Helbig, 2006; cf. Fig. 9B). Several of these pollen/spore types are produced by taxa from both dry and wet habitats, whereas others are indisputably produced by dryland taxa that can occur as pioneer plants at open spots. In some cases it is difficult to correlate these fluctuations positively to the Laacher See eruption or the deposition of the LST, since incidentally they already seem to start below the tephra. This is the case with BETULA NANA TYPE pollen in Reinberg C (Fig. 7), with BETULA and PINUS in diagram Horst 2/2 (Fig. 9B), and with ARTEMISIA in diagram Plauer Stadtwald (Fig. 9A). These pollen types probably reflect (minor) changes in the vegetation at the end of the Gerzensee fluctuation. If all pollen fluctuations were independent of the Laacher See eruption, however, these would all show as a gradual uninterrupted trend, which is not the case: the gradual trends in the relevant pollen diagrams are interrupted above the LST by other superimposed fluctuations that are likely to be connected to the Laacher See eruption or the deposition of the LST. The fluctuations predominantly occur in pollen diagrams from small basins, indicating that these mainly represent (extra)local pollen deposition of the parent plants directly surrounding the studied basins. This explains why the effect is absent from many large basins

in NE Germany where the trajectory above the LST has been studied with sufficient sample resolution for such fluctuations to have been found (cf. Müller, 1961b; Kleissle and Müller, 1969; Kloss, 1980; Zerbe et al., 2000). The small pollen fluctuations, however, are also not recorded in several other small basins with sufficient sample resolution above the LST (cf. Müller, 1961b; Brande et al., 1990; Theuerkauf, 2003). This shows that the opening of the vegetation is not a general feature but occurred at plots scattered throughout the region. Even patterns deviating from the general trends occur, such as fluctuations several samples above the LST instead of in the direct overlying sample (Kloss, 1994), a fluctuation with increased PINUS and decreased BETULA values above the LST instead of the other way around (Schulz and Strahl, 2001), and a prominent increase in BETULA pollen already at some depth below the LST (e.g. Müller, 1965; Böcker et al., 1986; Böse et al., 1993; Wolters, 2002). Interpolating the age of rise in BETULA pollen in these latter studies between the beginning of the Allerød and the LST suggest that they all date differently, indicating that these do in fact reflect different phenomena. This rise in BETULA pollen might result from the expansion of birch carrs along the basin margins that is not synchronous in the different basins (cf. Wolters, 2002; Theuerkauf, 2003; De Klerk, 2008). Several causes are imaginable for the small fluctuations directly over the LST. Unlikely, however, is a reaction to decreased temperatures in the Northern Hemisphere because the duration and magnitude will have been too small (cf. Section 1). In addition, a reaction to an eutrophication of the environment (cf. Edmondson, 1984) will probably not have occurred since such an eutrophication is not indicated in the diatom record of the Reinberg basin (cf. Section 4.2). The hypothesis of Müller (1961b) that the dryland tree vegetation suffered from ashfall (thus opening a niche for pioneer vegetation types) can be rejected, since trees were observed to have hardly been affected by a 15 cm thick ash deposit of the 1980 Mount Saint Helens eruption (cf. Mack, 1981; Zobel and Antos, 1997). Possible is a dying of trees due to acidification of the environment, thus opening a niche for pioneer vegetation types. Historical sources indicate extensive damage to the vegetation in Europe by acid volatiles in the lower atmosphere after the Icelandic AD 1783 Laki eruption (Grattan and Charman, 1994; Brayshay and Grattan, 1999). Since the diatoms of the Reinberg basin also indicate an acidification after the Laacher See eruption, such an effect may have played a role. Since this acidification can only have been slight (cf. Section 4.2), the effect can only have been minor. The wetland vegetation in NE Germany might have reacted to rises in water-levels as a result of the assumed increased precipitation after the Laacher See eruption (cf. Schmincke et al., 1999; Baales et al., 2002). An actual increase in precipitation in NE Germany is proven by two phenomena. 1) Small peaks of assumed exotic pollen directly over the LST at several localities, e.g. Reinberg C (Fig. 7), and in the studies of Müller (1961b) and Peterss et al. (2002), are attributable to increased soil erosion after the Laacher See eruption. Such increased soil erosion may have resulted from surface runoff induced by extreme precipitation. Higher soil erosion after the deposition of the LST is also reported in other studies (Merkt, 1991; Merkt and Müller, 1999), indicating that increased precipitation occurred widely in northern Germany. 2) A small gyttja layer immediately covering the LST at locations where the tephra is embedded in peat shows that water-levels of basins without outflows had risen, e.g. Horst 2 (Fig. 9B) and Crednersee (Fig. 9C). For such a rise only increased precipitation can be responsible: short-lived increased precipitation strongly influences water-levels in basins without outlet (Timmermann, 1999; Theuerkauf, 2003). After water-levels had fallen again, renewed peat formation occurred at the Horst locality (Fig. 9B). The accumulation rate of the gyttja at the Crednersee (Fig. 9C) was so large that the fluctuations of the pollen types are recorded in several samples instead of just one. In one case in northern Brandenburg peat underlies the tephra, but only gyttja occurs in the total Lateglacial section over the tephra (Endtmann,1998), indicating intense inundation

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Fig. 9. A: Pollen percentage diagram from Plauer Stadtwald, selected types modified after Theuerkauf (2002); B: Pollen percentage diagram of core Horst 2/2,modified after De Klerk and Helbig (2006); C: Pollen percentage diagram Crednersee, modified after Lange et al. (1986); the LST was erroneously omitted in the original study, but could be inferred from Kliewe (1995) who stated that the LST was dated at 11,018 ±120 14C years B.P: this date was included in the original diagram; D: Pollen percentage diagram from Schwarzer See, modified after Müller (1961b).

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Fig. 9 (continued ).

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Fig. 9 (continued ).

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Fig. 9 (continued ).

after the Laacher See eruption prohibiting renewed peat formation. Similar rises in water-levels will have occurred in basins where the LST is fully embedded in gyttja (such as the Reinberg basin), but these cannot be inferred from the sediment record because a lithological change does not occur. For several of such localities, however, palaeobotanical evidence is found (listed by Theuerkauf, 2003) that demonstrates inundation after the Laacher See eruption. The observed fluctuations in pollen attributable to Betula, Salix and Cyperaceae above the LST might result from an expansion of these taxa on moist dried-up areas after water-levels had fallen again (cf. Theuerkauf, 2003). This does not explain, however, the peaks of pollen types that unambiguously originate from dryland plants: pioneer vegetation types must have expanded on the dry grounds as well. A process that immediately creates space for pioneer vegetations is fire. In the Reinberg basin an increase of charcoal particles above the LST demonstrates that here fires had actually occurred after the Laacher See eruption. Unfortunately no charcoal particles were studied in any of the relevant pollen diagrams from NE Germany, with exception of the sites of Theuerkauf (2003): these all contain an increase in charcoal particles above the LST (cf. Figs. 5, 6 and 9A), showing that an increase in forest

fires after the Laacher See eruption occurred widely in NE Germany. Increased amounts of charcoal particles above tephra layers are also reported from New Zealand (Wilmshurst and McGlone,1996; Newnham et al., 1998). At large distances from the erupting volcanoes, only an increase in thunderstorm-frequency and connected lightning can be responsible for increased forest fires, which are also reported as an effect of some recent volcanic eruptions (e.g. Anderson et al., 1965; Christiansen, 1980) and which probably relates to the specific conductivity of volcanic ash (cf. Kenedi et al., 2000). These observations, however, were only made at relative short distances to the volcano, not over hundreds of kilometres. It is, however, known that after the AD 1783 sulphur-rich Laki eruption tremendous thunderstorms swept over Europe, of which an unusual high amount of ball lightning regularly set on fire in buildings and agricultural fields (Grattan and Brayshay, 1995; Brayshay and Grattan, 1999). This shows that increased lightning can actually occur at several hundreds of km distance from the erupting volcano. This was, thus, very likely also the case in NE Germany after the Laacher See eruption. Next to increased ignition resulting from lightning, the abundant forests during the Allerød provided a large amount of inflammable material, especially when the tree vegetation was

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additionally affected by acidification and soil erosion. The enlarged precipitation, on the other hand, will have controlled the spatial restriction of the fires without allowing large areas to burn down. 6. Conclusions and summary Climate and environment were already unstable in the area of NE Germany prior to the Laacher See eruption. The minor cooler Gerzensee fluctuation came to an end and temperatures had started to rise. At several localities local hydrological conditions might have triggered the expansion of birch carrs along the basin margins. When the Laacher See volcano erupted around 12,900 cal. yr. B.P., a tephra layer of uncompacted thickness over 0.5 cm thickness covered NE Germany. In limnic environments the bottom part of the Laacher See tephra was deposited from atmospheric input by the fall-out itself, whereas the upper part was washed-in from the surrounding catchments. Limnic diatom populations profited greatly from the input of silica by the tephra, which functioned as a nutrient source for the building of frustules and simultaneously prevented dissolution of dead diatoms. The tephra also provided a favourable habitat for many epipelic and epilithic taxa. This effect was only temporal and terminated when the LST was covered by new lake sediments. The water became temporarily slightly more acid and had a slightly higher specific conductivity. Although the eruption of the Laacher See volcano certainly caused temperatures to decrease, this decrease was too small and too short to greatly affect vegetation and environment. Eutrophication and direct damage by the tephra to the vegetation also seem not to have played a role in NE Germany. Temporarily increased precipitation caused increased soil erosion and rises of water-levels. A pioneer vegetation of plants growing in moist environments might have expanded on moist areas that emerged after water-levels had fallen again. Dryland pioneer plants inhabited sites that were opened by fire as the consequence of increased lightning during violent thunderstorms. Acknowledgments This research was partly financed by the Deutsche Forschungsgemeinschaft (DFG) (Project Bi 560/1–5 “Specification of the earliest vegetation development at the site Reinberg (time slice I)” within the priority program “Changes in the geo-biosphere during the last 15,000 years”) and supervised by Konrad Billwitz, was partly part of a MSc-thesis at the Institute of Botany and Landscape Ecology of the Greifswald University supervised by Hans Joosten, and was partly carried out voluntarily. Pollen and diatom samples were prepared by Henrik Helbig, Hannelore Rabe and Erika Retzlaff. Brigitta Ammann, Hans Joosten and an anonymous reviewer are greatly acknowledged for the valuable comments on the text. References Anderson, R., Björnsson, S., Blanchard, D.C., Gathman, S., Hughes, J., Jónasson, S., Moore, C.B., Survilas, H.J., Vonnegut, B., 1965. Electricity in volcanic clouds. Science 148, 1179–1189. Baales, M., Jöris, O., Street, M., Bitmann, F., Weninger, B., Wiethold, J., 2002. Impact of the Late Glacial eruption of the Laacher See volcano, central Rhineland, Germany. Quaternary Research 58, 273–288. Beckmann, T., 1997. Präparation bodenkundlicher Dünnschliffe für mikromorphologische Untersuchungen. Hohenheimer Bodenkundliche Hefte 40, 89–103. Bertrand, C., Van Ypersele, J.-P., Berger, A., 1999. Volcanic and solar impacts on climate since 1700. Climate Dynamics 15, 355–367. Bigler, C., Hall, R.I., Renberg, I., 2000. A diatom-training set for palaeoclimatic inferences from lakes in northern Sweden. Verhandlungen Internationale Verein Limnologie 27, 1–9. Birks, H.J.B., Birks, H.H., 1980. Quaternary Palaeoecology. Edward Arnold, London. Birks, H.J.B., Lotter, A.F., 1994. The impact of the Laacher See Volcano (11000 yr B.P.) on terrestrial vegetation and diatoms. Journal of Paleolimnology 11, 313–322. Böcker, R., Brande, A., Sukopp, H., 1986. Das Postfenn im Berliner Grunewald. Abhandlungen aus dem Westfälischen Museum für Naturkunde 48, 417–432.

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