The record of climatic change in the geological archives of shallow marine, coastal, and adjacent lowland areas of Northern Germany

Quaternary Science Reviews 22 (2003) 101–124

The record of climatic change in the geological archives of shallow marine, coastal, and adjacent lowland areas of Northern Germany b . G. Gerdesa, B.E.M. Petzelbergerc, B.M. Scholz-Bottcher , H. Streifd,* b

a Marine Laboratory of the Institute of Chemistry and Biology of the Marine Environment, Schleusenstra
e 1, D 26382 Wilhelmshaven, Germany Institute of Chemistry and Biology of the Marine Environment, University of Oldenburg, Carl von Ossietzki-Stra
e 9-11, D 26111 Oldenburg, Germany c Rua Nunes Valente, 1571; Apt. 501; 60125-070 Fortaleza-CE, Brazil d Niedersachsisches Landesamt fur . . Bodenforschung, Stilleweg 2, D 30655 Hannover, Germany

Abstract The record of climatic change in shallow marine, coastal, and adjacent lowland areas has been investigated by three different approaches. A mass balance study focused on the interaction between sea-level rise and Holocene sediment accumulation in the coastal lowland area between the Ems and Weser rivers on the German North Sea coast. This region, which comprises various sedimentary environments, such as barrier islands, sheltered and open tidal flats, bay flats, and estuaries, is highly suitable for such quantitative studies, which can be used to create a model for general mass transport and accumulation processes connected with transgressions over coastal lowlands. An integrated geochemical and microfacies study was made to assess the response of shallow marine, intertidal, and limnicsemiterrestrial environments to the climate-controlled Holocene sea-level rise. The factors controlling the development of various palaeoenvironments were estimated from the distribution of biomarkers, major and trace elements, diatoms, foraminifera and sedimentary structures observed at high resolution in core sections. These data complement those of conventional geological, lithostratigraphical, archaeological and geobotanical investigations. The extensive raised bogs which occur in the Pleistocene hinterland, adjacent to the coastal zone, provided an excellent opportunity to examine peat formation in response to climatic changes in the past. On the basis of a large number of 14 C-age determinations special attention was paid to the onset and regional expansion of raised bogs in this region and to the question of whether or not the formation of raised bog peat started synchronously in one or in a series of different phases. r 2002 Published by Elsevier Science Ltd.

1. Introduction Cyclic climatic variations between phases of cold (glacial) and phases of warm (interglacial) climate are characteristic for the Quaternary period of the last 2.6 million years. Superimposed on these major cycles are minor oscillations of moderately warm climate or short phases of warm climate, the so called interstadial periods. Besides the great variety of other effects produced by climatic variations, they also had significant influence on the earth’s ice–water balance. During the stages of cold climate, precipitation was partly trapped in the growing inland-ice sheets which resulted in a global lowering of sea level. In contrast, during the warm climate phases, melting of the inland-ice sheets caused a rise and high stands of sea level. Thus, the *Corresponding author. E-mail address: [email protected] (H. Streif). 0277-3791/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 2 7 7 - 3 7 9 1 ( 0 2 ) 0 0 1 8 3 - X

variation between glacial and interglacial conditions is reflected in sea-level changes with a vertical amplitude of about 100 m (Jelgersma, 1979; Long et al., 1988). These processes, which were triggered by external forces, had major effects on shoreline displacements as well as on the geo- and biosphere of shallow marine and adjacent lowland areas. The record of these changes can be analysed in the geological archives of shallow marine and coastal environments as well as in the adjacent lowland areas. During the phase of extreme climatic deterioration of the Weichselian glacial maximum, which lasted from about 22,000 to 18,000 years 14 C BP, the sea level of the North Sea was lowered to about 110–130 m below its present-day level. Due to the smooth relief which is characteristic for the North Sea basin, such a lowering of the sea level led to a seaward retreat of the coastline over a distance of about 600 km: Extended portions of the formerly shallow marine area became dry land and

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were exposed to periglacial conditions. In contrast, climatic amelioration in the subsequent deglacial hemicycle of the Weichselian late glacial and Holocene periods initiated a sea-level rise of about 130 m: This caused a landward and upward shift of the coastline leading to an accumulation of sediments in the marine and coastal environments. As the marine inundation progressed, extended areas of the present-day North Sea shelf passed through several stages of development. These started with increased wetness and swampiness, bog growth and peat formation, then turned into lagoonal, brackish, tidal flat sedimentation, and finally ended with shallow marine conditions. From about 7500 14 C BP onwards, during the final stage of the Holocene sea-level rise, the geological development of the coastal landscape began, and from about 2000 14 C BP the present-day pattern of geomorphological elements, such as the barrier islands, tidal flats, and coastal lowlands, were formed (Fig. 1). Information about the sea-level rise and related changes of the geo-biosphere are documented in the internal make up as well as in the sedimentological, chemical and biological characteristics of shallow marine and coastal deposits. These have been investigated by different approaches. A mass-balance study aimed at a quantification of coastal accumulation processes of the last 8500 years has been made. In addition to that, an integrated geochemical and microfacies analysis was focused on an assessment of the sedimentological characteristics and of the depositional environments which temporarily existed in the coastal zone. The hilly hinterland of the coastal zone, built up of Pleistocene glacial, glaciofluvial, fluvial and eolian deposits, was only indirectly influenced by the sea-level rise. In this landscape, the landward shift of the zone of oceanic climate which coincided with the shoreline displacement, led to drastic changes of the vegetation cover. The primary forest vegetation was replaced by regionally expanding bogs. Therefore, studies were carried out on the onset and progress of the formation of raised bog peat.

2. Mass balance study of the coastal Holocene A major aim of the mass-balance study was to determine the total volume of deposits which accumulated in the transition zone between the North Sea and Pleistocene hinterland as a result of the middle to late Holocene transgression. Additional interest was paid to quantify the import of clastic components from marine or riverine sources and the amount of organic material accumulated in situ in peat bogs within the coastal marshland area. The results were compared with the suspension load of the Ems, Weser, and Elbe rivers during the last 8500 years to estimate the mass transport from river

catchment areas into the coastal zone. The relationship between the rate of sea-level rise and the accumulation of coastal deposits was also investigated. The study was carried out in the coastal lowlands from the Netherlands to the eastern margin of the lower Weser river valley (Fig. 1). Extending across the northern part of the area are the East Friesian barrier islands and sheltered tidal flats on their landward side. To the east both units merge into open tidal flats which extend from the tidal inlet of the Jade as far as the Cuxhaven area. Additional morphological elements are the Dollart and Jadebusen bay flats, the Ems and Weser estuaries, and extended areas of coastal marshland. Thus, the study area can serve as a model for general mass transport and accumulation of sediment connected with transgression over lowland areas. 2.1. The sedimentary pattern in the offshore area and in the coastal zone At the bottom of the North Sea, the marine transgression of the late Weichselian to Holocene deglacial hemicycle caused major erosion, mass-transport, and redeposition. It left behind a layer of marine fine- to medium-grained shell rich sand, which in general is less than 5 m thick (Fig. 2). In most places this unit rests with an erosive contact on Pleistocene fluvial to glacifluvial sand or morainic deposits. Locally intercalated between both strata occur relics of soil horizons, limnic sediments, and peat as well as brackish to tidal flat sediments, which stem from the initial phase of submergence. As a whole the sedimentary sequences in the offshore area are indicative of an unidirectional landward shift of the shoreline during the early phase of sea-level rise. Totally different conditions are characteristic of the coastal lowland area which is in general 10–20 km wide and stretches 80–100 km into the funnel shaped estuaries of the Weser and Elbe rivers. This entire region is built up by a huge wedge like body of Holocene coastal deposits (Fig. 2) which reaches its maximum thickness of more than 35 m in the area of the barrier islands or at the seaward margin of the open tidal-flats. On the islands, these deposits are locally overlain by eolian dune sand up to 25 m thick. In the present-day tidal flat area and the coastal lowlands the deposits become thinner and have an average thickness of about 12–14 m: On the landward margin of the coastal marshlands the Holocene deposits wedge out against the Pleistocene hinterland or merge into alluvial floodplain sediments. The seaward part of the Holocene accumulation wedge consists of pure sand deposited in high-energy environments like the foreshore area or tidal inlets. Its middle part is built up of fine sand, silt, and clay deposited in tidal-flat and brackish-water environments.

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Fig. 1. Geomorphological units and landscape elements of the coastal marshlands and the adjacent Pleistocene hinterland between the Ems and Elbe Rivers, northwest Germany. W ¼ Wangerland; S ¼ Schweiburg; L ¼ Loxstedt (see Section 3).

Layers of fen peat and raised bog peat locally occur at the base of the clastic Holocene sediments as well as intercalated within or on top of them. In many places, the individual layers of basal, intercalated, and overlying peat merge into thicker units or into a continuous peat sequence close to the Pleistocene hinterland (Fig. 2). The internal make up of the Holocene accumulation wedge shows a characteristic cyclic alternation of transgressive overlaps (marine and brackish deposits overlying peat) with regressive overlaps (semiterrestrial peat overlying brackish and marine sediments), which is indicative of repeated shoreline displacements, both landward and seaward, over several kilometers. 2.2. Geometry and calculated volumes of the Holocene accumulation wedge For the mass-balance study (Hoselmann and Streif, 1998) it was necessary to determine the different

interfaces defining the Holocene accumulation wedge in relation to the German Ordnance Datum NN (Normalnull which is nearly identical with mean sea level). Each of the three interfaces had to be reconstructed separately and on the basis of different types of data. In the terrestrial areas (barrier islands, salt marshes, embanked coastal marshlands) the surface of the accumulation wedge is identical with the land surface. Therefore, its relief has been reconstructed based on the Digital Elevation Model DEM 5 (Washausen, 1992) of the ordnance survey of Lower Saxony. In areas where such data were not available, the morphology was determined by data from well locations and by manually digitizing contour lines as well as point elevations on topographic map sheets. At least 10 points per km2 were digitized from the 1:25,000 topographic map (TK25) in the flat coastal marshlands. The rough morphology of dune landscapes on the barrier islands required a much

G. Gerdes et al. / Quaternary Science Reviews 22 (2003) 101–124 Fig. 2. Schematic cross section through the accumulation wedge of Holocene coastal deposits from the foreshore area through the barrier islands, tidal flats and coastal marshlands to the Pleistocene hinterland.

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denser data set, which was obtained by digitizing approximately 250 points per km2 from 1:5000 topographical map sheets (DGK 5 N). In the foreshore area, the subtidal and intertidal zones as well as in the estuaries and lakes the surface of the Holocene accumulation wedge is identical with the sediment–water interface. This interface was reconstructed on the basis of the digital coastal model . Schleider and Buziek, 1991). Supplementary (DIGEKU; digital data sets were obtained from the Dutch–German research project WADE (‘‘Wadden Sea morphological development due to an accelerated relative sea-level rise’’; Niemeyer et al., 1995). Remaining gaps were filled by digitizing contour lines from 1:25,000 coastal map sheets of the German Coastal Engineering Research Council (KFKI). Thus, at least 10 elevation values per km2 were available for reconstructions of the surface relief of the accumulation wedge in the subaquatic and intertidal areas. All information about the base of the accumulation wedge was derived from the borehole database of the Geological Survey of Lower Saxony (NLfB). For this study, more than 25,000 borehole records from the coastal lowlands were evaluated. On this database, structure contours were manually constructed for each of the 60 map sheets covering the study area (Streif, 1998). Afterwards, the structure contour lines, which display the geological interface between Pleistocene and Holocene deposits in 1-meter depth intervals, were digitized. This information on the base of the Holocene accumulation wedge was supplemented by corresponding digital data of the Netherlands Institute of Applied Geosciences (NITG-TNO) for the German–Dutch border zone. In further steps, the three sets of digital scattered data of the different interfaces were mathematically transferred in regular grids and the ISM (Dynamic Graphics) program was used to calculate the total volume of Holocene coastal sediments. Furthermore, calculations were made of the volumes of each of the 1 m-thick sediment slices between þ25 and 42 m NN: A total volume of 40:09  109 m3 was calculated for the Holocene sediments below NN in the study area and 3:71  109 m3 for the total volume of Holocene sediments above NN. The 3:15  109 m3 volume of water bodies and the volume of morphological depressions had to be subtracted from the sum of these two volumes. Thus, the volume of the Holocene accumulation wedge in the study area is 40:65  109 m3 : The dunes on the East Friesian barrier islands have elevations of up to þ25 m NN. However, in the entire mass balance, the volume of eolian deposits does not play a significant role, amounting to only 1.3%. The set of borehole data mentioned above was also used for manual constructions of sequence maps at a scale of 1:25,000 which display a general view of the

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spatial distribution of clastic and organic layers of the coastal Holocene (Streif, 1998). After digitizing, this information, as well as the thickness of the strata recorded in the boreholes, served as a basis for calculations of the cumulative volumes of clastic and organic units of the Holocene accumulation wedge. The results show that clastic deposits make up about 90% of the total volume whereas organic formations make up only 10%. This figure has to be regarded as the net result of peat formation and subsequent compaction or erosion. Originally, the volume of the peat bogs was much larger. However, it has been reduced due to dewatering, loading, and compaction by overlying younger clastic sediments. A clear zonation can be seen on a regional basis in the peat occurrences within the coastal environment. In areas near the Pleistocene hinterland, up to 89% of the Holocene accumulation wedge consists of organic material. In contrast, the percentage of sedentary peat decreases from the hinterland towards the coast. On some map sheets, the proportion of organic deposits is zero or near zero. 2.3. Sources of the material and dynamics of accumulation From the shape and the internal make up of the Holocene accumulation wedge (Fig. 2), it is evident that it has acted as a sink for a tremendous amount of sediments during the last 8500 years. Several sources that contributed material to the accumulation wedge have to be taken in consideration. Autochthonous organic matter of peat bogs makes up about 10% of its volume. The remaining 90% of allochthonous clastic material (fine sand, silt, and clay) stem from eroded and redeposited Pleistocene and Holocene marine sediments of the North Sea, as well as from the riverine catchment areas. In contrast to clear lithological evidence of a mass transport from marine sources, there is only scanty evidence for mass flow from riverine sources into the coastal system. Therefore, the present-day suspension load of the Ems, Weser, and Elbe rivers (published in the German Hydrological Annual (Deutsches Gew.asserkundliches Jahrbuch, 1967–1995) for the period from 1967 to 1995) was used to calculate rough estimates of the riverine contribution to coastal sediment accumulation. The quality of these data has been discussed by Hinrich (1975). For this study, annual averages of the suspension load from the measuring station closest to the coast were evaluated. The data was extrapolated over the last 8500 years. Due to possible error sources, such as deforestation, heavy rain events, canalisation of rivers, etc., this calculation can be regarded as only a rough estimate of the total amount of the suspension load of the rivers during the last 8500

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years. The published data are in tons per year and must be translated for the mass balance study into m3 per year. Therefore, the many density-depth profiles investigated by the NLfB in the coastal zone of Lower Saxony were used to determine an average value of 1:17 t=m3 for the clastic coastal deposits. A total sediment volume of about 41  109 m3 has been calculated in this mass balance study (Section 2.2) to have been deposited in the 4950 km2 study area during the last 8500 years. For the same time span, the volume calculated to have been supplied by the rivers Ems and Weser is about 4:95  109 m3 ; which is a relatively small amount. Even if the suspension load of the River Elbe with 5:8  109 m3 is added, the fluvial component is a maximum of 25% of the accumulation wedge. However, this figure is not realistic, because of the current regime in the German Bight. It can be concluded that the suspension load of the River Elbe is of negligible importance for the investigated area. Most of the material was transported into deeper water or towards the coastal zone of Schleswig–Holstein. Only via complex detours by marine erosion and redeposition could a small portion of this material have reached the coastal zone between the Ems and Weser rivers. Therefore, it is probable that only about 10–15% of the total volume of the Holocene sediments is fluviatile. To include knowledge of the dynamics of the sea-level rise into the mass balance study, the band-like sea-level curve published by Streif (1989b), has been evaluated. This curve is based on conventional radiocarbon dates from basal and intercalated peat layers in the coastal zone between the Ems and Elbe rivers. For use in this study, the conventional radiocarbon dates were calibrated with the software CALIB Rev. 3.0.3c of Stuiver and Reimer (1993). For each 1 m depth interval from þ2 to 25 m NN, a mean calibrated 14 C age and, if possible, a calibrated minimum and maximum 14 C age was determined. These data were used for further calculations of sedimentation rates (cm/year) and volume increase ðm3 =yearÞ for the time interval between 9100 and 1400 cal BP. With regard to the time slices studied by the Deutsche Forschungsgemeinschaft (DFG) priority program ‘‘Changes of the geo-biosphere during the last 15,000 years—Continental sediments as evidence for changing environmental conditions’’ (Andres and Litt, 1999), the following results were obtained. An average sedimentation rate of about 0:99 cm per year was characteristic for time slice II, which covers the span from 9100 to 6400 cal BP. However, in time slice III, the interval between 3800 and 1400 cal BP it amounted to only about 0:16 cm per year. This difference is explained by the rapid rise of the sea level during time slice II and the significantly lower rate of sea-level rise in time slice III. This latter period has been characterized by repeated phases of retardation and acceleration, which among

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other things, is also reflected by lower sedimentation rates. The rates of volume increase ðm3 =aÞ were calculated from the 1 m depth intervals. Highly variable values are characteristic of time slice II. However, this may be partly due to the large number of measuring points, reworking and short distance transport of the coarse to medium-grained Pleistocene sand incorporated into the lower portions of the Holocene sediments. The average increase in volume of clastic and organic sediments in the study area amounted to 8.74 million m3 per year in time slice II. This value dropped to about 3.88 million m3 per year in time slice III, which is indicative of the strong influence of sea-level rise on sediment accumulation in the coastal zone. Separate sediment balances were calculated for each of the landscape elements of the coastal zone, such as sheltered tidal flats, bay flats, and estuaries. Here, the results for time slices II and III are particularly interesting. Considering the study area as a whole, the sediment accumulation in slice II amounts to 70% and in slice III it amounts to 30% of the total during these two time slices. Almost identical results are obtained for the sheltered tidal flats (71–29%) and the estuaries (72– 28%). In contrast to that, the accumulation volumes for the bay flats are 61% for time slice II to 39% for time slice III. This may indicate that the Dollart and Jadebusen bay flats, formed during medieval storm surges, acted as sediment traps in which a tremendous amount of clastic deposits accumulated within a relatively short time span. In the Dollart area, this was distinctly influenced by the compaction of thick peat sequences which were buried by clastic tidal-flat deposits. This started a self-accelerating series of processes of loading, compaction, and formation of new accommodation space.

3. Integrated geochemical and microfacies assessment of Holocene coastal deposits A major goal of this study was to assess responses of shallow marine and coastal lowland areas to the climatecontrolled Holocene sea-level rise. Different disciplinary fields were integrated in this study. The factors controlling the marine and terrestrial regime within the coastal palaeoenvironment were estimated from the distribution of biomarkers, major and trace elements, diatoms, foraminifera and sedimentary structures observed at high resolution in core sections. The data complements those of a variety of geological, lithostratigraphical, archaeological and geobotanical investigations performed in this area to date (Barckhausen et al., 1977; Behre and Streif, 1980; Streif, 1989a, 1990; Behre, 1995; Behre and Ku$can, 1999).

3.1. Indicators of marine and terrestrial regimes in Holocene coastal environments The studies were initiated by coring at three sites in the coastal marshlands of northwestern Germany; five boreholes were drilled in the Wangerland area and two each in the Schweiburg and Loxstedt areas (Fig. 3). All cores showed a typical alternation of transgressive and regressive phases. In addition, each borehole reflects regional modulations: the Loxstedt core mirrors estuarine deposition, the Wangerland sites reveal conditions of a sheltered tidal bay, and the Schweiburg profile indicates an open bay environment. Hence even on a small regional scale, a diachronous effect of the driving events like transgressive or regressive phases took part. In Table 1, datings (conventional radiocarbon age and calibrated age [cal AD and cal BC]) of selected peat layers are presented. To obtain signatures of marine and non-marine facies from quantitative data, samples from both sedimentary clastic and sedentary organic material were taken at mainly centimeter intervals (for analytic methods see Dellwig, 1999; Dellwig et al., 1998, 1999; Watermann, 2000; Watermann and Gerdes, 2002). 3.1.1. Indications of marine facies In the area of the present-day southern North Sea coast, palaeoenvironmental changes occurred due to the transgression of the North Sea over former land areas and their burial by a sequence of shallowmarine to tidal flat and brackish-lagoonal sediments. Chemofacies, lithofacies and biofacies characteristics indicate that the Holocene transgression caused irreversible environmental changes of the former limnic to semiterrestrial facies zones. The clastic cover of the basal peat usually reveals facies characteristics of a brackish-lagoonal environment which gradually changed into shallow marine facies as confirmed by Ca/Sr ratios, dominance patterns of diatoms and foraminifera, and sedimentological features (Figs. 4 and 5). Ca/Sr ratios can be used to differentiate between marine deposits usually enriched in biogenic carbonate and terrestrial deposits dominated by terrigenous detrital clay. Ratios > 210 are standard values of calcite, while ratios o70 define the range of shale (Wedepohl, 1971). Marine environmental conditions, indicated by average Ca/Sr ratios within the calcite range, prevailed throughout the time of deposition of the lower clastic units of the investigated cores due to the initially rapid North Sea transgression (Figs. 4A, D, 5A, 6A). Higher up in the sedimentary sequences, Ca/Sr ratios in the calcite range mirror later transgressive events. In the Wangerland W2 core (Fig. 4D), two thin horizons intercalated in lagoonal facies and revealing calcite dominance suggest relatively short-lived marine flooding

7°E

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source: administration of the national park of Lower Saxonian Wadden Sea; original Landsat data, ESA (1992)

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Fig. 3. (A) Map of the study area and the locations of drill sites for geochemical and microfacies investigations at the southern North Sea coast. (B) Close-up of the Wangerland site and position of the individual cores. The arrow marks an assumed former tidal channel.

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Table 1 Age determinations of peat layers of the Holocene sequences (Loxstedt borehole, Wangerland W5 and W2 boreholes, Schweiburg borehole) Depth below areal surface (m)

Sampling positions

Conventional radiocarbon ages

Calibrated years

Loxstedt borehole (depths are consistent with Fig. 6) 7.31–7.35 Organic-clastic interfingering unit 9.25–9.33 Organic-clastic interfingering unit 10.66–10.68 Organic-clastic interfingering unit 17.28–17.32 Basal peat 17.55–17.59 Basal peat

26307200 26407170 52507155 75407205 99207410

985–420 BC 925–535 BC 4315–3825 BC 6540–6170 BC 10,255–8535 BC

Wangerland W5 borehole (depths are consistent with Figs. 4 and 7) 2.74–2.76 Organic-clastic interfingering unit 2.92–2.94 Organic-clastic interfingering unit 6.10–6.12 Basal peat 6.32–6.35 Basal peat

2380775 2850790 4595765 58807295

525–385 BC 1125–900 BC 3495–3135 BC 5195–4400 BC

Wangerland W2 borehole (depths are consistent with Fig. 4) 4.90–4.92 Organic-clastic interfingering unit 8.24–8.25 Organic-clastic interfingering unit 8.33–8.36 Organic-clastic interfingering unit 10.67–10.71 Basal peat

46407100 58507125 59107190 62857300

3615–3335 4895–4540 4995–4540 5470–4850

Schweiburg borehole (depths are consistent with Fig. 8) 1.18–1.20m Upper peat 1.37–1.41 Upper peat 1.51–1.56 Upper peat 2.09–2.14 Upper peat 2.13–2.14 Upper peat, above lower Intercalated clastic layer 2.17–2.19 Upper peat, below lower Intercalated clastic layer 2.17–2.2 Upper peat 2.22–2.23 Upper peat, below lower Intercalated clastic layer 7.43–7.48 Basal peat 7.83–7.88 Basal peat 8.23–8.28 Basal peat n

BC BC BC BC

14207105 15107115 1660795 2175795 2383731

550–685 AD 425–655 AD 260–535 AD 375–60 BC 536–394 BCn

1359728

642–690 ADn

15407100 2245729

420–635 AD 393–202 BCn

4160780 48007120 58257125

2880–2586 BC 3700–3380 BC 4830–4530 BC

Data from AMS analyses for comparison.

(Dellwig, 1999). Longer-lasting marine conditions are indicated by a clastic unit of about 4 m thickness towards the top of the W2 profile (Fig. 4D). This applies in a similar way also to the transgressive sections of the upper clastic unit of the Loxstedt core (Fig. 6A). The differences between the boreholes can be traced back to local geomorphological variations. Dominance patterns of marine littoral diatoms correlate well with the geochemical results. The polyhaline littoral species Achnanthes delicatula is frequent in tidal flat units of the W2 core (Fig. 4E) and also in the Schweiburg core (not shown). In modern North Sea tidal flats, this species lives in sandy lower intertidal and upper subtidal zones. Similarly, the dominance patterns of euryhaline tidal flat foraminifera are consistent with the geochemical and diatom data (Figs. 4C and F). In the Wangerland boreholes W5 and W2, this group of organisms dominates at two different levels, the lower clastic unit of the W5 core and the upper clastic unit of the W2 core

(compare Figs. 4C and F). This difference sheds light on the importance of regionally modulating topography which becomes evident even at small distances between boreholes. The sedimentary sequence of the Wangerland W2 borehole reflects direct marine influence since about 6700 14 C BP, whereas the contact between the topmost portion of the basal peat of the W5 core and the overlying clayey silt was dated to 5300 14 C BP. The locality of this latter borehole may have been too far inland to have been affected by tidal channels which developed as a consequence of the transgression. In our study area, sedimentary Al2 O3 concentrations, grain size distributions and sedimentary fabrics also are helpful tools to estimate the extension of marine conditions during the formation of clastic sediments. The Al2 O3 content, which represents the clay content, is generally lower in marine than in non-marine sediments (Fig. 5B) due to the dilution with quartz. Thus, the higher depositional energy (tidal and wave actions)

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Fig. 4. Vertical distribution of Ca/Sr, diatoms and foraminifera in the Wangerland boreholes W5 and W2. (A)–(C) borehole W5: (A) Ca/Sr ratios (range > 210 indicates calcite dominance, range o70 indicates detrital shale dominance), (B) relative abundance of the littoral diatom species Delphineis surirella indicating polyhaline salinity conditions, (C) euryhaline tidal flat foraminifera (all scales of foraminifera refer to individuals/g sediment); (D)–(F) borehole W2: (D) Ca/Sr ratio, (E) relative abundance of the polyhaline littoral diatom Achnanthes delicatula, (F) euryhaline tidal flat foraminifera (legend for lithology see Fig. 7).

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calcite

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calcite

Wangerland core 5 (W5)

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Fig. 5. Vertical distribution of Ca/Sr and sedimentological parameters in the Schweiburg borehole (legend for lithology see Fig. 7). (A) Ca/Sr ratios, (B) Al2 O3 content (C) grain size median ðmmÞ; (D) and (E) microphotographs of sedimentary structures: (D) characteristic cloudy structure of bioturbated intertidal sediments showing elongated and rounded silt patches in clay matrix, (E) quartz/clay interbedding.

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euryhaline [Ind/g] foraminifera

Fig. 6. Vertical distribution of Ca/Sr, sterathiols, diatoms and foraminifera in the Loxstedt borehole (legend for lithology see Fig. 7). (A) Ca/Sr ratios, (B) concentration of sterathiols, (C) and (D) relative abundance of pelagic marine diatoms (Actinoptychus senarius) and pelagic fluviatile diatoms (Cyclotella meninghiniana), (E) agglutinated brackish water foraminifera, (F) euryhaline tidal flat foraminifera (E and F logarithmic scale).

(A)

depth [m]

0

Ca/Sr

calcite

Loxstedt

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under fully marine conditions, leads to an enrichment of coarser material like quartz and heavy minerals. This difference is also reflected in the median grain size and in the sedimentary fabrics of the Schweiburg core (Fig. 5C–E). In the Schweiburg profile the first marine incursion into the present-day Jade Bay is prominent and occurred at about 7000 14 C BP (Behre and Ku$can, 1999). The erosive contact at the top of the organic basal unit indicates a hiatus. The clastic sediments overlying the basal peat contain various pebbles of reworked organic material. Clayey and silty layers at the base of the lower clastic unit partly contain single coarser quartz grains, furnished with significant cleavages indicative of reworked Pleistocene material. The Loxstedt locality (Fig. 6) received direct marine contact at approximately 8300 14 C BP. The lower clastic unit shows signals of high hydrodynamic energy at the base followed by a gradual change towards less energetic conditions. The upper clastic unit indicates two transgressive overlaps. The first of these transgressions following the organic-clastic interfingering unit contains Ca/Sr ratios in the calcite range (Fig. 6A), abundant marine pelagic diatoms (Fig. 6C), euryhaline tidal flat foraminifera (Fig. 6F), marine allogenes, and clay/silt interlayering typical of a marine environment. An estuarine criterion is the presence of pelagic oligohaline diatoms (Cyclotella meninghiniana, Fig. 6D). In the youngest transgression recorded in the core, the marine signals are less pronounced, and the importance of fluvial signals increases. This indicates a greater distance or a more sheltered position of the sampling location from the sea at that time. Most important is the dominance of Cyclotella meninghiniana which is the present-day type species of the river Weser. Both transgressive overlaps are followed by brackish sediments with a semiterrestrial to limnic inventory. Finally, biomarkers confirm the results obtained from inorganic geochemical and microfacies analysis. The influence of sulphate-rich seawater at the contact zone between TOC-rich sedentary layers and marine sediments led to a significant enrichment of sulphur through the action of sulphate-reducing bacteria. Hence diagenetically formed organosulphur compounds are indicative of this process. For example, sterathiols are only detectable at the lower clastic—basal peat unit boundary (minor amount) and the organic-clastic interfingering unit of the Loxstedt core (Fig. 6B). Sterathiols in the salt marsh and the lower brackish/ marine depositional units are due to high contents of eroded peat material. Low mannose/fucose ratios in most of the Wangerland W5 core (Fig. 7C) indicate provenance of the organic matter from marine planktonic algae in the marine facies, despite the strong overprint by eroded terrestrial organic matter.

3.1.2. Marine invasions into the low-lying hinterland (minor sea-level oscillations) Within time slice II (9100–6400 cal BP), the accumulation of sediments kept pace with the rising sea level, particularly when the sea-level rise gradually slowed down towards the end of time slice II. The continuous deposition and accumulation caused a seaward progradation of the coastline. In the vertical successions studied, the progradation is indicated by an increasing admixture of terrestrial components into marine deposits and finally by clear facies changes towards brackish supratidal and ultimately lagoonal to semiterrestrial depositional environments. Distinct features of continuous progradation are terminations of clastic strata by peat layers. Usually, these are intercalated in clastic sediments indicative of repeated marine inundation. The sum of facies characteristics of the intercalated clastic strata suggests that the floods were short-lived events and did not cause significant facies changes towards marine conditions. Some examples from the different fields of the interdisciplinary project may illustrate that during time slices II and III (3800–1400 cal BP), the hinterland repeatedly received storm-related pulses of sea water, which faced only little resistance in the topographically low-lying coastal marshland: (a) Although Ca/Sr ratios generally tend to reveal shale dominance in the non-marine strata, there are occasional excursions of the ratio into the calcite range (an example is shown in Fig. 6A between 11 and 12 m depth). (b) There are distinct occurrences of organosulphur compounds (e.g. sterathiols) in non-marine strata (Fig. 6B). (c) Dominant allochthonous pelagic marine diatoms indicate transport into the hinterland areas (Fig. 6C between 11 and 12 m depth). Since the Loxstedt borehole locality represents estuarine conditions, the marine pelagic diatoms here are also intermixed with fluvial species (Fig. 6D). (d) Foraminifera distributed in the interfingering peat and clastic layers correspond to the geochemical and diatom signals (Fig. 6E and F). (e) Abundant FeS2 (pyrite, here calculated from total sulphur) in basal and intercalated peats (Fig. 7A) may be explained by two different scenarios: The first applies to a moderate sea-level rise and is typical of reed peats where pyrite formation coincides with bog growth. The peat-forming palaeo-environments probably were influenced by iron-rich freshwater and sulphate-rich seawater. The second scenario applies to extensive pyrite formation at the interface of thin clastic intercalations in the originally limnic-semiterrestrial basal peats. This pyrite formation may be indicative of tidal channel

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Legend

peat

freshwater mud

ICL (intercalated clastic layer)

natural levee

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tidal channel

tidal flat

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lagoonal deposit

Fig. 7. Vertical distribution of parameters in the Wangerland core W5. (A) content of FeS2 (pyrite), (B) d34 S; (C) Mannose/Fucose ratio, (D) abundance of the polyhaline marine diatom Paralia sulcata.

(A)

depth [m]

Wangerland core 5 (W5)

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activities which caused a partial flotation of the basal peat and allowed the input of marine suspended particulate matter. When the peat settled down, clastic layers remained whose interface may later have acted as an aquifer for waters of higher salinity (tidal pumping). Therefore, enhanced formation of pyrite was possible due to the combination of sulphate-rich groundwater and iron-rich peatland water (Dellwig et al., 2001a). Both scenarios also explain the distribution of stable sulphur isotope ratios (Fig. 7B) and redox-sensitive trace metals (e.g. As, Mo, Re, U; Dellwig et al., 2001b). In the investigated peats, d34 S values coincide with the availability of sulphate, i.e., the more negative the values are the higher was the sulphate availability. Thus, the pyrite-rich sections of the intercalated and basal peat of core W5 show the most negative values. This can only be explained by a sufficient influence of sulphate-rich seawater as described in the two scenarios above. 3.1.3. Semiterrestrial successions The semiterrestrial successions of the cores can be divided in two units: First, a basal organic unit of limnic wetland peats built up prior to the Holocene transgression (Loxstedt) or as an indirect response to the rising sea level (Wangerland, Schweiburg). Most of the sedentary layers are fen peat with, among others, reed, sedge and fen woodland vegetation sometimes along with raised bog elements pointing to transitional conditions. The second type is a horizontal interfingering of peat layers mainly formed of reed (Loxstedt, Wangerland) with siliciclastic deposits. These characteristic series are due to a slowing down or even stagnation of the sea-level rise and minor sea-level oscillations (see Section 3.1.2). In the Schweiburg core, the upper semiterrestrial succession from fen to a raised bog peat reflects a temporary lowering of the sea level (Behre, 1993). The organic geochemical data allow a molecular characterization and differentiation particularly of the TOC-rich sedentary successions. Biomarker analyses provided clues to the development of the dominant plant community in general and to some species in detail as well as their variations with depth (Gramberg et al., . 1995; Koller, 2002). These variations are closely connected to the local hydrological regime and the nutrient supply. As examples, three parameters are chosen: 1. C31 n-alkane ratio (quotient of n-C31 to the sum of C27 ; C29 and C31 n-alkanes in %): This ratio allows the differentiation between fen and raised bog peat . . (Koller, 2002; Koller et al., 2002) with an arbitrary separation line at 45, where ratios o45 indicate a fen and ratios > 50 raised bog peat. This is based on a representative number of own results and literature

data. Besides the general trend minor variations of the plant community within fen peat are indicated too. 2. C24 n-alkane ratio (quotient of n-C24 to the sum of C21 to C35 n-alkanes in %): The n-C24 -alkane is a . specific marker for reed vegetation (Koller, 2002; . Koller et al., 2002). Values X4 reflect a significant reed abundance. 3. Three triterpenoid types (lupanol/-on, lupeol/-on, glutenon and betulin) are indicative of birch (Betula) and alder (Alnus), typical trees of fen woodland peat. Their quotient to the sum of all (polar) triterpenoids is a useful indicator for the occurrence of these trees and accordingly a fen woodland peat indicator (WPI; . . Koller, 2002; Koller et al., 2002). The depth profiles of these parameters and of total organic carbon (TOC) are shown for the organic sequences of the Schweiburg profile in Fig. 8. The trend of the C31 n-alkane ratio (see Fig. 8B) of the core reflects fen conditions in the basal and the top sequence up to the upper intercalated clastic layer and a clear shift to raised bog conditions above this layer. This shift is described in the literature (Behre, 1993) and is indicative of sea-level lowering in this area; the clastic intercalations are due to secondary marine impacts (‘‘Klappklei’’; Behre and Ku$can, 1999). Smaller variations of this parameter indicate vegetation changes, e.g. in the upper peat unit at 1:75 m: The two other parameters underline that a change from a former fen woodland (see WPI, Fig. 8D) to a fen peat with a higher reed abundance (see C24 n-alkane ratio, Fig. 8C) has occurred. A steep increase of the C31 n-alkane ratio (Fig. 8B) below the lower interfingering clastic layer ð2:15 mÞ indicates an abrupt vegetation change to more raised bog conditions, as does the drop in WPI. Botanical analysis underlines this result and describes this layer as a raised bog or transitional bog. This result and a much younger 14 C age of this sample point to a secondary impact not discussed here. In the basal organic sequence of the Schweiburg core a high abundance of reed material with peaks below and at the top of the mud layer is reflected in (Fig. 7C). A clear drop of reed content towards the base and an increase of the WPI value indicate the fen woodland character of these layers. At the top of the basal sequence a shift to transitional bog vegetation is indicated by the elevated C31 n-alkane ratio (Fig. 8B). 3.2. Sea-level rise and facies changes Much of time slice II (postglacial warming optimum) was characterized by a high rate of sea-level rise which reached the lower-lying valleys of the present-day southern North Sea coast. Towards the end of time

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Schweiburg n-C24

n-C31 [%] n-(C27+C29+C31)

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Fig. 8. Vertical distribution of total organic carbon (TOC) and biomarker profiles in the TOC-rich sections of the Schweiburg core (lithology of the total core see Fig. 5, slightly variation in depth of the layers due to parallel coring). (A) TOC (%), (B) C31 n-alkane ratio (quotient of n-C31 to the sum of C27 ; C29 and C31 n-alkanes in %), (C) C24 n-alkane ratio (quotient of n-C24 to the sum of C21 to C35 n-alkanes in %) a specific marker for reed vegetation, values X4 reflect significant reed abundance, (D) Quotient of the three triterpenoid types (lupanol/-on, lupeol/-on, glutenon and betulin) to the sum of all (polar) triterpenoids as a woodland peat indicator (WPI). For legend see Fig. 7.

slice II, a distinct deceleration of the sea-level rise occurred. The inventory records of the vertical successions studied reflect phases of accelerated and slower sea-level rise. Marine facies is characteristic of channels and subtidal point bars, mixed and muddy tidal flats. Nonmarine facies units include those of salt marshes, lagoonal backswamps, fluvial levees, fen (reed) and raised bog (Sphagnum) peat. The occurrence of terrigenous allogenes in marine, and marine allogenes, e.g. pelagic diatoms, in nonmarine strata provides clues on minor sea-level undulations. Pelagic diatoms can be easily transported far away from their original habitats, e.g., due to storm surges into the hinterland. Since the coastal marshland was a lowland area, it was easily accessible by landward pulses of the sea. Fine clastics and diatom valves suspended in

the sea–water may have found their way into the swampy hinterland where the marine water became mixed with the terrestrial freshwater runoff. Marine sulphate induced sulphate reducing bacterial activity mirrored in the occurrence of organic sulphur compounds. The minor sea-level undulations were short enough not to change the whole environment which remained semiterrestrial. This is different to the transgressive overlaps. The considerable fluctuations indicated in the vertical successions may have hampered intense human utilisation of the coastal marshlands as long as into time slice III. In time slice III, several colonisation phases occcurred and were interrupted again by transgression periods. The sum of facies characteristics indicates a great biotope diversity in this transitional zone, probably due to the interfingering of marine and terrestrial impacts

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marine input:

North Sea salinity >30‰

short-term floods

natural levee shifting zone

salinity=10-15‰

terrestrial

salinity ≤5‰

input

reed peat sedge/woodland peat

gully raised bog peat

freshwater salinity <1‰ Fig. 9. Model of Holocene coastal environments controlled by marine and terrestrial regimes.

(Fig. 9). These kinds of extended brackish coastal marsh and backswamp environments represented by the cored material are rare in the NW German coastal area, because dikes nowadays effectively protect the low lying coastal marshlands from marine flooding.

4. The appearance and spread of raised bogs as indicators of environmental change in the lowland area adjacent to the coastal zone of northwestern Germany Lower Saxony is the region containing most of the bogs in Germany. More than 6500 km2 were progressively covered by bogs, during the Holocene period. Since medieval times subsequent human activities lead to a significant reduction of this area. Today, the size of these bogs varies between a few hectares and many square kilometers; originally, some of them had an area of more than 100 km2 (Schneekloth et al., 1970–1983). About 50% of the bogs of Lower Saxony consist of raised bog peat. This is of particular interest, because the formation and spatial distribution of raised bogs is closely related with specific climatic conditions, favourable conditions existing only where precipitation is significantly higher than evaporation. The vegetation of

raised bogs covers its nutrient demand exclusively from precipitation. Nowadays in northern Germany, such conditions exist mainly in the zone of oceanic (Atlantic) climate, which reaches from the coast southward to about 521 northern latitude. Besides that, favourable conditions exist only locally in the Harz mountains, where, under high precipitation rates small raised bogs were formed at an elevation of about 800–1000 m above sea level (Beug et al., 1999). The close relationship between climatic conditions and formation of raised bogs provides excellent opportunities to examine peat formation in response to climatic changes in the past. This was undertaken in a regional study dealing with the onset and regional expansion of raised bogs. Additional interest was paid to the question of whether or not the formation of raised bog peat started synchronously in one or in a series of different phases. Finally, the extent to which other environmental factors significantly affected the landscape was examined. 4.1. Data available on paludification The formation of fen peat is bound to the filling up of lakes or other geomorphologic depressions by plant

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growth. In coastal areas (see paragraph 2 and 3) it can also be triggered by a rise of the groundwater-level, which is closely related with the sea-level rise. Under these conditions, the onset of fen peat formation is not synchronous or indicative of climatic changes. Studies on the formation and composition of coastal peat bogs (Grosse-Brauckmann, 1962; Cordes, 1967; Behre, 1970, 1979, 1986; Behre and Streif, 1980; Behre and Ku$can, 1999) demonstrated close relations between bog vegetation and contemporary sea-level fluctuations. Totally different conditions are characteristic for the hinterland of the coastal zone which consists of Pleistocene morainic, glaciofluvial, and eolian deposits. Extended raised bogs occur in this region, most of them directly resting on the clastic sediments without any intercalation of fen peat. These raised bogs, which develop independently from the ground- and surfacewater level, are highly suitable for investigations of the climatically induced onset of bog formation. In northern Germany, there exists a number of pollen diagrams from bogs dealing with the history of vegetation and human settlements. Only a few of the older pollen diagrams are supplemented with 14 C-data (Overbeck, 1975). Even in more recent studies practically no 14 C-data are available for the base of the raised bog peat. Only a few and rather indeterminate ages can be given to the beginning of raised bog formation, which, according to this literature, began in the late Atlantic to early Subatlantic periods. Published pollen and 14 C-data from a few places in the neighbouring Netherlands (Zagwijn, 1986) indicate that the paludification started in the late Atlantic period. Prior to the formation of raised bogs the Pleistocene landscape of the Geest was covered with extensive mixed-oak forests. Relic oak stumps can occasionally be found in exposures at the base of the peat. The expanding Sphagnum bogs subsequently, replaced the primary forest vegetation. This drastic natural change of the biosphere seriously affected the living space of human beings which was progressively reduced by the bogs from the Neolithic period until medieval times. Thereafter, large scale cultivation of the bogs started, which led to the present-day pattern.

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for areas where the German style of raised bog cultivation was practised. In such areas, the entire peat profile is preserved except for the topmost unit (Overbeck, 1975). Throughout these areas the basal bog sequences are preserved and offer suitable conditions for the purpose of this project. Systematic mapping of the bogs of Lower Saxony (Schneekloth et al., 1970–1983) offers detailed information on the occurrence and spatial extension, the thickness, the stratigraphic sequence and the substratum of fen and raised bog peats, as well as on their development and present-day use. This information was used for the selection of areas suitable for this research project which completely covers northern Lower Saxony (Fig. 10). Field checks demonstrated that in some regions the present-day thickness of the bogs considerably differed from the results of the earlier mapping. This may be due to progressive peat exploitation, oxidative decomposition, shrinkage, and compaction resulting from drainage. Other areas turned out to be unsuitable for the project because deep ploughing had disturbed the base of the bogs. Large raised bog areas were investigated during drilling campaigns. Then, specific areas of 5–10 km2 were selected for the collection of samples for 14 Cdating. At each site, a test borehole was sunk with a guts-auger to get information on the substratum, the thickness, and the stratigraphic sequence of the bog. When the base of the profile was undisturbed, cores for 14 C-dating were taken from a parallel borehole with a 50 cm long Russian peat-auger. The precise elevation of each sampling site was determined by altimetric surveying, and from the observed thickness of the bogs. Finally, the altimetric surveying allowed a mapping of the relief of the landscape which existed at the beginning of the bog growth. This, together with information from recent topographical maps, was used for a regional comparison of the specific conditions of bog growth. A total number of more than 500 drillings were made, and from more than 340 locations, cores have been collected from the lowermost meter of the peat profiles. These units were used to take samples for 300 14 C-age determinations of the beginning of raised bog growth.

4.2. Areas of investigation and methods 4.3. Results In extended bog areas in Lower Saxony, where deep ploughing for agricultural measures has been carried out, the base of the bogs is usually completely destroyed down to a depth of about 2 m below the surface. Areas of extensive industrial peat exploitation offer more favourable conditions for studies of the base of the peat bogs. This is because the exploitation of peat is mainly concentrated on the light peat in the upper part of the sequences, whereas the lower unit of black peat generally remains undisturbed. The same holds true

According to the field observations, the bogs to the west of the Weser river predominantly rest on well sorted fine-grained sand with a small portion of silt and/ or medium-grained sand. In most cases this is eolian sand of the Weichselian and early Holocene periods. Boulder clay and pure silty to clayey deposits of previous glaciations have only limited outcrops. In general, poorly developed podsolic soils are developed on the top of the sand. They are characteristically less

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Fig. 10. Location of the investigated bogs and their average age in 14 C BP-Oldenburg–Ostfriesland ridge: Gro
es Moor near Aurich (1), WiesmoorNorth (2), Wiesmoor-South (3), Lengener Moor (4) Ihausener Moor (5); Hunte–Leda catchment area between Papenburg and Oldenburg: Wildes Moor/Kloster Moor (6), Timpemoor (7), Wester Moor (8), Oster Moor (9), Vehnemoor (western part) (10) and Vehnemoor (eastern part) (11); Elbe– Weser Region: Ahlen-Falkenberger Moor (12), Hymenmoor (13), Langen Moor (14), Bulter . Moor (15), Hahnenknooper Moor (16), Kurzes Moor (17), Posthausener Moor (18), Borchelsmoor (19), Lohmoor (20), Gnarrenburger Moor (21), Wei
es Moor (22), Pietzmoor (23) and Hohes Moor (24).

than 5 cm thick, with hardly visible eluvial horizons; an indurated B-horizon, the so-called Ortstein, has not been found in any of the boreholes. In contrast, badly sorted fine- to medium-grained sand occurs underneath the peat bogs of the Elbe–Weser region, which most probably is a glacial to glacio-fluvial deposit of the Younger Drenthe period. In undisturbed profiles, the transition zone from the underlying soil into the peat is about 5 cm thick. In most places the lowermost portion of the peat is strongly decomposed. Within the peat sequences, the ‘‘recurrence horizon’’ was difficult to see in the drillings. In some areas, where the light peat was already exploited, it was not preserved. 4.3.1. Oldenburg–Ostfriesland ridge In the bogs on the Oldenburg–Ostfriesland ridge more than 200 drillings were sunk, 150 of them were cored in the lowermost meter, and 117 radiocarbon datings were measured. Based on the large number of boreholes and the subsequent altimetric survey it was possible to reconstruct the relief of the subsurface underneath the bogs. It turned out that the variations in elevation in general were less than 5 m: An exception is the southern-

most bog of this area, the Ihausener Moor (Fig. 10, No. 5) with height variations of more than 10 m: On a regional scale, the base of the bogs on the Oldenburg– Ostfriesland ridge becomes more elevated from the north-west toward the south-east. Here, again the Ihausener Moor is an exception, because its base lies below the level of the neighbouring Lengener Moor (Fig. 10, No. 4). For the Oldenburg–Ostfriesland ridge the results of radiocarbon dating (Fig. 11) range between 7825765 and 2365775 14 C BP (Fig. 10). The highest ages stem from the Gro
es Moor near Aurich in the north (Fig. 10, No. 1), the lowest from the Ihausener Moor (Fig. 10, No. 5) in the south. The average age for both northern bogs, Gro
es Moor and Wiesmoor-North (Fig. 10, Nos. 1, 2), is 6650 14 C BP. Towards the south, the ages obtained for the onset of the bog growth become progressively younger. Average values of 5800 14 C BP, 4950 14 C BP, and 4400 14 C BP were obtained from Wiesmoor-South, Lengener Moor, and Ihausener Moor (Fig. 10, Nos. 3–5). These results demonstrate that the later beginning of raised bog formation correlates with increasing distance from the coast. An explanation for this phenomenon

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could be the lateral shift of the belt of oceanic climate which is connected with the shoreline displacement in the North Sea region in the course of the Holocene transgression (Petzelberger et al., 1999). A hint for such interactions is suggested by data on the beginning of marine sedimentation in the present-day coastal area (Streif, 1990) which is close to the earliest onset of raised bog formation on the Oldenburg–Ostfriesland ridge. With the subsequent landward progradation of the shoreline, influences of oceanic climate progressively encroached into the southern part of the investigation area. Another remarkable result is that the natural environmental changes which are documented in the onset and spread of raised bogs took place much earlier than assumed at the beginning of the project (Fig. 10). 4.3.2. Hunte–Leda catchment area The Hunte–Leda catchment area lies to the south of the Oldenburg–Ostfriesland ridge and extends from the towns of Papenburg in the west to Oldenburg in the east. In the bogs of this region 155 drillings were sunk. Cores of the lowermost meter were taken from 113 boreholes, and 106 14 C-age determinations were measured (Fig. 11). An altimetric survey of the drilling sites demonstrated that the average elevation of the base of the bogs lies 1:50 m below the corresponding level from the Oldenburg–Ostfriesland ridge. With height differences of about 3 m; the substratum has a more shallow relief. An exception is the Vehnemoor (Fig. 10, No. 10) with height variations of up to 7 m: The size of the examined bogs is similar to those on the Oldenburg–Ostfriesland ridge, the same holds true for their substratum. The 14 C-ages obtained for the beginning of raised bog formation in the Hunte–Leda catchment area range between 79057105 14 C BP and 3180770 14 C BP (Fig. 10). From west to east the average values vary between 5500 14 C BP in Wildes Moor/Kloster Moor, 5950 14 C BP in Timpemoor, 6350 14 C BP in Wester Moor, 4800 14 C BP in Oster Moor, 5550 14 C BP in Vehnemoor (western part), and 6250 14 C BP Vehnemoor (eastern part) (Fig. 10, Nos. 6–11). According to these results, the onset of bog growth progressively becomes older in the first three sites of this area (Fig. 10, Nos. 6– 8). The youngest ages for the beginning of peat formation stem from the Oster Moor (Fig. 10, No. 9). Further east, older ages occur again. Compared with the finds on the Oldenburg–Ostfriesland ridge, the ages obtained from the bases of the raised bogs of the Hunte–Leda catchment area show some differences. No evidence can be found to confirm the trend of a delayed onset of peat formation with an increasing distance from the coast. Thus other factors must have influenced the independent development in this area. An explanation may come from the geographical setting of these bogs which are located in the deep

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seated zone between the lower courses of the Ems and Weser rivers. In the course of the Holocene, both of these funnel shaped valleys acted as estuaries, so that the sea, at an early stage, could easily penetrate far into the hinterland. This may have affected the local climatic conditions prior to the seaward sites on the Oldenburg– Ostfriesland ridge. In the area to the south of the present-day tidal bay of the Dollart, which was formed by medieval storm surges, evidence can be found that the beginning of marine-brackish influence coincides with the onset of bog formation (Streif, 1990). Another effect may be attributed to the specific hygroclimatic situation in the Hunte–Leda catchment area which lies in front of the ridge of the Hummling . and the hilly regions of the Oldenburger Munsterland . with altitudes of about 40–50 m above sea level. For both areas to the west of the Weser river, the Oldenburg–Ostfriesland ridge and the Hunte–Leda catchment area, the following results can be summarized. At the end of the Atlantic period 60% of the total peat area was already covered by raised bogs (Fig. 10). So, the formation of raised bogs started significantly earlier than it had been assumed at the start of the project. In both areas the bogs rest on a levelled and monotonous surface with the highest elevations of þ14 m NN.

4.3.3. Elbe–Weser region Compared with the areas mentioned above, the raised bogs of the Elbe–Weser region are significantly smaller, normally not exceeding 20 km2 : This made it difficult to find continuous bog areas which were suitable for sampling with a grid of 400  400 m: A series of 140 drillings was sunk in this region. Cores have been taken from the lowermost meter of 94 boreholes, and a total number of 77 14 C-datings have been made on the onset of raised bog formation (Fig. 11). Altimetric surveying demonstrated that the base of three raised bogs occurs in exceptionally high positions. In the Borchelsmoor it occurs at a height of þ26:80 m NN, in the Lohmoor and Pietzmoor at þ35:10 m and even þ81:30 m NN (Fig. 10, Nos. 19, 20 and 23). In contrast to that, the bog bases of the Ahlen–Falkenberger Moor and Hahnenknooper Moor (Fig. 10, Nos. 12 and 16) were found at exceptionally low positions of 1:07 m NN. In most of the other bogs of this region it lies at elevations below þ16:00 m NN, which corresponds with the situation to west of the Weser river. The same is true for the relief of the substratum which shows height differences of up to 6:50 m for the individual bogs. However, in contrast to the investigation areas to the west of the Weser river, a greater variety of lithology and soils is characteristic for the Elbe–Weser region. On the top of fine- to medium-grained glacial and glaciofluvial sediments, podsolic soils, cambisols and gleysols

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(predominantly stagnic gleysols) are developed with a series of intermediary soil types. The 14 C-ages on the onset of raised bog formation in this region range between 8400765 14 C BP in Hohes Moor and 1980770 14 C BP Lohmoor (Fig. 10, Nos. 20 and 24). A comparison of the data obtained form the Elbe–Weser region with data from the west of the Weser river shows that the onset of bog formation covers a wider time span and according to the average values occurred later than in the other areas. Besides the relief and the lithology of the substratum the soils also seem to have affected the bog formation in the Elbe–Weser region. The relief, however, plays a role only within the individual bogs. No influence of the absolute height of the terrain has been observed so far. Stronger influences are ascribed to the variety of the lithology and the soils which seem to be less favourable for peat formation in this area. The age range of the data from the Elbe–Weser region is open to a wider scope of interpretations. Nevertheless, some general points can be made. All bogs which are located to the south-east of an imaginary line connecting the towns of Bremen and Stade (Fig. 10) yielded starting ages which were younger than 5700 14 C BP. In contrast, most of the bogs lying on the north-western side of this line yield significantly higher ages. An exception is the Hymenmoor (Fig. 10, No. 13) with a starting age of 3500 14 C BP. Compared with this latter result, only the Posthausener Moor and Lohmoor Moor (Fig. 10, Nos. 18 and 20) in the south-eastern part of the Elbe–Weser Region yield younger average ages for the beginning of raised bog formation. However, for such comparisons, it has to be taken into consideration that the total number of 14 C-ages is very low. Only three age determinations are available from Posthausener Moor and two from Lohmoor, whereas 8 exist from Hymenmoor. Although the results obtained from the Elbe–Weser region are ambiguous to some extent, they generally confirm the finds from the areas to the west of the Weser river. There are some hints that the growth of raised bog started early in the northern part of the Elbe–Weser region which, according the shoreline displacements, came under the influence of oceanic climate at an early stage of the Holocene sea-level rise. It was also found that the growth of raised bogs started much earlier than it was assumed at the beginning of the project.

5. Conclusions and human impact on the coastal zone and the adjacent bog landscape The geological archives of the coastal region and adjacent areas are of special interest with regard to changes of the geo-biosphere in the course of the last 15,000 years because they record influences from both, marine and terrestrial environments. Coastal evolution

is dominated by a climate induced sea-level rise which began with the climatic amelioration at the end of the Weichselian glacial maximum at about 18,000 14 C BP. This sea-level rise of about 110–130 m was linked with a landward and upward shift of the coastline over a distance of about 600 km: As the marine inundation progressed, extended areas of the present-day North Sea shelf passed through several stages of development which started with increased swampiness, bog growth and peat formation, then turned into lagoonal, brackish, tidal flat environments, and finally ended with shallow marine conditions (this is generalized in Figs. 9 in a schematic picture). Human beings, who lived in this area in the Mesolithic and Neolithic periods were totally at the mercy of the sea-level rise and were forced to escape in a landward direction. As shown, the accumulation of coastal deposits is determined by the interaction of sea-level rise, stability of the coastline, formation of accommodation space, the availability of clastic material, and the growth of peat bogs. From about 7500 14 C BP onwards, during the final stage of the Holocene sea-level rise, the coastal landscape came into being, and since about 2200 14 C BP the present-day pattern of the barrier islands, tidal flats, and coastal marshlands (Behre, 1999) was formed. It is made up of clastic marine, tidal flat, and brackish-lagoonal sediments with intercalated layers of semiterrestrial peat. Such sedimentary sequences indicate repeated phases of transgressive and regressive development and shoreline displacements over some kilometers, both landward and seaward (Streif, 1989a, 1990). This investigation confirms a lot of earlier studies about the development of the coastal geo-biosphere. In addition, the application of interdisciplinary methods and high sampling resolution offers a potentially large database of complementary information on processes controlling the specific depositional and palaeoecological conditions of the amphibic coastal area, such as transport and flooding, erosion and sedimentation, salinity changes, and small-scale interfingering between marine and terrestrial facies. Despite these distinct changes, the surface of the Holocene coastal sediments always closely adjusted itself to the elevation of the mean high tide level. In other words, the vertical growth of the coastal accumulation wedge could keep pace with the rise of the sea level. The earliest human settlements in the coastal lowland areas of Germany stem from late Bronze Age to the beginning of the Iron Age, which corresponds with the 10–9th century BC (Strahl, 1998). Relics of them were found on the surface of natural levees of the Weser river. Comparable settlements are also known from the PreRoman Iron Age (700–600 BC) from the banks of the Ems as well as the Weser river and its tributaries . (Haarnagel, 1969; Forst, 1991). All these settlements had to be abandoned after some time. In some cases it

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was possible to demonstrate that this was due to the direct influence of intensified inundation of the area; at other locations other reasons have also to be taken into consideration. Nevertheless, all of these early settlements erected on the natural surface of the coastal marshlands were subsequently covered by some decimeter thick layers of clastic sediments of marine or fluvial origin. A wide spread of numerous human settlements over the natural surface of the coastal marshland started at about 100 BC. This indicates a distinct regressive phase with no (or very rare) marine inundations. This period was followed by the first phase of the construction of numerous dwelling mounds which date from about 50 to 450 AD. Such human activities, which resulted in the erection of up to 5 m high artificial hills, are indicative of a rising storm-flood level coinciding with repeated inundations. They also can be regarded as the first technical measure to prevent the farm houses or villages from inundation by storm surges. This period was followed by an interruption of human occupation of the coastal zone which lasted for about 200 years. Reoccupation of the area started at about 700 AD with new settlements either located on the natural coastal marshland surface or on the pre-existing dwelling mounds. Immediately after that a second phase of construction of dwelling-mound began which lasted until 1100 AD (Behre, 1995, 1999). The time span from 1000 BC to 1100 AD is characterized by sensitive reactions of human beings to the natural processes in the coastal zone. Phases of occupation coincide with regressive coastal development, whereas phases of dwelling-mound construction or even abandonment of the area are indicative of transgressive development and inundation. Although the great number and volume of the dwelling mounds shows remarkable human activities, the effect of human interaction with natural processes seems to be negligible. This is mainly due to the fact that the dwelling mounds form only isolated hills in the extended coastal lowland area. Totally different effects resulted from the construction of dykes. This measure started at about 1100 AD with the erection of ring dykes around isolated areas and formed a continuous system of sea dykes protecting most of coastal lowlands against inundation by the 13th century. This had severe consequences on the subsequent development of the landscape. The natural silting up processes were restricted to a significantly reduced accommodation space on the seaward side of the dikes, whereas the embanked areas were cut off from natural processes. This led to a reduction of the great variety of primary facies units and of biotope diversity in the coastal zone. In addition, the construction of dykes required systematic drainage of the embanked areas by an artificial system of gullies, sluices and pumping stations,

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otherwise they would be inundated by fresh water from the adjacent hinterland and precipitation. The drainage measures led to a compaction of the coastal sediments, to enhanced oxidation of peat and thus to a significant lowering of the primary land surface. This had disastrous consequences when parts of the dykes collapsed leading to flooding of embanked areas which occurred repeatedly during storm surges of the medieval period as well as in modern times. The mass balance gives a quantitative insight in these processes. A comparison of the past accumulation processes and the present-day morphological behaviour of the intertidal zone demonstrates that the coastal sedimentary system is able to react in a flexible manner to changing boundary conditions. Even the significant anthropogenic modifications of the natural environment by the construction of dykes, massive coastal protection works, the embankment of former tidal bays, and the exploitation of sand and shells at the sea bed or of gas from deeper strata, did not repeal the basic rules of sedimentary systems. The observed high flexibility enables the system to adapt to or to counteract even the worst case scenario of future sea-level rise for the coming century. Therefore there is no reason for assuming that the sea-level rise could not become balanced by sediment accumulation in the coastal zone. A regional study of the raised bogs on the Pleistocene hinterland yielded a total number of 300 new 14 C-dates for the onset of raised bog formation in the area adjacent to the coast. According to earlier publications it had been expected that these bogs started to grow in the late Atlantic to the early Subboreal periods because of climatic deterioration. This assumption is contradicted by the new data sets, which give clear evidence that raised bog formation in the Elbe–Weser region, the Hunte–Leda catchment area, and on the Oldenburg– Ostfriesland ridge started already as early as 8400, 7900 14 C BP, and 7800 14 C BP (Fig. 11). It was found that the dominant factor for the formation of raised bogs was a change of climatic conditions triggered by the Holocene sea-level rise and shoreline displacement (see paragraphs 2 and 3). Both initiated a landward shift of the belt of oceanic climate, thus progressively creating favourable conditions for raised bog formation in the hinterland adjacent to the coastal zone. To some extent, this general trend has been modified by the morphology of the landscape. The lower courses of the Ems and Weser rivers acted as entrances for marine inundations. This might explain the fact that the beginning of raised bog formation in the southern part of the Oldenburg–Ostfriesland ridge occurs later than the onset in the more landward Hunte–Leda catchment area. Although the bases of the bogs occur between altitudes of 1 and þ81 m NN, the height difference does not play any role in the onset of peat formation in general. In contrast, the

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Fig. 11. Summarizing graph of all 14 C-datings from the base of the raised bogs. The bars show the percentage; numbers of available data also given.

pre-existing relief has some small effects on the growth of the individual raised bogs in closely neighbouring areas. Additional influence comes from the lithology of the substratum. In the investigation areas to the west of the Weser river it mainly consists of uniform fine-grained and decalcified sand, with a podsolic soil on its top, both offering highly favourable conditions for raised bog formation. Badly sorted sediments and a greater variety of soils are characteristic for the substratum in Elbe– Weser region. Both may be of some influence on the growth of the bogs in this area, which in general are smaller. The regional study shed new light on the onset and further development of the raised bogs in the Pleistocene hinterland of the coast. It demonstrated that these natural changes of the environment started already at about 8400 14 C BP and progressively affected landward areas until 2000 14 C BP. In the course of this process, the primary forest vegetation was replaced by the expansion of raised bogs and in addition to that significantly reduced the living space of human beings settling in this region. This is indicated by artifacts found underneath or within the peat, for example megalithic graves. Artificial constructions of wooden bridges across the expanding bogs demonstrate the efforts of men to maintain a system of tracks and roads through the

peatland. Such constructions are preserved from the late Neolithic period, about 2800–2100 BC, until the Roman period. However, clear evidence of them is missing from the medieval period (Hayen, 1985; Schwarz, 1995). Unearthed relics of such wooden constructions and other remains indicate that they have been used only for a time span of a few decades and were subsequently covered by the growing bogs. As a whole, this demonstrates that human beings were only able to react to the natural changes of the environment. Artificial drainage, mainly of fen peat areas, started at about 1200 AD, however, systematic drainage and cultivation of the extensive raised bogs began in the 17th century. So it can be stated that over most of the last 8400 years, the natural and climate induced processes dominated the evolution of the bogs and human activities played a minor role. Only during the last 400 years has human impact significantly affected the geo-biosphere of the bog landscape and subsequently led to present-day conditions.

Acknowledgements The authors thank Dr. J. Barckhausen, Geological Survey of the Federal State of Lower Saxony, for coring and many helpful discussions, Professor M.A. Geyh,

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Geological Survey of the Federal State of Lower Saxony, for radiocarbon measurements, Professor P.M. Grootes, Leibniz Labor fur . Altersbestimmung und Isotopenforschung, Christian-Albrechts-University, Kiel, for AMS-dating, W. Bartels, LUFA, Soil-Physical Laboratory, for botanical macro residual analyses, and . Dr. M. Bottcher, Max Planck Institute for Marine Biology, Bremen for measurements of stable sulphur isotopes. With regard to the mass-balance study, we wish to extend our thanks to the ordnance survey of Lower Saxony (LGN), to the Wasser- und Schiffahrtsdirektion Nordwest, Aurich, and the Bundesanstalt fur . Wasserbau, Karlsruhe, which furnished digital topographical data. Similar contributions were provided by the Forschungsstelle Kuste, . Norderney, of the Nieder. s.achsische Landesamt fur . Okologie, Hildesheim. Supplementary geological information came from the Netherlands Institute of Applied Geosciences (NITGTNO), which contributed structure contour maps of the base of the Holocene from regions neighbouring our study area. Prof. Dr. J. Vandenberghe and Prof. Dr. K. Barber are thanked for their constructive comments which greatly improved the manuscript. All studies were funded by the Deutsche Forschungsgemeinschaft (DFG) through grants No. Scho 561/2-1, 2, 3, Scho 561/4-1, GE 64/4-2, GE 64/7-1, Li 325/6-3, No. Str 142/3-1, 2, 3, 4 and No. BE 598/12-1, 2, 3, 4. The investigations were carried out as contributions to the interdisciplinary special research program ‘‘Changes in the Geo-Biosphere during the last 15,000 years— continental sediments as an expression of changing environmental conditions’’.

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