Jan Risberg

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadialstadial variability. Transitions between different ecological states occurred in as little as 40–230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Postglacial history and paleoecology of Wampum, Manitoba, a former lagoon in the Lake Agassiz basin

A small paleolagoon along the margin of the glacial Lake Agassiz basin near Wampum, Manitoba, 130 km southeast of Winnipeg, contains an unusually thick and continuous sequence of late-glacial and Holocene organic sediment. Cores at 17 sites through this sequence provide insight into the fluctuations of the level of Lake Agassiz, the timing of the construction of the lake9s most extensive strandline (the Upper Campbell beach), and the paleoecology of this area during the past 10 000 14 C yr. The history of this area is established from 26 new AMS radiocarbon dates, siliceous microfossils, plant macrofossils, mineralogy, sediment characteristics, and stratigraphic relationships. Following deglaciation at Wampum about 11 14 C ka, deposition of largely unfossiliferous silty clay with occasional ice-rafted clasts represents relatively deep-water conditions in Lake Agassiz. Overlying this is a spatially and temporally variable mixture of silt, sand, peat, gyttja, and marl, containing an allochthonous assemblage of siliceous microfossils and plant macrofossils with a wide range of radiocarbon dates. These sediments were deposited in an embayment of Lake Agassiz, mainly in shallow, quiet waters, as the lake transgressed about 10 ka over the Moorhead low-water phase surface. Between about 9.7 and 9.3 ka, the Wampum embayment was progressively isolated from Lake Agassiz by growth of the Upper Campbell spit, resulting in the formation of a lagoon. Organic-rich deposits on the lakeward side of the lagoon were buried 9.3–9.4 ka by sand and gravel of the spit, and a thin overwash of sand marks this event throughout the lagoon. At about this time, Lake Agassiz fell below the Upper Campbell beach level. Subsequent accumulation at Wampum consists mainly of algal gyttja during the early Holocene and marl during the middle Holocene; only minor fluctuations in hydrological conditions occurred, except for slightly warmer and drier conditions between 6.2 and 4.0 14 C ka. These stable, shallow-water conditions may be due partly to persistent groundwater contributions to the site. Progressive encroachment of rooted aquatics led to present-day fen conditions; peat accumulation across the basin began by about 4.0 ka.

Stratigraphic evidence for a high marine shore‐line during the late Weichselian deglaciation on the Kullen Peninsula, southern Sweden

Mineral magnetic properties and the carbon content of a sediment sequence in Lake Kullatorpssjon on the Kullen Peninsula in northwest Scania, southern Sweden, were investigated. Diatom and ostracod analyses were undertaken for palaeoecological reconstruction and a chronology was constructed from accelerator mass spectrometry (AMS) radiocarbon dating of terrestrial plant remains and mosses. Five stratigraphical units were identified (units 1-5, from oldest to youngest). The two lowermost units consist of clay. The lowest clay unit (1) is strikingly black, 2 m thick and has magnetic properties dominated by high concentrations of authigenic greigite (Fe3S4). In contrast, the overlying clay unit (2) is grey, 1.18 m thick and has magnetic properties dominated by low concentrations of detrital magnetite (Fe3O4). A major palaeoecological change is reflected in the diatom flora and ostracod fauna at the transition from unit 1 to unit 2. The sediment stratigraphy, geochemical, mineral magnetic and palaeoecological analyses also point to a significant change in depositional environment at this transition, which is proposed to represent the isolation of Lake Kullatorpssjon from the Kattegat Sea. Earlier investigations have determined the marine limit to ca. 65 m a.s.l. on the Kullen Peninsula. The separate independent lines of evidence presented here, however, suggest that the Late Weichselian marine limit developed at 85-90 m a.s.l. at the deglaciation ca. 17 200 cal. yr BP (ca. 14 500 yr BP). This limit is ca. 25 m higher than assumed previously. Lake Kullatorpssjon formed ca. 1000 cal. yr later when the basin became isolated from the Kattegat Sea. Copyright (C) 1999 John Wiley & Sons, Ltd. (Less)

Siliceous microfossils as late-Quaternary paleo-environmental indicators at Braamhoek wetland, South Africa

A peat-sequence covering the last 16 ka (16 000 cal. yr BP) from Braamhoek wetland, eastern South Africa, was analysed in terms of phytolith and diatom composition. The fossil peat was rich in phytoliths, while diatoms were less prominent, probably as a result of degradation during wetland plant growth associated with silica uptake. With this study we present the first continuous phytolith and diatom record from South Africa covering the Late Pleistocene and Holocene period. The phytolith assemblages indicate a clear dominance of C3-grasses within the wetland throughout the sequence. The fossil diatom record infer changes in past moisture conditions. Unlike the modern wetland, which is dominated by benthic and aerophilic diatoms, the Late Pleistocene—early Holocene wetland favoured growth of planktonic species. Abundance of planktonic diatoms suggests three main phases when water depth was deeper than today; at c.13.6 ka, 11.3 ka and 10.4—10.0 ka. These indications of past fluctuations in humidity mostly provide confirmation of previously published indications of pollen, charcoal fragments and isotopes in the same core, but the siliceous microfossil data also help to refine the paleo-environmental interpretation of the sequence.

Deglaciation, holocene shore displacement and coastal settlements in Eastern Svealand, Sweden

Abstract Detailed mapping of the clay-varve sediments and stratigraphical analysis of isolated lake basins on the Sodertorn peninsula have been undertaken as part of the research project “Eastern Svealand: Development of the Holocene Landscape”. The varve studies have identified three distinctive stratigraphical units and indicate that deglaciation took place between 11,400 and 10,400 clay-varve years BP. Initially recession of the ice margin was slow but subsequently it became more rapid. Study of 15 isolation basins has allowed a tentative shore displacement curve to be constructed for the area. This curve is marked by varied rates of regression and possibly displays 3 Litorina transgressions. The curve suggests markedly different ages for the transgression events than those proposed by earlier authors and this is assessed in terms of curve construction and dating errors. Archaeological evidence indicates that coastal settlements changed their altitudinal location in response to changes in sea level, this evidence supporting the data derived from isolation basins.

Early Holocene shore displacement and evidence of irregular isostatic uplift northwest of Lake Vänern, western Sweden

In the 1920s the Swedish geologist Lennart von Post found evidence of irregular early Holocene isostatic uplift in the region northwest of Lake Vänern in western Sweden. von Post based his conclusions on levellings of ancient shore lines. These were indirectly dated by pollen stratigraphical investigations in basins situated at approximately the same altitudes as the shore lines. To test von Posts hypothesis, we have adopted a different methodological approach. The sediments in twelve small lake basins, within five minor areas in this region, have been investigated with respect to when they became isolated from ancient Lake Vänern. Initially all the lake thresholds were levelled. Altitudes range between 165 and 96 m above sea level. By combining the results of mineral magnetic and diatom stratigraphical analyses it has been possible to determine the level in the sediment when the basin became isolated. The 14C age of this isolation level has been inferred from AMS dates based on the NaOH-soluble fraction of bulk sediment, partly with very low organic content. Altogether, 36 radiocarbon datings have been carried out, ranging from 10080 to 8800 14C years BP. Pollen analyses, and subsequent correspondence analysis (CA) of all pollen spectra, were carried out around the isolation level in each basin as a complement to the AMS datings.If the region had been subject to irregular isostatic uplift, lakes at the same altitude in the five areas should have been isolated at different times. The AMS determined isolation ages, ‘corrected’ according to CA of the pollen stratigraphy and compensated for the relatively higher rebound of the northerly situated sites support von Posts conclusion, from the beginning of this century, that the region northwest of Lake Vänern has been subjected to irregular isostatic uplift.

Magnetic susceptibility in sediment records of Lake Ådran, eastern Sweden: correlation among cores and interpretation

Magnetic susceptibility was used to test the representativity of the lithostratigraphy of a master sediment sequence from Lake Ådran, eastern Sweden. Five further sediment cores from the same lake were correlated and compared with the master sequence using magnetic susceptibility records. Mineral magnetic correlations are generally based on matching prominent susceptibility features but may be significantly improved by using slot sequence analyses. The result of these analyses show that the sediment in the Lake Ådran master sequence can be considered representative for the basin. The variations in the magnetic susceptibility in relation to pollen and diatom analyses also seem to reflect water level changes and shore displacement in the Baltic between 9900 14C years B.P. and the isolation at 5900 14C years B.P. The first emergence of scattered non-vegetated islands is characterised by high susceptibility values. Along with a continuous water lowering and the development of the vegetation during the Ancylus fresh water stage, susceptibility values gradually decrease. A rapid phase of the Ancylus regression between 9200 and 9000 14C years B.P. is seen as an increase in magnetic susceptibility. During the following Baltic brackish water stage, the Litorina stage, susceptibility values are at their lowest, followed by higher values at the isolation from the Baltic with a maximum around 5000 14C years B.P.Shore displacement between 9900 and 5900 14C years B.P. is discussed based on the magnetic susceptibility measurements and their relationship to the pollen and diatom analyses.

Environmental changes and human impact as recorded in a sediment sequence offshore from a Viking Age town, Birka, southeastern Sweden

The Viking Age town of Birka, which existed between AD 750 and 975, on the island of Björkö in the Mälar archipelago of the Baltic Sea, is known as the oldest centre used for large-scale international trading in Sweden. Birka had a strong defence: town rampart, hillfort and a water palisade. Studies of lithology, and microand macroremains in a sedimentary sequence accumulated offshore from Birka, outside the water palisade, revealed a stratigraphically distinct refuse stratum, referred to as the Birka Layer. We argue that the bulk material of the Birka Layer emanates from human activities in the town, e.g., latrine cleanings, cuttings of shore vegetation, cleaning of ditches and storage areas, and domestic animal dung. Organic remains from vegetation and animals, deposited during activities on the ice offshore from the town, e.g., winter markets, may also have contributed to the composition. The material constituting the Birka Layer started to accumulate after the construction of the protecting water palisade. In the Birka Layer and the underlying clay gyttja, there seems to be a succession of five activity phases recorded during the Birka period. After the initial establishment of the town, an expansion took place, followed by a retrogression. The most intense phase with culminating activities is registered prior to abandonment of the site, which approximately coincides with the onset of the extended final isolation process of Lake Mälaren (AD 1000-1300). The sailing routes to Birka from the south and east became too shallow and were finally cut off by the emerging thresholds. After the abandonment of Birka, the town area was utilized for cultivation and cattle-breeding.

Deglaciation, shore displacement and early-Holocene vegetation history in eastern middle Sweden:

The sedimentary record at Lake Vibysjo lake basin is situated close to the Närke Strait where the main water exchange occurred between the Vänern basin and Baltic basin during the middle and late phases of the Yoldia Sea stage of the Baltic Sea (c. 10 000– 9500 14C years BP). The area was deglaciated in a glaciomarine environment, as reflected by the occurrence of benthic calcareous fossils (foraminifera, ostracods and the mollusc Portlandia arctica (Gray)) in the lower-most clay units. The estimated date of deglaciation, 10 440 clay-varve years BP, suggests that a saline bottom-water current entered the Baltic basin shortly after the high areas south of Lake Vibysjön became free of ice. The brackish water fauna occurs during a regional Betula-Empetrum-Hippophaë pollen zone, which in southern Sweden is correlated with the early Preboreal. The isolation of the Lake Vibysjön basin is dated to 8100 14C years BP, and occurred during the last part of the Ancylus Lake stage. Based on investigations of other basins in Närke, it is concluded that the Mastogloia Sea reached c. 62 m a.s.l. and that the Litorina Sea reached at least 60 m a.s.l. in the area studied.

High-resolution environmental reconstruction in SW Peloponnese, Greece, covering the last c. 6000 years: Evidence from Agios Floros fen, Messenian plain:

A paleolimnological record from the central Messenian plain (southwestern Peloponnese, southern Greece) indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the Taygetos Mountains during the last c. 6000 years. The analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7.5-m-long sediment core retrieved from the drained fen of Agios Floros, at the northern banks of the ancient River Pamisos. The sequence consists of fen peat in the uppermost section underlain by lacustrine sediments, which are punctuated by two layers of clay with diatomaceous silt bands. High accumulation rate is recorded in the oldest part of the section (up to 23 mm/yr), particularly during two decadal-long periods centered at c. 5700 and c. 5300 cal. BP. The diatom record reveals pronounced peaks in the planktonic taxon Cyclotella distinguenda, which correspond to the laminated sequences, reflecting the rapid development of a deep lake with an open water environment during these two time periods. Another two events with intermediate water levels are inferred at c. 5200 and c. 4600 cal. BP. These short-lived phases were probably, to a large extent, caused by local tectonic processes and the consequent hydrological anomalies of the nearby karst springs, although abrupt climatic changes with enhanced precipitation might have also played a role. At c. 4500 cal. BP, our data suggest the development of terrestrial conditions in this area, which can be attributed to the decreasing activity/dry up of springs, probably associated with more arid climate. After c. 2500 cal. BP, the diatom record infers a return to wetter conditions, probably as a response to more humid climate with marked seasonality and human activities, developing the present-day environment with cultivated and seasonally semi-flooded fields.