Mats Rundgren

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

The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10 000-9900 and 9600-9500 14 C plateaux (c. 11 300-11 150 calendar yr BP) it is also difficult to 14 C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch-pine forest in Germany-Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased 2 H and 13 C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric 14 C/ 12 C ratio. A slow-down of the thermohaline circulation may tempor- arily have pushed the Polar Front further south.

Glacial and climatic events in iceland reflecting regional north atlantic climatic shifts during the Pleistocene-Holocene transition

Abstract This paper presents a summary of the evidence for glacial and climatic changes during the late Pleistocene-early Holocene transition in Iceland. The deglaciation during the Bolling-Allerod event was interrupted by a short-lived Older Dryas glacial advance. A biostratigraphical record from northern Iceland shows significant climate warming in late Allerod, when mean July temperatures were at least as warm as those of today. An abrupt cooling marked the beginning of the Younger Dryas event. It was characterised by a cold and stable polar climate and an extensive glaciation, before the postglacial warming of climate set in. The Icelandic paleoclimatic record is discussed in the light of climatic oscillations recorded from the GRIP ice-core, from the Greenland Inland Ice, and with reference to major shifts in the oceanic front systems, recorded in the Troll 8903 marine sediment core from the North Sea. The Vedde Ash gives a unique opportunity to address the chronological problems and correlate event stratigraphies of the different proxies. It is concluded that the Icelandic record of glacial and climatic changes during the late Pleistocene—early Holocene transition largely reflects the climatic development in the North Atlantic region.

Late-glacial and early Holocene variations in atmospheric CO2 concentration indicated by high-resolution stomatal index data

Data from ice cores suggest that Late-glacial and early Holocene atmospheric CO2 variations were rather conservative, the most important change being a gradual Younger Dryas increase. By contrast, palaeo-CO2 records based on the inverse relationship between CO2 Partial pressure and stomatal frequency of terrestrial plant leaves reflect a more dynamic CO2 evolution, including an abrupt decrease at the Allerod/Younger Dryas transition. Here we present a Late-glacial and early Holocene CO2 record based on stomatal index data from leaves preserved in the sediments of a small lake in southwestern Sweden. Three independent records constructed from stomatal index data of Salix polaris, Salix herbacea and Betula nana leaves were combined to form a high-resolution CO2 reconstruction for the period 12 800-10 800 cal yr BP. Atmospheric CO2 concentrations were found to have decreased rapidly from c. 260 ppmv to 210-215 ppmv within 200 years during the Allerod (GI-1)/Younger Dryas (GS-1) transition. After 100-200 years, CO2 concentration started to gradually increase to 270-290 ppmv at the end of the Younger Dryas stadial (GS-1). CO2 concentrations were relatively stable during the early Holocene, except for a short-lived period of lower (240-250 ppmv) values c. 11350-11200 cal yr BP. This Late-glacial and early Holocene CO2 evolution partly resembles previous stomatal-based CO2 reconstructions, and the overall trend is almost identical to that seen in ice-core records. The amplitude of change is, however, markedly higher in the Swedish stomatal-based record compared to the ice cores. This difference may partly be accounted for by the inherent smoothing of ice-core CO2 records caused by diffusion, but a major part of the difference in amplitude between ice-core and stomatal-based records still remains to be explained. Based on our reconstruction, atmospheric CO2 may have played an important role in climate dynamics during the last deglaciation

Early-Holocene vegetation of northern Iceland: pollen and plant macrofossil evidence from the Skagi peninsula

Pollen and plant macrofossil records from two lakes on northernmost Skagi peninsula, northern Iceland, reflect a progressive closing of the vegetation cover during the early Holocene. This development was connected with the succession from an initial herb-tundra phase characterized by Oxyria digyna, Poaceae and Caryophyllaceae, through an intermediate dwarf-shrub phase dominated by Salix and Empetrum nigrum,toa shrub and dwarf-shrub phase with Juniperus communis, Betula nana, Salix and Poaceae. This development is similar to that recorded at other sites in northern Iceland, both with respect to the taxa involved and the timing of transitions between successional phases, which suggests that early-Holocene vegetational development in northern Iceland was forced by broad-scale climatic changes. The palaeobotanical data suggest a gradual early-Holocene warming in northern Iceland, which probably was connected with northward displacements of Atlan tic waters and North Atlantic air masses. The deposition of the Saksunarvatn ash at 9000 BP caused a transitory phase of vegetational succession on northernmost Skagi.

Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe Islands

Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvotn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 mu m. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300-7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes. (Less)

Age, geochemistry and distribution of the mid-Holocene Hekla-S/Kebister tephra

The middle Holocene Hekla-S/Kebister tephra originates in the Hekla volcanic system on SW Iceland. The distal distribution of the tephra includes the Faroe Islands, Shetland and Central Sweden, indicating a main dispersal towards the east. The chemical composition of the tephra follows the pattern of other major eruptions of Hekla, and ratios between selected oxides may in some cases allow separation from other major Holocene tephras from Hekla. Tephra from the Plinian phase dominates in eastern sites, while tephra also from later phases is found in the Faroe sites. Wiggle-matching of radiocarbon dates around the tephra in a Swedish peat-bog suggests an age around 3720 cal. yr BP (3750—3700 cal. yr BP), which is in accordance with previous attempts to date this tephra. This is within a period with significant climate changes in NW Europe and opens possibilities for exact comparisons of peat and lake sediment records from different geographical areas.

Variability and seasonality of North Atlantic climate during the early Holocene: evidence from Faroe Island lake sediments

Based on their position in relation to major ocean currents, palaeoclimatic archives in the Faroe Islands are expected to be sensitive recorders of variations in North Atlantic circulation. The multiproxy analysis (magnetic susceptibility, total carbon, nitrogen and sulphur, biogenic silica, grains >255 µm) of a lacustrine record with both winter and summer climate indicators illustrates the variability of Faroese climate during the early Holocene (c. 11 300—10 240 cal. yr BP) and allows comparison with other records in the region. Our high-resolution record suggests a period of predominantly winter cooling and increased storminess centred at c. 11 150 cal. yr BP followed by a period of stability between c. 11 000 and 10 680 cal. yr BP, which correlates with a time when the North Atlantic was particularly free of meltwater incursions. After c. 10 680 cal. yr BP, both winter and summer indicators show an increase in variability broadly synchronous with increased variability at other North Atlantic sites. Within this time period, a predominantly winter cooling centred at c. 10 600 cal. yr BP, a winter/summer cooling centred at c. 10 450 cal. yr BP and a winter/summer cooling centred at c. 10 300 cal. yr BP are recorded. These distinctive coolings correspond to periods of ice core δ18O inferred lowered atmospheric temperatures over Greenland and the reoccurrence of meltwater outbursts at c. 10 600, 10 500 and 10 350 cal. yr BP.

Late-Holocene expansion of a south Swedish peatland and its impact on marginal ecosystems: Evidence from dendrochronology, peat stratigraphy and palaeobotanical data

In this study, a reconstruction of the long-term development and lateral expansion of a south Swedish peat bog was performed using a multi-proxy approach, including dendrochronology, peat stratigraphy and macrofossil and pollen analyses. By combining mapping of cross-dated subfossil trees with radiocarbon-dated peat sequences, an improved approach to reconstruction of lateral peat expansion was applied. Apart from providing approximate ages of tree burial episodes, the ring-width records offer information on hydrological variations prior to the bog expansion. New bog oak, pine and alder chronologies are presented and their potential as a dating tool for peatland expansion as well as for local to regional environmental interpretations is examined. Our tree-replication records show that increased amounts of bog trees in the central parts can be linked to drier bog-surface conditions, whereas an increase in wood remains in the marginal zone is related to enhanced preservation due to lateral bog expansion. Our reconstructions of the development of the peat deposit and associated changes in the distribution of vegetation communities provide new insight into peatland responses to climate change at the end of the ‘Holocene Thermal Maximum’ (5000–4000 cal. yr BP).

Century-scale changes of atmospheric CO2 during the last interglacial

The crucial role of atmospheric CO2 in glacial-interglacial transitions is demonstrated by recent ice-core studies that highlight the importance of accurate CO2 records for our understanding of Quaternary climate dynamics. Previous estimates of CO2 levels during the last interglacial stage (the Eemian) relied on measurements on air trapped in the Antarctic Vostok ice core. Due to uncertainties associated with in situ chemical alteration and gas diffusion, there is a need for independent estimates of past CO2 levels. Here we report 33 Eemian CO2 estimates based on the stomatal index of Salix herbacea L. leaves preserved in Greenland sediments. We reconstruct Eemian CO2 levels centered on 250-280 parts per million by volume (ppmv), in general agreement with ice-core data. Two deviating (lower) stomatal estimates may reflect diffusional smoothing of Vostok CO2, data and indicate century-scale Eemian CO2 variability. (Less)

Fossil leaves: Effective bioindicators of ancient CO2 levels?

[1] Past variations in the concentration of the greenhouse gas CO2 are thought to have played a major role in controlling Earth’s climate on preQuaternary and Quaternary timescales. To identify the contribution of CO2 to past climatic change requires accurate quantification of its content in the ancient atmosphere, and a number of proxies have been developed for this purpose (for a review see Royer et al. [2001a]). For the Late Quaternary, there is the unique opportunity to measure directly the composition of fossil air samples trapped in bubbles preserved in the polar ice sheets. Results from Antarctic ice cores reveal that the glacialinterglacial changes characterizing Quaternary climate were accompanied by variations in the atmospheric concentration of CO2 [Petit et al., 1999]. Although detection of phase relations between isotope-derived temperature estimates and trace gas concentration values is hampered by the difference in age of ice and air from the same ice sample, it is believed that CO2 lags glacial-interglacial temperature change and has acted as an amplifier of orbitally forced changes in temperature [Petit et al., 1999; Shackleton, 2000; Mudelsee, 2001].