MARINE GEOLOGY AND GLACIOLOGY
MOLLUSC RESEARCH

Characterizing the Sediments from the last Drainage Event of the Baltic Ice Lake

by

Kaare Lund Rasmussen kaare.lund.rasmussen@natmus.dk and Kaj Strand Petersen ksp@geus.dk

Submitted to Boreas, 5-OCT-2000

Abstract

We have characterized the sedimentary outpour from the final drainage of the Baltic Ice Lake. The characterization was done by a newly developed method, which consist of measuring two material parameters of the sediment: the magnetic susceptibility and the thermoluminescence sensitivity. The results reveal the detailed sequence of successive clay sources in the Danish basin during the Pleistocene/Holocene transition.

Introduction

The Skagen well was sunk 1992-93 just north of the town of Skagen in Northern Jutland, Denmark, and produced a cored section of 192 m with Quaternary marine deposits resting on Lower Cretaceous non-marine sands. Two sequences were found: Eemian/Early Weichselian and Late Weichselian/Holocene marine deposits, the latter with a thickness of ca. 130 m of which the Holocene constituted 115 m resting on clayey arctic Younger Yoldia clay. From studies of microfossils (Knudsen et al. 1996) and macrofossils (Petersen 2001) it appears the boundary between the arctic Yoldia clay and the temperate Holocene part is very sharp. A feature which might be induced by a change in the palaeocurrent system at that time.

The change in fauna is, however, not reflected in the sedimentary record, which shows a very homogenous clayey grain size distribution with nothing coarser than fine sand. Only at one sample at the sharp boundary between Late Weichselian and Holocene is observed medium sand, coarse sand and gravel. On this homogenous sequence of clay to fine sand we have conducted our measurements of magnetic susceptibility and thermoluminescence sensitivity.

Methods

The method of characterizing clay sediments has proven effective in studies of provenancing archaeological ceramics, pot sherds and red brick (Feveile et al. 1978, Rasmussen 1999, Rasmussen 2001). We measure the magnetic susceptibility on gram size samples with a KLY-2 susceptibility-meter capable of reaching ca. 1 10-6 SI-units.

The thermoluminescence sensitivity is measured on four 10 mg aliqoutes of the crushed and sieved grain size fraction 100-300 g.m. For the TL-measurements we use a TL/OSLsystem TL-DA-12 manufactured at Risø National Laboratory, Denmark. The palaeosignals are erased by heating the samples to 400°C. Subsequently the samples are irradiated for 60 sec under a 1.5 GBq 'Sr-source. The samples are then annealed for 30 sec at 200°C, and the TL-signal measured from 202-235°C and integrated in order to yield our TLsensitivity. Drift in photodetector high tension, optical transmission and other system parameters are monitored by measuring four aliqoutes of a standard sample daily.

The two material parameters are shown in Fig. la and lb as a function of depth in the core. The analyses showed four distinct facies (see Fig. 2). Three of them in the Late Weichselian and one in the Holocene. This is within a time span of ca. 7000 years, from ca. 14450±155 14C-years BP to middle Boreal at 8120±80 14C-years BP (Petersen 2001). The first source of sediments is recognized at the bottom of the analyzed interval. This is interrupted by a sequence of distinctly different sediments from 121.3 m to 119.2 m, after which sedimentation from the first source recommences. This was a time with a high sedimentation rate where the Danish area was still affected by the waning NE-ice and the Younger Baltic Ice subsequently covered only the southern part of Denmark, but melting away from Bornholm before the start of Bølling.

As can be seen from Fig. la and 2 there is a dramatic change in magnetic susceptibility at the very end of the Late Weichselian, the Younger Dryas, where a distinctly different sediment source is mapped in two samples at 115.3-115.0 m depth. In Younger Dryas no glaciers have been found calwing into the Danish seas, so the recorded peak must be connected with another event than calwing glaciers and/or meltwater runoff. Throughout the Skagen core grain size analyses has been conducted and within the section from 100-130 m 24 such analyses have all reveal clay to fine sand except for one sample at 115.1 m, 14C-dated to 10420±130 14C-years BP (Knudsen et al. 1996). Here the medium sand, coarse sand and gravel are found with 3.9, 0.6 and 0.2 percent respectively - not much - but a clear signal on the background of the 23 other samples above and bellow - all being more clayey and without coarser material than fine sand.

When looking into the millennia following the last glaciation the dynamic history of the Baltic Sea is in focus (Bjorck 1995) and especially the sudden drop from the highest Baltic coastline by 26 m when the ice margin retreated from Mt. Billingen in Sweden (see Fig. 3). This final stage of the Baltic Ice Lake has been dated to occur at ca. 10,400-10,500 '4C-years BP (Stromberg 1992). Mt. Bipingen is situated a little more than 200 km to the east of the Skagen well, but when taking into consideration that ca. 7000 km 3 of icedammed water (Stromberg 1992) must have drained through to the Western Sea - Kattegat - at that time, it is not unreasonable to think that a major imprint is visible at the Skagen site. The coarser material with high magnetic susceptibility and high thermoluminescence sensitivity found at 115.1 m depth in the Skagen core is identified as the erosional product of this huge amount of water.

It is worth noticing that in the diagram of magnetic susceptibility versus TL-sensitivity (Fig. 2) the two samples forming the peak in the last part of the Late Weichselian also represent the more immature sediment (high susceptibility and high TL-sensitivity). In contrast the whole series of samples from the lower part of the Holocene seems more mature (low susceptibility and low TL-sensitivity). So, in this way the peak can also be connected with the sudden break through of the water from the Baltic Ice Lake at Mt. Billingen, whereas the mature sediments from the Holocene may reflect the long transported sediments introduced by the new current system from the Atlantic bringing in the new temperate fauna in the early part of the Holocene and replacing the arctic fauna of Late Weichselian age.

Acknowledgements

The Carlsberg Foundation are thanked for support of the equipment. Karen Skov Jensen, Christina Jepsen, Lisbeth Jepsen and Peter Kristensen are thanked for technical assistance.

References

Bjorck, S. 1995: A review of the history of the Baltic Sea, 13.0-8.0 ka BP. Quaternary International 27, 19-40.

Feveile, C., Jensen, S. & Rasmussen, K.L. 1998: Produktion af drejet keramik i Ribeområdet i sen yngre germansk jernalder. KUML, vol 1997-98, 143-159.

Knudsen, K.L., Conradsen, K., Heier-Nielsen, S. & Seidenkrantz, M.-S. 1996: Quatenary palaeooceanography and palaeogeography in Northern Denmark: a review of results from the Skagen cores. Bull. Geol. Soc. Den. 43, 22-31.

Petersen, K.S. 2001: Late Quatenary environmental changes recorded in the Danish marine molluscan faunas. Geol. Den. Surv. Bull. in press.

Rasmussen, K.L. 1999: Ny arkæometrisk metode til proveniensbestemmelse af keramik. Arkæologiske udgravninger i Danmark 1998. 1999, 17-26.

Rasmussen, K.L. 2001: Provenance of ceramics revealed by magnetic susceptibility and thermoluminescence. Journal of Archaeological Science in press.

Stromberg, B. 1992: The final stage of the Baltic Ice Lake. Sveriges Geologiska Undersokning, Ser. Ca 81, 347-353.