Laminated Lake Sediments and their Impact on Paleoclimatology

Outline

1 Introduction

2 Laminated lake sediments
2.1 Different types of varved sediments
2.2 Driving forces of varve sedimentation

3 Reconstructions of climate variability during transition Pleistocene/ Holocene in Central Europe using laminated Lake Sediments from four sites
3.1 Methods and results
3.2 Correlation and synchronisation

4 Reconstructions of climate variability during at LGM in the Near East
4.1 Previous studies at Lake Lisan
4.2 Work placement studies
4.3 Implications of Lake Lisan on the understanding of climate dynamics

5 Conclusion

6 References

1 Introduction

Zolitschka (1998) defined varves as laminated sediments occurring in stationary water bod­ies. Their striking feature is a seasonal, rhythmical build-up of thin, horizontal layers with a changing composition.

Since the early 20th century, this type of sediment has been used for establishing exact geochronologies. In the course of the climate change debate, and the subsequent demand for high-resolution paleoclimatic data, they have come back into the focus of earth scien­tists.

A crucial advantage of varved sediments is that they provide two different kinds of informa­tion on a sediment profile: an absolute chronology and high-resolution paleoclimatic infor­mation. Combining these two attributes offers an absolute dated time series of paleoclimatic proxy-data. Furthermore, additional analyses of the same profile can be dated.

Varved sediments can be found in recent lakes as well as in paleolakes. However, their occurrence is limited to only a few sites as a consequence of the special circumstances necessary for their generation. Varved sediments in recent lakes are especially found in lakes, which are small and deep. These preconditions are often met by maars, where a lot of investigations are done (Brauer et al. 1999a, 1999b; Litt et al. 2001). Additionally, varved sediments are even found in ancient proglacial lakes (Moscariello et al. 2000) to give an example.

2 Laminated lake sediments

2.1 Different types of varved sediments

Varves occur under different environmental circumstances; hence different varve types can be distinguished. Theoretically, it can be differentiated between three major types, however, in reality a combination of two or more can come across as well (see 4.).

Organic varves occur mainly in middle latitudes areas with vegetation covered catchments. Evaporative Varves are limited to regions with a distinctive dry season, which allows the precipitation of autochthonous minerals. Currently, varves with high clastic content predo­minantly occur in high latitudes and throughout mountain regions. Furthermore, seasonal clastic layers can be found in lakes/ paleolakes with a low grade of vegetation cover (sea- sonal dry areas, see section 4) with a high potential for erosion.

Fig. 1 shows the types of varved sediments: A) shows a combination of biological (organic)

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Fig. 1: Different types of varved sediments and their seasonal layering.
Negendank & Zolitschka (1997)

and chemical (evaporative) types; B) shows the physical (clastic) type. Within one varve intraannual sublayers can be easily distinguished. In this case the spring/summer layer of A) appears light as a consequence of deposited algae blooms, whereas during the winter season amorphous organic deposits result in a dark appearance. The summer layer of B) is coarse grained as an effect of different seasonal sediment input due to changing runoff rates (snow melt during spring/summer). In contradiction, during winter the runoff and ero­sion/ accumulation rate considerably decreases. That is the cause why the winter layer is noticeably finer-grained.

2.2 Driving forces of varve sedimentation

Fig. 2 summarizes the main factors influencing the varve sedimentation as well as some examples of resulting varve features. The main driver of changing varve sedimentation is the climate. It acts directly during the generation of evaporative varves, for example, be­cause temperature and precipitation are important drivers of evaporation. Furthermore, cli­mate affects varve composition indirectly via several effects on other factors within the

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Fig.2: Factors and their interactions that affect the varve composition. On the right hand side a selection of features, which can be analyzed is shown.
Own] schema

catchment like soil, vegetation, land use or glaciations to name just a few. But there are even other, climate independent (at least on time scales of centuries and millennia’s) fac­tors affecting the sedimentation of varves. The relief influences the flow velocity of water and therefore the input of different kinds of material into the lake. Geology affects the min­eralogy of the lake sediments as well as factors in the catchment. The right hand side of Fig.2 shows some selected features, which can subsequently be analyzed: varve thickness, content of organic material, pollen composition or mineralogy to name a few.

3 Reconstructions of climate variability during transition Pleistocene/ Holocene in Central Europe using laminated Lake Sediments

Analyses on Greenland ice cores GRIP and GISP2 have greatly contributed to our under­standing of climatic history of the North Atlantic region during the last glacial. The climate was marked by several oscillations on varying time-scales. The data presents a high tem­poral-resolution and is based on 818O isotope analysis (Dansgaard et al. 1993).

Nevertheless, to evaluate climate variability in central Europe during the late glacial further analysis of continental records, which present an archive from the concerning area itself is necessary. Laminated lake sediments seem to be a promising approach. The great poten- tial of this archive relies on the possibility of establishing accu­rate chronologies and a subse- quent comparison with Greenland ice core data.

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Litt et al. (2001) suggest devel­oping regional stratotypes on the continents instead of using Greenland ice core as a strato-type for terrestrial European records. This approach offers an enhanced understanding of regional environments and their climate variabilities as well as the possibility to recognise tele- connections. A paper of Litt et al. (2001) provides an overview of varved lake sediments from four sites: Meerfelder Maar (western Germany), Hämelsee (northern Germany), Lake GoĞcią* (central Poland) and Lake Perespilno (eastern Poland). The study focuses on the correlation and synchronisation of the information that these sediments provide. The analy­ses are based on varve chronology, tephrochronology, palynostratigraphy and stable iso­topes.

3.1 Methods and results

Varve chronolgy and tephrochronolgy

The Meerfelder Maar sediments are laminated to a large extent and provide 12,000 counted varves. However, the uppermost part is not continuously varved. Hence, the need for an independent dating method arises. The connection to an absolute timescale is done by using the tephra of the Ulmener Maar eruption, which is present in all maar lakes in this region. The Ulmener Maar tephra (UMT) has been dated by varve counting in the Holzmaar (varved until today) record to 11,000 calendar years BP. The Laacher See tephra (LST), the most widespread tephrochronological feature in late Weichselian sediments in Europe, has been varve dated in the Meerfelder Maar record at 12,880 calendar years BP (independent Ar/Ar dated 12,900 ± 400 years). Unfortunately, the LST was not found in the polish lakes.

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