International Geologiical Congress - Oslo 2008

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CGC-09 Glacial-interglacial vegetation dynamics

 

Formation of peat layers in Estonia from the Holocene till now

 

Mall Orru, Geological Survey of Estonia (Estonia)
 

 

Estonia is among the areas richest in mires in northern Europe - 9836 mires make up 22.3% of the territory. Accumulation of mire deposits began soon after the retreat of the continental glacier, first in southern (upland) Estonia and somewhat later in northern (lowland) Estonia. The postglacial climate has been favourable for development of mires and accumulation of peat.
The geological investigations included all mires in Estonia with an area more than 1 hectare. The entire peat sequence was investigated. To establish the genesis and age of the peat layers, from 18 mires were collected altogether 360 samples for pollen and spore analyses; the same samples were used also for C14 dating. The samples were taken every 10 cm within the whole sequence. The changes in botanical composition and content of mineral matter during the Holocene were analysed.
General climate change in the postglacial period caused changes in vegetation, what can be seen on peat composition as well. The latter changes occurred simultaneously in large areas.
Based on the results the development of mires in Estonia can be described as follows:
- the oldest postglacial deposit in Estonia is sapropel which underlies peat in Remmeski mire; accumulation of the above sapropel began 10 740 years ago;
- In late Preboreal or early Boreal (9 100-8 800 years ago) the formation of peat began by accumulation of fen peat. In the peat samples dominates pollen of birch (76%), while pine pollen makes up 20%;
- In the Boreal pine forests dominated (60-65%); besides, the share of Gramineae plants and Cyperus was considerable as well. The Bryales, reed and sedge peat accumulated. During the first half of the Boreal (8 800-8 200 years ago) the formation of large mires started, while in the late Boreal (7800 years ago) also the formation of high moor peat began as well as transition from feeding from groundwater to feeding from precipitation.
- In the following Atlantic, Subboreal and Subatlantic climate periods in the mires of central and southern Estonia intense growth of high moor peat took place;
- In the West-Estonian Archipelago where the sea retreated later, the first peat layers deposited 3 200 years ago. During this period of time dominated alder, birch, and linden, later also spruce;
- In coastal areas the formation of high moor peat began only 1 000-2 000 years ago;
- The formation of peat continues today; in large mires accumulates moss peat and in minor closed depressions ? fen sedge-reed peat;
The development of mires and vegetation, their distribution and formation of peat and content of pollen in samples are controlled by several factors (relief, groundwater level, character of the Quaternary cover peat accumulation rate). Comparison and correlation of palynozones over extensive areas allows identifying the reasons which have influenced the dynamics of distribution of plant species and to prognosticate the future climate change, as well as the character and rate of peat accumulation.

 

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