Peat Stratigraphy and Changes in Peat Formation During the Holocene in Latvia

Abstract

Mires represented by raised bogs, fens, and transitional mires cover approximately 10 % of Latvia. They started to form towards the end of the last glacial period and the beginning of the Holocene under varying geological and palaeoecological conditions. The aim of the study was to evaluate how the results of peat palaeobotanical investigation can reveal changes in the conditions of peat formation and allow a subdivision to be made of the Holocene according to the scheme suggested by the INTIMATE group (Walker et al. 2012). Records from 21 mires have been evaluated to find evidence of changes in mire vegetation and peat formation, and to discover whether these records define boundaries allowing the Holocene deposits to be subdivided into the early Holocene, middle Holocene or the Holocene Thermal Maximum (HTM), and the late Holocene. Information from earlier studies and data from recent investigations are used for the revision of peat stratigraphy revision. Peat accumulation and fen formation during the early Holocene took place in the oldest mires, which started to develop by the infilling of shallow lake depressions. During the second part of the early Holocene, before 9500 cal yr BP, the climate became warmer and less humid, and the eutrophic plant species were gradually replaced by mesotrophic species. At the end of the early Holocene, parts of fens gradually turned to transitional mires, and the first layers of raised bog peat started to form. The boundary between the early and middle Holocene, or the HTM, can be established at the interval of 8100 to 8000 cal yr BP based on sharp changes in peat formation conditions. The further development of raised bogs and fens, including coastal areas, where ground paludification and bog development in the depressions caused by groundwater rise because of water level rise due to transgression of the Baltic Sea Littorina Sea stage took place. The boundary between the HTM and the late Holocene in this study is established at 4200 cal yr BP, as suggested by the INTIMATE group. During the late Holocene, the bog plant phytocenoses were formed essentially of Sphagnum and cotton-grass, which, on decay, produced slightly decomposed Sphagnum Magellanicum, Sphagnum fuscum, and Sphagnum-cotton-grass peat. An intensive accumulation of peat occurred all over, followed by intensive development of bog cupolas. After the regression of the Littorina Sea stage of the Baltic Sea, terrestrialisation processes and peat formation were initiated in the former lagoonal areas. The interdune mires started to form in the north-western coastal area of Latvia before 3500–3000 cal yr BP. This study shows that pollen, nonpollen, and the botanical composition of peat in mire deposits, together with 14C dating, can yield valuable information about environmental conditions during peat accumulation and provide insights into peat stratigraphy, which help to understand the development of mires and mire ecosystems. The findings of this study and comparison with earlier work enable us to conclude that peat stratigraphy and the subdivision of the Holocene in Latvia can generally use the scheme suggested by the INTIMATE group (Walker et al. 2012). However, the local peculiarities of mires should also be taken in account.