Patterns of mid-Holocene climate change: evidence from the peat archive

Abstract

With growing concerns over future climate change, the need to understand past climate variability has become an important topic of research. In this thesis, a gap in knowledge of the transition to modern climatic conditions during the mid-Holocene is identified and addressed. Mid-Holocene atmospheric moisture availability has been reconstructed from five raised mires (Kortlandamossen, Tore Hill Moss, Raeburn Flow, Gällseredsmossen, and Misten Bog) located on geographical and climatological gradients across northwest Europe.

In this thesis, a new testate amoebae-based palaeohydrological transfer function for south-central Sweden and an improved indicator-weighted Hydroclimatic Index (HCI) were created in order to establish univariate measures of mid-Holocene bog surface wetness (BSW). Together with detrended correspondence analysis, the HCI and testate amoebae transfer function were applied to palaeoecological data from the five bogs, in order to reconstruct climatic variability across northwest Europe from c. 9000 before present (BP) until c. 3500 BP. A series of wet and dry excursions of the bog surfaces were identified on all five bogs. These BSW changes were compared with palaeoclimatic evidence from the wider literature on a local, regional, continental and hemispheric scale.

Throughout the early- to mid-Holocene, increased BSW on Misten Bog closely coincided with episodes of reduced SST in the North Atlantic. A similar link was established between SST and BSW on Raeburn Flow and Gällseredsmossen, following the end of the Holocene Thermal Maximum (HTM). The results from the analysis demonstrate that the HTM occurred asynchronously across northwest Europe. Reconstructions from all five bogs, except Misten Bog, showed a prolonged dry phase on the bog surface starting prior to c. 7000 BP. The end of the HTM occurred between c. 5600 BP and c. 4800 BP on all sites except Kortlandamossen, where regional environmental mechanisms delayed the end of the HTM. A climatic boundary at the end of the HTM was detected around 57ºN, with sites south of 57ºN experiencing the termination between c. 5600 BP and c. 5300, while it occurred between c. 4800 BP and c. 4400 BP on the sites north of this latitude. The only period when all five sites displayed similar BSW changes was between c. 4400 BP and c. 4000 BP, a period of global climatic change. A major wetting on the bogs during this time coincided with increased BSW and glacial advances across Europe as well as high lake levels in France and Switzerland.

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ORCID for P.D.M. Hughes: orcid.org/0000-0002-8447-382X

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