Palaeoecological Study

Monograph No. 1


A Palaeoecological Study of Raised Mires in the Humberhead Levels

by Brian M Smith, P C Buckland (Issue editor), Martin Limbert (Issue editor)

Based on the author’s doctoral thesis, this study is the first of the Thorne and Hatfield Moors Monographs. Brian Smith looks at the development of the Humerhead Levels through pollen analysis, stratigraphic evidence and radiocarbon dating. The conclusions that he reaches trace changes in the vegetation of the Thorne and Hatfield Moors, the impact of agricultural activities and woodland regeneration. 154p, 24 tbs, 43 b/w figs (BAR 336, 2002)

 

A palaeoecological study of raised mires in the Humberhead Levels

Brian M Smith, P C Buckland (Issue editor), Martin Limbert (Issue editor), Erica Hedges (Issue editor) & — (Editor)

BAR Brit Ser, 336, 2002, 154pp, figs, tables, many refs, pb, price £29.00, ISBN 1 84171 307 4

Abstract

On the Humberhead Levels between the Rivers Ouse and Trent, the former raised mires of Hatfield Moors and Thorne Moors have been studies using pollen analytical, stratigraphic and radiocarbon dating techniques. Pollen analysis of nine sites from both mires has revealed a series of changes in vegetation which can be attributed to the activities of people. A series of phases of agricultural activity have been delimited from the Bronze Age through to the Middle Ages. These periods of agricultural activity are separated by phases of forest or woodland regeneration. Five regional pollen assemblage zones have been proposed encompassing a period of time from c.4500 BP to c.500 BP.

Microstratigraphic study of the ombrotrophic peat from both mires has revealed a number of recurrence surfaces, of which the unhumified peat component corresponds to phase shifts to wetter mire conditions. These recurrence surfaces have been placed into five major groups which appear to reflect, in the main, deteriorations in the prevailing climate, although sea-level changes and autogenic vegetational processes are also of importance.

Macrostratigraphic investigation of both mires has enabled the peat deposits to be categorised into three distinct stages of mire development: rheotrophic, mesotrophic and ombrotrophic mire. Thorne Moors shows all three stages, although there are temporal variations in the initiation and persistence of these stages in different areas of the mire. On Hatfield Moors, only a very limited development of the first two stages was encountered, and ombrotrophic peats are dominant. The differences in ontogenesis may be due to hydroedaphic variations in the vicinity of each mire. Includes ‘Appendix: radiocarbon dates from the Humberhead Levels’ (97–100).

Review from The Holocene 13 (2003)

This research monograph is based on a PhD thesis completed in 1985 at the Department of Plant Sciences, University of Wales Cardiff. The work has finally been published mainly because of its fundamental importance in understanding the nature and palaeoclimatic implications of the stratigraphic features of raised, ombrotrophic mires known as recurrence surfaces (RYs). The term ‘recurrence surface’ was proposed by Granlund (1932) for the boundary between relatively unhumified, light-coloured, Sphagnum peat and relatively humified or decomposed, dark-coloured peat below. A laterally continuous recurrence surface signals widespread reactivation of mire growth in response to the mire surface becoming wetter. Since the recognition by Granlund of five main recurrence surfaces in Sweden, other studies have variously expanded their number and con- firmed or denied their existence within and between raised mires across northwestern Europe. Both the concept of a recurrence surface and possible relationships to climatic variations have been much discussed and often criticized, especially on the grounds of variations in the age of the same surface across particular mires and a failure to detect a consistent pattern between mires in the same climatic region. The interpretation of recurrence surfaces, however, has been highly dependent on the methods used to describe and investigate them and on the available information relating to their age and chronology derived from radiocarbon dating. The new light shed on the topic by Smith derives from his painstaking and sophisticated micro- and macrostratigraphical methods combined with intensive radiocarbon dating.
Two separate raised mires – Thorne Moors and Hatfield Moors – were studied in detail. These are the largest remnants of the once-extensive mires of the Humberhead Levels (HHL) at the head of the Humber Estuary, which make up the largest surviving area of lowland ombrotrophic mire in eastern England. Several sites across each mire were used to investigate the pattern of development of each recurrence surface. Microstratigraphical techniques (including pollen, rhizopod and plant macrofossil analyses and accurate determination of humus content) were used systematically to define individual recurrence surfaces and to confirm their association with shifts in mire-surface wetness. Radiocarbon dating was employed to date 1 cm thick contiguous samples from the unhumified and humified peat, above and below each recurrence surface. Table 1 summarizes the five recurrence surfaces indentified for the last c.3000 years and the dates of the probable climatic shifts to wetter mire conditions that they represent.

The youngest recurrence surface (RY.HHL/I) had been removed by peat cutting at all but one site. RY.HHL/II was dated at six sites and, at Thorne Moors, the sequence of dates across this recurrence surface shows a consistent decrease in age from 1295 ±60 to 725 ±55 14C years BP towards the margin of the mire, indicating the ‘lateral spreading’ of wet-mire communities outwards from the centre. The older recurrence surfaces are almost invariably present at all sites where oligotrophic peat was accumulating and, with the possible exception of RY.HHL/IV, which may have been influenced by a sea-level rise, are consistent with earlier climatic deteriorations. The absence of recurrence surfaces older than RY.HHL/V in the Humberhead Levels can be attributed to the late initial development of the mires around 4500–4300 14C years BP, followed by varying periods of formation of rheotrophic and mesotrophic peat deposits before the climate- sensitive ombrotrophic peat deposits began to accumulate. Overall, the results support the climatic interpretation of recurrence surfaces as an aspect of the ‘Phasic Theory’ of raised mire development (cf. Barber, 1981). Once the mire grows sufficiently high to become independent of groundwater conditions, oligotrophic conditions prevail and mire-surface wetness becomes highly dependent on climate. Although the mire surface may dry out slowly as climatic variations pass from wet to dry phases, peat humification may also increase under a constant climate as peat accumulation raises the mire surface farther above the water table. Shifts to relatively dry mire-surface conditions are therefore not necessarily indicative of a drier climate. An increasingly wet climate, however, tends to lead to a relatively abrupt transition in the peat following regeneration of peat accumulation, especially when the mire surface is flooded. This transition may vary over the mire surface and between mires, in terms of extent and timing, due to differences in local topography and geoecological conditions.

The most important message from this study is that recurrence surfaces deserve to be given greater consideration in the context of relationships between mire development, peat humification and climate. A detailed history of human activity on surrounding higher land in the region is also provided by the pollen analyses. Phases of forest clearance and agricultural activity are indicated by decreases in arboreal pollen (which coincide with charcoal layers indicative of forest burning) and increases in ‘agricultural indicator’ species, respectively. Phases of agricultural activity from the Bronze Age to the Middle Ages are separated by phases of woodland regeneration. On the whole, the study successfully differentiates the climatic effects from the human impacts.

Table 1 Recurrence surfaces and climatic wet-shifts proposed for the Humberhead Levels

Recurrence surface
(RY.HHL/)
Date of wet shift
(14C years)
Age-equivalent
recurrence surface
of Granlund (1932)
I c. 650–550 BP I
II c. 1300–1250 BP
III c. 1650–1550 BP II
IV c. 2500–2300 BP
V c. 2600–2800 BP III

References

Barber, K.E. 1981: Peat stratigraphy and climatic change. A palaeoecological test of the theory of cyclic peat bog regeneration. Rotterdam: A.A. Balkema.
Granlund, E. 1932: De svenska högmossarnas geologi, deras bildningsbetingelser, utvecklingshistoria och utbedning jämte sambandet mellan högmossbildning och försumpning. Sveriges Geologiska Undersökning. Avhandlingar och uppsatser Series C, No. 373, 1–193.

John A. Matthews

(University of Wales Swansea)

Review from The Naturalist (Yorkshire Naturalists Union) No. 1046 vol 128 (July-Sept 2003)

This monograph is based on a PhD thesis produced in 1985 but published now in a form available to a wider audience. The original doctoral research has been much cited over the years and has been updated in the current publication to include more recent work on the palaeoecology of mires. It contains much Svaluable data, including nine pollen diagrams from Thorne and Hatfield Moors, covering the period 4,500 to 500 BP. Topics covered include the history of drainage in the Humberhead levels, the structure and hydrology of raised mires, climate change and ‘recurrence surfaces’, human impact on vegetation, and a discussion of the historical basis for the classification of mires. Scientific techniques employed include studies of peat stratigraphy, pollen, rhizopods and plant macrofossils. Radiocarbon dating provides a firm chronology for the environmental reconstructions. There is an extensive and useful bibliography. Illustrations are clear and helpful, but are presented at the back of the monograph rather than interleaved with the text. This book will be essential reading for all those involved in the conservation of the Humberhead Levels or palaeoecological research in other areas, but the price and academic style will make it less attractive to the general reader.