Chris Caseldine

A comparative study of optical properties of NaOH peat extracts: implications for humification studies

Assessment of the degree of decay of peat (humification) in ombrotrophic mires has become a standard technique for palaeoclimatic reconstruction, based on the finding that decay is primarily determined by surface wetness and temperature at the time of peat deposition. Determination of humification is undertaken by colorimetric measurement of an alkali extract of the peat at 540 nm. Humification is proportional to the amount of humic matter dissolved by this extraction process, although few researchers convert results to a quantitative measure of humification expressing results as percentage light transmission through the peat. This paper uses luminescence spectroscopy to assess the chemical composition of these extracts. Luminescence excitation and emission wavelengths suggest that high molecular weight acids (‘humic acids’) are altered by the extraction procedure to form lower molecular weight acids (‘fulvic acids’), amino acids and polysaccharides. Percentage transmission is principally related to luminescence emission wavelength and thus to molecular weight of the compounds present. Luminescence emission shows much more sensitivity to peat composition and demonstrates that different plant species may be affected to different degrees by the NaOH extraction process. The findings broadly support the underlying principle of colorimetric determination of ‘humification’ whereby transmission levels decrease with increasing plant breakdown, but show that it is based on an inadequate understanding of the chemical processes occurring in peat decay and preparation procedures. Luminescence spectroscopy provides a technique for resolving these issues.

Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland

Multi-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 14C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 14C yr B.P. and that July temperatures were around 4°C at ca. 9500 14C yr B.P. Temperatures then rose to ca. 8°C at the time of the deposition of the Saksunarvatn tephra (9100 14C yr B.P.), reaching ca. 10°C by 8500 14C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 14C yr B.P. There is some evidence for cooling immediately preceding 9100 14C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 14C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 14C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 14C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone.

Pollen-based quantitative reconstructions of Holocene regional vegetation cover (plant-functional types and land-cover types) in Europe suitable for climate modelling

We present quantitative reconstructions of regional vegetation cover in north-western Europe, western Europe north of the Alps, and eastern Europe for five time windows in the Holocene [around 6k, 3k, 0.5k, 0.2k, and 0.05k calendar years before present (bp)] at a 1° × 1° spatial scale with the objective of producing vegetation descriptions suitable for climate modelling. The REVEALS model was applied on 636 pollen records from lakes and bogs to reconstruct the past cover of 25 plant taxa grouped into 10 plant-functional types and three land-cover types [evergreen trees, summer-green (deciduous) trees, and open land]. The model corrects for some of the biases in pollen percentages by using pollen productivity estimates and fall speeds of pollen, and by applying simple but robust models of pollen dispersal and deposition. The emerging patterns of tree migration and deforestation between 6k bp and modern time in the REVEALS estimates agree with our general understanding of the vegetation history of Europe based on pollen percentages. However, the degree of anthropogenic deforestation (i.e. cover of cultivated and grazing land) at 3k, 0.5k, and 0.2k bp is significantly higher than deduced from pollen percentages. This is also the case at 6k in some parts of Europe, in particular Britain and Ireland. Furthermore, the relationship between summer-green and evergreen trees, and between individual tree taxa, differs significantly when expressed as pollen percentages or as REVEALS estimates of tree cover. For instance, when Pinus is dominant over Picea as pollen percentages, Picea is dominant over Pinus as REVEALS estimates. These differences play a major role in the reconstruction of European landscapes and for the study of land cover-climate interactions, biodiversity and human resources.

Surface Pollen Studies Across Bankhead Moss, Fife, Scotland

Surface pollen studies across a small, wooded raised bog at Bankhead Moss in Fife, Scotland, have revealed a pattern of pollen dispersal and accumulation showing how representative the present pollen record of the bog is of the surrounding area. Pollen accumulation in the central open area of the bog is dominated by local pollen, mainly Calluna, and Betula pollen from the woodland. Of all the pollen types produced outside the bog only Gramineae is well represented, and of all the weed taxa found locally only Plantago and Rumex consistently appear. The implications of this work for the interpretation of early periods of forest clearance using such deposits are also discussed emphasizing the likelihood of not recognizing or underestimating the extent of land cleared, particularly if clearance was taking place some distance from the site.

Stalagmite luminescence and peat humification records of palaeomoisture for the last 2500 years

Abstract Recent research has suggested that both raised and blanket bogs can provide proxy climate signals from variations in peat humification. In particular, oceanic margin sites have provided sensitive records that demonstrate century scale variations in humification. However, previous research has not compared records of peat humification with other terrestrial palaeoclimate proxies. Here, two records of climate change from an oceanic marginal site in NW Scotland are analysed. One, from a blanket bog, is derived from peat humification and covers the period 2100–100 BP. A second, from two stalagmites in a cave overlain by the bog, is derived from stalagmite luminescence wavelength variations for the samples deposited over 2500–0 BP. Both peat humification and stalagmite luminescence records demonstrate 90–100 year oscillations in bog wetness, that are attributed to variations in rainfall intensity or totals over this time period. It is argued that this is probably generated by a southward shift of the path of northern hemisphere depression tracks, possibly linked to variations in solar output.

Changes in Betula in the Holocene record from Iceland—a palaeoclimatic record or evidence for early Holocene hybridisation?

Tree birch in its mountain form is the only woodland-forming tree found in the Holocene record of Iceland. Given the close relationship between tree-lines and summer temperature in Fennoscandia, it should therefore provide a valuable proxy temperature indicator for the Holocene in Iceland. Following a review of issues relating to the taxonomy of mountain birch, and the identification of tree birch in the palynological record, data are presented from the Trollaskagi area of northern Iceland relating to the development of birch woodland in the early Holocene. It is argued that following the development of communities in which dwarf birch, Betula nana L., was important, true woodland communities only became established after a period of relative instability in the vegetation cover. Data from morphometric analyses of Betula pollen from this period are interpreted as representing probable hybridisation between the different forms of birch, rather than reflecting a clear climatic signal. The question is also raised of whether the progenitor of tree birch in Iceland was an immigrant form of mountain birch, (Betula pubescens Ehrh. ssp. tortuosa (Lebed.) Nyman as defined in Scandinavia), or whether Icelandic mountain birch developed by hybridisation in Iceland between Betula nana L. and Betula pubescens Ehrh. during the Holocene. The use of birch as a palaeoclimatic indicator, especially prior to the establishment of birch woodland at suitable locations and altitudes, should not therefore be accepted unless accompanied by other independent climatic evidence.

Reconstruction of Holocene Variations of the Upper Limit of Tree or Shrub Birch Growth in Northern Iceland Based on Evidence from Vesturardalur-Skioadalur, Trollaskagi

For paleoclimatic reconstructions based on vegetation history in Iceland, the upper limit of tree or shrub birch growth has been proposed as an indicator of summer temperature. Plant macrofossil and pollen analyses of a series of sections and cores from Vesturárdalur on Tröllaskagi show that Betula pubescens grew up to an altitude between 450 and 500 m a.s.l. during optimum conditions in the Holocene. The birch pollen and macrofossil record of core Vesturárdalur 2 at ca. 450 m a.s.l., which covers the time from ca. 9200 BP to present, thus represents the first continuous high-resolution reconstruction of the variations of Betula pubescens at the ecological upper limit of tree or shrub birch in northern Iceland. Between ca. 6700 and ca. 6000 BP, a distinct maximum in the influx of Betula pubescens pollen at this site indicates a high position of the shrub line. This can be distinguished from a low influx of tree or shrub birch pollen from ca. 6000 to ca. 5600 BP, and a very pronounced minimum of Betula pubescens in the pollen record around ca. 3300 BP. The inferred depressions of the shrub line can be correlated with evidence for glacier advances and increased slope activity in northern Iceland.

14. The Holocene vegetation history of Iceland, state-of-the-art and future research

In the Late Preboreal and Early Boreal Chronozones dwarf-shrub heath and shrub heath, followed by juniper and mountain birch copses, replaced snow beds and fellfield vegetation characteristic of the Lateglacial/Early Preboreal newly deglaciated landscape of Iceland. During the Late Boreal and Early Atlantic Chronozones birch woodland established itself in the more favourable places, especially fjord lowlands and inland valleys. The development of birch woodland suffered a setback due to a transient climatic oscillation some 7500 14 C years ago, but recovered again relatively quickly and more than 6000 14 C years ago birch woodland covered the lowland areas both in northern and southern Iceland. At that time it reached its highest altitude, at least in northern Iceland. During the Late Atlantic and Subboreal Chronozones the birch woodland showed a retrogressive succession towards a more open landscape with expanding mires and heaths. There is some conflict between the evidence from pollen percentages, which indicate that the woodland regenerated several times during the latter half of the Holocene, and pollen influx values which reflect no such regeneration of the woodland. New habitats were created for birch after a period of cool climate and instability during the Early Subatlantic Chronozone as fresh screes and sandur plains became vegetated, at least partly, by woodland. This development was halted at the beginning of the Norse settlement, which resulted in further opening of the woodland. The birch woodland closest to the farms in the lowland of Iceland was cut and utilized for timber and fuel. Grazing of domestic animals opened the landscape still further and the previous woodland never re-established itself. This happened within only half a century from the arrival of the first settlers. During the ensuing 1100 years of human influence the sub-alpine birch woodland has been so intensively utilized that only in fenced, protected areas and at the most inaccessible and remote places has birch survived. The shrub and dwarf-shrub heaths, widespread mires, fell fields, and hay fields so characteristic for the Icelandic landscape at present thus developed as a relatively recent phenomenon in the form in which they appear today. A number of areas are identified for future research: elucidating the origins of the Icelandic flora, deriving palaeoclimatic data from the vegetation record, correlating terrestrial with marine and ice core records and expanding our understanding of the human impact on vegetation.

The development of high moorland on Dartmoor: fire and the influence of Mesolithic activity on vegetation change

1. Results of pollen and charcoal analysis of blanket peat sites from northern Dartmoor demonstrate the influence of burning and grazing on the transition from hazel woodland to blanket peat during the early and mid Holocene. 2. Within a general phase of enhanced burning between 7700 and 6300 BP the site at Pinswell at an altitude of 461 m shows how woodland was transformed over a period of 600–1000 years into blanket peat via a phase of acid grassland, in some ways similar to heath-derived grassland found on Dartmoor today 3. The results clearly implicate Mesolithic communities, and their use of the high moorland, in the development of the open peat-covered landscape that is so characteristic of the present Dartmoor environment.

NEOGLACIAL GLACIER VARIATIONS IN NORTHERN ICELAND: EXAMPLES FROM THE EYJAFJORDUR AREA

Results of lichenometry and rock-weathering studies on outer moraines in Skidadalur, Eyjafjordur, northern Iceland show a late Little Ice Age 19th century maximum for all glaciers except the Klaengsholl glacier. Retreat from these maximum positions occurred in three distinct phases: 1896-1908, 1915-1926, and post-1930. Evidence suggests, however, that many glaciers must have reached a position close to the Little Ice Age maximum on at least one other occasion earlier in the Neoglacial.