1,721,135 research outputs found
A palaeoecological test of the theory of cyclic peat bog regeneration
No full textThe theory of peat bog growth commonly known as the cyclic regeneration theory, was formulated in Sweden by von Post & Sernander in 1910 and elaborated by Osvald from 1923 onwards. The history of the development and spread of the theory is traced in some detail and previous relevant research is dritically evaluated. The present work, based, mainly at Bolton Fell Moss, Cumbria, tests the theory by palaeoecol.ogical methods involving the recording of several sections of peat stratigraphy, the analysis of macrofossil remains from 21 monolith profiles and the dating and correlation of stratigraphic changes by pollen analysis and radiocarbon dating. A distinct link between peat stratigraphy and climatic change is demonstrated, apparently effecting such small-scale changes as outgrowths of individual hummocks. Periods of warm, dry summers such as prevailed between 1200 and 1300 A.D. are clearly associated with the formation of drier surfaces dominated by Calluna, Eriophorum vaginatum and Sphagnum imbricatum. Pool formation, due to a change to a wetter and/or colder climate, is widespread around 900-950 A.D., 1400-1450 A.D.,and 17501800 A.D., and is correlated closely with known climatic deteriorations. New evidence is presented regarding the recent extinction of SDhagnum imbricatum at the site, and the interactions of, various Sphagnum species, and peat accumulation rates in different microenvironments, are commented upon. In the absence of any evidence for an autogenic cycle of huraocks and hollows, and with the clear demonstration of climatic control over the relative areas of pools and drier surface, the 'Osvald Theory' is rejected. A new theory, the Phasic Theory, is proposed. This states that bog growth occurs in phases closely related to climatic variations, and recognises the importance of phase-shifts to a wetter bog surface which initiate changes in vegetation and peat stratigraphy.</p
Human impact signals from peat bogs: a combined palynological and geochemical approach
No full textAbstract This research compares palynological evidence for changes in land use histories with a geochemical method for reconstructing past soil erosion. Changes in land use have significant effects on soil erosion. It has been shown elsewhere that silicon (Si) and titanium (Ti) are good proxies for soil erosion. Ombrotrophic peat bogs are useful archives in which to measure Si and Ti depositions as they only receive inorganic erosional inputs through atmospheric deposition and they contain very low background levels of mineral matter. The correlation between geochemical and pollen analytical reconstructions of past human activity from three raised bog sites in Great Britain and Ireland is discussed here, with reference to examples from four particular time periods: the mid-to-late Bronze Age/Iron Age, the late Iron Age/Roman period, the Middle Ages/Tudor period and the more recent past. The results generally indicate a close correlation between the palynological and geochemical proxies, with the combination of both methods allowing a more comprehensive interpretation of the palaeoenvironmental record. Plantago lanceolata and Poaceae pollen frequencies appear to correlate particularly well with the geochemical proxies. A multi-proxy approach such as this may be particularly useful for identifying and interpreting low-level prehistoric human impact
Multiproxy late Holocene peat records from Ireland: towards a regional palaeoclimate curve
No full textTwo new peat-based climate records from Ireland covering the late Holocene are presented. The sequences are dated by a strong chronological framework formed by AMS radiocarbon dates and SCPs. Three proxy indicators (testate amoebae, macrofossils and humification) have been determined allowing the limitations and strengths of each to be identified and utilised to provide a bog surface wetness (BSW) record for both sites. Age-depth models take into account the potential for accumulation rates to vary with bog vegetation. The records from each site have been used to derive a combined BSW record that displays changes to a wetter/cooler climate from ca. AD 30 (1920 BP), ca. AD 310 (1640 BP), ca. AD 805 (1145 BP), ca. AD 1040 (910 BP) and ca. AD 1300 (650 BP). Changes follow closely those identified in a northern Britain composite BSW record and largely correspond with lake-level data in central France suggesting the main changes in water balance were coherent over a large region. Correspondence with increases in IRD and slower Iceland-Scotland Overflow Water (ISOW) suggests that these changes were related to oceanic forcing influencing the track of dominant westerly air flow over Ireland. Copyright © 2007 John Wiley & Sons, Ltd
Can rapidly accumulating Holocene peat profiles provide sub-decadal resolution proxy climate data?
No full textTo test the potential for developing sub-decadal resolution records of ombrotrophic bog surface wetness (BSW), selected abrupt climate events were identified in cores from four sites in north-west Europe and time-slices over each event were investigated at 2–5-mm resolution using peat humification, testate amoeba and plant macrofossil analyses. Age–depth models based on radiocarbon, spheroidal carbonaceous particle and tephra dates were applied and the error in these models was used to determine the effective temporal sampling resolution, which in most cases was sub-decadal. Fine-resolution results revealed some inconsistencies in the co-variability of individual proxies and there was a lack of correspondence between some results from adjacent cores. However, we found evidence of abrupt shifts in BSW occurring over ca. 5–20 years, indicating a more rapid proxy response than identified in previous research. To maximize potential temporal resolution and produce reliable sub-decadal BSW data in future research, we recommended that peat-based studies should obtain the best possible chronological control, focus on sites with high species diversity and rapid accumulation rates and perform replicate coring, or use open peat sections where the stratigraphy can be clearly seen, to assess intra-site differences in microtopography. Site-specific factors should be a primary consideration in the selection of sites for future fine-resolution research
A paleoecological perspective of vegetation succession on raised bog microforms
No full textPeat cores were taken from eight primary and one secondary raised bog sites throughout the United Kingdom in order to determine the pathways of succession between the microform (hummock, ridge, lawn, hollow, and pool) plant communities of the mid to late Holocene. All of these sites have undergone drainage and/or cutting in the recent past, at least. The plant communities were reconstructed by using plant macrofossil analysis, and they are defined primarily by the dominant species of Sphagnum. While plant macrofossils may be used to infer hydrological conditions, an alternative proxy, using testate amoebae, was utilized. This group of organisms was used to avoid circular arguments when relating the vegetation changes to hydrological events. Consequently, a conceptual model of late Holocene succession on British raised mires is constructed that elucidates succession between the microform plant communities according to changes in the water table depth, its stability, quality, and physical disturbance of the vegetation cover. The general sequence of ombrogenous communities was elucidated to be, with increasing height above the water tables, S. cuspidatum and S. auriculatum > Sphagnum sect. Cymbifolia taxa > Sphagnum sect. Acutifolia taxa > a return of Sphagnum sect. Cymbifolia taxa > non-Sphagnum mosses.A major pathway toward degradation of the mire is apparent in modern times, and recovery is typically via a phase of dominance by S. tenellum prior to the establishment of either S. magellanicum or S. papillosum. Although this disturbance is of a large magnitude, it was found that equable or more severe perturbations to the water table had occurred in the past, especially at coastal sites where marine regression led to the establishment of Sphagnum-poor communities dominated by Calluna and Eriophorum vaginatum. The long- term persistence of hummock–hollow vegetation communities is demonstrated, but the appearance of persistent, more homogeneous phases suggests that this surface topography is not always well developed
Comparing and cross-validating lake and bog palaeoclimatic records: a review and a new 5,000 year chironomid-inferred temperature record from northern England
No full textThis paper reviews the past history of comparisons between lacustrine records of chironomid-inferred summer temperatures (CI-T) and peat bog-derived records of surface wetness (BSW), and develops a new summer temperature record for the mid-late Holocene for Northern Britain from Bigland Tarn, Cumbria. The CI-T reconstruction is well dated, and corresponds well with the only two other CI-T records for Britain. Comparisons are made with BSW records from northern Britain, which show consistent similarities over centennial to millennial timescales, demonstrating that at this resolution cooler summers equated with a wetter climate, at least during the mid- to late-Holocene
Climatic influence upon early to mid-Holocene fire regimes within temperate woodlands: a multi-proxy reconstruction from the New Forest, southern England
No full textA combined pollen, charcoal and climatic record is presented from Cranes Moor, southern England, covering the period c. 10 500–5850 cal a BP. It is shown that the occurrence of burning is closely related to natural processes, including prevailing climatic conditions and vegetation composition. These burning events are often linked to an increase in the summer moisture deficit, implying that the timing of burning events is linked to periods of warmer/drier climate during the Holocene Thermal Maximum (c. 11 000–5000 cal a BP). These events play an important role in the vegetation composition and succession around the site. The nature of the burning recorded at the site shows strong similarities with other records from northern Europe. This study throws caution on suggestions that fire in the Holocene record from areas such as the British Isles is linked only to human activity, and enhances the possibility that natural fire incidence played an important role in natural woodland structure dynamics
A high-resolution pollen and geochemical analysis of late Holocene human impact and vegetation history in southern Cumbria, England.
No full textThe historic era, which in Cumbria begins with the Roman invasion of AD 71, is a frequently neglected period in palaeoecological research, but its study can bring benefits in improving knowledge of landscape history and in understanding the significance and limitations of palaeoecological records. Pollen and geochemical data are presented for late Holocene records from Deer Dyke and Hulleter Mosses in southern Cumbria. The records show initially low levels of anthropogenic impact, followed by a phase of forest clearance and mixed agriculture from the 7th to 11th centuries AD. The timing of these clearances suggests that they were initially Anglo-Saxon in origin, rather than Norse. Further clearances in the 16th century AD are interpreted as a response to monastic dissolution and late Tudor population pressures; the landscapes reached their contemporary form following extensive clearances in the 17th century AD. Silicon and titanium concentrations at Deer Dyke Moss were used to reconstruct past levels of atmospheric dust loading, which is broadly related to soil erosion. Geochemical influx was found to peak during periods of landscape transition rather than from established land use. This relationship with pollen data is thought to reflect the predominantly low levels of anthropogenic impact in the region, which changes as substantial woodland clearances during the 16th century AD and continuous land use pressure since then have greatly increased the supply of airborne dust
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