An investigation of vegetation and environmental change in the Comeragh Mountains Dr Bettina Stefanini An investigation of vegetation and environmental change in the Comeragh Mountains Prepared by Dr Bettina Stefanini 8 Middle Mountjoy Street Phibsboro Dublin 7 Phone: 087 218 0048 email: [email protected] February 2013 2 Introduction Ireland has a dense network of over 475 palaeoecological records of Quaternary origin. However, there are few late-Holocene vegetation records from Waterford and the extant ones are truncated (Mitchell et al. 2013). Thus the county presents a blank canvas regarding prehistoric vegetation dynamics. Likewise, possible traces in the environment of its well documented 18th century mining and potential earlier mining history have not been found so far. This study was commissioned by the Metal Links project, at the Copper Coast Geopark, which is part funded by the European Regional Development Fund (ERDF) through the Ireland Wales Programme 2007 - 2013 (Interreg 4A). It aims to investigate vegetation dynamics, mining history and environmental change in the Comeragh Mountains. Site description and sampling Ombrotrophic peat is the most promising medium for geochemical analysis since metal traces are thought to be immobile in this matrix (Mighall et al. 2009). Such deposits are equally well suited for microfossil analysis and for this reason the same cores were chosen for both analyses. The selection of potential study sites presented difficulties due to extensive local disturbance of peat sediments. Initial inspection of deposits on the flanks of the Comeragh Mountains revealed that past peat harvesting had removed or disturbed much of this material and thus rendered it unsuitable for analysis. A possible site in the central depression of the Com Tae corrie was rejected because sporadic inwash had produced bands of mineral material in the deposits. This material might potentially influence the geochemical profile and thus plans to investigate that site had to be abandoned. This left the deep blanket peat deposits of the central ridge of the Comeragh plateau. The mountains consist of Old Red Sandstone and are covered by largely intact blanket peat. However, there are quite extensive areas where the peat is eroding. Two cores were extracted in September 2012. The first measured 0.72m and was taken in intact ombrotrophic blanket peat of total depth 2.12m at Irish Grid 228400, 108600 (S 284 086) and at ca 715m altitude using a Wardenaar peat cutter (Wardenaar 1987). The second site was an eroding peat bank or hagg ca 1km north-east of the first. Here a monolith measuring 1.75m was extracted from a total peat depth of 1.85m at Irish Grid 229200,109100 (S 291 091), Figure 1 and Figure 6. 3 Figure 1 Comeragh ridge, left, extraction of Wardenaar core in intact blanket peat. Middle and right, cutting of peat monolith from eroding peat bank. Materials and methods Pollen and microfossil extraction followed standard methods (Faegri and Iversen 1989). A Lycopodium spore tablet, batch number 177745 containing on average 18584 spores, was added to facilitate concentration calculations of the sample material (Stockmarr 1971). Samples were mounted in silicone oil and scanned at x400, x600 and x1000 magnification. At least 400 pollen grains and fern spores were identified for each sample. This excluded aquatics and local bog taxa. The aim here was to analyse a large and relatively stable pollen sum of non-local origin, that is, from habitats outside the local blanket bog and heath vegetation. However, grass and sedge pollen formed part of the 400 grain count, even though their origin is probably partly local. Pollen, spores and other microfossils were analysed along transects across each slide. Identification was based mainly on keys and image database material. Nomenclature followed Beug, Feeser and van Geel (Moore 1991; van Hoeve 1998; Beug 2004; Feeser 2009). Results were computed and displayed using the Tilia computer programme (Grimm 2011). Results Dating and peat accumulation Three bulk samples, one from the Wardenaar core and two from the monolith, were prepared for AMS analysis which was carried out by the CHRONO centre, Queens University, Belfast. The resultant dates indicate that the peat started to form in the early Bronze Age at ca 4300 cal BP (2350 BC), Table 1 and Figure 2. From the age-depth relationship of these samples it appears that the shorter overall peat depth of the peat hagg may be due to water loss and shrinkage rather than to greater peat accumulation at the Wardenaar site. In the absence of more dates, the chronology is based on a simple linear interpolation model. This indicates that peat accumulation averaged 32 years cm-1 in the older section and 13.5 years cm-1 in the last millennium. More recent peat often has a faster accumulation rate than older sediments. This is due to the gradual 4 breakdown of peats as well as to compaction of older material. Dates in the text are quoted as calibrated before present (cal BP) where ‘present’ refers to 1950. Depth UBA Lab AMS Cal BP (2 (cm) Core number 14C Age δ13C sigma) AD/BC 69 Wardenaar UBA-21270 950 ± 29 -27.7 921 – 979 AD 1038–1132 75 Monolith UBA-21271 1141 ± 25 -27.0 1116 – 1166 AD 874 – 946 162 Monolith UBA-21272 3569 ± 31 -30.1 3538 – 3600 1886–1968 BC 0 20 40 Wardenaar 60 Monolith 80 100 Depth (cm) (cm) Depth 120 140 160 180 -200 300 800 1300 1800 2300 2800 3300 3800 Calibrated years before present (present= 1950) Figure 2 Age-depth relationship of the Wardenaar core and peat bank monolith based on three AMS dates and linear interpolation between these and the peat surface. The approximate location of the dated levels is marked with a star. Geochemistry The Wardenaar core was scanned by an ITRAX core scanner in the School of Geography, Planning and Environmental Policy, University College Dublin. This analysis would indicate the presence of geochemical traces which could have originated in mining pollution. The ITRAX scanner produces high resolution photo and x-ray images as well as x-ray fluorescence analysis (XRF) and profiles elements in the range of Al-U. Figure 3 shows the ITRAX output plot. From left to right this shows an optical image, X-ray, the count rate (kcps), the mean square error (MSE) and XRF plots of elements selected for their potential to indicate metal mining pollution signals. The scanner settings are shown below the plots. The MSE values show a consistent fit throughout the run. Copper content, Cu integrals, show little variation apart from where the counts, indicated by the kcps curve, drop. Their count rate is quite high, 500-600 counts per second, somewhat elevated compared with copper signals from minerogenic sediments. This may be due to the good sorption properties of peat (Turner, pers. comm.). The scanner did not detect any lead in the sample. 5 Calcium and iron show slightly elevated levels in the top of the core. This is possibly due to airborne dust deposition. The ‘Mo inc’ plot indicates that there was little change in water and carbon content throughout this core. The results do not indicate copper mining pollution in the Wardenaar core within the period of known mining activity in the region. However, the detection of the rather strong copper signal shows traces of this element in the core. The absence of metal mining pollution indicators may be a function of the large distance between the coring site and the mining locations. The Wardenaar coring site is located 18.5 km from Bunmahon. It may be worthwhile to investigate other sediments closer to known 19th century mining sites (Cowman 2006), to see if this could produce evidence for earlier mining. Alternatively it may be possible to provenance copper from the copper smelting finds that have turned up at Cootubbrid East in connection with the excavations of the Kilmacthomas realignment scheme (Fairburn 2008). Figure 3 ITRAX plot showing from left to right photographic, X-ray, count rate, error, XRF plots of Cu, Pb, Zn, Fe, Mn and Cn count integrals as well as Compton scattering (Mo inc) and scan settings. Microfossil analysis The microfossils come from 17 samples, 8 from the Wardenaar core and 9 from older strata within the monolith. For a full list of the pollen and spore types identified in the samples, see Appendix A whereas a full list of non-pollen microfossils is tabulated in Appendix B. By the time peat started to accumulate on the ridge local deforestation was already extensive. The deforestation removed the main pollen producers from the locality enlarging the ‘source area of 6 pollen’ (PSAs). At the onset of peat accumulation in the early Bronze Age much of the pollen may have come from a considerable distance from the core locations. Hence there is probably a substantial overlap between the PSAs of two sites. For convenience the microfossils are plotted and discussed as if they had come from the same core. This way of looking at them is also supported by the CONISS cluster analysis, Figure 4. This is based on the tree, herb and fern assemblages and shows major divisions between 40-50cm, between 155-165cm, between 10- 20cm, between 115-125cm and only the next largest division between 70-85cm. This latter division lies between the Wardenaar and the monolith cores. The diagram has been divided into zones A-F based on the CONISS divisions. Trees and shrubs Herbs FernsCharcoal SCP - FLY ASHZone CONISS cluster analysis 0 10 COM - F 20 30 COM - E 40 500 50 60 COM - D 70 1000 80 90 1500 100 2000 COM - C 110 Depth (cm) Depth 120 2500 130 3000 140 Age (calibrated years before present) before years (calibrated Age COM - B 150 3500 160 COM - A 4000 20 40 60 80 100 20 40 20 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Increase in sum of squares Figure 4 Composite percentage diagram with zoning based on CONISS cluster analysis.