Abstract
A compositional multivariate approach was used to analyse regional-scale soil geochemical data obtained as part of the Tellus Project generated by the Geological Survey of Northern Ireland. The multi-element total concentration data presented comprise X-ray fluorescence (XRF) analyses of 6862 rural soil samples collected at 20-cm depth on a non-aligned grid at one site per 2,hbox {km}^{2}. Censored data were imputed using published detection limits. Each soil sample site was assigned to the regional geology map, resulting in spatial data for one categorical variable and 35 continuous variables comprised of individual and amalgamated elements. This paper examines the extent to which soil geochemistry reflects the underlying geology or superficial deposits. Since the soil geochemistry is compositional, log-ratios were computed to adequately evaluate the data using multivariate statistical methods. Principal component analysis (PCA) and minimum/maximum autocorrelation factors (MAF) were used to carry out linear discriminant analysis (LDA) as a means to discover and validate processes related to the geologic assemblages coded as age bracket. Peat cover was introduced as an additional category to measure the ability to predict and monitor fragile ecosystems. Overall prediction accuracies for the age bracket categories were 68.4 % using PCA and 74.7 % using MAF. With inclusion of peat, the accuracy for LDA classification decreased to 65.0 and 69.9 %, respectively. The increase in misclassification due to the presence of peat may reflect degradation of peat-covered areas since the creation of superficial deposit classification.
Highlights
A diversity of rock types observed across Northern Ireland represents three basement terranes (Fig. 1a after Mitchell 2004)
An application of clr-based Principal component analysis (PCA) showed that several elements were near-collinear and prompted an amalgamation of the data based on the following proxies: light rare-earth elements (LREE), La, Ce, Nd along with Th; heavy rare-earth elements (HREE), Yb and Hf; elements associated with mafic minerals (Mafic), Fe, V, Cr and Sc; alkali elements associated with feldspars (Ba_Na_K_Rb), Ba, Na, K and Rb; and lastly elements associated with a feldspar silicate framework (Al_Ga), Al and Ga
To consider an improvement over PCA that uses the spatial relationships of the data, a classification based on maximum autocorrelation factors (MAF) analysis was undertaken using the first eight dominant factors (Fig. 3a)
Summary
A diversity of rock types observed across Northern Ireland represents three basement terranes (Fig. 1a after Mitchell 2004). The Grampian Terrane and associated rocks in the northwest have metamorphic igneous and sedimentary origins spanning the Proterozoic Era. Psammites and semi-pelites are the dominant rock type, with subordinate sandstones and conglomerates. Monitoring peat coverage has become important in calculating soil carbon stocks due to the relatively high carbon density of peat and organic-rich soils. This is important for Ireland (and other Northern European countries), where some 16 % of the land surface is covered by peat bog. In Northern Ireland, previous work has estimated the total amount of carbon stored in vegetation to be 4.4 Mt compared with 386 Mt stored within soils such as peat (Cruickshank et al 1998)
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