OR/18/005 Creation of the dataset

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Evans, J A, Mee, K, Chenery, C A, Cartwright, C E, Lee, K A, and Marchant, A P. 2018. User guide for the Biosphere Isotope Domains GB (Version 1) dataset and web portal. British Geological Survey. (OR/18/005).

Isotope methodology

Overview

This dataset brings together the oxygen isotope groundwater map published by Darling et al. (2003)[1], an update to the Sr isotope biosphere map, published by Evans et al. (2010)[2], a new sulphur dataset for plants in England and Wales, and the oxygen isotope composition of human tooth enamel based on Evans et al. (2012)[3]. These four layers can be interrogated to produce a distribution map of the different isotope compositions that can be found across Britain. This is Version 1 of the dataset, which will be developed over time, as more samples are collated.

Sample types

A variety of sample types have been used in this study. They represent different aspects of the biosphere and hydrosphere and are sampled at different scales.

Water (Sr study). This includes river water, pond/lake water, borehole, and tap waters. River water samples provide an average value for the catchment areas of the stream/river. They can have variable isotope composition depending upon season and rainfall (Shand et al., 2007[4]) and may introduce values into an area that are typical of the upper catchment rather than water from the immediate area of interest. Borehole water will provide an aquifer value most appropriate to wells and mineral water. Modern tap water will be the average of a large modern catchment system or possibly desalinated water in some parts of the world. Lake water will be a mixture of rainwater, river feeder system and equilibration with the lake bed.

Groundwater (O study). Groundwater δ18O represents a long term bulk rainfall composition and is reasonably representative of long-term rainfall across Britain. British groundwater samples were collected from shallow boreholes, local wells and pumping stations across England, Scotland, Wales and Northern Ireland (see Darling et al., 2003[1]).

Plants (Sr study). Plants provide a direct biosphere measurement. The advantages of plant samples are that they are ubiquitous and reliably geo-located. However, they only sample a small area of land and assumptions have to be made about the relationship between the isotope composition of a plant and that of fauna which consume them. In addition, they may reflect modern, rather than historic compositions. The plants collected for Sr analysis are generally collected away from agricultural land to avoid fertilizer contamination.

Plants (S study). Vegetation sample sites were positioned within fields of at least 25 x 50 m at a minimum distance of 200 m from railways and major roads, and at least 100 m from high power electricity cables, to avoid anthropogenic contamination. Samples were composed of mixed grassland species dominated by grasses for the Geochemical Mapping of Agricultural Soils of Europe project (GEMAS) and grass/mainly mixed herbaceous plants for the remainder.

Archaeological dentine/bone (Sr study). This material has the advantage that, when buried, it will equilibrate with strontium in groundwater close to the time of burial (Trueman et al., 2004[5]) and thus provides a method for looking at the strontium isotope composition of diagenetic fluids from the past and provides a comparison with modern data. It is commonly used as a reference sample for the local burial environment for provenance studies based on tooth enamel and hence a number of dentine or bone analyses have been accumulated though time.

GIS methodology

Strontium isotope domains

The Sr isotope domains were created by first dividing Great Britain into 5 broad geological groups: clay, igneous, limestone, sandstone and organic material. These were then subdivided into 56 domains based on geology, age and geography. Geological and age groupings were derived from the BGS Geology GB (Version 8) and the BGS Parent Material GB (Version 6) datasets, whilst their Sr isotope ranges were derived from ~900 samples from plants, water and bones (Figure 1), as well as theoretical interpolations. Some geological units, such as many of the smaller igneous intrusions in Wales, Scotland and the English Lake District, could not be assigned to strontium isotope domains and therefore have no associated strontium data. These areas are represented by values of -999 in the following fields of the attribute table: SR_MEDIAN, SR_Q1, SR_Q3, SR_INTERQR, SR_L_WHISK, SR_U_WHISK, SR_COUNT, SR_MIN, SR_MAX, SR_MEAN and SR_1SD. The full strontium isotope dataset is available to download from the BGS website (Evans, 2018[6]).

Figure 1    Distribution of samples collected across Great Britain used in defining Sr isotope domains.

Once the domains were determined, the map of Great Britain was divided into 1 km hexagons, (each hexagon side length being 1 km). Each hexagon was assigned strontium values based on the domain that covered the largest area of that cell. Table 1 lists the attributes attached to each cell. Figure 2 shows the 57 different domains used for attributing strontium isotope values across Britain.

Figure 2    87Sr/86Sr isotope domains.

Sulphur isotope domains

The sulphur isotope domains comprises five domains: three that have sufficient data to characterize them individually based on geology (Jurassic Clay, Jurassic Oolitic Limestone and Chalk); one based on the effects of sea spray (Coastal Effects) and a final domain for all other regions of England and Wales (Other Geological Substrates, England and Wales). The geological domains are derived from the strontium isotope domains. A buffer zone around the GB coastline was used to create the ‘Coastal Effects’ domain (sea spray), which typically has higher sulphur isotope values than the other domains and is allocated the range δ34S =21‰ to 8‰. The Coastal Effects domain refers to the region inland from the coastline that is regularly affected by spray from the sea, either directly or incorporated in rain. Figure 3 illustrates a cut off value of 8‰ (horizontal dashed line) to be taken as the lower limit of costal influence. This influence occurs up to 15 km in from the west coast and 10 km in from the east coast (vertical dotted lines).

Figure 3    The change in δ34S (VCDT) values in plants over distance from the coast.

The final sulphur isotope domain, ‘Other Geologic Substrates England & Wales’, is a composite of the remaining data from England and Wales. NB: There are currently no data for Scotland except in the Costal Effects zone. These hexagons are represented by values of -999 in the following fields of the attribute table: S_MEDIAN, S_Q1, S_Q3, S_INTERQR, S_L_WHISK, S_U_WHISK, S_COUNT, S_MIN, S_MAX, S_MEAN and S_1SD. Figure 4 shows the distribution of the sulphur isotope domains across Britain.

Figure 4    Distribution of sulphur isotope domains across Great Britain.

Oxygen isotope domains

The oxygen isotope domains comprise two components: data derived from ground water samples δ18Odrinking water ‰ (VSMOW) and data derived from the measurement δ18Ophos ‰ (VSMOW) in human tooth enamel. The groundwater data were first published by Darling et al. (2003)[1] as a series of contours across Britain reflecting specific ranges of oxygen isotope values (Figure 5).

Figure 5    δ18Odrinking water ‰ (VSMOW) contours derived from the analysis of groundwater samples across Britain (after Darling et al., 2003[1]).

Fractionation of oxygen isotope occurs when water is ingested and this is why there is a significant difference in the isotope composition between the measured groundwater and the tooth enamel values Levinson et al. (1987)[7]. The boundary in the tooth enamel dataset is taken as the -7.0‰ groundwater contour, with values of 16.6‰ to 17.9‰ to the east of -7.0 contour and values of 17.7‰ to 18.8‰ to the south and west of the contour (Figure 6). For further details of these data see Evans et al. (2012)[3].

Figure 6    The datasets for the δ18Ophos ‰ (VSMOW) human enamel which characterize the twofold subdivision of Great Britain.

Using the web portal

The webpage that hosts this site is: www.bgs.ac.uk/sciencefacilities/laboratories/geochemistry/gtf/environmentalTracers.html And should be referenced as:

J A Evans, C A Chenery, K Mee, C E Cartwright, K A Lee, A P Marchant, and L Hannaford (2018): Biosphere Isotope Domains GB (V1): Interactive Website. British Geological Survey. (Interactive Resource). [1]

The Biosphere Isotope Domains can be interrogated by typing a value (or values) into the appropriate query boxes. The query will highlight in orange on the map all the areas of GB in which fall within the designated statistical ranges. The highlighted areas therefore represent regions which cannot be excluded as a source/match for the interrogation.

The results can be download by generating a report within the webpage.

Dataset history

This is Version 1 of the Biosphere Isotope Domains GB dataset.

  • Much of the strontium data were published in Evans et al. (2010)[2] but this has been added to considerably from both published and unpublished datasets, as well as data from the NERC Isotope Geosciences Laboratory (NIGL).
  • Groundwater oxygen isotope data is from Darling et al. (2003)[1].
  • Human tooth enamel data is from Evans et al. 2012[3].
  • Plant sulphur isotope data from a new dataset produced by NIGL.

Coverage

The Biosphere Isotope Domains GB (V1) dataset covers all of Great Britain, including the Shetland Islands, Orkney Islands, Outer Hebrides, Isles of Scilly and the Isle of Wight. It does not currently extend to Northern Ireland.

Strontium and oxygen isotope coverage is for all of Great Britain but coverage is inconsistent and variable. The sulphur domains cover England and Wales only; inland Scotland has no coverage, however the Coastal Effects domain has been extrapolated into Scotland (Figure 7).

Figure 7    The coverage of the Biosphere Isotope Domains (V1) dataset: (a) coverage of the strontium and oxygen domains; (b) coverage of the sulphur domains (no coverage in Scotland except for the Coastal Effects domain).

Data format

The Biosphere Isotope Domains GB (V1) dataset has been created as vector polygons and are available in ESRI ArcGIS (.shp) format. More specialised formats may be available upon request. The strontium and sulphur isotope data that underpin this dataset are also available as separate spread sheets from the BGS.

Limitations

There are a number of limitations that should be taken into account when using the Biosphere Isotope Domains GB (V1) dataset.

Data density and statistical reliability

  • The strontium domains that have been constructed have variable numbers of samples with which to define their statistical characteristics. Some of the large datasets e.g. Chalk (n=85) and Jurassic Clay (n=100) can be viewed as statistically robust, whereas others display a large range of values over a limited number of samples, such as the Southwest England Batholith (n=2).
  • Areas for which there are no data have been extrapolated using comparable datasets. For example, there are no data from sediments in the Lake District: all of these lithologies are defined by Welsh data from comparable lithologies.
  • Where Count is given as 0, there are no measured data and the values have been estimated. These domains include:
- Igneous – Caledonian Granite (England)
- Igneous – Palaeozoic Granite (South Britain)
- Igneous – Paleoproterozoic Intrusions (Scotland)
- Igneous – Proterozoic Intrusions (Wales)
- Several carbonate domains – for these examples, average strontium isotope values have been derived from seawater curves from McArthur et al. (2001)[8] and an interquartile range of +/- 0.001 has been assigned.
  • All of the strontium domains represent a mixture between 87Sr/86Sr originating from soils with a variable contribution from rainwater and therefore it is expected that the extent of rainfall and water logging will affect the bioavailable values. Some sense of this variation can be derived from comparison of the upper quartile map, which tends towards the more geologically dominant biosphere values and the lower quartile map, which will reflect a greater rainwater contribution of strontium with a value of 0.7092 (McArthur et al., 2001[8]).
  • Sulphur isotope data are only available for England and Wales.
  • The Coastal Effects domain is the extent that sea spray and sulphur rainout are likely to affect plant δ34S.
  • The upper limit for the Coastal Effects domain is defined as the δ34S value of seawater (21.0 ± 0.2‰ VCDT).
  • The lower limit for the Coastal Effects domain is fixed at 8.0‰VCDT and was determined as a major inflection point in Probability and Kernel Density plots of all the vegetation δ34S data.
  • The sea spray aerosol δ34S contribution to the Coastal Effects domain diminishes rapidly the further away from the coast one goes. As a result of prevailing wind directions and the topography, not all coastal areas will be affected in the same way, usually due to the rain shadow effect.
  • The physical extent of the Coastal Effects domain is defined as the area up to 15 km inland from the west coast and 10 km inland from the east coast, see Figure 3.
  • δ34S values >15.0‰ VCDT analysed in this study, only occur in samples collected within the tidal zone.
  • Areas where the substrate is permanently or intermittently waterlogged or is poorly draining (such as clay substrates) tend to have low δ34S values (<0.0‰) due to the effects of sulphur reducing bacteria. These areas include marshes, bogs and fluvial settings, and can be found along the coast as well as inland. Where reducing conditions exist within the Coastal Effects domain, the sea spray effect will have a less obvious impact on the δ34S values. In areas where the substrate is porous/freely draining, the sea spray sulphur aerosol effect will also be negligible as the residence time in the soil will be very short.

References

  1. 1.0 1.1 1.2 1.3 1.4 Darling, W G, Bath, A H, Talbot, J C. 2003. The O and H stable isotope composition of freshwaters in the British Isles. 2. Surface waters and groundwater. Hydrology and Earth System Sciences Discussions, European Geosciences Union, 2003, 7 (2), pp.183–195.
  2. 2.0 2.1 Evans, J A, Montgomery, J, Wildman, G, and Boulton, N. 2010. Spatial variations in biosphere Sr-87/Sr-86 in Britain. Journal of the Geological Society 167(1): 1–4.
  3. 3.0 3.1 3.2 Evans, J A, et al. 2012. A summary of strontium and oxygen isotope variation in archaeological human tooth enamel excavated from Britain. Journal of Analytical Atomic Spectrometry 27(5): 754–764.
  4. Shand, P, Darbyshire, D P F, Gooddy, D and Haria, A H. 2007. Sr-87/Sr-86 as an indicator of flowpaths and weathering rates in the Plynlimon experimental catchments, Wales, UK. Chemical Geology 236(3–4): 247–265.
  5. Trueman, C N G, Behrensmeyer, A K, Tuross, N and Weiner, S. 2004. Mineralogical and compositional changes in bones exposed on soil surfaces in Amboseli National Park, Kenya: diagenetic mechanisms and the role of sediment pore fluids. Journal of Archaeological Science 31(6): 721–739.
  6. Evans, J A. Biosphere Isotope Domain Map GB (V1): strontium isotope data. DOI 10.5285/ba36de6f-5a20-476b-965d-48182166114a
  7. Levinson, A A, Luz, B and Kolodny, Y. 1987. Variations in oxygen isotope compositions of human teeth and urinary stones. Applied Geochemistry 2: 367–371.
  8. 8.0 8.1 McArthur, J M, Howarth, R J, and Bailey, T R. 2001. Strontium isotope stratigraphy: LOWESS version 3: Best fit to the marine Sr-isotope curve for 0-509 Ma and accompanying look-up table for deriving numerical age. Journal of Geology 109 (2): 155–170.