Abstract
Geochemical baseline studies are important for geological and environmental purposes such as investigating current spatial differences in soil composition and monitoring current, previous or future landuse activities. It is necessary to separately establish baseline data for each region because soil forming factors, such as parent rock type and age, cause the chemical compositions in soil to vary. Geochemical surveys of elemental compositions and their isotopes have previously been completed on various scales in countries around the world. However, to date, a national geochemical baseline survey to characterize elemental concentrations and other geochemical parameters across New Zealand has not yet been conducted. As a first step towards achieving this goal, a geochemical baseline survey of southern New Zealand, led by GNS Science, was carried out in 2015.
The overarching objective of this MSc study is to measure the strontium isotopic composition (recorded by the 87Sr/86Sr isotope ratio) and elemental concentrations of partially extracted soils from a subset of the sites used for the 2015 Geochemical Baseline Survey of Southern New Zealand Project led by GNS Science. Strontium isotopes are a powerful diagnostic tracer of provenance because the 87Sr/86Sr ratio varies at the 10 %-level in nature due to changes in local geology, and is ca. three orders of magnitude larger than precision during measurement.
In this study, 166 soil samples from A (0-30 cm depth) and B (50-70 cm depth) soil depths, representing 108 sites in a 16 km grid system, were digested using an ammonium nitrate solution to extract the “easily soluble” soil fraction, mimicking the pool of elements which are “bioavailable” during natural processes such as mineral leaching of soils during rainfall, the subsequent transfer of these elements to groundwaters and rivers, and their incorporation into plants and, in turn, animals (including humans) during uptake processes. For each digest, the concentrations of a suite of 43 elements were obtained by quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS). A chemical separation method using ion exchange chromatography was tested and optimized to isolate and purify Sr from matrix elements. The 87Sr/86Sr isotope ratio of each digest was then determined using multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS).
The Sr isotope datasets acquired for the sampling sites of the A-depth and B-depth soil samples were used to make colour interpolation maps using ArcGIS software. These Sr isotope soil maps show systematic variations across the Otago-Southland study area that are similar for both the A-depth and B-depths and closely reflect the underlying Otago-Southland geology. The modelled Sr isoscape results for the A-depth soil compared with excellent agreement to independently published vegetation and water results, therefore, these new Sr ‘isoscapes’ offer significant potential as a predictive tool for applications such as tracking migration of animals such as birds or fish and the forensic tracing of commercially marketed foods and drinks. These applications are particularly valuable for the successful dairy industry in the Otago-Southland region and Otago wineries. The Elemental distributions across the Otago-Southland region were also interrogated using Principal Components Analysis. However, the PCA biplots showed significant overlap of sites from distinct geological terranes mainly due to group size limitations.
A “Transect study” focusing on a 290 km west-east transect, extending from Fiordland National Park on the west coast to Dunedin on the east coast was completed to investigate, in more detail, the second-order factors controlling 87Sr/86Sr variations in the extracted soil samples of the study area, in addition to local geology. The influences of anthropogenic activities, such as farming practices involving fertiliser application, and the influence of seaspray at coastal areas, were, in some cases, difficult to determine solely based on the 87Sr/86Sr signature, but these influences could be distinguished if considered in combination with elemental concentration datasets.
This study provides informative 87Sr/86Sr isoscape baseline for the Otago-Southland region, and the first maps of this type for New Zealand. This could extend to agricultural uses such as forensic tracing studies, archaeological and anthropological uses such as investigation into the diet, birth place and mobility of modern and ancient humans or animals, as well as geological uses, such as providing information on the basement geology. In future this Sr isoscape could be extended to provide a better contour predictor by the addition of soil 87Sr/86Sr measurements at other sites for areas or terranes which currently have a limited number of measurements, such as coastal areas as well as the Brook Street and Māui Terranes.
If the pilot geochemical baseline study of the Otago-Southland region was extended by GNS Science in the future to include sites from the whole of New Zealand, then the characterisation of 87Sr/86Sr values in soil at a spatial resolution of 32 km would provide a useful and accurate reflection of the underlying geology of the region to inform forensic investigations of animals and vegetation in years to come.