Introduction to the thematic collection: a review of continental-scale geochemical mapping projects in Australia, Europe, and the conterminous United States

Thematic collection: This article is part of the Continental-scale geochemical mapping collection available at: https://www.lyellcollection.org/topic/collections/continental-scale-geochemical-mapping

V.M. Goldschmidt, generally considered the father of modern-day geochemistry, stated that the purpose of geochemistry is to determine quantitatively the chemical composition of the Earth and its parts, and to discover the laws that control the distribution of the individual elements (Goldschmidt 1937, 1954). Studies to address Goldschmidt's stated purpose can be conducted at a variety of scales ranging from the microscopic to the global. One of the major science issues for geochemistry is to determine the abundance and spatial distribution of chemical elements in the Earth's near-surface environment at the continental, or global, scale.

The tool to address this issue is, currently, large-scale geochemical mapping involving the collection of physical samples of a selected medium or media and the analysis of these samples in a chemical laboratory for many chemical parameters. Unfortunately, there is no current remote sensing technology that can accomplish such a task. These types of geochemical surveys, covering several millions of square kilometres of the Earth's surface, are necessarily conducted at a very low sampling density (usually 1 site per 1000 km² to 1 site per 10 000 km²). The most likely sample media to be considered for such a broad-scale survey include (1) sediments collected from active or recently active streams, (2) soils, and (3) floodplain or overbank sediments (also called catchment outlet sediments).

These low-density geochemical surveys produce patterns of element distribution that reflect physical and chemical processes acting at the broad scale of sampling. These processes are related to such factors as the geochemical and mineralogical composition of the original parent material, soil formation, topography, continental-scale glaciation, regional-scale alteration and local-scale mineralization, climate, tectonics, weathering, and, for some surveys, human activity. The production of a multi-element geochemical atlas of the Earth's land surface based on such low-density surveys has been discussed by the geochemical community since at least the 1980s. During the past 25 years, continental-scale geochemical surveys have been conducted in China, Europe, Australia, India, Mexico, and the USA. The data and maps from these studies have proven to be useful as tools for environmental and resource management.

In the following three papers, the project leaders for continental-scale geochemical surveys of Australia (Patrice de Caritat), Europe (Clemens Reimann), and the USA (David B. Smith) discuss each of their projects from inception through sample collection, analysis, publication of the results and, finally, impact.

The projects in Australia, Europe, and the USA used very low to low sampling densities: 1 site per 5500 km², 1 site per 2500 km², and 1 site per 1600 km², respectively. The sample target populations were catchment outlet sediments in Australia; soil from agricultural fields and grazing lands in Europe; and soils, regardless of land use, in the USA. In Australia, depth-based sampling was conducted with a top sample collected from 0–10 cm and a bottom sample collected, on average, between 60 and 80 cm. Europe's sampling protocols were dictated by the requirements of REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals). Thus, agricultural soils were sampled at a depth of 0–20 cm, considered to be the plowing zone; grazing lands were sampled at a depth of 0–10 cm. In the USA, a combination of depth-based sampling and horizon-based sampling was used. The sampling protocol included, at each site, a sample from a depth of 0–5 cm, a composite of the soil A horizon, and a deeper sample from the soil C horizon.

Sample preparation and analytical protocols were also somewhat different for each of the three studies. In Australia, two size fractions were prepared for analysis: <2 mm and <75 $μ$m. The <2 mm fraction was ground to a much finer fraction prior to analysis by most methods. In Europe, the samples of both agricultural soils and grazing land soils were sieved to <2 mm as required by REACH. This <2 mm fraction was not ground further prior to chemical analysis. In the USA study, all samples were sieved to <2 mm and then ground to <150 $μ$m prior to chemical analysis.

In the Australian study, samples were analysed for up to 68 elements by a variety of methods. Some of these methods provided total element content, whereas others used a partial digestion (aqua regia and, for the top coarse sample, a weak ligand extractable method called mobile metal ion (MMI®) extraction) prior to analysis. In Europe, the REACH requirements stated that an aqua regia digestion must be used. In addition, the project also determined total-element content. In Europe, all samples were analysed for 52 chemical elements following aqua regia extraction, 41 elements by XRF, and 57 elements in the MMI® extraction. In the USA study, 45 elements were determined by methods that yielded the total, or near-total, element content. These methods used multiple-acid extraction prior to chemical analysis. Each study utilized a rigorous quality control program that involved analysis of international reference materials, internal project standards, and numerous field and laboratory duplicate samples.

In addition to chemical analyses, each of the three projects performed other types of analyses as budget would allow. In Australia, several bulk properties were determined such as Munsell® colour, field- and lab-pH, electrical conductivity, and grain size distribution of bulk samples. Multiple additional analyses were conducted in collaboration with other institutions after delivery of the main project. In Europe, lead and strontium isotopes were determined on the agricultural soil samples and magnetic susceptibility studies were conducted. In addition, cation exchange capacity, pH, total organic carbon, total carbon and sulfur, soil colour and particle size were determined. Metal and metalloid partitioning coefficients (Kd) in soil were determined for selected elements using mid-infrared diffuse reflectance spectrometry. These were the basis for a risk assessment of metals in soil under the European REACH regulation. In the USA, quantitative mineralogy by X-ray diffraction was done on the soil A- and C-horizon samples and selected soil pathogens (anthrax, plague, and tularemia) were determined on samples collected from 0–5 cm.

Although these three continental-scale geochemical mapping projects differed in some respects, the broad conclusions reached were quite similar. These include:

• low-density geochemical mapping works, in that it delivers robust geochemical patterns that can be related to natural processes or, in some cases, human activities;

• low-density sampling and mapping not only work for geochemistry, but also for properties such as magnetics, isotope systems, mineralogy, microbiology (genomics), and others;

• low-density geochemistry works for a large variety of sample media (soil, sediments, water, plants) and would likely also work for other types of natural samples;

• low-density geochemistry provides a rather cheap tool to effectively map large tracts of land for a first ‘overview’ (it could, for example, also be used to map the geochemistry of the ocean floor);

• creating an archive of sample material for future research testing new ideas, potentially implementing unforeseeable technologies, is paramount;

• engaging with a wide range of stakeholders early-on in the project development is important;

• a strong publication strategy maximizes early uptake and interest in the new data, attracts industry and invites collaboration with academia;

• individual geochemical mapping projects should exchange project standards and organize round-robin tests to ensure comparability of data to the maximum extent possible.

In the following three papers, the authors describe in much more detail the evolution of each project. Conducting such successful continental-scale geochemical surveys is dependent on a variety of factors in addition to the required financial and human resources. Numerous lessons were learned in each respective study, and it is hoped that these lessons may assist future applied geochemists in planning and conducting similar geochemical mapping projects.