『Abstract
In 2004, the US Geological Survey (USGS) and the Geological Survey
of Canada (GSC) initiated a pilot study that involved collection
of more than 1500 soil samples from 221 sites along two continental
transects across Canada and the United States. The pilot study
was designed to test and refine protocols for a soil geochemical
study of North America. The two transects crossed a wide array
of soil parent materials, soil ages, climatic conditions, landforms,
land covers and land uses. Sample sites were selected randomly
at approximately 40-km intervals from a population defined as
all soils of the continent. at each site, soils representing 0
to 5 cm depth, and the O, A, and C horizons, if present, were
collected and analyzed for their near-total content of over 40
major and trace elements. Soils from 0-5 cm depth were also collected
for analysis of organic compounds. Results from the transects
confirm that soil samples collected at a 40-km spacing reveal
coherent, continental- to subcontinental-scale geochemical and
mineralogical patterns that can be correlated to aspects of underlying
soil pattern material, soil age and climate influence. The geochemical
data also demonstrate that at the continental-scale the dominance
of any of these major factors that control soil geochemistry can
change across the landscape. Along both transects, soil mineralogy
and geochemistry change abruptly with changes in soil parent materials.
However, the chemical influence of a soil's parent material can
be obscured by changing climatic conditions. For the transects,
increasing precipitation from west to east and increasing temperature
from north to south affect both soil mineralogy and geochemistry
because of climate effects on soil weathering and leaching, and
plant productivity. Regional anomalous metal concentrations can
be linked to natural variations in soil parent materials, such
as high Ni and Cr in soils developed on ultramafic rocks in California
or high P in soils formed on weathered Ordovician limestones in
central Kentucky. On local scales, anomalous metal concentrations
recognized in soil profiles, such as high P in soils from animal
confinement sites, are consistent with local anthropogenic disturbances.
At a larger scale, the distribution of Hg across the west to east
transect demonstrates that it can be difficult to distinguish
between natural or anthropogenic contributions and that many factors
can contribute to an element's spatial distribution.
Only three samples in a subset of seventy-three 0-5 cm depth
soil samples from the north to south transect had organochlorine
pesticides values above the method detection limit, apparently
related to historic usage of the pesticides DDT and dieldrin.』
1. Introduction
2. Sampling protocols and analytical methods
2.1. Sample site selection
2.2. Sample collection protocols
2.3 Analytical methods
3. Results and discussion
3.1. West-east transect mineralogy and geochemistry
3.2. North-south transect mineralogy and geochemistry
3.3. Geochemical patterns for paired A- and C-horizon soils -
climate effects
3.4. Geochemical patterns for paired A and C-horizon soils -
local anthropogenic effects
3.5. Geochemical patterns for paired A and C-horizon soils -
continental-scale effects
3.6. Pesticides in 0-5 cm depth soil samples from the north-south
transect
4. Conclusions
Acknowledgments
References