The purpose of these interpretive discussions is to provide a perspective on regional- and national-scale variations in element and mineral distributions in soils and their likely causes. The significant spatial variations shown by most elements and minerals can commonly be attributed to geologic sources in underlying parent materials, but other spatial variations seem clearly related to additional factors such as climate, the age of soils, transported source material, and anthropogenic influences. We attempt to distinguish the influence of these various factors on a regional and national scale. Numerous more local features might similarly be related to these same factors, but these features also have some probability of being an artifact of a random sampling of variable compositions, so that there is some probability of samples with similar compositions occurring in clusters of two or more adjacent sites by chance. Distinguishing such random occurrences from true variability is beyond the scope of the data from which these maps are constructed. Some caution, therefore, is advisable in interpreting the significance of these more local features unless some unique sources or processes can clearly be related to them.
Cobalt (Co) is a metallic trace element used in the production of rechargeable battery electrodes; superalloys, which are used to make parts for gas turbines; catalysts for the petroleum and chemical industry; dyes and pigments; and high–grade steel. Cobalt can have both beneficial and harmful effects to humans. Vitamin B–12 plays an essential role in human health and is unique among vitamins because it contains a metal ion, Co. However, exposure to high concentrations of Co can cause lung and heart problems. More information about the toxicity of Co and its potential negative human health impacts can be found at the Agency for Toxic Substances and Disease Registry (ATSDR) website, or click to download a fact sheet about Co.
Cobalt occurs mostly in ferromagnesian minerals such as olivine, pyroxene, and amphibole where it substitutes for iron (Fe) and magnesium (Mg). It is also incorporated into common sulfide minerals such as pyrite, arsenopyrite, and sphalerite. In soils, Co is often associated with Fe and especially manganese (Mn) oxides and hydroxides.
The average abundance of Co in the Earth's upper continental crust is estimated to be about 15 milligrams per kilogram (mg/kg) (Hu and Gao, 2008). Ultramafic rocks, such as serpentinite and peridotite, have highly elevated concentrations of Co with values up to a few hundred mg/kg. Mafic rocks, such as basalt, have an average Co concentration of about 45 mg/kg. Among the other common rock types, shale has elevated Co with an average concentration of about 20 mg/kg. The average Co concentration is about 4 mg/kg in granite and about 0.1 to 0.3 mg/kg in sandstone and limestone.
The distribution of mineral resource deposits with Co as a commodity (major or minor) in the United States, extracted from the U.S. Geological Survey (USGS) Mineral Resource Data System (MRDS) website, can be seen by hovering the mouse here. Statistics and information on the worldwide supply of, demand for, and flow of Co–bearing materials are available through the USGS National Minerals Information Center (NMIC) website.
In our data, the median Co concentration is slightly higher in the soil C horizon (8.4 mg/kg) than in the soil A horizon (7.8 mg/kg) and the top 0- to 5-cm layer (7.7 mg/kg) (see the summary statistics [open in new window]). This indicates that Co is somewhat mobile in the near-surface weathering environment. In general, the geochemical maps for the three sample types are quite similar for Co.
The distribution of Co in soils of the conterminous United States is primarily controlled by the composition of underlying soil parent materials. Areas of elevated Co concentrations in soils are seen in:
- Northern California, Oregon, Washington, extreme southeastern Pennsylvania, central Maryland, central and western North Carolina, northeastern Minnesota, and central Arizona where the parent materials for soils are, at least in part, ultramafic or mafic rocks or glacial deposits derived from these rock types;
- Eastern Montana, North Dakota, South Dakota, Iowa, eastern Nebraska, eastern Kansas, northern Missouri, Illinois, Indiana, Kentucky, Ohio, and Pennsylvania where parent materials are dominantly shale, clayey till, or glacial deposits containing a significant amount of shale; and
- Southern Blue Ridge, Southern Piedmont, and Northern Piedmont (USDA, 2006) where soil parent materials comprise, in part, igneous and metamorphic rocks of mafic composition (for example, basalt and amphibolite).
The Gulf and Atlantic Coastal Plain (Fenneman and Johnson, 1946) is bisected by the Southern Mississippi River Alluvium and the Southern Mississippi Valley Loess (USDA, 2006). Alluvial sediments have deposited in the Mississippi River valley as the river flooded in recent geologic time. When these sediments dried, winds picked up the fine material and deposited it in thick loess sheets, mainly along the east side of the river valley. The youngest loess sheets are about 10,000 years old. A pattern of higher Co concentrations in soils developed on these young sediments reflects long–range transport of Co–bearing material from the upper part of the Mississippi River drainage basin.
Areas of low Co concentrations in soils include:
- Atlantic Coastal Plain (Fenneman and Johnson, 1946) where parent materials are dominantly quartz–rich sedimentary rocks and unconsolidated sediments;
- Nebraska Sand Hills (USDA, 2006), where parent materials are quartz– and plagioclase–rich, unconsolidated sand dunes and sand sheets;
- Southern High Plains (USDA, 2006) of eastern New Mexico and Texas where parent materials consist of quartz–rich eolian sand and alluvial sediments; and
- Northern and western Michigan where parent materials comprise quartz–rich glacial deposits.
Statistics - 0 TO 5 CM
| Number of samples | 4,841 |
| LLD | 0.1 mg/kg |
| Number below LLD | 24 |
| Minimum | <0.1 mg/kg |
| 5 percentile | 0.9 mg/kg |
| 25 percentile | 4.4 mg/kg |
| 50 percentile | 7.7 mg/kg |
| 75 percentile | 11.1 mg/kg |
| 95 percentile | 21.2 mg/kg |
| Maximum | 216 mg/kg |
| MAD | 4.89 mg/kg |
| Robust CV | 63.5% |
Histogram
Boxplot
Empirical cumulative distribution function
Statistics - A Horizon
| Number of samples | 4,813 |
| LLD | 0.1 mg/kg |
| Number below LLD | 34 |
| Minimum | <0.1 mg/kg |
| 5 percentile | 1 mg/kg |
| 25 percentile | 4.6 mg/kg |
| 50 percentile | 7.8 mg/kg |
| 75 percentile | 11.3 mg/kg |
| 95 percentile | 21.6 mg/kg |
| Maximum | 184 mg/kg |
| MAD | 4.89 mg/kg |
| Robust CV | 62.7 % |
Histogram
Boxplot
Empirical cumulative distribution function
Statistics - C Horizon
| Number of samples | 4,780 |
| LLD | 0.1 mg/kg |
| Number below LLD | 24 |
| Minimum | <0.1 mg/kg |
| 5 percentile | 1.5 mg/kg |
| 25 percentile | 5.0 mg/kg |
| 50 percentile | 8.4 mg/kg |
| 75 percentile | 12.3 mg/kg |
| 95 percentile | 24.4 mg/kg |
| Maximum | 316 mg/kg |
| MAD | 5.34 mg/kg |
| Robust CV | 63.5 % |
Histogram
Boxplot
