Eastern Mineral and Environmental Resources Science Center
Prior to its inception as a National Park in 1940, Isle Royale, a large island in Lake Superior, primarily was a commercial fishing and resort area, with only sporadic historic copper mining, and insignificant logging. Isle Royale is now a National Park and a designated wilderness region with only minimal human impact, mostly concentrated along the coast. However, it recently has been discovered that six inland lakes on Isle Royale have game fish with high mercury levels; other inland lakes which have been studied do not show significant mercury impact.
Bedrock on Isle Royale is either basalt or sandstone and conglomerates of the 1100 million year old Midcontinent rift. Much of the island is covered with a thin layer of glacial material. A number of small native copper mines were active in the 1800's, but mineralization is not extensive and mining was never prosperous. Recent analyses by the USGS of both unmineralized basalt and copper-mineralized rock show that a small amount of naturally-occurring mercury is associated with mineralization. A limited soil survey conducted by the USGS in 1999 showed variable and occasionally high mercury levels in soils, but no link with copper mineralzation.
To gain a better understanding of the distribution of mercury and the causes of the large variations in mercury content in soils on the island a survey of soils and lichens has been initiated by scientists in the Geologic Division and Biological Resource Division, with cooperation from the National Park Service. Most of the mercury in the upper part of the soil profile is probably a result of atmospheric deposition from off-island sources, as there are no known significant mercury sources on the island to account for the widespread distribution of mercury in soils, and it is well known that atmospheric deposition is an important contributor of mercury in the general region.
Lichens have long been known to extract many elements from the atmosphere as a part of their life process and therefore record relative levels of atmospheric composition over their life span. Lichens that grow on trees (epiphytic lichens) derive all their contained components from the atmosphere and are especially useful in mapping relative element concentrations in the atmosphere. The survey on Isle Royale is collecting three different species of epiphytic lichens on traverses along and across the island and will map relative differences in mercury content in the atmosphere and precipitation on the island.
A soil survey linked to the lichen survey is collecting samples of organic surface layers and A and C soil horizons at each lichen sample site. The survey will establish the degree to which variations in mercury content in various soil horizons are related to variations in atmospheric mercury concentrations of the extent of the island. Soil mercury content will also be compared to other possible controlling factors such as soil type, forest type and maturity, and physiography and drainage.
The ultimate aim of this research is to assess the degree to which geologic materials play a role in controlling the movement of mercury in a largely undisturbed forest ecosystem and whether there is a direct correlation between mercury content in geological materials in a watershed and the degree of mercury accumulation in the aquatic food-web of the watershed. Results should prove useful both in understanding the cause of elevated levels of mercury in fish on Isle Royale and may also be of more general use in studying widespread mercury problems across the region.
|Mineral Resources Program||National Minerals Information / Geology, Minerals, Energy and Geophysics|
|Alaska / Crustal Geophysics and Geochemistry / Central Mineral and Environmental Resources / Spatial Data|