The US Geological Survey has made publicly available a large repository of geochemical data (63 of the 91 naturally occurring elements) that it has acquired through a continuing nation-wide survey of stream sediments (available at http://tin.er.usgs.gov/geochem/doc/home.htm). The data coverage is incomplete and involves several generations of previous surveys. The most revealing stream sediment surveys involve collection of panned sediment samples in every small stream that has no upstream tributary, but that is a daunting task for such a vast area as the USA. That method allows the analyses to be treated as accurate representations of stream sediment composition in upstream catchments around 1 x 1 km in size. The USGS data are a mixed bunch, some dating from the National Uranium Resource Evaluation (NURE) of the 1970s when there was a scramble to find new uranium ore bodies. The NURE survey involved a sample density based on a 17 x 17 km grid, and made no distinction between stream order. The latest USGS survey is based on sample collection that uses 10 x 10 km grids drawn in the UTM co-ordinate system. Each 10 x 10 km cell is divided into four quadrants, and one is selected at random for sampling. In that one small stream selected at random is chosen for analysis. The data set is too coarse and too varied to create meaningful gridded interpolations that can be displayed as continuous tone images, unlike comparable geochemical atlases based on systematic, small-stream sampling, such as that developed for commercial leasing by the British Geological Survey. The NGS data will be a useful resource for scanning broad geochemical features of the country, such as for high levels of potentially toxic elements in water, bearing in mind that the analyses are of solid minerals not the water itself.
The last issue of EPN showed that the debate over mantle plumes, their sources, and even their existence is hotting up (see Geoscience consensus challenged in EPN January 2004). However that pans out, vast areas of continental and submarine flood basalts compel geoscientists to ponder over them, the more so because they represent events never witnessed by humans and are therefore unimaginable. Now they have their own website (http://www.mantleplumes.org/) that has been compiled by Gillian Foulger of Durham University. It is an impressive and highly useful resource, the outstanding feature being pages on most aspects of large igneous provinces written by experts who are also excellent communicators. There is even a linked site at the Geological Society that hosts discussion on the Great Plume Debate, as well as a letters page, links and up to date news. For information, without unnecessary frills, this is the place to go, especially if you have to write an essay!