ELEMENTAL ANALYSES OF MODERN DUST IN SOUTHERN NEVADA AND CALIFORNIA
by Marith C. Reheis1, James R. Budahn1, and Paul J. Lamothe1
Open-File Report 99-531
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
Selected samples of modern dust collected in marble traps at sites in southern Nevada and California (Reheis and Kihl, 1995; Reheis, 1997) have been analyzed for elemental composition using instrumental neutron activation analysis (INAA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectroscopy (ICP-MS). For information on these analytical techniques and their levels of precision and accuracy, refer to Baedecker and McKown (1987) for INAA, to Briggs (1996) for ICP-AES, and to Briggs and Meier (1999) for ICP-MS. This report presents the elemental compositions obtained using these techniques on dust samples collected from 1991 through 1997.
The dust-trap sites were established at varying times; some have been maintained since 1984, others since 1991. For details on site location, dust-trap construction, and collection techniques, see Reheis and Kihl (1995) and Reheis (1997). Briefly, the trap consists of a coated angel-food cake pan painted black on the outside and mounted on a post about 2 m above the ground. Glass marbles rest on a circular piece of galvanized hardware cloth (now replaced by stainless-steel mesh), which is fitted into the pan so that it rests 3-4 cm below the rim. The 2-m height eliminates most saltating sand-sized particles. The marbles simulate the effect of a gravelly fan surface and prevent dust that has filtered or washed into the bottom of the pan from being blown back out. The dust traps are fitted with two metal straps looped in an inverted basket shape; the top surfaces of the straps are coated with a sticky material that effectively discourages birds from roosting.
Individual dust samples were usually analyzed separately, but in some cases samples from adjacent dust traps at the same site were combined before analysis; these are denoted by two sample numbers, for example T2+T3, or T26+26A (Tables 1-3). In addition, samples from different years or seasons were sometimes combined to obtain enough material for analysis. The analyses were performed on the <50µm fraction (silt plus clay) of the samples after removal of soluble salts and organic matter. The duration of time over which the dust accumulated is shown for each sample (Table 1). The year of collection, and the month of collection in the case of Owens Valley sites T62 through T68, follows the site number; for example, T15-91 was collected in 1991, and T62-0592 was collected in May 1992.
Most of the samples were analyzed using only one technique, but some were analyzed using both INAA and ICP. These two techniques yield results for different, but overlapping, suites of elements and provide results with different levels of precision. Table 1 is a summary of all of the analyses listed in order by sample site and time of collection, showing geographic location and altitude. The different levels of precision for the techniques are generalized in this summary table to facilitate comparison of the data. Tables 2 and 3 are the standard laboratory reports for the INAA and both types of ICP analyses, respectively; these tables show the reported errors (precision) and comparisons to standards (accuracy). Table 3 also shows whether measurements for a specific element were made using ICP-AES or ICP-MS.
This report is not intended to provide interpretation of the data, but users should be advised of two important points. First, all of the samples were collected in dust traps that employed galvanized metal screens (these have now been replaced by stainless steel screens) to support the marbles. The amounts of Zn (present in quantities as large as 6 percent) clearly demonstrate that these metal screens were corroding and contributing to the samples. Small amounts of Cu and Ni may also have been contributed to the samples in this way. Second, Table 3 presents elemental concentrations for samples scraped from the outer parts of three glass marbles that are typical of those used in the dust traps. Although most of the elements we analyzed are present in very small amounts in these marbles, they variably contain notable amounts of As, Ba, Cd, and Cu. Reheis and Kihl (1995) noted that in very alkaline settings (such as playa margins), the glass marbles used in the dust traps became visibly pitted over a period of several years, suggesting the possibility of sample contamination with respect to these elements. However, As and Ba show very large variations in samples from across the study area regardless of the local site environment; if these elements were entirely derived from weathering of marbles, the sample concentrations should be more consistent. We also caution users that the presumably non-reactive surfaces of the cake pans have not yet been analyzed for elemental composition.
Briggs, P.H., 1996, Forty elements by inductively coupled plasma-atomic emission spectrometry, in Arbogast, B.F., ed., Analytical methods manual for the U.S. Geological Survey: U.S. Geological Survey Open-File Report 96-525, pp. 77-94.
Briggs, P.H. and Meier, A.L., 1999, The determination of 42 elements in geological materials by inductively coupled plasma mass spectrometry: U.S. Geological Survey Open-File Report 99-166, 15 p.
Reheis, M.C., 1997, Dust deposition downwind of Owens (Dry) Lake, 1991-1994-Preliminary findings: Journal of Geophysical Research (Atmospheres), v. 102, no. D22, p. 25,999-26,008.
Reheis, M.C., and Kihl, Rolf, 1995, Dust deposition in southern Nevada and California, 1984-1989--relations to climate, source area, and source lithology: Journal of Geophysical Research (Atmospheres), v. 100, no. D5, p. 8893-8918.
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