Rates, Timing, and Cyclicity of Holocene Eolian Activity in North-Central United States: Evidence from Varved Lake Sediments

Most of the sediment components that accumulated in Elk Lake, northwestern Minnesota, during the Holocene are autochthonous or biogenic, delivered to the sediment-water interface on a seasonal schedule, preserved in distinct annual laminae (varves). The main allochthonous component is detrital clastic material, as measured by bulk-sediment concentrations of aluminum, sodium, potassium, titanium, and quartz, that enters the lake mostly as eolian dust. The eolian clastic influx to Elk Lake was considerably greater during the mid-Holocene (8–4 ka) than it has been for the past 4000 yr, when periods of increased eolian activity correspond to the time of the Little Ice Age and the dust bowl. Geochemical records of eolian activity exhibit distinct cyclicities with dominant periodicities of 400 and 84 yr. 1Data from Bradbury and Dean (1993) and Dean et al. (1994) are available over the Internet as the “Elk Lake Data Set” from NOAA, National Geophysical Data Center Paleoclimate Program, Boulder, Colorado (ftp://ftp.ngdc.noaa.gov/paleo/elklake), or from W. E. Dean. Figure 1. Maps showing (A) airstream regions in North America; (B) vegetation zones of Minnesota; (C) Itasca State Park with location of Elk Lake; (D) bathymetry of Elk Lake. INDICATORS OF EOLIAN ACTIVITY Because no permanent streams enter Elk Lake, the small amount of detrital clastic material that is present in the sediments most likely was transported by wind. Concentrations and mass accumulation rates (in mg/cm2/yr) of many elements usually associated with detrital clastic material (e.g., aluminum, potassium, sodium, titanium, and several trace elements) all reached maximum values during the middle Holocene; much lower amounts of sediments were deposited during the past 3000 yr (Dean, 1993). These elements are represented in Figure 2 by aluminum (Al) and sodium (Na). In addition, I use Na as a proxy for available moisture because the principal source of Na is plagioclase feldspar that is easily decomposed by hydrolysis with sufficient available moisture, releasing Na. During the dry midHolocene prairie period in Minnesota when available moisture was lower, less plagioclase was decomposed and the concentration of Na relative to the total detrital fraction increased. This lower decomposition of plagioclase is confirmed by mineralogical studies (Dean, 1993) that show that plagioclase was most abundant in mid-Holocene sediments in Elk Lake. Other proxy variables that indicate greater influx of eolian dust to the site of Elk Lake during the mid-Holocene are increased concentrations of silt-size quartz and increased magnetic susceptibility (Sprowl and Banerjee, 1989; Bradbury et al., 1993; Dean et al., 1996). The increase in influx of eolian clastic material during the mid-Holocene also produced varves that are much thicker than those formed over the past 4 k.y. from predominantly autochthonous components (Anderson, 1993). This evidence for increased mid-Holocene eolian activity from the sediments of Elk Lake is set in a background of evidence for active dune migration in Minnesota and the Great Plains (Dean et al., 1996). Figure 2, A and D, shows that the concentrations and mass accumulation rates of Al and Na in sediments that accumulated in Elk Lake over the past 3 k.y. are considerably less than those in sediments that accumulated between 8 and 4 ka. Specifically, the average Al accumulation rate in sediments deposited in the mid-Holocene between 7.8 and 5.5 ka is seven times higher than the average Al accumulation rate in sediments deposited over the past 3 k.y. By comparing Na accumulation rates between the same two periods, the average mid-Holocene Na accumulation rate is 17 times higher, reflecting the lower decomposition of plagioclase during the mid-Holocene. Even at the coarse sampling interval of the data plotted in Figure 2, A and D, there are peaks in concentrations and mass accumulation rates of Al and Na in sediments deposited during the past 1000 yr. When we examine the last 1500 yr of the Elk Lake record (Fig. 2, B and E), cyclic variations at several different scales are apparent, but the most obvious are three groups of peaks about 400 yr apart centered at 1200–1000 yr, 800–600 yr, and 400–200 yr ago. Half of a fourth 400-yr peak can be seen at 1500 yr ago. The youngest and largest group of peaks corresponds in time (400–200 yr ago) to the main phase of the Little Ice Age (A.D. 1550–1700; Lamb, 1977), and the highest concentrations correspond in time to the Maunder sunspot minimum (A.D. 1640–1710). The interpretation of greater eolian activity to explain most of the increased influx of detrital clastic material to Elk Lake during the midHolocene is supported by evidence of increased eolian activity at the same time from numerous localities (Dean et al., 1996). If this interpretation is correct and can be extended to the late Holocene, then the Elk Lake record suggests that northwestern Minnesota was windier and dustier at several times during the past 1500 yr, relative to 3000 to 332 GEOLOGY, April 1997 Figure 2. Profiles of (A) concentration (pattern) and mass accumulation rate (MAR in mg/cm2/yr; black) of aluminum (Al) in sediments of Elk Lake for the past 10,000 yr; (B) %Al for the past 1500 yr; (C) %Al for the past 114 yr; (D) concentration (pattern) and MAR (in mg/cm2/yr; black) of sodium (Na) for the past 10,000 yr; (E) %Na for the past 1500 yr; and (F)%Na for the past 114 yr. All values are on carbonate-free weight percent basis. Note difference in scales for %Al between A and B + C, and for %Na between D and E + F. 1500 yr ago, and that this eolian activity reached a maximum during the Little Ice Age. The Little Ice Age actually is the culmination of a series of late Holocene glacial advances (neoglaciation; Porter and Denton, 1967). Several lines of evidence suggest that the beginning of the mid-Holocene “prairie period” in Minnesota was cold and dry, at least until about 6.8 ka (Bradbury et al., 1993), not warm and dry as is usually interpreted for the midHolocene “altithermal.” The point is that aridity, not temperature, is the main variable controlling eolian activity. The most recent periods of eolian activity recorded in Elk Lake may or may not be related directly to neoglaciation, but they almost certainly are related to regional aridity in North America that caused active dune migration over large areas of the Great Plains several times during the past several thousand years (see review by Dean et al., 1996) and also caused increased salinity of lakes in the Northern Great Plains (Fritz et al., 1994). An examination of the Elk Lake record since A.D. 1870 (Fig. 2, C and F) shows peaks in concentrations of both Al and Na during the first half of the twentieth century. It is tempting to relate these increases to logging in Itasca Park between 1903 and 1919, but the greatest concentrations occur in sediment deposited between about 1930 and 1950, suggesting that greater clastic influx is related to increased regional aridity associated with the dust bowl decades. This dust bowl peak in eolian activity is confirmed in Figure 3B by another eolian proxy, quartz concentration as measured by relative X-ray diffraction peak height. This period is also marked in the Elk Lake cores by an increase in concentration of plagioclase feldspar (Dean et al., 1994) suggesting a decrease in available moisture. Both Al concentration (Fig. 3A) and, especially, quartz peak height (Fig. 3B) suggest that eolian activity was greater during the dust bowl years than during the Little Ice Age, and both periods of activity were not as great as mid-Holocene eolian activity (Fig. 2, A