Little Rock Lake sediment core

Figure 1. Sediment map of Little Rock Lake. Sediment characteristics were determined from 10-cm cores obtained at 5-m intervals along 28 transects extending from the shoreline towards the center of the lake, plus several samples from deep stations. Individual cores were extruded on site and samples were analyzed for wet weight, dry weight, and loss on ignition. This information was augmented by sonar mapping by Bill Rose (U.S. Geological Survey, Madison, WI).

Numerous laboratory studies involving 35S(>42 additions to mixed sediments and intact sediment-water cores from all areas of the lake support the conclusion that sulfate reduction is an important process in Little Rock Lake sediments. 

For Little Rock Lake, a single core from each of its two basins was analyzed and dated in stratigraphic detail. The remaining cores were analyzed for Hg content in three coarse intervals as described, but none of these profiles was actually dated. Instead the sedimentation rates were inferred from a series of five nearby cores that had been dated by 210Pb for other purposes (16). The mean sedimentation rates from dated cores collected at similar depth in the same basin were used to calculate Hg accumulation for each undated profile. 

Figure 4. Plots of sediment-accumulation rate versus age for the cores in Figure 3. Error bars represent one standard deviation propagated from counting uncertainty. The apparent increase in recent sediment accumulation in Little Rock Lake is atypical of other cores from this site.

Hg Concentrations in Sediments A typical Hg concentration profile in profundal sediment cores from Little Rock Lake Treatment Basin is shown in Figure 3. Mercury concentrations range from about 50-185 ng/g (dry weight). Similar concentrations were observed by Rada et al. (35) in Little Rock Reference Basin (6-205 ng/g for surface grabs across the lake, including sandy sediments) and by R. Rada (University of Wisconsin, LaCrosse, personal communication) for Little Rock Treatment Basin (3-220 ng/g for similarly retrieved surface grabs). The decrease in concentration toward the top... 

Figure 3. Sediment core from Little Rock Lake (3 m) depicting dry bulk particle (A) and pore-water (A) HgT concentration. Sediments were dated by 13rCs and 210Pb. Inset Coal use in the United States according to a 1986 report to the National Academy of Sciences.

Figure 7. Sulfur transformations of seston-S and microbially reduced S in sediments of Little Rock Lake. This diagram is based on information collected from the lake (cores and sediment traps) and from laboratory sediment-water microcosms to which 35S was added as labeled algae or as 35S042 Sizes of circles and arrows are roughly proportional to magnitudes of pools and fluxes, respectively. ROSO3 = ester-S MeS = metal sulfide, C-S = carbon-bonded S.

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