Sedimentation rates in Lake Sempach

Figure 2. Time series of the particle sedimentation rate in Lake Sempach. Fluxes are higher near the sediment surface in 86-m depth because of resuspension. The large maxima during summer correspond to algal blooms and subsequent biogenic calcite precipitation.

Figure 3. Mn sedimentation rates in Lake Sempach. Part a Flux of particulate manganese from the epilimnion. Part b Mn sedimentation at 81 m and at the sediment surface (86 m).

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Several factors may account for large resuspension rates. The retrieval and deployment of the trap at the sediment surface may resuspend some particulate matter. Natural resuspension may result from storms and sediment-focusing mechanisms. Postdepositional remobilization may increase the sedimentation rate of210 Pb at the deepest point of Lake Sempach (41). Because we cannot discriminate among different resuspension processes, we assumed that the Mn concentration in the resuspended material is equal to that in the sedimenting particles at a depth of 86 m. Particulate MnO, is rapidly reduced at the sediment surface therefore, this procedure tends to overestimate the resuspension term. 

Burial Rate of Mn. Analysis of sediment cores provided a closer look at the dynamics of Mn within the sediments. Figure 5 combines results from three sediment cores taken at the deepest site of Lake Sempach. Two cores were taken in January 1991 and one in May 1988. The arrows indicate the flux-averaged Mn concentrations of settling material at depths of 20 and 81 m, respectively. The time scale in Figure 5 was calculated with 137Cs dating (41) by using Ps = 1.84 g/m2 per day as the sediment accumulation rate. 

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