210Pb Geochronology in Lake Superior Sediments: Sedimentation Rates, Organic Carbon Deposition, Sedimentary Environments, and Post-Depositional Processes

Abstract

210Pb geochronology is used to determine sedimentation rates, 210Pb flux rates, and organic carbon deposition rates from 17 sediment box cores in Lake Superior, U.S.A. These data, in conjunction with organic carbon, PCB, trace metal, benthic organism, and sedimentary structure data, are used to investigate depositional and postdepositional processes. Sedimentation rates vary from 0.01-0.20 cm/yr in Lake Superior. A dynamic model is presented which emphasizes: (1) very high (greater than 0.15 cm/yr) sedimentation rates in marginal bays, (2) moderate to very high (0.07-0.19 cm/yr) open lake sedimentation rates in regions adjacent to marginal bays, these regions are affected by plumes of suspended sediment that originate in marginal bays by wave-stirring of bottom sediments, and enter the open lake, (3) moderate to high (0.05-0.11 cm/yr) sedimentation rates adjacent to the Red Clay Area, where shoreline recession rates are high, (4) moderate to high (0.05-0.12 cm/yr) sedimentation rates in the deepest portions of the Lake Superior Troughs region, with downslope sediment movement off the adjacent shoals and into the troughs, (5) low to moderate (0.04-0.05 cm/yr) sedimentation rates from cores with current bedding features in the Keweenaw Current region, and (6) very low (0.01-0.03 cm/yr) sedimentation rates in the central lake basins due to isolation from sediment sources. Organic carbon deposition rates (K) range from 0.0001 to 0.0032 g Carbon/cm2/yr, and K varies as a power function of sedimentation rate W (g/cm2/ yr) such that K = 0.04 W1.03. This result may imply that higher sedimentation rates favor organic carbon preservation with rapid removal from the oxidizing conditions at the sediment-water interface through burial. However, the exponent is very close to 1.00, which implies that a constant proportion of organic matter is deposited with sedimentation at any site. Calculations using primary productivity measurements and average K values indicate that about 77-87% of primary production carbon is oxidized in the water column during deposition. The 210 Pb flux rate P (dpm/cm2/ yr) is directly related to the organic carbon deposition rate, such that K = 4 x 10-4P. This indicates that the main transfer mechanism for 210Pb through the water column is via association with organic particles. Organic carbon concentrations (C) decline exponentially with increasing sediment age from surficial values of 1-5% to "background" values of 0.5% in 9,000 year old sediment. The decay phenomena can be described by C = C0 e-λt, with values for the decay constant (λ) ranging from 0.2 to 1.7 x 10 -2/yr. These decay constants are 2 to 3 orders of magnitude higher than the oceans, probably due to the presence of younger and more chemically reactive organic components in Lake Superior sediments. The sedimentation rate (W) is inversely related to the decay constant (λ), such that λ = 5.33 x 10 -4 w -0.53. This may indicate that rapid burial promotes organic matter preservation. Independent evidence for biological mixing of Lake Superior sediments includes surficial zones of constant 210Pb activity, the presence of PCB substances below the sediment horizon corresponding to its first usage in commercial quantities, benthic organism studies, and lack of lamination in the upper portions of cores on x-radiographs. Oligochaete displacement rates are calculated which equal or exceed sediment accumulation rates in many cores. This indicates that oligochaetes cause significant sediment mixing at some sites. At other sites, additional mixing by burrowing amphipods may increase sediment mixing. Mixing is considered as a mechanistic analogue to diffusion phenomenon, and mixing (eddy diffusion) coefficients are calculated (Db = 0.002-10.54 cm2/yr). The highest of these compare to other studies from lakes and nearshore marine regions, while the lowest compare to rates from abyssal regions. Zones of constant 210Pb activity at depth in the sediment correspond to the time intervals 1900-1910, 1910-1920, and 1940-1950. These are interpreted as storm deposit layers, and may correspond to major storms which occurred in the Lake Superior region during November 27-28, 1905, November 22-24, 1918, and November 10-12, 1940. Diagenetic horizons are described from the sediments, these include 1-2 mm thick black laminations, 1.0-1.5 cm thick orange-colored "crusts", and layers 3-5 cm thick of many 1-2 mm diameter micronodules. The available evidence indicates that the black laminations are Mn enrichments and the orange-colored crusts may be Fe enrichments.