R. Jack Cornett

Evaluation of210Pb dating in lake sediments using stable Pb,Ambrosia pollen, and137Cs

We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of210Pb derived dates was determined using common sediment event markers: the stable Pb rise, the137Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on210Pb derived dates. When comparing the concordance of two210Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between137Cs and210Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to ‘immobilize’ Cs.

Climatic changes in northwestern Ontario have had a greater effect on erosion and sediment accumulation than logging and fire: Evidence from 210Pb chronology in lake sediments

Sedimentation patterns in nine lake basins were examined where catchments were either clearcut, burned in recent history, or where there has been no recorded disturbance and the catchments consist of mature forests. Pronounced declines in sedimentation rates were observed in eight of eleven cores after 1980, in reference, clearcut, and burn lakes. The degree of change was positively correlated with the drainage ratio (catchment area: lake area), but was unrelated to land use history. The decline in sedimentation rates after 1980 coincide with a 60% decrease in catchment runoff and precipitation measured over the same time interval at the nearby Experimental Lakes Area. These results indicate that climatic changes over the past 20 years have had a greater effect on catchment erosion than either clearcutting or fire.

Carbon isotopic disequilibrium in the shell of the freshwater mussel Elliptio complanata

Isotopic analyses of shell material from the freshwater mussel Elliptio complanata produced mean C concentrations that were depleted in 14 C and 13 C relative to the lake water DIC. Depleted C isotope values were attributed to the incorporation of 12 C-enriched metabolic C into the shell. Microsampling of longitudinal sections of shell revealed a seasonal pattern in the δ 13 C values. The seasonal pattern matched predictions that DIC δ 13 C and metabolic rate determine the final shell δ 13 C concentration. The greatest depletion of δ 13 C occurred in the summer. Similar δ 13 C patterns in marine and freshwater mollusks probably record not only temperature fluctuations, upwelling events, and phytoplankton blooms as reported, but also the changes in metabolic activity brought on by such events.

Nickel diagenesis and partitioning in lake sediments

Abstract Nickel discharged into the Moira River has accumulated in the sediments of Moira Lake during the past 125 years. Depth profiles of total Ni in the sediments reflect variations in Ni input into the lake. In the porewaters the Ni concentration decreased below the mud-water interface and was not correlated with the concentration of Ni on the solid phase. Subsamples of each core section were leached sequentially with progressively stronger agents to identify the geochemical fractions with which the Ni was associated. The partitioning of anthropogenic Ni between fractions changed with depth. The fraction of Ni in the exchangeable (1–8%) and in the carbonate-ound (1–45%) fractions decreased with depth, and the percentage in the organic and sulphide fractions (15–60%) increased deeper in the sediments. Very little ( 80% of the total Ni is found in the residual extract. A simple diagenetic model using the changes in redox conditions and NiS solubility can explain the anthropogenic Ni distribution in the sediments.

Trace metal, radionuclide and sediment accumulation in littoral zones

There is a growing awareness of the importanee of roored maerophyres in the biogeoehemieal cycling of nutrients, rraee metals and organie eontaminants in lake eeosystems. While it has been long understood that fine-grained sediment particles and rheir assoeiated contaminants within the littoral zone ean be transporred to, and eventually aeeumulate in, deeper parts of the lake, two paradigms have evolved to deseribe rhe role o f maerophytes in sediment, nurrient and eontaminant distribution within aquatie systems.