Depositional Characteristics of Lake Sediments in Canada as Determined by Pb-210 and Cs-137

Abstract

The geographical variation in atmospheric input fluxes of pollutants is of paramount importance for understanding the implications of atmospheric pollution. In regions with high levels of industrial and agricultural activity, the natural distribution of some pollutants could be affected by existing point sources of chemicals. Due to its geographic location and size, Canada offers opportunities for studying geographic variations in fluxes of airborne materials independently of local point sources. The sub-Arctic covers 53 percent of Canada and is very sparsely populated, hence most of Canada is not affected by point-source anthropogenic impacts. Most changes in loadings of contaminants reflect changes in regional atmospheric inputs. Some of the most convincing examples of this include synthetic organic compounds never used in the Arctic. To reconstruct histories of chemical contamination over the past 150 years, sedimentary records of 20 lakes have been examined. Fifty-four sediment cores collected from 1987 to 1993 were analyzed using Pb-210 (t 1/2 =22.26 yr) and Cs-137 (t 1/2 =30 yr) dating methods. A wide range of depositional characteristics including sedimentation rates, mixing depths, geochronologies of sedimentation, radionuclide inventories and fluxes were obtained. It was assumed that sediments from basins with no local industrial or domestic sources served as natural collecting systems recording the history of net inputs from the atmosphere. Since radionuclide inventories in the sediments are affected by physical, chemical and geological conditions, estimates of sediment focusing factors were made. Focusing factors were determined by dividing the measured excess Pb-210 flux in the core by the predicted atmospheric flux of excess Pb-210. The predicted flux values were determined experimentally from terrestrial soil profiles.

210Pb and 137Cs in margin sediments of the Arctic Ocean: Controls on boundary scavenging

“…In Figure b, the inventories of 210 Pb ex calculated from the profiles in the cores (7.6–61.9 dpm cm −2 ) are compared to the total supply of 210 Pb ex to the water above each core by atmospheric deposition and 226 Ra decay estimated from the literature (compare solid bars and hatched bars). Atmospheric deposition is probably uniform within the study area within a factor of about 3, with the variation reflecting proximity to land, regional rates of radon emanation from soil, dominance of terrestrial versus marine air masses, rates of precipitation, and extent and persistence of sea ice cover, which inhibits deposition into the water [ Hermanson , ; Omelchenko et al ., ]. At the low end, an atmospheric deposition rate of 0.06 dpm cm −2 yr −1 has been used in previous studies of 210 Pb ex distribution in the interior Arctic Ocean [ Chen et al ., ; Huh et al ., ; Lepore et al ., ; Smith et al ., ].…”

Interactions of polychlorinated biphenyls and organochlorine pesticides with sedimentary organic matter of retrogressive thaw slump‐affected lakes in the tundra uplands adjacent to the Mackenzie Delta, NT, Canada

“…137 Cs activity was used to validate 210 Pb dating profiles, due to its maximum in 1963 from atmospheric fallout caused by nuclear weapon-testing detonations [Appleby, 2008]. Focus factors (FF) were calculated by dividing the product of the deposition rate of the total unsupported 210 Pb inventories of each sediment core and the 210 Pb decay constant by the estimated atmospheric unsupported 210 Pb deposition rate at this latitude (50 Bq m À2 yr À1 ) [Omelchenko et al, 2005]. Radioisotopic activity for these study lake cores has previously been published in Deison et al [2012] and Thienpont et al [2013].…”

Climate Change and Mercury Accumulation in Canadian High and Subarctic Lakes

“…Sediment particle focusing factors (FF) were therefore estimated for all cores dated by dividing the observed 210 Pb flux (Bq m -2 y -1 ) by the predicted 210 Pb flux for the same latitude based on soil 210 Pb measurements from 41.5 to 81.5 degrees latitude as in ref (3). This method was previously used in a study of latitudinal gradients of PCBs (7) and is discussed in detail in an unpublished study (8). ∆F F,adj = F Frecent -F Fpre-ind -(F Fpre-ind x sedimentation ratio-F Fpre-ind ) ( 5)…”