Sediment geochemistry reveals the Anthropocene history of metals in the
Great Lakes
Abstract
Geochemical analyses applied to lake sedimentary records can reveal the
history of pollution by metals and the effects of remedial efforts.
Lakes provide ideal environments for geochemical studies because they
have steady deposition of fine grained material suitable for fixation of
pollutants. The Laurentian Great Lakes are the most studied system in
this field and they have well-preserved chronological profiles. To date
this important system has been considered in parts, hampering basin-wide
conclusions regarding metal contamination. Therefore, we synthesized all
relevant literature to create a central understanding of geochemical
research to date in the Great Lakes. We also filled spatial and temporal
gaps in a comprehensive geochemical analysis of 11 sediment cores
collected from all five Great Lakes. Hierarchical cluster analysis of
all Great Lakes samples divided the metal analytes into five functional
groups: (1) carbonate elements; (2) metals and oxides with diverse
natural sources, including a subgroup of analytes known to be
anthropogenically enriched (Cd, Pb, Sn, Zn, and Sb); (3) common crustal
elements; (4) metals related to coal and nuclear power generation; and
(5) all of the rare earth elements. Two contamination indices (Igeo and
EF) applied to sedimentary metals indicated that Na, Co, Mn, Cd, Pb, Ta,
and Cu were the most enriched metal pollutants in this system, at some
point during the Anthropocene. Land uses correlated with the metal
analytes, such as increases in contaminant metals with the rise in
catchment population and increases in carbonate elements (e.g. Ca) with
agriculture. Certain contamination trends were observed basin-wide, such
as for the atmospheric pollutant Pb, which followed a rise associated
with fossil fuel combustion and a decline following the ban of leaded
gasoline. Other trends were lake-specific such as recent high
concentrations of Na in Lake Superior due to road salt applications and
a late-20th century peak in Ca associated with algal whiting events in
Lake Ontario. The Great Lakes are sensitive to environmental changes
such as pollution by metals and it clear that, while there has been
remedial success, results from the uppermost intervals of cores indicate
ongoing problems.