25th International Meeting on Organic Geochemistry IMOG 2011

P-412
Radiocarbon distributions in Lake Superior, the world’s largest
freshwater lake (by area)
Prosper Zigah 2 , Elizabeth Minor 1 , Josef Werne 1
1 Large Lakes Observatory and Dept of Chem and Biochem, University of Minnesota Duluth, Duluth, MN,
United States of America, 2 Water Resources Science and Large Lakes Observatory, University of Minnesota
Duluth, Duluth, MN, United States of America (corresponding author:eminor@d.umn.edu)
Lake Superior, as a deep (average depth 150 m;
maximum depth 406 m), oligotrophic large lake
dominated by planktonic processes, provides a
unique opportunity to understand biogeochemical
processes occurring at meso-to-synoptic scales, and
at basin-wide integrated level, something that is
logistically difficult to do in the world ocean. Lake
Superior‘s biogeochemistry appears similar to openocean
marine systems, with low water-column
nutrient levels, an ecosystem weighted toward
microbial processes, and low inputs of terrestrial
material due to a low watershed to surface area ratio.
The lake‘s mixing regime, however, is radically
different from that of the open ocean, with complete
water column overturn occurring in both spring and
fall and stable summer stratification. Thus Lake
Superior becomes a very interesting system for
studying interactions between the various pools within
the aquatic carbon cycle and seeing how the
biogeochemistry reacts when placed within very
different timescales of physical mixing processes.
Here, we report the ∆ 14 C, δ 13 C values and the
concentrations of dissolved inorganic carbon (DIC),
dissolved organic carbon (DOC), and particulate
organic carbon (POC) at multiple offshore sites during
mixed and stratified conditions in Lake Superior. DIC
in the lake was marked by homogeneity in
concentration and in ∆ 14 C and δ 13 C values across the
lake. DOC concentrations exhibited spatial
patchiness, ranging from 88-122 μM in the surface
water during stratification in August. While ∆ 14 C of
DOC did not exhibit considerable variation, the values
in the surface waters at the southern and northern
provinces of the lake were consistently, and uniquely,
low. ∆ 14 C of POC was consistently and significantly
depleted relative to DIC and DOC and was spatially
very heterogeneous (range, modern to 2840 BP yrs).
Its variable age appears to be the result of sediment
resuspension, which reaches even into surface
waters at certain sites and appears more prevalent in
the eastern lake and during the mixed water-column
period. Thus in this lake burial in the sediments does
not necessarily lead to sequestration; as lake
sediments are often resuspended back into regions of
higher reactivity, e.g., the water column and surface
sediment zones.
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