Characterization of Legacy Labile Organic Carbon Reserves
Rasika Gawde, Phillip A. DePetro, Kenneth J. Windsand, and Martin T. Auer
Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, MI 49931
Objectives & Approach
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The susceptibility of the particulate organic carbon (POC) pool
to diagenesis covers a broad range (labile → refractory), but for
practical purposes may be divided into two or more fractions
(cf. Berner 1980). A 2-G approach is often applied, i.e.
dC
  kG1  CG1  kG 2  CG 2
dt
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0
1970
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1980
1990
2000
Redrawn from Matthews and Effler 2006
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1
1980
1990
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and the total organic carbon content at any time after
deposition is given as,
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Yielding a sediment profile as represented by the figure at the
left.
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The profile presented above is for a constant
rate of POC deposition.
Where those
deposition rates have changed, i.e. in response
to reductions in phosphorus loading and
attendant primary production, the profile and
its interpretation become more complex.
The consumption of oxygen by the
sediment decomposition processes places
a demand on the oxygen resources of the
hypolimnion. The onset of anoxia may
trigger the flux of a variety of chemical
species (e.g. phosphorus, methylmercury,
see figure at right) from the sediment to
the water.
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Core collected in 1995
adjusted to 2008 datum
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Overlay of 1995 and 2008 cores.
Exposed yellow reflects diagenesis.
The results of a comparison of TOC profiles from 1995 and 2008 stands in marked contrast to the results of
the labile organic carbon assays. Here, reductions in carbon content occurred largely over the depth
interval 0-30 cm, with sediment at depths below 30 cm having a similar carbon content.
Evidence for Depletion of Legacy Deposits
Electron Transport System Activity
(gO2∙gDW-1∙hr-1)
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In the panel at right, TOC is plotted as a
function of depth with two domains evident: a
near-surface, modern domain (green) that
exhibits the exponential decay expected for a
constant rate of deposition and a deeper,
legacy domain (yellow) that reflects major
fluctuations in the rate of POC deposition. In
the Onondaga Lake sediment profile, the peak
in TOC observed at ~30 cm corresponds to
maximum in nutrient loading and primary
production observed in this system in the mid1970s.
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Limit of ETSA
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Objective: This research seeks to quantify the
contribution of legacy deposits of POC to present day
oxygen and nitrate demand, applying that result in
resolving the when and to what extent question.
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C ( H 2O)  O2  CO2  H 2O
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C ( H 2O)  NO3  N 2  CO2  HCO3  H 2O
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C ( H 2O)  Mn 4  Mn 2  CO2  H 2O
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2
C ( H 2O)  Fe  Fe  CO2  H 2O
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Redrawn from
Stromquist 1996
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C ( H 2O)  SO
 H 2 S  CO2  H 2O
Methane (mg∙L-1)
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Labile Organic Carbon Profile
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Depth in Sediment (cm)
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1.0
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Labile Organic Carbon Content (mgC∙gDW )
Our research group, studying Onondaga Lake in Syracuse, New York, has estimated that it takes ~20 years for
sediment nitrogen (Wickman 1996) and 19-26 years for sediment phosphorus (Lewis et al. 2007) to reach
steady state with new external loads. Here, we turn our attention to organic carbon and the role of legacy
sediment deposits in mediating the extent and duration of hypolimnetic anoxia.
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The response of a lake to management of external pollutant loads may be retarded by the presence of legacy
pollutants in the sediments. As Cooke et al. (2005) point out, the question is not whether lakes will improve
following external loading reductions, but when and to what extent.
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This suggests that diagenesis at depths >30 cm, mediated solely by methanogenesis, is less capable of
mineralizing organic carbon than that at shallower depths.
Total Organic Carbon (%DW)
Sedimented particulate organic carbon
consists of a labile fraction that
participates in biogeochemical reactions,
collectively termed diagenesis, and a
refractory fraction that is ultimately
buried. The diagenetic reactions include
oxidations mediated by oxygen, nitrate,
iron, manganese and sulfate and a
fermentation, methanogenesis.
These
reactions consume oxygen directly or
indirectly (oxidation of reduced species
end products) leading to a sediment
oxygen demand.
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Core collected in 2008
2000
Loads
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Ct  CG1,t 0  e kG1t  CG 2  e kG 2 t  Crefractory
Depth in Sediment (cm)
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External
Loads
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Depth in Sediment (cm)
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Organic Carbon Diagenesis
External
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Depth in Sediment (cm)
Redrawn from Effler et al. 2008
Depth in Sediment (mm)
The time course and extent of further recovery of
Onondaga Lake, as manifested to date in changes in
AHOD and, by association, in sediment oxygen
demand, will be governed by the reservoir of labile
organic carbon present as legacy deposits.
METRO Effluent TP (mg∙L-1)
P-removal technologies, implemented over the past
four decades have reduced the METRO effluent P
concentration from >10 mgP∙L-1 in 1970 to its
present concentration of <0.25 mgP∙L-1 (figure at
right, top panel). This reduction in P loading has
been accompanied by an almost two-fold reduction
in the areal hypolimnetic oxygen demand (AHOD,
figure at right, bottom panel).
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TOC (%DW)
Total Organic Carbon (%DW)
Total Organic Carbon (%DW)
Total Organic Carbon (%DW)
Areal Hypolimnetic Oxygen
Demand (gO2∙m-2∙d-1)
Onondaga Lake is a dimictic, eutrophic system
located in Syracuse, NY. Historically, more than 75%
of the phosphorus (P) loading to the lake has
originated from the Syracuse Metropolitan
Treatment Plant (METRO).
Decadal Scale Changes in Total Organic Carbon
The labile organic carbon profile, as
determined using aerobic bioassays,
mirrors the pattern presented for TOC
(see figure at extreme lower left). The
amount of TOC present in the labile
form ranged from 60-70% at depths >
60 cm to 80-100% above that.
C ( H 2O)  CH 4  CO2
Peak in
methanogenesis
Depth in Sediment (cm)
Study Site
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Redrawn from
Stromquist 1996
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These results would suggest that legacy
deposits account for very large fraction
of the labile organic carbon and that a
long approach to equilibrium may be
expected.
Other measures of sediment biogeochemistry stand in agreement with the 1995-2008 profile
comparison. Electron transport system activity, a measure of oxidative diagenesis (i.e. excluding
methanogenesis) reaches an asymptote at ~10 cm. Similarly, methane production reaches its peak value
at ~20 cm. Taken together, this information suggests that legacy deposits are essentially exhausted at a
depth of 30 cm. We can, thus, consider the organic carbon below a depth of 30 m to be preserved.
Therefore, the response time of sediment organic carbon in Onondaga Lake is on the order of 35 years,
longer than that determined for nitrogen (~20 years) or phosphorus (20-25 years).