The document discusses a study that aimed to identify historical periods of redox equilibrium in lake sediments by analyzing vertical profiles of redox-sensitive metals. Sediment cores from Elk Lake were analyzed for concentrations of manganese, uranium, molybdenum, arsenic, and iron. The results showed the metals followed the expected redox pattern near the surface but lacked distinct patterns at depth, possibly due to chemical changes over time erasing the initial pattern. Improving the study would require thinner sediment sections and pore water analysis to better preserve and identify historical redox conditions.

1. Mary Hingst
David Long, Ryan Vannier, Matt Parsons
Identifying Historical States ofIdentifying Historical States of
Balance (SteadyBalance (Steady
State/Equilibrium) in LakesState/Equilibrium) in Lakes
Using Sediment ChronologiesUsing Sediment Chronologies
of Redox-Sensitive Metalsof Redox-Sensitive Metals

2. Purpose
• If biogeochemical cycles in a lake attain a
balance with the flow of chemicals from
watersheds, will redox conditions in the
lake sediments enter a steady-state and
will this pattern be reflected in vertical
chemical profiles?
• Or simply put, can historical periods of
redox equilibrium be identified lake
sediments?

3. Why Lake Sediments?
• Lake sediments have
proven excellent records
for past environmental
changes (e.g. climate
change, logging,
pollution…)
• Preservation of elemental
profiles in lake sediments
may yield insight into
historical balances.
• Similar patterns have
been observed in Lake
Superior
ElkElk

4. (Eby 2004)
What is Redox?
• Reduction-oxidation
• Oxidation is the loss of electrons
(becomes more positive)
• Reduction is the gain of electrons
(becomes more negative)
• Redox reaction signifies a transfer of
electrons

5. (Eby 2004, Langmuir 1997)
What is Eh?
• “…the electromotive force of any reaction
measured relative to the standard hydrogen
electrode.” (Eby 2004)
• Reduction or redox potential
• Measures the tendency for an element to
acquire electrons (volts)
• A greater positive value means a greater
electron affinity (more likely to be reduced)
• A greater negative value corresponds to a lower
electron affinity (more likely to be oxidized)

6. (Brookins 1988)
Redox Conditions/Reactions
• Manganese
– Mn2+
↔ MnO2 (Mn(II) ↔ Mn(IV)) Eh: 0.54
• Uranium
– UO2OH+
↔ UO2(OH)3 (U(IV) ↔ U(VI)) Eh: 0.2
• Molybdenum
– MoO2 ↔ MoO4
2-
(Mo(IV) ↔ Mo(VI)) Eh: -0.1
• Iron
– Fe2+
↔ Fe3O4 (Fe(II) ↔ Fe(III)) Eh: -0.2
• Arsenic
– H3AsO3 ↔ H2AsO4
-
(As(III) ↔ As(V)) Eh: -0.18

7. Theoretical Sediment Concentration Profiles at Redox
Equilibrium
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 1 0
Concentration
Eh(V)
Mn
U
Mo
As Fe

8. Hypothesis
• Once the system has reached a balance
with chemicals off the watershed, redox
conditions will also enter a steady-state
and vertical sediment concentration
patterns would reflect this

9. Approach
• Sediment cores
collected from the
deepest part of Elk
Lake
• Concentrations were
determined and then
normalized to the
highest concentration
in the core in order to
plot on one graph
• Peaks in
concentration were
compared to
thermodynamic
patterns

10. Methods
• Cores were
collected aboard the
U.S. EPA R/V
Mudpuppy in 1999
• Immediately
sectioned onshore
into 0.5cm intervals
for the top 5cm then
1.0cm intervals
• Sediments were
freeze-dried
• Nitric acid
digestions were
used to dissolve
sediments
• Samples were
analyzed via ICPMS

11. Elk Lake
0
5
10
15
20
25
30
35
40
45
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized Concentrations
Depth(cm)
Fe
Mn
U
Mo
As

12. Elk Lake
0
1
2
3
4
5
6
7
8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Normalized Concentrations
Depth(cm)
Mn
U
Mo
As
Fe

13. Results
• Mn peaks at first sample (0.5cm)
• Mo, Fe, and As peak at second sample
(1.0cm)
• U does not peak until 8cm
• The order from the Eh diagram
Mn, U, Mo, As, Fe

14. A Closer Look at Molybdenum
• Mo peaks at the second sample (0.36
mg/kg). The first sample’s concentration
was 0.35, while the third was 0.11
• Having such similar concentrations in the
top two samples suggests the ‘true’ peak
layer lies between 0.5-1.0cm

15. A Closer Look at Arsenic
• The top 3 samples for As had the values
of 12.01(0.5cm), 36.81(1.0cm), and
23.01(1.5cm)
• The peak concentration is only slightly
closer to the lower sample meaning the
‘true’ peak layer lies just below the 1.0cm
mark

16. A Closer Look at Fe
• Fe had values of 10,973.39, 19,587.21,
and 16,091.84 for the first 1.5cm of
sediment
• The peak concentration is much closer in
value to the sample below than to the
sample above
• The ‘true’ peak layer lies somewhere
between 1.0cm and 1.5cm

17. So…
• Using the estimated ‘true’ peaks, the
order from top to bottom is…
Mn > Mo > As > Fe > U
• The Eh diagram had the order of…
Mn > U > Mo > As > Fe

18. The problem with Uranium
• When Mn and Fe are at their peaks, U is
at a minimum
• The peaks of Mn and Fe signify an
oxidized zone
• Uranium (VI) easily forms carbonates
which are very soluble

19. Conclusions
• From the graphs and numerical values, the
elements follow the redox pattern at the top of
cores
• The patterns are temporal and not preserved at
depth; hypothesis is not supported
• The lack of patterns at depth is possibly do to
chemical changes in the sediment that worked to
erase the pattern (conditions changed over time)

20. How to improve this study?
• For a more definite pattern, thinner
sections would need to be collected
• Pore water needs to be collected and
analyzed to prove U dissolves out
• A more oligotrophic lake may preserve
pattern in deeper sediment

21. References
• Brookins, Douglas G. Eh-PH diagrams for geochemistry.
Berlin: Springer-Verlag, 1988.
• Eby, G. Nelson. Principles of environmental
geochemistry. Pacific Grove, Calif: Thomson-
Brooks/Cole, 2004.
• Japan. Geological Survey. Atlas of Eh pH Diagrams -
Intercomparison of Thermodynamic Databases. By
Naoto Takeno. May 2005. Nat. Institute of Advanced
Industrial Science and Technology. 10 Apr. 2009
<http://www.gsj.jp/GDB/openfile/files/no0419/openfile419
e.pdf>.
• Langmuir, Donald. Aqueous environmental
geochemistry. Upper Saddle River, N.J: Prentice Hall,
1997.