Mary Hingst David Long, Ryan Vannier, Matt Parsons Identifying Historical Identifying Historical States of Balance States of Balance (Steady (Steady State/Equilibrium) in State/Equilibrium) in Lakes Using Sediment Lakes Using Sediment Chronologies of Redox- Chronologies of Redox- Sensitive Metals Sensitive Metals
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Indentifying Redox Steady States In Lake Sediments
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Mary HingstDavid Long, Ryan Vannier, Matt Parsons
Identifying Historical States of Identifying Historical States of Balance (Steady Balance (Steady
State/Equilibrium) in Lakes State/Equilibrium) in Lakes Using Sediment Chronologies Using Sediment Chronologies
of Redox-Sensitive Metalsof Redox-Sensitive Metals
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?
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
(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
(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
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
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
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
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
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
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
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)
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
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/openfile419e.pdf>.