indentifying redox steady states in lake sediments

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

electron affinity (more likely to be oxidized)

(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

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

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0 2 4 6 8 1 0

Concentration

Eh (V

)

Mn

U

MoAs Fe

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

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

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

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 1Normalized Concentrations

Dep

th (c

m)

Fe

Mn

U

Mo

As

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 ConcentrationsD

epth

(cm

)

Mn

U

Mo

As

Fe

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>.

• Langmuir, Donald. Aqueous environmental geochemistry. Upper Saddle River, N.J: Prentice Hall, 1997.