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TRANSCRIPT
Diel redox changes of Rio Agrio, Argen4na
Jackie Wrage
NDSU Geochemistry December 11, 2014
Outline
• Background • Introduc4on • Study • Results • Conclusion • Further Research
Where are we?
• Copahue Volcano • Stratovolcano • Ac4ve volcano:
– Been erup4ng since 2012 – Latest: 4th of July, 2014
• Name means “sulphur waters” in indigenous Mapuche
Study area Rio Agrio river: • Naturally acidic, starts at acid
hot springs at the top of Copahue Volcano
• Owes acidity to influx of vola4les (SO4, HCl, & HF)
• Enters Glacial Lake Caviahue • Con4nues down to waterfall:
Salto del Agrio • Lower Rio Agrio
• Increasing pH downstream
Diel redox changes of the Rio Agrio
• Diel: Chemistry changes that occur within 24 hours (day to night differences)
• Redox: Chemical reac4ons in which atoms have their oxida4on states changed
• I used data from Parker et al. (2008) at the sampling point of Salto del Agrio, a waterfall with pH of 3.2
Data
Diel varia4ons of total dissolved Fe2+ and Fe3+
(Parker et al. 2008)
Element analysis from Parker et al. (2008). Concentra4on in mmol.
The study
• Many studies done here, found that concentra2on and specia2on of Fe changed from day to night
• Why?
• Does this affect redox state of river?
Why?
• Iron has 2 valence states: 3+ and 2+ – Fe3+ is oxidized state (ferric) – Fe2+ is reduced state (ferrous)
• Studies have shown that in hyperacidic rivers (like Rio Agrio), photoreduc1on during the day causes the increase in Fe2+ and decrease in Fe3+
• Photoreduc4on: A reduc4on reac4on that takes place in the presence of light
This reac2on occurs in the UV to near UV region of Electromagne2c Spectrum (200-‐450 nm) & is max with pH of 2-‐4
Summary of diel Fe specia4on
• Fe3+ increases at night – open to atmosphere, oxidizing environment
• Photoreduc4on during the day decreases Fe3+ and increases Fe2+
Does this effect the redox state? Using PHREEQ-‐C:
• I simulated photoreduc4on by changing the ra4o of Fe species in the Salto del Agrio input file
• Used average element concentra4ons through 24 hours
• Ran 9 solu4ons to determine Eh (redox state)
Results
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7 8 9 10
Eh
Solu1on
Eh changes from day to night • Eh varied from 0.727 to 0.767
• Not a great difference
• Increased slightly at night
• Trend matched concentra4on of Fe3+ because it’s the more oxidized species
Graph of diel Eh changes (Blue box represents nighkme values)
Conclusion
• Although the Fe specia4on and concentra4on varied throughout the day, it did not effect the redox state of the en4re river
Further Research
• Rio Agrio is a modern analog to the Mar4an environment and possibly early Earth
• Understanding interac4ons between organic & inorganic processes is important to astrobiology
• Fe-‐oxidizing bacteria are present in the Rio Agrio, studying their effect on the river chemistry can have important implica4ons
References
Parker, S., Gammons, C., Pedrozo, F., and Wood, S. (2008) Diel changes in metal concentra4ons in a geogenically acidic river: Rio Agrio, Argen4na. Jornal of Volcanology and Geothermal Research 178. 213-‐223.
Gammons, C., Nimick, D., Parker, S., Snyder, D., McCleskey, B., Amils, R., and Poulson, S. (2008) Photoreduc4on fuels biogeochemical cycling of iron in Spain’s acid rivers. Chemical Geology 252. 202-‐213.
Ques4ons?