Arctic Methane Our most immediate climate change problem is Arctic methane.
Methane
description
Transcript of Methane
Methane
• CH4
• Greenhouse gas (~20x more powerful than CO2)
• Formed biologically (methanogenesis)
• Huge reservoir as methane clathrate hydrate in cold soils and ocean bottom – stable structure at low T, high P
• 2x1016 kg of C in these deposits
• What happens if the oceans warm??
• ‘Clathrate gun’ hyothesis – warming seas ‘melt’ these clathrates, CH4 released en masse to atmosphere…
Microbes and methane production
• Methanogenesis – Reduction of CO2 or other organics to form CH4 (also CH4 generation from special fermentative rxns)– Only certain groups of Archaea do this,
specifically with the Euryachaeota subdivision– Called methanogens
• These organisms do not compete well with other anaerobes for e- donors, thus they thrive where other alternate e- acceptors have been consumed
Methane cycle
Microbial methane oxidation• Organisms that can oxidize CH4 are
Methanotrophs – mostly bacteria• All aerobic methanotrophs use the enzyme
methane monooxygenase (MMO) to turn CH4 into methanol (CH3OH) which is subsequently oxidized into formaldehyde (HCHO) on the way to CO2
• Anaerobic methane oxidation – use SO42- as
the e- acceptor – this was long recognized chemically, but only very recently have these microbes been more positively identified (though not cultured)
Phosphorus cycle
• P exists in several redox states (-3, 0, +3, +5) but only +5, PO4
3-, stable in water• 1 microbe to date has been shown to grow
on PO33- (phosphite, P3+) as a substrate
• P is a critical nutrient for growth, often a limiting nutrient in rivers and lakes
• Most P present as the mineral apatite (Ca5(PO4)3(F,Cl,OH)); also vivianite (Fe3(PO4)2*8H2O)
P sorption
• P strongly sorbs to FeOOH and AlOOH mineral surfaces as well as some clays
• P mobility thus inherently linked to Fe cycling
• P sorption to AlOOH is taken advantage of as a treatment of eutrophic lakes with excess P (alum is a form AlOOH) – AlOOH is not affected by microbial reduction as FeOOH can be.
P cycling linked to SRB-IRB-MRB
activity
Blue Green Algae blooms
FeOOH
PO43- PO4
3-
PO43-PO4
3-
Org C + SO42-
H2SFeS2
PO43- PO4
3-
PO43-
PO43-
Sulfate Reducers
Redox ‘Fronts’
• Boundary between oxygen-rich (oxic) and more reduced (anoxic) waters
• Oxygen consumed by microbes which eat organic material
• When Oxygen is gone, there are species of microbes that can ‘breathe’ oxidized forms of iron, manganese, and sulfur
AnoxicOxic
St. Albans Bay Sediments
0.341
-0.058
0.000
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-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400
0.106
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-0.010
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-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400
Mn2+ + 2e- --> Mn0(Hg)
Fe3+ + 1e- Fe2+
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-0.004
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-0.100-1.800 -1.600 -1.400 -1.200 -1.000 -0.800 -0.600 -0.400
O2 + 2e- + 2H+ H2O2
H2O2 + 2e- + 2H+ 2H2O
FeS(aq)
Results: Seasonal Work
• Sediments generally become more reduced as summer progresses
• Redox fronts move up and down in response to Temperature, wind, biological activity changes
6-23-04 Core 2 Profile 2
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Current (nA)
Dep
th (
mm
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O2 (nA)
7-19-04 Core 1 Profile 1
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Current (nA)
Dep
th (
mm
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O2 (nA)
Fe3+ (nA)
FeS (nA)
7-26-04 Core 2 Profile 1
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Dep
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O2 (nA)
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8-12-04 Core 1 Profile 1
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Current (nA)
Dep
th (
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O2 (nA)
Mn (nA)
FeS (nA)
Fe3+ (nA)
Seasonal Phosphorus mobility
• Ascorbic acid extractions of Fe, Mn, and P from 10 sediment cores collected in summer 2004 show strong dependence between P and Mn or Fe
• Further, profiles show overall enrichment of all 3 parameters in upper sections of sediment
• Fe and Mn would be primarily in the form of Fe and Mn oxyhydroxide minerals transformation of these minerals is key to P movement
Profiles seasonal sample averages
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Conc. (mg/g sediment)
Dep
th (
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of
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, cm
)
AA P
AA Fe / 10
AA Mn
NaOH P
Ascorbic AcidDepth P-Fe P-Mn0-1 cm 0.863 0.8941-2 cm 0.933 0.9212-3 cm 0.829 0.5673-4 cm 0.604 0.5594-5 cm 0.732 0.7775-6 cm 0.889 0.8956-8 cm 0.866 0.8048-10 cm 0.894 0.876
P Loading and sediment deposition
• Constantly moving redox fronts affect Fe and Mn minerals, mobilize P and turn ideal profile into what we actually see…
Profiles seasonal sample averages
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Conc. (mg/g sediment)D
epth
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cm)
AA P
AA Fe / 10
AA Mn
NaOH P