stable water isotopes - mit opencourseware · carbon isotopes inorganic carbon organic carbon 30m...
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Stable Water Isotopes
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The fundamental control on the isotopic composition of precipitation is temperature. With increasing temperature, precipitation becomes enriched in the heavier isotopes,18O and 2H, in a linear relationship. Temperature affects fractionation at a rate of approximately 0.5‰ for every C° for oxygen. Similar effects are shown with increasing elevation and increased distance from the equator (both of which correspond to lower temperature). Because precipitation becomes progressively enriched in light oxygen as it moves toward the cold polar regions, polar ice constitutes a reservoir of 16O enriched water as compared to sea water.
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4.7E-05
3.6E-042.5E-04
8.2E-05
2.2E-04
1.8E-04
1.2E-04
2.4E-04
3.9E-04
7.2E-05
3.9E-05
0
20
80
60
40
100
160
140
120
Clay
0 2E-04 4E-04
?
0 2-4-6-10 -8
-2
May-01
G. Relative Head (mm difference from 19m value)F. Hydraulic Conductivity (m/s)
-100-200 3002001000
M ay- 0 1
D ec- 0 1
Jan - 0 2
Feb - 0 2
M ar- 0 2
A p r- 0 2
-4.0-3.0-2.0-1.00.0
Depth (m)
H. δ18O (per thousand)
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Tritium and Helium
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3H⎡⎣ ⎤⎦ = 3H0⎡⎣ ⎤⎦e−λt
3H0⎡⎣ ⎤⎦ = 3Hetrit⎡⎣ ⎤⎦ + 3H⎡⎣ ⎤⎦
3 3
3
3/
[ ]1 ln 1[ ]
tritH He
HetHλ
⎛ ⎞= +⎜ ⎟
⎝ ⎠
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0
20
40
60
80
1000 100 200 300 400 500 600
0 1 2 3 4 5
As
Tr
0
20
40
60
80
1000 100 200 300 400 500 600
0 1 2 3 4
As
As As
Tr
depth
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Carbon Isotopes
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Dissolved Carbon
Dissolved Organic Carbon (DOC) (mM)
0 2 4 6 8 10
20
40
60
80
100
120
401
601
180
0 0.4. 0.8. 1.2. 1.6. 2
Methane
DOC
DICDepth
(m )
Inorganic (DIC) and Methane (mM)
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Carbon Isotopes
InorganicCarbon
OrganicCarbon
30m19m3mMethanogenesis
CaCO3
(Atmospheric valuesover last 50 years)
60m
14C0/00
-1000
-500
0
500
1000
-30 -25 -20 -15 -10 -5 0-70
Methane
Surface Water Carbon• Inflow of young
carbon
• Young carbon drives biochemistry
• Mixture of young and old carbon is not the result of pore water mixing, but mobilization of old organic carbon
0700
40002000
Radiocarbon A
ge
13C0/00
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At Waquoit Bay
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Waquoit Bay, MA
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Saline Circulation Mechanisms
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q = −K dhdz
= −kρgµ
dhdz
q = −kµ
dPdz
− ρg⎛⎝⎜
⎞⎠⎟
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Initial Salinity ProfileElapsed Time 2:20Elapsed Time 21:00Elapsed Time 33:00Elapsed Time 47:46Elapsed Time 67:30Elapsed Time 76:00Elapsed Time 80:00