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Palaeoclimate Change SOES 3015
Lecture 8:
Tools & Insights-5: Sr isotopes, global weathering &
subduction vs. mountain building (PAW)
Lecture outline:
• CO2: Subduction vs. Mountain-building
-
BLAG & ocean crust cycling
Himalayan uplift
-
Isotope systematics
Consequences & uses
• Sr isotopes
• Global weathering debate
-
Critique of uplift hypothesis
www.oceanography.ac.uk
(1) CO2: Subduction vs. Mountain-building
• BLAG & ocean crust cycling
• Mountain building as alternative CO2 drawdown
(i) BLAG & ocean crust cycling
Key lesson from last Lec:
At long (multi-million yr) timescales atms pCO2 is controlled by the balance
between:
• CO2 production
@spreading centres
(outgassing) & subduction
boundaries (metamorphic
decarbonation)
• CO2 consumption by
continental weathering of
silicate rocks
Paul Wilson, University of Southampton
Question: Does the system ever
get out of balance?
Answer: Yes - to an extent- look
@Cretaceous spreading rates;
high eustatic sea level-maximum;
no ice
Courtesy of Geological Society of America: Larson, R.L., (1991), Geological
consequences of superplumes. Geology October, v. 19, p. 963-966.
But: ~ 3.5 Billion year record of Life on Earth
something must act as a -ve feedback to keep climate within reasonable
limits (compare Mars & Venus)
Recall, BLAG’s -ve feedback:
• inc. pCO2 inc. T inc. weathering check ‘run away greenhouse’
University of Southampton
For figure please follow
link to full article below:
Problems:
• lack of evidence for -ve
feedback (cold regions
intense weathering)
From: Huh, Y., Edmond, J.M., (1999), The fluvial
geochemistry of the rivers of Eastern Siberia: III.
Tributaries of the Lena and Anabar draining the
basement terrain of the Siberian Craton and the
Trans-Baikal Highlands, Geochimica et
Cosmochimica Acta, v. 63, p. 967-987. (Figure
10a)
OBSERVED COOLING LAGS BLAG PREDICTION
• pCO2 & Temp
• CCD (predicted vs.
observed)
GREEN
HOUSE
• mismatch in Cenozoic
shifts
ICE HOUSE
Problems:
From: Zachos, J.C., Pagani, M., Sloan, L., Thomas, E.,Billups, K.,(2001) ‘Trends, rhythms,
and aberrations in global climate 65 Ma to present’, Science, v. 292, p. 686–693.
Reprinted with permission from AAAS. This figure may be used for non-commercial,
classroom purposes only. Any other uses requires the prior written permission from AAAS.
Largest  in pCO2, global temp For Composite of CCD curves for Indian, Pacific,
& CCD:
and Atlantic Oceans, Please see figure 4 in
• BLAG- 100 to 50 Ma
(pre-Eoc)
• Palaeoceanog evidence50 to 0 Ma (post-Eoc)
Andel, V., (1975) Mesozoic and Cenozoic calcite
compensation depth and the global distribution
of calcarious sediments. Earth and Planetary
Science letters, v. 26 p187-194.
(ii) Mountain building as alternative CO2 draw-down
Mountain uplift: Physical effects:
• Phanerozoic-- glacials
& orogenies associated
• Cenozoic-- Himalayan
Tibetan Plateau Uplift
Courtesy of Geological Society of America: Raymo, M.E., (1991),Geochemical evidence supporting T. C. Chamberlin's theory of glaciation,
Geology, v. 19, p. 344-347.
Mountain uplift: Physical
effects:
• Topographic AGCM
experiment:
• middle lat. Jet stream
blocked
• onset Asian monsoon
circulation
Courtesy of Chicago Publishers: J. E. Kutzbach, J.E., Prell, W.L., Ruddiman, W.F., (1993), Sensitivity of
Eurasian Climate to Surface Uplift of the Tibetan Plateau,The Journal of Geology, v. 101, p. 177-190.
• Results:
Qualitative fit w/ palaeotemp. record but too warm (esp high lat summer)
HTP signif. but not whole story
• Mountain uplift: Chemical effects:
(MIT - Raymo & Edmond)
• Uplift
 intense rains; fresh rocks; steep slopes
 inc. weathering rates
 CO2 draw-down & global cooling
• Evidence:
• HTP ~25% global Total Dissolved Load
• ‘transport-limited’
• Debate:
• Yale school (BLAG): slow-down ocean crust cycling
predicts:  Temp  Weathering  CCD
• MIT school: speed-up continental crust weathering
predicts:  Temp  Weathering  CCD
Question: How can we test?
Answer: We need a proxy for global weathering rates–
…sea water 87Sr/86Sr?
(2) Sr isotopes
• Isotope systematics
• Consequences & uses
(i) Isotope systematics
88Sr
38
82.53
87Sr
38
86Sr
38
7.04
84Sr
38
9.87
87Rb
37
0.56
 87Sr38 + - + v + Q
T1/2 Rb ~ 48.8 Ga
 Sr ~ 5 Ma
• Ocean mixing: ~ 1.5 ka
• No fractionation pptn. CO3’s
Sr (Å = 1.13) Ca (Å = 0.99) Ca0.99Sr0.1CO3

Rb (Å = 1.48) Ca (Å = 0.99) marine CO3’s

Rb incompatible
(all stable)
%
87Sr/86Sr
high
low
old (T1/2 Rb)
acidic (Rb K)
continental
marine CO3’s
young
ultramafic
oceanic
(ii) Consequences & uses
• Consequences
• 87Sr/86Sr uniform oceans world wide (S ≥ 18‰)
• marine CO3’s equilibrium
• 87Sr/86Sr does not degrade
• sea water 87Sr/86Sr controlled by mixing 3 sources:
• young volcanics (JH)
• old sialics (JR)
• marine carbonates (JB) (DSr; diagenetic suscept.)
• Uses
Integrated proxy weathering
(oceanic vs. continental crust)
Sr isotope tests of uplift-driven CO2 draw-down
• Phanerozoic correlation w/ orogenies & icehouses
but:
• JR vs.  JH?
• ’s in JR87Sr/86Sr ≠ weathering rate
… Gangese-Brahamaputra.
• JRCO3 ≠ pCO2 drawdown.
Courtesy of Geological Society of America: Raymo, M.E., (1991),Geochemical evidence supporting T. C. Chamberlin's theory of glaciation,
Geology, v. 19, p. 344-347.
Figure is available by accessing the full article here: Richter, F.M., Rowley, D.B.,
DePaolo D.J., (1992), Sr isotope evolution of seawater: the role of tectonics,
Earth and Planetary Science Letters, v. 109, p. 11-23. (Figure 3a)
From: Edmond, J.M., (1992), Himalayan Tectonics, Weathering Processes, and the Strontium Isotope Record in Marine Limestones, Science, v.
258, p. 1594-1597. Reprinted with permission from AAAS. This figure may be used for non-commercial, classroom purposes only. Any other uses
requires the prior written permission from AAAS.
Figure is available by accessing the full article here: Richter, F.M., Rowley, D.B.,
DePaolo D.J., (1992), Sr isotope evolution of seawater: the role of tectonics,
Earth and Planetary Science Letters, v. 109, p. 11-23. (Figure 3b)
(3) Global weathering debate
• Critique of the uplift hypothesis
(i) Critique of the uplift hypothesis
Chicken or Egg?
• Evidence for uplift:
• Geomorphology (high, incised terraine)
• Sedimentation (Bay of Bengal)
• Palaeobotany-- taxonomy vs. physiognomy
is actually evidence of climate 
& climate D  erosion  uplift (isotasy)
Need -ve feedback:
• BLAG: dec. PCO2  dec. T  dec. weathering (check ‘run away icehouse’)
• MIT: alternatives- imbalance in C cycle
• SL erode shelves
• nutrient supply
• Himalayan uplift Tethyan Corg
• Metamorphic decarbonation in orogenic belts
Question: are these alternative -ve feedback mechs convincing?
Answer: BLAG: No - re-state CO2-dependence silicate weathering
-ve feedback must work globally & long time-scales
 we need:
• other weathering proxies (Ge/Si; Nd; Os)
• better constraints on ’s in JR vs. JR87Sr/86Sr
Cenozoic sea water Sr/Ca
• ~1.5 x modern in late K
• min = ~40 Ma
• steady inc to Present
Lear, C., Pers. Comm. (2010), Data publicised in: Lear, C.H., Elderfield, H., Wilson, P.A., (2003), A Cenozoic
seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering
fluxes, Earth and Planetary Science Letters, v. 208, p. 69-84.
• combine w/ sea water
87Sr/86Sr
• calculate ’s in JR & JR87Sr/86Sr
JR87Sr/86Sr inc by ~ 0.001
• 35 to 0 Ma assoc w/ 2
fold inc JR
• 75 to 35 Ma assoc w/ 2-3
fold dec JR
Lear, C., Pers. Comm. (2010), Data publicised in: Lear, C.H., Elderfield, H., Wilson, P.A., (2003), A Cenozoic
seawater Sr/Ca record from benthic foraminiferal calcite and its application in determining global weathering
fluxes, Earth and Planetary Science Letters, v. 208, p. 69-84.
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