The 231Pa/230Th Paleoproxy:

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The 231Pa/230Th Paleoproxy How should we
interpret the growing observational dataset?
Gideon Henderson Alex Thomas Mark Siddall James
Rae Ben Hickey University of Oxford
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A proxy for past rates of ocean circulation
Lynch Stieglitz et al. 2007
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Oceanic behaviour of U-series nuclides
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Oceanic behaviour of U-series nuclides
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Fluxes of 231Pa and 230Th
Scavenging of nuclide Downward transport Continued
equilibration
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Fluxes of 231Pa and 230Th
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Fluxes of 231Pa and 230Th
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Residence time of waters in North Atlantic
Rhien et al. 2001
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Some real 230Th data
South Atlantic
Labrador Sea
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Fluxes of 231Pa and 230Th
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Fluxes of 231Pa and 230Th
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231Pa/230Th in the oceans
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Glacial NADW formation rates
Suggests similar rates of NADW flow in glacial
and Holocene
Yu et al. 1996
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Sensitivity of the proxy
Marchal 2000
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Millennial changes in NADW flow
McManus et al. 2004
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• So thats how its supposed to work.
• but
• 1. Which depth water-mass is it telling us about?
• 2. Doesnt it have a big particle flux effect as
  well?
• 3. How about preferential scavenging by opal?
• 4. Do other things reset Pa/Th except for
  ventilation?
• 5. Does it look as though it works in other
  settings?
• 6. Do we really understand it?

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1. Which depth of water mass?

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Southern Indian Ocean

Thomas et al. 2006
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Indian Ocean seawater profiles

230Th
231Pa
232Th
Thomas et al. 2006
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Where does the proxy signal come from?

Is sedimentary 231Pa/230Th representative of
whole water column?
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2. The role of particle flux?
Chase et al. 2003
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3. Preferential opal scavenging
Chase et al. 2003
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Modelling the behaviour of Pa and Th
Distribution coefficients after Luo and Ku (Pa
with strong preference for lithogenic particles)
Distribution coefficients after Chase et al (Pa
with preference for opal, Th for carbonate)
Siddall, Henderson, et al. (2005)
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4. What else resets it?

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Indian Ocean seawater profiles

230Th
231Pa
232Th
Thomas et al. 2006
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Southern Indian Ocean

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Benthic d18O
Reconstruction of deep inflow to Indian Ocean
Down core record from East of Madagascar
indicates little or no change in ABW flow into
Indian Ocean during full climate cycle
(231Pa/230Th)xs
Authigenic U
Thomas et al. 2007
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5. Does it work in other settings?
Present day model - 14Sv MOC
Siddall et al. 2007
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Modelling ocean Pa/Th - MOC shutdown
Shut down model - 2Sv MOC
Siddall et al. 2007
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Effect of MOC shutdown on Pa/Th
Difference between full and shutdown MOC
Siddall et al. 2007
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Effect of MOC shutdown on Pa/Th
Difference between full and shutdown MOC
Siddall et al. 2007
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Preliminary data from the Argentine Basin
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Rae et al
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And some other cores
Different water masses? Or different sediment
composition?
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Effect of MOC shutdown on Pa/Th
Difference between full and shutdown MOC
Siddall et al. 2007
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Preliminary Cape Basin results
Difference between full and shutdown MOC
Negre et al. 2007
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Preliminary Cape Basin results
Negre et al. 2007
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Upstream silica scavenging?
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6. Do we really understand it?
Model of sediment Pa/Th compared to water-column
observations
Siddall, Henderson, et al. (2005)
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Interpretation of data by 3D modeling
Model of sediment Pa/Th compared to sediment
observations
Siddall, Henderson, et al. (2005)
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(No Transcript)
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U-Series Geochemistry
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Dont get fooled by models