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The 231 Pa/ 230 Th Paleoproxy: How should we interpret the growing observational dataset? Gideon Henderson Alex Thomas Mark Siddall James Rae Ben Hickey University of Oxford. A proxy for past rates of ocean circulation. Lynch Stieglitz et al. 2007.
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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
A proxy for past rates of ocean circulation Lynch Stieglitz et al. 2007
Fluxes of 231Pa and 230Th Scavenging of nuclide Downward transport Continued equilibration
Residence time of waters in North Atlantic Rhien et al. 2001
Some real 230Th data South Atlantic Labrador Sea
Glacial NADW formation rates Suggests similar rates of NADW flow in glacial and Holocene Yu et al. 1996
Sensitivity of the proxy Marchal 2000
Millennial changes in NADW flow McManus et al. 2004
So that’s how it’s supposed to work. but… 1. Which depth water-mass is it telling us about? 2. Doesn’t 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?
` Southern Indian Ocean Thomas et al. 2006
` Indian Ocean seawater profiles 230Th 231Pa 232Th Thomas et al. 2006
` Where does the proxy signal come from? Is sedimentary 231Pa/230Th representative of whole water column?
2. The role of particle flux? Chase et al. 2003
3. Preferential opal scavenging Chase et al. 2003
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)
` 4. What else resets it?
` Indian Ocean seawater profiles 230Th 231Pa 232Th Thomas et al. 2006
` Southern Indian Ocean
` 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
5. Does it work in other settings? Present day model - 14Sv MOC Siddall et al. 2007
Modelling ocean Pa/Th - MOC shutdown Shut down model - 2Sv MOC Siddall et al. 2007
Effect of MOC shutdown on Pa/Th Difference between full and shutdown MOC Siddall et al. 2007
Effect of MOC shutdown on Pa/Th Difference between full and shutdown MOC Siddall et al. 2007
Ice core 18O Benthic 18O McManus Pa/Th Fast New Pa/Th Slow Fe/Ca Sea level Rae et al
NADW AIW/NADW AIW And some other cores… Different water masses? Or different sediment composition?
Effect of MOC shutdown on Pa/Th Difference between full and shutdown MOC Siddall et al. 2007
Preliminary Cape Basin results Difference between full and shutdown MOC Negre et al. 2007
Preliminary Cape Basin results Negre et al. 2007
6. Do we really understand it? Model of sediment Pa/Th compared to water-columnobservations Siddall, Henderson, et al. (2005)
Interpretation of data by 3D modeling Model of sediment Pa/Th compared to sediment observations Siddall, Henderson, et al. (2005)