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

Lynch Stieglitz et al A proxy for past rates of ocean circulation

Oceanic behaviour of U-series nuclides

Fluxes of 231 Pa and 230 Th Scavenging of nuclide Downward transport Continued equilibration

Fluxes of 231 Pa and 230 Th

Residence time of waters in North Atlantic Rhien et al. 2001

Some real 230Th data South Atlantic Labrador Sea

Fluxes of 231 Pa and 230 Th

231 Pa/ 230 Th in the oceans

Yu et al Glacial NADW formation rates Suggests similar rates of NADW flow in glacial and Holocene

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?

` 1. Which depth of water mass?

` Southern Indian Ocean Thomas et al. 2006

` Indian Ocean seawater profiles 230 Th 231 Pa 232 Th Thomas et al. 2006

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

` Southern Indian Ocean

` Reconstruction of deep inflow to Indian Ocean Benthic  18 O ( 231 Pa/ 230 Th) xs Authigenic U Down core record from East of Madagascar indicates little or no change in ABW flow into Indian Ocean during full climate cycle 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

Preliminary data from the Argentine Basin

McManus Pa/Th Benthic  18 O Ice core  18 O New Pa/Th Fe/Ca Sea level Fast Slow 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

Upstream silica scavenging?

6. Do we really understand it? Model of sediment Pa/Th compared to water-column observations 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)

U-Series Geochemistry

Don’t get fooled by models