The The 231231Pa/Pa/230230Th Paleoproxy: Th Paleoproxy: How should we interpret the How should we interpret the growing observational growing observational dataset? dataset?

Gideon HendersonGideon HendersonAlex ThomasAlex ThomasMark SiddallMark SiddallJames RaeJames RaeBen HickeyBen Hickey

University of OxfordUniversity of Oxford

Lynch Stieglitz et al. 2007

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A proxy for past rates of ocean circulation

Oceanic behaviour of U-series nuclides

238U -> 234Th

234Pa -> 234U -> 230Th

226Ra

235U -> 231Th -> 231Pa

227Ac

particlescavenging

releasefromsediment

Oceanic behaviour of U-series nuclides

238U -> 234Th

234Pa -> 234U -> 230Th

226Ra

235U -> 231Th -> 231Pa

227Ac

particlescavenging

releasefromsediment

Fluxes of 231Pa and 230Th

F = P

concentration

Scavenging of nuclideDownward transportContinued equilibration

Fluxes of 231Pa and 230Th

F = P

concentration

F = P F < PF < P

concentration concentrationconcentration

230Th

Fluxes of 231Pa and 230Th

Residence time of waters in North Atlantic

Rhien et al. 2001

Some real 230Th data

0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

230Th (dpm/1000 litres)

Depth (km) SouthAtlantic

LabradorSea

F = P F < PF < P

concentration concentrationconcentration

230Th

Fluxes of 231Pa and 230Th

F = P F < PF < P

concentration concentrationconcentration

F < PF < PF < P

230Th231Pa

Fluxes of 231Pa and 230Th

231Pa/230Th in the oceans

231Pa230Th

Advection

NADW

234U 230Th

235U 231PaProduction

231Pa/ 230Th = 0.093

NorthAtlantic

SouthernOcean

lowproductivity

highproductivity

230Th231Pa

231Pa230Th

Sedimentary231Pa/ 230Th

< 0.093i.e. 0.06

Sedimentary231Pa/ 230Th

> 0.093i.e. 0.15

Yu et al. 1996

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

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

0.01

0.10

1.00

10 100 1000

Mass Flux (mg m-2 day-1)

3. Preferential opal scavenging

0.01

0.10

1.00

0.01 0.1 1 10 1000.01

0.10

1.00

10 100 1000

Opal/Carbonate (%/%)

Particulate 231Pa/230Th

Mass Flux (mg m-2 day-1)

Southern OceanEquatorial PacificOther PacificNorth Atlantic

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

`

Reconstruction of deep inflow to Indian Ocean

Benthic 18O

(231Pa/230Th)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 18O

Ice core 18O

New Pa/Th

Fe/Ca

Sea level

Fast

Slow

Rae et al

0.01

0.03

0.05

0.07

0.09

0.11

0.13

0.15

0 5000 10000 15000 20000 25000

Age (yr)

(Pa/Th)xs 0

2.5

3.0

3.5

4.0

4.5

5.0

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 modelingModel of sediment Pa/Th compared to sediment

observations

Siddall, Henderson, et al. (2005)

238U

206Hg

206Tl

206Pb

210Po

210Bi

210Pb

210Tl

214Po

214Bi

214Pb

218Rn

218At

218Po

222Rn

226Ra

230Th

234U

234Pa

234Th

U-Series Geochemistry

238U

234U

234Pa

234Th

α4.5 Gyrβ

β

α250 kyr

230Thα

76 kyr

Don’t get fooled by models