4He and 232Th

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4He and 232Th

• Continental Dust Proxies

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Continental Dust

• Mass accumulation rate (MAR) records for aeolian
  dust are important for reconstructing
• 1) Past continental climates
• dust flux is controlled primarily by aridity
• 2) Past earth climates
• - dust and radiation budget
• 3) Past wind patterns
• 4) Looking that the glacial-interglacial link to
  earths dustiness
• 5) Iron delivery to oceans (and subsequent
  impact on ocean organism productivity)

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4He Proxy

• Measuring MARs for continental dust was
  traditionally done by chemical leaching of the
  nonauthigenic, inorganic, crystalline matter in a
  sample
• Problem volcanic ash can survive the leaching
  and mess up the record
• This is especially problematic in the Pacific
• 4He to the rescue!
• Acts as a proxy for the flux of old continental
  crust
• 4He volcanic ash 5 ncc/g
• 4He continental crust 2000 ncc/g

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4He and Zircons

• 4He is a decay product
• 238U, 235U, and 232Th all undergo ?-decay (?
  4He)
• Zircon ZrSi04
• High U and Th concentrations
• Ubiquitous, dense and durable
• -Other minerals are also U, Th bearing, but
  zircons are most prevalent

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Calculating terrigenous 4He

• Equation assuming terrestrial and ET endmembers

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4He as a continental dust flux proxy

• Considerations
• Continuous decay of U, Th and accumulation of 4He
  post-transport?
• Endmember estimates (terrestrial vs ET)
• Fractionation during transport?
• Problem if multiple source areas of varying
  distances
• Advantages
• Dominantly terrigenous source
• Eliminates issues with volcanic ash in MAR
  calculation
• Simultaneous CFP and dust flux proxy measurements

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232Th

• 232Th (common thorium) concentrations are
  significantly higher in continental crust(10-15
  ppm) than oceanic crust (1.2 ppm)
• Relatively constant concentration found in
  terrigenous materious (i.e. not a function of age
  like 4He)
• Use in conjunction with the 230Th-normalized
  method (to eliminate problems discussed last
  week, e.g. sediment focusing)

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McGee et al (2007) Fig. 3
Upper Continental Crust Average
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232Th as a continental dust flux proxy

• Considerations
• Andesitic rocks have high 232Th
• Advantages
• Reliable, but not precise, proxy for dust
• Volcanic contributions can be significant
• Internal consistency between proximal cores
• Simultaneous CFP and dust proxy measurements
  (230Th and 232Th)

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Previous studies have shown that

• central and eastern China is dominant source of
  dust in NW and central Pacific (Pye and Zhou,
  1989)
• north Pacific MAR records show strong 100 kyr
  cyclicity for past 530 kyr (Hovan et al.,
  1989,1991)
• equatorial central Pacific records are more
  complex than those in the north, indicating
  possible ITCZ interference (Rea, 1994)

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Patterson et al (1999) Fig. 3
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Patterson et al (1999) Fig. 7
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Anderson et al (2006) Fig. 3

• Decrease in 232Th as
• the cores are located further
• South (i.e. farther from
• dust source)
• High 232Th during glacial
• periods

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Winckler et al (2008) Fig. 1
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4He and 232Th

• Winckler et al (2008). Figure S1

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