Theoretical analysis of the p-ratio

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Theoretical analysis of the p-ratio
S. Bradley Moran, URI
OUTLINE

• Key variables particle size and density.
• Derivation of p-ratio and sensitivity analysis.
• Summary and recommendations.

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Key variables particle size and density
salp
copepod
1 mm
euphausiid
Courtesy of Alice Alldredge
Courtesy of Debby Steinberg
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Some questions regarding POC/234Th ratios and POC
flux

• How should we sample POC/234Th ratios to get an
  accurate POC export flux? Filtration what size?
  Traps what kind?
• What is the size-distribution of POC and 234Th?
• Are all phases of POC in equilibrium with 234Th?
• How are POC/234Th ratios (and hence POC flux)
  related to changes in particle density, size and
  hence sinking speed?
• Given range and variability in POC/234Th ratios,
  how do we assess the accuracy of POC export flux?

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Model derivation of the p-ratio
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Model derivation of the p-ratio
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Model derivation of the p-ratio
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Constant Density w/ Low Flux (LF) and High Flux
(HF)
p-ratio
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Variable Density w/ Low Flux (LF) and High Flux
(HF)
p-ratio
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Particle Assemblage 1 POC density 1060
p-ratio
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Particle Assemblage 2 POC density 1300
p-ratio
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Summary

• The p-ratio is an indicator of the accuracy of
  234Th -derived POC fluxes.
• For p-ratio (PPOC/FPOC) 1, average sinking
  speed of POC- and Th-carrying particles must be
  equal.
• Accurate measurement of POC export flux requires
  p-ratio 1 at base of euphotic zone.
• p-ratio 1 does not mean that residence times of
  POC and 234Th are equal in the euphotic zone.

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What does all of this mean?

• Open Ocean
• Biogenic particles dominate POC- and Th-carrying
  particles may have reasonably similar sinking
  speeds
• p-ratio approaches 1.
• However, dust/mineral inputs, org. C
  remineralization will increase p-ratio.
• Shelf-Slope
• More inorganic (small, high density) particles
  that scavenge Th
• p-ratio will increase.

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Questions to think about

• How do we best determine the p-ratio?
• To what extent do operationally defined sampling
  methods affect the p-ratio?
• How much does the p-ratio vary for different
  oceanic regions (opal, carbonate, lithogenic)?

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Future Work

• Determine particle size-distributions, densities
  and sinking speeds of POC- and 234Th-carrying
  particles.
• Relate measurements of p-ratio to density of
  size-fractionated particles over range of
  particle types.
• Develop models of particle-234Th interactions and
  POC/234Th size-distributions and test against
  p-ratio.