d13C in alkenones as a PaleoCO2 Proxy

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d13C in alkenones as a Paleo-CO2 Proxy

• Paleo-proxy seminar
• March 12, 2007
• Jenny Arbuszewski

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Motivation

• Past CO2 changes are not well understood
• Understanding the link between CO2 and climate
  change is vital for studies of the past and the
  future
• This proxy may additionally give information
  about nutrient utilization and availability

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As a Paleo-CO2 proxy

• Fractionation is a function of CO2 aq,
  cellular growth rates (closely tied to nutrient
  availability), cell geometry
• Problems
• estimate PO43- in past
• Alkenones can grow at various depths?estimate
  depth of growth using Uk37 T estimate to
  determine depth of max production for calibration
• For coretop calibrations, must adjust to
  preindustrial CO2
• E. huxleyi (major alkenone producer) only
  appeared 250 ka

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Photosynthetic carbon isotope fractionation (ep)
in alkenones
ef
Fin
Fout
ep ef b/CO2 ef m/CO2 ef Fout/Fin
ef C-fractionation during enzymatic carbon
fixation, e.g 29 for Rubisco b empirical
factor combining the effects of physiological
influences m growth rate
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Useful definitions and equations

• ep fractionation during marine photosynthetic
  carbon fixation
• ef C-fractionation during enzymatic carbon
  fixation, e.g 29 for Rubisco
• ep ef b/CO2 ((dCO2 aq1000)/(dorg1000))-11
  03
• b empirical factor combining the effects of
  physiological influences
• µ growth rate

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Seasonal Variability of haptophyte production
depth
Pagani et al. 2002
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Calibrations Pagani et al. 2002

• compared CO2 aq values estimated from di and
  tri unsaturated alkenones from Holocene age
  sediments to NOPACCS study results
• PO43- constrained using T range from Uk37 and
  depth-PO43- profiles
• Need to correct modern CO2 values to
  preindustrial values for comparison
• d13C mainly function of PO4 3- but can still
  use to estimate pCO2

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Pagani et al., 2002

• Max CO2aq (4.14PO43- 125.48PO43-
  107.85) / (27-ep)
• Min CO2aq(116.12PO43-81.5) / (25-ep)

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Pagani et al., 2002
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Pagani et al., 2002
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Applications Pagani et al., 2005

• Calculated ep as described earlier
• Mean ep for each site used along with a range of
  PO43- values
• pCO2 calculated from CO2aq using Henrys law and
  assuming S35 and T from d18O of marine
  carbonates
• Calculated max, min, and intermediate values
  using max, min and intermediate values of T and
  PO43-
• Errors 20 for Miocene, 30-40 for Paleogene

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Calibrations Benthein et al., 2007

• Studied bloom mesocosms
• 3 pCO2 settings glacial (180 ppmv), today (380
  ppmv), and year 2100 for IPCC business as usual
  (710 ppmv)
• All had same light, nutrients, etc.
• Only 1-2 shift for variable CO2
• 5-6 shift during exponential growth phase?
  nutrients are primary factor
• NOT useful as paleo-CO2 proxy

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Mesocosm study set up
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Benthein et al., 2007
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Maybe as a paleo PO43- proxy?

• Schulte et al., 2004 primary control on
  fractionation is growth rate and shows strong
  correlation to PO43-
• Possible global relation of ep and PO43-
• Application as a paleo PO43- proxy with
  relative error of 10.4

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Schulte et al., 2004
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Schulte et al., 2004
PO43- (15.1 ep) / 4.6
Relative error of 10.4
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Some Complications

• Small change of d13C with CO2
• Need to constrain PO4 3- and production depth
  for use as paleo CO2 proxy
• Post depositional diagenetic alterations not
  completely understood (dissolution)
• Quite a few assumptions (i.e. method of CO2
  delivery, depth of max production, etc.)
• In the past, possible cell geometries, C
  transport mechanisms were different from today
• Need to have reconstructed T and S values for the
  past as well
• We still dont know what alkenones actually do in
  the cell!