Assessment of Paleoproductivity via 13C

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Assessment of Paleoproductivity via ?13C?

• Alessandro Tagliabue

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?13C Paleoproductivity ??

• Ratio of 13C12C in calcareous shells (typically
  foraminifera)
• ?13C dependant on
• Photosynthesis/respiration
• CO2 Gas Exchange
• Removal of C via Export Production
• Resupply of DIC to surface waters
• Surface waters generally enriched in ?13C

Broecker Peng (1982)
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Surface waters - Deep waters?13C disparity

• 12C of DIC used preferentially during
  photosynthesis
• Reduces Euphotic Zone 12C - i.e. increases
  ?13Csurf
• Ensures export flux is also rich in 12C (i.e low
  ?13Cexport)
• Limited by NO3 and PO4 (generally)
• Upwelling of nutrients also enriches in 12C -
  from remineralization of sinking OrgC

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Technique No. 1Quick and Dirty

• Examine the changing ?13C of the preserved / in
  situ phytoplankton over time.
• Modern example during IronExII
• ?13C increased after Fe additions
• Supports use as proxy for production

(Bidigare et al. 1999)
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Initial Problems

• Fractionation is temp dependant
• Cold waters have higher ?13C
• Upwelling of remineralized OrgC with low ?13C in
  areas of strong mixing
• Understanding the balance between ?13C of
  exported POC and ?13C of upwelled DIC is key
• Depth of remineralization
• Global long term changes in ocean-atmosphere ?13C
  due to changes in biosphere size

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Initial Solutions

• Examine maximum seasonal range i.e. relative
  difference more valid.
• Gradient from surface waters to deep
• ? ?13C between different planktonic spp.
• Forams from different depths can allow study of
  thermocline attributes

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Technique No. 2pelagic - benthic

• Look at ? ?13C between planktonic and benthic
  forams
• Gives info on C pump efficiency
• Changes between G-IG could indicate changes in
  nutrient utilization (Broecker 1982)

(Shackelton et al. 1983)
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Technique No. 3Benthics

• ? ?13C between Cibicidoides wuellerstorfi
  (epifanal) and Uvigerina peregrina (infaunal)
• Epifaunal spp. record bottom water ?13C
• Infaunal spp. record pore water ?13C
• ? ?13C thought to reflect differences in surface
  ocean productivity (Zahn et al. 1986 McCorkle et
  al. 1990)

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Cont...

• Assumes that productivity increases with
  increasing C flux to sediment (non linear)
• Knowledge of the benthic ? ?13C allows back
  calculation of primary production.
• Empirical Algorithms estimate surface production
  from sediment flux
• Associated caveats of calculating flux to depth
  from primary prod and vice versa (Lutz et al. in
  press)

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Benthic Problems

• ?13C of benthic forams impacted by dissolution
• This preferentially removes ?13C-enriched tests
• e.g. McCorkle et al. (1995)
• Bethic forams at depths of lt2km 0.2 per mil
  enriched in 13C (relative to bottom water)
• Whereas same sp. gt4km 0.2 per mil depleted in
  13C
• Are samples biased against low ?13C? (lt7o/oo are
  rare)
• If so, does a threshold for U. peregrina exist?
• i.e. is it just a record of this threshold flux?

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All-round problems

• Interspecific variability of forams is comparable
  to oceanic variability
• Lower ?13C in infaunal rather than epifaunal
  tests.
• Suggests remineralization of OrgC within seds
• Can complicate interpretation.
• Upwelling of DIC depleted in ?13C

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More problems

• Vital effects such as photosynthesis,
  respiration, calcification can alter ?13C
• Isotopic comp of calcite is dependant on CO32-
  chemistry of sea water, independent of any
  isotopic change in ?CO2
• Increase in oceanic pH of 0.2-0.4 units (Sanyal
  et al. 1995) , could ??13C by 0.3-0.4 (Spero et
  al. 1997)

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Example 1- Mortlock et al. (1991)

• Planktonic forams (Neogloboquadrina pachyderma)
• Lower planktonic ?13C at LGM ( 1 o/oo), pH?
• Qualitative agreement with Opal
• ? Lower Antarctic production during LGM
• Problem of preservation
• Non-silicious/carbonate spp. could be dominant.
• E.g. Phaeocystis antarctica

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Example 2 - Core V19-30(Shackelton et al. 1983,
Curry and Crowley 1987)

• Pelagic - Benthic technique
• Re-evaluation with different pelagic spp.
• Similar increases in ? ?13C of 0.5 ppt between
  studies.
• Contrary to Mortlock et al. 1991
• 20 increase in PO4 input or utilization
  (Broecker and Peng 1982)

(Curry and Crowley 1987)
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