Tracing Ecosystem Processes with Stable Isotopes

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Tracing Ecosystem Processes with Stable Isotopes

• Marilyn L. Fogel
• Geophysical Laboratory
• Carnegie Institution of Washington
• 2000

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Major areas of investigation

• Animal Ecology and Migration
• Aquatic Ecosystems
• Paleoecology
• Soil and Microbial Processes
• Large-scale Ecosystem Processes

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Animal Ecology

• Nutritional ecology
• Trophic relationships
• Foraging strategies
• Migration
• Birds
• Butterflies
• Fish

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Aquatic Ecosystems

• Estuarine Ecology
• Primary Production
• Nutrient Effects
• Limnology
• Bio-magnification
• Benthic Ecology

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Paleoecology

• Climate Change
• Environmental Reconstruction
• Origins
• Life
• Humans
• Agriculture
• Archaeology

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Soil and Microbial Processes

• Biogeochemical Cycling
• Soil formation
• Decomposition
• Microbial biochemistry
• Methane Production and Oxidation
• Sulfur cycling
• Nitrification, Denitrification, and N2 fixation

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Large-scale Ecosystem Processes

• Gas Fluxes
• Methane
• Carbon Dioxide
• Trace Gases
• Atmospheric-Terrestrial-Marine Connections
• Atmospheric Deposition
• Transfer of matter from estuaries to oceans

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Research Projects

• Animal Ecology
• Food web studies of the Blue crab
• Redhead Duck Migration
• Aquatic Ecosystems
• Estuarine processes
• Freshwater Marsh Nitrogen Fluxes
• Paleoecology
• Climate Change in the Australian Outback
• Origins of Life
• Archaeology

• Soil and Microbial Processes
• Decomposition at the molecular level
• Biochemical pathways in bacteria
• Large Scale Ecosystems Processes
• Atmospheric deposition in estuarine and marine
  environments

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Stable Isotope Meaurements1 millimeter in a
Kilometer

• 12C 98.89 14N 99.7
• 13C 1.11 15N 0.3
• Isotopic Composition (?)
• ?13C () 13C/12C sample/ 13C/12C standard -1
  x 1000
• Atmos. CO2 ?13C -8 Grass ?13C
  -12.5
• Leaves ?13C -26

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OLearys Laws of Isotopes--Its All Physics and
Chemistry!

• First Law The lighter isotope reacts faster and
  requires less energy of activation.
• Second Law The heavier isotopes make the
  strongest bonds.
• Zeroth Law Beware of changes less than 1 !

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Biological Laws of Isotopes

• Key reactions (e.g. Photosynthesis) label the
  bulk of the organism.
• You are what you eat--with some modification.
• Every biochemical reaction imparts an isotopic
  label on the products and reactants.

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Tracers of Present and Past

• Food web of the juvenile Blue Crab, Callinectes
  sapidus
• Paleoclimate of the Australian continent with
  isotope tracers in emu eggshell and termite mounds

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Climate Change in the Australian Outback

• Carbon and nitrogen isotopes
• in bird eggshell and termite mounds
• as paleoproxies of past environments
• Collaborators Giff Miller and Beverly Johnson ,
  Univ. of Colorado.
• John Magee, Australian National University.

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Questions

• What changes occurred in the climate of the
  interior of Australia after humans arrived 60,000
  years ago?
• Did human activity change the climate?
• Burning was a frequent practice, how did this
  affect the climate longterm?

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Study Structure

• Geological study at Lake Eyre and Lake Gregory--
  Lakes with continuous records of deposition for
  at least 100,000 kA
• Isotopic studies of paleoproxies--Emu eggshell
  and termite mound material
• Extensive dating--Amino acid, TL, U series

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Analytical Procedures

• ?13C and ?15 N of bulk samples--soils, plants,
  modern emu eggshells, and termite mounds
• Compound specific isotope analysis
• Amino acids in eggshells
• Lignin monomers and fatty acids in termite
  material

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Ratite Eggshell as Paleoproxy
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Development of the Proxy
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PhotosynthesisKinetic Isotope Effect
6CO2 6H2O
C6H12O6 6O2
?13C -8
-13 to -28
12CO2
12C
13CO2
Rubisco
13C
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Australian Plants C-3 and C-4 grasses
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?13C relationship to MAP
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Australian PlantsNitrogen fixers important
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?15N relationship to MAP
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Nitrogen in soils and plants are linked.
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Fractionation between Bird Diet and Carbonate in
Eggshell

• Emu (Dromaius)
• ?13C diet-ES carbonate 102
• Genyornis
• ?13C diet-ES carbonate 142
• Ostrich (Struthio)
• ?13C diet-ES carbonate 141

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Modern Emu Eggshell
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Inorganic Carbon in ARES at Madigan Gulf,Lake Eyre
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Eggshell Isotopic Interpretations

• Diet of emu changed remarkably during the last 70
  kA!
• At Last Glacial Maxiumum, shift to more C-3
  vegetation.
• Climate never rebounded after Ice Age ended.
• Genyornis and emu coexisted. Geny went extinct
  _at_45-50 kA. Why?

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Compound Specific Isotope Analysis in Eggshell

• Distinguish protein source from bulk carbon
  source.
• Detect alteration that might affect carbonate
  isotope values.
• Reflect level of dietary stress during egg laying.

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Carbon isotope analysis of Amino acids in
eggshells

• Measure ?13C of amino acids in major diet items.
• Plants
• Grasshopper
• Measure ?13C of amino acids in modern emu with
  known diet
• Determine isotope fractionation
• Measure ?13C of amino acids in fossil bird
  eggshells
• Calculate diet from known paramters above.

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Two species coexisting with different diets
55-60 kA
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At extinction diets overlap 50 kA
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Genyornis diet changed at extinction.
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Genyornis Extinction

• Two similar species occupied the same region
  before the arrival of humans.
• Diets were different at this time. Emu, a
  generalist Geny, more a specialist.
• At the time of extinction, Geny switched its
  diet--to one very similar to emu.
• Competition or predation or climate change--
  which factor(s) contributed to extinction event?

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Compound Specific Isotope Analysis of Termite
Metabolism
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Lignin Monomers in Plants
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TMAH lignin monomer

• Peaks found only in grasses.
• Sorghum
• ?13C -19.0
• Spinifex
• ?13C -17.0

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TMAH Lignin Monomer

• S5 and S6 peaks found in wood samples of C-3
  plants.
• Eucalyptus
• ?13C -25
• Acacia
• ?13C -26

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Conclusions

• Ratite Eggshell is a very strong tracer of
  changes in vegetation and climate.
• Egg laying season reflects winter climate.
• Are there fossil ES in North Australia?

• Termite mounds are more direct isotopic tracers
  of vegetation.
• Are they widely preserved in the fossil record?

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Answers toBurning
Question(s)

• Ecosystems in Australia changed dramatically.
• Extinctions occurred when climate was not
  changing.
• These and other climate changes are not linked to
  atmospheric or geologic forcing.
• Humans (and burning)were factors.

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New Developments in Stable Isotope Ecology

• Compound Specific Isotope Analysis
• Individual compounds in less than a 1 ?g of
  sample!
• Automated technology
• Large scale studies will not impact the
  laboratory.
• New methods for analyzing H and O isotopes
• Automated and avoid analytical problems.