N Isotope Geochem

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N Isotope Geochem

• Provide overview of Diversity of approaches
• Marine N Cycle Background
• Observational approaches w/r to Natural
  Abundance of N
• Biological oceanog/ Paleoecology
• Experimental approaches
• Tracer methodology
• Web site
• http//www.usc.edu/dept/LAS/biosci/tricho

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Disclaimer

• Im NOT an isotope geochemist!
• (thats why Im in the class!)
• Experimentally-oriented microbial ecologist who
  stumbled in
• First w/ tracers
• More recently using natural abundance approaches

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MARINE N CYCLE(why do we care?)

• Primary limiting nutrient in most of ocean
• Major control on primary productivity and C
  export
• Control on climate? Paleo analysis
• Regional, basin scale and atmospheric
  perturbations

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N Cycle Pools

• N2 dominates- but largely unavailable
• Large deep pools of NO3-, DON
• Pacificward enrichment

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• Oceanic N Concentrations and Inventories

TOTAL gN x1015
Species
Surface
Deep
Estuarine µM
Coastal
Reefs
22,000
N
800
1150
700 - 1100
700 - 1100
-
2
_ 3
570
NO
0.2
35
0 - 30
0 - 350
0.1 - 2.7
_ 2
-
NO
0.1
lt 0.1
0 - 2
0 - 30
0.02 - 0.16
4
7
NH
lt 0.5
3
0 - 25
0 - 600
0.02 - 1.7
550
DON
5
3
3 - 10
5 - 150
2- (70)
PON
0.4
lt 0.1
0.1 - 2
1 - 100
-
3 - 24






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N Cycle Features

• Phase transitions
• Oxidation/ reduction reactions
• Largely biological/ microbial
• Trophic Structure
• Habitat/ Environment Specific Components
• Photic/ aphotic (or shallow/ deep)
• (e.g. NO3- uptake vs. nitrification)
• Aerobic/ anaerobic (nitrification vs.
  denitrification)
• Pelagic/ benthic
• Coastal/ pelagic (quantitative aspects)

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New Production Model

• Dugdale Goering (1967)
• Eppley Peterson (1979)
• Recognized different forms of N
• Recycled (regenerated N)
• New (from external to the system
• Production based on new N set a limit on
  exportable production
• Research has largely focused on NO3-

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New Production Paradigm (after Dugdale Goering
1967)
N2 / CO2 (atm)
algae
zoop
New N
NH4
Recycled N
thermocline
NO3- / CO2 (deep) _at_ Redfield ratio
POC/PONdown
NH4
NO3-
? PONdown ? uptake NO3- N2 since CO2 /
NO3- upwell ? C/N POMdown POCatm-down ?
uptake N2 C/Ndown
Sea Floor
0 15 30 NO3-, µM
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Oceanic N demand- large flux

• 7200 Tg Total primary production
• 2500 Tg N/ y for net primary production
• (i.e. export) (NPP)
• 250 Tg N/ y for sequestered production
• (i.e. that required for ocean biology to fix 1.5
  Pg C)

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Natural Abundance Approach

• Trophodynamic structure/ interactions
• Nutrient (NO3-) use, tracking
• e.g. sewage
• Paleoecology
• Nutrient limitation
• N2 fixation/ Denitrification over
• glacial-interglacial periods
• N2O sources/ sinks

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15N Natural Abundance

• Natural abundance of 15N 0.3663 atm
• Slight natural variation through isotopic
  discrimination
• Determined by isotope ratio mass spectrometry
• Generally expressed in del units
• ?15N (RS- RR )/RR 1000 or
  Rs/RR-1 1000
• where RS 15N/14N in sample, RR 15N/14N in
  reference relative to N2 in air, where N2 in air
  (0.3663 atm ) defined as 0
• N2 fixation effects little isotopic
  discrimination (relative to other N
  transformation pathways) and results in biomass
  with a ?15N near 0

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Reminders

• Fractionation factor
• ? Rp / Rs
• Enrichment factor (generally negative)
• ? (? -1) 1000 ?p- ?s x 1000
• 
  ?s 1000
• Isotopic discrimination
• ? ?p- ?s

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Limitations

• Mass requirements
• Low levels of many pools, e.g. NO3-, NH4
• Imposed fractionation with concentration and pool
  separation
• Require 100 efficency
• Operational definitions
• e.g. PON gt 0.6 ?m (GF/F)

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Nitrogen Isotope Fractionation
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NO3- Pool Dynamics

• Strong fractionation w/ uptake
• Rayleigh fractionation
• Concentration dependent
• Basin differences
• Denitrification/ N2 fixation effects
• Typically low concentrations in upper water
  column make it difficult to measure

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Global average 4.5 to 5.0
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Pelagic denitrification ? -20
?15N
Nitrate uptake ? -5
10
Addition of diazotroph N to nitrate pool ?
0 to -1
NO3
Sediment denitrification ? -0
Nitrification
0
0 100 200 of Original Pool
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?15N
Nitrification ? 0 to -20
10
Ammonium uptake ? (-5 ??)
Addition of diazotroph N to NH4 pool ? 0 to
-1
NH4
Ammonification ? (-3 to -5 ??)
0
0 100 200 of Original Pool
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Brandes et al. 1999
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Sedimentary Imprint of NO3- Utilization- Altabet
et al.

• Inverse correlation between surface NO3-
  concentration (more specifically utilization) and
  sediment surface ?15N
• Greater drawdown, less fractionation
• Regional differences in apparent fractionation
• Pacific - -2.5
• Southern Ocean- -7 to -11
• Residence time issues

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Classical food chain
N2 fixation-based food chain
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Trichodesmium spp.Best Known Planktonic
Diazotroph
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Paleoecology/ -oceanography

• Imprint of Processes on PON
• NO3- utilization efficiency
• (higher del w/ greater use
• Denitrification (increase)
• N2 fixation (decrease)

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Take home Depends where you are!
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N2O Production in the Sea

• Potent Greenhouse gas
• Radiative forcing/ ozone depletion
• Increasing in atmosphere
• Oceans are a slight source
• Denitrification originally thought to be source
• Nitrification also
• denitrifying Nitrifiers??
• Net Ocean flux ? 4 Tg Global sum ? 16 Tg

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Nitrification
NH4 (lt 4.5)
NH2OH
NO2
NO3
NO
NOH
Nitrifier denitrification
N2O
N2
NO
NO2
Denitrification
NO3 (? 4.5 )
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