4

Origins of elements and isotopes

• Hydrogen burning FUSION: E = mc2

– 1H + 1H = 2H + + 0.422 MeV

– 2H + 1H = 3He + + 5.493 MeV

– 3He + 3He = 4He + 1H + 1H + 12.859 MeV

• Helium burning

– 4He + 4He + 4He = 12C

5

• CNO cycle – 12C + 1H = 13N +

– 13N = 13C + + +

– 13C + 1H = 14N

– 14N + 1H = 15O

– 15O = 15N + + +

– 15N + 1H = 12C + 4He

• Carbon burning – 12C + 4He = 16O

• Oxygen burning – 16O + 4He = 20Ne

• Neon burning – 20Ne + 4He = 24Mg

16

17

Isotope notation and fractionation

• δ notation (in “parts per thousand” or

“permil”)

δHX = [(Rsample/Rstandard – 1)] * 1000

Where H = heavy isotope mass, X = element,

R = ratio of heavy to light isotope of the element

Natural ranges in isotope ratios:

ca. 600‰ for δ2H

ca. 100‰ for δ13C, δ18O, δ34S

ca. 30‰ for δ15N

19

Further notes on δ notation

• Values can be positive or negative.

• Linearly related to percent (%) abundance of the heavier isotope.

• Convenient means of dealing with relatively small % changes (i.e. 1% change in abundance is 10‰).

• However, δ notation is not “exact” in all mathematical applications and isotopic ranges and Atom Percent (HAP), Fractional (F) and Ratio (R) nomenclature is typically used (see Fry chapter).

• However, as Biologists, δ-notation will cover all your needs unless you delve into “spiking” experiments!

20

Stable Isotope Standards

Standards by definition have a 0‰ value of the -scale

of interest. Internal lab standards must be corrected to

International reference standards. International

reference materials are distributed by the US National

Institute of Standards and Technology (NIST; formerly

the US National Bureau of Standards), and by the

International Atomic Energy Agency (IAEA).

NIST: <www.nist.gov>

IAEA: <www.iaea.or.at >

38

Some examples of isotopic fractionations in

the Biosphere

• Elements and isotopes circulate in the atmosphere and fractionation and mixing bring about characteristic isotope distributions.

• Large (well buffered) pools provide points of “stability”

– E.g. Ocean (H,O,S,C), atmosphere (N).

• Fractionation is the agent of change.

• Plants, microbes fix nutrients and change isotope distributions for C,N,S.

39

Carbon

40

Carbon cycle

• Active exchanges between atmosphere, terrestrial ecosystems, sea surface.

• Atmospheric CO2 (-7 to -8 o/oo)

• C3 Photosynthesis ~-20 o/oo fractionation (-28 o/oo plant tissue).

• C4, CAM Photosynthesis ~-5 o/oo(-13 o/oo).

• Ocean: dissolved CO2 ~ +8 o/oo, bicarbonate production ~ +1 o/oo (+1 o/oo).

• Planktonic photosynthesis ~-20 o/oo.

41

Plant C fixation

Enzymatic

fixation CO2

Diffusion

C3: Calvin cycle, RUBISCO

ribulose biphosphate carboxylase

C4: Hatch-Slack cycle, PEP

Phosphoenolpyruvate carboxylase

Diffusion Δδ = ~-4 o/oo

Rubisco Δδ = ~-29 o/oo

Diffusion Δδ = ~4 o/oo

Rubisco Δδ = ~6 o/oo

CAM: Crassulacean acid metabolism,

PEP into C4 acids at night, refixed by

Rubisco during day

42

C4

C3

Model estimates of plant organic 13C

Suits et al. (2005)

Worldwide distribution of C4 plants (C4 fraction)

Still et al. (2003)

What is the typical range of values for plant tissues?

47

But, water stress affects isotopic

discrimination …

Stewart et al. (1995)

13C of the atmosphere is changing

Francey et al. (1999)

49

Dave Keeling

A: photosynthetic assimilation

R: respiration

R

R

A

A

Bowling et al. (2005)

Isotopic patterns in a

subalpine forest: 3-

month averages

0

5

10

15

20

25

Ingeborg Levin, University of Heidelberg

http://www.iup.uni-heidelberg.de/institut/forschung/groups/kk/en/14CO2_html

59

Nitrogen

Utility of Nitrogen Isotopes

• isotope ratios are frequently used to identify sources of nitrate

• isotope ratios give information about geochemical processes and chemical reactions such as nitrification or denitrification

• for NO3- - isotopes of N and O can be

used - dual isotope tracer

Decadal changes…

Fertilizer Nitrate

-5 0 5 10 15 20 25 30 35

15

N - NO3(AIR)

-5

0

5

10

15

20

25

30

1

8O

- N

O3 (

VS

MO

W)

2004

1993

Denitrificatio

n -->

DO < 4 mg/L

Fertilizer Ammonium

Animal Waste

•Significant shift in 15N to lower values

(switch to inorganic

fertilizer use from

manure – BMP )

•Denitrification –

limited to suboxic

riparian zones and

deep (>30yr)

groundwater

62

Nitrogen Cycling

• Atmospheric reservoir of 0‰

• Because N is limiting, fractionation is generally low.

• Faster loss of 14N than 15N in particulate N

decomposition leads to increase in 15N with depth in

oceans and soil.

• So, plants that rely on soil N tend to be more enriched

than those depending on atmospheric N.

• Nitrification and denitrification are the key sources of

fractionation.

• Phytoplankton use N2 gas, ammonia and nitrate.

N2-Fixation Deposition

Soil 15N

N Losses

Transformations

Fertilizer

Transformations

Input

Models and Patterns of Soil 15N

Plant 15N

Patterns and Gradients of Plant 15N

Inorganic N

Mycorrhizae

Lecture – Part 1

What Controls Plant 15N?

Variation in Soil and Plants

Observations from Fry (1991)

1. Large variation

2. No correlation with precipitation

3. Soils more enriched than plants

4. N2-fixers near 0 ‰

General Trends in Soil 15N

Values are usually positive (but there are exceptions)

Amundson et al. (2003)

Soil

Depth

(m

)

1.2

1.0

0.8

0.6

0.4

0.2

0

0.5 0.4 0.3 0.2 0.1 0.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0

Soil 15 N (‰) Soil Nitrogen (mg N / g Soil)

Juniperus

Artemisia

Inter-Canopy

General Trends in Soil 15N

From: Evans and Ehleringer (1993)

Soil Nitrogen Transformations S

oil

Org

anic

Ma

tter Active Pool

PassivePool

Slow Pool NH4+ NO3

-

Mineralization Nitrification

Plants Microbes

Volatilization

NH3 N2O, NO NO, N2O, N2

Denitrification

AminoAcids

Högberg (1997)

Shearer and Kohl (1990)

Process Observed Discrimination (‰)

Mineralization 0

NH4+ : NH3 Equilibrium 20 to 27

Volatilization 29

Diffusion in Solution 0

Nitrification 0 to 35

Denitrification 0 to 33

Högberg (1997)

Shearer and Kohl (1990)

% Substrate Remaining

020406080100

15N

(‰

)

-20

0

20

40

60

Substrate

Product

Nitrogen Loss: Volatilization

Ammonia

Ammonium

Plant 15N Patterns and Gradients

Swap et al. (2004) Annual Precipitation (mm)

0 500 1000 1500

Le

af

15N

(‰

)

-2

0

2

4

6

8

10C3 (R

2=0.63)

C4 (R2=0.19)

Kalahari Transect

Nutrient availability varies

inversely with precipitation

N cycles in arid sites are

more open

72

Sulfur

73

Sulfur Cycle

• Sulfate in the ocean is the primary reservoir

that is 21o/oo heavier than primordial sulfur

(e.g. Canyon Diablo Troilite).

• Fixation by plants has a small isotope effect

but reduction in sediments and anaerobic

conditions has a large effect (30-70o/oo).

• Continental vegetation (+2 to +6o/oo) vs

marine plants (+17 to +21o/oo).

Utility of Sulfur Isotopes

• S isotope ratios are frequently used to identify sources of dissolved species

• isotope ratios give information about geochemical processes and chemical reactions

• for SO4= - isotopes of S and O can be

used

• Animal studies: marine vs. terrestrial, estuaries, marshes; S-amino acids …..

Common Dissolved Sulfur Forms

• sulfate - SO4=

• hydrogen sulfide (H2S), elemental S,

bi-sulfide

• other sulfur forms are generally

insignificant (sulfite, thiosulfite)

Sources of Sulfate

• dissolution of evaporites (gypsum,

anhydrite)

• oxidation of pyrite

• atmospheric precipitation (minor)

• volcanic emissions

• hydrogen sulfide from bogs, fossil fuel

combustion

78

Pichlmayer et al. (1998)

The hydrologic cycle

Evaporation

Condensation

Sublimation

Percolation

Infiltration

& Transpiration - of -

PRECIPITATION

Linking it to soils and plants

81

In preparation for the

Hydrospehere

93

The Global Meteoric Water Line, GMWL

After Clark and Fritz, 1997; GMWL defined by Craig, 1961

Condensation is an equilibrium process

so

Most precipitation values lie along a Global Meteoric

Water Line (GMWL) of slope ~8

(e2H / e18O = 8)

94

d-excess & the Global Meteoric Water Line

Isotopes Reflect Soil Water Use Patterns

Community (Interspecific) Water-use

Ehleringer et al., Oecologia, 1991

Central Pacific

High

Warmer Temperatures

Cold Subarctic Currents

Upwelling

Summer fog formation off the coast of west-central North America occurs when subsidence air

moved by the Central Pacific high pressure cell meets the warm air moving off of the continent

and cold water from subarctic Alaskan currents and deepwater upwelling

H L

Fog Drip in Redwood-Forest Communities

-80

-70

-60

-50

-40

-30

-20

-10

0

101992 1993 1994

Fog

Rainfall

Sequoia sempervirens

Oxalis oregana

Rhododendron macrophyllum

Polysticum munitum

Gaultheria shallon

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D

Month of the Year

Sequoia sempervirens

Oxalis oregana

Rhododendron macrophyllum

Polysticum munitum

Gaultheria shallon

90

80

70

60

50

40

30

20

10

(a)

(b)

Complete dependence

On FOG! FOG

Dawson, 1998

Hydrogen exchange:

H C

H N

H O

Strong bonds

Weak bonds

Saskatoon Seasonality

5/7/90 1/31/93 10/28/95 7/24/98 4/19/01 1/14/04

-240

-200

-160

-120

-80

-40

D

(V

SM

OW

)

5/7/90 1/31/93 10/28/95 7/24/98 4/19/01 1/14/04

-35

-30

-25

-20

-15

-10

-5

(V

SM

OW

)

“Hydrogen Exchange” Problem

• 10-20% of keratin H will quickly (<24 hrs)

exchange hydrogen with ambient moisture

• D results vary among season and

between labs in different geographic

locations

• Dorg results not comparable among labs!

• Require new standardized procedures

Intercomparison Results

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Passerine Individual #

D

NHRC SINLAB

CPSIL

•18 passerine

feathers

•Cut along vein

and stem

•Keratin references

+/-2 at all labs

•Intra-sample

heterogenetity may

be an issue!

•Heterogeneity

may be larger than

the geographic

variance!

Other topics?

• Lipid Extraction

– C/N ratio (see Post et al.) vs removal.

• Your projects?