Iron isotope constraints on Iron isotope constraints on Fe cycling and mass Fe cycling and mass

balance in oxygenated balance in oxygenated Earth oceansEarth oceans

Brian L. Beard, Clark M. Johnson, Karen Brian L. Beard, Clark M. Johnson, Karen L. Von Damm, Rebecca L. PoulsonL. Von Damm, Rebecca L. Poulson

20032003

GeologyGeology, , 31/731/7, 629-632, 629-632

Geochemistry III Seminar SessionsGeochemistry III Seminar Sessions John Chapman, February John Chapman, February 20052005

Seminar ProgrammeSeminar Programme

• Brief intro to iron stable isotope theoryBrief intro to iron stable isotope theory

• Paper presentationPaper presentation

• Your QuestionsYour Questions

• Structured discussionStructured discussion

• Model answersModel answers

Iron Stable Isotope TheoryIron Stable Isotope Theory

• 5454Fe (5.84%), Fe (5.84%), 5656Fe (91.76%), Fe (91.76%), 5757Fe (2.12%), Fe (2.12%), 5858Fe (0.28%)Fe (0.28%)

• Standard Standard -notation used: parts per 10-notation used: parts per 1033, ‰, ‰

• 5656Fe = [ (Fe = [ (56/5456/54FeFesamp samp / / 56/5456/54FeFestdstd) -1 ] * 1000) -1 ] * 1000

• +ve enriched in heavy, -ve enriched in light+ve enriched in heavy, -ve enriched in light

IntroductionIntroduction

• Modern ocean [Fe] very low: <1 nMModern ocean [Fe] very low: <1 nM

• Residence of Fe very short: 70 – 200 yrResidence of Fe very short: 70 – 200 yr

• Fe-Mn crusts show Fe-Mn crusts show 5656Fe = - 0.69 ‰ rising Fe = - 0.69 ‰ rising to + 0.04 ‰ after ~1.7 Mato + 0.04 ‰ after ~1.7 Ma

• Effect correlates with Pb, not diageneticEffect correlates with Pb, not diagenetic

Inputs to Marine Iron BudgetInputs to Marine Iron Budget

0.0040.004

1010

292933

Atmospheric InputsAtmospheric Inputs

MORMOR

110110

11

Riverine Riverine InputsInputs

( x 10( x 101212 g/yr g/yr ))

Isotopic ContributionIsotopic Contribution

InputInput 5656FeFe Contribution %Contribution % Alternate %Alternate %

Riv. ParticulateRiv. Particulate 0.02 ± 0.10.02 ± 0.1 7272 --

Riv. DissolvedRiv. Dissolved ?? 0.650.65 2.32.3

Atmos. Atmos. ParticulateParticulate 0.01 ±0.070.01 ±0.07 1919 67.467.4

Atmos. Atmos. DissolvedDissolved ?? 22 77

MOR MOR HydrothermalHydrothermal -0.21 to -0.54-0.21 to -0.54 6.56.5 23.223.2

ExtraterrestrialExtraterrestrial +0.5 to -0.8+0.5 to -0.8 0.0030.003 --

Oceanic Fe Mass BalanceOceanic Fe Mass Balance

• Assume modern contribution proportionsAssume modern contribution proportions

Ocean island runoffOcean island runoff

Rapid spreading Rapid spreading rates at ridgesrates at ridges

Control ??Control ??

High atmospheric fluxHigh atmospheric flux

Missing Fe?Missing Fe?

• How to account for highly –ve Fe-Mn?How to account for highly –ve Fe-Mn?

• Lower Lower 5656Fe in some vent fluids??Fe in some vent fluids??

• Previous slide ignored dissolved fluxesPrevious slide ignored dissolved fluxes

• Maybe these might be more significant Maybe these might be more significant than previously assumed?than previously assumed?

Dissolved Riverine FluxesDissolved Riverine Fluxes

• Fe is nonconservative during freshwater-Fe is nonconservative during freshwater-seawater mixing.seawater mixing.

• Small fractionation between dissolved Fe Small fractionation between dissolved Fe and ppt may cause large shift in and ppt may cause large shift in 5656FeFe

• Some evidence for ppt fractionationSome evidence for ppt fractionation

• Most Fe is FeMost Fe is Fe3+3+ so very low frac. potential so very low frac. potential

Rayleigh DistillationRayleigh Distillation5

656 F

e (‰

)F

e (‰

)

00

+ ve+ ve

- ve - ve

Bulk SolidBulk Solid

Instantaneous Solid

Instantaneous Solid

Dissolved Fraction

Dissolved Fraction

% Fe % Fe in in solidsolid

50% 100%

dis-soldis-sol

Dissolved Atmospheric FluxesDissolved Atmospheric Fluxes

• Most dissolved Fe is FeMost dissolved Fe is Fe2+2+

• If all iron is dissolved from particulates If all iron is dissolved from particulates there will be no overall fractionationthere will be no overall fractionation

• Equilibrium frac. between Fe(II) and Fe(III) Equilibrium frac. between Fe(II) and Fe(III) may produce may produce 5656FeFeFe(II)Fe(II) = -1.4 to -0.5 ‰ = -1.4 to -0.5 ‰

• Fe(III) lost immediately as insolubleFe(III) lost immediately as insoluble

Conclusions – 1Conclusions – 1• Highly sensitive monitor of ocean Highly sensitive monitor of ocean 5656FeFe

• Homogenous input values, source Homogenous input values, source variation is highly unlikely on short scalevariation is highly unlikely on short scale

• GlaciationsGlaciations = high detrital input (ice raft, = high detrital input (ice raft, mechanical weathering) mechanical weathering) → → 5656Fe ~ 0 ‰Fe ~ 0 ‰

• InterglacialInterglacial = more MOR and chemically = more MOR and chemically weathered Fe → weathered Fe → 5656Fe = - 0.2 to – 2 ‰Fe = - 0.2 to – 2 ‰

Conclusions – 2Conclusions – 2

• However!However!

• Snowball Earth completely removes Snowball Earth completely removes detrital input so values shift to –vedetrital input so values shift to –ve

• Alternative is variation due to ocean Alternative is variation due to ocean mixingmixing

• Climate changes may lead to rapid Climate changes may lead to rapid reorganisation global ocean currentsreorganisation global ocean currents

Questions…Questions…

1.1. Given that RAMGiven that RAMFeFe is 55.847, what is 1 nM is 55.847, what is 1 nM

expressed in ppm terms?expressed in ppm terms?

2.2. Will the instantaneous solid always show Will the instantaneous solid always show 5656Fe = 0 ‰ at the 50% precipitation point?Fe = 0 ‰ at the 50% precipitation point?

3.3. If the iron budget is affected by riverine If the iron budget is affected by riverine inputs, what global events may have inputs, what global events may have caused caused 5656Fe excursions?Fe excursions?

Q1 - AnswerQ1 - Answer

• RAMRAMFe Fe = 55.847 = 55.847 ≡ 55.847 g/mol≡ 55.847 g/mol

• 1 nmol Fe1 nmol Fe= 5.5847 x 10= 5.5847 x 10-8-8 g g

• 1 nM Fe1 nM Fe = 5.58 x 10= 5.58 x 10-8-8 ‰ ‰

= 5.58 x 10= 5.58 x 10-5-5 ppm ppm

≈ ≈ 56 ppt56 ppt

Thermohaline CirculationThermohaline Circulation

Sr Isotopic EvidenceSr Isotopic EvidenceRiverine influx:Riverine influx:

8787Sr/Sr/8686Sr = 0.711Sr = 0.711

MOR influx:MOR influx:

8787Sr/Sr/8686Sr = 0.703Sr = 0.703

MOR Spreading RateMOR Spreading Rate

Further Reading!Further Reading!Beard, B.L. Beard, B.L. et alet al., 2003. Application of Fe isotopes to tracing ., 2003. Application of Fe isotopes to tracing

the geochemical and biological cycling of Fe. the geochemical and biological cycling of Fe. Chemical Chemical GeologyGeology, 195: 87-117., 195: 87-117.

Johnson, C.M. Johnson, C.M. et alet al., 2003. Ancient geochemical cycling in ., 2003. Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton. iron formations from the Transvaal Craton. Contributions to Contributions to Mineralogy and PetrologyMineralogy and Petrology, 144: 523-547., 144: 523-547.

Levasseur, S., Levasseur, S., et alet al., 2004. The global variation in the iron ., 2004. The global variation in the iron isotope composition of marine hydrogenetic isotope composition of marine hydrogenetic ferromanganese deposits: implications for seawater ferromanganese deposits: implications for seawater chemistry? chemistry? Earth & Planetary Science LettersEarth & Planetary Science Letters, 224: 91-105., 224: 91-105.

Rouxel, O., Rouxel, O., et alet al., 2003. Iron isotope fractionation during ., 2003. Iron isotope fractionation during oceanic crust alteration. oceanic crust alteration. Chemical GeologyChemical Geology, 202(1-2): 155-, 202(1-2): 155-182.182.

Severmann, S. Severmann, S. et alet al., 2004. The effect of plume processes on ., 2004. The effect of plume processes on the Fe isotope composition of hydrothermally derived Fe in the Fe isotope composition of hydrothermally derived Fe in the deep ocean as inferred from the Rainbow vent site, the deep ocean as inferred from the Rainbow vent site, Mid-Atlantic Ridge, 36°14'N. Mid-Atlantic Ridge, 36°14'N. Earth & Planetary Science Earth & Planetary Science LettersLetters, 225(1-2): 63-76., 225(1-2): 63-76.