Iron isotope constraints on Fe cycling and mass balance in oxygenated Earth oceans Brian L. Beard, Clark M. Johnson, Karen L. Von Damm, Rebecca L. Poulson 2003 Geology, 31/7, Geochemistry III Seminar Sessions John Chapman, February 2005

Seminar Programme Brief intro to iron stable isotope theory Brief intro to iron stable isotope theory Paper presentation Paper presentation Your Questions Your Questions Structured discussion Structured discussion Model answers Model answers

Iron Stable Isotope Theory 54 Fe (5.84%), 56 Fe (91.76%), 57 Fe (2.12%), 58 Fe (0.28%) 54 Fe (5.84%), 56 Fe (91.76%), 57 Fe (2.12%), 58 Fe (0.28%) Standard  -notation used: parts per 10 3, ‰ Standard  -notation used: parts per 10 3, ‰  56 Fe = [ ( 56/54 Fe samp / 56/54 Fe std ) -1 ] * 1000  56 Fe = [ ( 56/54 Fe samp / 56/54 Fe std ) -1 ] * ve enriched in heavy, -ve enriched in light +ve enriched in heavy, -ve enriched in light

Introduction Modern ocean [Fe] very low: <1 nM Modern ocean [Fe] very low: <1 nM Residence of Fe very short: 70 – 200 yr Residence of Fe very short: 70 – 200 yr Fe-Mn crusts show  56 Fe = ‰ rising to ‰ after ~1.7 Ma Fe-Mn crusts show  56 Fe = ‰ rising to ‰ after ~1.7 Ma Effect correlates with Pb, not diagenetic Effect correlates with Pb, not diagenetic

Inputs to Marine Iron Budget Atmospheric Inputs MOR Riverine Inputs ( x g/yr )

Isotopic Contribution Input  56 Fe Contribution % Alternate % Riv. Particulate 0.02 ± Riv. Dissolved ? Atmos. Particulate 0.01 ± Atmos. Dissolved ?27 MOR Hydrothermal to Extraterrestrial +0.5 to

Oceanic Fe Mass Balance Assume modern contribution proportions Assume modern contribution proportions Ocean island runoff Rapid spreading Rapid spreading rates at ridges rates at ridges Control ?? High atmospheric flux

Missing Fe? How to account for highly –ve Fe-Mn? How to account for highly –ve Fe-Mn? Lower  56 Fe in some vent fluids?? Lower  56 Fe in some vent fluids?? Previous slide ignored dissolved fluxes Previous slide ignored dissolved fluxes Maybe these might be more significant than previously assumed? Maybe these might be more significant than previously assumed?

Dissolved Riverine Fluxes Fe is nonconservative during freshwater- seawater mixing. Fe is nonconservative during freshwater- seawater mixing. Small fractionation between dissolved Fe and ppt may cause large shift in  56 Fe Small fractionation between dissolved Fe and ppt may cause large shift in  56 Fe Some evidence for ppt fractionation Some evidence for ppt fractionation Most Fe is Fe 3+ so very low frac. potential Most Fe is Fe 3+ so very low frac. potential

Rayleigh Distillation  56 Fe (‰) 0 + ve - ve Bulk Solid Instantaneous Solid Dissolved Fraction % Fe in solid 50%100%  dis-sol

Dissolved Atmospheric Fluxes Most dissolved Fe is Fe 2+ Most dissolved Fe is Fe 2+ If all iron is dissolved from particulates there will be no overall fractionation If all iron is dissolved from particulates there will be no overall fractionation Equilibrium frac. between Fe(II) and Fe(III) may produce  56 Fe Fe(II) = -1.4 to -0.5 ‰ Equilibrium frac. between Fe(II) and Fe(III) may produce  56 Fe Fe(II) = -1.4 to -0.5 ‰ Fe(III) lost immediately as insoluble Fe(III) lost immediately as insoluble

Conclusions – 1 Highly sensitive monitor of ocean  56 Fe Highly sensitive monitor of ocean  56 Fe Homogenous input values, source variation is highly unlikely on short scale Homogenous input values, source variation is highly unlikely on short scale Glaciations = high detrital input (ice raft, mechanical weathering) →  56 Fe ~ 0 ‰ Glaciations = high detrital input (ice raft, mechanical weathering) →  56 Fe ~ 0 ‰ Interglacial = more MOR and chemically weathered Fe →  56 Fe = to – 2 ‰ Interglacial = more MOR and chemically weathered Fe →  56 Fe = to – 2 ‰

Conclusions – 2 However! However! Snowball Earth completely removes detrital input so values shift to –ve Snowball Earth completely removes detrital input so values shift to –ve Alternative is variation due to ocean mixing Alternative is variation due to ocean mixing Climate changes may lead to rapid reorganisation global ocean currents Climate changes may lead to rapid reorganisation global ocean currents

Questions… 1. Given that RAM Fe is , what is 1 nM expressed in ppm terms? 2. Will the instantaneous solid always show  56 Fe = 0 ‰ at the 50% precipitation point? 3. If the iron budget is affected by riverine inputs, what global events may have caused  56 Fe excursions?

Q1 - Answer RAM Fe = ≡ g/mol RAM Fe = ≡ g/mol 1 nmol Fe= x g 1 nmol Fe= x g 1 nM Fe= 5.58 x ‰ 1 nM Fe= 5.58 x ‰ = 5.58 x ppm ≈ 56 ppt

Thermohaline Circulation

Sr Isotopic Evidence Riverine influx: 87 Sr/ 86 Sr = MOR influx: 87 Sr/ 86 Sr = 0.703

MOR Spreading Rate

Further Reading! Beard, B.L. et al., Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chemical Geology, 195: Johnson, C.M. et al., Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton. Contributions to Mineralogy and Petrology, 144: Levasseur, S., et al., The global variation in the iron isotope composition of marine hydrogenetic ferromanganese deposits: implications for seawater chemistry? Earth & Planetary Science Letters, 224: Rouxel, O., et al., Iron isotope fractionation during oceanic crust alteration. Chemical Geology, 202(1-2): Severmann, S. et al., The effect of plume processes on the Fe isotope composition of hydrothermally derived Fe in the deep ocean as inferred from the Rainbow vent site, Mid-Atlantic Ridge, 36°14'N. Earth & Planetary Science Letters, 225(1-2):