Iron isotope fractionation in Arctic Ocean sediments.

(1)

IRON ISOTOPE FRACTIONATION IN ARCTIC OCEAN SEDIMENTS

Alexandre Royer-Lavallée

1 , Charles Gobeil

1 and André Poirier

2

1

_{INRS-ETE, 490, rue de la Couronne, G1K 9A9, Quebec, Canada}

2

_{GEOTOP-UQAM, P.O. Box 8888, Station Centre-ville, H3C 3P8, Montreal, Canada}

Figure 1. Sediment sampling sites with a box-corer in the Arctic Ocean

Introduction

This sequential extraction scheme has been followed:

To better document sources and sinks of Fe across the well-oxygenated Canada Basin in the Arctic Ocean, profiles of the concentrations and isotopic compositions of total Fe (Fe_TOT), 1M HCl extractable Fe (Fe_HCl), and residual Fe (Fe_RES) remaining after the HCl extraction were determined in sediment cores collected at 51, 619 and 3130 m depth, respectively in the shelf, slope and abyssal portion of this basin. Concentrations of Fe associated to pyrite (Fe_PY) were also determined in each of the cores through an operationally defined extraction protocol.

Sampling sites

Beard BL et al. 2003 Application of Fe isotopes to tracing the geochemical and biological cycling of Fe. Chemical Geology 195(1-4):87-117

Scholz F et al. (2014) On the isotope composition of reactive iron in marine sediments: Redox shuttle versus early diagenesis. Chemical Geology 389:48-59.

Severmann S et al. (2010) The continental shelf benthic iron flux and its isotope composition. Geochimica et Cosmochimica Acta 74(14):3984-4004

The isotopic composition of Fe_TOT is slightly lighter in shelf sediments than in slope and deep basin sediments. In the shelf core, where the degree of pyritization (i.e., DOP=Fe_PY/Fe_HCl+Fe_PY) progressively increases below the sediment-water interface reaching up to 42% at 25 cm depth, there is no pronounced difference between the isotopic composition of Fe_TOT and those of Fe_HCl and Fe_RES in samples exhibiting significant pyrite enrichment. In contrast, the Fe_HCl pools in the slope and deep basin cores are characterized by a light isotope composition relative to that of Fe_TOT, undetectable or negligible concentrations of Fe_PY, and much higher concentrations and inventories of Fe_HCl than in shelf sediments.

References

Results

0 5 10 15 20 25 30 35 0 10 20 30 40 50 D ept h (c m ) Fe (mg/g) UTN5 0 5 10 15 20 25 30 35 40 45 0 20 40 60 80 D ept h (c m ) Fe (mg/g) CG2 0 5 10 15 20 25 30 35 40 45 0 20 40 60 80 D ept h (c m ) Fe (mg/g) S26

Figure 2. Vertical profiles of Fe_TOT(blue), Fe_HCl(red), Fe_RES(green) and Fe_PY(purple) in the

sediments core

Digested sediments has been analysed for element concentration (ICP-AES) and Fe isotopic fractionation (MC-ICP-MS in high-resolution).

Station Latitude Longitude Depth (m) UTN5 67°40.2 (N) 168°57.5 (O) 51

CG2 70°42.0 (N) 142°49.9 (O) 619 S26 84°03.8 (N) 175°05.3 (E) 3130

Methodology

Conclusion

Acknowledgments

Table1. Sediment sampling locations and water depth

Figure 4. Mean iron isotopic fractionation of the sediment cores. Fill dots represent the Fe_TOT

fractions and empty dots, Fe_HCl fractions

We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds Québécois de Recherche Nature et technologies (FQRNT) for their financial support. P. Girard, B. Patry and A. Bensadoune for their technical support.

Freeze-dried sediments Fe_TOT Fe_HCl Fe_RES Fe_PY HNO₃ HClO₄ HF HNO₃ HClO₄ HF HCl 1M 12h Supernatant Pellet HCl 1M HF HNO₃ UTN5 (51 m) CG2 (619 m) S26 (3130 m)

Figure 3. Vertical profiles of δ56_Fe

TOT (blue), δ56FeHCl (red), δ56FeRes (green) and δ56FePY (purple) in the

sediments core 0 5 10 15 20 25 30 35 40 -2.50 -1.50 -0.50 0.50 D ept h (c n) δ56Fe (°⁄ₒₒ) S26 0 5 10 15 20 25 30 35 -2.50 -1.50 -0.50 0.50 D ept h (c m ) δ56Fe(°⁄ₒₒ) UTN5 0 5 10 15 20 25 30 35 40 -2.50 -1.50 -0.50 0.50 D ept h (c m ) δ56Fe(°⁄ₒₒ) CG2