Iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Henkel, Susann, Kasten, Sabine, Hartmann, Jan, Busso, Adrian S., Staubwasser, Michael
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: PERGAMON-ELSEVIER SCIENCE LTD 2018
• Subjects: Antarc*; Antarctic; Antarctic Peninsula; King George Island; Southern Ocean
• Online Access: https://epic.awi.de/id/eprint/46878/; https://epic.awi.de/id/eprint/46878/2/Henkel_et_al_2018_post-print.pdf; https://hdl.handle.net/10013/epic.dafcf214-fceb-4818-9f2f-44d5a236f121

Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe ) are considered a proxy for Fe sources and reaction pathways, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions, and biogeochemical processes. We present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the icefree coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile ...