Sources and reaction pathways of soluble Fe from the Western Antarctic Peninsula to the Southern Ocean
Final Report Abstract
During two campaigns on King George Island (Antarctica) in 2012 and 2013, numerous sediment and pore water samples were collected in Potter Cove and Maxwell Bay. Especially Potter Cove is strongly affected by glacier retreat, which is assumed to affect the biogeochemical processes in the area. Based on pore water profiles the degradation of sedimentary organic matter in the sediments in proximity to the marine-terminating Fourcade Glacier was found to be dominated by dissimilatory iron reduction (DIR). In contrast, sulfate reduction was apparent at shallow sediment depths in those parts of Potter Cove, where surficial meltwater streams discharge. Sediments in proximity to the glacier fronts contain significantly higher amounts of easily reducible (amorphous) Fe oxyhydroxides than stations in the central part of the bay or in discharge areas of surficial meltwater streams. Stable iron isotopes are considered a proxy for Fe sources, but respective data are scarce and Fe-cycling in complex natural environments is not understood well enough yet to constrain respective delta56Fe “endmembers” for different types of sediments and environmental conditions. In order to enhance the usability of iron isotopes as proxies for iron sources and reaction pathways, we developed a new method that allows to measure delta56Fe on sequentially extracted sedimentary Fe phases and applied the new protocol to sediment from King George Island. We suggest that easily reducible Fe in proximity to the glacier front is mostly delivered from subglacial sources, where iron liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (pyrite oxidation or DIR). Our strongest argument for a subglacial source of the highly reactive iron pool in sediments close to the glacier front is its overall negative delta56Fe signature that remains constant over the whole ferruginous zone. This pattern implies that the supply with easily reducible Fe exceeds the fraction that afterwards undergoes early diagenetic DIR by far. The light delta56Fe values of easily reducible Fe oxides imply pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. Interestingly, the strongest 56Fe-depletion in pore water and of the most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. In terms of the potential of delta56Fe as a proxy for benthic Fe fluxes, the study demonstrates limitations due to a large variability of pore water delta56Fe deriving from DIR in the marine sediments at small spatial distances. The controlling factors are multi-fold and include the availability of reducible Fe oxides and organic matter, the isotopic composition of the primary ferric substrate, sedimentation rates, and physical reworking (bioturbation, ice scraping). Whereas delta56Fe may prove a valuable parameter to further investigate biochemical weathering of glacier beds, a quantification of benthic Fe fluxes based on delta56Fe seems complicated for Antarctic bays and requires in-depth knowledge about the general status and the variability of early diagenesis in the investigated area. As a side-project, iron concentrations and isotopes in the hemolymph of the Antarctic bivalve "Laternula elliptica", sampled during the campaign in 2013, were analysed in order to determine the Fe assimilation pathway (uptake of pore water Fe2+ vs. reactive Fe oxides) of the bivalve. The pathway of iron uptake is of interest for assessing effects of environmental change on bivalve populations. delta56Fe values of bivalve hemolymph at two sites were -1.19 ± 0.34‰ and 1.04 ± 0.39‰ and lighter than the pool of reducible Fe oxides in surface sediments. In case of assimilation of iron oxide particles, this would be in agreement with a preferential assimilation of light isotopes from nutrition. However, delta556Fe hemolymph values showed a high variability. In addition, mass dependent Fe fractionation related to physiological processes within the bivalve could not be ruled out. Further studies are needed until delta56Fe in hemolymph of L. elliptica (and other bivalves) can be used as a proxy for different types of Fe uptake.
Publications
- Iron assimilation by the clam Laternula elliptica: Do stable isotopes (δ56Fe) help to decipher the sources?
Chemosphere, Vol. 134. 2015, pp. 294-300.
Poigner, H., D. Wilhelms-Dick, D. Abele, M. Staubwasser, and S. Henkel
(See online at https://doi.org/10.1016/j.chemosphere.2015.04.067) - Determination of the stable iron isotopic composition of sequentially leached iron phases in marine sediments.
Chemical Geology, Vol. 421. 2016, pp. 93-102.
Henkel, S., S. Kasten, S.W. Poulton, and M. Staubwasser
(See online at https://doi.org/10.1016/j.chemgeo.2015.12.003)
