Constraints on the source of reactive phases in sediment from a major Arctic river using neodymium isotopes

Larkin, Christina S.; Piotrowski, Alexander M.; Hindshaw, Ruth S.; Bayon, Germain; Hilton, Robert G.; Baronas, J. Jotautas; Dellinger, Mathieu; Wang, Ruixue; Tipper, Edward T.

Abstract

Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.

Citation

Larkin, C. S., Piotrowski, A. M., Hindshaw, R. S., Bayon, G., Hilton, R. G., Baronas, J. J., …Tipper, E. T. (2021). Constraints on the source of reactive phases in sediment from a major Arctic river using neodymium isotopes. Earth and Planetary Science Letters, 565, Article 116933. https://doi.org/10.1016/j.epsl.2021.116933

• Journal Article Type: Article
• Acceptance Date: Apr 2, 2021
• Online Publication Date: Apr 16, 2021
• Publication Date: Jul 1, 2021
• Deposit Date: Dec 19, 2023
• Journal: Earth and Planetary Science Letters
• Print ISSN: 0012-821X
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 565
• Article Number: 116933
• DOI: https://doi.org/10.1016/j.epsl.2021.116933
• Keywords: Space and Planetary Science; Earth and Planetary Sciences (miscellaneous); Geochemistry and Petrology; Geophysics
• Public URL: https://durham-repository.worktribe.com/output/2048544