Preprints
https://doi.org/10.5194/egusphere-2023-2991
https://doi.org/10.5194/egusphere-2023-2991
09 Jan 2024
 | 09 Jan 2024
Status: this preprint is open for discussion.

The macronutrient and micronutrient (iron and manganese) signature of icebergs

Jana Krause, Dustin Carroll, Juan Höfer, Jeremy Donaire, Eric Pieter Achterberg, Emilio Alarcón, Te Liu, Lorenz Meire, Kechen Zhu, and Mark James Hopwood

Abstract. Ice calved from the Antarctic and Greenland Ice Sheets or tidewater glaciers ultimately melts in the ocean contributing to sea-level rise. Icebergs have also been described as biological hotspots due to their potential roles as platforms for marine mammals and birds, and as micronutrient fertilizing agents. Icebergs may be especially important in the Southern Ocean where availability of the micronutrients iron and manganese extensively limits marine primary production. Whilst icebergs have long been described as a source of iron to the ocean, their nutrient signature is poorly constrained and it is unclear if there are regional differences. Here we show that 589 ice fragments collected from floating ice in contrasting regions spanning the Antarctic Peninsula, Greenland, and smaller tidewater systems in Svalbard, Patagonia and Iceland have similar characteristic (micro)nutrient signatures with limited or no significant differences between regions. Icebergs are a minor or negligible source of macronutrients to the ocean with low concentrations of NOx (NO3 + NO2, median 0.51 µM), PO4 (median 0.04 µM), and dissolved Si (dSi, median 0.02 µM). In contrast, icebergs deliver elevated concentrations of dissolved Fe (dFe; mean 82 nM, median 12 nM) and Mn (dMn; mean 26 nM, median 2.6 nM). A tight correlation between total dissolvable Fe and Mn (R2 = 0.95) and a Mn:Fe ratio of 0.024 suggested a lithogenic origin for the majority of sediment present in ice. Total dissolvable Fe and Mn retained a strong relationship with sediment load (both R2 = 0.43, p<0.001), whereas weaker relationships were observed for dFe, dMn and dSi. Sediment load for Antarctic ice (median 9 mg L-1, n=144) was low compared to prior reported values for the Arctic. A particularly curious incidental finding was that melting samples of ice were observed to rapidly lose their sediment load, even when sediment layers were embedded within the ice and stored in the dark. Our results demonstrated that the nutrient signature of icebergs is consistent with an atmospheric source of NOx and PO4. Conversely, high Fe and Mn, and modest dSi concentrations, are associated with englacial sediment, which experiences limited biogeochemical processing prior to release into the ocean.

Jana Krause, Dustin Carroll, Juan Höfer, Jeremy Donaire, Eric Pieter Achterberg, Emilio Alarcón, Te Liu, Lorenz Meire, Kechen Zhu, and Mark James Hopwood

Status: open (until 27 Mar 2024)

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Jana Krause, Dustin Carroll, Juan Höfer, Jeremy Donaire, Eric Pieter Achterberg, Emilio Alarcón, Te Liu, Lorenz Meire, Kechen Zhu, and Mark James Hopwood
Jana Krause, Dustin Carroll, Juan Höfer, Jeremy Donaire, Eric Pieter Achterberg, Emilio Alarcón, Te Liu, Lorenz Meire, Kechen Zhu, and Mark James Hopwood

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Short summary
Icebergs are a mechanism via which the cryosphere and ocean interact. Here we analyzed ice samples from both Arctic and Antarctic polar regions to assess the variability in the composition of calved ice. Our results show that low concentrations of nitrate and phosphate in ice are primarily atmospheric in origin, whereas sediments impart a low concentration of silica and modest concentrations of trace metals, especially iron and manganese.