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Abstract. Ice calved from the Antarctic and Greenland ice sheets or tidewater glaciers ultimately melts in the ocean, contributing to sea-level rise and potentially affecting marine biogeochemistry. Icebergs have been described as ocean micronutrient fertilizing agents and biological hotspots due to their potential roles as platforms for marine mammals and birds. Icebergs may be especially important fertilizing agents in the Southern Ocean, where low 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 load is poorly constrained and it is unclear if there are regional differences. Here we show that 589 ice fragments collected from calved ice in contrasting regions spanning the Antarctic Peninsula; Greenland; and smaller tidewater systems in Svalbard, Patagonia, and Iceland have similar (micro)nutrient concentrations 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 of 0.51 µM), PO43- (median of 0.04 µM), and dissolved Si (dSi; median of 0.02 µM). In contrast, icebergs deliver elevated concentrations of dissolved Fe (dFe; median of 12 nM) and Mn (dMn; median of 2.6 nM). The sediment load for Antarctic ice (median of 9 mg L−1, n=144) was low compared to prior reported values for the Arctic (up to 200 g L−1). Total dissolvable Fe and Mn retained a strong relationship with the sediment load (both R2=0.43, p<0.001), whereas weaker relationships were observed for dFe (R2=0.30, p<0.001), dMn (R2=0.20, p<0.001), and dSi (R2=0.29, p<0.001). A strong correlation between total dissolvable Fe and Mn (R2=0.95, p<0.001) and a total dissolvable Mn:Fe ratio of 0.024 suggested a lithogenic origin for the majority of sediment present in ice. Dissolved Mn was present at higher dMn:dFe ratios, with fluxes from melting ice roughly equivalent to 30 % of the corresponding dFe flux. Our results suggest that NOx- and PO43- concentrations measured in calved icebergs originate from the ice matrix. Conversely, high Fe and Mn, as well as occasionally high dSi concentrations, are associated with englacial sediment, which experiences limited biogeochemical processing prior to release into the ocean.
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GEOTRACES: Ironing Out the Details of the Oceanic Iron Sources?
Dissolved iron (dFe) is an essential micronutrient for phytoplankton, with vanishingly low oceanic dissolved concentrations (pico- to nanomoles per kg) known to limit growth—and thus influence primary productivity and carbon cycling—over much of the surface ocean. However, because of the considerable challenges associated with contamination-free sample collection and accurate analysis of such low dFe concentrations, the first reliable dFe measurements came only in the 1980s. Further, by 2003, despite several decades of research, there were only ~25 full-depth oceanic dFe profiles worldwide, with dust considered to be the main oceanic dFe source. Since 2008, facilitated by the extensive field campaign and rigorous intercalibration of the international GEOTRACES program, there has been an “explosion” in the availability of oceanic dFe data, with hundreds of profiles now available. Concurrently, there has been a paradigm shift to a view of the marine Fe cycle where multiple sources contribute, and some forms of dFe can be transported great distances through the intermediate and deep ocean. Here, we showcase the GEOTRACES dFe datasets across the different ocean basins, synthesize our current multi-source view of the oceanic Fe cycle, spotlight sediments as an important dFe source, and look to future directions for constraining oceanic dFe boundary exchange.
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- PAR ID:
- 10534300
- Publisher / Repository:
- The Oceanography Society
- Date Published:
- Journal Name:
- Oceanography
- Volume:
- 37
- Issue:
- 2
- ISSN:
- 1042-8275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5670/oceanog.2024.416
