Limited atmospheric iron availability increase during the Pleistocene-Holocene transition in the Northern Hemisphere

Burgay, François; Derrod, Haley; Erhardt, Tobias; Scoto, Federico; Segato, Delia; Maffezzoli, Niccolò; Dallo, Federico; Zannoni, Daniele; Spagnesi, Azzurra; Kjær, Helle-Astrid; Fischer, Hubertus; Varin, Cristiano; Barbante, Carlo; Spolaor, Andrea

doi:10.5194/egusphere-2025-6339

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https://doi.org/10.5194/egusphere-2025-6339

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27 Jan 2026

| 27 Jan 2026

Status: this preprint is open for discussion and under review for Climate of the Past (CP).

Limited atmospheric iron availability increase during the Pleistocene-Holocene transition in the Northern Hemisphere

François Burgay, Haley Derrod, Tobias Erhardt, Federico Scoto, Delia Segato, Niccolò Maffezzoli, Federico Dallo, Daniele Zannoni, Azzurra Spagnesi, Helle-Astrid Kjær, Hubertus Fischer, Cristiano Varin, Carlo Barbante, and Andrea Spolaor

Abstract. Iron (Fe) availability modulates phytoplankton blooms in High-Nutrient Low-Chlorophyll (HNLC) regions, i.e., ocean areas characterized by an abundance of major nutrients but low marine productivity. Fe can be delivered to the oceans through atmospheric dust deposition, making ice cores unique archives for reconstructing past changes in aeolian Fe deposition. However, while it is known that during dustier periods atmospheric Fe depositions increased, uncertainties remain regarding the fraction of Fe actually available to phytoplankton. Here, we present evidence from the EGRIP ice core (Greenland), which allows insights into atmospheric aerosol deposition over the Fe-limited North Pacific Ocean, during the Pleistocene-Holocene transition (10.3–13.0 ka). Results show that, in contrast to the 17-fold enhancement in total Fe concentration, dissolved Fe increased only modestly (+29 %) during the Younger Dryas compared to the Early Holocene, likely due to prevailing alkaline aerosol conditions reducing its solubility. This finding supports the hypothesis that factors other than atmospheric Fe deposition (e.g., stronger water stratification, sea-ice extent, volcanic eruptions, iron remobilization from sediments), play a more relevant role in regulating marine net primary productivity in the HNLC North Pacific Ocean over the last glacial transition.

Received: 18 Dec 2025 – Discussion started: 27 Jan 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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RC1:
'Comment on egusphere-2025-6339', Anonymous Referee #1, 12 Feb 2026 reply
This manuscript presents an important and timely contribution to our understanding of atmospheric iron deposition and its potential role in regulating marine productivity during the Pleistocene–Holocene transition. By generating the first continuous records of FeICP and operationally defined dissolved iron (DFe) from the EGRIP ice core, the authors provide a high-resolution perspective on how iron speciation, rather than total iron flux alone, varied across a major climatic transition. The study is carefully executed, clearly written, and well situated within the long-standing debate surrounding the “iron hypothesis” and its regional expression in HNLC systems. In particular, the finding that dissolved iron increased only modestly during the Younger Dryas, despite a large enhancement in total iron, represents a valuable constraint on the effectiveness of aeolian iron fertilization in the North Pacific region.

Overall, this study represents a significant methodological and conceptual advance. By shifting the focus from total iron flux to iron solubility and chemical form, the authors provide a more nuanced framework for evaluating the climatic impact of atmospheric iron deposition. With minor clarifications regarding bioavailability and broader oceanographic implications, this manuscript will be of high interest to the paleoclimate, biogeochemistry, and Earth system science communities.
1. Age model and chronological constraints
How sensitive are the observed millennial- to centennial-scale variations in FeICP and DFe to uncertainties in the EGRIP age model?

Could short-term depositional variability or age-model smoothing influence the alignment between iron speciation, acidity, and climatic transitions (e.g., the Younger Dryas)?

2. Analytical methods and proxy interpretation
How robust is the operational definition of CFA-derived DFe as a “labile” iron fraction across different aerosol sources and depositional conditions?

Are there potential analytical or post-depositional processes in snow and firn that could alter iron solubility or speciation prior to measurement?

Can the authors further clarify how volcanic versus dust-derived iron is distinguished analytically, and whether their dissolution behavior differs under the CFA protocol?

3. Bioavailable iron and global implications
How do the authors evaluate the extent to which CFA-DFe represents iron that is truly bioavailable to marine phytoplankton, rather than merely chemically soluble?

What additional constraints—such as iron speciation analyses, ligand-binding considerations, or experimental dissolution and incubation studies—would be required to directly link ice-core DFe to biological uptake in the ocean?

Given that the ice-core record integrates long-range atmospheric transport, how representative are the inferred iron solubility patterns for North Pacific?

Can the authors elaborate on the pathways by which similar iron species are deposited in both marine sediments and ice cores, and how post-depositional processes in snow, firn, and seawater may differentially modify iron speciation?

4. Lastly, one minor comment: although the study period extends slightly beyond the Holocene, it represents only a very limited interval of the late Pleistocene. As such, the term “Pleistocene–Holocene” in the title may be somewhat misleading with respect to the actual temporal scope of the study. I suggest revising the title to refer more specifically to the “last deglaciation” or to explicitly highlight the focus on the Younger Dryas interval.

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Citation: https://doi.org/10.5194/egusphere-2025-6339-RC1

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Short summary

With this work, we provide important evidence that the Iron Hypothesis is less effective than originally thought in the Northern Hemisphere due to alkaline aerosol conditions. Therefore in the High-Nutrient Low-Chlorophyll North Pacific Ocean, other factors such as sea-ice extent or iron remobilization from sediments may have played a more relevant role than atmospheric iron fertilization in modulating marine primary productivity.


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