Brief communication: Rb-Sr dating and provenance of ice-drafted dropstones from the mid-Atlantic ridge at mid-latitudes (36&deg;12&rsquo;N; 33&deg;53&rsquo;W)

Gyomlai, Thomas; Andreani, Muriel; Caujolle, Emeline; Debret, Baptiste; Puzenat-Vassor, Valentine; Ildefonse, Benoit; Fellah, Clémentine

doi:10.5194/egusphere-2026-3184

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https://doi.org/10.5194/egusphere-2026-3184

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09 Jul 2026

| 09 Jul 2026

Status: this preprint is open for discussion and under review for The Cryosphere (TC).

Brief communication: Rb-Sr dating and provenance of ice-drafted dropstones from the mid-Atlantic ridge at mid-latitudes (36°12’N; 33°53’W)

Thomas Gyomlai, Muriel Andreani, Emeline Caujolle, Baptiste Debret, Valentine Puzenat-Vassor, Benoit Ildefonse, and Clémentine Fellah

Abstract. Understanding past iceberg activity during the Quaternary is key to constraining the spatial pattern and dynamics of ice sheets, as well as the amount and distribution of freshwater released during iceberg melting. These mechanisms significantly influence global climate and are primarily constrained by the records of ice-rafted deposits concentrated around ~40–50° N. The significance of iceberg transport at lower latitudes and its potential impact on the Atlantic Ocean circulation remain poorly constrained. In this study, we document three ice-rafted dropstones recovered from the Mid-Atlantic Ridge at mid-latitudes (~36°12’N), which could originate from various localities within the Laurentia or Baltica cratons. Their latitude of deposition is consistent with areas of low‑iceberg concentration during Heinrich events, according to published numerical models of iceberg transport and melting. The samples form a calc‑alkaline magmatic series (monzogranite, granodiorite, and tonalite) and yield in‑situ Rb–Sr ages of ~1700–1630 Ma. This suggests that they derive from the Late Paleoproterozoic Labradorian arc‑accretion orogeny exposed in eastern Canada. This provenance further suggests a relatively low‑altitude iceberg source in the Gulf of Saint Lawrence, favoring iceberg transport to lower latitudes.

Received: 02 Jun 2026 – Discussion started: 09 Jul 2026

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Thomas Gyomlai, Muriel Andreani, Emeline Caujolle, Baptiste Debret, Valentine Puzenat-Vassor, Benoit Ildefonse, and Clémentine Fellah

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'Comment on egusphere-2026-3184', Pierre Dietrich, 25 Aug 2026 reply
I have read and reviewed the brief communication by Thomas Gyomlai and co-authors, titled ‘Rb-Sr dating and provenance of ice-rafted dropstones from the mid-Atlantic Ridge at mid-latitude’ submitted to The Cryosphere. The paper presents dating of felsic clasts collected from the mid-Atlantic Ridge, interpreted as ice-rafted debris transported by icebergs calved from landmasses during Heinrich events and drifted to mid-latitudes in the Atlantic Ocean. Based on these dates, the authors infer the provenance of the icebergs and the ice masses that sourced and calved them. As such, this contribution brings new light on these particular episodes of iceberg armada release and the associated climatic events. First, I must clarify that, as a glacial sedimentologist, I lack the expertise to critically assess the petrological aspects of the paper or the robustness of the dating techniques. I am however well-positioned to evaluate the glacial interpretations and related discussions. Overall, this manuscript represents an important contribution to the understanding of iceberg drift at low latitudes and I therefore recommend the manuscript for publication with moderate revision for which I have listed below a few major and minor concerns that should in my opinion be addressed before the paper can be published.
Major concerns
The authors describe and have dated dropstones, which they interpret as ice-rafted debris (IRD) linked to Heinrich events—massive iceberg discharge episodes in the Atlantic Ocean, originating from the decaying Laurentide or Fennoscandian ice sheets. However, the manuscript lacks critical information about age constraints for the deposition of these IRD. Without this, readers are left uncertain as to which specific Heinrich event(s) (or if they relate to Heinrich Event at all, or to other episodes of iceberg release) the samples correspond to. Over the last ~60,000 years (i.e., the Last Glacial Period), at least six Heinrich events have been identified (H1–H6; e.g., Martin et al., 2023, Nature https://doi.org/10.1038/s41586-023-05875-2), and similar events likely occurred during earlier glacial stages. I therefore strongly recommend that the authors add radiometric or stratigraphic age data for the sedimentary layers containing the IRD, if available, to clarify the temporal context. The only mention to an age would be line118 “The three felsic samples were dredged on the young seafloor (<0.78Ma)” but this range remains too loose, as over the last 780 kyr, 7-8 glacial periods occurred. If dating is not feasible, I would suggest to discuss the possible age range of the deposits and its implications for iceberg drift pathways and provenance. This would help readers assess whether the IRD relates to known Heinrich events or to previously undocumented episodes (?). Additionally, the authors should clarify whether the lonestones were embedded within sedimentary layers (whose age would correspond to the deposition of the IRDs) or recovered as isolated clasts. Given that the original dredging targeted – if I am correct – the structural and hydrothermal features of the Mid-Atlantic Ridge (Andreani et al., 2014), this sedimentary context seems missing (although elusively mentioned, line 52-53 ‘pebbles associated with pelagic sediments’) yet essential to interpret the depositional environment and rule out alternative transport mechanisms. Finally, pictures of the dropstones would also be appreciated in this context, if possible, especially as most sedimentary witnesses for Heinrich layers are sand-sized sediments scattered within oceanic mud and therefore pebbles that large showcased in this contribution are quite unusual (does that mean that icebergs drifted very rapidly?)

The authors interpret the pebbles they dredged from the MAR as IRDs partly because of their chemical composition and state line 121 that ‘This explains their exotic composition for an oceanic setting notably their higher potassium content (K2O>1wt%)’. However, this assertion lacks clarity for non-specialists in the geochemistry of oceanic crust. To strengthen their interpretation, the authors should clarify why a K₂O content > 1 wt% is considered exotic in this context. For example, they could compare it to the typical K₂O content of local oceanic crust materials and showcase that IRDs composition clearly departs from these local sources. Also, it seems that felsic rocks can be found on the oceanic sea floor (?) and therefore a felsic composition alone does not seem diagnostic of an exotic provenance. While the ancient age (1.6-1.7 Ga) of these samples strongly suggests they are not locally sourced from the Mid-Atlantic Ridge (lines 118-122), the authors’ emphasis on geochemistry warrants further clarification. In line with this, the authors may also briefly clarify why the original explanation for these outstanding lonestones was of local origin (line 52-53 ‘They were first interpreted as oceanic felsic rocks’).

Improvement suggestion
Based on the chemical composition and ages of the lonestones, the authors propose Canada and Fennoscandia as the two most likely source regions for the IRDs. To further refine this interpretation, I recommend that the authors examine maps of ice streams that drained these ice sheets during different stages of retreat. Ice streams—fast-flowing corridors that can span tens (to hundreds!) of kilometers in width and thousands in length—fed floating ice shelves, whose collapse released vast quantities of icebergs into the Atlantic Ocean, triggering Heinrich events. Comparing the potential source regions of the MAR lonestones with the spatial and temporal distribution of ice streams could help favor one hypothesis over the other. For the Laurentide Ice Sheet ice streams, Margold et al., 2015 (http://dx.doi.org/10.1016/j.earscirev.2015.01.011) provide detailed reconstructions of ice stream activity, including shifts in their locations during deglaciation (see their Figure 1). For the Fennoscandian Ice Sheet, Patton et al. 2017 (http://dx.doi.org/10.1016/j.quascirev.2017.05.019) offer insights into surface velocity patterns (eg Figure 4), which could help assess the likelihood of IRD transport pathways. This point also ties back to my primary concern regarding the dating of the deposits: ice streams are highly dynamic features, with their spatial extent and activity shifting rapidly over the course of a deglacial cycle (e.g., Margold et al., 2015, Figure 1). Conversely, if the authors can provide robust source area constraints and deposit ages, these lonestones could also serve as an additional proxy for reconstructing ice stream dynamics.
For example, if the lonestones are dated to 20–10 kyr (ie HE1 dated to ca. 16 kyr), the Trans-Labrador Batholith may have been eroded and pieces of it were dragged under the Laurentian Channel Ice Stream or one of its tributaries, which ultimately fed the Laurentian Ice Shelf. The collapse of this ice shelf released icebergs into the North Atlantic, aligning with the Heinrich event framework (you may also have a look at the quite old now paper by J. Shaw et al., 2006 https://www.sciencedirect.com/science/article/abs/pii/S0277379106001326 their figure 12 picturing the collapse of the Laurentian Ice Shelf and the release of icebergs). Of course this interpretation is preliminary, done very rapidly with the info I had at hand and should be treated with caution but exemplifying how the reasoning could be done. In contrast, the Transcandinavian Igneous Belt appears a less likely source for the IRDs on the MAR because it was situated near (beyond?) the head of the North Sea Ice Stream drainage basin, which directed icebergs toward the North Sea and, ultimately, the Atlantic. This region seems to have been more drained by the Baltic Ice Stream, ultimately draining into the Baltic Sea, making it a less probable pathway for IRD reaching the Mid-Atlantic Ridge. Again, this assessment is tentative and requires further validation.
Minor concerns
Line 23 ‘important volumes’: please clarify what important volumes mean when referring to ice-rafted debris.
Line 23: You may want to add up a little on the ‘polar gyre’
Line 26, in line with my major comment 1, you may expand a bit on the timing of Heinrich events.
Line 164-166 ‘Their latitude of deposition is compatible with modelled low-concentration iceberg zones during Heinrich events’ => this sentence seems to partly conflict with the paragraph lines 34-40 where it is being implied that the presence of low-latitude icebergs is not well-reproduced by numerical modeling. Perhaps the sentences need clarification.
Pierre Dietrich,
Rennes, France, August 24, 2026

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Citation: https://doi.org/10.5194/egusphere-2026-3184-RC1

Thomas Gyomlai, Muriel Andreani, Emeline Caujolle, Baptiste Debret, Valentine Puzenat-Vassor, Benoit Ildefonse, and Clémentine Fellah

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https://doi.org/10.5194/egusphere-2026-3184-supplement

Thomas Gyomlai, Muriel Andreani, Emeline Caujolle, Baptiste Debret, Valentine Puzenat-Vassor, Benoit Ildefonse, and Clémentine Fellah

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Short summary

Understanding past iceberg activity during the Quaternary is key to constraining ice‑sheet dynamics and freshwater release during melting. We document ice‑rafted dropstones dredged at 36° N on the Mid‑Atlantic Ridge that form a calc‑alkaline magmatic series and yield ages of ~1700–1630 Ma. This indicates derivation from the Labradorian arc‑accretion orogeny in eastern Canada, implying a source in the Gulf of Saint Lawrence that favored southward iceberg transport to lower latitudes.


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