the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jul 2024
In situ Cosmogenic 10Be and 26Al in Deglacial Sediment Reveals Interglacial Exposure, Burial, and Limited Erosion Under the Quebec-Labrador Ice Dome
Abstract. To understand the erosivity of the eastern portion of the Laurentide Ice Sheet and the isotopic characteristics of the sediment it transported, we sampled buried sand from deglacial features (eskers and deltas) across eastern Canada (n = 10), a landscape repeatedly covered by the Quebec-Labrador Ice Dome. We measured concentrations of 10Be and 26Al in quartz isolated from the sediment and, after correcting for sub-surface cosmic-ray exposure after Holocene deglaciation, used these results to determine nuclide concentrations at the time the ice sheet deposited the sediment. To determine what percentage of sediment moving through streams and rivers currently draining the field area was derived from incision of thick glacial deposits as opposed to surface erosion, we used 10Be and 26Al as tracers by collecting and analyzing modern river sand sourced from Holocene-exposed landscapes (n = 11).
We find that all ten deglacial sediment samples contain measurable concentrations of 10Be and 26Al equivalent on average to several thousand years of surface exposure – after correction, based on sampling depth, for Holocene nuclide production after deposition. Error-weighted averages (1 standard deviation errors) of measured 26Al/10Be ratios for both corrected deglacial (6.1 ± 1.2) and modern sediment samples (6.6 ± 0.5) are slightly lower than the production ratio at high latitudes (7.3 ± 0.3) implying burial and preferential decay of 26Al, the shorter-lived nuclide. However, five deglacial samples collected closer to the center of the former Quebec-Labrador Ice Dome have much lower corrected 26Al/10Be ratios (5.2 ± 0.8) than five samples collected closer to the former ice margins (7.0 ± 0.7). Modern river sand contains on average about 1.75 times the concentration of both nuclides compared to deglacial sediment corrected for Holocene exposure.
The ubiquitous presence of 10Be and 26Al in eastern Quebec deglacial sediment is consistent with many older-than-expected exposure ages, reported here and by others, for bedrock outcrops and boulders once covered by the Quebec-Labrador Ice Dome. Together, these data suggest that glacial erosion and sediment transport in eastern Canada were insufficient to remove material containing cosmogenic nuclides produced during prior interglacial periods both from at least some bedrock outcrops and from all glacially transported sediment we sampled. Near the center of the Quebec-Labrador Ice Dome, ratios of 26Al/10Be are below those characteristic of surface production at high latitude. This suggests burial of the glacially transported sediment for at least many hundreds of thousands of years and the possibility that ice at the center of the Quebec-Labrador Ice Dome survived many interglacials when more distal ice melted away.
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RC1: 'Comment on egusphere-2024-2233', Jason Briner, 25 Aug 2024
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Cavnar et al produced 10Be and 26Al measurements in sediment deposits sampled from varying distances along paleo-flowlines of the former Quebec-Labrador dome of the Laurentide Ice Sheet. Varying amounts of nuclide inheritance in both modern sediments and early Holocene landforms allow the authors to make inferences about limited glacial erosion in this region during past glaciations. Based on prior work in this and other ice-sheet dome centers, it is expected to find inheritance signals; however, the exact quantities may be relevant for crn tracer studies in offshore sediment cores.
That this paper does not relate to geochronology leads me to be somewhat neutral about the journal choice.
I liked this paper a lot. I enjoyed reading it and found it grammatically clean. The results were interesting to read about and mull over. The figure and table support is solid.
I have several comments that they authors could consider.
1) This is great work, I wonder if the pitch for relevance to offshore work could be amped up. The title could be more informative, for example, by including a conclusion. As I write in my synopsis above, it strikes me that the findings are not highly novel, perhaps even only mildly informative, when couched in terms of inferences on past ice sheet exposure/burial/erosion. I don’t think anyone would expect a story from the sediments to be any different than that from surface boulder and bedrock crn data, in addition to other geomorphological datasets, that already exist. That said, it seems to me that the real new information, and more important motivation for this work, is to inform new offshore sediment records like those of LeBlanc et al (2023) and others. To this end, I would suggest being more up front about this motivation (like including this impetus in the abstract and giving it more emphasis throughout the paper; it’s there, but I found it not front and center).
2) I don’t see how the Al/Be data in this study can be used to support claims that ice persisted throughout interglacials (and I sort of disagree that the community thinks that such persistence would be unexpected). It seems to be a conclusion from the LeBlanc work, but the data in this study standing alone, I don’t see the evidence. In fact, when considering the Al/Be ratios at the proximal deglacial sites (5.2±0.8) against Figure 3 in LeBlanc et al (2023), it seems more plausible for every interglacial to have experienced many thousands of years of exposure. If making this point and further supporting LeBlanc is important to the team, I suggest some exposure/burial history modeling to see if these crn data uniquely support persistent interglacial ice cover. If not, it just seems like rather unsupported bolstering of a previous finding by this research group. Either way, I think the study stands just fine without pushing this conclusion.
Line by line comments
13, don’t really need the word ‘buried’ here
18, ‘surface erosion’ of bedrock? Or of sediments, too?
22, ‘average several thousand years’. The average and standard deviation of other measurements are reported numerically; I suggest doing the same with this one – that is, report the average and standard deviation of inheritance in terms of years.
22, could remove words from ‘after correction..’ I find this multiple paragraph abstract long and wordy. I think it could be made much more succinct. Readers can find the details in the manuscript body.
31, could just say ‘reported by others.’ ‘reported here’ is a little redundant in this sentence since the first part states that there is inheritance in this study’s dataset.
44, I don’t think anyone honestly thinks land-based ice domes like these ‘collapsed’, suggest other wording. Also, find other wording for the ‘northern Canadian ice domes’. What are these, like the Barnes and Agassiz ice caps?
48, ‘Because’ sentence.. suggest rewording. ‘far north’ but then study Rice study, which is arguably the southernmost of the three LIS domes. Be specific.
58, to one of my main points, this paragraph closes to say results could be important for offshore work. I think this paper would be stronger this point was more directly as a major impetus of the work. I guess after reading the entire manuscript, I felt that the work was done as a companion for offshore studies, but I did not get that sense while reading it.
80, section 2.1, this is subjective not objective comment, but covering the basics of Al/Be methodology seems potentially below the readership. Critical to include in student writings, but not necessary in disciplinary manuscript.
115, reword, not sure ‘for inception to deglaciation, the LIS was…’ makes grammatical sense.
122, 137, the content in these two paragraphs are the same, but are separated by a paragraph on a different topic, suggest combining and streamlining section 2.2
126, suggest write ‘not as well constrained by comparison’
128, I’m not sure I would characterize this as “it is debated.” The Zhou and McManus reference is missing, I didn’t look up what they write.
131, What is meant by “these”? Prior sentence cites LeBlanc but the data aren’t mentioned in order to justify use of “these”
135, ‘commonly held assumption’ I disagree with. The Holocene technically isn’t fully ice free (eg, Barnes ice cap contains Pleistocene ice) anyway. As a rule, I don’t think it’s entirely responsible to push a controversy to elevate importance of one’s work (I wince a little reading this after I wrote it, but I do think it can be said here, sorry if harsh). And as LeBlanc point out, one look at those mild middle Pleistocene interglacials, why would one think that every interglacial was absolutely ice free in the first place? I, for one, think the persistence of small ice caps/domes in the three LIS centers, like today on Baffin but even a larger Barnes, during may Pleistocene interglacials is actually highly likely.
166, who’s ‘they’?
197, after reading this section, some relevant research could be included. I would think that the abundant literature on bedrock crn inventories from ice sheet settings showing inheritance is worth citing. As are locations within ice sheet boundaries where pre-Holocene sediment has been preserved (Miller et al last interglacial sediments in lakes on Baffin and Greenland, and maybe look into old sediments preserved in the Manicouagan Reservoir, I seem to recall a presentation showing old seds right there in the heart of Labrador).
234, love this sentence, to repeat the above, would add emphasis to this earlier in the paper
Figure 1, Panel A says “text” in N Atlantic. Box showing area of B is incorrect. Overall could be improved significantly, make larger for starters, no need to be so small – really important figure here. The dashed ice sheet extent lines are pretty difficult to decipher (why dashed lines?). Panel B is crucial for this work, should be improved, put on shaded DEM perhaps, maybe even color, would be nice to see elevation of the transects and the overall terrain. Dome center does not coincide with ‘last ice location’ - why not? Wouldn’t the retreat center (location of last ice) make more sense if interested in exposure/burial durations?
322, would be nice to see in this table - the % correction for Holocene exposure. So many samples are from generally great depths, I would not expect there to be much correction, still, would be nice to show to readers.
361, ‘with distance from center’ note comment above, I question the choice of ‘center.’ Furthermore, distance from center is a little awkward, it only makes partial sense. Plotting distance along flowline would be better. Or by retreat age would be best. Figure 3c shows two deglacial samples within the 9 cal ka ice limit that have ratios >8. In the south transect, samples in similar positions (just within 9 ka limit) have ratios of 5.5 and 3.5. Seems like data plotted along paleo-flowline distances would make most sense?
Figure 5, could be two side by side plots, one with modern and one with deglacial. Would be cleaner, for example, to symbols don’t look different in grayscale (how I printed it out). Also suggest for all plots, sticking with blue/red/purple color scheme in map figures, why not carry that color scheme over to the data plots? Can you add the bedrock sample?
403, I find it awkward to take an ‘all sample average’ for samples that are at different distances and with obviously different exposure/burial histories. It only occurred to me later that you would even bother taking a total average because you could consider this an integrated value for what might make it to the ocean. This is one example of how stronger emphasis on the offshore work (and included as a justification in the abstract, if not the title) might make more of your thinking here in the thick of data reporting make better sense. To me anyway. In the abstract, upon first read, I was like, ‘why in the world would you average this value?’
404, neither 6.1±1.2 nor 6.6±0.5 are statistically different than 7.3±0.3.
405, ‘consistent with burial after initial exposure’ It could also be that the sediments themselves were emplaced at time zero with a burial signal. Isn’t that the nature of sediment work, you don’t know what’s post deposition vs. pre deposition? Or?
Figure 6, love seeing the LeBlanc panel here, again emphasizes the importance of this work for the offshore stuff. Panel C I don’t see any justification for the splitting of these samples into two groups. Why not three groups, or five groups? What is the threshold that allows distinction of “proximal” v “distal,” and again consider distance as was measured versus flowline distance.
465, ‘cosmogenic dating’ seems lazy phrasing
469, Briner and Swanson 1998 was neither on the LIS or Fennoscandia/Greenland.
472, There are probably >1000 of crn data published from Baffin, some papers included many data on sediments (boulders, cobbles, pebbles; some of mine, reach out if you have questions), why point to one cobble result from the Davis study? Throughout this paper, there is a bias toward citing authors’ own work. To some degree this is difficult to avoid, certainly I’m guilty too at times.
483, this paragraph and previous paragraph, difficult to summarize all this work in 1-2 paragraphs. That said, the tactic taken, to cite individual studies and individual samples, seems destined to be highly exclusive. Is there a way to discuss this more broadly and lean more on synthesis papers? It’s kinda like the main author knew a few papers really well, so leaned toward citing those again and again instead of a more comprehensive view of the literature. Of course this is easy for me to write, and much more difficult implement, but I’ll throw it out there.
507, not really ‘remnants’ per se. If ice shrinks to become smaller but in balance with a new climate state, does it make it a ‘remnant’?
507, ‘must’ seems strong. Language in the LeBlanc paper uses no such strong language, careful to re-write history.
515, replace ‘taken from outcrops’ with ‘bedrock’ (could add “eg” in front of Marsella ref, again back to comment about citing authors’ own work a little heavily)
526, I do not believe your data allow you to make these conclusions. See earlier comment at top of review. Concentrations and ratios of ~5 can be easily explained with burial during glacials, even figure 3 in LeBlanc shows this. If there is anything close to a flaw in this great paper, it is here in my opinion. Unless you perform some modeling that says the data can are uniquely explained by ice cover persisting throughout during interglacials could you use language used in this paragraph.
561, to repeat, I do not see this statement being supported by data, and I fear this work will be cited in the future to support such a notion, but careful reading reveals no such support.
Jason Briner
Citation: https://doi.org/10.5194/egusphere-2024-2233-RC1
Data sets
Measuring in situ Cosmogenic Beryllium-10 and Aluminum-26 in Deglacial Sediment Reveals Limited Erosion Under the Quebec-Labrador Ice Dome, Canada, 2022 - 2024. Arctic Data Center Peyton Cavnar, Paul Bierman, Jeremy Shakun, Lee Corbett, Marc Caffe, Gillian Galford, and Danielle LeBlanc https://www.doi.org/10.18739/A2X34MT66
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