the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Oct 2024
Terrestrial Cosmogenic Nuclide depth profiles used to infer changes in Holocene glacier cover, Vintage Peak, Southern Coast Mountains, British Columbia
Abstract. The majority of glaciers in North America reached their maximum Holocene downvalley positions during the Little Ice Age (1300–1850 CE), and in most cases, this expansion also destroyed earlier evidence of glacier activity. Substantial retreat in the 20th and early 21st centuries exposed bedrock that fronts many glaciers that may record early-to-mid Holocene exposure and later burial by ice which can be elucidated using multiple-nuclide cosmogenic surface exposure dating. Furthermore, cores of bedrock allow the measurement of cosmogenic nuclide depth profiles to better constrain potential exposure and burial histories. We collected four bedrock surface samples for 10Be and 14C surface exposure dating and shallow bedrock cores from Vintage Peak, in the southern Coast Mountains of British Columbia, Canada. We apply a Monte Carlo approach to generate combinations of exposure and burial duration that can explain our data. Vintage Peak became uncovered by the Cordilleran Ice Sheet between 14.5 and 11.6 ka, though higher reaches on Vintage Peak retained ice until 10–12 ka before retreating to smaller than modern positions. Glaciers on Vintage Peak advanced within 100 m of late Holocene maximum positions around 4–6 ka. Poorly constrained subglacial erosion rates, possible inheritance, and variable mass shielding complicate our ability to more robustly interpret bedrock cosmogenic surface exposure histories. Nine 10Be ages on late Holocene moraines reveal that glaciers reached their greatest Holocene extents ca. 1300 CE. Our results agree with other regional glacier records and demonstrate the utility of surface exposure dating applied to deglaciated bedrock as a technique to help construct a record of Holocene glacier activity where organic material associated with glacier expansions may be absent or poorly-preserved. Further work to increase exposure/burial history modeling complexity may help to better constrain complex exposure histories in glaciated alpine areas.
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RC1: 'Comment on egusphere-2024-2900', Anonymous Referee #1, 04 Nov 2024
My general comments:This paper presents TCN-based geochronology, near Vintage Peak in the southern Coast Mountains of British Columbia, Canada, using nine moraine boulder samples, one erratic block, and four shallow bedrock cores. The main purpose of the study was to reveal the early-to-mid Holocene exposure/burial history of the sampled sites using a multiple-nuclide approach (14C, 10Be) and bedrock depth profiles. However, the complex history of ice, possible inherited nuclide inventory, and likely snow/ice/till cover make the study's outcome somewhat ambiguous. However, authors tried to resolve the glacier history at Vintage Peak, and found out (i) major ice sheet deglaciation roughly between 14.5 and 11.6 ka, ii) Holocene advancement at around 4-6 ka, and iii) further advance of ice to maximum position at the LIA (ca 1300 CE). The paper is well-written, concise, and provides all the necessary details for the community. It certainly deserves publication in the Geochronology journal but requires some major revisions, as suggested by my specific comments.Specific comments:• Explaining the complex history of alpine ice is a challenging task, but the authors should address and deal with specific complexities individually, such as till cover, inheritance, bedrock erosion, etc. to show the outcome of the study is valid. For instance, till coverage on bedrock surfaces has not been discussed, and may have influence on accumulation of cosmogenic nuclides depending of the thickness and density of till on top of sampled bedrock profiles.• The Monte Carlo approach has not been discussed in detail. Some information is presented in Ch. 4.4 and Figure 9, but it's method and results are unclear.• The connection between the LIA extent and Holocene glaciers is poorly explained. The spatial extents should be elaborated, on maps and in the text.• Cosmogenic 14C bedrock depth profiles have not been discussed in sufficient detail.Technical corrections:• Title: Since the depth profiles were taken from the bedrock, you may want to add word “bedrock” after the Nuclide, but before the depth, so that the first line of the title will read “Terrestrial Cosmogenic Nuclide bedrock depth profiles used to infer…” There are not only depth profiles in the study, 9+1 boulders, so, the current title is somewhat misleading.• L93: The Vintage peak is located at 1800 m. Its impressions look like it is located at 1600 m.• L95: On the Table 2, where the snow survey data is presented, the altitude of snow stations are different than the numbers here.• L140: SM Table 1 is related to blank samples, not batch of 10Be samples, so move the referring (SM Table 1) to the end of the sentence• The erratic boulder (17-VP-07), how would you sure that this is an erratic boulder? a rockfall from top?• L323: Most of the columns of this table (Table 1) has no units and last four columns of Table 1 have no explanations.• Table 3: It is not clear what are the numbers at the last 3 columns of Table 3?Citation: https://doi.org/
10.5194/egusphere-2024-2900-RC1 -
RC2: 'Comment on egusphere-2024-2900', Anonymous Referee #2, 20 Nov 2024
Overview:
The manuscript describes the utility and application of Terrestrial Cosmogenic Nuclides Beryllium-10 and in situ Carbon-14 (C-14) from boulder, bedrock surface, and bedrock depth profiles from cores to quantify and model glacier exposure/burial scenarios during the Holocene at Vintage Peak, Southern Coast Mountains, British Columbia. At the time of sample collection in 2017, very few, if any, published studies of portable backpack drilling for cosmogenic depth profile analysis had been done, so the application is a novel technique. Modelling for exposure/burial/erosion scenarios were leveraged to characterize glacier size in concert with modern observations of glacier extent and snow depth partial shielding. Results show uncovering from the Cordilleran Ice Sheet around 11.6-14.5 ka, and Vintage peak retaining local ice until 10-12 ka before retreating smaller than modern limits. The ice then began to regrow during the late Holocene/Little Ice Age, and then retreated to modern limits.
The technique of bedrock depth profile shows promise, and C-14 has applications for complex exposure/burial modelling due to the short half life and small mass of quartz necessary compared to Be-10. Difficulties observed during the study were limits of drill penetration due to bit type (this was a common problem with the early portable drill systems cutting bits designed for sedimentary rock) that prevented recovery of the deeper muon dominant depth section, and the relatively larger uncertainty with C-14 compared to Be-10.
The study is generally well written with occasional grammatical inconsistencies or regional word spelling variation. It is in my opinion that the geochronologic techniques applied, in particular the application of in situ C-14 in bedrock depth profiles, is a novel and powerful technique that can and should be applied in more studies. This study tested the method and experienced some difficulties but exhibits the utility and need for additional efforts in furthering in situ C-14 and depth profiles. There are additional analyses that could have been done regarding C-14 that are currently in the literature but were not discussed as part of this study and may provide merit, but the authors state there is future work that could be applied to better understand the data. The scientific quality is good, and additional analyses would improve the study, but the results and techniques presented are appropriate for the conclusions made. There is a concern about inheritance and incomplete nuclide resetting that may be addressable through additional evaluation of C-14 modelling from Secular equilibrium (ex Schweinsberg et al,2018; Graham et al, 2019; Sbarra et al, 2022) or consideration of deep muon long term production (Briner et al, 2016; Balter-Kennedy et al, 2021). The presentation quality is good, and minor revision would improve the quality of the manuscript.
General notes:
There are some parts of the manuscript that could be improved. In general, the application mindset of the utility of cosmogenic nuclide analysis appears to be strictly focusing on the method as a chronology technique, however, studies referenced in the paper expand on using the method for more than just chronology (ex Balter-Kennedy et al, 2021, Graham et al, 2023). Part of the utility of depth profile analysis and modelling is the a priori knowledge of the glacial history to constrain the modelling parameters. When referring to the depth profile, it may be more accurate to refer to it as “cosmogenic nuclide analysis” or “analysis” rather than “dating” because the methods can infer more than an age. Alternatively, the authors could refer to the measured concentrations as “relative exposure ages” for comparison of shielded samples that are lower than the expected age if the sample was not affect by additional mass depth (example Graham et al, 2023). “Relative exposure ages” can be useful in relating a more audience understandable metric of difference.
Another general note is to uniformly use the term “modelling” vs “modeling”. I recommend using “modelling” throughout as it appears to be more universal amongst international audiences whereas “modeling” is more generalized to the United States.
Comments per line number:
31 – You reference the Little Ice Age for the first time in the body of the paper outside the abstract, but do not provide the shorthand of (LIA). Add (LIA) after you name Little Ice Age, and give the age range. (1300-1850 CE).
36 & 39 – “Cosmogenic surface exposure dating” - consider changing to “Cosmogenic exposure analysis” because the methods applied in the manuscript to evaluate more than just the surface and dating.
43 – Consider stating an approximate duration of burial required to reset or “forget” previous exposure (Hippe, 2017, Graham et al, 2019). This assumption that C-14 is completely reset during LGM burial may not be valid in some samples.
45 – Consider referencing previous field based applications to model burial (Goehring et al, 2011, 2013; Schweinsberg et al, 2018; Pendleton et al, 2019; Graham et al, 2019; Sondergard et al, 2020; Sbarra et al, 2022; etc.)
50-52 – Include references to those studies
Something not clearly addressed is the potential value-added information from collecting bedrock cores or subsurface samples for depth profile analysis. Why should you go to the trouble of bringing 30 kgs of equipment up the side of a mountain, scrounge for cooling water, and spend all day manually drilling to collect the core? What does the core give you that a bedrock surface sample doesn’t? Please explain. Consider referencing Schaefer et al, 2016 in addition to Goehring et al 2013, Balter-Kennedy et al, 2021 and Graham et al, 2023
110 – Is the lidar data shown in the manuscript? Please include the figure number.
116 – What type of drill was used? A portable electric hand drill or a gasoline powered system?
120 – Consider changing “dating” to “measurements”. The samples from the core are not being measured for ages, but concentrations that are correlated and represent the same exposure history with different shielding parameters. The concentrations from the core are what is used in the model, not the age. In general, expand on the utility of cores.
184 – Identify in the text which erratic boulder was used.
187 – State the Figure number for the two isotope plot.
Additional details about the Monte Carlo simulation would be helpful.
194 – 50 meter ice thickness is considered sufficiently thick to cease c-14 production from muons. The authors state below (Line 228) that ice was likely 35-40 m thick. Production likely did not cease, but was probably not significant (beyond the uncertainty of the measurements) at the maximum extent. As ice thinned, and if it maintained thickness below 10 meters, it can become significant (Pendleton et al, 2019).
220-221 – modelled and modeled used in the same sentence. Fix throughout.
245 Figure 4 caption – Consider removing in situ from the organic in situ 14C samples because it is more common to see in situ referring to in situ cosmogenic C-14 rather than organic. Stating organic 14C should be sufficient and likely more consistent with other literature.
260 Figure 5 – This is a great figure and it is very informative that this data was made available for this study.
264 – Is there a classic canonical definition for the LIA? State clearly when the beginning is.
266, 270, 271, 272 – The authors flip between ka (years before present) and CE (years after 0 CE). Please be consistent throughout the paper and reference events in ka or a as defined earlier in the paper. If CE is beneficial to link to other worldly events, accompany with ka and have the CE date in ().
276 – consider replacing “some” with “approximately”. Also, “returned” is very informal and colloquial, as a boulder did not deliver the age. Consider replacing with “has a calculated age of… for 10Be and …
285 – This is a great analysis and it supports the snow data over longer time periods.
Table 1 – It appears that a second row in the headers is missing.
If there isn’t a second row missing, what is the difference between Exposure and Exposure Age?
For the 10Be/9Be ratio, 1 sigma, Blank-corrected, Blank-corrected (assuming 1 sigma), consider using the same 10^x for each column. Cosmo tables should be formatted to easily copy and paste from the manuscript into a spreadsheet for uniform analysis. It also maintains significant figures between samples and allows the reader to quickly see which samples are orders of magnitude different. I recommend: 10^-14, 10^-15, 10^04, 10^02, respectively.
Figure 8 – Consider showing the measurement points as a box with depth to represent the integrated measurement thickness of each sample. Refer to Schaefer et al, 2016 as an example.
384 – Consider simplifying to insolation, as “solar insolation” is redundant as insolation refers to the input from the Sun.
401 – Consider replacing “outside” with “outboard”
An assumption made throughout the paper is minimal sub-glacial erosion, and no inheritance. If there is inheritance of Be-10 in bedrock samples, is the duration of burial during the LGM and depth of erosion during the LGM sufficient enough to reset the C-14 system to background levels? If it is not, can you model to account for that?
There are other assumptions made that may not be entirely valid, however, if those assumptions are not fully valid, the effect of the assumptions likely does not significantly impact the results presented. Consider reviewing all assumptions and state the effect if those assumptions are not valid.
Citation: https://doi.org/10.5194/egusphere-2024-2900-RC2
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