the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 01 Feb 2024
The Laurentide Ice Sheet in southern New England and New York during and at the end of the Last Glacial Maximum – A cosmogenic-nuclide chronology
Abstract. We present 40 new 10Be exposure ages of moraines and other glacial deposits left behind by the southeastern sector of the Laurentide Ice Sheet (LIS) in southern New England and New York, summarize the regional moraine record, and interpret the dataset in the context of previously published deglaciation chronologies. The regional moraine record spans the Last Glacial Maximum (LGM), with the outermost ridge of the terminal complex dating to ~26–25 ka, the innermost ridge of the terminal complex dating to ~22 ka, and a series of smaller recessional limits within ~50 km of the terminal complex dating to ~21–20.5 ka. The chronology generally agrees with independent age constraints from radiocarbon and glacial varves. A few inconsistencies among ages from cosmogenic-nuclide measurements and those from other dating methods are explained by geologic scatter where several bedrock samples and boulders from the outer terminal moraine exhibit nuclide inheritance, while exposure ages on large moraines are likely affected by postdepositional disturbance. The exposure-age chronology places the southeastern sector of the LIS at or near its maximum extent from ~26 to 21 ka, which is broadly consistent with the LGM sea-level lowstand, local and regional temperature indicators, and local summer insolation. The net change in LIS extent represented by this chronology occurred more slowly (<5 to 25 m yr-1) than retreat through the rest of New England, consistent with a slow general rise in insolation and modeled summer temperature. We conclude that the major pulse of LIS deglaciation and accelerated recession, recorded by dated glacial deposits north of the moraines discussed here, did not begin until after atmospheric CO2 increased at ~18 ka, marking the onset of Termination 1.
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RC1: 'Review of The Laurentide Ice Sheet in southern New England and New York during and at the end of the Last Glacial Maximum – A cosmogenic nuclide chronology', Christopher Halsted, 01 Mar 2024
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Summary
In this manuscript, Balter-Kennedy et al. date the timing of southeastern Laurentide Ice Sheet (LIS) maximum advance and initial retreat using 40 new cosmogenic nuclide exposure ages from southern New England and New York. They provide a comprehensive background of the geomorphology of ice-contact features marking the maximum extent and initial retreat of the southeastern (LIS), including the extensive terminal moraines and prominent recessional moraines, and existing chronologic constraints for these features. Their new exposure ages generally agree with previous dating constraints, albeit with some isolated instances of old or young ages that are attributed geologic scatter. Importantly, Balter-Kennedy et al. infer a lack of more widespread inheritance in their sample populations based on the normal distribution of exposure ages at each sample site. Skewed distributions characterize sample populations at other sites near the Laurentide Ice Sheet terminal moraine, and samples from these sites are thus believed to contain some unknown amount of nuclide inheritance. The ages presented here confirm previous estimations that the southeastern Laurentide Ice Sheet remained near its maximum extent from 26 – 21 ka, with marginal areas exhibiting minor fluctuations during this time. The exact timing of initial retreat likely varied across this region, but generally occurred around 22 ka. This initial ice recession was slow (5 – 25 m/yr) and likely triggered by regional climatological changes such as the steady increase in local summer insolation that began around 24 ka and the slow increase in local summer temperatures that began around 26 ka. This ice margin recession represented a small areal loss of the overall Laurentide Ice Sheet (<2%) and most likely did not represent a shift in behavior of the entire ice sheet. Widespread Laurentide Ice Sheet deglaciation did not occur until the rise in atmospheric CO2 around 18 ka that signaled the onset of Termination 1.
General Impressions
This manuscript and the data within are much-needed and valuable additions to deglacial chronologies in this region. The timing of initial ice recession from the local terminal moraines has been a thorny issue for some time, complicated by large disagreements between available cosmogenic exposure ages and radiocarbon ages, and the relative lack of other constraints. I have personally hypothesized that some of the existing exposure ages did not correspond exactly to the timing of ice recession due to inherited nuclides and/or post-glacial disturbance of ice-contact features. Here, the authors did a fantastic job of assessing the geomorphic context of each sampled ice-contact feature and estimating ice recession ages, providing reasonable justification for discarded ages that likely contain inherited nuclides, and are thus too old, or were subject to post-glacial disturbance, and are thus too young. The decision to remove certain ages was done carefully and in line with established practices. By adding confident date constraints to many ice-contact features near the local terminal moraines, Balter-Kennedy et al. have provided a much clearer picture of initial ice recession in southern New England and New York.
The manuscript itself reads very well. I particularly appreciated the background section providing a summary of the regional glacial geomorphology. These are old, complex moraines with discontinuities, cross-cutting relationships, and a long history of interpretation. I found this detailed background helpful for orienting myself with the study area. The sampling and processing procedures, choices of scaling scheme and calibration data set, and normalization of older measurements all look good. The interpretation of the data makes sense to me, and I appreciated the in-depth discussion about how you used the age distributions to gauge the presence of inheritance or post-glacial disturbance.
Overall I recommend this manuscript for publication with minor revisions. Most of my suggested revisions are small line edits, as this manuscript already reads very well. I agree with the authors’ interpretations and the data supports their conclusions.
Line Edits (indicated by line number)
Figure 3 – Peekamoose Mt. samples are identified as bedrock, but were actually boulder samples
222 – There is a big jump here between detailed deglacial chronology in southern New England up to ~18 ka and the timing of ice recession from northern New England around 13.5 ka. It might be worth adding a line or two in here to briefly summarize what happened in the intervening 5 kyr. Something as simple as “systematic retreat between 50 – 300 m/yr with relatively minor standstills in northern New Hampshire and Maine around 14 ka” would give the reader just a bit more insight into what happened in the rest of the region.
232 – “Jones Point, New York” is repeated twice in this sentence. Remove the second occurrence.
294 – Elaborate what the “Glaciotectonic structures” were that indicate the moraine depositional history
Figure 5 – I really like this figure, especially the inset showing retreat rates as slopes, but I wonder if there is a way to identify which specific moraines are represented by the solid symbols. For example, the outer terminal moraine appears to have two solid circles, but in Figure 6 (and in the text), four moraines segments are grouped into the “outer terminal moraine” classification (Martha’s Vineyard, Ronkonkoma, Budd Lake, Staten Island). This might not be easily do-able in the figure itself, so maybe providing some more information in the caption would help.
567 – Again, the samples from Peekamoose Mt. were boulders, not bedrock
Supplementary Tables
Table S3:
- The header row in this table says “Table S2”, check for consistency
- Add units to the “Distance from terminal moraine” column
- Units for age columns should be kyr, correct?
Citation: https://doi.org/10.5194/egusphere-2024-241-RC1 -
AC1: 'Reply on RC1', Allie Balter-Kennedy, 24 Apr 2024
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We thank Chris Halsted for his review of the manuscript and insightful comments. Below, we address referee comments, supplied in bold, with our responses in regular text.
Line Edits (indicated by line number)
Figure 3 – Peekamoose Mt. samples are identified as bedrock, but were actually boulder samples
Great catch, thank you! Reference to the “Mt. Peekamoose bedrock ages” will be updated to “Peekamoose Mountain boulder ages” in Figure 3 and all instances in the text.
222 – There is a big jump here between detailed deglacial chronology in southern New England up to ~18 ka and the timing of ice recession from northern New England around 13.5 ka. It might be worth adding a line or two in here to briefly summarize what happened in the intervening 5 kyr. Something as simple as “systematic retreat between 50 – 300 m/yr with relatively minor standstills in northern New Hampshire and Maine around 14 ka” would give the reader just a bit more insight into what happened in the rest of the region.
We will update these sentences to read:
“The NAVC reveals systematic ice retreat through New England at 50–300 m yr-1 (Ridge et al., 2012), with relatively minor advances or stillstands at least in the White Mountains and Maine (e.g., Borns et al., 2004; Bromley et al., 2015; Davis et al., 2015; Dorion et al., 2001, Hall et al., 2017; Kaplan, 2007; Koester et al., 2017; Thompson et al., 2017). The position of the retreating ice margin is also marked by annual DeGeer moraines spaced 100 to 300 m apart in northern New England (Sinclair, 2018; Todd, 2007; Wrobleski, 2020). The LIS margin retreated north of New England by 13.6 ka (Ridge et al., 2012).”
232 – “Jones Point, New York” is repeated twice in this sentence. Remove the second occurrence.
We will remove the second occurrence.
294 – Elaborate what the “Glaciotectonic structures” were that indicate the moraine depositional history
We will update this sentence to read “Glaciotectonic structures, such as imbricated thrust sheets and dislocated strata, within the moraine stratigraphy indicate that the moraine was likely deposited during a readvance of the ice margin, rather than a representing a standstill (Oldale and O’Hara, 1984; Boothroyd and Sirkin, 2002).”
Figure 5 – I really like this figure, especially the inset showing retreat rates as slopes, but I wonder if there is a way to identify which specific moraines are represented by the solid symbols. For example, the outer terminal moraine appears to have two solid circles, but in Figure 6 (and in the text), four moraines segments are grouped into the “outer terminal moraine” classification (Martha’s Vineyard, Ronkonkoma, Budd Lake, Staten Island). This might not be easily do-able in the figure itself, so maybe providing some more information in the caption would help.
Figure 5 depicts ages only for the Connecticut-Narragansett-Buzzards Bay Lobes of the LIS, where there are enough dated landforms to estimate retreat rates between limits, while Figure 6 shows the chronology across the entire region separated by ice lobe.
We agree that adding moraine names to Figure 5 will make it easier to follow along with the text. Below each grouping of symbols, we will add shortened versions of the moraine names, in order from oldest to youngest, and update the caption to explain this (see figure in attachment).
567 – Again, the samples from Peekamoose Mt. were boulders, not bedrock
Updated!
Supplementary Tables
Table S3:
- The header row in this table says “Table S2”, check for consistency
- Add units to the “Distance from terminal moraine” column
- Units for age columns should be kyr, correct?
Thank you for catching this. We will update Table S3 to include the correct units and table number.
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