Dating Deglaciation

Retreat Rate of Laurentide Ice Sheet in New England with regard to three methodologies.

The dating ages are divided into those data points north of 43.00° N and those data points south of 41.75°. No data points between these two latitudes are available for study. The three methodologies are 1. Radiocarbon dating of sediments, 2. Radiocarbon dating of macrofossils and 3. Cosmogenic dating. Specific cosmogenic method used was not specified.

Bulk Sediment

34 data points

North Latitude mean calibrated radiocarbon age (13 data points): 13355 yr

South Latitude mean calibrated radiocarbon age (21 data points): 15126 yr

Northern Latitudes mean: 44.35°

Southern Latitudes mean: 41.26°

Degree distance between the two mean latitudes:

44.35 – 41.26 = 3.09° One degree equals 111 km.

3.09° x 111 km per degree = 342 km

Rate of retreat = 342.9 km/(15126-13355) yr = 342.9/1771 = 0.1936 km/yr = 193.6 m/yr

Macrofossils

26 data points

North Latitude mean calibrated radiocarbon age (22 data points): 13037 yr

South Latitude mean calibrated radiocarbon age (4 data points): 12950 yr

Northern Latitudes mean: 43.94°

Southern Latitudes mean: 41.52°

Degree distance between the two mean latitudes:

43.94 – 41.52 = 2.42°

2.42° x 111 km/degree = 268.6 kms

Rate of retreat = 268.6 kms/(13037 – 12950 yrs) = 268.6/87 = 3.087 kms/yr = 3087 m/yr

Cosmogenic Nuclide Exposure age

8 data points

Northern Latitudes mean age (3 data points): 13566 yrs

Southern Latitudes mean age (5 data points): 22880 yrs

Northern Latitudes mean: 44.40°

Southern Latitudes mean: 41.33°

Degree distance between the two mean latitudes:

44.40 – 41.33 = 3.07 °

3.07 x 111 km/degree = 340.8 km

Rate of retreat = 340.8 km/(22880 – 13566 yr) = 340.8/9314 = 0.0366 km/yr = 36 m/yr

Thus:

Data Set Estimate of Rate of Ice Sheet Retreat

Bulk Sediment 193 m/yr

Macrofossil 3087 m/yr

Cosmogenic 36 m/yr

Given that there are Bulk Sediment data points having the same ages but different latitudes, as well as data points sharing the same latitude but having a 12,000 yr range of ages, it is impossible to make a confident judgement as to the rate of retreat. Such a conclusion is further complicated by the lack of data points between the northern and southern groups. The problem of overlap of latitude and age is also present in the Macrofossil group, along with a dearth of data between the latitudes 41.75° and 43.00°. The data is uninterpretable. Moreover, the rate generated by this data set is far outside of the range given from other sources. See Discussion below. More than the other two data sets, the Cosmogenic set suffers from a low number of data points. However, there is no overlap, with a clear set of points at approx. 41.00° to 41.55° having an age of 20,300 to 25,200 yr and another set of points at 44.31° to 44.50° having a tight age range of 13,200 to 13,800 yr.

The Table above presents the very wide range of rate estimates for the Laurentide Ice Sheet Retreat among the three methodologies. It is possible that if data points were present between the latitudes 41.75° and 43.00° for the radiocarbon methodologies, one would see a similar spread. The radiocarbon methodologies assume a constant rate at which basins were denuded of material and then vegetated, which is very unlikely. The data could be interpreted to mean that the glacier exposed the area from 41.00° to 45.00° essentially at the same time. This would give a rate of at least 444 km, which is well outside of the accepted range from another source. That source is a talk given by Stephen Wright in May of 2018 at Smilie School in Bolton. The glacier was present in Vermont 13,000 years ago. He stated that the glacier had a retreat rate of 110 m/yr in northern Vermont. The retreat rate decreased with decreasing lower latitude. At Long Island, the retreat rate was 20 m/yr. Therefore, I think that the Cosmogenic methodology offers the only credible data set, despite the very small numbers.

Figure 1A of Lambeck et. al.’s paper (above) indicates that the sea level began to drop about 32,000 years ago, reaching a nadir at about 25,000 years ago. There it remained stable for about 5,000 years. At about 20,000 years ago, sea level rose slightly, then stabilized for about 5,000 years. Then, beginning about 15,000 years ago, there was a steady, arithmetic increase in sea level over the course of about 9,000 years. The sea level has remained steady for the last 7,000 years. The data from the cosmogenic methodology suggest that ice retreat in New England was happening early in the final sea level rise. This would make sense since New England was situated in the southern reaches of the Ice Sheet. Retreat occurring there was early in the full history of the Ice Sheet retreat, and thus early in the melting and subsequent rise in sea level.