Timing and Nature of Deglaciation of Southern New England
The absence of constraining radiocarbon ages and other accurate and precise dating techniques has left the chronology of initial deglaciation from the maximum position of the southeastern Laurentide Ice Sheet in New England only crudely estimated. This is in marked contrast to areas further from the terminal margin, where ice retreat is tied to abundant radiocarbon ages and a well-dated glacial varve chronology.
General model of Late Wisconsinan advance and retreat of the Laurentide Ice Sheet in New England.
Reconstruction of the Laurentide Ice Sheet in southern New England: 26,000 yBP.
We compiled a 265-year varve series from Glacial Lake Narragansett collected from the Providence River, Narragansett Bay, Rhode Island. was not correlated either with the North American Varve Chronology, or with other varve sequences from southern New England or southeastern New York. However, this uncorrelated sequence represents the minimum time of deposition within the northern segment of Glacial Lake Narragansett.
Approximate extent of Glacial Lake Narragansett
Vibracoring in the Providence River (Glacial Lake Narragansett) to collect varve sequences. The vessel is a specially outfitted pontoon boat owned by the USDA/NRCS Rhode Island office. Pictured is Jim Turenne, Rhode Island State Soil Scientist (United States Department of Agriculture, Natural Resource Conservation Service, Warwick RI), Chris Troskosky (in the water) and Tyson Bottenus (Both formerly of URI Geosciences)
Varves with thick (~ 1 cm) winter layers from core PVD-1. Numbers indicate the Glacial Lake Narragansett varve year. Black arrows point to examples of some of the sedimentary features commonly seen in the Providence Cores. A: Sand parting at the top of the winter layer representing a late winter-early spring melting, overturning or a storm runoff event. B: Fault in the core, likely not induced during coring. C: Scour at the top of the winter layer. D: Thin (1 mm) sandy layer at top of summer layer that represents a late-season storm E: Prominent sandy layer capped by thin (1 mm) silt/clay layer at the base of the summer layer representing early season melting or storm runoff followed by a period of little/no sediment input into the lake.
Correlation of varve records based on close annual matches in total couplet thickness. A. Correlation of varve records collected in the Providence River for Narragansett Bay varve years 5000 to 5150. A constant offset was added to some of the records to display them on one graph without overlap. EW-1 was originally collected by Pickart (1987). B. Correlation of varve records collected in the Providence River for Narragansett Bay varve years 5150 to 5260. A constant offset was added to some of the records to display them on one graph without overlap.
Correlation of varve records based on close annual matches in total couplet thickness. C. Correlation of varve records from the Providence River and Seekonk River (Antevs, 1928). Arrows point to two discrepancies, where it appears one varve may be missing from each sequence. The Providence composite curve was offset by 3 cm to display the records on one graph without overlap. D. Correlation of varve records from Pawtuxet Cove (Pickart, 1987) and Gaspee Point Varve thickness of PC-6 was offset by 3 cm to display the records on one graph without overlap.
Used in conjunction with the calibrated North American Varve Chronology and cosmogenic exposure ages from the Wolf Rocks and Congdon Hill recessional end moraines, minimum (> 19,400 yBP) and maximum (< 20,500 yBP) ages for Glacial Lake Narragansett can be established.
NGRIP ice core chronology and δ18O profile of the NGRIP ice core (Andersen et al., 2006; Rasmussen et al., 2008). Red arrows indicate cosmogenic exposure ages of the Charlestown-Buzzards Bay (CM-BB), Ledyard-Congdon Hill (L-CH) and Old Saybrook-Wolf Rocks (OS-WR) recessional end moraines in southern New England (Balco et al., 2009; Balco and Schaefer, 2006). Dashed black lines refer to potential correlation with Greenland Ice Cores LIS readvance/still stands
The volume of sediment deposited and presence of overlapping lacustrine fans in Glacial Lake Narragansett indicate that the ice sheet was warm-based, or at least ‘warm poly-thermal’, with subglacial drainages that did not reorganize every year. The relatively rapid retreat of the ice sheet along with models of the landscape prior to the onset of isostatic rebound approximately 16,000 yBP, support the trend of a thinner, ‘warmer’ Laurentide Ice Sheet in southeastern New England.
Proposed minimum ages of Glacial Lake Narragansett shown in comparison with the base of the North American Varve Chronology and other uncorrelated varve series in southern New England and eastern New York. Red arrows indicate cosmogenic exposure ages from (Balco et al., 2009; Balco and Schaefer, 2006; Balco et al., 2002). Green arrows indicate proposed correlated ages of the Congdon Hill and Wolf Rock end moraines. Blue arrow marks the maximum range of Glacial Lake Narragansett.
While many studies refer to the last glacial maximum occurring at 18,000 to 20,000 yBP, the constrained age of Glacial Lake Narragansett suggests that at least for the southeastern portion of the Laurentide Ice Sheet, deglaciation was well underway by this time.
Significant questions remain regarding the initial deglaciation of southeastern New England, particularly the timing of ice retreat from the terminal moraine near Block Island, Rhode Island and across present-day Block Island and Rhode Island Sounds prior to the southern margin of the Laurentide retreating from the cosmogenically dated Charlestown moraine position around 21,300 yBP.
Modified from: Constraining the Age of Deglaciation of Southeastern New England (Northeast GSA Meeting, 2013).
Many of the figures are from Oakley and Boothroyd (2013) Journal of Paleolimnology. 50(3):305-317