Permafrost

A number of age determinations are derived from the highly wind-abraded sand from
within the wedges. These vary from >55 ka to >146 ka. Because these ages span the duration of
the last interglacial (Eemian, or OIS-5), they may indicate two distinct periods (i.e. ~50-70 ka
or Early Wisconsinan, and ~140-200 ka or OIS-6) when cold-climate (permafrost) conditions
prevailed. However, we are unsure of an OIS-6 interpretation and prefer, at this point, to discuss
the evidence for permafrost that is associated with the climatic deterioration that occurred at the
transition from the Eemian interglacial to the Wisconsinan glacial. This allows a better
interpretation of the thermokarst structures that we have observed.
At the end of the Eemian interglacial, global sea level and the regional groundwater table
in Southern new Jersey would have been relatively high. Thus, as climate progressively
deteriorated and as permafrost aggraded, water would have migrated, by cryosuction, through
the permeable sand and gravel substrate towards the downward-advancing freezing plane. Icy
beds might also have formed immediately beneath the relatively impermeable bog ironstone
beds. Permafrost was probably continuous in nature.
A second group of ages concentrate around ~30 ka. They have been obtained from the
sandy material that infills sediment-pots and deformed wedge-like structures. If our
thermokarstic interpretation of these phenomena is correct, the dates suggest an amelioration of
climate between ~40-30 ka led to their formation by fluvio-thermal erosion processes. At the
same time, degradation of permafrost would have led to mass displacements and disturbed
bedding. The ‘foundering' of bog-ironstone beds would also have occurred in response to the
melt of icy beds.
A third group of age determinations of ~13 ka to ~17 ka relate to the more heterogeneous
and loosely-packed sandy infill of a number of wedges that have been studied near the southern
extremity of the Pine Barrens. Here, the fine sand fraction is much less highly wind abraded and
more local in origin, suggesting less wind action. These wedges suggest thermal-contraction
cracking occurred when the regional climate deteriorated during the LGM. At one locality, near
Newtonville, Atlantic County, a sand-wedge infill that was dated at ~ 16ka was observed to
penetrate a sediment-pot, the infill of which was dated at ~30 ka. The type locality for these
younger wedges is near Port Elizabeth, Cumberland County. Geomorphological considerations
indicate it was probably an exposed, snow-free site. This suggests that permafrost may have
been thin and discontinuous in nature. However, we cannot exclude the possibility that these
wedges are ‘ground’ wedges and reflect deep seasonal frost. There is little evidence of any
thermokarst activity associated with this period of permafrost conditions.
This sequence of permafrost-related events in Southern New Jersey appears to mirror the
large-scale climatic changes that have been deduced from the paleoecologic and
paleoenvironmental record from the eastern and mid-continental USA. These are based on
inferences from the loess depositional record and from well-dated cave carbonates. They
indicate climatic cooling post OIS-5a, at about 70 ka, followed by warming between 40-35 ka
(OIS-3), with cooling commencing at 30-35 ka, a slight warming between 30-25 ka, and then
the onset of the LGM.
Key words: Relict sand wedges, relict thermokarst, OSL dating
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