Firestone et al. sampled sedimentary sequences at many sites across North America, Europe, and Asia [Firestone RB, et al. (2007) Proc Natl Acad Sci USA 106:16016–16021]. In sediments dated to the Younger Dryas onset or Boundary (YDB) approximately 12,900 calendar years ago, Firestone et al. reported discovery of markers, including nanodiamonds, aciniform soot, high-temperature melt-glass, and magnetic microspherules attributed to cosmic impacts/airbursts. The microspherules were explained as either cosmic material ablation or terrestrial ejecta from a hypothesized North American impact that initiated the abrupt Younger Dryas cooling, contributed to megafaunal extinctions, and triggered human cultural shifts and population declines. A number of independent groups have confirmed the presence of YDB spherules, but two have not. One of them [Surovell TA, et al. (2009) Proc Natl Acad Sci USA 104:18155–18158] collected and analyzed samples from seven YDB sites, purportedly using the same protocol as Firestone et al., but did not find a single spherule in YDB sediments at two previously reported sites. To examine this discrepancy, we conducted an independent blind investigation of two sites common to both studies, and a third site investigated only by Surovell et al. We found abundant YDB microspherules at all three widely separated sites consistent with the results of Firestone et al. and conclude that the analytical protocol employed by Surovell et al. deviated significantly from that of Firestone et al. Morphological and geochemical analyses of YDB spherules suggest they are not cosmic, volcanic, authigenic, or anthropogenic in origin. Instead, they appear to have formed from abrupt melting and quenching of terrestrial materials.

Significance This study ties the spherules recovered in Pennsylvania and New Jersey to an impact in Quebec about 12,900 y ago at the onset of Younger Dryas. Our discovery resulted from an exhaustive search that examined the question of whether there is any evidence of extraterrestrial platinum group metals present in the bulk sediments, magnetic grains, and spherules recovered from the Younger Dryas boundary (YDB). We find that the spherules are likely quenched silicate melts produced following the impact at the YDB. The source of spherule osmium, however, is likely terrestrial and not meteorite derived.

Relict sand wedges, up to 2.5 m deep and 0.4 m wide, are present in the Pine Barrens of southern New Jersey. They indicate the previous existence of permafrost. The wedges are composed predominantly of sand that shows evidence of wind transport and abrasion. Optically-stimulated-luminescence dating of infill material indicates that thermal-contraction-cracking and emplacement of the sand infill must have occurred during two separate periods during the Late Pleistocene. The most recent was in Late Wisconsinan times, $15-18 ka. An earlier period of permafrost conditions is indicated by dates >55-65 ka. On both occasions, the Late-Pleistocene ice sheets would have advanced as far south as northern New Jersey and strong winds would have occurred in the lower mid-latitudes. The sandy soils of the Pine Barrens would have allowed the ice-marginal periglacial zone to extend southwards into southern New Jersey. The sparse tundra vegetation on the sandy substrate, with its relatively high thermal conductivity, would have permitted deep frost penetration because the 'thermal offset' would have been minimized. A mean annual air temperature of between À3.0 C and À4.0 C is inferred. Permafrost was probably discontinuous and less than 10-15 m in thickness. Episodes of permafrost thaw are indicated by the widespread occurrence of deformed sediments ('thermokarst involutions') and by various small-scale non-diastrophic structures associated with bog ironstone beds. The presence of soil (ground) wedges in southern New Jersey and adjacent Delaware also suggest conditions of deep seasonal frost, probably when the most recent permafrost degraded.

The ‘frost‐thaw’ or thermokarst‐lake‐basin hypothesis, first invoked by P. E. Wolfe in 1953 to explain the enclosed depressions and shallow basins (‘spungs’) of southern New Jersey, is re‐examined. The most probable explanation is that they formed in late Wisconsinan times as deflation hollows, or ‘blowouts’, when strong katabatic winds flowed southwards from the continental ice margin across the sparsely vegetated, tundra terrain of the Pine Barrens. Wedge structures and cryoturbation phenomena suggest the existence of either permafrost or deep seasonal frost, and imply mean annual air temperatures of between −0.5 °C and −6 °C. When the groundwater table rose in late‐glacial times, the hollows became ponds or wetlands. These were utilized as early as 12,000 years ago by palaeoindian and early archaic cultures as hunting camp sites. Today, many of these wetlands are drying up as the regional water table falls in response to increased water usage from agriculture and urbanization. Copyright © 2001 John Wiley & Sons, Ltd.

Sand‐wedge casts, soil wedges and other non‐diastrophic, post‐depositional sedimentary structures suggest that Late‐Pleistocene permafrost and deep seasonal frost on the Mid‐Atlantic Coastal Plain extended at least as far south as southern Delaware, the Eastern Shore and southern Maryland. Heterogeneous cold‐climate slope deposits mantle lower valley‐side slopes in central Maryland. A widespread pre‐existing fragipan is congruent with the inferred palaeo‐permafrost table. The high bulk density of the fragipan was probably enhanced by either thaw consolidation when icy permafrost degraded at the active layer‐permafrost interface or by liquefaction and compaction when deep seasonal frost thawed. Copyright © 2009 John Wiley & Sons, Ltd.