Lake sediment records of deglaciation and post-glacial paleoenvironmental conditions, northeast Pennsylvania, USA

Sediment cores from two glacial lake basins in northeast Pennsylvania, USA, record the timing of Laurentide Ice Sheet (LIS) retreat along with post-glacial climate and environmental conditions. Basal radiocarbon ages from Nuangola Lake and Cranberry Pond, located ∼10 km north of the late-Wisconsinan age terminal moraine complex, indicate the onset of post-glacial sedimentation sometime prior to ∼19,260 calibrated years before present (cal yr BP) (∼18,840 to ∼19,780 cal yr BP range) and ∼18,010 cal yr BP (∼16,400 to ∼19,460 cal yr BP range), respectively. These calibrated radiocarbon ages are statistically similar, given the overlap of the uncertainty range, and therefore indicate LIS retreat from the terminal moraine position and deglaciation across the study lakes as early as ∼19,300 and ∼18,000 cal yr BP. However, given the radiocarbon-dated charcoal materials were isolated from post-glacial sediments located stratigraphically above basal glaciolacustrine sediments, the ages only represent a minimum estimate of deglaciation. These are the oldest known ages for proglacial lakes in Pennsylvania, and, interestingly, lie between published 10Be exposure and radiocarbon ages in the broader region. To reconstruct paleoenvironmental conditions, we focused on the analysis of a composite 11.7-m length core sequence from Nuangola Lake and identified four distinct lithologic units based on variations in sedimentology, stratigraphic characteristics, and proxy data including dry bulk density, magnetic susceptibility, sediment color, alumina content, organic matter, biogenic silica, carbon to nitrogen mass ratios (C/N), and organic matter stable carbon isotopes (δ13C). Age control of the sediment sequence is provided by eight radiocarbon dates and five independently dated non-visible volcanic ash (cryptotephra) deposits. The basal glaciolacustrine sediments were deposited prior to ∼19,260 cal yr BP and are characterized by high density minerogenic material with very low organic content and laminated couplets that likely represent varves. The transition to banded and homogeneous post-glacial sediments is abrupt, marking the retreat of the LIS out of the lake catchment. Transitional post-glacial sediments, deposited between > ∼19,260 and ∼14,400 cal yr BP, are characterized by high density, mineral rich material with low organic content, and define a prolonged period of low ecosystem productivity and cold, periglacial climate conditions. Increasing and variable sediment organic content and biogenic silica levels, combined with decreases in bulk density and mineral matter, occur by ∼14,400 cal yr BP and define the latest Pleistocene through early Holocene. A marked increase in organic content and gradually increasing C/N ratios document increasing terrestrial ecosystem productivity and overall warmer and wetter conditions by the onset of the Holocene at ∼11,700 cal yr BP. The subsequent transition to fibrous organic matter rich, peaty sediments by ∼5900 cal yr BP defines a transition to a bog-wetland (peatland) environment that persisted until the present day.