A mile or so of glacial ice covering much of North America and plowing down from the north once terminated in the New York metropolitan area, at a front stretching roughly from exit 13 on the New Jersey Turnpike (Rahway), on across southern Staten Island, the Bensonhurst neighborhood of Brooklyn, and northeastward through Long Island. But exactly when that ice started to seriously melt has long been an enigma. Depending on methodology, geologists have estimated 21,000 to 28,000 years ago–a range that does not seem to match records of sea-level rise and other oceanographic indicators, which place the turning point more recently, at maybe 17,000 years. Knowing the timing is important, because it applies to how the ocean, atmosphere and ice might interact today during the increasingly rapid melting of glaciers farther north.
Now, a group of researchers at Columbia University’s Lamont-Doherty Earth Observatory says they have found evidence that the more recent date is right. Led by geochemist Dorothy Peteet, the group analyzed plant microfossils that they removed in cores drilled into the bottoms of more than a dozen lakes and bogs in the region. These cores suggest that vegetation did not appear around Connecticut, New York’s Hudson Valley, northern New Jersey and southeastern Pennsylvania until just 15,000 or 16,000 years ago. The absence until then suggests that ice still covered the landscape, or at the very least that temperatures were still so cold, plants could not survive. Their study appears in the latest issue of the journal Geophysical Research Letters.