Try taking a gallon of hot water and pour it on the carpet on one section towards the end. Some carpet glues are water based. Let it sti for an hour and try pulling on that section. Unless you want to introduce chemicals into the basement, scraping may be the best way. Your local carpet installer may have a scraping machine. Call them for a quote.

I'd invest a few bucks in having an accredited lab sample the mastic/glue to make sure it doesn't contain asbestos first.