This is a very thin title, a French title, the main protaginist is a Frenchman, no surprise there. It is called "The Ice People" and written by Rene Barjavel. At least, that is the title in English. The copyright page lists it originally published as Las Nuit des Temps in 1970.
The team that locates the Sphere is ostensibly international, but even back then you can detect an Anti-AMerican bias in the book, with the main US character on the team depicted as slovenly, and greedy, while the European characters are all generally noble of purpose, and genuine. There is a subplot that echoes the story line from Elea's time (the female lead character from the distant past) of gathering forces that will lead to the destruction of civilization, and there is an implication that the US is one of the plotters of the current tension.
Incidently, I too first read this over 30 years ago, probably from our local library bookmobile.