Non-algebraic geometrically trivial cohomology classes over finite fields

Abstract

We give the first examples of smooth projective varieties $X$ over a finite field $\mathbb{F}$ admitting a non-algebraic torsion $\ell$-adic cohomology class of degree $4$ which vanishes over $\overline{\mathbb{F}}$. We use them to show that two versions of the integral Tate conjecture over $\mathbb{F}$ are not equivalent to one another and that a fundamental exact sequence of Colliot-Thélène and Kahn does not necessarily split. Some of our examples have dimension $4$, and are the first known examples of fourfolds with non-vanishing $H_{\text{nr}}^3(X,{\mathbb{Q}}_2/{\mathbb{Z}}_2(2))$.

23 pages, comments welcome, v3. final version, to appear in Advances in Mathematics