Classes de cycles motiviques étales

Abstract

Let X be a smooth variety over a field k, and l be a prime number invertible in k. We study the (étale) unramified H^3 of X with coefficients Q_l/Z_l(2) in the style of Colliot-Thélène and Voisin. If k is separably closed, finite or p-adic, this describes it as an extension of a finite group F by a divisible group D, where F is the torsion subgroup of the cokernel of the l-adic cycle map. If k is finite and X is projective and of abelian type, verifying the Tate conjecture, D=0. If k is separably closed, we relate D to an l-adic Griffiths group. If k is the separable closure of a finite field and X comes from a variety over a finite field as described above, then D = 0 as soon as H^3(X,Q_l) is entirely of coniveau > 0, but an example of Schoen shows that this condition is not necessary.