Arithmetic of Gysin kernels

Abstract

Let $k$ be a field, and let $X$ be a smooth projective surface over $k$. Fix a Lefschetz pencil on $X$, and let $C$ be its fibre at the generic point of ${\mathbb{P}}^1$. The closed immersion of $C$ in to $X_{k(t)}$ induces the Gysin homomorphism from the Jacobian $A$ of the curve $C$ to the Chow group $A_0(X_{k(t)})$ of $0$-cycles of degree $0$ on $X_{k(t)}$. Embedding $k(t)$ in to an uncountable universal domain $\mathbb{k}$, we obtain the corresponding homomorphism from $A(\mathbb{k})$ to $A_0(X_{\mathbb{k}})$, whose kernel is either countable or the union of translates of a certain abelian subvariety inside $A_{\mathbb{k}}$, due to the Deligne-Katz irreducibility of monodromy action on vanishing cycles. We prove three dichotomy theorems on the structure of the kernel of Gysin homomorphism on $0$-cycles, in terms of étale monodromy action, and working over a field of arbitrary characteristic.

62 pages