  We construct an infinitesimal invariant for cycles in a family with
cohomology class in the total space lying in a given level of the Leray
filtration. This infinitesimal invariant detects cycles modulo algebraic
equivalence in the fibers. We apply this construction to the Ikeda family,
which gives optimal results for the Beauville decomposition of the 1-cycle of a
very general plane curve in its Jacobian.
