Rigid local systems and motives of type G2

Using the middle convolution functor MC chi introduced by N. Katz, we prove the existence of rigid local systems whose monodromy is dense in the simple algebraic group G(2). We derive the existence of motives for motivated cycles which have a motivic Galois group of type G(2). Granting Grothendieck's standard conjectures, the existence of motives with motivic Galois group of type G(2) can be deduced, giving a partial answer to a question of Serre.