We consider volume-preserving flows \documentclass[12pt]{minimal}
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\begin{document}$${(\Phi^f_t)_{t\in\mathbb{R}}}$$\end{document} on \documentclass[12pt]{minimal}
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\begin{document}$${S \times \mathbb{R}}$$\end{document} , where S is a compact connected surface of genus g ≥ 2 and \documentclass[12pt]{minimal}
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\begin{document}$${(\Phi^f_t)_{t\in\mathbb{R}}}$$\end{document} has the form \documentclass[12pt]{minimal}
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\begin{document}$${\Phi^f_t(x, y) = (\phi_{t}x, y + \int_0^{t}f(\phi_{s}x)\,ds)}$$\end{document} where \documentclass[12pt]{minimal}
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\begin{document}$${(\phi_t)_{t\in\mathbb{R}}}$$\end{document} is a locally Hamiltonian flow of hyperbolic periodic type on S and f is a smooth real valued function on S. We investigate ergodic properties of these infinite measure-preserving flows and prove that if f belongs to a space of finite codimension in \documentclass[12pt]{minimal}
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\begin{document}$${\fancyscript{C}^{2+\epsilon}(S)}$$\end{document} , then the following dynamical dichotomy holds: if there is a fixed point of \documentclass[12pt]{minimal}
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\begin{document}$${(\phi_t)_{t\in\mathbb{R}}}$$\end{document} on which f does not vanish, then \documentclass[12pt]{minimal}
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\begin{document}$${(\Phi^f_t)_{t\in\mathbb{R}}}$$\end{document} is ergodic, otherwise, if f vanishes on all fixed points, it is reducible, i.e. isomorphic to the trivial extension \documentclass[12pt]{minimal}
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\begin{document}$${(\Phi^0_t)_{t\in\mathbb{R}}}$$\end{document} . The proof of this result exploits the reduction of \documentclass[12pt]{minimal}
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\begin{document}$${(\Phi^f_t)_{t\in\mathbb{R}}}$$\end{document} to a skew product automorphism over an interval exchange transformation of periodic type. If there is a fixed point of \documentclass[12pt]{minimal}
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\begin{document}$${(\phi_t)_{t\in\mathbb{R}}}$$\end{document} on which f does not vanish, the reduction yields cocycles with symmetric logarithmic singularities, for which we prove ergodicity.
