Genus 3 mapping class groups are not Kahler

We prove that finite index subgroups of genus 3 mapping class and Torelli groups that contain the group generated by Dehn twists on bounding simple closed curves are not Kahler. These results are deduced from explicit presentations of the unipotent (aka, Malcev) completion of genus 3 Torelli groups and of the relative completions of genus 3 mapping class groups. The main results follow from the fact that these presentations are not quadratic. To complete the picture, we compute presentations of completed Torelli and mapping class in genera at least 4; they are quadratic. We also show that groups commensurable with hyperelliptic mapping class groups and mapping class groups in genera at most 2 are not Kahler.