Explicit form of the Mann-Marolf surface term in (3+1) dimensions

The Mann-Marolf surface term is a specific candidate for the “reference background term” that is to be subtracted from the Gibbons-Hawking surface term in order make the total gravitational action of asymptotically flat spacetimes finite. That is, the total gravitational action is taken to be: (Einstein-Hilbert   bulk   term)+(Gibbons-Hawking   surface   term)-(Mann-Marolf   surface   term). As presented by Mann and Marolf, their surface term is specified implicitly in terms of the Ricci tensor of the boundary. Herein, I demonstrate that, for the physically interesting case of a (3+1)-dimensional bulk spacetime, the Mann-Marolf surface term can be specified explicitly in terms of the Einstein tensor of the (2+1)-dimensional boundary.