  Delta-matroids are "type B" generalizations of matroids in the same way that
maximal orthogonal Grassmannians are generalizations of Grassmannians. A
delta-matroid analogue of the Tutte polynomial of a matroid is the interlace
polynomial. We give a geometric interpretation for the interlace polynomial via
the K-theory of maximal orthogonal Grassmannians. To do so, we develop a new
Hirzebruch-Riemann-Roch-type formula for the type B permutohedral variety.
