My big brick wall

My most annoying brick wall concerns a set of great-great grandparents, Thomas Jackson POLK and his wife Sophia (HARNESS).

Thomas J. was born 20 Feb1830 in Botetourt Co., VA to Joseph and Catherine (BECKNER) POLK.
Sophia (HARNESS) POLK was born 14 Oct 1828 in Clinton Co., IN to George W. and Harriet (SOWARDS) HARNESS.

They married 28 Nov 1852 in Carroll Co., IN

In 1860, they were living in Ervin Township, Howard Co., IN, where they also lived in 1870 & 1880. They had 10 children, however 5 died in infancy. They five infants are interred in Bell Mound Cemetery, Ervin township, Howard Co., IN. Thomas' parents are also interred in the same lot.

There is no death record in Howard County of either Thomas J. or Sophia Polk, but both signed a deed tranfer to son-in-law, Edward M. Rioth on 10 Aug 1886. On 24 Sep 1887, Edward M and Martha J Rioth sell land to only Sophiah Polk, leading me to believe that Thomas has died in the interim. On 11 Feb 1889 that piece of land is sold at delinquent tax sale to James D Johnson. Document states land returned delinquent in the name of Sophia Polk etal. The etal part makes me wonder if Sophia has died at that point in time.

I have been unable to locate an obituary for either. I have run into many descendants of this couple, however, no one has found any information about the deaths of either. I descend through Edward M and Martha J (POLK) RIOTH. Any help would be appreciated.