How can we accurately carbon-date samples from La Brea?
According to Radiocarbon dating of petroleum-impregnated bone from tar pits at Rancho La Brea, California. Science  30 May 1969:
Vol. 164, Issue 3883, pp. 1051-1052 :


  Extensive bone dating was not possible before a method was devised for the
  removal of the impregnating petroleum...Bone is now routinely dated from its collagen