Here we have an instance where the preposition is on rather than the much more frequent upon.
There is one other example:
Elsewhere in the text, when upon or on precedes “the face of ”, we get upon 111 times but on only twice. This disparity in frequency suggests the possibility that here we have two cases where upon was accidentally written as on. The original manuscript is not extant for the second case, but the on is extant in the original manuscript for the first example. In one of the 111 examples with upon, the 1906 LDS edition replaced upon with on (Helaman 3:9, quoted below), but in all other cases there has been no variation between upon and on for this expression involving “the face of”. More generally, there are nine cases where upon has been replaced by on in the textual history:
On the other hand, there are three cases where the opposite has occurred (an original on being replaced by upon):
The 1837 edition and 1874 RLDS edition are responsible for the large majority of these prepositional switches. For further discussion, see under each of these 12 passages. Obviously, confusion between the two prepositions on and upon is fairly frequent.
But in 1 Nephi 12:4 and Alma 16:16, there is no textual variation in any of the (extant) textual sources. Thus it is probably best to keep the preposition on in these two examples, even though upon is much more frequent in the phrase “(up)on the face of ”.
Summary: Retain in 1 Nephi 12:4 and Alma 16:16 the preposition on in the phrase “on the face of the land” since it is found in the earliest textual sources for these two passages; the preposition on, although infrequent when compared with upon, is possible.