I don't know if this is official, since you'd think it would be bigger news if it were, but according to Larry LaRue:
[Branyan] doesn’t have great range, but he’s a big target and, guarding the line late, the Mariners think he can do the job. Mike Sweeney will be the ‘other’ first baseman on the roster.
If this is indeed the way the Mariners are going, it doesn't come as much of a surprise, given all the positive press Sweeney's received from the start. And with Shelton having been one of the best hitters of the month, this also suggests that the job was Sweeney's to lose from the get-go - that he was the guy the Mariners wanted all along, provided he was healthy and capable of swinging the bat. He's been healthy and swung the bat, so there you go. Tough break for Shelton, but if/when Sweeney or Griffey breaks down, he won't be far away.
I'm not wild about this decision, but as we talked about a few days ago, it's unlikely to make much of a difference. While Sweeney's the worse bat, the playing time is limited, and he's as good a bet as anyone to be good for the clubhouse. I should also point out that this is the kind of move we probably would've held against Bill Bavasi, but the difference now is that Zduriencik has made enough other good moves that he's earned the benefit of the doubt. Choosing Sweeney would've been part of the pattern for Bill. For Jack, it's something else. That's why I'm not really upset or anything.
So that looks like one position battle wrapped up. The meaningful decisions remaining: closer, catcher, Griffey's position. They're really milking this long-ass Spring Training for all it's worth.