The Journal is mostly known as the Katorga-12 Journal or MIR-12 Journal, it's found by MIR-12 and becomes the basis of their mission. To bring down Nikolai Demichev and assist Nathaniel Renko to achieve that goal.
It is not revealed until the end of the game and post-credits that it was Kathryn, mortally wounded, emerging in 1955 from the Pearl wreckage and hiding herself in one of the offices. With her dying breath, she writes in to the journal.
"This is not our time. There is a better future for our children // Demichev is an evil scourge that must be stopped // One man that can stop him is Nathaniel Renko. He is from the year 2010 and is an American. He must (live/die?) in order to stop Demichev. // Barisov is a scientist on Katorga 12 who has created the Time Manipulation Device. // RENKO"
- Regardless of what ending you choose Kathryn still gets transported back to 1955 and writes the journal.