Rashek had the power of Preservation for the time he held it. He could technically do anything Preservation was capable of. From what I understood, Preservation's mind was busy keeping Ruin's mind under control. That's why Preservation's power needed a mind to control and use it. But before Rashek, there was no Atium and no Pits for the Atium to grow as far as I remember. I am of the opinion that if what believe is true, it had to be Rashek who did it.
I'm pretty sure Atium existed before Rashek ascended. If I recall correctly, it was Preservation that hid Ruins body in the Pits when Ruin was first trapped. This was to weaken Ruin in order to balance the two since Preservation had become weaker as a result of creating humans and imprisoning Ruin. Rashek merely became aware about the atium during the short time he held Preservations power.
Also, it was mentioned many times in the book that the power in the well was only a fraction of Preservation's power since other portions of its power were also located in the mists, humans, and Preservations original body. If we take this one step further, I think its safe to assume that Vin's ascension was greater the Rashek's, since she took in Preservation's power located in both the mists, what was released at the well, and waht ever was released when Preservation's body died. This would also explain why she was able to become one with Ruin since they were evenly matched.