Ok just stick with me on this. They needed something to split the Alpha. The sarcophagus came up right before Delta's "birth." the markings on the sarcophagus are the same that are on the engineers in Halo. When the sarcophagus started freaking out, it was making the same noises as engineers do. I honestly think that the sarcophagus is an engineer that they used to spilt the alpha.