XIETIAN
We can consider those pagodas as giant treasure chests. Those, and I assume many other ancient ruins scattered across the planet. I'm guessing as long as something could perfectly seal the soul power within, it was protected when the earth was reverted to a previous state / mana was removed from all living beings. As for why Sirius could ask for such a favor from the Soul Record- I wonder if it was the other way around, and Sirius/earth was part of SR's plan. Earth was the birth place of some of the most powerful beings currently living in the known universe (outside of that war zone that easily destroys higher beings), and the Soul Record is searching for a "perfect being". By re-doing the earth's (and humanity's) evolution, it may be able to recreate Sirius, one that it could guide and nourish.