The most important part of the solution is taking into account that both the Bloop and the Julia do not sound organic at all. The "illusion" of the sounds being organic is due to the fact that the sped-up versions get played more than the real-time versions. When listening to the 1X speed versions, you can clearly tell that they don't sound organic in nature at all. So, what are they? Well, that's quite easy to figure out.

All you have to do is look at the spectrograms. The Bloop's spectrogram has clearly visible "column". This indicates a sudden spike in sound. Now, we have to think: Which geophysical processes generate sudden sounds? That's right: Ice calving. The sudden cracking of ice from the bigger iceberg causes, obviously, a sudden sound. Furthermore, the estimated location of the source lies somewhere near the western side of the southern tip of South America - which is really cold, so the origin being related to ice is not really far-fetched.

Now, what caused the "Julia"? That's also quite easy to firgure out: The curve which is clearly visible in the spectrogram tells us that whatever caused the sound was probably moving, before suddenly and quickly decelerating. Now, what does that, that is also near Antarctica? A phenomenon called "seabed gouging by ice". In short terms, it's when an iceberg collides with the ocean floor, and gets stuck. You can imagine how loud that is.