It'd be considered a part of the geosphere.
Here's all of their definitions though:
Geosphere: any of the almost spherical concentric regions of matter that make up the earth and its atmosphere, as the lithosphere and hydrosphere.
Cryosphere: The cryosphere is the frozen water part of the Earth system.
Hydrosphere: all the waters on the earth's surface, such as lakes and seas, and sometimes including water over the earth's surface, such as clouds
Biosphere: the regions of the surface, atmosphere, and hydrosphere of the earth (or analogous parts of other planets) occupied by living organisms.
