Carbonate compensation depth (CCD)
CaCO3 shells (tests) sink from surface waters
Tests may reach a depth where water is significantly undersaturated with respect to CaCO3
At this depth, called the lysocline, shells begin to dissolve
In the modern oceans, there is also a depth at which there is no longer any free CaCO3
This depth, called the carbonate compensation depth (CCD) = ~ -4 km
CaCO3 tests accumulate only if they settle on sea floor above the CCD