This is because benthic forams are outnumbered by planktic forams 99 to 1. It is a laborious task and a slow process to then try to find all the benthic tests. This record is prefered to land records because of the large area it covers. Shells of forams are refered to as tests. Some tests are more resistant than others.
Species have their own response to dissolution.
This will influence the relative abundance of species accumulated in the sediment as well as the number of species preserved. Some other limitations of using oxygene 18 ratios in forams is that sometimes the CaCO3 secreted by the forams is not always in isotopic composition equilibrium with that of sea water. What may also affect correlation accuracy is the reworking of the sediments by bioturbation or bottom current activity, or slow sedimentation. It is then quite possible that a mixing of the sediments will occur thus mixing younger forams with older forams. To overcome the limitations of bioturbation, there is now a new method that is a mathematical model made to simulate the effects of bioturbation on paleoceanographic time series.