Ungrounding of the Ross Ice Shelf approximately 5000 years ago resulted in extreme changes in watermass characteristics and the resultant faunal assemblages of McMurdo Sound, Antarctica. What was an abundant and diverse calcareous foraminiferal assemblage in the western sound is now a diverse and abundant arenaceous assemblage. Waters currently bathing Explorers Cove are corrosive to CaCO3 due to the presence of supercooled waters, enriched in CO2. Enrichment in CO2 is the result of the formation of perennial ice cover reducing photosyntheses and outgassing of CO2. Development of informal zones for the Explorers Cove area was possible on the basis of diversity, percentage of calcareous vs. arenaceous specimens, total foraminiferal number and species present. Zone 1, the Cassidulinoides-Cribrostomoides assemblage-zone, represents current benthic assemblages living at or near the surface. Zone 2, the Portotrochammina-Trochammina assemblage-zone, is the altered remains of .zone 1, the result of post-mortem calcareous dissolution. Zone 3, the Lagena-Rosalina-Glandulina assemblage-zone has an increase in calcareous percentages and calcareous species of a solution prone, very delicate nature are present. Faunal variations described are the result of development of CaC03 corrosive waters, evidenced by the condition of benthic calcareous foraminiferal tests and the essentially sudden change from calcareous to arenaceous assemblages. An estimate of removal of calcareous material, on the basis of calcareous test condition and total removal of calcareous specimens at Explorers Cove is approximately 65 to 85%. Estimated sedimentation rates for the nearshore area of Explorers Cove are 3 to 4 mm/year, and 0.02 mm/year in the "starved basin" area offshore. At McMurdo, waters are only slightly corrosive to CaCO3, the result of mixing of Ice Shelf Water and High Salinity Shelf Water. Informal zones were established for this area on the basis of total foraminiferal numbers and calcareous percentages. Zone 1, the Globocassidulina-Cassidulinoides assemblage-zone, represents current foraminiferal assemblages living in the surface sediment. Zone 2, the Ammodiscus assemblage-zone is considered to be the altered remains of zone 1 by calcareous dissolution. Zone 3, the Astrononion-Rosalina-Pseudobolivina assemblage-zone is characterised by increases in calcareous numbers, total foraminiferal numbers and diversity. The percentage of material lost to dissolution at McMurdo is much lower, estimated at 10 to 20% on the basis of calcareous test condition. Estimated sedimentation rates for the McMurdo area are 0.10 mm/year near shore and 0.01 to 0.02 mm/year further offshore. Several measurable characteristics can be examined and used to estimate the amount of alteration an assemblage may have been exposed to: 1) the condition of calcareous foraminiferal tests; 2) calcareous diversity of unaltered assemblages are moderate and poor in dissolution altered assemblages; 3) exceptionally large changes in total foraminiferal number can be indicative of a change in environmental conditions.