An association of benthic foraminifera and gypsum in Holocene sediments of estuarine Chesapeake Bay, USA

Two cores of Holocene sediments recovered from the Cape Charles Channel of Chesapeake Bay yielded radiocarbon ages of about 6.8 to 5.8 ka for the lower intervals. Fossil foraminifera preserved in these lower sediments are dominated by species of Elphidium, which make up about 90% of the assemblage throughout, and probably signify deposition in hypersaline waters. Buccella frigida and Ammonia beccarii are the only other species commonly present. Hypersalinity of bottom waters seems to have been main- tained by water-density stratification in a basin-like section of the channel. In core PTXT-4-P-1 transition to modern Chesapeake conditions, in which numbers of Ammonia beccarii exceed those of Elphidium, com- menced about 400 years ago. In core PTXT-3-P-2 hypersalinity is further signified by the presence of abun- dant euhedral crystals of gypsum in association with the fossil Elphidium. This occurrence of gypsum is not attributed to palaeoclimatic aridity, but rather to inflow of groundwater from adjacent gypsiferous Mio- cene strata. The study shows that in palaeoclimatic investigations the significance of the presence of gyp- sum should be evaluated with cautionit does not necessarily signify an evaporative regime.