FULL-GLACIAL CLIMATE OF ST. LOUIS, MISSOURI, RECONSTRUCTED FROM POND-DWELLING OSTRACODES

A 4.8-m succession of silty, fossiliferous sediment and three clay beds dating from 19,660 to about 17,000 yr B.P. were sampled at the Bellefontaine Quarry near St. Louis, Missouri. The succession yielded abundant ostracodes that are collectively found today in south-central Canada, including Cyclocypris sharpei, C. ampla, C. ovum, Cypridopsis vidua, Candona crogmaniana, C. distincta, and C. ohioensis. The ostracode habitat formed when a sinkhole became plugged by a 1-m thick clay bed deposited at 19,660 ± 65 yr B.P. By about 17,000 yr B.P, the pond had desiccated and was buried by loess. The material assayed for radiocarbon included in situ stumps and redeposited fragments of Picea.

An ostracode-based modern analog database was used to reconstruct the physical and chemical conditions of the ostracodes' environment, and the ambient climate. The results imply gradual cooling and drier conditions from 19,660 to 17,000 yr B.P. The water in the hydrologically-open, spring-fed pond was less than 1 m deep, and the total dissolved solids progressively increased from about 150 to 250 mg/L. The shallow water depth indicates that the temperature reconstruction is robust with mean annual temperatures (MAT) that varied from 0.8 to 3.9°C , and mean July temperatures from 16.8 to 18.1°C. Other estimated climatic parameters include mean annual precipitation (MAP; 430 to 840 mm/yr), and moisture balance (P-E; -111 to 298 mm/yr). The estimated MAP is about 400 mm less than modern; P-E values are similar to those measured today in St. Louis. Full-glacial MATs were about 10°C cooler than today. These conclusions are consistent with other published reconstructions based on modern analog analysis of fossil beetles and pollen, and paleothermometry based on amino acid racemization.