UC Santa Barbara

Distributions of stable isotopes of water, radiocarbon, noble gases, and chloride (C1) in groundwater from the Upper Floridan aquifer in southeastern Georgia suggest that down gradient of the Gulf Trough this aquifer contains waters representative of both regional and local groundwater flow systems. In this area, locally recharged waters tend to remain near the top of the aquifer and do not mix substantially with the regional groundwater flow system. Noble gas temperatures suggest that this region of Georgia was 4.0 ± 0.6°C cooler during the last glacial period (LGP). Similar temperature changes have been reported for southern Texas and northern New Mexico, suggesting that the southern United States cooled uniformly during the LGP. Stable isotopes of water became enriched down gradient from the recharge area. These enrichments which have been observed before appear to result from local influx of shallow groundwater into the regional aquifer system rather than representing a climate change signal. An inland gradient of the stable isotope composition (0.60 ± 0.14‰/100 km in δ18O) was found in young (Holocene) water. After correcting for the change in the stable isotope composition of the ocean during the LGP, water that was recharged during the LGP was found to be slightly depleted in stable isotopes relative to modern recharge (Δδ18O = 0.6 ± 0.4‰). Assuming the modern inland gradient, the change in δ18O is consistent with a shift in the coastline, which was caused by the lower sea level during the LGP.