STRONTIUM ISOTOPIC COMPOSITIONS OF THE NEUSE AND CAPE FEAR RIVER BASINS, NORTH CAROLINA

River water, bedrock, saprolite, and groundwater samples from the Neuse and Cape Fear rivers in North Carolina were collected and analyzed for their strontium (Sr) concentrations and ⁸⁷Sr/⁸⁶Sr ratios. The ⁸⁷Sr/⁸⁶Sr ratios of the Neuse river begin at approximately 0.705 in the headwaters; ratios then rise and stabilize to near 0.709 in the coastal plain. The Cape Fear river shows a similar trend in ⁸⁷Sr/⁸⁶Sr ratios. Both rivers have [Sr] between 30 and 90 ppb upstream of the estuaries (at which Sr concentrations increase by up to two orders of magnitude). Bedrock and saprolite samples from the three geologic provinces in the Neuse and Cape Fear river catchments have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.705 to 0.720, and there is no consistent correlation between degree of weathering and isotopic composition. Groundwater samples from the Neuse headwater catchment have ⁸⁷Sr/⁸⁶Sr ratios ranging from approximately 0.704 to 0.710. These data indicate that dominant surficial bedrock is not the primary source of Sr to the headwaters of the Neuse and Cape Fear rivers, and that unexposed bedrock can exert a strong local control on the groundwater component that enters into streams. Both the Neuse and Cape Fear rivers deliver water with ⁸⁷Sr/⁸⁶Sr 0.7092 to the marine environment, indicating that the sediments of coastal plain environments can rapidly overwhelm upstream Sr isotopic compositions.