Unexpected Sources of Strontium to the Neuse and Cape Fear River Basins, North Carolina: Implications for the Global Strontium Isotope Budget in Seawater

Abstract

Water, bedrock, and saprolite samples from the Neuse and Cape Fear River basins, North Carolina, were analyzed for 87Sr/86Sr and [Sr] to evaluate the control of exposed bedrock on fluvial Sr isotopic compositions and the influence of geology on Sr delivered to the ocean. The 87Sr/86Sr and [Sr] of the two rivers start low and rise downstream, eventually approximating recent ocean isotopic compositions before entering their estuaries. Groundwater samples from the headwaters have 87Sr/86Sr ratios that are lower than expected from the dominant exposed bedrock. The isotopic compositions of bedrock and saprolite samples vary predictably with rock type and age and show no correlation with degree of weathering. The data indicate that dominant surficial bedrock is not the primary source of Sr to the headwaters of the rivers. Rather, mafic dikes that focus groundwater flow and are more easily weathered than their silicic hosts impact the 87Sr/86Sr of the waters more than their limited exposure might suggest. Furthermore, the Sr isotopic composition of the water delivered to the marine environment is buffered by groundwater from coastal plain sedimentary rocks, leaving no evidence of upstream geology. The data suggest that rock type and structure exert significant control on the Sr isotope geochemistry of groundwater that enters into streams. Considering the global fluvial Sr budget, these results emphasize that, in some settings, (1) rock exposure area can be a poor indication of the geologic influence on Sr isotopic compositions of surface waters, and (2) downstream Sr isotopic compositions may not reflect upstream geology.