Unexpected Sources of Strontium to the Neuse and Cape Fear River Basins, North Carolina: Implications for the Global Strontium Isotope Budget in Seawater. Issue 5 (12th May 2019)

• Record Type: Journal Article
• Authors: Watts, Elena M.; Coleman, Drew S.; Colon‐Ramirez, Angel M.; Walsh, Aleah R.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 5(2019)
• Page Start: 1160
• Page End: 1174
• Publication Date: 2019-05-12
• Subjects: Sr isotopes -- Neuse river -- Cape Fear river; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2018JF004797
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Water, bedrock, and saprolite samples from the Neuse and Cape Fear River basins, North Carolina, were analyzed for 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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. Plain Language Summary: In this study we characterize how the chemical signatures (Sr isotopic ratios) of underlying rock influence the chemical signatures of river water. Rivers contribute a significant amount water to the ocean, and thus, the chemical signatures of rivers help determine the overall chemical signature of the ocean throughout geologic time. This is important for understanding how the composition of the oceans reflects geologic processes such as mountain building, volcanism, and weathering. By studying the relationships between bedrock and rivers, we can better understand how continental geology influences the geochemistry of the ocean. We found that, in some cases, bedrock types that are not abundant on the surface can impact groundwater chemical signatures disproportionately, and that rivers which run through large, young coastal geologic provinces can ultimately deliver water with an isotopic signature similar to that of the ocean. Key Points: Bedrock structure and rock type can exert stronger local control on the Sr isotopic composition of groundwater than exposure area Coastal sedimentary rocks can buffer the isotopic composition of water delivered to the ocean The Sr isotopic composition of estuarine output is not necessarily representative of the Sr isotopic composition upstream