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Unexpected Sources of Strontium to the Neuse and Cape Fear River Basins, North Carolina: Implications for the Global Strontium Isotope Budget in Seawater
Author(s) -
Watts Elena M.,
Coleman Drew S.,
ColonRamirez Angel M.,
Walsh Aleah R.
Publication year - 2019
Publication title -
journal of geophysical research: earth surface
Language(s) - English
Resource type - Journals
eISSN - 2169-9011
pISSN - 2169-9003
DOI - 10.1029/2018jf004797
Subject(s) - geology , bedrock , geochemistry , isotopes of strontium , groundwater , weathering , strontium , saprolite , fluvial , sedimentary rock , hydrology (agriculture) , geomorphology , structural basin , physics , geotechnical engineering , nuclear physics
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.