Radiogenic isotopes of strontium (87Sr/86Sr) and neodymium (144Nd/143Nd) are widely used to trace sediment across source-to-sink networks, with samples typically collected from outcrops at basin headwaters and from sediments along the channel margin, floodplain, and/or seafloor. Here, we established the Sr-Nd isotope systematics of recent (in the past 1 k.y.) Mississippi River (USA) basin alluvial sediments, evaluated the sensitivity of these isotope systems to the presence of artificial impoundments that trap sediments behind them, and tested their ability to provenance mixed sedimentary records. Sediment cores collected from floodplain depressions and oxbow lakes along the Mississippi River and its major tributaries, where an extensive lock and dam system was constructed during the mid-twentieth century, show that the isotopic signatures of major tributaries are distinct, and that some of these signatures shift following dam closure. We then used mixing models to demonstrate that, near the confluence of major tributaries, Sr and Nd isotope signatures can be used to ascertain provenance of sediments deposited in floodplain lakes during overbank floods. Further downstream, where sediments are well mixed, the provenance of overbank deposits is more challenging to evaluate using Sr-Nd isotope systematics. Given the global pervasiveness of artificial impoundments on rivers, our findings imply that widely employed sediment fingerprinting techniques based on modern conditions may not be representative of conditions from as recently as a century ago.
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