Radiogenic fingerprinting reveals anthropogenic and buffering controls on sediment dynamics of the Mississippi River system

Abstract

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. INTRODUCTION Sediment transport from source to sink is a fundamental process that shapes landscapes and the geological record (Allen, 2008). Radiogenic isotopes of strontium and neodymium are widely used as sediment tracers to identify source area and patterns of erosion and deposition within and across drainage networks (e.g., McLennan et al., 1989; Clift et al., 2008; Padoan et al., 2011). Basinwide systematics of Sr and Nd isotopes are typically evaluated by collecting samples from outcrops at basin headwaters (i.e., source rocks), along channel margins, from the floodplain surface, and/or in offshore areas (i.e., mixed sediments). Over the past century, artificial impoundments that trap sediments have been built on many of the world’s major rivers and their tributaries, profoundly altering the downstream delivery of sediments (Syvitski et al., 2005). The potential for these artificial impoundments to influence Sr and Nd isotope systematics has previously been recognized (e.g., Padoan et al., 2011), but, to our knowledge, the magnitude of this influence has yet to be evaluated. Here, we examined the Sr and Nd isotope systematics for the Mississippi River system, USA (Fig. 1). The Mississippi River is the largest river in North America and one of the world’s most heavily engineered drainage networks (Knox, 2007), where dams constructed primarily during the mid–twentieth century on the Missouri, Upper Mississippi, and Ohio Rivers (Fig. DR1 in the GSA Data Repository1) have reduced average annual sediment fluxes to the Gulf of Mexico by 50%–70% (Horowitz, 2010). We measured isotopes of Sr and Nd on sediment samples collected from the floodplains of the Missouri, Upper Mississippi, and Ohio Rivers to evaluate the isotopic variation among the major tributaries of the Mississippi River system before and after dam closure. Using these data, we constructed Bayesian mixing models (Parnell et al., 2013) to evaluate the provenance of flood deposits preserved in oxbow lakes previously used in paleoflood reconstructions of the Upper Mississippi (Munoz et al., 2015) and Lower Mississippi Rivers (Munoz et al., 2018). Together, our analyses established Sr-Nd isotope systematics for the largest drainage network in North America, tested their ability to establish provenance in mixed sedimentary records, and allowed us to evaluate the sensitivity of these radiogenic tracers to artificial impoundments. MATERIALS AND METHODS We collected sediment cores (in October 2016, using a gouge auger) from three infilling depressions in the floodplains of the Missouri River (38.664869°N, 90.702690°W; core length: 97 cm), Upper Mississippi River (39.112535°N, 90.695270°W; core length: 137 cm), and Ohio River (37.166491°N, 89.064583°W; core length: 96 cm) that are periodically inundated by floodwaters and are downstream of most dams (Fig. 1; Fig. DR2). To establish chronological control on these cores, we measured 137Cs activity on bulk sediment samples at 5 cm resolution on the upper 60 cm of each core in a Canberra GL2020RS well detector for lowenergy germanium radiation (Fig. DR3). We 1GSA Data Repository item 2019097, supplemental data tables (Tables DR1 and DR2) and figures (Figures DR1–DR4), is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Munoz, S.E., et al., 2019, Radiogenic fingerprinting reveals anthropogenic and buffering controls on sediment dynamics of the Mississippi River system: Geology, v. 47, p. 271–274, https:// doi .org /10 .1130 /G45194.1 *E-mail: s.munoz@ northeastern .edu Manuscript received 24 May 2018 Revised manuscript received 10 December 2018 Manuscript accepted 12 January 2019 https://doi.org/10.1130/G45194.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 6 February 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/4650251/271.pdf by guest on 31 October 2019 272 www.gsapubs.org | Volume 47 | Number 3 | GEOLOGY | Geological Society of America collected five to seven samples from each core that were stratigraphically >20 cm below the base of 137Cs activity (i.e., A.D. 1954) and above the peak of 137Cs activity (i.e., A.D. 1963) to represent the depositional environment before and after the closing of the majority of artificial impoundments on the Mississippi River system (Fig. DR1). To test the significance of difference between preand post-dam samples, we used Welch’s t-test implemented in R.Studio v.1.1.423 software (https:// www .rstudio .com /products /rstudio/) (t.test function). To minimize grain-size and hydraulic sorting effects, the <63 μm filtrate was used for grainsize and isotopic analysis. For grain-size analysis, organics were removed by incineration at 360 °C in a muffle furnace, and then the sample was dispersed in water before 5 s of sonication and analysis in a Beckman Coulter LS 13 320 laser diffraction particle-size analyzer. Nd and Sr isotopic analysis was performed with conventional ion chromatography. Strontium was separated and purified from samples using Sr-Spec (Eichrom) resin. Nd chemistry was measured with LN resin (Eichrom) following the method described by Scher and Delaney (2010). Sr and Nd analyses were conducted on a NEPTUNE multicollector–inductively coupled plasma– mass spectrometer (MC-ICP-MS) with internal precision of ~10–20 ppm (2σ). The 143Nd/144Nd and 87Sr/86Sr ratios for unknowns were normalized by the offset between our average measured value of the Nd La Jolla reference standard and the Sr NBS987 standard during the analytical session and the preferred 143Nd/144Nd value of 0.511847 (White and Patchett, 1984) and 87Sr/86Sr value of 0.710240 (Jackson and Hart, 2006), and external precision is estimated to be 15–25 ppm (2σ). The 143Nd/144Nd isotopic composition is expressed further as εNd (DePaolo and Wasserburg, 1976) units relative to (Nd/Nd)CHUR = 0.512638 (CHUR—chondritic uniform reservoir). We tested the influence of grain size on Sr and Nd isotopes (McLennan et al., 1989) using regression to find a significant linear relationship between εNd and the mode of grain size in Missouri River and oxbow lake samples (R2 = 0.7711, p < 0.001), but not in the other tributaries (Fig. DR4) nor between grain size and 86Sr/87Sr ratios. We thus used linear regression to normalize εNd of Missouri River and oxbow lake samples to the pooled mode of grain-size measurements (Table DR1). To compare the isotopic composition of the mixed sediments collected as part of this study with that of the source rocks that underlie the Mississippi River basin, we extracted the Sr and Nd isotope measurements within 500 km of the basin (n = 1356) from the GEOROC database (http:// georoc .mpch -mainz .gwdg .de /georoc/; Sarbas and Nohl, 2008). To evaluate the potential of Sr-Nd isotopes to provenance sedimentary paleoflood records, we collected two samples from previously dated cores at Horseshoe Lake, Illinois (HRM), ~20 km below the confluence of the Missouri and Mississippi Rivers (Munoz et al., 2015), and four samples from Lake Mary, Mississippi (MRY), ~1000 km below the confluence of the Ohio and Mississippi Rivers (Munoz et al., 2018), and performed grain-size and isotope analysis on these samples using the same approach as described above (Fig. 1). Chronologies of these oxbow paleoflood records were developed using a Bayesian age model informed by 137Cs, 210Pb, 14C, optically stimulated luminescence (OSL), and stratigraphic markers; additional details of their age models can be found in their original publications (Munoz et al., 2015, 2018). At HRM, one sample came from a fine-grained deposit dated to A.D. 1160 ± 90 yr (reported as median age ± 2σ confidence interval), interpreted by Munoz et al. (2014) as the suspended load from a large Mississippi River flood event, and one sample came from the same core, dated to A.D. 1450 ± 160 yr, that was not associated with a flood. At MRY, we collected samples of fine-grained sediment immediately overlying prominent flood deposits dated to A.D. 1917 ± 13 yr, 1934 ± 12 yr, 2011 ± 7 yr, and 2014 ± 6 yr, interpreted by Munoz et al. (2018) to represent major historic floods in A.D. 1927, 1937, 2011, and 2016, respectively. To evaluate the provenance of the oxbow lake sediments, we constructed Bayesian mixing models using the simmr package (ht