Chemical Geology 285 (2011) 203–214

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Chemical Geology

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Strontium isotope constraints on solute sources and mixing dynamics in a largeimpoundment, South-Central USA

Li Sun a, Matthew I. Leybourne b,⁎, Nathan R. Miller c, Roger E. Denison a

a Department of Geosciences, University of Texas at Dallas, Richardson, Texas 75083, USAb GNS Science, Lower Hutt, New Zealandc Department of Geological Sciences, University of Texas, Austin, Texas, USA

⁎ Corresponding author.E-mail address: m.leybourne@gns.cri.nz (M.I. Leybou

0009-2541/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.chemgeo.2011.04.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 September 2010Received in revised form 12 April 2011Accepted 15 April 2011Available online 28 April 2011

Editor: J. Fein

Keywords:Lake water geochemistryMetalimnionRed RiverWashita RiverUpper Red River BasinSr isotopes

Strontium concentration and isotope composition (87Sr/86Sr) are important proxies for tracing solute andsuspended load origins in surface water systems, but have not been widely applied to lakes. We determinedthese proxies, in addition to major ion compositions, in waters collected from Lake Texoma, a large crescent-shaped (concave-west) impoundment on the border of Texas and Oklahoma that marks the downstreamculmination of the Upper Red River Basin (URRB), and its major influent rivers. These rivers enter the lake atits southerly and northerly arms and have distinctly different salinities and discharges, respectively: the moresaline Red River (mean total dissolved solids, TDS=2930 mg/L; 1290 mg/L weighted for discharge,Gainesville gauging station) and the fresher Washita River (TDS=634 mg/L; 376 mg/L weighted fordischarge, Dickson gauging station). These two influent river systems combine to mix in the deepest (main)part of the lake. The URRB involves a heterogeneous mix of bedrock geologies, notably high solubility Permianmarine evaporites (gypsum, halite) and carbonates, and riverine solute load and composition can varyaccordingly with regional precipitation. Most river and lake waters show an strong positive correlationbetween 87Sr/86Sr and 1/Sr. Red River system waters have variable 87Sr/86Sr values, ranging from 0.7076 to0.7097 (n=34), whereas the Washita River system appears to be less isotopically variable, with 87Sr/86Srvalues ranging from 0.7082 to 0.7087 (n=16). Lake Texoma 87Sr/86Sr varies spatially between 0.7087 and0.7092, reflecting mixing of the two input rivers. Variations in Sr isotope composition, Ca/Na and Sr/Ca of riverand lake waters are consistent with the majority of the solute load being derived from dissolution of Permianmarine chemical sediments, dominantly halite with lesser gypsum. Waters with Sr isotope composition moreradiogenic than Permian seawater reflect the contribution of Permian and Pennsylvanian siliciclasticsediments in the case of the Red River watershed, and Precambrian granite in theWashita River watershed. Inthe main lake water column, Sr concentrations and filtered 87Sr/86Sr increase substantially with depth duringsummer stratification (from 0.7087 to 0.7091), likely reflecting desorption of Sr from suspended solid surfacesand breakdown of oxyhydroxides below the metalimnion. The differences in 87Sr/86Sr between filtered andunfiltered aliquots suggest that Sr associated with the suspended load (calcite) is either less radiogenic thanthe dissolved load or more radiogenic than surface waters but only released to the dissolved pool underdeeper reducing conditions below the metalimnion. Either possibility has important implications for Srcycling in binary mixing models and paleolake reconstruction from Sr isotopes.

rne).

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1. Introduction

The concentration and isotopic composition of aqueous Sr (Sr and87Sr/86Sr) have been used as lithochemical proxies to characterizesources of dissolved solutes, the geochemistry of surface water–groundwater interactions, and mixing dynamics in rivers, groundwa-ters, and estuaries (Palmer and Edmond, 1989; Krishnaswami et al.,1992; Négrel et al., 1993; Négrel and Dupré, 1993; Andersson et al.,

1994; Abrerg, 1995; Négrel et al., 1997; Négrel and Grosbois, 1999;Aubert et al., 2002; Ojiambo et al., 2003; Xu and Marcantonio, 2004;Xu and Marcantonio, 2007). However, relatively few studies haveutilized Sr isotopes to understand water sources, mixing andstratification in lakes and impoundments (Stein et al., 1997; Nakanaoet al., 2005; Eckardt et al., 2008; Jin et al., 2010).

Watershed studies demonstrate the utility of Sr isotope systematicsfor: (1) understanding the influence of silicate and carbonate weath-ering on the solute budget of rivers and watersheds (Clow et al., 1997;Jacobson et al., 2002; Bickle et al., 2005; White et al., 2005; Pett-Ridgeet al., 2009; Musgrove et al., 2010; Souza et al., 2010), in addition to(2) identifying sources and fluxes of solutes for management of urban

204 L. Sun et al. / Chemical Geology 285 (2011) 203–214

water quality (Nakano et al., 2008; Hosono et al., 2010). The Sr isotopecomposition of the silicate end-member is typically more radiogenicthan the carbonate end-member, so that in some systems, Sr isotopescan be a powerful tool for determining relative proportions of silicateand carbonate weathering. Although most large river systems aredominated by silicate and marine carbonate end-members, sulfate andchloride evaporite weathering also influence a smaller number ofsystems (Gaillardet et al., 1999). Different correlation patterns betweenriver discharge and Sr isotope compositions havebeen reported (Palmerand Edmond, 1989; Abrerg, 1995; Clow et al., 1997). In some studies, aninverse relationship between discharge and 87Sr/86Sr value in streamwater has been observed, whereas in others a positive correlationbetween discharge and 87Sr/86Sr value is observed for large catchmentscoveringmultiple lithologies, and in a small catchment draining graniticrocks (Palmer and Edmond, 1989; Krishnaswami et al., 1992; Abrerg,1995; Clow et al., 1997; Aubert et al., 2002).

The great utility of Sr isotopes in tracing solute sources and watermixing is that the 87Sr/86Sr value is unaffected by low-temperaturesurface processes. Note however, that recent work on 88Sr/86Sr valueshas suggested that these may be fractionated by weathering andpedogenesis (Halicz et al., 2008). Solutes in rivers and lakes generallyreflect their lithological sources, which typically have significantlydifferent Sr concentrations and 87Sr/86Sr compositions in marinechemical sediments compared to siliciclastic sediments and igneousrocks. Marine chemical sediments typically have less radiogenic 87Sr/86Sr values relative to siliciclastic rocks, which provides the proxypower of Sr isotopes. Although diagenesis and surface processes mayalter primary 87Sr/86Sr compositions of watershed lithologies,differences between primary seawater compositions (carbonates/evaporites≈0.708) and radiogenic old continental crust (siliciclas-tics≈0.712 or greater) are so large that initial solutes (and suspendedloads) are unlikely to vary significantly from their primary lithologicalorigins (Burke et al., 1982; Denison et al., 1994; Denison et al., 1998;Kirkland et al., 2000). Differences in strontium isotope ratios betweendissolvedand suspended loads canoccur inenvironmentswhere Fe–Mnoxyhydroxides formor dissolve (Andersson et al., 1994). In riverwaters,differences of 87Sr/86Sr have been observed between particulates anddissolved load (Négrel et al., 1993; Semhi et al., 2000). However,whether there are differences in the Sr isotope composition betweensuspended load and dissolved load in lakes is still unclear.

Herein, we report results of a systematic 3-year study of the Srisotope composition of Lake Texoma, a large impoundment on theborder between Texas and Oklahoma (South-Central United States).Lake Texoma is unusual in that two rivers of highly contrastingsalinities provide its main inflows. This inflow dichotomy results fromdiffering upstream bedrock lithologies and degrees of chemicalweathering, and these unique solute end-members thus provide anexcellent opportunity to use Sr isotopes to characterize the LakeTexoma geochemical system in terms of temporal and spatial solutemixing behavior. In addition to geologic variations in headwaterreaches, Lake Texoma chemistry is seasonally affected by summerstratification and bottom anoxia. In this dynamic lake system, wesurveyed Sr concentrations and 87Sr/86Sr for the two influent rivers,the lake, and its outflow water to establish the relative influences ofcatchmentweathering, hydrology and summer anoxia on Sr chemistryand isotopic composition. In order to determine differences betweentotal load and dissolved load in Lake Texoma waters, we measured87Sr/86Sr for both filtered and unfiltered samples. The two mainquestions addressed in this paper are: (1) what are the sources of Sr inthe lake and its major influent rivers, and (2) what are the majortemporal and spatial controls of 87Sr/86Sr in Lake Texoma waters?

2. Geological setting

Lake Texoma is a large crescent-shaped (concave-west) impound-ment on the border of Texas and Oklahoma that marks the

downstream culmination of the Upper Red River Basin (URRB).Constructed in 1943 for flood control, power generation, conserva-tion, and recreation, it is Texas' third largest impoundment (360 km2)with a drainage area of 15,336 km2 (Fig. 1). The deepest part of themain lake is up to 35 m in summer and the mean depth of the wholelake is 9.3 m. Inflows to the lake are mainly from two rivers, the RedRiver and the Washita River. Headwaters of both rivers within theURRB share similar bedrock lithologies, including Mississippian andPennsylvanian siliciclastics and Permian evaporites, but the WashitaRiver uniquely intersects siliceous igneous rocks of the WichitaMountains closer to Lake Texoma. URRB bedrock in lowest reaches ofthe Red River and Lake Texoma region is mainly upper Cretaceouslimestone (Fig. 1). The Red River flows into Lake Texoma from thewest along the Texas–Oklahoma border, whereas the Washita Riverenters the lake from the northwest in south-central Oklahoma. Lakebottom waters are released from Denison Dam (at the SE end of thelake) to the lower Red River, and ultimately to the lower MississippiRiver.

The Red River constitutes about 60% of the inflow to Lake Texomaand is the dominant source of dissolved solids (Table 1). Theheadwaters of the Red River system begin as springs at the base ofthe Llano Estacado, the southernmost (W. Texas) portion of the HighPlains. The River crosses Permian marine chemical sediments(gypsum, anhydrite and carbonates, with halite in the subsurface)as well as paralic and terrestrial siliclastic rocks (Permian andPennsylvanian shales, siltstones, redbeds) and Cretaceous sandstone,limestone and shale, and Cenozoic terrestrial sediments. The WashitaRiver also originates in the Texas Panhandle, but flows through south-central Oklahoma before entering Lake Texoma from the north.Although chemical sedimentary rocks are abundant in headwaterreaches of both rivers (Fig. 1), Washita River waters entering LakeTexoma are typically fresher than the Red River (Red River:TDS=2930 mg/L; 1290 mg/L weighted for discharge, Gainesvillegauging station; Washita River: TDS=634 mg/L; 376 mg/L weightedfor discharge, Dickson gauging station). “Weighting for discharge”adjusts the salinity based on inverse relationship between dischargeand TDS, and is calculated as:

TDSdischarge weighted =∑n

idischargei × TDSið Þ∑ dischargeð Þ :

Lake Texoma, like many other reservoirs in the southern andwestern United States, undergoes thermal stratification in summerand becomes almost completely mixed during the winter (Leifesteet al., 1971; Sun et al., submitted for publication-b). Limnologically,the lake also possesses strong horizontal chemical concentrationgradients (salinity, major and trace ions) with higher variability in themore riverine areas compared to deeper areas of the main lake nearthe Denison Dam (Work and Gophen, 1999; Sun et al., submitted forpublication-b).

3. Sampling and analytical methods

Samples were collected at several stations around Lake Texoma(1998–2005), the URRB (1998–2005; Fig. 1), and from multiplestations in Lake Texoma (2003–2005; Fig. 2). For Lake Texomasamples, depth, temperature, dissolved oxygen (DO), oxidation–reduction potential (ORP) and pH were measured in situ using aHydrolab Quanta-multiprobe before sampling to ensure depth-representative samples were obtained. The number of samples takenfrom each station, varied from 1 to 8 samples, depending on stationwater depth and vertical profile of physical and chemical parameters.Lake water was collected using a depth discrete (Niskin-type) watersampler and transferred to high-density polyethylene (HDPE) bottles,which were rinsed three times with sample prior to collection. Both

Fig. 1. Geologic map of Lake Texoma and the Upper Red River basin (modified after King and Beikman, 1974) showing locations of water samples throughout the basin. Single samplelocations are shown with corresponding 87Sr/86Sr values. Multi-sampled locations are indicated by site names listed on Table 3. The rectangular box outlines the Lake Texoma area.

205L. Sun et al. / Chemical Geology 285 (2011) 203–214

filtered and unfiltered samples for each location were collected forcation analyses. For filtered samples, 250 mL volumes were passedthrough 0.45 μm Millipore Sterivex membrane filters. Cation sampleswere acidified to pHb2 with 70% ultrapure HNO3. For the Red Riversystem, we collected samples for Sr isotope analysis from the mainstem of the Red River in the headwaters in West Texas (Pease andWichita rivers), from several tributaries of the Red River inWest Texas,and from the USGS gauging station at Gainesville, the last stationbefore theRedRiver enters Lake Texoma(Fig. 1). The largest number ofsamples was collected from the USGS gauging station at Gainesville(n=26) over the period 1998 to 2004. These URRB samples (andsamples collected below Denison Dam and the Washita River duringthis period) were collected without filtration in pre-cleaned bottles

Table 1Total annual average discharge to Lake Texoma by the Red River and the Washita Riverin six consecutive years from USGS real-timewater data set (http://waterdata.usgs.gov/nwis). Locations of USGS gage stations at Gainesville and Dickson are shown on Fig. 1.Unit: m3/s.

Year Gainesville(Red River)

Dickson(Washita River)

Washita R. inflow as a% of the Red R. inflow

1998 39,350 26,138 60%1999 18,923 14,420 57%2000 26,786 18,642 59%2001 32,421 23,612 58%2002 16,749 12,849 57%2003 9643 8192 54%2004 27,726 16,782 62%Mean 24,514 17,234 58%SD 10,090 6206 3%

and stored in a cooler after collection in thefield before refrigeration inthe lab.

Water samples for aqueous geochemical studies are typicallyfilteredusing either 0.45 μmor0.2 μmfilters. Filteredanalyte concentrations arethen commonly considered as the dissolved load, whereas thesuspended load is considered the difference between filtered andunfiltered aliquots. However, thesefilter pore sizes and the classificationof b0.45 μm as the dissolved load are only operationally defined (Hallet al., 1996); the b0.45 μm portion contains not only dissolved species,but also fine colloidalmaterial. To determine the truly dissolved portionrequires the use of ultrafiltration (Degueldre et al., 1996; Zanker et al.,2002) or centrifugation (Leybourne et al., 2000) to 0.005 μm or less.With the caveat that 0.45 μm filters do not represent truly dissolvedspecies,wemeasured the Sr isotope composition of LakeTexomawatersin filtered (b0.45 μm) and, in a subset of samples, unfiltered aliquots.

Strontium and major ion concentrations were analyzed byInductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES;Perkin Elmer Optima 3300DV), ion chromatography (DionexDX 600),and end-point titration (alkalinity). Complete analytical details areprovided in Sun et al. (submitted for publication-b).

Two methods using two different thermal ionization massspectrometers (TIMS) were used to determine 87Sr/86Sr values. Forthe first method, 5 mL of 0.5M Na2CO3 was added to 50 mL unfilteredaliquots to precipitate CaCO3 and SrCO3. Subsequently, samples werecentrifuged and the supernatant decanted. The remaining solids weredried and dissolved in 0.5 mL of 3N HNO3. Strontium was separatedusing a strontium specific resin and collected with deionized (Milli-Q)water. Sr solutions were dried and solid samples were dissolved in0.5 mL 3N HNO3, loaded onto etched Ta filaments, and dried. The

Table 287Sr/86Sr ratios of carbonates and gypsum in the URRB.

Sample ID 87Sr/86Sr ± Description

Carbonate SF-1C 0.708694 6 Salt flat, surface carbonatesand, Hudspeth Co. TX

SF-2C 0.708681 10 Salt flat, core carbonateSF-3C 0.708739 9 Salt flat, core carbonateSR-4C 0.708695 9 Salt flat, core carbonateMean 0.708702 SD 0.000025

Gypsum SF-1G 0.708726 18 Salt flat, surface gypsum sandSF-2G 0.708740 13 Salt flat, core gypsumSF-3G 0.708664 10 Salt flat, core gypsumSR-4G 0.708690 9 Salt flat, core gypsum13173G 0.707919 17 White sands gypsum,

Dona Ana Co., NMWS-1 0.707930 10 White sands NM gypsumMean 0.708445 SD 0.000404

Fig. 2. Lake Texoma map showing locations of sampling sites (0–20) and USACEchloride zones the Main Lake (ML), the Red River zone (RRZ), and the Washita Riverzone (WRZ) and two transition zones (RRTZ and WRTZ) (MC = mean chloride)(Atkinson et al., 1996).

206 L. Sun et al. / Chemical Geology 285 (2011) 203–214

isotope ratios weremeasured on a second order, double focusingmassspectrometer with a 60°, 33.0 cm radius of curvature magnetic sectorand a 91°, 40.1 cm radius of curvature electric sector. Masses 85, 86,87, and 88 were measured simultaneously in four faraday cups. The87Sr/86Sr values were normalized to 86Sr/88Sr=0.1194. The isotoperatios of the unknown samples were measured by comparison to NISTstandard SRM-987, normalized to a value of 0.710240.

For the second method, filtered and unfiltered Lake Texomasamples containing ~3 μg Sr were evaporated under HEPA-filtered air.Unfiltered samples were shaken thoroughly before processing toensure the incorporation of both particulates and dissolved solutes indried solids. Dried samples were dissolved in 0.2 mL of 3N HNO3, andthen loaded into polypropylene columns containing 0.3 mL of Sr-specific resin (Eichrom). After washing the resin column with 1.8 mL3N HNO3, Sr was eluted with 1 mL of deionized (Milli-Q) water. Twodrops of 1N phosphoric acid were added to visualize the sample afterevaporation; again under filtered air. Samples were loaded onto Refilaments as tantalum oxide slurries, which promotes ionization, andanalyzed in static mode for Sr isotopes on a Finnigan MAT261 TIMS.NIST SRM-987, analyzed every three samples, yielded an average 87Sr/86Sr composition of 0.71029±7. Sample LS-LT0803 1–2, analyzedtwice over 4 months yielded 87Sr/86Sr values of 0.708756±21 and0.708789±23, respectively. After the first group of tests on both TIMS,differences in 87Sr/86Sr value between filtered and unfiltered aliquotsof identical samples were observed. In order to rule out the possibilityof machine bias, two identical unfiltered aliquots (LS-LT0803 9–2)were analyzed on both TIMS. The two instruments yielded 87Sr/86Srvalues of 0.708742±20 and 0.708744±20, respectively, indicating nosystematic bias.

Descriptive statistics for strontium concentration and isotope datawere calculated and evaluated using the Minitab statistical analysisprogramv. 14.1 (Minitab, Inc., State College, PA). Correlation coefficientswere also calculated using student and paired sample t-tests todetermine 95% (unless otherwise noted) significance levels of differ-ences between selected variables. Speciation and saturation indices (SI)for the lake waters were calculated using PHREEQCi (Parkhurst andAppelo, 1999), with pe controlled by measured Eh.

4. Results

4.1. Catchment geology and regional waters

Permian marine chemical sediments, including evaporites (gyp-sum outcrops and halite in subcrop) and carbonates, dominate thegeology of the headwater regions for both the Red andWashita rivers.Samples of gypsum analyzed in this study and elsewhere in thewatershed (Miller and Leybourne, 2002; Miller et al., in preparation)(Table 2) are consistent with values for Permian seawater, with 87Sr/86Sr of ~0.7079. Gypsum and carbonate from Salt Flat, Texas, are moreradiogenic than Permian seawater, with 87Sr/86Sr of ~0.7087. Closer toLake Texoma both of the main influent rivers flow over Cretaceousmarine carbonate-dominated lithologies (Fig. 1). Although we did notanalyze Cretaceous carbonates in the catchment, seawater during thistime had a narrow range of 87Sr/86Sr (0.7070–0.7075), which bracketsthe strontium isotope composition of major bedrocks in the catch-ment area.

Based on data collected in theWichita River watershed (Miller andLeybourne, 2002; Leybourne et al., in preparation) and USGS gaugingstations, highest salinities in the Red River system occur in headwaterregions—with some tributaries having salinities in excess of seawater(e.g., South Fork Wichita River). For the samples reported here, TDSranges from as high as 11,900 mg/L in the Prairie Dog Town Fork of theRed River to as low as 319 mg/L at the downstreamGainesville station(Table 3). Salinities decrease along the Red and Washita riverstowards Lake Texoma, with salinities variable as a function ofdischarge. At the Gainesville gauging station, just above Lake Texomaon the Red River, salinity variations as a function of discharge andrelative solute contributions from headwater tributaries follow apower law: Ln[SpC]=−0.289 Ln[m3/s]+9.2, r=−0.69. During lowflow periods, salinity at the Gainesville station commonly exceeds3000 mg/L. By contrast, at the Dickson gauging station on theWashitaRiver, river salinity is typically much lower than at Gainesville,although salinity and discharge still follow a power law: Ln[SpC]=−0.182 Ln[m3/s]+7.5, r=−0.51. The dampening effect on waterchemistry of Lake Texoma is evident for the Denison Dam gaugingstation,where salinity decreases only slightlywith discharge: Ln[SpC]=−0.030 Ln[m3/s]+7.5, r=−0.28.

Table 3Sr concentration and 87Sr/86Sr values in surface waters collected in Lake Texoma and the Upper Red River Basin from 1998 to 2004 (mostly unfiltered unless otherwise indicated).

Location Sample ID Collectiondate

87Sr/86Sr ± TDS(mg/L)

Sr(mg/L)

Ca(mg/L)

Sr/Ca(molar)

Ca/Na(molar)

Discharge(m3/s)

Upstream Red River Red River-24 07/07/99 0.707867 7 3730.00 6.73 591.47 0.0052 0.0758Red River-18 02/20/99 0.708156 17 3870.00 4.09 331.05 0.0056 0.1387Red River-19 02/20/99 0.707797 15 1550.00 4.59 463.23 0.0045 1.1203Red River-20 02/20/99 0.707659 13 11900.00 10.86 779.05 0.0064 0.0940Red River-21 02/20/99 0.707982 15 4250.00 3.22 232.18 0.0063 0.1810Mean 0.707892 5060.00 5.90 479.40 0.0056 0.32196SD 0.000188 3966.95 3.06 215.49 0.00079 0.44815

Red River tributaries Pease River 03/21/99 0.707610 17 10000.00 12.11 926.62 0.0060 0.1278Wichita Falls 03/21/99 0.709654 17 2760.00 5.36 305.07 0.0080 0.1785 0.40Wichita Falls 09/07/99 0.708967 12 2800.00 4.53 268.93 0.0077 0.1694 1.56Wichita Falls 05/05/00 0.708373 8 2900.00 3.77 252.43 0.0068 0.1874 0.76Mean 0.708651 4615 6.4425 438.26 0.00713 0.16578 0.96SD 0.000869 3590.48 3.8337 326.31 0.00091 0.02636 0.57

Red River at Gainesville Red River-1 03/03/98 0.708400 14 1870.00 1.70 119.46 0.0065 0.1906 168.66Red River-2 05/26/98 0.708322 14 2480.00 2.65 188.33 0.0064 0.1908 42.08Red River-4 07/10/98 0.708694 9 2120.00 3.62 229.82 0.0072 0.1787 21.92Red River-5 08/06/98 0.708772 10 2200.00 3.05 178.25 0.0078 0.1604 10.62Red River-9 09/10/98 0.708538 17 2580.00 3.73 247.77 0.0069 0.2017 7.05Red River-10 10/23/98 0.708611 8 2450.00 10.82Red River-13 11/24/98 0.708382 14 2850.00 3.72 280.33 0.0061 0.1684 13.42Red River-15 12/29/98 0.708369 16 2380.00 3.45 260.07 0.0061 0.1873 11.02Red River-15 01/28/99 0.708356 7 2950.00 3.63 270.08 0.0061 0.1746 12.66Red River-17 02/25/99 0.708365 15 3010.00 3.42 247.54 0.0063 0.1566 14.67Red River-22 04/22/99 0.708963 15 1150.00 1.51 119.34 0.0058 0.2146 27.33Red River-23 06/03/99 0.708390 8 732.00 1.10 101.99 0.0049 0.3010 200.43Red River-25 10/12/99 0.708695 15 2430.00 3.30 215.29 0.0070 0.1578 7.53Red River-27 01/13/00 0.708524 15 2390.00 2.54 185.69 0.0063 0.1639 11.30Red River-29 02/03/00 0.708466 10 2920.00 3.31 246.15 0.0061 0.1744 9.74Red River-30 02/17/00 0.708486 17 3180.00 3.61 263.17 0.0063 0.1582 6.77Red River-33 09/17/00 0.708565 13 2890.00 3.93 265.26 0.0068 0.1750 2.69Red River-35 11/10/00 0.709349 7 319.00 0.51 57.33 0.0041 0.3405 462.05Red River-36 01/26/01 0.708855 18 1550.00 1.96 125.32 0.0071 0.1418 45.08Red River-37 10/20/01 0.708903 16 1280.00 11.19Red River-38 10/20/01 0.708893 14 1360.00 11.19Red River-39 04/26/02 0.708578 11 1210.00 0.2453 53.01Red River-40 11/19/02 0.708325 11 2920.00 14.10Red River −41 04/10/03 0.708536 11 2920.00 9.17Red River-42 11/06/03 0.708982 17 2170.00 4.33Red River-44 11/11/04 0.709211 12 458.00 44.03Mean 0.708636 2106.5 2.82 200.07 0.0063 0.1938 47.40SD 0.000281 841.74 1.03 68.75 0.0008 0.0510 97.01

Red River below Denison Dam Red River-3 05/26/98 0.708688 15 952.00 1.44 120.86 0.0055 0.2995 13.22Red River-6 08/06/98 0.708693 15 823.00 1.37 117.32 0.0053 0.3464 120.29Red River-8 09/10/98 0.708668 12 761.00 1.37 116.84 0.0054 0.3472 64.48Red River-11 10/23/98 0.708598 15 838.00 1.42 89.27 0.0073 0.2445 61.19Red River-12 11/24/98 0.708603 13 1220.00 1.72 111.13 0.0071 0.1954 46.24Red River-14 12/29/98 0.708497 11 870.00 1.48 101.18 0.0067 0.2678 57.94Red River-14 01/28/99 0.708594 16 878.00 1.50 104.08 0.0066 0.2736 58.50Red River-16 02/25/99 0.708620 16 922.00 1.64 120.30 0.0062 0.3036 57.14Red River-26 01/13/00 0.708560 13 1070.00 1.51 108.27 0.0064 0.2591 75.97Red River-28 02/03/00 0.708513 12 1090.00 1.54 116.35 0.0061 0.2796 60.91Red River-31 04/02/00 0.708560 17 1040.00 1.57 121.13 0.0059 0.2955 160.16Red River-32 04/04/00 0.708567 16 1060.00 194.90Red River-34 09/17/00 0.708472 18 1010.00 1.65 127.99 0.0059 0.2546 7.45Red River-43 10/18/04 0.708666 14 853.00 70.76Red River-45 01/26/05 0.708639 7 813.00 159.08Red River-46 02/16/05 0.708746 13 809.00 158.52Mean 0.708605 938.06 1.52 112.89 0.0062 0.2806 85.43SD 0.000762 129.93 0.11 10.70 0.00065 0.0424 55.70

Downstream Red River Red River-7 08/06/98 0.708673 18 740.00Washita River at Davis Washita-1 03/03/98 0.708382 13 847.00 2.49 144.97 0.0079 1.0983

Washita-2 (filtered) 05/05/98 0.708600 18 565.00 1.51 93.08 0.0074 1.1113Washita-12 04/22/99 0.708246 13 740.00 2.57 190.92 0.0062 1.7623Washita-13 10/12/99 0.708456 16 935.00 2.49 182.50 0.0063 0.8866Mean 0.708421 771.75 2.265 152.87 0.0070 1.21463SD 0.000148 159.23 0.5047 44.583 0.00083 0.37937

Washita River at ChickasawRecreation Area

Pavilion Spring 09/17/00 0.711143 9 420.00 1.82 43.29 0.0193 0.3218Vendome Well 09/17/00 0.711142 15

Washita River at Tishomingo Washita-3 05/26/98 0.708368 15 711.00 2.16 158.25 0.0063 1.3376 35.23Washita-4 07/10/98 0.708514 10 800.00 2.27 138.21 0.0075 0.7398 10.42Washita-5 08/06/98 0.708380 15 885.00 2.48 157.60 0.0072 0.7496 4.81Washita-6 09/10/98 0.708431 17 810.00 2.36 151.16 0.0071 0.7089 2.69Washita-7 10/23/98 0.708715 18 547.00 1.17 70.54 0.0076 0.7639 17.39Washita-8 11/24/98 0.708189 16 920.00 3.03 215.51 0.0064 1.7506 17.19

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207L. Sun et al. / Chemical Geology 285 (2011) 203–214

Table 3 (continued)

Location Sample ID Collectiondate

87Sr/86Sr ± TDS(mg/L)

Sr(mg/L)

Ca(mg/L)

Sr/Ca(molar)

Ca/Na(molar)

Discharge(m3/s)

Washita-9 12/29/98 0.708188 16 756.00 2.64 183.94 0.0066 1.5580 28.68Washita-10 01/28/99 0.708342 14 764.00 2.33 188.61 0.0057 1.4251 15.35Washita-11 02/25/99 0.708376 16 780.00 2.63 203.09 0.0059 1.5638 20.76Washita-14 01/13/00 0.708559 16 762.00 1.75 132.23 0.0060 1.0424 12.83Washita-15 02/03/00 0.708538 14 800.00 1.91 159.77 0.0055 1.1827 11.38Mean 0.708418 775.91 2.25 159.90 0.0065 1.1657 16.11SD 0.000158 96.10 0.50 39.58 0.0007 0.3862 9.57

Washita River at Tishomingo

208 L. Sun et al. / Chemical Geology 285 (2011) 203–214

Waters collected from Red River tributaries provide somemeasureof the Sr flux and range of 87Sr/86Sr associated with Red River input toLake Texoma. On the main stem of the upper Red River, 87Sr/86Srvaries between 0.7077 and 0.07082 (average=0.7079±0.0002,n=5). These values are similar to Permian seawater. Four UpperRed River tributary samples (one on the Pease River and three on theWichita River at Wichita Falls) have Sr isotope compositionscomparable to or more radiogenic than Permian seawater (87Sr/86Sr=0.7076–0.7097). A more comprehensive study of the Sr isotopesystematics of theWichita River system is in progress, but preliminarydata indicate that 87Sr/86Sr values range from 0.7074 to 0.7126 (Millerand Leybourne, 2002; Miller et al., in preparation); the easternWichita River near its confluence with the Red River (Wichita Falls) ismore radiogenic and typically less saline than its western headwaters.

The Red River at Gainesville (above Lake Texoma) is variable interms of Sr isotope composition, salinity and discharge. Unfilteredwater samples have 87Sr/86Sr values that fluctuate through time be-tween 0.7083 and 0.7093. Although there is no statistically significantrelationship between Sr isotope composition and discharge, the RedRiver at Gainesville is generally more radiogenic during periods ofincreased discharge. For this station, Ca/Namolar values vary inverselywith discharge. The Ca/Namolar values, along with Sr isotope compo-sition, are primarily controlled by bedrock lithology.

Washita River waters collected at Davis, Oklahoma (about 100 kmnorthwest of Lake Texoma), and Tishomingo, Oklahoma (proximal totheWashita input; Fig.1) have 87Sr/86Sr ranging from 0.7082 to 0.7087(n=15). The highest Washita River 87Sr/86Sr values (0.7111, n=2)were found in Pavilion Spring and Vendome Well, Oklahoma (about70 km NNW of Lake Texoma) (Fig. 1).

Outflow from Lake Texoma (i.e., the Red River belowDenisonDam)has relatively uniform 87Sr/86Sr close to 0.7086±8 (unfiltered, n=16;Table 3); slightly less radiogenic than Lake Texoma (0.708818±0.000180, unfiltered, n=8; Table 4) (t=5.83, pb0.001).

River and lake samples show a general inverse relationshipbetween 87Sr/86Sr and both Sr concentration and TDS (Fig. 3). Riverwaters above Lake Texoma, from the Red River at Gainesville and

Table 4Sr and Ca concentrations and Sr isotope compositions of water samples from Lake Texoma

Sample ID Collectiondate

Sample depth(m)

pH Water columndepth (m)

Filtered87Sr/86Sr ±

LS-LT0803 1–2 08/09/03 4 8.23 12 0.708756 2LS-LT0803 5–2 08/09/03 3 8.23 15 0.708747 2LS-LT0803 5–4 08/06/03 15 7.27 15 0.708944 2LS-LT0803 7–2 08/06/03 6 8.07 22.5 0.708722 1LS-LT0803 7–5 08/06/03 20 6.99 22.5 0.708740 1LS-LT0803 8–2 08/06/03 5 8.21 21 0.708871 2LS-LT0803 9–2 08/06/03 6 8.06 28 0.708931 2LS-LT0803 9–3 08/06/03 15 7.66 28 0.708694 1LS-LT0803 9–4 08/06/03 18 7.39 28 0.708861 1LS-LT0803 9–5 08/06/03 24 7.2 28 0.709106 1LS-LT0803 9–6 08/06/03 27 7.12 28 0.708955 1LS-LT0803 14–2 08/06/03 6 7.75 14 0.708998 2LS-LT0803 16–2 08/08/03 1 7.73 1 0.709233 1Mean 7.69 0.708889SD 0.45 0.000162

Washita River at Tishomingo, exhibit negative correlations betweenSr concentrations and 87Sr/86Sr values (r=−0.54; and r=−0.91,respectively) (Fig. 3). TDS is also negatively correlated with 87Sr/86Srvalues at Gainesville (r=−0.712; n=18). However, samples fromthe upper Red River main stem show much greater variation in Srconcentration, with only subtle changes and least radiogenicsignatures in Sr isotopes (Fig. 3), except that there is an positivetrend for three samples from the Wichita River at Wichita Falls thatare characterized by more radiogenic 87Sr/86Sr with increasing Srconcentration (Table 3; Fig. 3).

Strontiumconcentrations decrease downstream in theURRB catch-ment area above, within, and below Lake Texoma. The highest mea-sured Sr concentration of 12.1 mg/L was found for the Pease River, aRed River tributary (500 km west of Lake Texoma). Sr concentrationsbetween 11 and 3 mg/L (average: 5.90±3.06, n=5) occur in themainstemof the Red Riverwest of Lake Texoma (Table 3). River Sr input justabove Lake Texoma averages 2.8±1.0 mg/L (n=18) for Gainesville(Red River) and 2.3±0.5 mg/L (n=11) for Tishomingo (WashitaRiver). The Red River belowDenisonDamhas Sr concentrationswithina narrow range, 1.7–1.4 mg/L (average: 1.5±0.1 mg/L), reasonablycomparable to that in the lake (2003 average: 1.0±0.1, n=13;Table 4).

Ca concentrations range from 43 to 927 mg/L in the Red River,55–86 mg/L in Lake Texoma, 43–216 mg/L in the Washita River, and89–128 mg/L in the outflow below the Denison Dam in the Red River.Red River waters have Ca/Namolar values ranging from 0.09 to 0.35(average=0.21±0.19), with one sample N1.0 (Fig. 4). Lake Texomawater samples have broadly similar Ca/Namolar values, ranging from0.18 to 0.45 (average=0.26±0.06). By contrast,Washita River samplesexhibit more elevated Ca/Namolar values, typically N1 (average=1.13±0.42).

4.2. Lake Texoma

Sr concentrations vary both laterally and vertically within LakeTexoma. Highest concentrations in the range of 0.72 to 1.32

(August 2003).

Unfiltered87Sr/86Sr ±

Sr(mg/L)

Ca(mg/L)

Na(mg/L)

Sr/Ca(molar)

Ca/Na(molar)

TDS(mg/L)

1 0.709023 27 1.185 84.6 266.0 0.006407 0.1824 1105.121 1.06 79.6 206.0 0.006005 0.2216 939.492 0.708733 17 1.100 83.0 217.0 0.006035 0.2194 990.908 0.999 78.2 187.0 0.005765 0.2399 883.868 0.708663 19 1.090 84.0 189.0 0.005908 0.2549 896.20 1.040 78.6 200.0 0.005965 0.2254 930.122 0.708744 20 1.010 80.7 192.0 0.005685 0.2411 905.660 0.708699 18 0.986 80.7 185.0 0.005603 0.2502 893.069 1.030 81.8 194.0 0.006067 0.2419 913.068 0.708756 20 1.090 86.2 188.0 0.005731 0.263 913.877 0.708752 14 1.080 86.1 185.0 0.005711 0.267 894.734 0.930 75.0 133.0 0.005678 0.3235 746.097 0.709173 18 0.775 54.8 71.8 0.006365 0.4378 457.98

0.708818 1.029 79.5 185.7 0.005917 0.2591 882.320.00018 0.099 8.1 44.7 0.000258 0.0627 149.60

A

B

C

Fig. 3. Plots relating Sr isotope composition of Lake Texoma waters and catchmentrivers to salinity/TDS and Sr concentration.

209L. Sun et al. / Chemical Geology 285 (2011) 203–214

(average=1.06±0.12) occur near the inflow of the Red River (site 1;RRZ), whereas systematically lower concentrations are encounteredwith increasing proximity to Washita River inflow (site 16; WRZ,Table 4). Within the deep main lake, Sr concentrations decreaseslightly with depth toward the metalimnion (i.e., 15 m at site 9) andthen increase again toward the lake bottom (Table 4, Fig. 5).

Overall, 87Sr/86Sr values of filtered waters from Lake Texoma fromAugust 2003 range from 0.708694±10 (site 9 at 15 m) to 0.709233±17 (site 16 at 1 m) with a mean value at 0.708889±162 (Table 4).Regionally, Sr isotope compositions of Lake Texomawaters weremostradiogenic in the WRZ (August 2003) (Fig. 4). In the main lake area,87Sr/86Sr values vary with depth, temperature, pH, and DO (Fig. 5). Forexample, at site 9 above DenisonDam, the least radiogenic samplewasfound at 15 m (0.7087), whereas more radiogenic values occurred inthe surface and bottom waters (0.7088–0.7091; Fig. 5). Lake Texomawaters become more radiogenic, and Sr contents decrease, away fromthe Red River inflow and toward the Washita River inflow.

Several other geochemical indices (Ca, Ca/Na, and Sr/Ca) appear tofollow lateral and vertical Sr concentration and 87Sr/86Sr variationswithin Lake Texoma (Table 4, Fig. 4). For example Ca concentrations(range: 55 to 86 mg/L) are greater toward the Red River input andlower toward the Washita River input, and also increase with depth(e.g., sites 7 and 9, Table 4). Ca/Na is generally higher towardWashitaRiver input (Table 4; Fig. 4C) and with depth (sites 7 and 9). Sr/Cavariations within Lake Texoma appear to be somewhat morecomplicated. Although they are generally higher toward Red Riverinput, locality 16 closest to Washita River inflow has the secondhighest Sr/Ca value and is a notable exception to this trend. Inaddition, Sr/Ca seems to increase then decrease with depth near themetalimnion, but the depth of maximum enrichment is shallowerthan that for 87Sr/86Sr described above (Table 4).

4.3. Sr isotope composition of filtered and unfiltered samples

Regionally, filtered Lake Texoma samples (n=8) generally havemore radiogenic 87Sr/86Sr compositions compared to their unfilteredcompanions, with two notable exceptions. One sample from the mainlake zone (LS-LT0308 9–3) has identical filtered and unfiltered values,and one sample from the Red River zone (LS-LT0308 1–2) has a moreradiogenic unfiltered signature compared to its filtered aliquot(0.709023±27 vs. 0.708756±21; paired sample t-test: all samplesn=8, t=1.56, P=0.164; without the two exceptions n=6, t=4.22;P=0.008). Interestingly, compared to filtered samples from the mainlake zone that show vertical 87Sr/86Sr variations relative to themetalimnion, corresponding unfiltered samples have virtually iden-tical 87Sr/86Sr compositions (Site 9; Fig. 5).

Limited comparisons for riverine samples also demonstrate 87Sr/86Sr variations between filtered and unfiltered samples. Specifically,one filtered sample collected from the Washita River at Davis is moreradiogenic (0.708600±18) than three other unfiltered samples(0.708246±13 to 0.708456±16).

4.4. Mineral saturation calculations

All Lake Texoma waters are undersaturated with respect to gypsum(average SIgypsum=−1.34±0.03) and halite (average SIhalite=−5.89±0.20). In contrast, manywaters are saturatedwith respect to calcite (anddolomite) (Fig. 5c). LakeTexomawatersdonot showsignificant variationin major ion chemistry and this is reflected in the restricted range inSIcalcite, from−1.04 to 0.58. Furthermore, there is no significant regionalvariation in calcite saturation within the lake (e.g., Red River arm versusWashita arm and main lake). However, there is a clear distinctionbetween SIcalcite and depth, pH, dissolved oxygen and Eh (e.g., DO shownin Fig. 5).Waters below ~12 mdepth are undersaturated throughout thelakewith respect to calcite,with thedegreeof undersaturation increasingwithdepth. In contrast,waters above~10 mare saturatedwith respect tocalcite. In comparison, Leybourne et al. (in preparation) found thatessentially all surface waters in the Wichita River watershed and theupper Red River are saturated with respect to calcite. Thus, thehypolimnion (the lower or, in summer, the anoxic layer) is characterizedby calcite undersaturation, whereas themetalimnion and epilimnion arecharacterized by calcite saturation.

Siliciclastic weathering

Gypsum weathering

Gypsum weathering

Silic

icla

stic

wea

ther

ing

B

A

C

D

F

E

Fig. 4. Solute molar cation cross-plots showing probable mineralogical end-member associations. A) 87Sr/86Sr vs. Ca/Namolar, B) 87Sr/86Sr vs. Sr/Namolar, and C) Ca/Namolar vs. Sr/Namolar. Shaded boxes on A, B, and represent the respective insets for D, E, and F. Also shown are data for large rivers of the world, from Gaillardet et al. (1999).

210 L. Sun et al. / Chemical Geology 285 (2011) 203–214

5. Discussion

5.1. Solute sources and catchment control on the Sr isotope compositionof Lake Texoma

Surface water 87Sr/86Sr has been used as a lithogeochemical proxyto better understand the relative contributions of carbonate and

silicate weathering to the solute budget of rivers, lakes, and the ocean(Semhi et al., 2000; Jacobson et al., 2002; Bickle et al., 2005).Significant volumes of highly soluble chemical marine strata (Permiangypsum/anhydrite, sub-surface halite, Permian/Cretaceous carbon-ate) within headwater regions of the URRB complicate a similarassessment for Lake Texoma (Fig. 1) and its two main (Red/Washita)inflow systems.

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

86420

SIca

lcit

e

Main lake

Washita River arm

Red River armDissolved oxygen (DO)pH

Sr concentrations

87Sr/86Sr filtered87Sr/86Sr unfiltered

0.7086 0.7088 0.7090 0.709287Sr/86Sr

Sr (mg/L) DO (mg/L)

DO (mg/L)

0

4

8

12

16

20

24

28

Dep

th (

m)

0.95 1.00 1.05 1.10 1.15

7.00 7.50 8.00 8.50

pH

Epilimnion

Hypolimnion

Metalimnion

0 2 4 6 8

A B C

Fig. 5. Comparison of 87Sr/86Sr values between filtered and unfiltered samples collected at Site 9 in Lake Texoma in August 2003 (a) in the background of vertical variations of pH anddissolved oxygen (DO) profiles (b). Filtered samples had higher 87Sr/86Sr than their paired unfiltered samples except Sample 9–3. Also shown in the figure are three layers of waterstratification, epilimnion (upper or oxic layer), metalimnion (middle or transitional layer), and hypolimnion (lower or anoxic layer). This stratification normally appears in summerseasons in Lake Texoma from early June to late September. c) Plot showing the relationship between the saturation index of calcite (SIcalcite) and dissolved oxygen. The data suggest amajor change in calcite saturation at the chemocline, where waters become reducing and pH decreases; namely, waters above and below the chemocline are respectively saturatedand unsaturated with respect to calcite.

211L. Sun et al. / Chemical Geology 285 (2011) 203–214

Weathering related solute flux studies are also important forunderstanding the secular evolution of seawater 87Sr/86Sr relative tovariations in its major inputs — non-radiogenic Sr derived fromhydrothermal alteration of seafloor and radiogenic Sr derived fromchemical weathering of old continental crust (Burke et al., 1982). Forexample, Jacobson et al. (2002) used Sr isotopes and major ionconcentrations to show that Himalayan stream solutes derive mainly(~76%) from carbonate dissolution. For Lake Texomawaters, it is moredifficult to constrain the relative contributions from silicate andcarbonate weathering, as the dissolution of evaporites play a majorrole in composition of solute sources. This influence of evaporitedissolution on the URRB and Lake Texoma is evident by comparisonwith the composition of major rivers elsewhere (Fig. 4) (Gaillardetet al., 1999). Most of the world's large river systems primarilyrepresent mixing between carbonate and older silicate weatheringend-members in terms of Sr isotope compositions and major ionratios (Fig. 4). By contrast, both the Red and Washita rivers show thestrong influence of evaporite dissolution; for the Red River, halitedissolution is a major influence, whereas for the Washita River,gypsum dissolution is important (Fig. 4).

To apportion the halite dissolution contribution to Lake Texoma,an equivalent amount of Na to the concentration of Cl is assigned tohalite dissolution, and for SO4 derived from gypsum/anhydritedissolution; an equivalent amount of Ca is apportioned (Jacobsonet al., 2002). The remaining Na (Nasilicate) is then assumed to bederived from silicate weathering, with Casilicate calculated from wholerock Ca/Na values (e.g., Jacobson et al., 2002). However, Lake Texomawaters have Na/Clmolar indicating that essentially all of the Na and Cl isderived by dissolution of halite (average Na/Clmolar=0.99±0.029).There is an excess of SO4 over Ca (average Ca/SO4-molar=0.82±0.047), whereas adding Mg (from epsomite dissolution) produces anexcess of (Ca+Mg)/SO4 (average [Ca+Mg] /SO4-molar=1.37±0.067). Even if all of the DIC in the upper 10 m of Lake Texomacomes from carbonate dissolution, at most 3–5% of the dissolved loadcan be accounted for by congruent carbonate dissolution, andremoving the mole equivalent of Ca and Mg results in (Ca+Mg)/

SO4-molar closer to unity (average [Ca+Mg] /SO4-molar=1.15±0.048). These calculations are consistent with variation in the Srisotope compositions of Lake Texoma and its influent rivers (Fig. 4), asmost water samples are located closer to the halite than to other twoend members with respect to Ca/Namolar and 87Sr/86Sr (Fig. 4). Thevariations in Sr/Camolar, Ca/Namolar and 87Sr/86Sr, thus, indicate thatweathering in the headwater catchments to Lake Texoma isdominated by dissolution of halite and gypsum with Sr isotopecompositions consistent with Permian seawater (reflected in theupper Red River main stem and Pease River samples; Fig. 4), andweathering of clastic sedimentary rocks with substantially moreradiogenic Sr isotope compositions. More radiogenic Sr derived bysilicate weathering is indicated by the sample from Chickasaw, whichis locally influenced by chemical weathering of Precambrian granit-oids, and the samples from the Wichita River at Wichita Falls (Fig. 4)that flow through Paleozoic (Carboniferous) paralic siliciclastics.

Thus, Lake Texoma solutes and Sr isotope signatures are dominatedby chemical and clastic sediments in the headwater catchments; theextensive Cretaceous carbonates closer to Lake Texoma have a minorinfluence. However, the decrease in salinity from the headwaterregions from near seawater to values ~1000 mg/L is a function ofrunoff and base-flow dilution of the Red andWashita river systems asthey flow eastwards to Lake Texoma. Mixing calculations for thedifferent lithology end-members indicate that N95% dilution of highersalinity headwater streams has little influence on the Sr/Camolar, Ca/Namolar and Sr isotope composition of the mix.

The average 87Sr/86Sr values for all samples in the catchment areais significantly lower (0.0003 or 300 ppm less) than that of LakeTexoma (average=0.7086 vs. 0.7089) and modern seawater (87Sr/86Sr=0.7092) (DePaolo and Ingram, 1985). However, as shown bythe decrease in salinity with increasing discharge and the Sr isotopecomposition over time for the Gainesville gauging station, the Srisotope composition of Lake Texoma reflects the dampening effects ofthe lake on the chemistry of the rivers entering the lake. Our morelimited time series data for the Washita River suggests that 87Sr/86Srcompositions are less variable compared to Red River input. However,

212 L. Sun et al. / Chemical Geology 285 (2011) 203–214

at the time the samples from the Washita River arm of Lake Texomawere collected, the Washita River was anomalously radiogenic, likelyreflecting rainfall in the lower portion of theWashita Riverwatershed,with subsequent baseflow to the river and runoff more influenced bychemical weathering of the granitic rocks.

5.2. Sr isotopes in Lake Texoma waters

The negative correlation between 87Sr/86Sr values and Sr concen-trations, (similarly documented by (Semhi et al., 2000), can beexplained by variable solubilities of catchment lithologies. Higherriverine Sr concentrations with lower 87Sr/86Sr compositions arerelated to greater dissolution of soluble chemical sediments comparedto less soluble igneous and clastic sedimentary rocks, contributingmore radiogenic Sr. As shown above, the chemical sediment end-member appears to be dominated by dissolution of gypsum and haliteevaporites, with comparatively minor influence from the extensivecarbonate lithologies in the Red River and Washita River catchments.

In the study of erosion sources in the Congo Basin bymajor elementand strontium isotope ratios (Négrel et al., 1993), suspended anddissolved loads from the seven main tributaries were compared withrespect to their 87Sr/86Sr values. These Congo Basin results show that87Sr/86Sr values of the suspended load are 0.01 higher than that of thedissolved load. The reason is that the suspended loads originate fromweathering of old igneous rocks that bear a higher 87Sr/86Sr ratiobecause of 87Rb decay. In another study, the suspended load was alsofound to be higher than the dissolved load due to occurrences ofminerals enriched with Rb associated with particulates during a floodevent in theGaronne River and its tributaries in SWFrance (Semhi et al.,2000). Similarly, suspended sediment (N0.2 μm) in the Mississippi andAtchafalaya rivers at the Gulf of Mexico is more radiogenic than thedissolved load (Xu andMarcantonio, 2004; Xu andMarcantonio, 2007).In our study, for the sample LS-LT0803 1–2, collected at the stationclosest to where the Red River enters Lake Texoma (Fig. 2), theunfiltered aliquot has amore radiogenic 87Sr/86Sr value than thefilteredaliquot. Here, the suspended sediment is more radiogenic than the“dissolved” load because the suspended sediment in the Red River isdominated by low solubility silicates from Permian and Pennsylvanian(Fig. 1) redbeds in the headwater regions and sediments derived byrunoff during rainfall events more proximal to the lake.

In the central deep portion of Lake Texoma, the b0.45 μm filteredaliquots are more radiogenic than the unfiltered aliquots (Fig. 5).Filtered and unfiltered samples (although different) generally showsimilar trends in 87Sr/86Sr profiles, but show the greatest divergencein the waters of the hypolimnion. That both profiles become lessradiogenic and approach comparable 87Sr/86Sr values in the meta-limnion might suggest that this interval may represent the maximumdissolution of marine chemical particulates carrying low 87Sr/86Sr, orthat surface and bottom waters are effectively decoupled in terms ofSr isotope equilibrium (as possibly indicated by lower Sr in themetalimnion). Furthermore, that the profiles then become substan-tially more radiogenic than even their surface values indicates anincreased proportion of radiogenic Sr released to the dissolved phasewithin the hypolimnion, or that bottom waters reflect a time-dependent equilibrium mixing process that does not necessarilyreflect surface water chemistry i.e., depending on relative inputs fromthe two influent rivers and the development and maintenance ofsummer stratification and whole water column mixing in the fall andwinter. Some of this effect might be explained by the release of acidicspecies (such as HS- and organic acids) associated with organic matterdecomposition in the hypolimnion or early diagenesis of lakesediments. River waters collected below Denison Dam have consis-tently lower 87Sr/86Sr values than Lake Texoma waters (Fig. 4),consistent with the lower 87Sr/86Sr values in the unfiltered hypolim-nion lake waters, given that Denison dam is bottom released.

Watershed geology of the URRB above Lake Texoma can be dividedinto two groups; 1) marine chemical sediments, with Sr isotopecompositions that reflect seawater at the time they were deposited,and 2) igneous rocks and siliciclastic sedimentary rocks with moreradiogenic isotope signatures than the marine sediments. The firstgroup includes Permian evaporites (gypsum/anhydrite and halite)and Permian and Cretaceous carbonates. All of these chemicalsediments have Sr isotope compositions in the range 0.7070–0.7080.However, of these, only less soluble carbonates (limestone anddolostone) are likely to persist as suspended particulates in the inflowfrom Red andWashita rivers. Survivability of carbonate particulates isparticularly favored by high carbonate saturation indices (as indicatedearlier for the Wichita River sub-watershed of the Red River(Leybourne et al., in preparation). The surface waters of Lake Texoma(i.e., above the metalimnion) are also saturated with respect to calciteand dolomite (Fig. 5).

Differences in dissolved Fe and Mn concentrations in Lake Texomasuggest that Fe- and Mn-oxyhydroxides are present in the surfacewaters of the main lake during summer, but are dissolved in themetalimnion (Sun et al., submitted for publication-a). Strontiumassociated with particulate oxyhydroxides will be returned to thedissolved pool below the chemocline, resulting in an increase indissolved Sr concentrations below the metalimnion, as observed forsite 9 in the main lake (Fig. 5). Thus, if Sr associated with these phasesis more radiogenic than Sr associated with suspended carbonateminerals, this would account for the differences in isotope ratiobetween the suspended and dissolved pools for the deep waters inLake Texoma. Note, however, that calcite is typically slightlyundersaturated in these deeper waters, so the dissolution of thesesuspended carbonates cannot be discounted. Supporting evidence forthe role of suspended calcite and oxyhydroxides comes from a recentstudy of the Sr isotope composition of the suspended and dissolvedload of the Mississippi River at the Gulf of Mexico (Xu andMarcantonio, 2004). This study demonstrated, via sequential extrac-tion experiments, that suspended calcite was less radiogenic than thedissolved load, and that suspended Fe oxyhydroxides were slightlymore radiogenic than calcite. It may also be that highly radiogenicparticulates are preferentially adsorbed on Fe and Mn oxyhydroxides,clays, or OM, and are released to the dissolved pool when they settlebelow the redoxocline (due to reduction and possibly also dissolutionby organic acids). The fact that the sample collected from themetalimnion shows essentially no difference between the dissolvedand suspended pools may indicate that calcite has been partiallydissolved in this layer (Fig. 5; Table 4). However, the Sr concentrationis lowest in this sample for this site. For the sample collected in theepilimnion, the fact that the dissolved load is more radiogenic thanthe suspended load may be due to atmospheric loading (meteoricrunoff) of more radiogenic Sr. Alternatively, there may be Srassociated with organic matter that is introduced into the surfacewaters by runoff.

5.3. Implications for paleolimnological studies

Our data from Lake Texoma shows significant lateral and verticalgeochemical and isotopic variations. The total variation in 87Sr/86Srvalues for Lake Texoma is around 0.0007 (Fig. 4). Although its influentrivers have much larger 87Sr/86Sr ranges tied to differential weath-ering of different catchment rocks and downstream mixing, some ofthe regional and temporal isotopic variability is preserved, in additionvertically variations associated with development of chemicalstratification in the summer. These findings have important implica-tions for studies that rely on recent or fossil lake sediments tointerpret changes in regional hydrology, limnology and climate. Forexample, Li et al. (2008) investigated sediment from the Salton Sea inorder to understand changes in river inputs over the last 20,000 years.The total variation in 87Sr/86Sr recorded in the Salton Sea sediments

213L. Sun et al. / Chemical Geology 285 (2011) 203–214

wasaround0.0002 (Li et al., 2008),wellwithin the variation observed inLake Texomawaters. Similarly, a recent study of carbonate deposited bya lake in the Eocene in central Utah found total 87Sr/86Sr variationssimilar to those in Lake Texoma (Gierlowski-Kordesch et al., 2008).Although this study of paleolake conditions showed systematicvariations in 87Sr/86Sr with stratigraphic height, our work on LakeTexoma suggests that such data needs to be interpreted in light ofpotentially significant variations owing to seasonal stratification.

Conclusions

1. Sr concentrations and isotope compositions were determinedspatially within Lake Texoma and its two major input rivers (RedandWashita).Water sampleswith higher Sr concentrations tend tohave lower 87Sr/86Sr, and this pattern is observed in the grossmixing pattern for Lake Texoma. Specifically, lake waters becomemore radiogenic, and Sr contents decrease, away from the RedRiver inflow and toward the Washita River inflow. Theselithochemical proxy results concur with earlier studies (Millerand Leybourne, 2002; Leybourne et al., in preparation) that Srcycling in the URRB surface waters primarily involves three-component mixing between halite, gypsum/carbonate, and alumi-nosilicates, of which evaporites exert the greatest control on soluteload chemistry.

2. Differences in 87Sr/86Sr between filtered and unfiltered aliquots ofthe upper Lake Texoma water column, in addition to saturationconsiderations, suggest that the suspended load of surface watersmay include an appreciable marine carbonate component (withnon radiogenic 87Sr/86Sr), which dissolves at and immediatelybelow the metalimnion.

3. Below the metalimnion, dissolved (filtered) Sr increases withdepth and 87Sr/86Sr becomesmore radiogenic. Explanations for thisbehavior may be related to: (1) euxinia, organic decompositionand/or early diagenesis of lake sediments below the metalimnion/redoxocline, during which radiogenic Sr associated with lowsolubility aluminosilicate detritus is released to the dissolvedpool, or (2) that surface waters are out-of-phase with deeperwaters due to a time-dependent (seasonal stratification?) equilib-riummixing process i.e., depending on relative inputs from the twoinfluent rivers and the development and maintenance of summerstratification and whole water column mixing in the fall andwinter. This vertical solute-load phenomenon and its explanationhave important implications for Sr cycling in Lake Texoma, whichmay also be important for solute studies of other lake systems.

Acknowledgments

The authors thank Adam Franklin, Cornel Olariu, Jamil Sader,Willie Manton, Katherine Robertson, and Trey Hargrove for their helpwith field and lab work and Neil Basu for help with the Sr isotopeanalyses. This program was supported by NSF grant 0224551 (MIL)and a Geological Society of America Student Research Grant (LS).Journal editors Bourdon and Fein and two anonymous reviewers arethanked for comments that greatly improved the manuscript.

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