Nebraska Sandhills Research Articles

Growth and reproductive performance of yearling beef heifers implanted with Revalor G in the Nebraska Sandhills

ABSTRACT Crossbred beef heifers (n = 3,242), approximately 12 mo of age, were managed at 3 locations in the Nebraska Sandhills. Heifers were randomly assigned (May 1) to be implanted with 40 mg of trenbolone acetate plus 8 mg of estradiol (IMP; Revalor G, Merck Animal Health, Summit, NJ) or not implanted (control; CON). Heifers (238 ± 2 kg) grazed native Sandhills range for the duration of the trial (164 ± 4 d). Eighty-two ± 2 d following trial initiation, heifers were synchronized for estrus and received AI followed with clean-up bulls as part of a 25-d breeding season. Body weight was measured at the beginning and end of the trial. Pregnancy detection occurred 45 d following bull removal. During the second breeding season heifers were supplemented with 0.45 kg of a 32% CP supplement 15 d before and 15 d after placement with bulls for a 56-d breeding season. Implanted heifers gained more and were heavier ( P P P = 0.02; 93 vs. 96 ± 2%, IMP vs. CON, respectively). Implanting beef heifers with trenbolone acetate plus estradiol at approximately 12 mo of age increased ADG and summer BW gain; however, it decreased initial and subsequent pregnancy rate compared with heifers not implanted.

Ecophysiological responses of native invasive woody Juniperus virginiana L. to resource availability and stand characteristics in the semiarid grasslands of the Nebraska Sandhills

Vegetation in grasslands is changing at an unprecedented rate. In the Nebraska Sandhills, this shift is attributed in part to encroachment of the woody species Juniperus virginiana. We investigated changes in resource availability and their feedback on seasonal trends in photosynthetic characteristics of J. virginiana trees scattered in open grasslands vs. a dense 57-year-old stand. Dense stand exhibited lower volumetric soil water content, NH4+, NO3–, and δ13C, as well as foliage δ13C, δ15N, and N content, compared to grasslands. Water potential was higher in trees in grasslands compared to dense stand. J. virginiana in dense stand exhibited similar trends to trees in grasslands for net photosynthetic rate (PN), stomatal conductance, transpiration, maximum photochemical efficiency of PSII, maximum carboxylation velocity, and maximum rate of electron transport. PN peaked early summer and declined in the fall, with trees in open grasslands lagging behind those in dense stand. Plasticity of this species may place it at a competitive advantage in the Sandhills, further altering grasslands vegetation and ecosystem processes.

Olfaction as a Cue for Nest-Site Choice in Turtles

Nest-site choice in turtles has a demonstrated impact on their fitness. Previous studies of nest-site choice have focused on environmental factors potentially affecting that choice (e.g., temperature, insolation, soil type, or moisture). Observations of nesting of painted turtles (Chrysemys picta) at the Crescent Lake National Wildlife Refuge in the Nebraska Sandhills suggest that females might use olfactory cues (specifically odors of other nesting females) to choose nest sites. We tested this hypothesis indirectly by using the nearest neighbor (nest) distance algorithm in ArcMap 10.1. Our results for nests in 2012–2014 were mixed, with nesting at some sites, in some years, and by some turtles occurring nonrandomly, in very close proximity to previous nests. Preliminary experimental data from 2015 using urine-treated sites in primary nesting areas also suggested that females showed more interest in those sites than sites moistened with equal amounts of water. These data provide some support for the use of nest odor as an important cue for nest-site choice in turtles, but they are insufficient to reject the possibility of the simultaneous use of other fine-scale environmental cues.

0016 Growth and reproductive performance of yearling beef heifers implanted with Revalor G in the Nebraska Sandhills

0016 Growth and reproductive performance of yearling beef heifers implanted with Revalor G in the Nebraska Sandhills

0657 WS Methods to increase productivity of spring calving production systems in the Nebraska Sandhills

0657 WS Methods to increase productivity of spring calving production systems in the Nebraska Sandhills

Cow size and age as economic drivers of beef production systems in the Nebraska Sandhills

ABSTRACT Contribution of cow age and size to profitability of beef production systems in the Nebraska Sandhills was analyzed using hedonic methodology. Four years of cow-calf production systems data from 197 observations per year were used. Historical prices for inputs and production outputs were used. Pooled hedonic type regression models, representing 9 possible production system scenarios, evaluated the effect of cow size and age on profitability. Each model included production year, marketing year, calving season, calf BW at birth, calf weaning age, calf BW at weaning, average cow BW, cow age, wintering system, calf sex, and postweaning system as independent variables. Three variables—(1) calf BW at birth, (2) calf BW at weaning, and (3) calf weaning age—were used as dependent variables in 3 secondary regressions. The secondary estimated effects were added to the primary estimated effects providing an adjusted estimate of cow characteristics on net returns. In general, animals marketed at slaughter contribute more positively to profit if they are from heavier cows. Conversely, if animals are sold at weaning or as yearlings, calves born to the lightest dams contribute most to returns. Dam age also contributes to returns differentially depending on marketing scheme. Younger dams contribute more to profit if cattle are marketed on a hot carcass basis, whereas grid pricing favors contribution to returns of more mature dams. If profit is the motive, the contribution cow size and age make on profitability may or may not be worth considering depending on both production and marketing scheme.

The role of isoforms in the evolution of cryptic coloration in Peromyscus mice.

A central goal of evolutionary biology is to understand the molecular mechanisms underlying phenotypic adaptation. While the contribution of protein-coding and cis-regulatory mutations to adaptive traits has been well documented, additional sources of variation - such as the production of alternative RNA transcripts from a single gene, or isoforms - have been understudied. Here, we focus on the pigmentation gene Agouti, known to express multiple alternative transcripts, to investigate the role of isoform usage in the evolution of cryptic colour phenotypes in deer mice (genus Peromyscus). We first characterize the Agouti isoforms expressed in the Peromyscus skin and find two novel isoforms not previously identified in Mus. Next, we show that a locally adapted light-coloured population of P.maniculatus living on the Nebraska Sand Hills shows an upregulation of a single Agouti isoform, termed 1C, compared with their ancestral dark-coloured conspecifics. Using invitro assays, we show that this preference for isoform 1C may be driven by isoform-specific differences in translation. In addition, using an admixed population of wild-caught mice, we find that variation in overall Agouti expression maps to a region near exon 1C, which also has patterns of nucleotide variation consistent with strong positive selection. Finally, we show that the independent evolution of cryptic light pigmentation in a different species, P.polionotus, has been driven by a preference for the same Agouti isoform. Together, these findings present an example of the role of alternative transcript processing in adaptation and demonstrate molecular convergence at the level of isoform regulation.

Skewed Age Ratios of Breeding Mallards in the Nebraska Sandhills

Abstract Understanding demographic structures of populations allows managers to better evaluate factors affecting populations and increase efficiency of conservation efforts. Recent studies suggest low productivity, but high survival characterizes mallard (Anas platyrhnchos) populations in the Nebraska Sandhills. We studied age ratios of decoy-trapped and observer-shot mallards in the Sandhills during 2007–2008. Observed age distribution (second-year:after-second-year) of mallards was 2.8:1 for females and 0.8:1 for males. Age ratios of trapped females were skewed towards young females relative to other studies, and the age ratio of trapped females was not different than the age ratio of the sample we collected by shooting. The skewed age ratios we observed provide additional context to the low reproductive success that has been reported for nesting mallards in the Sandhills. Second-year mallard females tend to invest fewer resources and take fewer risks associated with nesting. As a result the Sandhills ...

IButton® Temperature Loggers Effectively Determine Prairie Grouse Nest Absences

Recent technological innovations allow the monitoring of avian nest attendance in ways that reduce disturbance and expense without altering nesting activities. Yet the efficacy of these techniques to assess nest absences has not been determined for ground-nesting prairie grouse. We sought to determine the timing of nest absences by means of accurate detection without intrusive human visitations technology. During the nesting season (May and June), we inserted iButton® Thermochrons (Maxim/Dallas Semiconductor Corp., Sunnyvale, CA) into active greater prairie-chicken ( Tympanuchus cupido ) nests in the Sandhills of Nebraska, USA. We simultaneously monitored the nests with solar-powered video cameras to determine if the lower-cost iButtons would record noticeable changes in nest bowl temperature when video indicated a hen absence. Our data showed that iButtons detected 88% of nest absences in 49 off-bouts (instances of nest absence) and accurately portrayed the length of absence from the nest. In total, average lag time, departure lag time, and arrival lag times were detected by iButtons less than 2 minutes after the actual time recorded by the camera. We also obtained valuable information regarding the timing of hen absence, predation events, and optimal iButton recording intervals. Our study suggests that iButtons could be effectively used to monitor absences at greater prairie-chicken and likely other species of prairie grouse nests in future studies.

Ecological factors responsible for the geographic distribution of Rhabdias joaquinensis: where do lungworms infect anurans in nature?

The lungworm Rhabdias joaquinensis (Nematoda: Rhabdiasidae) is a common parasite of anurans in eastern Nebraska, yet absent from the same hosts in western Nebraska. This study investigated the ecology of the lungworm's free-living stages that reside in host feces and surrounding soils to establish the worm's free-living niche in eastern Nebraska. Using a comparative method, this study also investigated the absence of lungworms in western Nebraska's Sandhills. Soil composition, moisture, and temperature were experimentally varied in the laboratory to assess their effects on juvenile development and survival. Field mesocosm and host defecation experiments were used to determine where in nature lungworms survive and eventually infect frogs and toads and to discover if vegetation facilitates lungworm transmission to hosts. The results found loam soils were amenable to lungworm development, whereas soils with high clay or sand content produced few infective lungworms. Soil moisture <50 % did not support lungworm development. Infective juveniles successfully developed between 5 and 35 °C, albeit at different rates, whereas only a limited number of non-infective worms developed at 40 °C. Field studies found that shoreline environments supported lungworm development, and the majority of lungworm infections appear to occur within a zone of infection adjacent to shorelines in eastern Nebraska. The prevalence in vegetation mesocosms was 100 %, and a significantly higher mean abundance was found in toads from containers with vegetation than without. Overall, these experiments suggest that the spatial distribution of R. joaquinensis in Nebraska is constrained by the worm's ability to survive and reproduce in a soil matrix.

Effects of stocking rate on forage nutrient composition of Nebraska Sandhills upland range when grazed in early summer.

Twelve 2-ha, Nebraska Sandhills upland range paddocks were used in a 2-yr study to determine effects of stocking rate on grazed forage nutrient value in early summer. Paddocks contain a mixture of native cool- and warm-season grasses with forbs as a minor component. Stocking rates were 0 (control), 0.57 (light), and 0.85 (heavy) animal unit months per ha, respectively. Three esophageally fistulated cows collected diet samples on May 14 (d 0), May 21 (d 7), May 28 (d 14), and June 4 (d 21) in 2013 and 2014. Ten quadrats per paddock were clipped and separated into current or previous year growth on each diet sampling date. All samples were analyzed for CP, NDF, and in vitro OM disappearance, which was adjusted to an in vivo OM basis (). For diet samples, treatment interacted ( < 0.001) with sampling date for CP, NDF, and IVOMD. However, treatment and date did not interact ( ≥ 0.32) for clipped samples. Diets collected in control paddocks had greater ( < 0.05) IVOMD and CP and lower NDF compared with light and heavy stocking rate paddocks on d 7, 14, and 21 but light stocking rate paddocks did not differ ( ≥ 0.26) from heavy stocking rate paddocks on any sampling day except d 21 NDF ( = 0.03). In general, previous year growth IVOMD, CP, and NDF were not affected ( ≥ 0.11) by treatment. Generally, current year growth IVOMD and CP were not affected ( ≥ 0.22) by treatment. Current year growth NDF did not differ ( = 0.23) among stocking rates on d 0, but control paddock had lower ( = 0.02) NDF than light and heavy on d 7, 14, and 21. In stocked paddocks, diet samples had greater ( < 0.01) IVOMD and CP and lower NDF than current and previous year growth except IVOMD on d 0 where diet and current year growth did not differ ( = 0.34). Generally, in control paddocks, current year growth and diet samples had greater ( < 0.01) IVOMD and CP than previous year growth. Control paddocks had greater ( < 0.01) forage accumulation than stocked paddocks on d 7, 14, and 21; however, light and heavy stocked paddocks did not differ ( ≥ 0.29). On d 0 it was estimated that 96% of the diet was current year growth and only 4% previous year growth, whereas on d 21, 70% of the diet was current year growth vs. 30% previous year growth. Stocking pastures decrease current year growth forage mass and therefore diet nutritive value by forcing cattle to consume diets containing previous year growth in early summer.

A Culinary History of the Nebraska Sand Hills: Recipes and Recollections from Prairie Kitchens by Christianna Reinhardt (review)

A Culinary History of the Nebraska Sand Hills: Recipes and Recollections from Prairie Kitchens by Christianna Reinhardt (review)

Habitat preference and survival for western meadowlark (Sturnella Neglecta) fledglings in a contiguous prairie system

Grassland bird populations in North America continue to decline according to count and vital-rate data, highlighting the importance of remaining grasslands. Conservation efforts may be enhanced by understanding fledgling habitat preference and survival in relation to age and ambient temperature. We radio-tracked 46 Western Meadowlark (Sturnella neglecta) fledglings in 2006 and 2007 to evaluate variation in habitat preference and survival in subirrigated meadows of the Nebraska Sandhills, one of North America’s largest contiguous grassland systems. Our research indicates that habitat preference varied with fledgling age and ambient temperature: at higher ambient temperatures, fledglings tended to select sites with deeper litter and shorter vegetation, while younger, mostly flightless fledglings tended to select sites with shallower litter and taller vegetation. We confirmed 23 fatalities, including 8 (35%) from predators, 2 (9%) from haying operations, and 13 (56%) from unknown causes. Seven of the eight (88%) depredated fledglings were tracked to live snakes, including six bullsnakes (Pituophis catenifer sayi). The probability of daily survival (0.981) was high compared to estimates for passerine fledglings in other grassland systems. Survival increased with age and ambient temperature but decreased with litter depth. Prior research indicates prescribed burning can decrease litter depth and associated abundance of snakes in grasslands, but neither are likely conservation strategies in subirrigated meadows of the Nebraska Sandhills, because hay production provides winter feed for cattle. We recommend maintaining mid to late summer hay harvest so first cohorts of nestlings can fledge, attain flight, and evade machinery.

Greater prairie‐chicken brood‐site selection and survival in the Nebraska sandhills

ABSTRACTThe greater prairie‐chicken (Tympanuchus cupido pinnatus) is a grassland bird species of conservation concern, although the region of the Nebraska Sandhills has the largest and most stable population in North America. The Sandhills region is semiarid and dominated by privately owned rangeland; vegetation in the region is typically less dense than the tall grass ecosystem. Most management recommendations for the greater prairie‐chicken are based on work in tall grass prairies, so our goal was to estimate the responses of brood‐site selection and survival to the unique vegetation characteristics of the Sandhills region. We studied prairie‐chickens on private rangelands in Rock and Brown Counties from 2009 to 2011. We radio‐marked 139 females and monitored females with broods from May through July using pickup‐mounted and handheld telemetry systems. We used discrete choice models to assess micro‐scale habitat selection within used pastures. Specifically, we investigated the effect of local heterogeneity of vegetation structure and composition on site selection. The contiguous grasslands of the Sandhills contain variation in soils, slope, and proximity to water table, so we also assessed selection of these ecological sites (the equivalent of land‐use categories in fragmented landscapes) for brood‐rearing within the broad landscape (macro‐scale). Last, we assessed variation in brood survival in response to weather and vegetation structure and composition. Prairie‐chicken females with broods selected sands ecological sites (upland, rolling hills sites) that had thicker vegetation with higher visual obstruction reading (VOR; = 7.7 cm, SE = 0.4), more variable VOR ( = 20.1, ±2.1), and greater litter depth ( = 0.07 cm, ±0.01) than at coupled random locations (VOR: = 6.6 cm, ±0.1; variance VOR: = 15.6, ±0.8; litter depth: = 0.05 cm, ±0.01). The selection function for brood sites peaked for sites with approximately 10 cm VOR, which was significantly less than the range of 20–30 cm of visual obstruction suggested for management of tallgrass prairies. In fact, only 5.0% of random sites and 5.4% of sites used by hens with broods had VOR &gt;20 cm. However, VOR and litter depth did not explain variability in brood survival, and we found considerable uncertainty in our analysis of daily survival. Models with forb cover were ranked highest in the survival analyses (combined ωAIC c = 0.63), which provided some support for a positive effect of forb cover on brood survival (βforb cover = 0.327, SE = 0.337). We suggest that land managers in the Sandhills region provide brooding habitat in grazed, upland ecological sites that is characterized by minimal bare ground, a consistent distribution of litter, a forb component, and VOR levels of 4–11 cm in pastures in the vicinity of leks. © 2015 The Wildlife Society.

Time of Grazing Effect on Subsequent-Year Standing Crop in the Eastern Nebraska Sandhills

Rotational grazing methods provide range managers opportunities to alter the time when grazing occurs on pastures in consecutive years. The effect of time of grazing on subsequent-year standing crop was evaluated within four pastures (42 ± 1 ha) rotated annually within a deferred rotation using a modified Williams design that included mean times of grazing within different pastures from 16 May to 14 June, 13 June to 20 July, 20 July to 29 August, and 27 August to 7 October (i.e., stages of grazing A, B, C, and D, respectively). Subsequent-year standing crop of different plant functional groups was collected from 2001 to 2008 at four topographic positions (i.e., north slopes, south slopes, dune tops, and interdunes) in mid-June and mid-August harvests. During the mid-June harvest, standing crop of subsequent-year warm-season grasses was 51 ± 24 to 117 ± 41 kg·ha-1 greater (P < 0.1) when grazing occurred during stage A compared with when grazing occurred during stages B and C on north- and south-facing slopes and at the pasture level. During the mid-August harvest on south-facing slopes, subsequent-year warm-season grass standing crop was 265 ± 105 and 341 ± 113 kg·ha-1 greater (P < 0.03) when grazing occurred during stage A compared with stages B and C, respectively. Overall, grazing on pastures during stage A tended to provide greater amounts of subsequent-year standing crop for warm-season and cool-season graminoids at specific topographic positions. Given that the effect of time of grazing on subsequent-year standing crop varied among topographic positions for different functional groups, a deferred grazing rotation provides a viable management tool to alter the yearly sequence of grazing on pastures to avoid negative effects of grazing Nebraska Sandhills vegetation at critical times in consecutive years.

Impacts of devegetation on the temporal evolution of soil saturated hydraulic conductivity in a vegetated sand dune area

Soil saturated hydraulic conductivity (K S ) is partly affected by vegetation activities, which can either increase K S by enhancing macropore flow or reduce K S by clogging pore space. Despite the complex interactions of K S with vegetation, the impact of devegetation on K S has not been adequately addressed, particularly in regions that are prone to drought-induced devegetation. In this study, the impacts of devegetation on K S in a native grassland-stabilized sand dune area were investigated by artificially controlling surface vegetation at an experimental site in the Nebraska Sand Hills. The experimental results revealed that the temporal evolution of K S at the site was mainly affected by the erosion processes triggered by devegetation. Over a short-term (about 1 year), the impact of devegetation on K S was negligible, owing to that the existence of dead root systems prevented erosion processes. By comparison, the long-term impact of devegetation on K S emerged when devegetation-induced erosion processes exposed deeper soil layers with higher K S . Particularly, the dunetop locations that experienced higher erosion rates had larger temporal changes in K S . Thus, the impacts of devegetation on K S mainly depended on two factors (i.e., time and topographic locations) that were related to erosion processes in this native grassland-stabilized sand dune area. To further investigate the ecohydrological implications of the temporal change in K S , a newly developed ecohydrological model was also employed, and the simulation results showed that the impacts of changes in K S on water balance components and biomass production were non-negligible and highly nonlinear. In spite of previous studies, the findings presented here demonstrate the close tie between near-surface hydrology and land surface evolution processes controlled by vegetation in sand dune areas, and highlight the importance of coupling eco-hydro-geomorphic interactions in the context of climate change.

Temporal and spatial variability in dune reactivation across the Nebraska Sand Hills, USA

The Nebraska Sand Hills is a stabilized dune field on the Great Plains of North America. Although it is well known that this dune field, like several others on the Great Plains, last experienced widespread activity during the Medieval Climatic Anomaly (MCA, ~AD 900–1300), spatial variation in the timing and nature of drought development is poorly constrained. To elucidate spatial trends in dune reactivation, samples potentially representing MCA activity across the Sand Hills were collected and dated using optically stimulated luminescence (OSL). Ages from the older part of the MCA were obtained from eolian sediments in the northwestern Sand Hills, while ages from later in the episode were obtained to the southeast, suggesting a geographic trend in the timing of revegetation of the dunes near the end of the drought. Revegetation likely occurred to the northwest initially as a result of renewed moisture availability from a rising water table in the interdunes, which serve as refugia for vegetation during times of drought. Vegetation then gradually spread to the southeastern Sand Hills. An additional spatial trend in ages is apparent in the chronology of linear dune mobilization across the Sand Hills. Linear dunes in the northwest are superimposed on megadunes and originated during the last reactivation, while linear dunes in the southeast are built around older cores of dunes and formed during several reactivations. Our geochronology reveals three episodes of eolian transport, including the MCA, in the formation of linear dunes in the southeast.

Determining the impacts of experimental forest plantation on groundwater recharge in the Nebraska Sand Hills (USA) using chloride and sulfate

Although impacts of land-use changes on groundwater recharge have been widely demonstrated across diverse environmental settings, most previous research has focused on the role of agriculture. This study investigates recharge impacts of tree plantations in a century-old experimental forest surrounded by mixed-grass prairie in the Northern High Plains (Nebraska National Forest), USA. Recharge was estimated using solute mass balance methods from unsaturated zone cores beneath 10 experimental plots with different vegetation and planting densities. Pine and cedar plantation plots had uniformly lower moisture contents and higher solute concentrations than grasslands. Cumulative solute concentrations were greatest beneath the plots with the highest planting densities (chloride concentrations 225–240 % and sulfate concentrations 175–230 % of the grassland plot). Estimated recharge rates beneath the dense plantations (4–10 mm yr−1) represent reductions of 86–94 % relative to the surrounding native grassland. Relationships between sulfate, chloride, and moisture content in the area’s relatively homogenous sandy soils confirm that the unsaturated zone solute signals reflect partitioning between drainage and evapotranspiration in this setting. This study is among the first to explore afforestation impacts on recharge beneath sandy soils and sulfate as a tracer of deep drainage.

Vadose zone lag time and potential 21st century climate change effects on spatially distributed groundwater recharge in the semi-arid Nebraska Sand Hills

Summary Deep drainage of water below plant root zones (potential groundwater recharge) will become groundwater recharge (GR) after a delay (or lag time) in which soil moisture traverses the vadose zone before reaching the water table. Depending on the thickness of the vadose zone, the magnitude of deep drainage, and soil hydraulic properties, lag times will vary broadly, exceeding decades to centuries in semi-arid and arid environments. Yet, studies of future climate change impacts to GR have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone. We investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of GR throughout the semi-arid Nebraska Sand Hills (NSH) region (∼50,000 km2). We propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate. Understanding of such effects is needed for groundwater modeling, analysis of climate change impacts on groundwater, and the effective management and sustainability of future water resources. Lag time estimates are made using the pressure-based vertical velocity of soil moisture changes, equivalent to a kinematic wave approximation of Richards’ equation. The underlying assumptions (unit hydraulic gradient and relatively slow changes in climate) are supported by observations in the High Plains aquifer region, encompassing the NSH. The analysis relies on four sources of input data, including: Spatially distributed high resolution (1.1-km) GR rates (as the difference in 2000–2009 mean precipitation and evapotranspiration—estimated using a novel MODIS-based approach); thickness of the vadose zone—based on 30-m digital elevation models of the land surface and water table elevations; statistically downscaled estimates of future (2010–2099) potential GR rates from two hydroclimate model projections (WRCP CMIP3) with opposing GR trends; and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van Genuchten–Mualem equation. We found that vadose zone lag times (with a mean GR rate of 78 mm/yr for 2000–2009) have an estimated spatial mean of 5.28 yrs (vertical velocity of 5.68 m/yr). End-of-century lag times (with mean potential GR rates of 128 and 61 mm/yr for 2090–2099) have estimated spatial means of 3.78 and 6.55 yrs (vertical velocities of 8.13 and 4.69 m/yr) for wet and dry scenarios, respectively. Findings suggest that the highly conductive materials (primarily sand deposits) and high potential GR rates in the NSH lead to relatively quick drainage of vadose zone water on the order of several years (less than a decade) for the entire 21st century. However, considerable spatial variations of lag times exist due to gradients in potential GR rates and the highly variable vadose zone thickness in the NSH.

Isotope geochemistry of Swan Lake Basin in the Nebraska Sandhills, USA: Large 13C enrichment in sediment-calcite records

This study presents isotope geochemical analyses conducted on water column samples and core sediments collected from the Swan Lake Basin. Water analyses include the dissolved methane (CH4) content and the ratio of carbon-13 to carbon-12 (δ13C) in dissolved inorganic carbon (DIC). The core sediments – sandy muds containing inorganic calcite, organic matter, and opal phases±ostracods – were examined by X-ray diffraction, dated by radiocarbon (14C), analyzed for wt% organic carbon, wt% organic nitrogen, wt% organic matter, wt% calcite, δ13C of bulk-sediment insoluble organic matter (kerogen), 18O:16O ratio (δ18O) and δ13C of bulk and ostracod calcite. Of particular significance is the large enrichment in carbon-13 (δ13C=+4.5 to +20.4‰ V-PDB) in the calcite of these sediments. The 13C-enriched calcite is primarily formed from DIC in the water column of the lake as a result of the following combined processes: (i) the incorporation of 13C enriched residual carbon dioxide (CO2) after partial reduction to CH4 in the sediments and its migration into the water column-DIC pool; (ii) the preferential assimilation of 12C by phytoplankton during photosynthesis; (iii) the removal of 13C-depleted CH4 by ebullition and of organic matter by sedimentation and burial. The 13C enrichment was low between 3624 and 2470yr BP; high between 2470 and 1299yr BP; and moderate since 1299yr BP. Low 13C enrichment was formed under low water-column carbon levels while higher ones were formed under elevated rates of biomass and calcite deposition. These associations seem to imply that biological productivity is the main reason for carbon-13 enrichments.