David C. Gosselin

Rare earth elements in chloride-rich groundwater, Palo Duro Basin, Texas, USA

Rare earth element (REE) data for groundwater samples from the Deep-Basin Brine aquifer of the Palo Duro Basin, Texas, USA, illustrates the potential use of REE for inferring groundwater flow paths through different geologic materials. The REE content of the groundwaters range over 2.5 orders of magnitude and are depleted by 102 to 105 relative to aquifer materials. The shale-normalized REE patterns for groundwater that have primarily interacted with arkosic sandstones (granite wash) are flat with similar heavy REE (HREE) enrichments [(LuLa)n = 0.60 to 0.80]. The samples with highest REE contents and REE patterns, which are enriched in the intermediate REEs (IREEs; Sm-Tb) reflect variable degrees of interaction with carbonate rocks. The IREE enrichment is the result of fluid interaction with Fe-Mn coatings on carbonate minerals and/or secondary minerals in fractures and vugs. The chloride complex, [LnCl2+] and free-ions are the predominant REE species, accounting for over 95% of the REEs. Carbonate and sulfate species account for the other 5% and have very little influence on the behavior of the REEs. Although this study indicates a potentially important role for the REEs in understanding geochemical transport and groundwater movement, it also indicates the necessity for developing a better understanding of REE speciation in high ionic strength solutions.

NDVI relationship to monthly evaporation

Note: 25(10): 1753-1756 Reference EFLUM-ARTICLE-1998-006doi:10.1029/98GL01176 Record created on 2005-09-08, modified on 2017-02-23

Strontium isotope geochemistry of groundwater in the central part of the Dakota (Great Plains) aquifer, USA

Abstract The Dakota aquifer of the central and eastern Great Plains of the United States is an important source of water for municipal supplies, irrigation and industrial use. Although the regional flow system can be characterized generally as east to northeasterly from the Rocky Mountains towards the Missouri River, locally the flow systems are hydrologically complex. This study uses Sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the Dakota aquifer in Nebraska and Kansas. The interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to: long-term water–rock interaction, during which varying degrees of isotopic equilibrium between water and rock has been achieved; and the alteration of NaCl fluids by water-rock interaction. Specifically, Sr isotopic data distinguish brines from Kansas and western Nebraska from those in eastern Nebraska: the former are interpreted to reflect interaction with Permian rocks, whereas the latter record interaction with Pennsylvanian rocks. The Sr isotopic composition of groundwater from other parts of Nebraska and Kansas are a function of the dynamic interaction between groundwater and unlithified sediments (e.g., glacial till and loess), followed by interaction with oxidized and unoxidized sediments within the Dakota Formation. This study illustrates the power of combining Sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist.

Archean rocks of the Black Hills, South Dakota: Reworked basement from the southern extension of the Trans-Hudson orogen

The Little Elk and the Bear Mountain terranes on the margin of the Precambrian core of the Black Hills provide the only known exposures of Archean rocks between the Archean Wyoming and Superior provinces. The Little Elk terrane consists of a supracrustal sequence dominated by biotite-feldspar gneiss (meta-arkose or metatuff) that is intruded by the slightly metaluminous to peraluminous, calc-alkaline Little Elk Granite, which has a U-Pb intercept age of 2,549 ± 11 m.y. A biotite-plagioclase schist (metagraywacke) dominates the Bear Mountain terrane and is intruded subparallel to foliation by the peraluminous granite at Bear Mountain. An isochron calculated from six severely discordant U-Pb zircon analyses yields an age of 2,393 ± 230 m.y. The Archean terranes participated in the Trans-Hudson orogenic event, suggested by a published Rb-Sr age for the Little Elk Granite (∼1,850 m.y.), 1.87-1.97 Gaigneous activity in the Black Hills, and the similarity in orientation of the Proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the Trans-Hudson orogen. The metamorphic conditions estimated for the Archean rocks may be the result of a thermal event that produced the Proterozoic Harney Peak Granite and thermal high northeast of Lead, South Dakota. The unexposed Precambrian basement is probably dominated by granites similar to the Little Elk Granite and supracrustal rocks consisting of continental margin deposits, based on the REE contents of the Archean schists and on the heavy-mineral assemblages in, the uraniferous nature of, and the metasedimentary clasts in, the lower Proterozoic Box Elder Creek Formation. The ages and the relatively long-lived crustal sources for these Archean granites, and the similarity of the inferred basement characteristics to those of rocks in the Wyoming province, suggest that the Black Hills Archean rocks may be the easternmost exposures of this province. The probable Trans-Hudsonian reworking of the Black Hills Proterozoic rocks and their similarity to those in northern Saskatchewan imply that the Black Hills occupies a tectonic position comparable to that of rocks described from the Cree Lake Zone of Saskatchewan.

Geochemistry of K-rich alkaline lakes, Western Sandhills, Nebraska, USA

Abstract The Alkali Lakes region of the western Sandhills, Nebraska, has lakes that range in composition from freshwater to brine with TDS exceeding 250,000 mg/1. An unusual geochemical feature of these lakes is the conservative behavior of K with concentrations exceeding 1,900 mmoles/kg (86,000 mg/1). The lakes are dominantly Na-K-CO3-(SO4)-(Cl) and Na-K-SO4-CO3-(Cl) waters. Lakes occupy interdunal areas where there is little or no surface runoff. Groundwater primarily from locally derived precipitation is the principle source of water and solutes. This origin for the source water contrasts with closed-basin saline lake complexes, where surface water from the adjacent areas flows directly into the lakes or recharges the groundwater system. The principle geochemical process controlling lake chemistry is evaporative concentration. Other processes are operating, but to a lesser extent; these include mineral precipitation and dissolution and organically mediated sulfate reduction. Geochemical mass-balance modeling indicates distinct differences in the amount of water that is required to be evaporated to produce the observed lake compositions. These differences are related to the groundwater inflow to outflow ratio for individual lakes. This emphasizes that, although evaporation and related processes control the geochemical evolution, the local hydrology of individual lakes regulates the extent to which these processes will proceed.

Major-ion chemistry of compositionally diverse lakes, Western Nebraska, U.S.A.: implications for paleoclimatic interpretations

The western Sand Hills, Nebraska, contains hundreds of compositionally diverse lakes with total dissolved solids ranging from 200 mg l-1 to over 100000 mg l-1 on occasion. This region has a semi-arid, mid-continental climate that is likely to be sensitive to future changes in climate. To interpret the record of Holocene climate in the lake sediments using available paleolimnological techniques, especially proxies for geochemical variations such as diatoms. ostracodes, and authigenic carbonates, requires an understanding of the modern spatial and temporal variability in lake chemistry. This paper provides: (1) a general description of the region and its many lakes; (2) a review of previous investigations related to the origin of these lakes; (3) major-ion data collected in 1992 and 1993 for 27 lakes in Sheridan County, NE; and (4) a discussion of these data in relation to a qualitative conceptual model, implications for lake history, and interpretations of paleoclimatic indicators that employ Mg/Ca ratios. Implications of the data include: (1) compositional differences between lakes are a function of local hydrologic variability, largely the magnitude of the input and output components and the lakes lifetime; (2) the lifetime of a lake is likely related to the complex geomorphic and hydrologic development of the ancestral Blue Creek basin; and (3) solutes in these lakes were not acquired from a high TDS groundwater source. The observed concentrations can be produced by evaporatively concentrating local groundwater. The available Mg and Ca data indicate that for low salinity lakes (e.g., Dillings and Krause lakes) there is a systematic relationship between seasonal increases in Mg/Ca ratios and salinity. Analyses of single ostracodes of the same species could provide a detailed record of seasonal salinity variations, as well as year-to-year hydrologic variability. In the more saline lakes (e.g., Potash, Shriner, Homestead, UNNJ, Wilkinson lakes), the Mg/Ca ratios in authigenic carbonate or ostracodes would be more difficult to interpret. In general, the Mg/Ca ratio decreases with increasing salinity. The dynamics of Ca- and Mg-bearing mineral phases need to be considered for a detailed interpretation of the Mg/Ca ratios in these lakes.

The Bholghati howardite: Chemical study

Bholghati is homogeneous on a cm scale in major element composition, but heterogeneous in trace elements such as the rare earth elements (REEs). The REEs are concentrated in accessory phosphates, and their sampling can influence the REE patterns of eucritic clasts which, in turn, can govern the bulk REE patterns. Bholghati can be reasonably modeled with 56% eucritic, 45% diogenitic, and 3% dark clast components; however, an additional component (feldspathic) is required to match the bulk composition. Dark clasts represent carbonaceous CM type material that was admixed with other components during regolith formation on the eucrite parent body (EPB). One eucritic clast (fine-grained) is light REE (LREE) depleted relative to heavy REE (HREE), which is unusual in normal eucrites. The LREE depletion pattern cannot be modeled from one-stage partial melting of a chondritic source; however, it can be modeled from a non-chondritic source depleted in LREEs or multi-stage melting of a chondritic source, provided that the LREE depletion is not from undersampling of phosphates. Bholghati has a complex history that involved early multiple magmatic events, later annealing, and fragmentation and low-temperature mixing of eucritic, diogenitic, and carbonaceous components on the howardite parent body.

Pre-/Post-Knowledge Assessment of an Earth Science Course for Elementary/Middle School Education Majors.

Many K-12 educators never experience active learning processes in their science courses that are part of their formal teacher preparation program because of the prevalence of the didactic teaching approach (lecture-read-answer) used in undergraduate science classrooms. From an educational perspective there are substantial benefits to experiencing the process of science through active learning strategies. A 16-week, one semester course entitled, “Earth Systems Science for Educators” has been designed at the University of Nebraska-Lincoln to use an active learning approach to improve student knowledge of basic Earth science concepts. To evaluate the extent to which their knowledge has improved, we developed a fairly simple assessment instrument, which consists of 38 statements in which the student responds true-false-“I do not know.” Based on pre- and post-course assessments of 108 elementary and middle-level education majors who took the class in 5 different semesters from 1998 to 2000, there was an average increase in their content knowledge of 30 percent.

Transforming the Teaching of Geoscience and Sustainability

The geosciences have an important role to play in addressing whether humans can live sustainably on Earth. From water to energy, from climate change to natural hazards, geoscience is central to solving a wide range of problems.

A reconnaissance study of oxygen, hydrogen and strontium isotopes in geochemically diverse lakes, Western Nebraska, USA

Reconnaissance δ18O,, δD, and δ87Sr data for fifteen lakes in the Western Lakes Region of the Sand Hills of Nebraska indicate dynamic hydrologic systems. The rather narrow range of δ87Sr from lake water (1.1 to 2.1) and groundwater (0.9 to 1.7) indicates that the groundwater is generally unradiogenic. Groundwater residence times and relatively unradiogenic volcanic ash within the dune sediments control the δ87Sr values. Based on the mutual variations of δ18O and δD, the lakes can be divided into three groups. In Group 1, both δ18O and δD values increase from spring to fall. The δ18O and δD values in Group 2 decreased from spring to fall. Group 3 are ephemeral lakes that went dry some time during 1992. The data and isotopic modeling show that variations in the ratio of evaporation relative to groundwater inflow, local humidity conditions, and the δa has substantial influence on the isotopic composition. In addition, isotopic behavior in ephemeral lakes can be rather unusual because of the changing activities of water and mineral precipitation and redissolution. The annual and interannual isotopic variability of these lakes which is reflected in the paleonvironmental indicators may be the rule rather than the exception in these types of systems.