Evaluation of the importance of clay confining units on groundwater flow in alluvial basins using solute and isotope tracers: the case of Middle San Pedro Basin in southeastern Arizona (USA)

As groundwater becomes an increasingly important water resource worldwide, it is essential to understand how local geology affects groundwater quality, flowpaths and residence times. This study utilized multiple tracers to improve conceptual and numerical models of groundwater flow in the Middle San Pedro Basin in southeastern Arizona (USA) by determining recharge areas, compartmentalization of water sources, flowpaths and residence times. Ninety-five groundwater and surface-water samples were analyzed for major ion chemistry (water type and Ca/Sr ratios) and stable (¹⁸O, ²H, ¹³C) and radiogenic (³H, ¹⁴C) isotopes, and resulting data were used in conjunction with hydrogeologic information (e.g. hydraulic head and hydrostratigraphy). Results show that recent recharge (<60 years) has occurred within mountain systems along the basin margins and in shallow floodplain aquifers adjacent to the San Pedro River. Groundwater in the lower basin fill aquifer (semi confined) was recharged at high elevation in the fractured bedrock and has been extensively modified by water-rock reactions (increasing F and Sr, decreasing ¹⁴C) over long timescales (up to 35,000 years BP). Distinct solute and isotope geochemistries between the lower and upper basin fill aquifers show the importance of a clay confining unit on groundwater flow in the basin, which minimizes vertical groundwater movement.     

Scales and structure of frontal adjustment and freshwater export in a region of freshwater influence

Sea surface temperature satellite imagery and a regional hydrodynamic model are used to investigate the variability and structure of the Liverpool Bay thermohaline front. A statistically based water mass classification technique is used to locate the front in both data sets. The front moves between 5 and 35 km in response to spring–neap changes in tidal mixing, an adjustment that is much greater than at other shelf-sea fronts. Superimposed on top of this fortnightly cycle are semi-diurnal movements of 5–10 km driven by flood and ebb tidal currents. Seasonal variability in the freshwater discharge and the density difference between buoyant inflow and more saline Irish Sea water give rise to two different dynamical regimes. During winter, when cold inflow reduces the buoyancy of the plume, a bottom-advected front develops. Over the summer, when warm river water provides additional buoyancy, a surface-advected plume detaches from the bottom and propagates much larger distances across the bay. Decoupled from near-bed processes, the position of the surface front is more variable. Fortnightly stratification and re-mixing over large areas of Liverpool Bay is a potentially important mechanism by which freshwater, and its nutrient and pollutant loads, are exported from the coastal plume system. Based on length scales estimated from model and satellite data, the erosion of post-neap stratification is estimated to be responsible for exporting approximately 19% of the fresh estuarine discharge annually entering the system. Although the baroclinic residual circulation makes a more significant contribution to freshwater fluxes, the episodic nature of the spring–neap cycle may have important implications for biogeochemical cycles within the bay.     

Estimate of biological production in some stream invertebrates

Annual production of invertebrates was investigated in two streams, the Hinau and Horokiwi, in the North Island of New Zealand. Estimates were based on the instantaneous growth rate method, with adjustments made according to the number of generations produced per year. Production of primary consumers (herbivores and detritus feeders) was 7.6–72.1 g.m^‐2.y^‐1, and of secondary consumers (predators) 0.8–11.9 g.m^‐3.y^‐1. In both streams the lowest production was found in tree‐shaded situations. Production: biomass ratios ranged from 5.3 to 7.9 for primary consumers, and from 3.7 to 6.5 for secondary consumers. These ratios fall within the range of results found by workers in other countries. Fish in the two streams studied feed mainly on the primary consumers and so may be competing for food with invertebrate secondary consumers.     

DDT in trout and its possible effect on reproductive potential

Eggs of rainbow trout (Salmo gairdneri Richardson) from five different lakes in North Island, New Zealand, were reared to discover whether they showed significant differences in survival which could be linked with DDT levels in the tissue. The muscle and gonads of pre‐breeding season fish and the parent females were assayed for DDT, as were the whole fry at the end of rearing. The highest DDT levels were found in fish from Lake Rerewhakaaitu, and eggs from these fish showed the least viability. Mortality was relatively low among eggs from fish out of the other lakes. Thus, DDT may possibly contribute to the high mortality found in eggs from Lake Rerewhakaaitu fish.     

The GIN Sea—A synthesis of its physical oceanography and literature review 1972–1985

A synthesis and review of the physical oceanography of the GIN Sea (Greenland, Iceland and Norwegian Seas) is presented. An accompanying bibliography is included from 1972 through 1985. Some 1986 works are also noted. Emphasis is placed on describing the GIN Sea as a major semi-enclosed basin that plays a role unique among the world's oceans: by providing a strong two-way advective exchange between the ice-covered Polar Sea and the North Atlantic Ocean and at the same time acting as the primary site for Northern Hemisphere bottom water formation. The water masses for the GIN Sea are defined, in a manner as consistent as possible with the literature. The large-scale oceanographic and meteorological factors influencing the water mass formation and circulation are described. A sample calculation of the various horizontal and vertical thermohaline-driven exchanges is given.     

Canyon Creek: A late Pleistocene vertebrate locality in interior Alaska

The Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000–17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf.O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode.     

An Anvilian (early pleistocene) marine fauna from western Seward Peninsula, Alaska

Cover sediments of the York Terrace exposed near the California River, western Seward Peninsula, Alaska, yield mollusks, ostracodes, and foraminifera that lived during the Anvilian transgression of early Pleistocene age. The fossiliferous sediments lie at the inner edge of the York Terrace, a deformed wave-cut platform that extends eastward from Bering Strait along much of the southern coast of Seward Peninsula. The seaward margin is truncated by the little-deformed Lost River Terrace, carved during the Pelukian (Sangamonian) transgression. The early Pleistocene sediments seem to have been deposited between the first and second of four glaciations for which evidence can be found in the California River area.The California River fauna includes several extinct species and several species now confined to areas as remote as the northwestern Pacific and north Atlantic. The fauna probably lived in water temperatures much like those of the present time but deeper water on the Bering Shelf is suggested.The presence of an early Pleistocene fauna at the inner edge of the York Terrace at California River shows that the terrace was largely carved before and during early Pleistocene time. However, a marine fauna apparently of middle Pleistocene age is found on the York Terrace near Cassiterite Peak, and this seems to indicate that the terrace remained low until middle Pleistocene time. Uplift of the York Terrace probably was accompanied by uplift of Bering Strait. The strait may have been deeper, and there may have been no land bridge between the Seward Peninsula of Alaksa and the Chukotka Peninsula of Siberia during most of early and middle Pleistocene time.     

A protracted timeline for lunar bombardment from mineral chemistry,Ti thermometry and U–Pb geochronology of Apollo 14 melt breccia zircons

New zircon U–Pb and trace element investigations from Apollo 14 lunar impact breccia sample #14311 reveal at least three distinct (Concordia, 2σ) age populations at 4334 ± 10, 4245 ± 10 and 3953 ± 10 Ma. Titanium-in-zircon thermometry (Ti ^xln ) results correlated with U–Pb ages range from ~800–1200 ºC. Lattice strain models used to infer zircon versus whole-rock rare earth element contents, and partitioning calculations against lunar impact breccia component compositions, quantitatively constrain formation conditions for the different age populations. A compilation of new data with published work shows that Apollo 14 zircons older than ca. 4300 Ma formed by igneous processes associated with lunar crust formation. Compositional variability in the ca. 4240 Ma zircon age population is interpretable, however, via a mixture of inherited and melt-generated components from one or more large impacts perhaps related to a marked increase in bombardment flux. Ages from the youngest zircon group at ca. 3950 Ma coincide with the classical “late heavy bombardment” (LHB) as documented from previous lunar geochronologies. These results lend support to the idea that instead of a simple unimodal LHB scenario, or a monotonic decline in impacts, the Moon was battered by multiple cataclysms since ca. 4240 Ma. Such a “Picket fence”-like bombardment to the Moon best describes the mode and tempo of impacts that accompanied the late stages of solar system formation and giant planet migration.     

Kersey–Kuner terrace investigations at the Dent and Bernhardt sites,Colorado

The Dent site provided the first association of fluted points with mammoth bones in the New World. However, the stratigraphic integrity of the site has remained in doubt since the original excavations in 1932 and 1933. Core sampling at the Dent Clovis site indicates that the site, on Kersey terrace gravel, extends under railroad tracks adjacent to the original area of excavation. Four hundred meters south the Kuner strath terrace has been exposed by a roadcut at the Bernhardt site. An Archaic hearth dated 4030 ± 60 B.P. is near the top of a 1-m-thick eolian sand overlying 1 m of fine-grained alluvium dated 5740 ± 60 B.P., which in turn overlies sand and gravel of the Kuner strath terrace with an AMS radiocarbon age of 10,105 ± 90 B.P. The South Platte River appears to have been quasistable at the Kuner level during the Younger Dryas when Paleoindians from Clovis to Cody hunted megafauna on the Kersey terrace. © 1998 John Wiley & Sons, Inc.