Late quaternary glacial history of the South Orkney Plateau, Antarctica

Piston cores from the South Orkney Plateau penetrated overcompacted diamictons in water depths of up to 250 m. Detailed textural and petrological analyses of these diamictons indicate that they are basal tills. Seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m. Piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material. The ice-rafted debris in these glacial-marine sediments is composed almost exclusively of material derived from the South Orkney Islands. This implies deposition beneath an ice shelf as opposed to iceberg rafting. In contrast, diatomaceous muds contain relatively abundant exotic iceberg-rafted material and reflect a glacial-maritime setting similar to that of today. The sharp contact separating diatomaceous surface sediments from basal tills and sub-ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly. Radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago. The homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time.     

Age and paleoclimatic significance of the Stansbury shoreline of Lake Bonneville, Northeastern Great Basin

The Stansbury shoreline, one of the conspicuous late Pleistocene shorelines of Lake Bonneville, consists of tufa-cemented gravel and barrier beaches within a vertical zone of about 45 m, the lower limit of which is 70 m above the modern average level of Great Salt Lake. Stratigraphic evidence at a number of localities, including new evidence from Crater Island on the west side of the Great Salt Lake Desert, shows that the Stansbury shoreline formed during the transgressive phase of late Pleistocene Lake bonneville (sometime between about 22,000 and 20,000 yr B.P.). Tufa-cemented gravel and barrier beaches were deposited in the Stansbury shorezone during one or more fluctuations in water level with a maximum total amplitude of 45 m. We refer to the fluctuations as the Stansbury oscillation. The Stansbury oscillation cannot have been caused by basin-hypsometric factors, such as stabilization of lake level at an external overflow threshold or by expansion into an interior subbasin, or by changes in drainage basin size. Therefore, changes in climate must have caused the lake level to reverse its general rise, to drop about 45 m in altitude (reducing its surface area by about 18%, 5000 km²), and later to resume its rise. If the sizes of Great Basin lakes are controlled by the mean position of storm tracks and the jetstream, which as recently postulated may be controlled by the size of the continental ice sheets, the Stansbury oscillation may have been caused by a shift in the jetstream during a major interstade of the Laurentide ice sheet.     

Flood hydrology and geomorphic effectiveness in the central Appalachians

This paper compares hydrologic records and geomorphic effects of several historic floods in the central Appalachian region of the eastern United States. The most recent of these, occurring in November 1985, was the largest ever recorded in West Virginia, with peak discharges exceeding the estimated 500-year discharge at eight of eleven stations in the South Branch Potomac River and Cheat River basins. Geomorphic effects on valley floors included some of the most severe and widespread floodplain erosion ever documented and exceeded anything seen in previous floods, even though comparable or greater rainfall and unit discharge have been observed several times in the region over the past 50 years. Comparison of discharge-drainage area plots suggests that the intensity and spatial scale of the November 1985 flood were optimal for erosion of valley floors along the three forks of the South Branch Potomac River. However, when a larger geographic area is considered, rainfall totals and discharge-drainage area relationships are insufficient predictors of geomorphic effectiveness for valley floors at drainage areas of 250 to 2500 km². Unit stream power was calculated for the largest recorded flood discharge at 46 stations in the central Appalachians. Maximum values of unit stream power are developed in bedrock canyons, where the boundaries are resistant to erosion and the flow cross-section cannot adjust its width to accommodate extreme discharges. The largest value was 2570 W m^?2; record discharge at most stations was associated with unit stream power values less than 300 W m^?2, but more stations exceeded this value in the November 1985 flood than in the other floods that were analysed. Unit stream power at indirect discharge measurement sites near areas experiencing severe erosion in this and other central Appalachian floods generally exceeded 300 W m^?2; reach-average values of 200-500 W m^?2 were calculated for valleys where erosion damage was most widespread. Despite these general trends, unit stream power is not a reliable predictor of geomorphic change for individual sites. Improved understanding of flood impacts will require more detailed investigation of interactions between local site characteristics and patterns of flood flow over the valley floor.     

Geological and seismological evidence of increased explosivity during the 1986 eruptions of Pavlof volcano,Alaska

We present results of study of the best-documented eruptions of Pavlof volcano in historic time. The 1986 eruptions were mostly Strombolian in character; a strong initial phase may have been Vulcanian. The 1986 activity erupted at least 8×10⁶ m³ of feldspar-phyric basaltic andesite lava (SiO₂=53–54%), and a comparable volume of wind-borne tephra. During the course of the eruption, 5300 explosion earthquakes occurred, the largest of which was equivalent to an M _L =2.5 earthquake. Volcanic tremor was recorded for 2600 hours, and the strongest tremor was recorded out to a distance of 160 km and had an amplitude of at least 54 cm² reduced displacement. The 1986 eruptions modified the structure of the vent area for the first time in over two decades. A possible pyroclastic flow was observed on 19 June 1986, the first time such a phenomenon has been observed at the volcano. Overall, the 1986 eruptions were the strongest and longest duration eruptions in historic time, and changed a temporal pattern of activity that had persisted from 1973–1984.     

Proterozoic metavolcanics from western Sierras Pampeanas terrane, Argentina

A series of medium grade metamorphic rocks of the western sector of the Sierras Pampeanas Terrane in central western Argentina are represented by amphibolites, gneisses and schists derived from sedimentary as well as from igneous rocks. The metavolcanics consist of amphibolites, quartz-K-feldspar-muscovite schists, and hornblende-biotite and biotite-epidote-plagioclase schists. Based on petrographic and geochemical data they are interpreted as originating as basaltic tholeiites, rhyolites and mesosilicic volcanics. The distribution and geochemical behavior are similar to present day western Pacific lavas, mainly those developed on island arcs or heavily attenuated continental crust. Based on these characteristics, an accretionary tectonic model involving a series of island-arc collisions is proposed for the Proterozoic. The complex Proterozoic tectonic history of the western Sierras Pampeanas has been partially obliterated by the emplacement of the Early Paleozoic magmatic arc rocks.     

Copper(II) carbonate complexation in seawater

Cupric carbonate and cupric bicarbonate complexation constants were determined in natural seawater and in a variety of synthetic media. The formation constants of CuHCO₃⁺, CuCO₃⁰ and Cu(CO₃)₂^2? at 25°C and zero ionic strength are: log β_H⁰ = 1.8, log β₁⁰ = 6.82 and log β⁰₂ = 10.6. Formation constants of these species appropriate to 0.7 molar ionic strength and 25°C are log β_H ～- 1, log β₁ = 5.73, log β₂ = 9.3. Our results indicate that the inorganic speciation scheme of Cu(II) in seawater is dominated by CuCO₃⁰ and that the ternary species, CuCO₃OH^?, is of substantial importance.     

Distribution of forms of sulfur in peats from saline environments in the Florida Everglades

Although there is published information on the mechanism of sedimentary pyrite formation resulting from bacterial reduction of sulfate, little is known about the distribution of forms of sulfur, including organic, in peats. In order to increase understanding of the geochemistry of sulfur in coals, we have determined distributions of forms of sulfur in five cores of saline peat and one of brackish peat, all from areas near the Shark River and Whitewater Bay in the coastal swamps of south Florida. All concentrations vary widely with depth. Total sulfur concentrations range up to 6% of dry solids. Minor amounts of sulfur are present as H₂S, S⁰, SO₄²⁻, and acid-soluble sulfide, but the principal forms are usually pyritic and organic. The ratio, organic/pyritic, is highly variable, but at a majority of levels in a profile is considerably greater than unity. It is inferred that topochemical factors are important in determining the distribution of sulfur forms in any element of volume.     

Aminostratigraphy of European marine interglacial deposits

Molluscan fossils collected from shallow water marine sediment across NW Europe and nearby Arctic regions have been analysed for the extent of isoleucine epimerization ( ratio) in indigenous protein residues. The ratios confirm that essentially all ‘classical’ Eemian sites from NW Europe are of the same age, and are correlative with the type locality near Amersfoort in the Netherlands; shells from interglacial marine sediment beneath the type Weichselian till in Poland also correlate with the type Eemian site. ratios in Holsteinian marine shells (0.29) are substantially higher than in their Eemian counterparts (0.17); ‘Late Cromerian’ shells yield even higher ratios (0.46). ratios in late glacial shells (0.06) and Middle Weichselian shells (0.09) permit differentiation from modern (0.01) and last interglacial material. Based on the position of the Matuyama-Brunhes boundary and the differences in ratios, the Eemian must correlate with isotope substage 5e, whereas the Holsteinian is most likely substage 7c, possibly stage 9 but certainly younger than stage 11. Intra-Saalian warm periods may be terrestrial equivalents of the younger substages of stage 7. Extensive pre-Eemian marine sediments along the SW coast of Denmark previously correlated with the Holsteinian are shown to be of ‘Late Cromerian’ age. The underlying till there is the first widespread evidence of a pre-Elsterian till in NW Europe. ratios in molluscs from last interglacial sites along the Arctic coast of the USSR, the Arctic Islands and eastern Greenland are substantially lower than in their European counterparts due to their low thermal histories. The combined mid- and high-latitude data are used to develop a predictive model for the expected ratio in any of several moderate epimerization-rate taxa for last interglacial sites with mean temperatures between −20 and +15°C.Not all sites could be unambiguously assigned to an established interglacial. The Fjøsanger (Norway) and Margareteberg (Sweden) sites previously thought to be Eemian, yield ratios higher than in secure nearby Eemian material. It is yet unresolved whether these are aberrant sites or if they predate the last interglacial. In situ shoreline deposits encountered in borings in SW Belgium and in exposures on the Belgium coastal plain contain molluscs that yield ratios intermediate between secure Eemian and Late Weichselian ratios, raising the possibility that a late stage 5 high-sea-level event attained near-modern levels in the southern North Sea basin. Resolution of these uncertainties is the focus of future work.