Calculation of uncertainties of U-Pb isotope data

Equations are derived for the estimation of errors and error correlations for various types of U-Pb isotope data, taking into account ion-beam instabilities, run-to-run variability in mass-discrimination, uncertainties in Pb and U concentrations, and uncertainties in initial-Pb and blank-Pb amount and isotopic composition. Equations are also given for the calculation of concordia intercept errors.     

Oceanographic distributions of cadmium,zinc, nickel,and copper in the North Pacific

Vertical profiles of Cd, Zn, Ni, and Cu have been determined at three stations in the North Pacific and in the surface waters on a transect from Hawaii to Monterey, California. The distributions found are oceanographically consistent and provide a needed confirmation and extension of several recent studies on the marine geochemistries of these metals. Cadmium concentrations average 1.4 pmol/kg in surface waters of the central North Pacific and show a strong correlation with the labile nutrients, phosphate and nitrate, increasing to values of 1.1 nmol/kg at depths corresponding to the phosphate maximum. Zinc is depleted in surface waters of the central gyre to an average value of 0.07 nmol/kg and increases to a deep maximum of 9 nmol/kg exhibiting a strong correlation with the nutrient silicate. Nickel concentrations average 2.1 nmol/kg in surface central gyre waters and increase to a deep maximum of 11 nmol/kg. Nickel is best correlated with a combination of phosphate and silicate. Copper averages less than 0.5 nmol/kg in surface waters of the central North Pacific and increases gradually to values of 5 nmol/kg in bottom waters. The Cu profiles show evidence of intermediate and deep water scavenging. The involvement of these metals in the internal biogeochemical cycles of the sea is responsible for their distributions which are predictable on the basis of oceanographic parameters.     

Geochronologic and isotopic study of the La Désirade island basement complex: Jurassic oceanic crust in the Lesser Antilles?

La Désirade, a small island east of Guadeloupe, is underlain by the only exposed pre-Tertiary basement rocks in the Lesser Antilles. The basement complex comprises spilitic and keratophyric flows and pillow lavas (with interbedded and overlying radiolarian cherts), swarms of mafic to silicic dikes, and subjacent plagiogranite. These features, and the absence of carbonates, terrigenous clastic sediments, or tuffaceous sediments from the complex indicate that it developed in a deep marine environment beyond the reach of terrigenous sedimentation or emergent island arc pyroclastic deposition. Previous workers have suggested that the Désirade basement complex originated either as oceanic crust or during an early (tholeiitic) stage of island arc growth. The isotopic compositions of Sr and Pb from the complex, and previously reported rare earth data (Johnston and Schilling, 1974) do not provide a clear distinction between these two possibilities. Nor does the presence of siliceous keratophyre in the complex rule out an oceanic crustal origin-such rocks are common in well studied ophiolites that originated as oceanic crust. Hence we turn to the age relationships of the complex, the surrounding ocean floor, and adjacent island arcs in an attempt to resolve this problem. The age of the complex strongly supports an oceanic crustal (ophiolitic) origin. The ages of zircons and a previously reported K-Ar age indicate that the complex is 145±5 m.y. old. The complex predates the next oldest volcanic rocks of the Lesser Antilles arc by ca. 110 m.y., and the oldest known rocks of the Aves Ridge, a possible Mesozoic precursor of the Lesser Antilles arc, by 50–60 m.y. This makes it unlikely that the Désirade complex is related to an early phase of either of these arcs. Instead, the age of the complex falls in the range of ages expected for oceanic crust in the vicinity of the Lesser Antilles prior to the development of any subduction zone and resulting arc. Thus we interpret the Désirade complex to be an uplifted segment of oceanic crust that represents the basement on which the later island arcs grew: first the Aves Ridge, an arc that was active in middle to late Cretaceous time (but whose exact mode of origin is enigmatic, and is considered in four alternate tectonic models), then the Eocene to Recent Lesser Antilles arc.     

A pollen record from Birket Qarun and the recent history of the Fayum,Egypt

Sediments and fossil pollen of two short cores from Birket Qarun, Egypt, reflect Nile floods, lake levels, and agricultural developments of the last 325 yr, and demonstrate the potential of a long and detailed record from the Fayum Depression. The chronology of these cores is inferred by correlation of historic events with changes in the fossil and sedimentary records.Subangular clay clasts and blocky structure resulting from occasional exposure, drying, and reworking of lake sediments reflect low Nile floods of the mid-1600s. Abundant pollen of shallowwater, rooted aquatic plants provides evidence for continued low lake levels through the 1700s. A high lake level, resulting from the extreme Nile flood of 1817–1818, is recorded by hystrichospheres, reworked by wave action, from Eocene marine sediments exposed on the north shore of Birket Qarun.Political administration, as reflected in agricultural policy, is also recorded in lake history. Changing lake levels are, in part, correlated with canal neglect during Mamluk and Ottoman control, and renewed canal maintenance under the agricultural policy of Mohammed Ali. Increased cattail (Typha) pollen dates from perenial irrigation after 1873. Olive and date pollen, and pollen of newly introduced exotic trees, are abundant after 1930 as a result of accelerated introduction and cultivation of fruit, lumber, fuel, and windbreak trees following World War I. The sequence of introduction of exotic plants is reflected in the pollen of Zea mays, from the New World, followed by Casuarina from Southeast Asia and Australia, and Eucalyptus from Australia.     

Concave-bank benches and associated floodplain formation

This paper describes the morphology, sequential development and general sedimentology of concave-bank benches on the Murrumbidgee River of southeastern Australia, and also notes their important role in floodplain formation on certain meandering rivers in western Canada. Benches form against the concave bank (cut-bank) of abruptly curving bends immediately upstream of the point of maximum curvature. As a result of flow deflection against the upstream limb of the convex bank, the channel widens here and produces a zone of expanded flow facilitating flow separation near the upstream limb of the concave bank. Sedimentation within this zone starts with a longitudinal-shaped bar of medium sand forming a platform isolated even at low flow by a narrow secondary channel against the concave bank. Aggradation of the longitudinal-shaped bar with fine sand, mud and organic matter permits the establishment of trees. Further sedimentation, particularly around the young trees, results in the formation of a fully developed bench isolated by the secondary channel from the remainder of the floodplain only during high flows. Observations on confined meandering rivers in western Canada provide evidence of substantial floodplain formation by concave-bank bench accretion, a process distinctly different in character to the more familiar mechanism of lateral point-bar accretion. Furthermore, the preservation of abundant organic debris means that extensive bench deposits may be a source of locally useful natural gas from within floodplain sediments.     

Cytosolic metal distribution as an indicator of toxicity in sea urchins from the Southern California Bight

Sea urchins (Strongylocentrotus purpuratus) were collected from intertidal stations at Point Dume, Redondo Beach and White's Point in the Southern California Bight. Cytosolic metal levels varied both within and between stations, with the highest levels found in organisms from White's Point. Examination of the cytosolic metal distribution by Sephadex G-75 gel chromatography indicated that organisms adapted to increased levels of trace metals by producing more of a metallothionein-like metal-binding protein. Histological and histochemical examinations indicated that these organisms were not stressed over the range of metal concentrations encountered in this study.     

Two-dimensional field of thermal turbulence at the edge of an escarpment

Acoustic sounder measurements of the temperature structure parameter were obtained at the edge of an escarpment which is part of a ridge of mountains. These measurements indicate that in mountainous terrain, the daytime two-dimensional field of thermal turbulence is strongly affected by relative sun-slope orientation and wind direction out to ranges of at least 200–300 m. For the geometry of this site, westerly flow results in a field which tends to decrease rapidly to the west in the morning with a much less rapid decrease in the afternoon. At night, easterly flow results in significantly higher thermal turbulence compared to that obtained during westerly flow.These measurements show an increase in thermal turbulence at horizontal ranges of 100–200 m to the west of the escarpment during early afternoon on days with deep mixed layers. It is conjectured that this is due to the mountain upslope wind.     

Contrasting styles of Archaean crustal evolution in parts of southern West Greenland

Aspects of the crustal evolution of three areas in the Archaean block of southern West Greenland are compared and new Rb and Sr isotope data are presented for one of them (inner Godthåbsfjord). It is concluded that major differences resulted from variations in crustal thickness with the early Archaean Amîtsoq gneisses controlling later events.