Indication of a geomagnetic low-inclination excursion in supposed Middle Weichselian interstadial marine clay at Rubjerg,Denmark

Palaeomagnetic measurements have been made on specimens from Late Pleistocene sediments of a piston boring at Rubjerg in Vendsyssel, northern Jutland. The stratigraphy of the deposits is based on content of foraminifera. A total of 70 relatively oriented specimens were investigated palaeomagnetically. Normal steep inclinations close to that of the axial dipole field were found in the Upper Saxicava Sand and in the Younger Yoldia Clay (radiocarbon dated at 14,650 ± 190?12,650 ± 180 B.P.), and a secular variation with an amplitude of 10–12° in the inclincation and a “period” roughly estimated at about 350–400 years was found in the Younger Yoldia Clay.Seventeen relatively oriented specimens from undisturbed older marine deposits revealed a stable low inclination of 11° with α₉₅ = 3°. The age of this apparent geomagnetic excursion falls somewhere between 23,000 and 40,000 B.P. (Older Yoldia Clay). Among other known geomagnetic excursions and events within this interval are Laschamp in France, Mono Lake in California and Lake Mungo in Australia. Until more definite ages have been obtained, the excursion is provisionally named the “Rubjerg Excursion”.     

Geochemistry of lead contaminated wetland soils amended with phosphorus

To remediate Pb contaminated soils it is proposed that phosphorus can be amended to the soils to transform the Pb into poorly soluble Pb-phosphate mineral phases. However, remediation strategies must account for variable Pb speciation and site-specific factors. In this study soil mineralogy and Pb speciation in soils from P-amended field trials at sites within the Coeur d’Alene River Basin in Idaho, USA were investigated. The soils are contaminated from mining activities and are enriched with Fe and Mn. Selective extraction of the soils indicated that the Fe oxides are poorly crystalline. XRD of the soil clay size fractions identified quartz, muscovite, kaolinite, siderite, lepidocrocite, and chlorite minerals. Amendment with P fertilizer dissolved the siderite. No Pb–phosphate minerals were detected by XRD. Electron microprobe analysis showed direct correlations between Pb, Fe, and Mn in the unamended soils, and negative correlations between Pb and Si. Lead and Mn were strongly correlated. In the amended soils Fe and P were strongly correlated. Results indicate that the Pb is associated with poorly crystalline Fe and Mn oxides, and that added P is primarily associated with Fe oxide phases. Comparisons of pore water Pb concentrations with chloropyromorphite and plumbogummite solubility suggest that in the phosphate-amended soils the pore waters are undersaturated in these phases, whereas several of the control soil pore waters were oversaturated, indicating the added phosphate suppressed the Pb solubility. Results from this research provide insight into the geochemistry occurring in the P-remediated soils that will help in making management and remediation decisions.     

Sediment macrobenthos off eastern Waiheke Island,Hauraki Gulf,New Zealand

Aspects of sea-bed structure and benthic-macroinvertebrate species composition, distribution, richness and diversity in coastal waters off eastern Waiheke Island, Hauraki Gulf, are reported. In contrast to the sole historical account of sea-bed community structure from this same region, no widely distributed assemblages of species are recognised throughout it; no two sites share the exact same complement of species; and almost all sites are less than 80% similar in their taxonomic composition, most considerably so. Species richness and diversity are reported to vary with substratum type and depth, and spatially; species occurring within muds are the least diverse and species rich, followed by those of muddy gravels, and then gravels; many taxa prove common to the three substratum types; and dominance of taxa is recognised to decrease with an increase in substratum complexity, from muds to gravels, and species richness. With the exception of invasive marine species, apparent changes in the composition of assemblages throughout this region over the eight-decade period that data span are considered artefacts of the way in which such assemblages were historically defined. We recommend historical accounts of sea-bed community distributions throughout Hauraki Gulf be interpreted with caution, especially when attempting to use such schematic depictions to determine whether changes have occurred in assemblage composition.     

Pleistocene stratigraphy in the Devils Hole Area,Central North Sea: Foraminiferal and amino-acid evidence

A detailed study of the foraminiferal assemblages from the 229.1-m-deep core 81/34 in the central North Sea has been combined with a series of measurements of the isoleucine epimerisation of foraminiferal tests. A total of 17 foraminiferal zones have been established and both the faunal compositions and the amino-acid values suggest that a major part of the sequence represents deposits of early and middle Pleistocene age. Only the uppermost zone is referred to the late Pleistocene. The sequence mainly comprises a series of marine zones from cold periods, but with some barren, possibly non-marine intervals in between. Only two of the foraminiferal zones can be referred to interglacial periods. The oldest one of these, defined here as the Devils Hole Interglacial, may belong to the latter part of the Cromerian Complex, while the upper warm interval is correlated with the Holsteinian of northwest Europe on the basis of its amino-acid values. A detailed stratigraphical correlation between core 81/34 and the neighbouring core 81/29 is suggested on the basis of their foraminiferal content, palaeomagnetic evidence and amino-acid measurements from both cores. A characteristic feature of both sequences is that most of the Quaternary record is missing. Similar episodic patterns of deposition and erosion have been reported previously from the North Sea area.     

Palaeontology and taphonomy of Late Quaternary fossil assemblages at Somersham,Cambridgeshire, England,and the problem of reworking

Assemblages of foraminifers, ostracods and molluscs from temperate Ipswichian Stage (last temperate stage) sediments and overlying cold Devensian Stage (last cold stage) sediments at Somersham in the southern Fenland of Cambridgeshire have been analysed. The Ipswichian sediments contain faunas consistent with temperate brackish water conditions under tidal influence. The Devensian assemblages were recovered from a series of sands and gravels laterally accreting in a channel cutting into Ipswichian sediments. In contrast to the Ipswichian faunas, the faunas of particular Devensian samples show a complex mixture of temperate freshwater, brackish and marine taxa. The molluscs are mainly freshwater, with few land snails; they occur together with foraminifers and ostracods. Freshwater, brackish water and marine ostracods are present with foraminifers. A sample of Devensian fine laminated sediment in the channel was analysed for pollen; only abundant pre-Quaternary spores were present, with abundant foraminifers in the same sample. The taphonomy of the assemblages and the difficulties of their interpretation in environmental terms are discussed. The importance of taphonomy in assessing environments, climate, range of taxa and dating is stressed.     

The P–T–X(fluid) evolution of meta‐anorthosites in the Eastern Granulites,Tanzania

Meta‐anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar‐rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts of these textures constitute high‐Al clinopyroxene – which is a common magmatic mineral in anorthosites – and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO₂–H₂O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual meta‐anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, 850–900 °C; Uluguru Mountains: 12–14 kbar, 850–900 °C. The P–T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid‐saturated assemblages at 10 kbar and 800 °C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid‐saturated and fluid‐undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated of the metamorphic fluid is ～0.3–0.5. Although the meta‐anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan‐African metamorphic overprint with a retrograde isobaric cooling path. Similar P–T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.     

Detrital zircon ages from southern Norway – implications for the Proterozoic evolution of the southwestern Baltic Shield

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have ²⁰⁷Pb/²⁰⁶Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core ²⁰⁷Pb/²⁰⁶Pb age of 2383 Ma. Two discrete, unzoned zircons have ²⁰⁷Pb/²⁰⁶Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U ratios and maximum ²⁰⁷Pb/²⁰⁶Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997     

Mid-Proterozoic magmatic arc evolution at the southwest margin of the Baltic Shield

Mid-Proterozoic calc-alkaline granitoids from southern Norway, and their extrusive equivalents have been dated by LAM-ICPMS U–Pb on zircons to ages ranging from 1.61 to 1.52 Ga; there are no systematic age differences across potential Precambrian terrane boundaries in the region. U–Pb and Lu–Hf data on detrital zircons from metasedimentary gneisses belonging to the arc association show that these were mainly derived from ca. 1.6 Ga arc-related rocks. They also contain a minor but significant fraction of material derived from (at least) two distinct older (1.7–1.8 Ga) sources; one has a clear continental signature, and the other represents juvenile, depleted mantle-derived material. The former component resided in granitoids of the Transscandinavian Igneous Belt, the other in mafic rocks related to these granites or to the earliest, subduction-related magmatism in the region. Together with published data from south Norway and southwest Sweden, these findings suggest that the western margin of the Baltic Shield was the site of continuous magmatic arc evolution from at least ca 1.66 to 1.50 Ga. Most of the calc-alkaline metaigneous rocks formed in this period show major- and trace-element characteristics of rocks formed in a normal continental margin magmatic arc. The exceptions are the Stora Le-Marstrand belt in Sweden and the Kongsberg complex of Norway, which have an arc-tholeiitic chemical affinity. The new data from south Norway do not justify a suggestion that the crust on the west side of the Oslo Rift had an early to mid-Proterozoic history different from the crust to the east. Instead, they indicate that the different parts of south Norway and southwest Sweden were situated at the margin of the Baltic Shield throughout the mid-Proterozoic. Changes from arc tholeitic to calc-alkaline magmatism reflect changes with time in the subduction zone system, or lateral differences in subduction zone geometry. The NW American Cordillera may be a useful present-day analogue for the tectonomagmatic evolution of the mid-Proterozoic Baltic margin.