Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event

Snowball, I., Muscheler, R., Zillén, L., Sandgren, P., Stanton, T. & Ljung, K. 2010: Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event. Boreas, Vol. 39, pp. 720–733. 10.1111/j.1502‐3885.2010.00167.x. ISSN 0300‐9483. A series of nine radiocarbon measurements obtained at increments of 50 years was used to wiggle match varved sediments in Lake Kälksjön (west central Sweden) to the calendar year time scale provided by the tree‐ring‐derived radiocarbon calibration curve. The anchor point for a series of 400 varves known to span the ‘8.2 kyr cold event’ was derived from a combination of correlation analysis, Monte Carlo statistics and Bayesian age modelling. The GRIP and NorthGRIP ice‐cores were matched to the same absolute time scale by comparing ¹⁰Be data and tree‐ring Δ¹⁴C. The radiocarbon‐based wiggle match, organic carbon measurements, mineral magnetic parameters and XRF data reconstruct a distinct period of enhanced erosion in Kälksjön's catchment between 8066 ± 25 and 7920 ± 25 cal. yr BP. Prior to human impact, the increased erosion in the boreal environment is assigned to increased winter precipitation in the form of snow, which caused spring meltwater discharge to intensify. Our results suggest that the abrupt onset of increased winter precipitation in west central Sweden started at least 50 years after the onset of the ‘8.2 kyr cold event’ as defined by oxygen isotope data from Greenland. The study highlights the value of synchronized time scales in the reconstruction of abrupt climate changes based on proxy data.     

Fountains in Magma Chambers

Cyclic layering is a common feature of the ultramafic zone oflayered intrusions and is usually attributed to the entry ofnew pulses of dense magma into the chamber. Since the crystallizationof olivine and bronzite lowers the density of the magma, a newpulse of the parent magma will be denser than the fractionatedmagma in the chamber. If the new pulse enters with excess momentumit will initially rise up into the host magma to form a fountain,then fall back around the feeder when negative buoyancy forcesovercome the initial momentum of the pulse. Laboratory experimentsusing aqueous solutions with both point and line sources havebeen conducted to obtain a quantitative understanding of thefluid-dynamical processes that are important in fountains. Itis observed that convection within the fountain is highly turbulent,resulting in appreciable entrainment of the host magma. A gravity-stratifiedhybrid layer develops at the floor and this breaks up into aseries of double-diffusive convecting layers if the new pulseis hotter than the host magma. The number of layers that formdepends on a number of factors, especially R, the ratio of thecontributions of composition and heat to the total density differencebetween the host magma and the new pulse. Raising the valueof R, results in the formation of more, thinner layers. The thickness of the hybrid layer at any time t is given byH = h₀+(V₀/A)t where V₀ is the volume flux through the feederand A is the horizontal area of the chamber. h₀ is related tothe initial steady-state height of the fountain and, for a linesource, is given by h₀=CU₀^4/3 d^–1(g/)^–2/3 whereU₀ is the volume flux per unit length, g is the accelerationdue to gravity, d is the width of the feeder, is the densityof the host magma, is the density difference between the magmasand C is a constant. Calculations based on these results and the consideration ofthe flow in the feeder dykes below the chamber indicate thata fountain will rise at least 350 m in a continental magma chamberif the feeder width is greater than 10 m. This will lead toextensive mixing between the new pulse and the fractionatedmagma in the chamber, producing a zoned hybrid layer at thefloor that is commonly over 1000 m thick. If the chamber receivesmany pulses of dense magma, the resulting zoning may persistthroughout much of the life of the chamber, especially if thefirst pulse to enter becomes contaminated by light magma releasedby melting at the margins. The highest Mg/Fe ratio for olivineand pyroxenes from cyclic units from the ultramafic zones oflayered intrusions is often well below the value expected forminerals crystallizing from a melt derived directly from themantle, supporting the hypothesis that new pulses of dense magmacan mix extensively with the fractionated magma in the chamber. The feeder dykes to some oceanic magma chambers, such as theBay of Islands Ophiolite, are believed to be narrower, so thatfountains do not rise more than a few metres above the floorof the chamber. This restricts mixing between the input magmaand the host magma and can result in the formation of a hybridzone that is only a few metres thick.     

Interaction between bimodal volcanism, fluvial sedimentation and basin development in the Permo-Carboniferous Saar-Nahe Basin (south-west Germany)

Widespread bimodal-calcalkaline magmatism and contemporaneously active faulting reflect the combined effects of late-orogenic collapse and strike-slip faulting during syn-rift development of the late-Variscan Saar-Nahe Basin. The intrabasinal drainage systems experienced major but differing influences from three prominent styles of volcanic extrusives: extensive basic lava flows; localized acidic domes; and widespread pyroclastic units. Sedimentary sections including basic to intermediate lavas show the transformation of pre-eruptive meandering fluvial systems into lacustrine systems following lava eruptions. These relationships are interpreted in terms of damming of the drainage systems by the topographic barriers represented by the basic lavas themselves. Siliciclastic sediments re-entered and ultimately filled the lakes through prograding, small-scale crevasse channel/mouth bar complexes. Uplift associated with the intrusive/extrusive emplacement of acidic domes affected both the accommodation space and the internal topography of the basin. Following an extrusive pulse, each dome collapse caused the progradation of voluminous volcaniclastic aprons onto the surrounding alluvial plain. Locally, the overload by large volumes of unconsolidated detritus converted the regional meandering drainage system into a braided one. The overall duration of acidic dome activity could have been longer than 10⁶ years, but was interrupted by repose periods varying probably between a few hundred years to several thousand years. Pyroclastic activity accompanied dome emplacement and registers their episodic activity in the stratigraphic record. Fine-grained ashes of phreatoplinian eruptions were spread widely within the basin and overloaded streams in the area of fallout. Rapid reworking of huge amounts of unconsolidated material supplied an abundance of volcanic detritus, which tended to fill topographic lows that were usually fault-controlled. The relationships between basin evolution, intrabasinal volcanism and contemporaneously active faulting, identified in the Saar-Nahe Basin, are important. They emphasize and further the understanding of how similar volcanic products in sedimentary basins worldwide impacted on the depositional system.     

Reindeer on Prins Karls Forland, Svalbard

Svalbard reindeer were exterminated on the island of Prins Karls Forland just after the turn of the last century. In July 1994, after a period of 90 years, reindeer were again observed on the island.     

Mesoclimatology: Present Themes and Future Prospects

Abstract. Some results of recent research into mesoscale phenomena and climate are reviewed –notably sea breezes, cyclonic eddies, dry lake circulations, urban climates and topoclimates –with particular reference to New Zealand. Extension of this work into modelling and prediction of climate change is likely, and the case for a more integrated geography –one which could fruitfully address questions relating to environmental inputs and societal adjustments which are currently neglected by physical geography –is made.     

Observations of walruses along the Norwegian coast 1967–1992

Extralimital observations of walruses are known to be quite common in Norway. The present review covers observations of walruses along the Norwegian coast between 1967 and 1992. A total of 34 different walruses have been recorded observed since 1967. These observations indicate a significant increase in the number of walrus observations in recent years, most likely due to an increase in the walrus population in the Barents Sea area. Most of the walruses observed are assumed to be subadult males.     

Petrological and isotopic implications of some contrasting Late Precambrian carbonates, NE Spitsbergen

The dominantly shallow-marine Vendian succession of NE Spitsbergen contains distinctive types of carbonate rock. Limestones deposited before Vendian glaciation resemble those described from other Upper Proterozoic successions, being high in Sr and inferred to have been originally aragonitic, including the distinctive 5–10 Jim equant polygonal calcite of cemented shrinkage cracks. In contrast, manganoan stromatolitic limestones within marginal-marine glacial-outwash deposits, and consisting of micrite, microspar and fascicular-optic calcite are interpreted as originally calcitic. The restriction of primary marine calcite to cold seawater is comparable with Recent and Permian carbonates, although the Precambrian example formed in a sea diluted with meltwater. There is good textural preservation of relatively ¹⁸O-rich oolitic dolostones which were cemented in a supratidal environment by artesian fluids. Nevertheless, early diagenetic replacement is inferred, immediately prior to a glacial episode. Post-glacial dolostones are either replacive marine, or evaporative lacustrine, but share rather more negative δ¹⁸O values, closer to the mean of Late Precambrian dolostones. The heaviest oxygen isotope values constrain seawater δ¹⁸O to no more negative than — 2 to — 4_SMOW. The main reason for the pronounced oxygen isotopic depletion of most Late Precambrian carbonates is their initial metastable mineralogy. The possibility of determining palaeolatitudes of the enigmatic widespread Late Proterozoic glaciations by isotopic analysis of freshwater periglacial calcareous precipitates is raised. Significant carbon isotope variations reflect changes in depositional water chemistry: some of these could be global in extent.     

Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe

Santonian-Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment-ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high- and low-productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well-oxygenated, current-swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate-fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment-seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post-oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo-Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.     

The Chemistry and Petrogenesis of a Suite of Pantellerites from the Ethiopian Rift

Data on the major and trace element chemistry of a suite oftwenty pantelleritic pitchstone lavas from the Quaternary Ethiopianvolcano Fantale is presented. This reveals a contrast betweenthe composition of the pre-caldera flows and the more siliceous,less peralkaline post-caldera lavas. Comparison with experimental and theoretical studies suggeststhat nearly all of the major and trace element variation withinthe two suites can be explained by assuming fractional crystallizationof alkali-feldspar, the most abundant phenocryst phase. Fractionationof the mafic phases appears to have been less significant. The trace element data strongly indicate that the lavas allbelong to a single suite. However, it is suggested that themajor element chemistry of the post-caldera flows was modifiedby the loss of volatiles at the time of the formation of thecaldera, an event which coincided with the eruption of a 2 km³welded ash-flow tuff.