Environmental change during the Allerød and Younger Dryas reconstructed from Swiss tree-ring data

Annually resolved tree-ring width variations and radiocarbon ages were measured from a collection of 120 Lateglacial pine stumps excavated on the Swiss Plateau. These data – representing the oldest absolutely dated wood samples worldwide – extend the absolute tree-ring chronology from Central Europe by 183 years back to 12 593 cal. yr BP (10 644 cal. yr BC). They also yield a 1420-year floating chronology covering the entire Allerød and the early Younger Dryas (14 170–12 750 cal. yr BP). Radiocarbon data suggest a 250-year jump in the ¹⁴C reservoir correction around the time of the Allerød to Younger Dryas transition, although calendric dating of the floating chronology – by filling a ∼150 year gap – is necessary for confirmation. Various subgroups, based on the year of germination, were used to assess temporal changes in growth characteristics along the Allerød to Younger Dryas transition. Comparison of these Lateglacial data with a reference data set of living and historic pines from the Swiss Valais (AD 940–2000) revealed differences in both growth trend and level. The generally slower Lateglacial growth was likely influenced by higher geomorphic activity and severe climatic conditions. After removal of the biological age-trend, a strong common signal found in the tree-ring data suggests some skill in estimating interannual to multidecadal Lateglacial climatic variations.     

Petrology and Cooling Rates of the Valhalla Complex, British Columbia, Canada

Rocks from the Valhalla metamorphic core complex, British Columbia,Canada, have experienced granulite facies metamorphism at conditionsof 82030C, 81 kbar. Peak metamorphism was accompanied bydehydration melting of muscovite, but not biotite, followedby minor back reaction of garnet $ K-feldspar $ H₂0= sillimanite$ biotite$plagioclase. At conditions very near those of thepeak, extensive shearing produced s-c (schistositS-cisaillement)fabrics, ribbon quartz and grain size reduction of garnet atseveral locations. Gamet-biotite Fe-Mg exchange thermometryyields temperatures that range from 580 to 1051C Low temperaturesare calculated from biotite modified dominantly by Fe–Mgexchange with garnet; high temperatures are calculated fromFerich biotites produced from the above retrograde reaction.Geothermometry is useless in these rocks to estimate peak temperaturea priori, but is very useful to help constrain the complex reactionhistory of biotites. Geochronology on monazite, zircon, allanite,titanite, hornblende, muscovite, biotite and apatite has beenused to constrain the timing of the metamorphic peak at 67–72Ma and the average cooling rate to 24 6C/Ma. Diffusion modelingof Fe–Mg exchange between biotite inclusions and hostgarnet yields cooling rates of either 3–80C/Ma or 200–2500C/Ma, depending on the choice of diffusion coefficients. Theformer value is consistent with the average cooling rate of24 C/Ma for the complex determined from geochronology, butthe faster rate cannot be ruled out and may indicate initialvery rapid cooling by thrusting of the complex onto cooler basementIt is suggested that cooling rates determined from geochronologicvs petrologic methods may not be directly comparable becausepetrologic methods sample near-peak nutamorphic cooling rateswhereas geochronologic methods sample post-peak to ambient coolingrates. KEY WORDS: geothermometry; geochronology; garnet diffusion; cooling rates; Valhalla complex ^*Corresponding author. Telephone: (518) 276-6103, Fax: (518) 276-8627. e-mail: spear{at}harold.geo.rpi.edu HTTP: //www.geo.rpi.edu/facstaff/spear/valhalla/valhalla.html     

A Strontium Isotopic Investigation of the Bjerkreim--Sokndal Layered Instrusion, Southwest Norway

The 230 km² Proterozoic Bjerkreim—Sokndal layered instrusionhas a monzonoritic bulk composition and comprises a 6 km thick,broadly leuconoritic Layered Series (LS) overlain by unlayeredmangerite and quartz mangerite. In the Bjerkreim lobe the LScomprises six megacyclic units (MCU 0, IA, IB, II, 11 and IV)in a syncline. This lobe is surrounded by migmatitic gneisses;the roof to the instrusion is missing. The mg-number opx, An% and whole-rock initial ⁸⁷Sr/⁸⁶Sr isotope ratios (Sr₀) displaybroadly parallel trends through MCUs II and III, and into MCUIV, with decreasing mg-number (75–58) and An % (50–40)accompanying increasing Sr₀ (0.7050–0.7085). This correlationbreaks down in the upper part of MCU IV and mangerite; the quartzmangerite has Sr₀ values of 0.7085. Abrupt reversals occur acrossthe MCU boundaries. The LS crystallized on the floor of a periodically replenishedmagma chamber that was continually assimilating country rockgneisses. Strong compositional zoning of the magma developedas a result of repeated replenishments with relatively dense,primitive ferrobasaltic magma (Sr₀ = 0.7049) along the floorand the development of extensive buoyant roof melts. Assimilationtook place on a massive scale (up to 50%) in the upper partsof the chamber. KEY WORDS: assimilation; fractional crystallization; hybrid magma; layered intrusion; Sr-isolopes ^*Corresponding author.     

Eocene fan delta-submarine fan deposition in the Wagwater Trough, east-central Jamaica

ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

Metamorphic P-T Paths from Cordillera Darwin, a Core Complex in Tierra del Fuego, Chile

P–T conditions and prograde P–T paths have beencalculated for amphibolite-grade pelites and amphibolites fromCordillera Darwin, Tierra del Fuego, Chile. Peak P–T conditionsare nearly all within the kyanite stability field; temperaturesgenerally show an increase with increasing grade, but pressureshave a less consistent trend, possibly increasing slightly fromgarnet to kyanite grade. P–T paths from pelites show heatingby 80–100C during loading of 0•2–3 kbar. Texturalanalysis and previous structural work indicate that this segmentof the path correlates with back-folding deformation. P–Tpaths from two Mg-rich garnet amphibolites suggest a decreasein pressure of as much as 3 kbar with 25–50C of heatingfrom the kyanite stability field to the sillimanite, and areconsistent with pervasive, minor development of fibrolitic sillimanitealong plagioclase grain boundaries. Together, the P–Tpath segments from pelites and amphibolites constitute a clockwiseP–T trajectory. The proposed clockwise P–T paths are consistent with theinterpretation that Cordillera Darwin represents an extensionallyexhumed metamorphic core complex, in which loading during garnetgrowth in the pelitic rocks was succeeded by differential upliftduring garnet growth in magnesian amphibolites. ^* Present address: Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706     

The influence of particle size variations on the magnetic properties of sediments from the north-eastern Irish Sea

This paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4ø as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/x values. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source types.     

CHANGES IN MORPHOLOGY AT THE MARGIN OF THE GREENLAND ICE SHEET (LEVERETT GLACIER), IN THE PERIOD 1943–1992: A QUANTITATIVE ANALYSIS

Changes in ice-marginal morphology near Leverett glacier, a small outlet glacier at the western margin of the Greenland ice sheet, are determined from a photogrammetrical analysis. To be able to compare two datasets from subsequent years with measurements at different coordinates, kriging was used for interpolation. In this study the kriging standard error is used to evaluate the relative accuracy of the resulting maps. Aerial photographs of 1943, 1968 and 1985 were compared. In the period 1943–1968 an area of 0.2 × 10⁶ m² was deglaciated. Approximately 1.1 × 10 m³ of material is deposited in this area. The southern part of the deglaciated area is characterized by ice-cored moraines, while moraines without ice core were formed in the north. Differences in depositional products reflect differences in meltwater activity and probably ice-marginal thermal regime. During deglaciation a small proglacial sandur decreased in altitude by 3.2 ± 0.1 m. From the early 1970s Leverett glacier advanced over a previously deglaciated area. During this advance, small ice-marginal accumulations were incorporated and eroded by the advancing glacier. Erosion products were for a substantial part stored in the proglacial sandur. About 1.2 ×10⁵ m² of the northern part of an ice-cored moraine complex decreased in altitude by −3.6 ± 0.1 m from 1943 to 1968 and over 2.7 × 10⁴ m² by −2.7 ± 0.1 m during 1968–1985. The spatial patterns of altitude change were analysed in relation to topomorphological parameters as exposition and slope angle and areas occupied by lakes. The estimated energy used to melt the subsurface ice of the ice-cored moraine is 1.4–2.2 W m² (1943–1968) and 1.0–1.6 W m² (1968–1985). These values are 30–50 times larger than the geothermal heat flux. For the expected average debris concentration of the ice core (< 10 per cent by volume) the deviation of the surface energy balance forced by climate change will be small and encompass an insignificant part of the total estimated energy used for melting.     

桂林水南洞洞穴沉积物的古地磁记录及其古环境意义

通过对桂林水南洞2.5m厚的洞穴沉积物剖面的古地磁初步研究,其结果及地质证据表明该沉积剖面形成期可能与贾拉米洛正向期对应。根据磁化率、磁性矿物的特征和沉积学特征,对该地区这一地质时期的古环境变化提出以下认识:1.第三暖湿期,以钙华沉积为特征;2.第二干冷期,以含有少量钙质的黄褐色粘土为特征,并以含有高矫顽磁性矿物和低磁化率为特征;3.第二暖湿期,以互层的红褐色粘土、含高含量钙质的粘土或钙华为特征,并以含有低矫顽磁性矿物和高磁化率为特征;4.第一干冷期,以褐灰色粘土为特征,并以含有高矫顽磁性矿物和低磁化率为特征;5.第一暖湿期,以地下河沉积相(砂卵石为主)为特征。      

黄、渤海漂移浒苔(Enteromorpha prolifera)脂肪酸组成及聚类分析的研究

于2008年7月在黄、渤海区域采集了17个漂移浒苔样品,结合3个本地种,对其脂肪酸组成进行了GC-MS分析。结果表明,所有浒苔中优势脂肪酸为十六碳酸,相对含量在27.82%—43.27%之间。藻体中还含有丰富的PUFA,主要为9,12,15-十八碳三烯酸、6,9,12-十八碳三烯酸、9,12-十八碳二烯酸、8,11,14-二十碳三烯酸,另外还含有EPA不饱和脂肪酸。以欧式距离平方为距离测量技术,类间距用平均链锁法,对20个不同地理位置的浒苔样品进行聚类分析,结果分为4大类,所有漂浮浒苔聚为第一类,青岛和连云港的本地种分别聚为另三类,说明青岛近海漂浮的浒苔不是青岛本地生浒苔,而是从外海漂移过来的,第一类的聚类结果显示了浒苔的漂移路径。结果表明,藻类脂肪酸的系统聚类分析为海洋绿藻分类与量化评价提供了一种好方法。