Holocene palaeoenvironments at the timberline in the French Alps—a multidisciplinary approach

An attempt is made to reconstruct the palaeoenvironmental Holocene history at the timberline on the basis of the analysis of various palaeoecological proxy indicators available on a marshy area and its surroundings in the Taillefer Massif (Isère. France). The multidisciplinary approach involves analyses of pollen assemblages. plant macrofossils, coleoptera remains. subfossil trunks extracted from lakes or peat-bogs. and charcoals derived from the surrounding soils. This enables definition of the respective roles of five forest taxa ( Pinus uncinata Mill., Pinus cembra L., Larix decidua Mill., Abies alba Mill, and Picea abies L. Karsten) in the evolution of high altitude forests during the Holocene. Pinus uncinata was present on the plateau throughout the Holocene. Larix and Pinus cembra were present only during two periods: 7500-5000 BP and 3500-2000 BP. All trees disappeared from the plateau at about 2000 BP, while, at lower altitude. Abies was replaced by Picea. The action of both climate and early human impact can explain these changes.     

The sulfides in the garnet pyroxenite xenoliths from Salt Lake Crater, Oahu

Trace amounts of sulfide, ranging from 0?06 to 0?26 volume percent, are present in the garnet pyroxenite xenoliths in nephelinebasalt at Salt Lake Crater, Oahu, Hawaii. Microscopic studyrevealed the existence of three chemically and physically distinctsulfide types, i.e. enclosed, interstitial, and vein sulfides.The enclosed sulfide forms spherical inclusions in primary pyroxene,garnet, ilmenite, and spinel. It has a bi-modal chemical character,which may be in part due to alteration of some of the enclosedsulfide grains by the nepheline basalt host magma, but moreprobably represent Cu-Ni-rich sulfide liquid coexisting withCu-rich monosulfide solid solution. The interstitial sulfideis uni-modal and occurs in the interstices of the primary silicates.It has been subjected to intense chemical alteration by thenepheline basalt host magma. As a result the interstitial sulfidebecame Cu-poor with a varying Ni content, which indicates thatcomplete chemical equilibrium with the basalt host magma hasnot been attained. The vein sulfide, present as fracture fillings, is of late genesisand appears to represent partly altered enclosed sulfide. Thealteration process is the same as that which affected the interstitialsulfide. The mineralogy and chemistry of the enclosed sulfide suggeststhat the xenoliths have been abruptly cooled from about 1000?C, which represents the ejection of the xenoliths from thehost basalt magma.     

Estimation des périodicités et de la tendance des températures moyennes mensuelles en Tunisie

Résumé

Des données de températures moyennes mensuelles rassemblées, entre 1901 et 1988, pour six stations météorologiques de Tunisie ont été étudiées en vue d'estimer leurs périodicités dominantes et leurs tendances. L'utilisation de la procédure de Siddiqui & Wang (1984) a permis de détecter dans les séries originales une seule période significativement dominante qui est celle des 12 mois de l'année. Le test de corrélation de rang de Kendall a montré que les séries ne sont pas tendancielles.     

Aeolian construction and alluvial dismantling of a fault‐bounded intracontinental aeolian dune field (Teruel Basin,Spain); a continental perspective on Late Pliocene climate change and variability

An aeolian dune field migrating to the east encroached on the toes of alluvial fans in the Teruel Basin (eastern Spain) during a short interval in the Late Pliocene (ca 2·9 to 2·6 Ma), when Northern Hemisphere glaciation and strong glacial–interglacial cycles began. Preservation of the dune field was controlled by syn‐sedimentary activity of a normal fault. Ephemeral water discharge eroded aeolian sands and formed V‐shaped channels in which aeolian sandstone blocks accumulated. The incorporation of loose aeolian sand in wadi waters modified the sediment/water ratio, changing the physical properties of the flows as they penetrated the aeolian dune field. The erosion and cover of aeolian dune foresets by sheetflood deposits suggest that dune‐damming caused the intermittent ponding of water behind the dunes and its flashy release. The arid climate in the Late Pliocene western Mediterranean realm favoured the transport of windblown sediments from northern Africa and western Mediterranean land masses into the Mediterranean. The formation of the studied aeolian dune field (2·9 to 2·6 Ma) and possibly others (for example, the Atacama, Namib and Sahara deserts) correlates with a strong increase of the influence of obliquity, which can be attributed to the combination of a regional expression related to the reduced effect of precession due to a minimum in the long‐period (2·3 Ma) eccentricity cycle and a remote expression of the onset of the Northern Hemisphere glaciation.     

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone,Guatemala: insights from phase equilibria and Raman spectroscopy

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.     

Dinoflagellate cysts reflecting surface-water conditions in Voldafjorden, western Norway during the last 11 300 years

Abundant dinocysts in a high-resolution core from Voldafjorden, western Norway, reflect changes in sea surface-water conditions during the last c. 11 300 BP. The period from c. 11 300 to 10 800 BP (Late Allerφd) was characterized by cool temperate surface-waters, high annual temperature variation and relatively strong stratification of the water column, which is characteristic of fjord environments. Due to the stratification of the surface waters, the uppermost layer may have warmed considerably. This generated a principal difference in temperature conditions between land and sea, with slightly higher temperatures in the marine environments. The period from c. 10 800 to 10 000 BP is characterized by very harsh conditions, with sea surface-water temperatures close to freezing and long lasting seasonal sea-ice cover. Similar temperature changes at the beginning and end of the Younger Dryas are characteristic for NW Europe, but those in Voldafjorden differ from those in the open sea and in the Norwegian Channel by being significantly larger. The stratification of the water column during the Late Allerφd was probably broken down because of incipient inflow of temperate normal saline waters, which caused a marked sea surface-water warming, at c. 10 000 BP. Surface-water conditions close to those of today were gradually established between c. 10000 and 9500 BP. However, these interglacial conditions were abruptly interrupted by a significant drop in winter sea surface-water temperature and salinity occurring around 9700 BP. From c. 9500 to 7000 BP the influence of temperate normal saline water masses increased stepwise until full interglacial conditions were established around c. 7000 BP. The change in the dinocyst assemblage around 7000 BP in Voldafjorden was probably related to the onset of the modern Norwegian Coastal Current, previously documented in cores from the Skagerrak and the Mid-Norwegian Continental Shelf. The last c. 7000 BP is characterized by relatively stable surface-water conditions, possibly interrupted by periods of cooling or decreased inflow of temperate normal saline water. Like several other dinoflagellate cyst records from the Norwegian-Greenland Sea, O. centrocarpum peak values are between 4000 and 5000 BP, suggesting a regional-scale oceanographic change.     

Moisture retrievals from simulated zenith delay "observations" and their impact on short‐range precipitation forecasts

The feasibility of assimilating the GPS total zenith delay into atmospheric models is investigated within the framework of the "Observing System Simulation Experiment." The total zenith delay is made up of two terms: one is proportional to the pressure at the site of the GPS ground‐based receiver and the other to the overlying amount of water vapor. Using the MM5 mesoscale model and its adjoint, a set of 4‐dimensional variational (4DVAR) experiments is performed. Results from the assimilation of simulated precipitable water observations are used as the benchmark. The model domain covers Southern California. The observations are simulated with a 10 km horizontal resolution model that includes full physics, while a 20‐km resolution and a less comprehensive physics package are used in the 4DVAR experiments. Both, the 10‐km and 20‐km models employ the same set of 15 vertical levels. Moisture fields retrieved from the total zenith delay are found to compare very well with those retrieved from the precipitable water. Verified against the observations, the vertically integrated moisture is found to be very accurate. An overall improvement is also achieved in the vertical profiles of the moisture fields. The use of the so‐called background term and model initialization are shown to greatly reduce the negative impact that the sole assimilation of the total zenith delay can have on the pressure field and integrated water vapor. The adverse effect stems from the poor resolution of the topography needed to evaluate the model pressure at the GPS sites. The analysis increments of all model fields are found to be similar to the counterparts obtained from the assimilation of the precipitable water. The same is true for the short‐range precipitation forecasts initiated from the 4DVAR‐optimal initial conditions.     

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Samples of a core (52 m) of ablating Antarctic ice were analysed for ¹⁴CO and ¹⁴CO₂ by accelerator mass spectrometry. The data were compared with a ¹⁴C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ ¹⁴C at depths below 10 m, which we attribute to ¹⁴C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 ¹⁴C ka BP.