Differential nocturnal cooling in the landscape detected by mobile measurements

Summary In June 1986 mobile air temperature measurements were performed in the Kivik area in Sweden. The measuring trips were repeated each hour during a night with calm and clear weather conditions. Along a loop, 8650 m long and traversing a hilly landscape, the air temperature was measured at four levels (0.2, 1.0, 2.0 and 3.0 m a g). In this study the results are focused on the measurements at the standard level, 2.0 m a g, in order to make it easier to compare this study with earlier mobile studies.Sudden rises in air temperature at 2.0 m occurred at a number of elevated locations along the loop, at a max. rate of 3.5°C/hour, indicative of the influence of dynamic turbulence. Lower parts of the loop did show a drop in air temperature at the same time, with a max. of 3.8°C/hour, indicative of cold air influence. In most earlier studies the mobile measurements were compensated for the nocturnal cooling in order to make it possible to directly compare the different measurement points. This study highlights the problems due to turbulence as mentioned above. It also stresses the fact that more reference measurements at more than one point along a loop are needed in hilly terrain.With 7 Figures     

Climatic and environmental reconstructions based on fossil assemblages from middle devensian (Weichselian) deposits of the river Thames at south kensington, central London, UK

Fossiliferous silts within the Late Pleistocene Kempton Park Gravel, of the River Thames Valley, were exposed in 1980 during foundation works for the Ismaili Centre in South Kensington, London. The results of a multidisciplinary study of the geomorphology, sediments, fossil plants, vertebrates, molluscs, ostracods and insects are reported. The silts were deposited under two distinct climatic regimes; a lower unit accumulated when the climate was arctic and an upper when the temperatures were at least as warm as those of the present day. Both these units occupy the same channel system and are separated from one another by less than a metre of sediment, implying that the climatic change was probably sudden and intense. The strongest evidence for this climatic difference comes from a study of the Coleoptera, which show an almost complete replacement of the arctic element in the fauna by a suite of temperate species. Palaeotemperature reconstructions using the Mutual Climatic Range method, based on the coleopteran assemblages from the lower unit, suggest that the mean temperature of the warmest month was 9±2 °C and that of the coldest month −22±10 °C. For the upper unit the mean temperature of the warmest month had risen to about 17 °C and that of the coldest month to about −4 °C. The episode represented by the lower unit, with its arctic climate, had not previously been recognized in the Thames Valley. The fauna from the upper, temperate, unit is very similar to that from other sites in the Kempton Park Gravel, such as that from Isleworth, 10 km upriver, which, like the upper unit at the Ismaili Centre, was characterized by the virtual absence of trees. It would appear that in such cases this treelessness does not indicate cold conditions, equivalent to those of the modern tundra, but may instead result from a combination of ecological and temporal factors. The value of multidisciplinary studies in reaching such conclusions is emphasized.The temperate episode described here is correlated with the thermal maximum at the early part of the Upton Warren Interstadial Complex. An earlier suggestion, based on amino acid epimerization ratios, that the Upton Warren Interstadial correlates with Oxygen Isotope Sub-stage 5a is not supported by the data, which show no evidence of the forested environments that characterized this period in both Britain and the adjacent Continent. It is thought that the temperate deposits at the Ismaili Centre belong to the Middle (Pleniglacial), rather than the Early, Devensian (Weichselian) and are equivalent to Oxygen Isotope Stage 3.     

Deep-sea avulsion and morphosedimentary evolution of the Rhône Fan Valley and Neofan during the Late Quaternary (north-western Mediterranean Sea)

The Petit-Rhône Fan Valley (north-western Mediterranean) is a broad, sinuous, filled valley that is deeply incised by a narrow, sinuous thalweg. The valley fill is differentiated into three seismic subunits on high-resolution seismic-reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high-amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass-flow deposits resulting from the disintegration of the fan-valley flanks. The upper bedded subunit shows an overall lens-shaped geometry and the seismic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhône levees. Piston core data show that the upper few meters of this upper subunit consist of thin turbidites, probably deposited by overflow processes. The few available ¹⁴C ages suggest that the upper stratified subunit filled the Petit-Rhône Fan Valley between 21 and 11 kyr BP. The upper bedded subunit is deposited within the Petit-Rhône Fan Valley downslope of a major decrease in slope gradient. This upper subunit and the thalweg are genetically related and represent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature resulting from a recent avulsion of the major channel course. Our interpretation implies that the thalweg is not a purely erosional feature but a depositional/erosional channel. This small channel/levee system is superimposed on a large muddy channel/levee system after the sediment supply changed from thick muddy flows during the main phase of aggradation of the Rhône Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (～16–18 ka BP). The pre-existing morphology of the Petit-Rhône Fan Valley played a determinant role in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow stripping resulting from the changes in the slope gradient along the thalweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit-Rhône Fan Valley. Coarser channelled sediment remaining in the thalweg were deposited as a ‘sandy’lobe (Neofan). As indicated by ¹⁴C dating, sedimentation on this lobe continued until very recently, suggesting a further evolution of the turbidity flows from small mixed flows to small sandy flows. the deposition of this study lobe and the sedimentary fill of the Petit-Rhône Fan Valley may be related to widespread shelf edge and canyon wall failures with a resulting downslope evolution of failed sediment into turbidity currents.     

Sixteen Statistical Tests for Outlier Detection and Rejection in Evaluation of International Geochemical Reference Materials: Example of Microgabbro PM-S

A totally objective procedure involving sixteen statistical tests (a total of thirty four single or multiple outlier versions of these tests) for outlier detection and rejection in a univariate sample is applied to a data base of sixty four elements in a recently issued international geochemical reference material (RM), a microgabbro PM-S from Scotland. This example illustrates the relative importance and usefulness of these tests in processing modern geochemical data for possible outliers and obtaining mean concentration and other statistical parameters from a final normal sample of univariate data. The final mean values are more reliable (characterized by smaller standard deviations and narrower confidence limits) than those obtained earlier using an accommodation approach (robust techniques) applied to this data base. Very high quality (certified value equivalent, cve) mean data are now obtained for eleven elements as well as high quality recommended values (rv) for thirty three elements in PM-S. Earlier work using the accommodation approach failed to establish even one cve value for any of the sixty four elements compiled here. The present procedure of outlier detection and elimination is therefore recommended in the study of RMs     

Two stages of zircon and monazite growth in anatectic leucogneiss: SHRIMP constraints on the duration and intensity of Pan-African metamorphism in Prydz Bay, East Antarctica

SHRIMP U–Pb data from two samples of foliated anatectic leucogneiss bracket the high-grade metamorphic history of metasediments from southern Prydz Bay, East Antarctica. Magmatic zircons in these two samples have mean crystallization ages of 535 ± 13 and 536 ± 35 Myr, and provide a good estimate for the timing of peak metamorphism and partial melting. Low-Th monazite grains in the same samples have mean ages of 528 ± 4 and 527 ± 11 Myr, which are within error of the zircon ages but may reflect a lower blocking temperature. High-Th monazite rims with younger mean ages of 518 ± 3 and 512 ± 13 Myr occur around low-Th cores in both samples, and were precipitated from hydrous fluids released during the crystallization of residual melts. These magmatic fluids were channelled along the retrograde foliation present in both samples. All mineral assemblages and fabrics preserved in the metasediments reflect intense Pan-African metamorphism and deformation, associated with high degrees of partial melting and significant vertical displacements of the crust. Continental reconstructions assuming limited Pan-African tectonism in East Antarctica should be reassessed in the light of these new data.     

Unusual biomarker maturation ratio changes through the oil window, a consequence of varied thermal history

Saturated and aromatic biomarker ratios continue to change systematically through the oil window and into the gas-condensate window to high vitrinite reflectances (R_o = 1.16%) in mature marine and lacustrine Mesozoic clastic samples from a South African basin. Two of the ratios reverse above R_o = 0.9%. These unusual maturation effects result from isolated periods of high rates of maturation increase. The basin cooled regionally after the break-up of Gondwana but high heating rates prevailed during the late Cretaceous-early Tertiary, as Africa moved across a hotspot, and again in the late Tertiary as a result of a possible hotspot and hydrothermal event.     

Comparison of source rock geochemistry of selected rocks from the Schei Point group and Ringnes formation, Sverdrup basin, arctic Canada

Organic-rich from the Schei Point group (middle to late Triassic in age) and the Ringnes formation (late Jurassic) from the Sverdrup basin of the Canadian arctic archipelago have been geochemically evaluated for source rock characterization. Most samples from the Schei Point group are organic-rich (> 2% TOC and are considered as immature to mature oil-prone source rocks [kerogen types I, I–II (IIA) and II (IIA)]. These kerogen types contain abundant AOM1, AOM2 and alginite (Tasmanales, Nostocopsis, Leiosphaeridia, acritarch and dinoflagellate) with variable amounts of vitrinite, inertinite and exinite. Samples from the Ringnes formation contain dominant vitrinite and inertinite with partially oxidized AOM2, alginite and exinite forming mostly immature to mature condensate- and gas-prone source rocks [kerogen type II–III (IIB), III and a few II (IIA)]. Schei Point samples contain higher bitumen extract, saturate hydrocarbons and saturate/ aromatic ratio than the Ringnes samples. Triterpane and sterane (dominant C₃₀) distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the Schei Point group are at the onset of oil generation and contain a mixture of sapropelic (algal) and minor terrestrial humic organic matter. Sterane carbon number distributions in the Ringnes formation also suggest a mixed algal and terrestrial organic matter type. There are some variations in hopane carbon number distributions, but these are apparently a function of thermal maturity rather than significant genetic differences among samples. Pyrolysis-gas chromatography/mass spectrometry of the two samples with similar maturity shows that the Schei Point sample generates three times more pyrolyzate than the Ringnes sample. Both samples have a dominant aliphatic character, although the Ringnes sample contains phenol and an aromaticity that is higher than that of the Schei Point sample.     

Timing and Extent of Late Quaternary Paleolakes in the Trans-Pecos Closed Basin, West Texas and South-Central New Mexico

The Trans-Pecos Closed Basin is a hydrographically closed region covering 20,000 km²centered on Salt Basin, 160 km east of El Paso, Texas. Geomorphic and limnetic evidence have been used to identify four major highstands for Lake King during the last glacial maximum (LGM). Additional geomorphic features from a second, recently identified, paleolake, Lake Sacramento, have been found in the Beargrass subbasin, a nested subbasin approximately 75 km northwest of Salt Basin. Radiocarbon ages of the organic material in Lake King sediments date four abrupt climate changes and rapid lacustrine transgressions during the LGM with a quasi-periodicity of 2000 yr. Geomorphic evidence in the Beargrass subbasin identifies lake cycles contemporaneous with those in Lake King. The dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in New Mexico. The quasi-periodicity of the events approximates that of Dansgaard–Oeschger events identified from Greenland ice cores. The contemporaneity of the Trans-Pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the LGM.