Urs Krähenbühl

Mass Mortality and Its Environmental and Evolutionary Consequences

The latest Mesozoic and earliest Tertiary sediments at Deep Sea Drilling Project site 524 provide an amplified record of environmental and biostratographic changes at the end of Cretaceous. Closely spaced samples, representing time intervals as short as 102 or 103 years, were analyzed for their bulk carbonate and trace-metal compositions, and for oxygen and carbon isotopic compositions. The data indicate that at the end of Cretaceous, when a high proportion of the oceans planktic organisms were eliminated, an associated reduction in productivity led to a partial transfer of dissolved carbon dioxide from the oceans to the atmosphere. This resulted in a large increase of the atmospheric carbon dioxide during the next 50,000 years, which is believed to have caused a temperature rise revealed by the oxygen-isotope data. The lowermost Tertiary sediments at site 524 include fossils with Cretaceous affinities, which may include both reworked individuals and some forms that survived for a while after the catastrophe. Our data indicate that many of the Cretaceous pelagic organisms became extinct over a period of a few tens of thousands of years, and do not contradict the scenario of cometary impact as a cause of mass mortality in the oceans, as suggested by an iridium anomaly at the Cretaceous-Tertiary boundary.

Interaction of sulfate assimilation with carbon and nitrogen metabolism in Lemna minor

Cysteine synthesis from sulfide andO-acetyl-l-serine (OAS) is a reaction interconnecting sulfate, nitrogen, and carbon assimilation. UsingLemna minor, we analyzed the effects of omission of CO2 from the atmosphere and simultaneous application of alternative carbon sources on adenosine 5′-phosphosulfate reductase (APR) and nitrate reductase (NR), the key enzymes of sulfate and nitrate assimilation, respectively. Incubation in air without CO2 led to severe decrease in APR and NR activities and mRNA levels, but ribulose-1,5-bisphosphate carboxylase/oxygenase was not considerably affected. Simultaneous addition of sucrose (Suc) prevented the reduction in enzyme activities, but not in mRNA levels. OAS, a known regulator of sulfate assimilation, could also attenuate the effect of missing CO2 on APR, but did not affect NR. When the plants were subjected to normal air after a 24-h pretreatment in air without CO2, APR and NR activities and mRNA levels recovered within the next 24 h. The addition of Suc and glucose in air without CO2 also recovered both enzyme activities, with OAS again influenced only APR.35SO4 2− feeding showed that treatment in air without CO2 severely inhibited sulfate uptake and the flux through sulfate assimilation. After a resupply of normal air or the addition of Suc, incorporation of 35S into proteins and glutathione greatly increased. OAS treatment resulted in high labeling of cysteine; the incorporation of 35S in proteins and glutathione was much less increased compared with treatment with normal air or Suc. These results corroborate the tight interconnection of sulfate, nitrate, and carbon assimilation.

Pre-Imbrian craters and basins: ages, compositions and excavation depths of Apollo 16 breccias

Breccia fragments have been analyzed from the 2–4 mm sieve fraction of three Apollo 16 soils collected in the vicinity of North Ray Crater (63503,17 at Station 13; 67603,1 and 67703,14 at Station 11). Ar39-Ar40 ages, Ar37-Ar38 exposure ages, abundances of major and certain trace elements, and petrographie data relevant to thermal history have been obtained for up to 48 individual fragments. Among the samples. 30 gave Ar39-Ar40 release patterns that allowed the assignment of a high- or intermediate-temperature plateau age and the recognition of three age groups. Group I (10 fragments) are 4.12-4.21 AE, Group 2 (13 fragments) are 3.89-4.02 AE, and Group 3 (6 fragments) are <2.5 AE in age. Only one fragment (3.60 AE) falls outside this grouping and possibly represents Theophilus ejecta. The probability that the gap between 4.12 and 4.02 AE is a statistical fluctuation is only ∼2%. The exposure ages cluster strongly around 50 × 106y. the age of North Ray Crater. The oldest, Group 1 fragments are all anorthositic metamorphosed breccias of light-matrix type. The younger. Group 2 fragments are noritic anorthosite and anorthositic norite breccias with textures indicative of greater annealing (melted matrix), one totally melted sample being of KREEP-basalt texture. The very young. Group 3 fragments are chiefly of glass or devitrified glass. There is a marked distinction between Groups 1 and 2 in compositional as well as textural properties. The Group 2 breccias are generally enriched in Mg, K and REE relative to the aluminous Group I breccias (eg. K ≤ 400 ppm in Group 1 and mostly ≥ 600 ppm in Group 2). This difference is attributed to the introduction of KREEP and mafic ANT components during the formation of the Group 2 breccias. The results are interpreted as reflecting two magnitudes of cratering. The older craters (>4.1 AE) were of medium size (diameters up to a few hundred kilometers), large enough to reset the ages but not capable of excavating deeper than predominantly feldspathic (anorthositic) layers of the crust. The younger craters (∼3.9-4.0 AE) were, in contrast, those ascribed to major basin-forming events and were therefore capable of excavating a deeper and wider spectrum of crustal lithologies. The latter resulted in admixture of KREEP and mafic ANT components with the feldspathic ANT, cover layer. KREEP was thus only excavated in abundance during the basin-forming events, from a sub-crustal layer formed initially at ∼4.4 AE but incorporated in the breccias at ∼4 AE. The KREEP-contaminated. Group 2 breccias have—except two fragments—ages between 3.95 and 4.02 AE. This group includes a crystallized melt (3.97 ± 0.04 AE), close in composition and texture to 14310 (3.87 ± 0.04 AE) which is generally attributed to the Imbrian basin-forming event (∼3.88 AE). The pre-Imbrian. Group 2 breccias of Apollo 16 can best be attributed to the Nectaris basin-forming event, which according to the clustered ages probably occurred at ∼3.98 AE. Our results support a multi-impact lunar cataclysm with the formation of Nectaris (3.98 AE). Humorum. South Serenitatis, Crisium and Imbrium (3.88 AE) within a 0.1 AE time interval.

A 700-YEAR PALEOECOLOGICAL RECORD OF BOREAL ECOSYSTEM RESPONSES TO CLIMATIC VARIATION FROM ALASKA

Recent observations and model simulations have highlighted the sensitivity of the forest-tundra ecotone to climatic forcing. In contrast, paleoecological studies have not provided evidence of tree-line fluctuations in response to Holocene climatic changes in Alaska, suggesting that the forest-tundra boundary in certain areas may be relatively stable at multicentennial to millennial time scales. We conducted a multiproxy study of sediment cores from an Alaskan lake near the altitudinal limits of key boreal-forest species. Paleoecological data were compared with independent climatic reconstructions to assess ecosystem responses of the forest tundra boundary to Little Ice Age (LIA) climatic fluctuations. Pollen, diatom, charcoal, macrofossil, and magnetic analyses provide the first continuous record of vegetation fire-climate interactions at decadal to centennial time scales during the past 700 years from southern Alaska. Boreal-forest diebacks characterized by declines of Picea mariana, P. glauca, and tree Betula occurred during the LIA (AD 1500-1800), whereas shrubs (Alnus viridis, Betula glandulosa/nana) and herbaceous taxa (Epilobium, Aconitum) expanded. Marked increases in charcoal abundance and changes in magnetic properties suggest increases in fire importance and soil erosion during the same period. In addition, the conspicuous reduction or disappearance of certain aquatic (e.g., Isoetes, Nuphar, Pediastrum) and wetland (Sphagnum) plants and major shifts in diatom assemblages suggest pronounced lake-level fluctuations and rapid ecosystem reorganization in response to LIA climatic deterioration. Our results imply that temperature shifts of 1-2 degrees C, when accompanied by major changes in moisture balance, can greatly alter high-altitudinal terrestrial, wetland, and aquatic ecosystems, including conversion between boreal-forest tree line and tundra. The climatic and ecosystem variations in our study area appear to be coherent with changes in solar irradiance, suggesting that changes in solar activity contributed to the environmental instability of the past 700 years.

History of the paired lunar meteorites MAC88104 and MAC88105 derived from noble gas isotopes, radionuclides, and some chemical abundances☆

Abstract Noble gas isotopes, radionuclides, and chemical abundances were studied in the lunar meteorites MAC88104 and MAC88105 collected in the MacAlpine Hills area of Antarctica. The concentrations of the noble gas isotopes and the radionuclide activities in the two meteorites are essentially identical, proving that the two meteorites are paired. From 40K-40Ar dating we obtain a gas retention age of 3550 ± 400 Ma, typical for lunar surface material. Probably before breccia compaction the MAC88104/5 material resided for 630 ± 200 Ma at an average shielding depth of 85 g cm 2 , that is, about 50 cm below the lunar surface in the lunar regolith, as judged from the concentration of cosmic-ray produced Kr and Xe isotopes. Although this duration of lunar regolith residence is relatively long, MAC88104/5 represent immature regolith material: the concentration of solar wind implanted noble gases are two orders of magnitude lower than those in mature lunar soil. The reason for that might be the relatively high average shielding. The 40 Ar 36Ar ratio of the trapped component is 5.7 ± 1.0, indicating an intermediate antiquity of the material; we estimate that the solar wind and lunar atmospheric particles were implanted about 2000 Ma ago. The radionuclide activities allow us to derive the exposure history of the MAC88104/5 material. The duration of the Moon-Earth transfer was ≦0.24 Ma. Production of the radionuclides on the lunar surface immediately before ejection is possible, but duration and shielding depth for such an exposure cannot be determined from our data. We obtain a terrestrial age of 0.1 ≦ tterr ≦ 0.6 Ma and an ejection time of MAC88104/5 from the Moon of 0.1 ≦ tejection ≦ 0.8 Ma. The exposure histories of the lunar meteorites discussed in this work indicate that at least two impact events are required for their ejection from the Moon. Our first noble gas results for lunar meteorite Yamato-793274 show that it represents mature lunar regolith material with relatively high concentrations of solar wind implanted noble gases and a duration of several hundred million years of exposure to cosmic rays.

Combined osmium and strontium isotopic study of the Cretaceous-Tertiary boundary at Sumbar, Turkmenistan: A test for an impact vs. a volcanic hypothesis

Osmium and strontium isotopic ratios at the Cretaceous-Tertiary boundary at Sumbar, Turkmenistan, display a negative hyperbolic covariation superimposed on the long-term trend, which displays a positive covariation. The minimum ratio for Os and the maximum ratio for Sr occur at the boundary clay. Volcanism with a mantle or crustal source cannot account for the isotopic data. The low 187 -O s / 188 -O s and the high 87 -Sr/ 86 -Sr ratios can be explained by an impact, whereby Os was derived from the bolide and the 87 Sr/ 86 Sr ratio was enhanced by acid rain and/or a tsunami following the event.

The reactor accident at chernobyl: A possibility to test colloid-controlled transport of radionuclides in a shallow aquifer

Radioactive fall-out from the damaged nuclear power station at Chernobyl (USSR) has been measured in May 1986 in the River Glatt (Zurich, Switzerland) and in a shallow groundwater stream. This aquifer is hydraulically connected to the river and recharged by river water. Ruthenium-103, I-131, Te-132, Cs-134 and Cs-137 were measured several times using gamma-ray spectroscopy. By filtration through 0.45 μm, 0.2 μm and 0.05 μm filters the radionuclides were partitioned between solution (filtrate < 0.05 μm) and particles/colloids. In the river, the main radioactivity for all the investigated nuclides was found in the water passing the 0.05 μm filter. Among the particulates the highest radioactivity was detected in the fraction > 0.45 μm, the two smaller sizes contributing only little. In the water infiltrating into the groundwater Ru-103, I-131 and Te-132 were found almost exclusively in the filtrate ( 0.05 μm suggesting their retention by the heterogeneous glaciofluvial outwash deposits (stones, gravel, sand, clays).

Trapped solar wind noble gases, kr81/kr exposure ages and k/ar ages in apollo 11 lunar material.

Grain size and etching experiments show that the fine lunar material contains large amounts of trapped solar wind particles. Elemental and isotopic compositions of the noble gases in solar material and in the terrestrial atmosphere are significantly different, except for the Ar36/ Ar38 and the Kr isotope ratios. Exposure ages of two rocks and of the fine material are between 380 and 510 x 106 years. Feldspar concentrates give K/Ar ages of 3220 and 3300 x 106 years, significantly higher than the unseparated rock.

Radiochemical separation of actinides from environmental samples for determination with DF-ICP-MS and alpha spectrometry

Summary The purpose of this study was to develop an optimised sample preparation method for the determination of Pu, Am, and Cm in environmental samples, soils, and sediments, and to compare the sensitivities of DF-ICP-MS, alpha spectrometry, and liquid scintillation alpha spectrometry. Borate fusion was used for sample material digestion. Two methods based on TOPO liquid–liquid extraction and combination of TEVA and TRU extraction chromatography showed highest yields of 92.7% for Pu, 86.8% for Am, and 87.6% for Cm. The detection limits were between 0.014–0.034 Bq/kg for 239Pu, 0.014–0.093 Bq/kg for 240Pu, and 0.011–1.20 Bq/kg for 241Am. Analytical results for 238Pu, 239Pu, 240Pu, 241Pu and 241Am and their isotope ratios in certified reference materials (IAEA-135 and IAEA-300) indicated very good accuracy of the methods proposed.

Halogen contamination in Antarctic H5 and H6 chondrites and relation to sites of recovery

Abstract The distribution of the elements F, Cl, Br and I was analysed in layers removed stepwise from Antarctic H5 and H6 chondrites. All meteorites show higher concentrations of these elements on their surfaces than in their interiors. The degree of halogen enrichment cannot be correlated with the degree of visual weathering and is proportional to the time the meteorites were residing on the surface of the Antarctic ice. During this period, aerosols, salts and gaseous components are deposited on the surfaces of the meteorites and diffuse into their interior. The observed contamination level of the meteorites is influenced by leaching properties and diffusion behaviour of the individual halogens. The major contamination source for F, Cl and probably Br is air-borne seaspray; for I it is the gaseous compound methyl iodide (CH3I) produced by biological processes in the sea. Methyl iodide and its oxidation products formed in the Antarctic atmosphere (e.g., I2) can be transported over longer distances to the interior of Antarctica than air-borne seaspray. Therefore, the ratio of the halogen contamination is related to the collection site of the meteorites. All meteorites that were found in the Antarctic interior are contaminated to a lesser degree by F, Cl and Br relative to I than those found near the coast. The measured enrichment ratio of I/F is a function of the distance between the collection site and the open sea, and increases from the Allan Hills to the Thiel Mountains. By revealing the degree of contamination of a meteorite it is possible to determine its maximum surface residence time on the Antarctic ice.