The radioracemization of isovaline. Cosmochemical implications

The optically pure d- and l-enantiomers of isovaline (I), which cannot be racemized by ordinary chemical mechanisms involving α-hydrogen removal, and which has been isolated in apparently racemic form from the Murchison meteorite, have been subjected to partial radiolysis by the ionizing radiation from a 3000 Ci ⁶⁰Co γ-ray source. Both in the anhydrous and hydrated solid states and as solid sodium or hydrochloride salts each enantiomer suffered significant radioracemization of the undestroyed residue during its partial radiolysis. The sodium salt of isovaline in 0.1 M aqueous solution suffered extensive radiolysis with relatively small radiation doses, but showed no detectable radioracemization. The significance of these observations with respect to the primordial enantiomeric composition of the isovaline (and other amino acids) indigenous to meteroties is discussed.     

The NdSr isotopic correlation in mantle materials and geodynamic consequences

The neodymium-strontium isotopic correlation observed in most of the Earth mantle materials is evaluated by means of direct modelling. Several geochemical models are quantitatively developed to explain the observations. The main results of this modelling are that such a correlation is not geochemically trivial and that it corresponds to specific conditions in chemical fractionation. These specific conditions seem to be satisfied by solid-liquid partitioning in magmatic conditions. The discussion of the experimental data supports a continuous convecting-magmatic fractionation model for a large proportion of the mantle.     

The importance of energetic particle precipitation on the chemical composition of the middle atmosphere

An assessment is made of the relative contribution of certain classes of energetic particle precipitation to the chemical composition of the middle atmosphere with emphasis placed on the production of odd nitrogen and odd hydrogen species and their subsequent role in the catalytic removal of ozone. Galactic cosmic radiation is an important source of odd nitrogen in the lower stratosphere but since the peak energy deposition occurs below the region where catalytic removal of O₃ is most effective, it is questionable whether this mechanism is important in the overall terrestrial ozone budget. The precipitation of energetic solar protons can periodically produce dramatic enhancement in upper stratospheric NO. The long residence time of NO in this region of the atmosphere, where catalytic interaction with O₃ is also most effective, mandates that this mechanism be included in future modelling of the global distribution of O₃. Throughout the mesosphere the precipitation of energetic electrons from the outer radiation belt (60°70°) can sporadically act as a major local source of odd hydrogen and odd nitrogen leading to observable O₃ depletion. Future satellite studies should be directed at simultaneously measuring the precipitation flux and the concomitant atmosphere modification, and these results should be employed to develop more sophisticated models of this important coupling.     

Comparison of isoleucine epimerization in a model dipeptide and fossil protein

The dipeptide isoleucyl-glycine in aqueous solution has activation energies for isoleucine epimerization and hydrolysis of 23.1 and 20.9 kcal mol^?1, respectively. The activation energy for epimerization of NH₂-terminal isoleucine is 5–8 kcal mol^?1 less than activation energies reported for epimerization of isoleucine in the free state or in various protein systems. Furthermore, the rate of isoleucine epimerization in the NH₂-terminal position exceeds the rate of hydrolysis at temperatures between 0 and 152°C. Consequently, a high rate of epimerization in fossils should only occur when isoleucine is in the terminal position. Partially epimerized isoleucine will be converted through hydrolysis from the terminal form to the slower epimerizing free isoleucine. Over the course of the total isoleucine epimerization reaction, either in dipeptides or proteins, the activation energy will increase and the reaction rate will decrease as terminal isoleucine is converted to free isoleucine. This process may explain the non-linear kinetics observed for isoleucine epimerization in carbonate fossils.The degree of isoleucine epimerization is low in different molecular weight fractions of a fossil protein from Mercenaria in all fractions greater than 500 mol. wt, where most isoleucine is likely to be in the interior position. In the less than 500 mol. wt peptide fraction, where a considerable portion of the isoleucine is likely to be in the terminal position, the degree of epimerization is significantly greater. These analytical results from fossil protein support the interpretation of isoleucine kinetics obtained by study of dipeptides.     

Partial chemical characterization of estuarine dissolved organic matter

Activated charcoal chromatography was used to isolate dissolved organic matter from a series of water samples collected in Narragansett Bay, Rhode Island, during 1975–1976. The range of dissolved organic carbon values in the samples was 1.46–3.75 mg OC/1. A relatively constant amount of organic carbon was isolated giving overall recoveries of 23 to 63% (mean of 45%). The isolated organic matter was characterized by spectrophotometric measurements, stable carbon isotope determinations, and copper solubilizing capacities. In general, the isolated material displayed considerable constancy in terms of the parameters measured. However, several chemical variations were detected that were probably related to the introduction of organic matter from diatom blooms and terrigenous runoff.     

Sedimentary processes and trace metal contaminants in the buccaneer oil/gas field,northwestern gulf of Mexico

Sediments are the ultimate sink for contaminants in the marine environment, and physical processes of sedimentation influence the distribution and accumulation of these contaminants. Evaluation of contaminant levels in sediments is one approach to assessing environmental impact; data interpretation depends on consideration of sediment texture and mineralogy, however, which profoundly influence chemical composition. In this study, comparison of potentially contaminated sediments from the production field with control populations was done only within the context of similar (as to texture and organic carbon and carbonate content) sample groups as determined by cluster analysis. Ba, Cd, and Sr are identified as contaminants. Supported by the identification of a well-crystallized expandable clay—possibly bentonite—drilling fluids are a potential source of Ba. Ba and Sr may be unnaturally high because of their abundance in discharged produced formation waters, but may also be naturally controlled by the unique faunal assemblage associated with the structures. Cd is probably derived from corrosion of the structures and assorted debris on the seafloor. In general, contamination is limited to an area within 100 m of the platforms. Furthermore, substantial erosion around platforms has probably effectively removed and dispersed the bulk of the contaminants introduced into the marine environment by the offshore exploration/production operations.     

Neodymium and strontium isotope study of ophiolite and orogenic lherzolite petrogenesis

Neodymium isotopic analyses have been measured on nine ophiolites and four orogenic lherzolites. ε_Nd varies from +12 to +3 in the ophiolites and from +18 to +2 in the orogenic lherzolites. The majority of the analyses plot on a ε_Nd-ε_Sr correlation line as defined by Nd and Sr isotopic analyses of oceanic basalts. However, certain ophiolitic and lherzolitic samples exhibit high⁸⁷Sr/⁸⁶Sr ratios and as such lie to the right of the correlation line towards seawater compositions.From these data one can postulate several origins for ophiolites including that of mid-ocean ridges and ocean islands. If the orogenic lherzolites are interpreted as representative of the mantle occurring below active ridges a more complex model is required involving mantle heterogeneity and multi-episodic chemical fractionation starting prior to 2 Ga ago.     

Pelagic tar off Georgia and Florida in relation to physical processes

Following the Ixtoc I oil rig blowout in Campeche Bay we hypothesized that resulting tarballs should eventually appear in the Gulf Stream off Gerogia and that because of dynamic barriers in the innershelf little would reach nearshore areas. To test these hypotheses, surface tows to collect floating tar were taken off the coasts of Georgia and Florida in October and December, 1979.No tar was found within 40 km of the shore. All samples more than 40 km offshore contained some tar. The mean concentration was 0.82 mg m^?2 with a range of 0.01–5.6 mg m^?2. Closely spaced sampling showed extreme variation but trends were consistent. Perylene was the most abundant compound in the tarballs.     

Hydrocarbons and fatty acids in two cores of Lake Huron sediments

Compositions of aliphatic hydrocarbons and of fatty acids were analyzed in two half-meter cores of postglacial Lake Huron sediment. One core represents a continuous record of the past 450 yr of sediment accumulation; the other consists of a surficial layer of modern sediment overlying 40 cm of 11,000–12,000 yr-old sediment. Concentrations of hydrocarbons are higher in the younger core than in the older one. Based upon n-alkane distributions, this reflects a smaller input of terrigenous material to Lake Huron 11,000 yr ago rather than diagenetic losses. Most of the hydrocarbons present in the 450 yr-old core are allochthonous while half are autochthonous in the older core. Fatty acids are primarily of aquatic character in both cores, and their concentrations decrease rapidly with depth. Unsaturated acids disappear more quickly than do their saturated analogs. Fatty acid degradation occurs mostly in the biologically active zone of these sediments, and little further alteration of fatty acids appears to happen over times as long as 12,000 yr.