Epimerization Ratios Research Articles

A 45,000-yr record of a tropical lowland biota: The land snail fauna from cave sediments at Coco Ree, Jamaica

Research Article| January 01, 1993 A 45,000-yr record of a tropical lowland biota: The land snail fauna from cave sediments at Coco Ree, Jamaica GLENN A. GOODFRIEND; GLENN A. GOODFRIEND 1Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, 76100 Rehovot, Israel Search for other works by this author on: GSW Google Scholar RICHARD M. MITTERER RICHARD M. MITTERER 2Geosciences Program, University of Texas at Dallas, Richardson, Texas 75080 Search for other works by this author on: GSW Google Scholar Author and Article Information GLENN A. GOODFRIEND 1Department of Environmental Sciences and Energy Research, Weizmann Institute of Science, 76100 Rehovot, Israel RICHARD M. MITTERER 2Geosciences Program, University of Texas at Dallas, Richardson, Texas 75080 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (1): 18–29. https://doi.org/10.1130/0016-7606(1993)105<0018:AYROAT>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GLENN A. GOODFRIEND, RICHARD M. MITTERER; A 45,000-yr record of a tropical lowland biota: The land snail fauna from cave sediments at Coco Ree, Jamaica. GSA Bulletin 1993;; 105 (1): 18–29. doi: https://doi.org/10.1130/0016-7606(1993)105<0018:AYROAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The land snail fauna from cave sediments in wet tropical lowland forest in central Jamaica was analyzed with regard to their implications for the biogeographic history of the late Quaternary. Eight radiocarbon dates on the shells range from 600 to 45,000 yr B.P. and show a close relationship to amino acid epimer ratios (D-alloisoleucine/L-isoleucine, or A/I, values). A/I values of individual shells indicate that the faunas span a continuous time sequence but that a significant mixture of ages occurs within each level of the cave sediments. The chronologies of individual species were therefore determined by A/I values of individual shells, calibrated against radiocarbon ages. The land snail fauna consists of 40 species (all endemic to the island) and shows remarkable stability over the span of time covered by the deposit, indicating the continuous persistence of forest. The much lower epimerization rate for the late Pleistocene compared to the Holocene indicates much cooler late Pleistocene temperatures, but this appears to have had little effect on the fauna. Many species that are presently endemic to restricted regions, which include the area of the cave, are present throughout the sequence. No changes are detected at the last glacial maximum, but some changes are seen in the Holocene. At 3000-4000 yr B.P., two species disappear from the sequence (one lives nearby at present, whereas the other presently has a distribution farther to the west). The first records of one species in the sequence occurs at this time, and for another species at ca. 10,000 yr B.P. No extirpations of species occur at the site following European settlement. Relatively stable climatic gradients, rather than climatic change, may be an important cause of the evolution of faunas endemic to different parts of the island. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Aminostratigraphy of Pliocene-Pleistocene high-sea-level deposits, Nome coastal plain and adjacent nearshore area, Alaska

Amino acid epimerization ratios (aIle/Ile) measured in 295 fossil molluscan shells of 3 genera provide a basis to evaluate the number and relative timing of high-sea-level events represented by deposits blanketing the Nome coastal plain and adjacent nearshore area, northwestern Alaska. Frequency-distribution analysis, along with a new approach that relies on the ranked-data distribution, was used to identify clusters (aminozones) in the aIle/Ile data, each of which presumably represents a discrete period of marine inundation of the study area. Six periods of high relative sea level were distinguished; some are recognized only by redeposited shells rather than in situ bio- or lithostratigraphic units. The oldest period is represented by shells recovered from the deepest sections of two offshore boreholes; aIle/Ile ratios in these shells (1.0) are much higher than in any found onshore and probably date to the late Miocene/early Pliocene. The oldest aminozone lies below nonfossiliferous sediments, which, in turn, are overlain by glacigenic and marine deposits containing largely fragmented shells with aIle/Ile ratios ranging from 0.35 to 0.65. This range coincides with aIle/Ile ratios from various onshore deposits previously ascribed to the middle Pliocene (∼3 m.y.) Beringian transgression. Ratios in this range can be subdivided into three evenly spaced aminozones, each of which may represent multiple, short-duration (10 4 yr) high-sea stands separated by a longer-period (>10 5 yr) sea-level oscillation. On the basis of a new empirical model of parabolic epimerization kinetics and an assumed age of 3.2 m.y. for the oldest Beringian aminozone (the approximate age of the initial submergence of Bering Strait), the estimated length of time represented by the three Beringian aminozones is between 0.5 and 1.7 m.y. The youngest Beringian highstand was followed in the late Pliocene or early Pleistocene by a lengthy period of nonmarine deposition during which local valley glaciers advanced onto the coastal plain at least once. Relative sea level apparently did not again surpass that of today until the middle Pleistocene Anvilian transgression, the oldest higher-than-present stand of the Pleistocene, yet considerably younger than previously thought. Glaciers overran the Nome coastal plain at least once following the Anvilian transgression, incorporating abundant shells into their drift. The drift is notched by a shoreline formed during the Pelukian transgression of the last interglaciation. Although well represented onshore, no Pelukian deposits have been found offshore.

Stereoselective carbohydate synthesis via palladium hydroxide catalyzed epoxide hydrogenolysis

2,5:3,4-Dianhydro-D-altritol ( 1) reacts with lithium aluminum hydride to produce a 50:50 mixture of the epimeric alcohols ( 2) and ( 3) in fairly good yield. However, when 1 is catalytically hydrogenated in the presence of palladium hydroxide on charcoal (Pearlman's catalyst), the ratio of epimers formed changes to 99:1 ( 2:3) as determined by GC of the corresponding acetylated derivative compounds. The stereochemistry of the epimers has been determined by NMR spectroscopy.

Synthesis and diastereofacial selectivity in Diels-Alder reactions of halogenated 2-aza-1,3-dienes

The synthesis of mono- and trihalogenated derivatives of 2-aza 1,3-dienes 2 and 3 is described. The first example of diastereofacial selectivity in Diels-Alder reactions of heterodienes of type 2 with electron-poor dienophiles is reported, and a model for the explanation of the observed epimeric ratios, based on the interactions between diene and dienophile, is proposed

Determination of the epimeric composition of ibuprofenyl-CoA

Ibuprofen [racemic2-(4-isobutylphenyl)propionic acid] is a 2-arylpropionic acid nonsteroidal anti-inflammatory drug which undergoes unidirectional, R to S chiral inversion in vivo. It has been proposed that this chiral inversion phenomenon occurs via a coenzyme A (CoA) thioester intermediate. To characterize the formation and metabolism of this metabolic intermediate, ibuprofenyl-CoA, reference standards were needed and thus the CoA derivatives of ( R)-, ( S)-, and racemic ibuprofen were chemically synthesized. An HPLC assay employing a C 18 reverse-phase column was developed to quantitate “total” ibuprofenyl CoA. Samples collected from this assay were then analyzed for ibuprofenyl-CoA epimeric composition by chiral chromatography employing a Chiral-AGP α 1-acid glycoprotein column. The applicability of these methods was demonstrated by assessing ( R)- and ( S)-ibuprofenyl-CoA hydrolysis and epimerization following incubation with rat liver homogenates. Rat liver homogenate catalyzed the complete and rapid epimerization of ibuprofenyl-CoA and the rate constants for ( R)- and ( S)-ibuprofenyl-CoA hydrolysis were equal. ATP and CoA were found to inhibit rat liver-catalyzed ibuprofenyl-CoA hydrolysis by 70–80% with no effect on epimerization. Additionally, it was demonstrated that traditional indirect ibuprofenyl-CoA assays which employ basic hydrolysis result in erroneous epimeric ratio determinations due to chemical epimerization.

Effects of source, thermal maturity, and biodegradation on the distribution and isomerization of homohopanes in petroleum

The distributions of C 31–C 35 17α,21β(H)-homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment. For example, high C 35-homohopane indices [C 35/(C 31–C 35)] are typical of petroleums generated from source rocks deposited under anoxic marine conditions, where the C 35 backbone of the precursor bacteriohopanetetrol is preferentially preserved. Homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock. Homohopane distributions are altered by secondary processes. For example, the C 35-homohopane index decreases with increasing API gravity and thermal maturity of related oils from the Monterey Formation, offshore California. Similar decreases in the homohopane index are observed for expelled oils from Monterey source rock during hydrous pyrolysis. Biodegradation can result in selective loss of high or low molecular weight homologs, apparently depending on the bacterial population in the reservoir. Bacterial demethylation of homohopanes appears necessary to explain 25-norhopanes in at least one biodegraded oil. Hydrous pyrolysis studies of the Phosphatic and Siliceous members in the Monterey Formation show that identical heating conditions result in different homohopane C-22 epimer ratios. Furthermore, a “reversal” in the trend of the epimer ratio at high experimental temperatures is observed for the Phosphatic, but not for the Siliceous lithology. This could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures, possibly mediated by rock mineralogy.

Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean (1)

Petroleum is associated with sulfide-rich sediment at two active oceanic spreading axes in the northeast Pacific Ocean. In the Guaymas Basin, a spreading axis in the Gulf of California, petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in Quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites. In Escanaba Trough, at the southern end of the Gorda Ridge spreading axis offshore northern California, petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary turbiditic river-derived sediment. Comparisons of the distributions of hydrocarbons--n-alkanes, isoprenoids, terpanes, steranes, and aromatics--show that chemical differences among four petroleum samp es are such that two samples from Guaymas Basin can be distinguished from two samples from Escanaba Trough. Distinguishing characteristics resulting from differences in sources include n-alkane distributions and certain sterane ratios; distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons. These oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes, and their generation, expulsion, and migration occurred simultaneously over an instantaneous geological time period. The potential economic significance of hydrothermally derived petroleum is uncertain, but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation, expulsion, and migration.

A comparison of U-series disequilibrium dates and amino acid epimerization ratios between corals and marine molluscs of Pleistocene age

A direct comparison of uranium-series dates of marine molluscs determined as pertaining to the last interglacial period ∼ 125 ka ago with U-series ages of coexisting corals and isoleucine epimerization ratios has been undertaken on the fossiliferous calcarenites at Il Fronte (Italy) and Son Grauet (Spain) which have long been recognized as important interglacial marine deposits. Two groups of mollusc shells from the Glycymeris species: G. violacescens and G. bimaculata have been studied, which show that, when compared to coral ages, G. violacescens produces more consistent results with respect to the U-series dating technique than does G. bimaculata. Isoleucine epimerization ratios from the same shells are internally consistent although the five Il Fronte shells show an anomalously low mean value when compared to previous site and regional data. This may be attributed to post-depositional changes of these samples indicated by their very high U content. It is concluded that, although the U-series dating technique may be valuable for some mollusc shells when corals are lacking, it must be used with caution, after a thorough consideration of the analytical data.

Source and heating rate effects upon maturity parameters based on ratios of 24-ethylcholestane diastereomers

Abstract Samples from two sedimentary sequences, one a typical shale and the other containing coals and dispersed coal particles, have been used to investigate the progress of epimerisation of 20R 5α(H),14α(H),17α(H) 24-ethylcholestane to the corresponding 20S diastereomer with increasing maturity of the sediments. The apparent activation energies and entropies for this reaction are different in these two sequences, and the differences have been attributed to differences in the rock matrixes in the two sequences. Furthermore, these kinetic parameters have been used to model the behaviour of the epimerisation process in coals and in shales under a range of sediment heating rates typical of those encountered in sedimentary basins. These calculations show that low heating rates cause the epimerisation to be more advanced in a coal than in a shale while the opposite applies under high heating rates. Some samples of closely occurring coals and shales were found to contain steranes with appreciably different relative abundances of 20S and 20R diastereomers. These observations are interpreted in terms of the different effects of different heating rates upon the epimerisation process in the two types of matrix. In turn, these results indicate that sediments which contain type III organic matter in the form of dispersed coals as the dominant source of hydrocarbons would produce petroleum with anomalously low sterane epimer ratios if they had experienced a high heating rate as a result of high geothermal gradients and high burial rates.

Short-term biomarker variability in the Monterey formation, Santa Maria Basin

Forty-eight samples of a 370-ft Miocene Monterey Formation core from the Union Leroy 51-18 well, Santa Maria Basin, California, are examined for source rock potential and molecular distributions. Total organic carbon content, Rock-Eval pyrolytic yield, vitrinite reflectance, and distribution of regular steranes, tricyclic and pentacyclic terpanes, and aromatic steroids are measured. The present-day temperature range through the sampled interval is estimated to be less than 5°C; maturity differences among samples are negligible. Source rock potential and molecular distribution differences are considered to arise entirely from the effects of source input, diagenesis (including lithologic effects) and migration. The sampled section is comprised of siliceous and phosphatic/carbonate lithofacies, and covariances between geochemical parameters (source potential and biomarker distribution) and lithofacies are recognized. Several molecular parameters change systematically with depth/lithology, including ββ( αα + ββ) ethylcholestane ratios and C-20 and C-22 sterane and hopane epimer ratios. The siliceous lithofacies possesses higher moretane/hopane, tricyclic/pentacyclic terpane, and sterane/terpane ratios, and lower mono/mono + tri aromatic steroid ratios. Bisnorhopane/hopane ratios are up to 40 times higher in the phosphatic/carbonate lithofacies than in the siliceous lithofacies. Increased relative concentrations of aromatic steroids are attributed to sulfur-sterol reactions during very early diagenesis, whereas increased relative bisnorhopane amounts are considered to result from massive input of specific anaerobic biota. Although kerogen elemental ratios change abruptly at the lithofacies boundary, molecular distributions within the extractable organic matter change gradually, beginning about 30 ft (9m) above this interface. This gradual change is tentatively attributed to upward migration of hydrocarbons originating in the phosphatic/carbonate lithofacies, probably through fractures within the lower part of the siliceous lithofacies interval.

Amino acid geochronology of selected interglacial sites in the North Sea area.

Isoleucine epimerization (alle/Ile) ratios have been measured on foraminiferal tests from interglaical sites in the southern North Sea region. In order to evaluate the possibilities of this amino acid method on benthic foraminifera, sites were chosen from areas where the stratigraphy was already well known. The study demonstrates that the isoleucine epimerization reaction in benthic foraminifera can be used for correlation and as a means of relative age estimation within a limited geographic area. Previous suggestions of a Holsteinian age for the Rugȧrd, Tornskov, and Wacken sites are supported for the Kås Hoved site. The age of most presumed Eemian sites is supported by the amino acid results; the Reighton site also seems to belong in this interglacial.

Climatic evolution of the eastern Canadian Arctic and Baffin Bay during the past three million years

The outer east coast of Baffin Island is characterized by a series of sedimentary forelands. These contain a variety of litho- and biofacies associated with glacial marine and marine deposition into sea levels higher than those of the present. These high relative sea levels were associated with glacial isostatic loading and unloading of the crust by the NE sector of the Laurentide Ice Sheet. On the basis of amino acid epimerization ratios, eleven chronologically distinct units are delimited. The youngest unit is less than or equal to 10 ka, but all others are at or beyond the limits of radiocarbon dating. Based on biostratigraphy and amino acid data, the oldest units exposed in the forelands may be Pliocene in age. Molluscs, Foraminifera, and the palynology of buried soils and organics, indicate that the vast bulk of the exposed sequences contain floras and faunas that represent environments warmer than those at present. An analysis of modern and fossil pollen spectra suggests a steady decrease in low arctic conditions throughout the Quaternary.

Late quaternary land snails from the north coast of Jamaica: Local extinctions and climatic change

Analysis of the land snail faunas and chronology ( 14C and amino acid epimer ratios) of six late Quaternary deposits from a site on the north coast of Jamaica provides a basis for reconstruction of local extinction events and their relation to paleoclimate. Paleotemperature estimates are derived from epimerization rates at an interior site and indicate an average Holocene-late last glacial temperature difference of c.4–5°C. Size variation in land snails indicates relatively dry conditions during the late last glacial and moist conditions in the late Holocene, followed by drier conditions beginning sometime during the last millennium. The rich faunas of the latter half of the last glacial are represented today at the same site by only a relatively few remnant species. Two major periods of extinction are identified. During latest last glacial to early Holocene time, a number of species became locally extinct, probably due to increased temperatures; many survive today in the cooler interior of the island. Extensive extinctions occurred again during the last millennium. These apparently resulted largely from habitat disturbance by man. A similar pattern of late Holocene extinctions has also been documented for vertebrates on other tropical islands and is also thought to result from human disturbance.

Thermal depolymerization of kerogen and formation of immature oil

Two Chinese immature oil shales from the continental deposits of kerogen type I and II have been thermally treated combined with the technique of supercritical fluid extraction at 630–650 K and 15–25 MPa in a semi-continuous laboratory scale apparatus. Toluence is selected as the solvent. Up to 70–80% of the kerogen matrix can be converted to a thermal bitumen and extracted simultaneously. The chemical structural parameters from the NMR, IR, XRD, ESR and EA analyses of the kerogen and the thermal bitumen show striking resemblance in nature. It implies that the thermal bitumen is primarily a depolymerized product of the kerogen. Based on the GC/MS spectra of the aliphatic eluate of the thermal bitumen, the predominance of the odd/even ratio of the alkanes and the epimeric ratios, such as 20S(20S + 20R) of C-29 steranes and 22S/(22S + 22R) of C-32 terpanes, show that the maturity of the thermal bitumen from these oil shales is comparable to that of commercial immature oils from East China. The thermal bitumen is thought to be an intermediate product of the thermal degradation of kerogen. Since the thermal bitumen is mainly composed of asphaltenes and resins, it has a structure of gel. The gel-state bitumen may turn to sol-state readily due to its low aromaticity and polarity, or due to selective adsorption of asphaltenes by clay minerals. Then the migration potential of the bitumen is enhanced. Consequently, under favorable geological conditions, the thermal depolymerization of kerogen seems to be a probable mechanism to explain the formation of immature oils.

Preparation and solvolysis of trans,trans,trans-tricyclo[7.3.1.0]tridecan-3-yl toluene-p-sulphonates in acetic acid and aqueous hexafluoropropan-2-ol. The role of non-chair conformers and the question of internal return in the limiting S N1 solvolytic mechanism

Details are reported of a general synthesis of the epimeric trans,trans,trans-tricyclo[7.3.1.0]tridecan-3-ols and the corresponding toluene-p-sulphonates, (5a and b). Both compounds react in buffered acetic acid slightly more slowly than simpler unbridged axial and equatorial cyclohexyl toluene-p-sulphonates, but the axial: equatorial epimeric rate ratio is very similar at 3.4 : 1 (50 °C) and is quite different from the divergent values obtained for bridged systems. The epimeric rate ratio is even smaller in 97 : 3 (w/w) hexafluoropropan-2-ol–water (97HFIP), 1 : 1 at 25 °C, and the equatorial compound is estimated to be the more reactive in this medium below ca. 6 °C. Remarkably, the standard enthalpy of activation for solvolysis in 97HFIP is lower for the equatorial than the axial isomer. These results, the role of non-chair conformers for the equatorially substituted isomer, and the nature of the rate-determining step in the limiting SN1 solvolytic mechanism generally are discussed.

Evaluation of amino-acid racemization/epimerization dating using radiocarbon-dated fossil land snails

The relation between age and amino-acid epimer ratios (alloisoleucine/isoleucine, A/I) of Holocene land snails was quantitatively evaluated through 14 C and amino-acid analysis of 33 samples from fluvial and colluvial sediments and rodent middens in the Northern Negev Desert of Israel. A/I is strongly correlated with 14 C ages in fluvial and rodent midden deposits ( r = 0.95 and 0.94, respectively), permitting age estimates from A/I ratios with precisions of ±700 and ±660 yr. The correlation is weaker in colluvial deposits ( r = 0.74), and age estimates from A/I ratios are correspondingly less precise (±1580 yr). This probably results from delayed burial, which exposes the shells to intense radiation on the desert surface. Because of the generally strong relation between age and A/I, amino-acid epimerization analysis of individual shells can be used to identify mixed-age deposits and to reconstruct species chronologies from mixed-age deposits.

Reduction of digoxin to 20R-dihydrodigoxin by cultures of Eubacterium lentum.

The anaerobic bacterium Eubacterium lentum, a common constituent of the intestinal microflora, inactivates digoxin by reducing the unsaturated lactone ring. Reduction of the cardiac glycoside by growing cultures of E. lentum ATCC 25559 proceeded in a stereospecific manner, with the 20R-dihydrodigoxin constituting more than 99% of the product formed. This is in contrast to the 3:1 ratio of 20R and 20S epimers formed in the chemical catalytic hydrogenation. Formation of the reduced glycosides proceeded quantitatively when an overall concentration of 10 micrograms/ml was added to the cultures. E. lentum did not hydrolyze the digitoxose sugars from C-3 of the parent glycoside. However, the synthetically prepared sugar-hydrolyzed metabolites (digoxigenin, digoxigenin monodigitoxoside, and digoxigenin bisdigitoxoside) were reduced to the corresponding dihydro metabolites. Repetition of the experiments with a feces sample from a volunteer who was known to be a converter of digoxin to dihydrodigoxin gave results identical to those obtained with pure E. lentum cultures.

Human pharmacokinetics and antimicrobial activities of the temocillin epimers.

The pharmacokinetics of the epimers of temocillin were investigated in 4 healthy male subjects following intravenous administration of 1g of temocillin disodium (free acid) which contains a R : S epimer ratio of approximately 65 : 35. The R epimer had a 2-fold greater total plasma clearance, a 23% larger volume of distribution and a shorter beta half-life than the S epimer. Intermediate values were obtained for total temocillin (R + S) from high pressure liquid chromatography (HPLC) data. In each plasma sample, the unbound fraction of the R epimer was generally 2-fold higher than that of the S epimer, which is suggested as the reason for the differences in the pharmacokinetic properties of the epimers. The temocillin pharmacokinetic parameters obtained from the microbiological assay data reflect most closely those for the R epimer derived from HPLC data. The resolved R epimer exhibited twice the potency of the S epimer against the microbiological assay organism Pseudomonas aeruginosa NCTC 10701. However, in tests for antibacterial susceptibility, for instance minimum inhibitory concentration (MIC) determinations involving prolonged incubation, there was little difference in the inhibitory activities of the resolved R and S epimers compared with temocillin (R + S), presumably as a consequence of the epimerization of the individual epimers. In contrast, in rapid tests for bactericidal activity, which minimise the effect of epimerization, the R epimer exhibited greater bactericidal activity than the S epimer.

Isoleucine epimerization in Quaternary benthonic foraminifera from the Norwegian Continental Shelf: A pilot study

The extent of epimerization of the protein amino acid L-isoleucine has been measured in Late Quaternary benthonic foraminifera found in cores from two areas of the Norwegian Continental Shelf in order to evaluate the potential of this reaction as a geochronological tool. Measurements were restricted to monospecific assemblages of Cibicides lobatulus and Elphidium excavatum from 14C-dated and seismostratigraphic units representing a range of ages. By comparing with data on the same species from last-interglacial and late-glacial sites in western Norway (Miller et al., 1983) and with the assumption of similar temperature history for the shelf, and coastal areas, the absolute ages of the samples are discussed. Of the nine samples analysed one was of Holocene, seven of Late to Middle Weichselian and one sample of pre-Weichselian age. From this pilot study we conclude that isoleucine epimerization ratios in benthonic foraminifera provide a reliable index of relative age that can be used to correlate sedimentary units that contain in situ faunas and have similar thermal histories. Absolute ages of the enclosing sediment can be approximated if suitable well-studied calibration sites are available.

2' And 3'-ketonucleosides and their arabino and xylo reduction products : Convenient access via selective protection and oxidation of ribonucleosides

A number of 2',5'- or 3',5'-diprotected ribonucleosides and 5'-protected 2'- or 3'-deoxy-β-D- erythro-pentofuranosyl nucleosides have been oxidized to the corresponding 3' or 2'-ketonucleoside derivatives using chromium trioxide/pyridine/acetic anhydride or dimethyl sulfoxide/ acetic anhydride. Reduction of the carbonyl functions with sodium borohydride gave the inverted arabino, xylo, or deoxy- threo isomers as predominant products by attack at the less hindered α-face of the sugar ring. Parallel reductions using sodium borodeuteride corroborated the epimeric ratios by demonstrating that complete oxidation of the original hydroxyl groups had occurred. The deuterium labeling also aided in making NMR spectral assignments.