Michael H. Engel

Isotopic evidence for extraterrestrial non- racemic amino acids in the Murchison meteorite

Many amino acids contain an asymmetric centre, occurring as laevorotatory, L, or dextrorotatory, D, compounds. It is generally assumed that abiotic synthesis of amino acids on the early Earth resulted in racemic mixtures (L- and D-enantiomers in equal abundance). But the origin of life required, owing to conformational constraints, the almost exclusive selection of either L- or D-enantiomers, and the question of why living systems on the Earth consist of L-enantiomers rather than D-enantiomers is unresolved. A substantial fraction of the organic compounds on the early Earth may have been derived from comet and meteorite impacts. It has been reported previously that amino acids in the Murchison meteorite exhibit an excess of L-enantiomers, raising the possibility that a similar excess was present in the initial inventory of organic compounds on the Earth. The stable carbon isotope compositions of individual amino acids in Murchison support an extraterrestrial origin—rather than a terrestrial overprint of biological amino acids—although reservations have persisted (see, for example, ref. 9). Here we show that individual amino-acid enantiomers from Murchison are enriched in 15N relative to their terrestrial counterparts, so confirming an extraterrestrial source for an L-enantiomer excess in the Solar System that may predate the origin of life on the Earth.

Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France

Results of a paleomagnetic, rock magnetic, geochemical, and petrographic study on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization (CRM) carried by magnetite. Where smectite has altered to other clay minerals, limestones are characterized by a prefolding, secondary, normal polarity magnetization throughout the basin. The magnetization is interpreted to be a CRM based on low burial depths which cannot cause thermoviscous resetting. Where significant smectite is still present, the CRM is absent/weakly developed, and where the clays show no evidence for burial alteration, the units are characterized by a primary magnetization. CRM intensity also varies with the amount of smectite and burial. Isothermal, anhysteretic, and natural remanent magnetization intensities increase where smectite has altered, both stratigraphically and geographically. This is interpreted to indicate magnetite authigenesis associated with clay diagenesis. Superparamagnetic magnetite is more dominant in highly altered units based on the results of low-temperature experiments. All sections away from the Alps have 87Sr/86Sr values that are similar to coeval seawater, and stable isotopes of carbon and oxygen show no sign of alteration. Orogenic-type fluids therefore are not a likely agent of remagnetization. Near the Alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the CRM through a reversal. A postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic-type fluids. We conclude that burial diagenesis of smectite is the likely cause for the development of the widespread CRM in the Vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere.

Kinetic fractionation of stable carbon and nitrogen isotopes during peptide bond hydrolysis: Experimental evidence and geochemical implications

Abstract Kinetic isotope effects associated with the hydrolysis of the dipeptide glycylglycine in unbuffered aqueous solution were investigated over a 60°C temperature range. The hydrolysis of the dipeptide resulted in the distinct fractionation of carbon and nitrogen isotopes. As the extent of the hydrolysis reaction increased, the residual peptide became increasingly enriched in 13 C and 15 N. The kinetic isotope effect for nitrogen ranged from 1.0025 to 1.0040 and in general increased with decreasing temperature. The experimental evidence indicates that isotope effects associated with peptide bond cleavage may complicate the interpretation of stable carbon and nitrogen isotope signatures of residual proteinaceous material preserved in fossils.

The Ice Man’s diet as reflected by the stable nitrogen and carbon isotopic composition of his hair

Establishing the diets of ancient human populations is an integral component of most archaeological studies. Stable isotope analysis of well‐preserved bone collagen is the most direct approach for a general assessment of paleodiet. However, this method has been limited by the scarcity of well‐preserved skeletal materials for this type of destructive analysis. Hair is preserved in many burials, but is often overlooked as an alternative material for isotopic analysis. Here we report that the stable carbon and nitrogen isotope values for the hair of the 5200 year‐old Ice Man indicates a primarily vegetarian diet, in agreement with his dental wear pattern. Whereas previous investigations have focused on bone collagen, the stable isotope composition of hair may prove to be a more reliable proxy for paleodiet reconstruction, particularly when skeletal remains are not well preserved and additional archaeological artifacts are unavailable.—Macko, S. A., Lubec, G., Teschler‐Nicola, M., Andusevich, V., Engel, M. H. The Ice Mans diet as reflected by the stable nitrogen and carbon isotopic composition of his hair. FASEB J. 13, 559–562 (1999)

Kinetics of peptide hydrolysis and amino acid decomposition at high temperature

Dipeptide hydrolysis and amino acid decomposition appear to follow a first-order rate law. The hydrolysis rate increases exponentially with increasing temperature in aqueous solution at both 265 atm and water steam pressures over the temperature range of 100 to 220 degrees C. Dipeptide hydrolysis has a lower apparent activation energy at 265 atm (44.1 KJ/mol) than at water steam pressure (98.9 KJ/mol). At lower temperatures (<200-220 degrees C), the rate of peptide bond hydrolysis is faster at 265 atm than at water steam pressure. At higher temperatures (>200-220 degrees C), however, peptide bond hydrolysis is slower at 265 atm than at water steam pressure. In aqueous solution, amino acid decomposition rates also increase exponentially with increasing temperature. Amino acid decomposition rates are much higher at 265 atm than at water steam pressure over the entire temperature range investigated.

Chemical remagnetization and burial diagenesis: Testing the hypothesis in the Pennsylvanian Belden Formation, Colorado

Lower Pennsylvanian Belden Formation carbonate rocks from Colorado were subjected to paleomagnetic, rock magnetic and geochemical studies to test whether there is a connection between a widespread chemical remanent magnetization (CRM), carried by authigenic magnetite, and burial diagenesis. Thermal demagnetization results indicate the presence of two components of natural remanent magnetization (NRM) after removal of a low unblocking temperature (NRM-250°C) remanence that is interpreted to be a modern, viscous magnetization. An intermediate unblocking temperature (250–400°C) remanence component with normal and reversed polarity Tertiary directions is interpreted to be a thermoviscous remanent magnetization. Many limestones also contain a high unblocking temperature (400–570°C) remanence component which is interpreted to be a CRM. Fold tests from different parts of the basin indicate that the CRM was acquired either before or during Laramide folding. This CRM is interpreted to be carried by authigenic magnetite that formed by replacement of pyrite. Hysteresis ratios are consistent with those reported for other remagnetized carbonates and indicate that the CRM is carried by single-domain/pseudo single-domain magnetite. Although elevated 87Sr/86Sr values indicate passage of radiogenic fluids through the limestones, the results of contact vein tests do not support the hypothesis that these fluids were responsible for the CRM. The time of CRM acquisition, which varies from late Paleozoic to Cretaceous, coincides with the modeled time of organic matter maturation in different parts of the basin. This suggests that diagenetic reactions, that were triggered by low to moderate burial temperatures, may have caused the magnetite authigenesis and probably gave rise to the CRM.

Secular, episodic changes in stable carbon isotope composition of crude oils

Abstract Stable carbon isotopic compositions of C15+ saturate and aromatic hydrocarbon fractions from 514 crude oils from known source rocks were examined for secular, episodic variations. The oils have been correlated by means of biomarker analyses and/or geological inferences to their respective source rocks and were further categorized into seven source rock types based on geochemical and petrologic indicators of depositional environments. Thus, these oil samples represent a wide range of source paleoenvironments from thirteen age divisions (Riphean to Neogene). The stable carbon isotopic compositions of both fractions become enriched in 13 C with decreasing geologic age. Three major, abrupt shifts toward 13 C enrichment occurred at the Precambrian–Cambrian/Ordovician, Triassic/Jurassic and Paleogene/Neogene boundaries. Analysis of smaller scale isotopic shifts reveals four distinct groups of oils, generally independent of organic facies and secondary alteration processes. Comparison of evolutionary changes in the biosphere to episodic changes in stable carbon isotopic compositions throughout the Phanerozoic indicates that isotopic shifts of crude oils may generally be related to the diversity of preserved phytoplankton.

The late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron

Abstract The geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron. The design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the Pennsylvanian age Belden Formation, an organic-rich carbonate in northwestern Colorado with pyrite grains rimmed with magnetite that contains a Cretaceous paleopole position. The geochemistry is applicable to both deep and near surface reactions involving pyrite, magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations (CRM). Aqueous solutions of ferric-ligand complexes and pyrite were heated (60°C) for 60 days in the absence of light and at low partial pressures of oxygen. Additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron (ligand only). Raising the pH to ⪖ 8.5 produced a ferrous hydroxide precipitate, and subsequent heating (90°C) resulted in the formation of magnetite. Ferric complexes of oxalate, salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation. The activity of strongly reducing ligands (e.g., catechol) or unsaturated Fe(III)-exchange capacity (particularly in experiments with bentonite or with solutions of uncomplexed ligands) can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and/or interfere with the crystallization of magnetite. The bentonite acted to adsorb dissolved species and also underwent direct dissolution, especially by the oxalate ligands. Although these reactions could occur at any depth, a continuing supply of molecular oxygen (more available in near surface settings) would result in the formation of hematite or the production of acidic sulfate conditions, whereas at greater depths (or restricted porosity), the redox gradation is halted at magnetite. The results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis.

The Precambrian nonesuch formation of the North American mid-continent rift, sedimentology and organic geochemical aspects of lacustrine deposition

Abstract The Precambrian Keweenawan Trough in the Lake Superior region of North America is part of a failed intractonic rift that is filled with a thick sequence of volcanic and sedimentary rocks. One unit in this sequence, the None-such Formation, is composed of black-to-green siltstones and shales and is conformable with the underlying alluvial Copper Harbor Conglomerate and the overlying fluvial Freda Sandstone. The formation is exposed as a lens in northern Michigan and Wisconsin and attains a maximum thickness of ∼ 200 m. Based on the integration of outcrop and core data, nine facies have been recognized in the formation; each has been assigned to one of three genetic facies assemblages. A marginal lacustrine assemblage, characterized by interbedded lithic sandstones, siltstones, mudstones and sandstone-shale couplets, is interpreted as a record of deposition on a sandflat-mudflat complex. A lacustrine assemblage is characterized by massive to well-laminated dark shaly siltstones, carbonate laminites, shales, siltstones and mudstones. These sediments were probably deposited in a progressively shallowing perennial lake. Bottom conditions were anoxic and became increasingly more oxic as lake regression progressed. A gradual transition from a lacustrine environment to a fluvial environment is represented by the red to brown, massive, horizontally laminated and rippled fine sandstones, siltstones and mudstones of the fluvial-lacustrine assemblage. Interactions among rates of subsidence, rates of sedimentation, fluctuations in lake level, possible changes in climate and differences in tectonic setting controlled sedimentation in the Nonesuch Lake and resulted in variable vertical facies sequences. Total organic carbon (TOC) analyses show a strong correlation between organic richness and the shale facies (lacustrine assemblage) and mudstone facies (marginal lacustrine assemblage). Detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two prevalent organic petrographies which could be distinguished using combined pyrolysis-gas chromatography-mass spectrometry (PY-GC-MS) and, to a limited extent, by stable carbon isotope mass spectrometry. The geographic and stratigraphic distribution of rocks containing the two organic petrographies suggests that differential preservation (e.g., microbial degradation) accounts for the difference rather than maturity or variable source input.

Early diagenesis and organic matter preservation — a molecular stable carbon isotope perspective

Abstract Through a new development in stable isotope capability, gas chromatography coupled to a stable isotope ratio mass spectrometer (GC-C-IRMS), the reaction pathways of organic matter diagenesis can potentially be traced by analysis of the isotopic compositions of individual molecular components. In this study, changes in bulk stable carbon and nitrogen isotope compositions, amino acid contents and carbon isotope compositions of individual amino acids are reported for seagrass and sediment degradation experiments using isotopically resolvable organic substrates. Seagrass showed slight enrichments in δ 15 N (+0.4‰) with little change in δ 13 C following 4 weeks of decomposition. During that period, the identifiable amino acid content decreased by ∼ 50% for each amino acid. Mixtures of marine sediment with the same seagrass showed enrichments in 15 N (+2.3‰) with associated depletions in carbon (−2.8‰) isotopic compositions over the same time span. These changes are attributed to hydrolysis, deamination, decarboxylation and condensation reactions. Control experiments on the sediments without added fresh seagrass showed no change in isotopic content. At the molecular level, using GC-C-IRMS, certain amino acids in the seagrass (e.g., Glu, Ala, Val, Gly) are seen to decrease in 13 C content during decomposition whereas others remained constant (e.g., Leu, Ile, Pro) or became increasingly enriched in 13 C (Asp). The molecular isotope approach indicates that the process of early diagenesis, leading to the eventual residual materials which are preserved, is significantly more complex than simple breakdown and loss. A portion of the organic matter consequently preserved in organic-rich deposits can be attributed to new production during early diagenetic stages.