¹I.Blanchard,¹D.C.Rubie,³E.S.Jennings,³I.A.Franchi,³X.Zhao,¹S.Petitgirard,¹N.Miyajima,⁴S.A.Jacobson,⁵A.Morbidelli
Earth and Planetary Science Letters 580, 117374 Link to Article [https://doi.org/10.1016/j.epsl.2022.117374]
¹Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany
²Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK
³School of Physical Sciences, Open University, Milton Keynes MK7 6AA, UK
⁴Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48824, USA
⁵Laboratoire Lagrange, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice, France
Copyright Elsevier

Carbon is an essential element for the existence and evolution of life on Earth. Its abundance in Earth’s crust and mantle (the Bulk Silicate Earth, BSE) is surprisingly high given that carbon is strongly siderophile (metal-loving) at low pressures and temperatures, and hence should have segregated almost completely into Earth’s core during accretion. Estimates of the concentration of carbon in the BSE lie in the range 100–260 ppm and are much higher than expected based on simple models of core–mantle differentiation. Here we show through experiments at the putative conditions of Earth’s core formation (49–71 GPa and 3600–4000 K) that carbon is significantly less siderophile at these conditions than at the low pressures (≤13 GPa) and temperatures (≤2500 K) of previous large volume press studies, but at least an order of magnitude more siderophile than proposed recently based on an experimental approach that is similar to ours. Using our new data along with previously published results, we derive a new parameterization of the pressure–temperature dependence of the metal–silicate partitioning of carbon. We apply this parameterization in a model that combines planet formation and core-mantle differentiation that is based on astrophysical N-body accretion simulations. Because differentiated planetesimals were almost completely depleted in carbon due to sublimation at high temperatures, almost all carbon in the BSE was added by the accretion of fully-oxidized carbonaceous chondrite material from the outer solar system. Carbon is added to the mantle continuously throughout accretion and its concentration reaches values within the BSE range (e.g. 140+-40 ppm) at the end of accretion. The corresponding final core and bulk Earth carbon concentrations are 1270+-300 ppm and 495+-125 ppm respectively.

¹L.Chimiak,²J.Eiler,²A.Sessions,³C.Blumenfeld,⁴M.Klatte,⁵B.M.Stoltz
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.01.015]
¹Department of Geological Sciences, University of Colorado—Boulder, Boulder, CO, 80309 USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, 91125, USA
³Dantari, Inc, Westlake Villiage, CA, 91361, USA
⁴Dottikon Exlusive Synthesis AG, Dottikon, 5605, CH
⁵Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, 91125, USA
Copyright Elsevier

Strecker synthesis creates α–amino acids from prebiotically plausible substrates (cyanide, ammonia, and aldehydes) and is widely hypothesized to be a key mechanism in the chemistry that led to life on Earth and on other planets. To better constrain the synthetic environments and precursors of abiotic α–amino acids, and to determine unique signatures of abiogenic amino acids, we measured the molecular-averaged and site-specific carbon and nitrogen isotope effects for the Strecker synthesis of alanine. The reaction steps of the Strecker synthesis can be divided into two groups: an initial series of reversible amination and nitrile-addition reactions (‘equilibration’) and a second series of irreversible hydrolysis reactions (‘hydrolysis’). The equilibration of cyanide, acetaldehyde, and ammonia with the intermediate, α–aminopropionitrile (α-APN), has a measured 55.1 ‰ equilibrium nitrogen isotope effect between the 15N–rich amine nitrogen in α-aminopropionitrile and the 15N–poor ammonia and a 20.0 ‰ equilibrium carbon isotope effect between the 13C-poor C–2 site in α–aminopropionitrile and the 13C–rich carbonyl carbon in acetaldehyde. The first irreversible hydrolysis step is inferred to have an up to 10 ‰ normal carbon fractionation (i.e., faster for 12C, slower for 13C) for the whole molecule, but it also has one or more side reactions that deplete the reactive α-APN reservoir by up to 15 ‰. The second hydrolysis step has a 15.4 ‰ normal kinetic isotope effect on the amide (C–1) site of alaninamide, which becomes the carboxyl site of alanine. Other α–amino acids will likely experience similar nitrogen isotope fractionations between ammonia and their amine sites, and similar carbon isotope fractionations between the carbonyl carbon in reactant aldehydes or ketones and the intermediate α–aminonitrile, and between cyanide and the carboxyl site. Therefore, these isotope effects allow us to predict the carbon and nitrogen isotopic contents and intramolecular structures of α-amino acids formed by Strecker synthesis based on their substrates’ isotopic compositions, or to infer the isotopic compositions of substrates from which amino acids formed, for example in the case of the amino-acid-rich carbonaceous chondrites. The site-specific C and N isotopic compositions of amino acids formed by Strecker chemistry contrast with those typical of terrestrial biosynthetic amino acids, so these data also provide a means of discriminating between biogenic and abiogenic α–amino acids.