Isotopic compositions of carbonate-forming elements (C, O, Mg, Ca) are widely used geological proxies. Non-traditional stable isotope ratios (Mg, Ca) of carbonates can constrain water–rock reactions and precipitation kinetics but are typically measured by bulk digestion techniques that lack textural context and can be contaminated by elements released from associated phases (e.g., phyllosilicates). This is particularly problematic for micron-scale, chemically zoned extraterrestrial carbonates from asteroid-returned samples, carbonaceous chondrites, and Martian meteorites. Secondary ion mass spectrometry (SIMS) provides a reliable in situ approach, but its application to Mg and Ca isotopes in carbonates has been limited by the lack of well-characterized standards spanning the dolomite-ankerite and magnesite-siderite solid-solution series. Here we present 22 carbonate standards (10 dolomite-ankerite, 11 magnesite-siderite, and 1 calcite) for coupled C–Mg–Ca isotope analysis at 10–20 µm spatial resolution using a Cameca IMS 7f-GEO ion microprobe. The instrumental mass fractionation (IMF) for each isotopic system varies non-linearly with Fe + Mn content of carbonates, parameterized as (Fe + Mn)# [= molar (Fe + Mn)/(Fe + Mn + Mg)], and is modelled using polynomial or sigmoidal functions (R2 ≥ 0.92) with calibration residuals ≤0.7‰ (1SD). Application of these calibrations to compositionally zoned secondary standards yields grain-averaged isotopic compositions that agree with independent bulk measurements within propagated 2SE uncertainties, validating the accuracy of the protocol at per-mille-level precision. This work enables accurate, texture-resolved coupled C–Mg–Ca isotopic microanalysis of complex terrestrial and extraterrestrial carbonates.
", "doi": "10.1039/d6ja00136j", "issn": "0267-9477", "publisher": "Royal Society of Chemistry", "publication": "Journal of Analytical Atomic Spectrometry", "publication_date": "2026-08-01", "series_number": "8", "volume": "41", "issue": "8", "pages": "2875-2900" }, { "id": "authors:ngerq-5qy97", "collection": "authors", "collection_id": "ngerq-5qy97", "cite_using_url": "https://authors.library.caltech.edu/records/ngerq-5qy97", "type": "article", "title": "Exposure and karst development on an aggradational shelf margin, Ediacaran Birba Platform, South Oman Salt Basin", "author": [ { "family_name": "Present", "given_name": "Theodore M.", "orcid": "0000-0002-4747-2174", "clpid": "Present-Theodore-Michael" }, { "family_name": "Adams", "given_name": "Erwin W." }, { "family_name": "Bergmann", "given_name": "Kristin D." }, { "family_name": "Al Harthy", "given_name": "Abdulrahman" }, { "family_name": "Al Rawahi", "given_name": "Zuwaina" }, { "family_name": "Grotzinger", "given_name": "John P.", "orcid": "0000-0001-9324-1257", "clpid": "Grotzinger-J-P" } ], "abstract": "Intraformational dolomite breccias observed in three cores retrieved from the subsurface Birba Formation in Oman are interpreted as exposure and karst features in the amalgamated Basal Ara Carbonate (BAC) units of the Ediacaran–Cambrian Ara Group of the South Oman Salt Basin (SOSB). Karst breccias developed during BAC exposure are differentiated from depositional breccias in supratidal and subtidal facies by core and petrographic observations. Well-developed karst-collapse breccias are limited in scale, and much of the brecciation is in-situ compaction of solution-enlarged fractures, forming mosaic breccias in already permeable intertidal-to-supratidal facies that constituted a shelf-crest shoal complex with positive relief. Petrographic fabrics and chemical imaging of breccias indicate that karsting was followed by reinundation of the BAC by suboxic marine or mixed marine–meteoric fluids, consistent with limited exposure durations at high-frequency or composite sequence-scale disconformities. The observations complement previous work on more protected evaporite–carbonate stringers that lack clear evidence of well-developed exposure surfaces. Integrated stratigraphic and petrographic analysis is necessary to identify sequence boundaries amid accumulated exposure inherent in supratidal facies. The supratidal barrier complex in the BAC developed in a stable and aggradational accommodation regime at a key tectonic transition in the SOSB, when, just before the advancement of a foreland thrust belt associated with the latest assembly of Gondwana, basin accommodation began increasing markedly. These conditions generated a well-developed hydrographic barrier, resulting in the predominant architecture of the Ara Group, in which lowstand evaporitic facies conformably overlie carbonate platforms.
", "doi": "10.2110/jsr.2025.098", "issn": "1938-3681", "publisher": "Society for Sedimentary Geology", "publication": "Journal of Sedimentary Research", "publication_date": "2026-08", "series_number": "4", "volume": "96", "issue": "4", "pages": "723-750" }, { "id": "authors:5qe3r-j9j37", "collection": "authors", "collection_id": "5qe3r-j9j37", "cite_using_url": "https://authors.library.caltech.edu/records/5qe3r-j9j37", "type": "article", "title": "Formation and spontaneous oxidation of neutral [4Fe\u20134S] clusters in prebiotic oceans", "author": [ { "family_name": "Present", "given_name": "Theodore M.", "orcid": "0000-0002-4747-2174", "clpid": "Present-Theodore-Michael" }, { "family_name": "Valentine", "given_name": "Joan Selverstone", "orcid": "0000-0002-7174-925X" }, { "family_name": "Johnson", "given_name": "Jena E." }, { "family_name": "Szilagyi", "given_name": "Robert K.", "orcid": "0000-0002-9314-6222" } ], "abstract": "