Stromatolites are sedimentary formations created by the layered growth of microorganisms in shallow marine settings. Fossil stromatolites constitute some of the earliest evidence for life on Earth. Allen Nutman et al. describe metamorphosed stromatolites deposited around 3,700 million years ago in what is now Greenland. This is more than 200 million years older than the previous record-holders for earliest-known fossils, so these stromatolites rank as the Earth's earliest fossils by some margin. Although there is indirect evidence from isotope geochemistry that the pedigree of life on Earth is even older, this report is likely to be controversial.

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Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

The fractionation factor (α) between the 18O/16O ratio in calcite and CO2 gas produced as a result of reaction with phosphoric acid has been determined as a function of temperature for two of the most widely used variations of the dissolution technique, reaction in a sealed vessel and reaction in an acid bath. Although these methods show approximately the same relationship between temperature and α, they are offset from one another by between 0.2 and 0.4% over the temperature range 25–90°C. There appears to be no temperature- or technique-dependent fractionation on carbon over the range of temperatures examined in this study. The difference between these two methods arises from three main processes: (1) isotopic exchange between the CO2, the phosphate polymers in the acid and water produced in the reaction; (2) dissolution of CO2 in the acid; and (3) incomplete removal of CO2 during the extraction process. Of these two methods the “acid bath” method probably provides the most accurate isotopic composition of the calcite. As a result of the fact the original fractionation factors were determined using the “sealed vessel” method, we have calculated our temperature [T (K)] - dependent fractionation factor (αT) taking into account the difference between the two methods at 25°C. For the “sealed vessel” method: (a) αT= [5.60 (± 0.33) × 102] T−2 + 1.003943 (± 0.000067). For the “common acid bath” technique: (b) αT = [5.25 (± 0.08) × 102] T −2 + 1.003970 (± 0.000034).

Fractionation of the stable isotopes of oxygen and carbon in carbon dioxide during the reaction of calcite with phosphoric acid as a function of temperature and technique

Selective chemical separation techniques for isotopic analysis of coexisting calcite, dolomite, siderite and magnesite have been investigated in this study. Comparison of reaction rates of all pure carbonate minerals with 100% phosphoric acid at 25° and 50°C demonstrated that a uniformly fine grain size (<200 mesh) is required. Tests with fine-grained (<200 mesh) pure calcite, dolomite, siderite and magnesite reacted at 25°C (calcite) and 50°C (others) show that δ18O of evolved CO2 increases during the course of reaction. This is probably due to a slight kinetic isotope effect associated with rapid dissolution of sub-micrometer carbonate crystallites adhered to the outer surfaces of the very fine-grained crystals. Isotopic analysis of carbonates based on CO2 from incomplete acid reactions is accurate only after minimum reaction times. Conversely, much longer reaction times than the minima are not required.

Experiments conducted on variable mixtures of calcite-dolomite, dolomite-siderite and calcite-dolomite-siderite demonstrated the magnitudes of cross-contamination on the inferred isotopic compositions of the individual minerals. The isotopic compositions and magnitudes of their uncertainty can be evaluated for individual carbonates selectively extracted from natural mixtures by variable time-temperature protocol.

Stable isotope analysis of multiple carbonate samples using selective acid extraction

Hydrate Ridge is part of the accretionary complex at the Cascadia margin and
is an area of widespread carbonate precipitation induced by the expulsion of
methane-rich fluids. All carbonates on Hydrate Ridge are related to the methanecarbon
pool either through anaerobic methanotrophy or through methanogenesis.
Several petrographically distinct lithologies occur in boulder fields or in massive
autochtonous chemoherm complexes which include methane-associated
diagenetic mudstones and venting-induced breccias. The mudstones result from
methane diagenesis in different sediment horizons and geochemical environments
related to very slow methane venting. Cemented bioturbation casts occur
as fragments, complex framework or as clasts together with bivalve shells as part
of intraformational breccias, which are restricted to chemoherm complexes.
Here, fluids ascend from the sub-seafloor and support aragonite-dominated carbonate
precipitation near or at the sediment surface. Voids within mudclast breccias
are either aragonite-rich indicating a formation near the surface at vent sites
or are cemented by dolomite, which indicates formation in deeper parts of the
sediment column. Brecciation is caused by tectonic or slump processes. In addition,
we recognized a direct relationship between gas hydrates and sediment fracturing
as well as the oxygen isotope composition of carbonate lithologies. Such
gas hydrate-associated carbonates either show layered megapores and veins as
relics of the original gas hydrate fabric or consist of aragonite-cemented intraclast
breccias formed by growing and decomposing gas hydrate near the sediment
surface. Both rock fabrics and the enrichment of 180 in high Mg-calcite
demonstrate carbonate-forming mechanisms of gas hydrate.

Gas Hydrate‐Associated Carbonates and Methane‐Venting at Hydrate Ridge: Classification, Distribution, and Origin of Authigenic Lithologies

Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism.

J Rosenbaum

Papers

An isotopic study of siderites, dolomites and ankerites at high temperatures☆