Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 +/- 4 degrees C in a near-surface aqueous environment

Experimental formation of carbonates from perchlorate and sulphate brines: Implications for Jezero crater, Mars

Extensive carbonate precipitation has occurred on Mars. To gain insight into the carbonation mechanisms and formation processes under ancient Martian aqueous conditions, we examine the precipitation of carbonates resulting from atmospheric carbon fixation, focusing on interactions between various brines and silicate and perchlorate solutions in alkaline environments. The micro-scale morphology and composition of the resulting precipitates are analysed using ESEM micrographs, EDX chemical compositional analysis, X-ray diffraction, and micro-Raman spectroscopy. Our findings indicate a significant atmospheric carbonation process involving chlorate and sulphate brines reacting with alkaline perchlorate solutions, leading to the precipitation of calcium carbonate polymorphs, including vaterite, aragonite, and calcite, as well as other carbonates like siderite (iron carbonate) and zaratite (nickel carbonate). Some precipitates exhibit biomorphic structures (such as globular spherical aggregates, fine branched tubes, and flower-like morphologies) that should not be mistaken for fossils. These experiments demonstrate that various precipitates can form simultaneously in a single reaction vessel while being exposed to different micro-scale pH conditions. We propose that systematic laboratory studies of such precipitate reactions should be conducted in preparation for the analysis of the Mars Sample Return collection on Earth, aiding in the interpretation of carbonate presence in natural brine-rock carbonation processes under Martian conditions while also helping to distinguish potential biosignatures from purely geochemical processes.

Highly enriched carbon and oxygen isotopes in carbonate-derived CO2 at Gale crater, Mars

Carbonate minerals are of particular interest in paleoenvironmental research as they are an integral part of the carbon and water cycles, both of which are relevant to habitability. Given that these cycles are less constrained on Mars than they are on Earth, the identification of carbonates has been a point of emphasis for rover missions. Here, we present carbon (δ13C) and oxygen (δ18O) isotope data from four carbonates encountered by the Curiosity rover within the Gale crater. The carbon isotope values range from 72 ± 2‰ to 110 ± 3‰ Vienna Pee Dee Belemnite while the oxygen isotope values span from 59 ± 4‰ to 91 ± 4‰ Vienna Standard Mean Ocean Water (1 SE uncertainties). Notably, these values are isotopically heavy (13C- and 18O-enriched) relative to nearly every other Martian material. The extreme isotopic difference between the carbonates and other carbon- and oxygen-rich reservoirs on Mars cannot be reconciled by standard equilibrium carbonate-CO2 fractionation, thus requiring an alternative process during or prior to carbonate formation. This paper explores two processes capable of contributing to the isotopic enrichments: 1) evaporative-driven Rayleigh distillation and 2) kinetic isotope effects related to cryogenic precipitation. In isolation, each process cannot reproduce the observed carbonate isotope values; however, a combination of these processes represents the most likely source for the extreme isotopic enrichments.

Paleomagnetic evidence for a long-lived, potentially reversing martian dynamo at ~3.9 Ga

The 4.1-billion-year-old meteorite Allan Hills 84001 (ALH 84001) may preserve a magnetic record of the extinct martian dynamo. However, previous paleomagnetic studies have reported heterogeneous, nonunidirectional magnetization in the meteorite at submillimeter scales, calling into question whether it records a dynamo field. We use the quantum diamond microscope to analyze igneous Fe-sulfides in ALH 84001 that may carry remanence as old as 4.1 billion years (Ga). We find that individual, 100-μm-scale ferromagnetic mineral assemblages are strongly magnetized in two nearly antipodal directions. This suggests that the meteorite recorded strong fields following impact heating at 4.1 to 3.95 Ga, after which at least one further impact heterogeneously remagnetized the meteorite in a nearly antipodal local field. These observations are most simply explained by a reversing martian dynamo that was active until 3.9 Ga, thereby implying a late cessation for the martian dynamo and potentially documenting reversing behavior in a nonterrestrial planetary dynamo.

Oxygen Isotope Exchange between Carbon Dioxide and Iron Oxides on Mars' Surface

An investigation of the fundamental processes leading to the incorporation of ¹⁸O isotopes in carbon dioxide and in iron oxides is critical to understanding the atmospheric evolution and geochemistry of Mars. Whereas signatures of ¹⁸O have been observed by the Phoenix Lander and the sample analysis at Mars for carbon dioxide, the underlying isotopic exchange pathways with minerals of the crust of Mars are still elusive. Here, we reveal that reactions of gaseous ¹⁸O-carbon dioxide over goethite (FeO(OH)) and hematite (Fe₂O₃) lead to an ¹⁸O transfer from the atmosphere that enriches the ¹⁸O content of the iron oxides in the absence of water and light. This proof-of-concept study shows that isotopic enrichment processes on Mars not only are limited to the atmosphere but also proceed via chemical interaction with dry iron oxides. These processes are decisive to comprehending the ¹⁸O cycle between the atmosphere and the surface on the planetary scale.

Organic synthesis associated with serpentinization and carbonation on early Mars

Water-rock interactions are relevant to planetary habitability, influencing mineralogical diversity and the production of organic molecules. We examine carbonates and silicates in the martian meteorite Allan Hills 84001 (ALH 84001), using colocated nanoscale analyses, to characterize the nature of water-rock reactions on early Mars. We find complex refractory organic material associated with mineral assemblages that formed by mineral carbonation and serpentinization reactions. The organic molecules are colocated with nanophase magnetite; both formed in situ during water-rock interactions on Mars. Two potentially distinct mechanisms of abiotic organic synthesis operated on early Mars during the late Noachian period (3.9 to 4.1 billion years ago).

Characteristics, Origins, and Biosignature Preservation Potential of Carbonate-Bearing Rocks Within and Outside of Jezero Crater

Carbonate minerals have been detected in Jezero crater, an ancient lake basin that is the landing site of the Mars 2020 Perseverance rover, and within the regional olivine-bearing (ROB) unit in the Nili Fossae region surrounding this crater. It has been suggested that some carbonates in the margin fractured unit, a rock unit within Jezero crater, formed in a fluviolacustrine environment, which would be conducive to preservation of biosignatures from paleolake-inhabiting lifeforms. Here, we show that carbonate-bearing rocks within and outside of Jezero crater have the same range of visible-to-near-infrared carbonate absorption strengths, carbonate absorption band positions, thermal inertias, and morphologies. Thicknesses of exposed carbonate-bearing rock cross-sections in Jezero crater are ∼75-90 m thicker than typical ROB unit cross-sections in the Nili Fossae region, but have similar thicknesses to ROB unit exposures in Libya Montes. These similarities in carbonate properties within and outside of Jezero crater is consistent with a shared origin for all of the carbonates in the Nili Fossae region. Carbonate absorption minima positions indicate that both Mg- and more Fe-rich carbonates are present in the Nili Fossae region, consistent with the expected products of olivine carbonation. These estimated carbonate chemistries are similar to those in martian meteorites and the Comanche carbonates investigated by the Spirit rover in Columbia Hills. Our results indicate that hydrothermal alteration is the most likely formation mechanism for non-deltaic carbonates within and outside of Jezero crater.

Carbonate clumped isotope analysis (Δ47 ) of 21 carbonate standards determined via gas-source isotope-ratio mass spectrometry on four instrumental configurations using carbonate-based standardization and multiyear data sets

RATIONALE

Clumped isotope geochemistry examines the pairing or clumping of heavy isotopes in molecules and provides information about the thermodynamic and kinetic controls on their formation. The first clumped isotope measurements of carbonate minerals were first published 15 years ago, and since then, interlaboratory offsets have been observed, and laboratory and community practices for measurement, data analysis, and instrumentation have evolved. Here we briefly review historical and recent developments for measurements, share Tripati Lab practices for four different instrument configurations, test a recently published proposal for carbonate-based standardization on multiple instruments using multi-year data sets, and report values for 21 different carbonate standards that allow for recalculations of previously published data sets.

METHODS

We examine data from 4628 standard measurements on Thermo MAT 253 and Nu Perspective IS mass spectrometers, using a common acid bath (90°C) and small-sample (70°C) individual reaction vessels. Each configuration was investigated by treating some standards as anchors (working standards) and the remainder as unknowns (consistency standards).

RESULTS

We show that different acid digestion systems and mass spectrometer models yield indistinguishable results when instrument drift is well characterized. For linearity correction, mixed gas-and-carbonate standardization or carbonate-only standardization yields similar results. No difference is observed in the use of three or eight working standards for the construction of transfer functions.

CONCLUSIONS

We show that all configurations yield similar results if instrument drift is robustly characterized and validate a recent proposal for carbonate-based standardization using large multiyear data sets. Δ₄₇ values are reported for 21 carbonate standards on both the absolute reference frame (ARF; also refered to as the Carbon Dioxide Equilibrated Scale or CDES) and the new InterCarb-Carbon Dioxide Equilibrium Scale (I-CDES) reference frame, facilitating intercomparison of data from a diversity of labs and instrument configurations and restandardization of a broad range of sample sets between 2006, when the first carbonate measurements were published, and the present.

Uninhabitable and Potentially Habitable Environments on Mars: Evidence from Meteorite ALH 84001

The martian meteorite ALH 84001 formed before ∼4.0 Ga, so it could have preserved information about habitability on early Mars and habitability since then. ALH 84001 is particularly important as it contains carbonate (and other) minerals that were deposited by liquid water, raising the chance that they may have formed in a habitable environment. Despite vigorous efforts from the scientific community, there is no accepted evidence that ALH 84001 contains traces or markers of ancient martian life-all the purported signs have been shown to be incorrect or ambiguous. However, the meteorite provides evidence for three distinct episodes of potentially habitable environments on early Mars. First is evidence that the meteorite's precursors interacted with clay-rich material, formed approximately at 4.2 Ga. Second is that igneous olivine crystals in ALH 84001 were partially dissolved and removed, presumably by liquid water. Third is, of course, the deposition of the carbonate globules, which occurred at ∼15-25°C and involved near-neutral to alkaline waters. The environments of olivine dissolution and carbonate deposition are not known precisely; hydrothermal and soil environments are current possibilities. By analogies with similar alteration minerals and sequences in the nakhlite martian meteorites and volcanic rocks from Spitzbergen (Norway), a hydrothermal environment is favored. As with the nakhlite alterations, those in ALH 84001 likely formed in a hydrothermal system related to a meteoroid impact event. Following deposition of the carbonates (at 3.95 Ga), ALH 84001 preserves no evidence of habitable environments, that is, interaction with water. The meteorite contains several materials (formed by impact shock at ∼3.9 Ga) that should have reacted readily with water to form hydrous silicates, but there is no evidence any formed.

Formation of Magnesium Carbonates on Earth and Implications for Mars

Magnesium carbonates have been identified within the landing site of the Perseverance rover mission. This study reviews terrestrial analog environments and textural, mineral assemblage, isotopic, and elemental analyses that have been applied to establish formation conditions of magnesium carbonates. Magnesium carbonates form in five distinct settings: ultramafic rock-hosted veins, the matrix of carbonated peridotite, nodules in soil, alkaline lake, and playa deposits, and as diagenetic replacements within lime-and dolostones. Dominant textures include fine-grained or microcrystalline veins, nodules, and crusts. Microbial influences on formation are recorded in thrombolites, stromatolites, crinkly, and pustular laminites, spheroids, and filamentous microstructures. Mineral assemblages, fluid inclusions, and carbon, oxygen, magnesium, and clumped isotopes of carbon and oxygen have been used to determine the sources of carbon, magnesium, and fluid for magnesium carbonates as well as their temperatures of formation. Isotopic signatures in ultramafic rock-hosted magnesium carbonates reveal that they form by either low-temperature meteoric water infiltration and alteration, hydrothermal alteration, or metamorphic processes. Isotopic compositions of lacustrine magnesium carbonate record precipitation from lake water, evaporation processes, and ambient formation temperatures. Assessment of these features with similar analytical techniques applied to returned Martian samples can establish whether carbonates on ancient Mars were formed at high or low temperature conditions in the surface or subsurface through abiotic or biotic processes. The timing of carbonate formation processes could be constrained by 147Sm-143Nd isochron, U-Pb concordia, 207Pb-206Pb isochron radiometric dating as well as 3He, 21Ne, 22Ne, or 36Ar surface exposure dating of returned Martian magnesium carbonate samples.

Photogeologic Map of the Perseverance Rover Field Site in Jezero Crater Constructed by the Mars 2020 Science Team

The Mars 2020 Perseverance rover landing site is located within Jezero crater, a ∼ 50 km diameter impact crater interpreted to be a Noachian-aged lake basin inside the western edge of the Isidis impact structure. Jezero hosts remnants of a fluvial delta, inlet and outlet valleys, and infill deposits containing diverse carbonate, mafic, and hydrated minerals. Prior to the launch of the Mars 2020 mission, members of the Science Team collaborated to produce a photogeologic map of the Perseverance landing site in Jezero crater. Mapping was performed at a 1:5000 digital map scale using a 25 cm/pixel High Resolution Imaging Science Experiment (HiRISE) orthoimage mosaic base map and a 1 m/pixel HiRISE stereo digital terrain model. Mapped bedrock and surficial units were distinguished by differences in relative brightness, tone, topography, surface texture, and apparent roughness. Mapped bedrock units are generally consistent with those identified in previously published mapping efforts, but this study's map includes the distribution of surficial deposits and sub-units of the Jezero delta at a higher level of detail than previous studies. This study considers four possible unit correlations to explain the relative age relationships of major units within the map area. Unit correlations include previously published interpretations as well as those that consider more complex interfingering relationships and alternative relative age relationships. The photogeologic map presented here is the foundation for scientific hypothesis development and strategic planning for Perseverance's exploration of Jezero crater.

In-situ preservation of nitrogen-bearing organics in Noachian Martian carbonates

Understanding the origin of organic material on Mars is a major issue in modern planetary science. Recent robotic exploration of Martian sedimentary rocks and laboratory analyses of Martian meteorites have both reported plausible indigenous organic components. However, little is known about their origin, evolution, and preservation. Here we report that 4-billion-year-old (Ga) carbonates in Martian meteorite, Allan Hills 84001, preserve indigenous nitrogen(N)-bearing organics by developing a new technique for high-spatial resolution in situ N-chemical speciation. The organic materials were synthesized locally and/or delivered meteoritically on Mars during Noachian age. The carbonates, alteration minerals from the Martian near-surface aqueous fluid, trapped and kept the organic materials intact over long geological times. This presence of N-bearing compounds requires abiotic or possibly biotic N-fixation and ammonia storage, suggesting that early Mars had a less oxidizing environment than today.

Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy

The high precision measurement of doubly substituted ("clumped") isotopologues in CO₂ is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the ¹²C¹⁶O₂ + ¹³C¹⁶O¹⁸O ⇆ ¹³C¹⁶O₂ + ¹²C¹⁶O¹⁸O exchange reaction. CO₂ samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 μmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ_16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.

Follow the Oxygen: Comparative Histories of Planetary Oxygenation and Opportunities for Aerobic Life

Aerobic respiration-the reduction of molecular oxygen (O₂) coupled to the oxidation of reduced compounds such as organic carbon, ferrous iron, reduced sulfur compounds, or molecular hydrogen while conserving energy to drive cellular processes-is the most widespread and bioenergetically favorable metabolism on Earth today. Aerobic respiration is essential for the development of complex multicellular life; thus the presence of abundant O₂ is an important metric for planetary habitability. O₂ on Earth is supplied by oxygenic photosynthesis, but it is becoming more widely understood that abiotic processes may supply meaningful amounts of O₂ on other worlds. The modern atmosphere and rock record of Mars suggest a history of relatively high O₂ as a result of photochemical processes, potentially overlapping with the range of O₂ concentrations used by biology. Europa may have accumulated high O₂ concentrations in its subsurface ocean due to the radiolysis of water ice at its surface. Recent modeling efforts suggest that coexisting water and O₂ may be common on exoplanets, with confirmation from measurements of exoplanet atmospheres potentially coming soon. In all these cases, O₂ accumulates through abiotic processes-independent of water-oxidizing photosynthesis. We hypothesize that abiogenic O₂ may enhance the habitability of some planetary environments, allowing highly energetic aerobic respiration and potentially even the development of complex multicellular life which depends on it, without the need to first evolve oxygenic photosynthesis. This hypothesis is testable with further exploration and life-detection efforts on O₂-rich worlds such as Mars and Europa, and comparison to O₂-poor worlds such as Enceladus. This hypothesis further suggests a new dimension to planetary habitability: "Follow the Oxygen," in which environments with opportunities for energy-rich metabolisms such as aerobic respiration are preferentially targeted for investigation and life detection.

Branching geometry of valley networks on Mars and Earth and its implications for early Martian climate

Mars' surface bears the imprint of valley networks formed billions of years ago. Whether these networks were formed by groundwater sapping, ice melt, or fluvial runoff has been debated for decades. These different scenarios have profoundly different implications for Mars' climatic history and thus for its habitability in the distant past. Recent studies on Earth revealed that valley networks in arid landscapes with more surface runoff branch at narrower angles, while in humid environments with more groundwater flow, branching angles are much wider. We find that valley networks on Mars generally tend to branch at narrow angles similar to those found in arid landscapes on Earth. This result supports the inference that Mars once had an active hydrologic cycle and that Mars' valley networks were formed primarily by overland flow erosion, with groundwater seepage playing only a minor role.

A Field Guide to Finding Fossils on Mars

The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface. This evidence has motivated National Aeronautics and Space Administration and European Space Agency to prioritize the search for any remains or traces of organisms from early Mars in forthcoming missions. Informed by (1) stratigraphic, mineralogical and geochemical data collected by previous and current missions, (2) Earth's fossil record, and (3) experimental studies of organic decay and preservation, we here consider whether, how, and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars. We conclude that Noachian-Hesperian Fe-bearing clay-rich fluvio-lacustrine siliciclastic deposits, especially where enriched in silica, currently represent the most promising and best understood astropaleontological targets. Siliceous sinters would also be an excellent target, but their presence on Mars awaits confirmation. More work is needed to improve our understanding of fossil preservation in the context of other environments specific to Mars, particularly within evaporative salts and pore/fracture-filling subsurface minerals.

Reducing contamination parameters for clumped isotope analysis: The effect of lowering Porapak™ Q trap temperature to below -50°C

RATIONALE

Carbonate clumped isotope thermometry examines the thermodynamic preference of ¹³ C-¹⁸ O bonds to form within the carbonate crystal lattice. The ¹³ C¹⁸ O¹⁶ O isotopologue in analyte CO₂ has a natural abundance of 44.4 ppm necessitating stringent purification procedures to remove contaminant molecules that may produce significant isobaric effects within range of the mass 47 isotopologue. Strict purifications of analyte CO₂ are thus required as well as reliable contamination indicators.

METHODS

CO₂ purification was carried out by vacuum cryogenic purification through a static trap packed with Porapak™ Q (PPQ). The correlation between mass excesses on m/z 47, 48 and 49 in CO₂ produced by acid digestion of 12 natural samples was measured by isotope ratio mass spectrometry (IRMS). CO₂ from two contaminated carbonate samples was then purified at PPQ trap temperatures between -25 and -65°C and measured by IRMS to determine changes in mass excesses on m/z 47, 48 and 49. Finally carbonate standards, Carrara marble (CM) and ETH3, were purified at PPQ trap temperatures of -35 and -60°C to identify isotopic fractionation associated with lowering trap temperature.

RESULTS

The correlation between mass excesses on m/z 47, 48 and 49 is determined to be sample dependent. Lowering the PPQ trap temperature to -60°C has a 78% success rate in decreasing Δ_48offset , a measure of sample contamination, to within an acceptable range (<1.5 ‰). Lowering the PPQ temperature in purification of CM and ETH3 is associated with decreases in the δ¹³ C and δ¹⁸ O values as a result of isotopic fractionation_. We demonstrate that we can correct for fractionation at a trap temperature of -60°C.

CONCLUSIONS

Lowering the temperature of the Porapak Q trap to -60°C results in improved sample cleaning. It is possible to correct for fractionation in δ¹³ C and δ¹⁸ O values at lower PPQ trap temperatures using identically prepared standards. This result has important connotations for laboratories using similar sample preparation methods.

Clumped Isotope Paleothermometry: Principles, Applications, and Challenges

Clumped isotopes geochemistry measures the thermodynamic preference of two heavy, rare, isotopes to bind with each other. This preference is temperature dependent, and is more pronounced at low temperatures. Carbonate clumped isotope values are independent of the carbonate δ13C and δ18O, making them independent of the carbon or oxygen composition of the solution from which the carbonate precipitated. At equilibrium, it is therefore a direct proxy for the temperature in which the carbonate mineral formed. In most cases, carbonate clumped isotopes record the temperature of carbonate formation, irrespective of the mineral form (calcite, aragonite, or bioapatite) or the organism making it. The carbonate formation temperatures obtained from carbonate clumped isotope analysis can be used in conjunction with the δ18O of the same carbonate, to constrain the oxygen isotope composition of the water from which the carbonate has precipitated. There are, however, cases of deviation from thermodynamic equilibrium, where both clumped and oxygen isotopes are offset from the expected values. Such carbonates must be characterized and calibrated separately. For deep-time applications, special care must be paid to the preservation of the original signal, in particular with respect to diagenetic alteration associated with atomic scale diffusion that may be undetectable by common tests for diagenesis.

Tracing the fate of carbon and the atmospheric evolution of Mars

The climate of Mars likely evolved from a warmer, wetter early state to the cold, arid current state. However, no solutions for this evolution have previously been found to satisfy the observed geological features and isotopic measurements of the atmosphere. Here we show that a family of solutions exist, invoking no missing reservoirs or loss processes. Escape of carbon via CO photodissociation and sputtering enriches heavy carbon (¹³C) in the Martian atmosphere, partially compensated by moderate carbonate precipitation. The current atmospheric ¹³C/¹²C and rock and soil carbonate measurements indicate an early atmosphere with a surface pressure <1 bar. Only scenarios with large amounts of carbonate formation in open lakes permit higher values up to 1.8 bar. The evolutionary scenarios are fully testable with data from the MAVEN mission and further studies of the isotopic composition of carbonate in the Martian rock record through time.

Mars likely evolved from a warmer, wetter early state to the cold, arid current climate, but this evolution is not reflected in recent observations and measurements. Here, the authors derive quantitative constraints on the atmospheric pressure through time, identifying a mechanism that explains the carbon data.

Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere

Carbonate minerals provide critical information for defining atmosphere-hydrosphere interactions. Carbonate minerals in the Martian meteorite ALH 84001 have been dated to ∼ 3.9 Ga, and both C and O-triple isotopes can be used to decipher the planet's climate history. Here we report Δ(17)O, δ(18)O, and δ(13)C data of ALH 84001 of at least two varieties of carbonates, using a stepped acid dissolution technique paired with ion microprobe analyses to specifically target carbonates from distinct formation events and constrain the Martian atmosphere-hydrosphere-geosphere interactions and surficial aqueous alterations. These results indicate the presence of a Ca-rich carbonate phase enriched in (18)O that formed sometime after the primary aqueous event at 3.9 Ga. The phases showed excess (17)O (0.7‰) that captured the atmosphere-regolith chemical reservoir transfer, as well as CO2, O3, and H2O isotopic interactions at the time of formation of each specific carbonate. The carbon isotopes preserved in the Ca-rich carbonate phase indicate that the Noachian atmosphere of Mars was substantially depleted in (13)C compared with the modern atmosphere.

Clumped isotope measurements of small carbonate samples using a high-efficiency dual-reservoir technique

RATIONALE

The measurement of multiply substituted isotopologues of CO2 derived from carbonate has allowed the reconstruction of paleotemperatures from a single phase (CaCO3 ), circumventing uncertainty inherent in other isotopic paleothermometers. Traditional analytical techniques require relatively large amounts of carbonate (3-8 mg per replicate), which limits the applicability of the clumped isotope proxy to certain geological materials such as marine microfossils, commonly used for paleoclimate reconstructions.

METHODS

Clumped isotope ratio measurements of small samples were made on a new, high-efficiency, dual-reservoir sample-preparation inlet system attached to a Thermo-Finnigan MAT 253 mass spectrometer. Sample gas produced on the inlet is introduced from a 10 mL reservoir directly into the source via a capillary. Reference gas fills an identical 10 mL reservoir installed between the reference bellows and the capillary. The gas pressures in the two reservoirs are initially balanced, and are allowed to decrease together over the run.

RESULTS

Carbonate samples from 1 mg to 2.6 mg produced Δ47 values equivalent to those from the traditional two-bellows method with identical single-sample precision (1 SE = 0.005-0.015‰) and external standard error (SE = 0.006-0.015‰, n = 4-6). The size of sample needed to achieve good precision is controlled by the sensitivity of the mass spectrometer and the size of the fixed reservoirs and adjacent U-trap installed on our inlet.

CONCLUSIONS

The high-precision clumped isotope measurements of small aliquots of carbonate obtained in this method allows for the application of this proxy to a wider range of geological sample materials, such as marine microfossils, that until now have been nearly impossible to analyze given sample size limitation.

Long-term performance of the Kiel carbonate device with a new correction scheme for clumped isotope measurements

RATIONALE

Carbonate clumped isotope thermometry is a powerful new technique increasingly used in many fields in earth science. Recently, it has been shown that clumped isotope measurements can be performed with a Kiel carbonate preparation device and micro-volume analyses, allowing measurements of small (1.5-2 mg) carbonate samples. However, common data correction schemes rely on measurements of gases prepared offline, potentially leading to unrecognized biases in the results.

METHODS

We propose a new correction scheme for the Kiel device method including: (1) A pressure-sensitive baseline correction (PBL) of the raw beam signals; (2) Transfer of data to the absolute reference frame; (3) Correction for acid fractionation; (4) Correction for average standard offsets; (5) When necessary, correction for Δ47 scale compression based on offsets among standards with different ordering state. The long-term performance of the new scheme was tested with a large set of standard measurements (N = 432) obtained over the course of 15 months.

RESULTS

The PBL correction reliably removes composition-dependent artifacts, which are commonly corrected for with gas measurements, and offsets observed in micro-volume measurements when ion beams are imbalanced. We show that the shape of the PBL can vary strongly and needs to be properly characterized. Combined PBL and standard correction resulted in long-term stability with standard deviations in Δ47 of 0.012-0.016 ‰ for the five standards over the whole period, close to the average error of 0.011 ‰ observed for individual measurements consisting of 10 replicate analyses.

CONCLUSIONS

Our correction scheme eliminates the need for routine gas measurements, allowing for equal treatment of samples and standards with the Kiel device setup. While the PBL and standard data obtained over 15 months reveal variable mass spectrometer behavior, they provide a robust means of correction, yielding reproducible results from small carbonate samples in the long term.

A modified procedure for gas-source isotope ratio mass spectrometry: the long-integration dual-inlet (LIDI) methodology and implications for clumped isotope measurements

RATIONALE

High-precision stable isotope measurements in gas-source isotope ratio mass spectrometry are generally carried out by repeated comparison of the composition of an unknown sample with that of a working gas (WG) through a dual-inlet (DI). Due to the established DI protocols, however, most of the sample gas is wasted rather than measured, which is a major problem when sample size is limited. Here we propose a new methodology allowing the measurement of a much larger portion of the available sample.

METHODS

We tested a new measurement protocol, the long-integration dual-inlet (LIDI) method, which consists of a single measurement of the sample for 200 to 600 seconds followed by a single measurement of the WG. The isotope ratios of the sample are calculated by comparison of the beam ratios of the WG and sample at equivalent intensities of the major ion beam.

RESULTS

Three isotopically very different CO2 samples were analyzed. The LIDI measurements of large samples (50 to 100 µmol of CO2) measured at quasi-constant beam sizes, and of small samples (1.5 to 2 µmol of CO2) measured in micro-volume mode, generated results that are indistinguishable from the standard DI measurements for carbon, oxygen and clumped isotope compositions. The external precision of Δ47 using the LIDI protocol (~±0.007‰) is similar to that of the state of the art DI measurements.

CONCLUSIONS

For traditional and clumped isotope measurements of CO2, the LIDI protocol allows the measurement of a much larger portion of the sample gas rather than only ~20% of it. In addition, the sample can be measured at higher signal intensity and for longer time, allowing the measurement of smaller samples while preserving precision. We suggest that other gases commonly used for stable isotope measurements with gas-source mass spectrometry would also benefit from this new protocol.

High-precision determination of ¹³C-¹⁸O bonds in CO₂ using multicollector peak hopping

RATIONALE

Rapid development of the CO2 'clumped-isotope' temperature proxy (Δ47 ) has involved both resource- and labor-intensive analytical methods. We report strategies for measuring Δ47 on a conventional mass spectrometer, with the usual CO2 set of three Faraday collectors while reducing the time devoted to standardization with temperature-equilibrated CO2.

METHODS

We measured Δ47 in CO2 on a mass spectrometer using only three Faraday cups by 'multicollector peak hopping', directing (46)CO2(+) and (47)CO2(+) ion beams into the Faraday cups normally used for m/z 44 and 45. Δ47 was calculated from the previously measured δ(13) C and δ(18) O values and we then measured (47)CO2(+)/(46)CO2(+). We invoke a protocol for correcting ion-beam intensities for secondary electrons to address common problems with compositional non-linearity and both long- and short-term measurement stability.

RESULTS

Our method eliminates the need for six Faraday cups for measuring Δ47, a possible advantage for many laboratories. In addition, by using a correction based on removing the effects of secondary electrons, we remove the Δ47 dependence on δ(47)CO2. As a result, our methods are robust against mass spectrometer operating conditions.

CONCLUSIONS

Δ47 can be precisely measured on mass spectrometers currently employed for measuring δ(13) C and δ(18) O values without significant resource investment. Accounting for ion scatter during measurements of CO2 results in improved external precision and long-term stability, while significantly increasing the machine time available for sample analysis.

Seeding life on the moons of the outer planets via lithopanspermia

Material from the surface of a planet can be ejected into space by a large impact and could carry primitive life-forms with it. We performed n-body simulations of such ejecta to determine where in the Solar System rock from Earth and Mars may end up. We found that, in addition to frequent transfer of material among the terrestrial planets, transfer of material from Earth and Mars to the moons of Jupiter and Saturn is also possible, but rare. We expect that such transfers were most likely to occur during the Late Heavy Bombardment or during the ensuing 1-2 billion years. At this time, the icy moons were warmer and likely had little or no ice shell to prevent meteorites from reaching their liquid interiors. We also note significant rates of re-impact in the first million years after ejection. This could re-seed life on a planet after partial or complete sterilization by a large impact, which would aid the survival of early life during the Late Heavy Bombardment.

Pressure baseline correction and high-precision CO2 clumped-isotope (∆47) measurements in bellows and micro-volume modes

RATIONALE

CO(2) 'clumped-isotope' measurements (tracking enrichment of (16)O(13)C(18)O, reported as ∆(47) values, on CO(2) derived from carbonate minerals or the atmosphere) are becoming central to a wide range of geochemical investigations. We present a novel approach to address problems with instrument stability, external precision, and the analysis of small samples that have hampered the advancement of Δ(47) measurements.

METHODS

We measured Δ(47) values on CO(2) gases introduced via dual inlet to an isotope ratio mass spectrometer. We developed a method for determining the 'pressure baseline' and integrating a correction to ion beam intensity measurements during analysis. We then tested this approach for both bellows and micro-volume modes of sample introduction. Heated gas and equilibrated gas lines (Δ(47) vs. δ(47)) established the effectiveness of this correction.

RESULTS

We have determined that drift in instrument calibration that compromises Δ(47) measurements results from a shift in the baseline signal on sensitive collectors (m/z 47, 48, and 49) that occurs when gas is admitted to the ion source. Applying a 'pressure baseline' (PBL) correction significantly stabilizes ∆(47) measurements and reduces the dependence of ∆(47) values on δ(47) values by up to an order of magnitude.

CONCLUSIONS

PBL-corrected heated gas and equilibrated gas calibrations in bellows and micro-volume modes are nearly identical and stable through time. Introduction of the PBL correction, a revision to the absolute reference frame approach to determining Δ(47) values, dramatically improves the external precision of Δ(47) measurements to near instrumental analytical uncertainty (6-8 ppm (1σ) in bellows mode; 10-12 ppm in micro-volume mode).