Search

Special Feature: The Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC).-- 42 pages, 16 figures, 3 tables, supplemental files https://doi.org/10.1525/elementa.2021.000046.-- Data accessibility statement: All data in this manuscript are publicly available from online repositories. Note that most data sets contain raw or preliminary data, while advanced versions will become available in future. The data may be found under the following references: drift track data (Figure 1, Nicolaus et al., doi:10.1594/PANGAEA.937204), observational dates (Figure 4, Nicolaus et al., doi:10.5281/zenodo.5898517), panorama photographs (Figure 5, Nicolaus et al., doi:10.1594/PANGAEA.938534), TLS data (Figure 6, Clemens-Sewall et al., doi:10.18739/A27S7HT3B), ROV radiation data (Figure 7, Nicolaus et al., doi:10.1594/PANGAEA.935688), surface albedo data on ground (Figure 8, Smith et al., broadband data under doi:10.18739/A2KK94D36 and spectral data under doi:10.18739/A2FT8DK8Z) and from the HELiX drone (Figure 8, Calmer et al., doi:10.18739/A2GH9BB0Q), on-ice RS data (Figure 10, Spreen et al., doi:10.5281/zenodo.5725870), surface images from thermal infrared and true color (Figure 11, Thielke et al, doi:10.1594/PANGAEA.934666), drift speed data from Polarstern (Figure 12, Nicolaus et al., doi:10.1594/PANGAEA.937204), deformation data from SAR (Figure 13, von Albedyll et al, doi:10.5281/zenodo.5195366), sea ice thickness and snow depth distribution (Figure 14, Hendricks et al., doi:10.5281/zenodo.5155244), sea ice physical properties (Figure 15, in Tables S2 and S3) with a sea ice core overview (Granskog et al., doi:10.5281/zenodo.4719905), snow pack properties (Figure 16, Macfarlane et al., doi: 10.1594/PANGAEA.935934), and ship radar video sequence (Jäkel et al., doi:10.5446/52953), Year-round observations of the physical snow and ice properties and processes that govern the ice pack evolution and its interaction with the atmosphere and the ocean were conducted during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition of the research vessel Polarstern in the Arctic Ocean from October 2019 to September 2020. This work was embedded into the interdisciplinary design of the 5 MOSAiC teams, studying the atmosphere, the sea ice, the ocean, the ecosystem, and biogeochemical processes. The overall aim of the snow and sea ice observations during MOSAiC was to characterize the physical properties of the snow and ice cover comprehensively in the central Arctic over an entire annual cycle. This objective was achieved by detailed observations of physical properties and of energy and mass balance of snow and ice. By studying snow and sea ice dynamics over nested spatial scales from centimeters to tens of kilometers, the variability across scales can be considered. On-ice observations of in situ and remote sensing properties of the different surface types over all seasons will help to improve numerical process and climate models and to establish and validate novel satellite remote sensing methods; the linkages to accompanying airborne measurements, satellite observations, and results of numerical models are discussed. We found large spatial variabilities of snow metamorphism and thermal regimes impacting sea ice growth. We conclude that the highly variable snow cover needs to be considered in more detail (in observations, remote sensing, and models) to better understand snow-related feedback processes. The ice pack revealed rapid transformations and motions along the drift in all seasons. The number of coupled ice–ocean interface processes observed in detail are expected to guide upcoming research with respect to the changing Arctic sea ice, This work was funded by the following: – the German Federal Ministry of Education and Research (BMBF) through financing the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI) and the Polarstern expedition PS122 under the grant N-2014-H-060_Dethloff, – the AWI through its projects: AWI_ROV, AWI_ICE, AWI_SNOW, AWI_ECO. The AWI buoy program and ROV work were funded by the Helmholtz strategic investment Frontiers in Arctic Marine Monitoring (FRAM), – the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through the Transregional Collaborative Research Centre TRR-172 “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3” (grant 268020496), the International Research Training Group 1904 ArcTrain (grant 221211316), the MOSAiCmicrowaveRS project (grant 420499875), the HELiPOD grant (LA 2907/11-1), and the SCASI (NI 1096/5-1 and KA 2694/7-1) and SnowCast (AR1236/1) projects, – the BMBF through the projects Diatom-ARCTIC (03F0810A), IceSense (BMBF 03F0866A and 03F0866B), MOSAiC3-IceScan (BMBF 03F0916A), NiceLABpro (BMBF 03F0867A), SSIP (01LN1701A), and SIDFExplore (03F0868A), – the German Federal Ministry for Economic Affairs and Energy through the project ArcticSense (BMWi 50EE1917A), – the US National Science Foundation (NSF) through the project PROMIS (OPP-1724467, OPP-1724540, and OPP-1724748), the buoy work (OPP-1723400), the MiSNOW (OPP-1820927), the snow transect work (OPP-1820927), the sea ice coring work (OPP-1735862), the HELiX drone operations (OPP-1805569), surface energy fluxes (OPP-1724551), Climate Active Trace Gases (OPP-1807496), and Reactive Gas Chemistry (OPP-1914781). The last 4 of these were also supported by the NOAA Physical Sciences Laboratory, – the European Union’s Horizon 2020 research and innovation program projects ARICE (grant 730965) for berth fees associated with the participation of the DEARice team and INTAROS (grant 727890) supporting the drone and albedo measurements, – the US Department of Energy Atmospheric Radiation Measurement (ARM) and Atmospheric System Research (ASR) programs (DE-SC0019251, DE-SC0021341), – the National Aeronautics and Space Administration (NASA) project 80NSSC20K0658, – the European Space Agency (ESA) MOSAiC microwave radiometer (EMIRAD2, ELBARA, HUTRAD), (EMIRAD2, ELBARA, HUTRAD), CIMRex (contract 4000125503/18/NL/FF/gp) and GNSS-R (P.O. 5001025474, C.N. 4000128320/19/NL/FF/ab) GNSS-R (contracts P.O. 5001025474 and C.N. 4000128320/19/NL/FF/ab) projects, – the Canadian Space Agency FAST project (grant no. 19FACALB08), – EUMETSAT support for microwave scatterometer measurements, – the Research Council of Norway through the projects HAVOC (grant no. 280292), SIDRiFT (grant no. 287871), and CAATEX (grant no. 280531), – the Fram Centre (Tromsø, Norway), from its flagship program on Arctic Ocean through the PHOTA project, – the UKRI Natural Environment Research Council (NERC) and BMBF, who jointly funded the Changing Arctic Ocean program (project Diatom Arctic, NE/R012849/1 and 03F0810A), – the UK Natural Environment Research Council (project SSAASI-CLIM grant NE/S00257X/1), – the Agencia Estatal de Investigación AEI of Spain (grant no. PCI2019-111844-2, RTI2018-099008-B-C22), – the Swedish Research Council (VR, grant no. 2018-03859), – the Swedish Polar Research Secretariat for berth fees for MOSAiC, – the Swiss Polar Institute project SnowMOSAiC, – the Werner-Petersen-Foundation for the development of a remotely operated floating platform (grant no. FKZ 2019/610).

Calcium isotope data ( δ 44 Ca , μ 42 / 44 Ca , and μ 48 / 44 Ca ) are reported for high-temperature metamorphic rocks and minerals, and compared with density-functional theory (DFT) estimates of equilibrium Ca isotope fractionation factors for plagioclase, garnet, clinopyroxene, orthopyroxene, olivine, and apatite. The data and calculations are used to evaluate equilibrium and kinetic fractionation effects that apply to high-temperature metamorphism, where extended residence at high temperature should promote equilibration, but where centimeter-to-meter scale Ca transport could produce diffusive kinetic effects. At upper-granulite facies conditions (T ≥ ∼900 °C), DFT-predicted equilibrium fractionations between minerals are ≤0.8‰, decreasing to ca. 0.6‰ at 1100 °C. We find much larger δ 44 Ca variations in both whole-rock samples (range of ∼4‰) and individual minerals (range of ∼8‰), and large variations in the Ca isotope fractionation between mineral pairs (e.g. Δ 44 Ca grt-plag from −1.5 to +1.5‰). Deviations from equilibrium tend to be larger in concert with indications of higher temperature, such as increasing whole-rock Mg#, plagioclase anorthite content, orthopyroxene Ca/Mg, and garnet Mg#. These large variations are inferred to be due to intragranular or grain-boundary diffusion during metamorphism, as this is the only mechanism that can produce such large isotopic variations. We can confirm the kinetic origin of the variations using measurements of μ 48 / 44 Ca by MC-ICP-MS to distinguish kinetic from equilibrium fractionation processes using a triple-isotope approach. A new variable ( Δ 48 Ca ′ ) quantifies deviations from the Ca-isotope equilibrium slope on a plot of 48Ca/44Ca vs. 42Ca/44Ca. Available geochronological constraints and numerical modeling indicate that observed kinetic isotope fractionations between adjacent high and low Ca rock layers require effective Ca diffusivities of 10−10 to 10−7 m2/yr, and a ratio of Ca isotope diffusivities of D 44 / D 40 ≈ 0.99. The diffusivities are consistent with Ca transport by volume- and grain-boundary diffusion. The apparent contrast in isotopic diffusivities is large and more consistent with silicate liquids than aqueous fluids. This study confirms that kinetic Ca isotope effects are abundant in nature and can overwrite small equilibrium effects, even at high temperatures and even when other techniques (such as Fe-Mg exchange and Ca-in-Opx thermometry) suggest the establishment of chemical equilibrium. Our results imply that kinetic fractionation effects may complicate the use of δ 44 Ca measurements for geothermometry or as a tracer of carbonate recycling into the mantle.

Reliably dated surficial deposits for reconstructing palaeoclimate are rare in the McMurdo Dry Valleys of Antarctica. While many tephra have been found and dated, none is well characterized. In the Wright Valley, the Hart Ash is poorly dated and described. This paper reports profiles through tephra, the chemical signature of the glass shards and new high-precision multi-crystal laser fusion of 40Ar/39Ar ages. Major and trace element analyses of glass shards indicate the tephra are phonolitic and most probably sourced from Mount Discovery in the Erebus volcanic province. Two chemically distinct and stratigraphically separate tephra layers within the Hart Ash were found in three closely spaced soil profiles. The complex stratigraphy between these profiles could not be delineated without the geochemistry of the tephra. Importantly, our data suggest that only one tephra may be an in situ fall-out deposit, which gave a robust age of 2.97 ± 0.02 Ma. This new age for the Hart Ash tephra, which is 10 cm thick and is preserved at the current surface, provides a maximum age for surface deposits in the lower Wright Valley. This study highlights that well-characterized tephra enhance stratigraphic correlations in the Dry Valleys and improve the accuracy of palaeoenvironmental interpretations.

Pebbles of millimeter sizes are abundant in protoplanetary discs around young stars. Chondrules inside primitive meteorites - formed by melting of dust aggregate pebbles or in impacts between planetesimals - have similar sizes. The role of pebble accretion for terrestrial planet formation is nevertheless unclear. Here we present a model where inwards-drifting pebbles feed the growth of terrestrial planets. The masses and orbits of Venus, Earth, Theia (which later collided with the Earth to form the Moon) and Mars are all consistent with pebble accretion onto protoplanets that formed around Mars' orbit and migrated to their final positions while growing. The isotopic compositions of Earth and Mars are matched qualitatively by accretion of two generations of pebbles, carrying distinct isotopic signatures. Finally, we show that the water and carbon budget of Earth can be delivered by pebbles from the early generation before the gas envelope became hot enough to vaporise volatiles., Comment: Science Advances, in press

Refractory inclusions [Ca-Al-rich Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs)] in primitive meteorites are the oldest Solar System solids. They formed in the hot inner protoplanetary disk and, as such, provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the Sun and its planetary system. Using the short-lived 26Al to 26Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct 26Al-rich (26Al/27Al ratios of ∼5 × 10−5) populations of refractory inclusions characterized by different initial 26Mg/24Mg isotope compositions (μ26Mg*0). Another 26Al-poor CAI records an even larger μ26Mg*0 deficit. This suggests that formation of refractory inclusions was punctuated and recurrent, possibly associated with episodic outbursts from the accreting proto-Sun lasting as short as 26Al enrichments and large nucleosynthetic Mg isotope effects. This suggests that refractory inclusions formed by incomplete thermal processing of presolar dust, thereby inheriting a diluted signature of their isotope systematics. As such, they record snapshots in the progressive sublimation of isotopically anomalous presolar carriers through selective thermal processing of young dust components from the proto-Solar molecular cloud. We infer that 26Al-rich refractory inclusions incorporated 26Al-rich dust which formed 26Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.

The large collection of howardite-eucrite-diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb-Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass-independent 26Mg isotope compositions (26Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb-Pb age of 4484.5 ± 7.9 Myr and has a 26Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb-Pb age of 4546.4 ± 4.7 Myr and a 26Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short-lived, early-formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.

The iron isotope composition of Earth’s mantle requires planetary growth within 5 Ma of solar system formation., Nucleosynthetic isotope variability among solar system objects provides insights into the accretion history of terrestrial planets. We report on the nucleosynthetic Fe isotope composition (μ54Fe) of various meteorites and show that the only material matching the terrestrial composition is CI (Ivuna-type) carbonaceous chondrites, which represent the bulk solar system composition. All other meteorites, including carbonaceous, ordinary, and enstatite chondrites, record excesses in μ54Fe. This observation is inconsistent with protracted growth of Earth by stochastic collisional accretion, which predicts a μ54Fe value reflecting a mixture of the various meteorite parent bodies. Instead, our results suggest a rapid accretion and differentiation of Earth during the ~5–million year disk lifetime, when the volatile-rich CI-like material is accreted to the proto-Sun via the inner disk.

Group IVA iron and siliate-iron meteorites record a large range of cooling rates attributed to an impact-related disruption of a molten and differentiated ca. 1000 km diameter planetary embryo of chondritic composition before re-accretion of mainly the metallic core with minor silicates. To better understand the timing of primary accretion, disruption, re-accretion and cooling of the Group IVA parent body, we have determined Pb-Pb and Al-Mg ages for the Group IVA silicate-iron Steinbach meteorite. A Pb-Pb age based on multiple fractions of late-phase, slowly-cooled orthopyroxene from Steinbach yields an absolute age of 4565.47 ± 0.30 Ma corresponding to a relative age of 1.83 ± 0.34 Myr after formation of calcium-aluminium-rich inclusions (CAIs). This is the oldest U-corrected Pb-Pb absolute age for a differentiated meteorite. We use the deficit Al-Mg dating method on one whole rock sample and two mineral separates to produce a model age of 1.3 − 0.3 + 0.5 Myr after CAI formation corresponding to the depletion age of Al relative to Mg in the source material for Steinbach. Assuming this fractionation event occurred in the pre-impact parent body, this provides a maximum time after CAI formation for the disruption of the original Group IVA parent body. Together, these ages require that the original parent body accreted very early and differentiated prior to the impact-related break up, re-accretion and cooling between 1.3 − 0.3 + 0.5 Myr and 1.83 ± 0.34 Myr after CAI formation. These ages are fully consistent with a growing body of evidence from meteorites and astronomical observation supporting the early and efficient growth of planetary embryos and with numerical models of pebble accretion that predict rapid growth of embryos in the presence of chondrules. This time frame for the efficient formation of planetary embryos by chondrule accretion is inconsistent with a proposed ∼1.5 Myr delay in chondrule formation, a contradiction that is resolved by a non-canonical abundance of 26Al in the inner Solar System during at least the first million years of the protoplanetary disk.

The decay of radioactive 26Al to 26Mg (half-life of 730,000 years) is postulated to have been the main energy source promoting asteroidal melting and differentiation in the nascent solar system. High-resolution chronological information provided by the 26Al-26Mg decay system is, therefore, intrinsically linked to the thermal evolution of early-formed planetesimals. In this paper, we explore the timing and style of asteroidal differentiation by combining high-precision Mg isotope measurements of meteorites with thermal evolution models for planetesimals. In detail, we report Mg isotope data for a suite of olivine-rich [Al/Mg ~ 0] achondritic meteorites, as well as a few chondrites. Main Group, pyroxene and the Zinder pallasites as well as the lodranite all record deficits in the mass-independent component of μ26Mg (μ26Mg*) relative to chondrites and Earth. This isotope signal is expected for the retarded ingrowth of radiogenic 26Mg* in olivine-rich residues produced through partial silicate melting during 26Al decay and consistent with their marginally heavy Mg isotope composition relative to ordinary chondrites, which may reflect the early extraction of isotopically light partial melts from the source rock. We propose that their parent planetesimals started forming within ~250,000 years of solar system formation from a hot (>~500 K) inner protoplanetary disk region characterized by a reduced initial (26Al/27Al)0 abundance (~1-2 × 10-5) relative to the (26Al/27Al)0 value in CAIs of 5.25 × 10-5. This effectively reduced the total heat production and allowed for the preservation of solid residues produced through progressive silicate melting with depth within the planetesimals. These 'non-carbonaceous' planetesimals acquired their mass throughout an extended period (>3 Myr) of continuous accretion, thereby generating onion-shell structures of incompletely differentiated zones, consisting of olivine-rich residues, overlaid by metachondrites and undifferentiated chondritic crusts. In contrast, individual olivine crystals from Eagle Station pallasites record variable μ26Mg* excesses, suggesting that these crystals captured the 26Mg* evolution of a magmatic reservoir controlled by fractional crystallization processes during the lifespan of 26Al. Similar to previous suggestions based on isotopic evidence, we suggest that Eagle Station pallasites formed from precursor material similar in composition to carbonaceous chondrites from a cool outer protoplanetary disk region characterized by (26Al/27Al)0 ≥ 2.7 × 10-5. Protracted planetesimal accretion timescales at large orbital distances, with onset of accretion 0.3-1 Myr post-CAIs, may have resulted in significant radiative heat loss and thus efficient early interior cooling of slowly accreting 'carbonaceous' planetesimals.

We report on the petrology, magnesium isotopes and mass-independent 54Cr/52Cr compositions (μ54Cr) of 42 chondrules from CV (Vigarano and NWA 3118) and CR (NWA 6043, NWA 801 and LAP 02342) chondrites. All sampled chondrules are classified as type IA or type IAB, have low 27Al/24Mg ratios (0.04–0.27) and display little or no evidence for secondary alteration processes. The CV and CR chondrules show variable 25Mg/24Mg and 26Mg/24Mg values corresponding to a range of mass-dependent fractionation of ∼500 ppm (parts per million) per atomic mass unit. This mass-dependent Mg isotope fractionation is interpreted as reflecting Mg isotope heterogeneity of the chondrule precursors and not the result of secondary alteration or volatility-controlled processes during chondrule formation. The CV and CR chondrule populations studied here are characterized by systematic deficits in the mass-independent component of 26Mg (μ26Mg∗) relative to the solar value defined by CI chondrites, which we interpret as reflecting formation from precursor material with a reduced initial abundance of 26Al compared to the canonical 26Al/27Al of ∼5 × 10−5. Model initial 26Al/27Al values of CV and CR chondrules vary from (1.5 ± 4.0) × 10−6 to (2.2 ± 0.4) × 10−5. The CV chondrules display significant μ54Cr variability, defining a range of compositions that is comparable to that observed for inner Solar System primitive and differentiated meteorites. In contrast, CR chondrites are characterized by a narrower range of μ54Cr values restricted to compositions typically observed for bulk carbonaceous chondrites. Collectively, these observations suggest that the CV chondrules formed from precursors that originated in various regions of the protoplanetary disk and were then transported to the accretion region of the CV parent asteroid whereas CR chondrule predominantly formed from precursor with carbonaceous chondrite-like μ54Cr signatures. The observed μ54Cr variability in chondrules from CV and CR chondrites suggest that the matrix and chondrules did not necessarily formed from the same reservoir. The coupled μ26Mg∗ and μ54Cr systematics of CR chondrules establishes that these objects formed from a thermally unprocessed and 26Al-poor source reservoir distinct from most inner Solar System asteroids and planetary bodies, possibly located beyond the orbits of the gas giants. In contrast, a large fraction of the CV chondrules plot on the inner Solar System correlation line, indicating that these objects predominantly formed from thermally-processed, 26Al-bearing precursor material akin to that of inner Solar System solids, asteroids and planets.

Tracking the secular evolution of 142Nd/144Nd anomalies is important towards\udunderstanding the crust-mantle dynamics in the early Earth. Excessive scatter in\udthe published data, however, precludes identifying the fine structure of\ud142Nd/144Nd evolution as the expected variability is on the order of few parts per\udmillion. We report ultra-high precision 142Nd/144Nd data for Eoarchean and\udPalaeoarchean rocks from the Isua Supracrustal Belt (SW Greenland) that show\uda well-resolved 142Nd/144Nd temporal variability suggesting progressive convective\udhomogenisation of the Hadean Isua depleted mantle. This temporally\uddecreasing 142Nd/144Nd signal provides a direct measure of early mantle dynamics,\uddefining a stirring timescale of

The mechanisms and timescales of accretion of 10–1000 km sized planetesimals, the building blocks of planets, are not yet well understood. With planetesimal melting predominantly driven by the decay of the short-lived radionuclide 26Al (26Al→26Mg; t1/2 = 0.73 Ma), its initial abundance determines the permissible timeframe of planetesimal-scale melting and its subsequent cooling history. Currently, precise knowledge about the initial 26Al abundance [(26Al/27Al)0] exists only for the oldest known solids, calcium aluminum-rich inclusions (CAIs) – the so-called canonical value. We have determined the 26Al/27Al of three angrite meteorites, D’Orbigny, Sahara 99555 and NWA 1670, at their time of crystallization, which corresponds to (3.98 ± 0.15)×10−7, (3.64 ± 0.18)×10−7, and (5.92 ± 0.59)×10−7, respectively. Combined with a newly determined absolute U-corrected Pb–Pb age for NWA 1670 of 4564.39 ± 0.24 Ma and published U-corrected Pb–Pb ages for the other two angrites, this allows us to calculate an initial (26Al/27Al)0 of (1.33−0.18+0.21)×10−5 for the angrite parent body (APB) precursor material at the time of CAI formation, a value four times lower than the accepted canonical value of 5.25 × 10−5. Based on their similar 54Cr/52Cr ratios, most inner solar system materials likely accreted from material containing a similar 26Al/27Al ratio as the APB precursor at the time of CAI formation. To satisfy the abundant evidence for widespread planetesimal differentiation, the subcanonical 26Al budget requires that differentiated planetesimals, and hence protoplanets, accreted rapidly within 0.25 ± 0.15 Ma of the formation of canonical CAIs.

The mass-independent calcium isotope composition of inner-Solar-System bodies is correlated with their masses and accretion ages, indicating a rapid growth for the precursors of Earth and the Moon during the protoplanetary disk’s lifetime. Variation in the isotopic composition of material within the early inner Solar System is usually thought to reflect spatial heterogeneity in the protoplanetary disk. Martin Schiller and co-authors find that the calcium isotope composition of samples from the parent bodies of ureilite and angrite meteorites, as well as from Vesta, Mars and Earth, are correlated to the masses of their inferred parent asteroids and planets. This provides a proxy for their accretion timescales and implies a rapid 'secular' evolution of the bulk calcium isotope composition of the disk in the rocky-planet-forming region. The authors infer that this secular evolution reflects the introduction of pristine outer-Solar-System material to the thermally processed inner protoplanetary disk associated with the accretion of mass to the proto-Sun. They also conclude that the indistinguishable calcium isotope composition of the Earth and the Moon implies that the Moon-forming impact involved protoplanets that completed their accretion near the end of the disk's lifetime. Nucleosynthetic isotope variability among Solar System objects is often used to probe the genetic relationship between meteorite groups and the rocky planets (Mercury, Venus, Earth and Mars), which, in turn, may provide insights into the building blocks of the Earth–Moon system1,2,3,4,5. Using this approach, it has been inferred that no primitive meteorite matches the terrestrial composition and the protoplanetary disk material from which Earth and the Moon accreted is therefore largely unconstrained6. This conclusion, however, is based on the assumption that the observed nucleosynthetic variability of inner-Solar-System objects predominantly reflects spatial heterogeneity. Here we use the isotopic composition of the refractory element calcium to show that the nucleosynthetic variability in the inner Solar System primarily reflects a rapid change in the mass-independent calcium isotope composition of protoplanetary disk solids associated with early mass accretion to the proto-Sun. We measure the mass-independent 48Ca/44Ca ratios of samples originating from the parent bodies of ureilite and angrite meteorites, as well as from Vesta, Mars and Earth, and find that they are positively correlated with the masses of their parent asteroids and planets, which are a proxy of their accretion timescales. This correlation implies a secular evolution of the bulk calcium isotope composition of the protoplanetary disk in the terrestrial planet-forming region. Individual chondrules from ordinary chondrites formed within one million years of the collapse of the proto-Sun7 reveal the full range of inner-Solar-System mass-independent 48Ca/44Ca ratios, indicating a rapid change in the composition of the material of the protoplanetary disk. We infer that this secular evolution reflects admixing of pristine outer-Solar-System material into the thermally processed inner protoplanetary disk associated with the accretion of mass to the proto-Sun. The identical calcium isotope composition of Earth and the Moon reported here is a prediction of our model if the Moon-forming impact involved protoplanets or precursors that completed their accretion near the end of the protoplanetary disk’s lifetime.

Polymict ureilites are meteoritic breccias that provide insights into the differentiation history of the ureilite parent body. We have sampled a total of 24 clasts from the polymict ureilite Dar al Gani 319, representing a variety of lithologies such as mantle residues, cumulates and crustal fragments that are genetically related to monomict ureilites. In addition, we sampled four non-indigenous dark clasts and two chondrule-containing clasts from the same meteorite. We report on the petrology and the bulk mass-dependent and mass-independent magnesium and chromium isotope systematics of these clasts. The DaG 319 polymict ureilite consists predominantly of clasts related to Main Group ureilite residues (MG clasts) with varying Mg#s (0.74–0.91), as well as a significant fraction of olivine-orthopyroxene clasts related to Hughes Type ureilites (HT clasts) with consistently high Mg#s (∼0.89). In addition, DaG 319 contains less abundant feldspathic clasts that are thought to represent melts derived from the ureilite mantle. A significant mass-dependent Mg-isotope fractionation totaling Δμ25Δμ25Mg = ∼450 ppm was found between isotopically light feldspathic clasts (μ25μ25Mg = −305 ± 25 to 15 ± 12 ppm), MG clasts (μ25μ25Mg = −23 ± 51 ppm) and HT clasts (μ25μ25Mg = 157 ± 21 ppm). We suggest that this isotopic offset is the result of equilibrium isotope fractionation during melting in the presence of an isotopically light magnesite component. We propose Mg-carbonates to be stable in the upper ureilite mantle, and pure carbon phases such as graphite to be stable at higher pressures. This is consistent with HT clasts lacking carbon-related phases, whereas MG clasts contain abundant carbon. The timing of differentiation events for the ureilitic clasts are constrained by high precision 53Mn-53Cr systematics and 26Al-26Mg model ages. We show that a dichotomy of ages exist between the differentiation of main group ureilite residues and HT cumulates rapidly after CAI formation and later remelting of cumulates with corresponding feldspathic melts, at 3.8 ± 1.3 Myr after CAI formation. Assuming an initial 26Al/27Al abundance [(26Al/27Al)0 = 1.33-0.18+0.21 × 10−5] similar to the angrite parent body, the early melting event is best explained by heat production from 26Al whereas the late event is more likely caused by a major impact. Variations in 54Cr between MG clasts and HT clasts agree with a carbonaceous chondrite impactor onto the ureilite parent body. This impactor may be represented by abundant dark clasts found in polymict ureilites, which have μ26Mg∗μ26Mg∗ and μ54μ54Cr signatures similar to CI chondrites. Similar volatile-rich dark clasts found in other meteorite breccias provide insights into the timing of volatile influx to the accretion region of the terrestrial planets.

Primitive meteorites are samples of asteroidal bodies that contain a high proportion of chemically complex organic matter (COM) including prebiotic molecules such as amino acids, which are thought to have been delivered to Earth via impacts during the early history of the Solar System. Thus, understanding the origin of COM, including their formation pathway(s) and environment(s), is critical to elucidate the origin of life on Earth as well as assessing the potential habitability of exoplanetary systems. The Isheyevo CH/CBb carbonaceous chondrite contains chondritic lithic clasts with variable enrichments in 15N believed to be of outer Solar System origin. Using transmission electron microscopy (TEM-EELS) and in situ isotope analyses (SIMS and NanoSIMS), we report on the structure of the organic matter as well as the bulk H and N isotope composition of Isheyevo lithic clasts. These data are complemented by electron microprobe analyses of the clast mineral chemistry and bulk Mg and Cr isotopes obtained by inductively coupled plasma and thermal ionization mass spectrometry, respectively (MC-ICPMS and TIMS). Weakly hydrated (A) clasts largely consist of Mg-rich anhydrous silicates with local hydrated veins composed of phyllosilicates, magnetite and globular and diffuse organic matter. Extensively hydrated clasts (H) are thoroughly hydrated and contain Fe-sulfides, sometimes clustered with organic matter, as well as magnetite and carbonates embedded in a phyllosilicate matrix. The A-clasts are characterized by a more 15N-rich bulk nitrogen isotope composition (δ15N = 200–650‰) relative to H-clasts (δ15N = 50–180‰) and contain extremely 15N-rich domains with δ15N 15N-rich domains show that the lithic clast diffuse organic matter is typically more 15N-rich than globular organic matter. The correlated δ15N values and C/N ratios of nanoglobules require the existence of multiple organic components, in agreement with the H isotope data. The combined H and N isotope data suggest that the organic precursors of the lithic clasts are defined by an extremely 15N-poor (similar to solar) and D-rich component for H-clasts, and a moderately 15N-rich and D-rich component for A-clasts. In contrast, the composition of the putative fluids is inferred to include D-poor but moderately to extremely 15N-rich H- and N-bearing components. The variable 15N enrichments in H- and A-clasts are associated with structural differences in the N bonding environments of their diffuse organic matter, which are dominated by amine groups in H-clasts and nitrile functional groups in A-clasts. We suggest that the isotopically divergent organic precursors in Isheyevo clasts may be similar to organic moieties in carbonaceous chondrites (CI, CM, CR) and thermally recalcitrant organic compounds in ordinary chondrites, respectively. The altering fluids, which are inferred to cause the 15N enrichments observed in the clasts, may be the result of accretion of variable abundances of NH3 and HCN ices. Finally, using bulk Mg and Cr isotope composition of clasts, we speculate on the accretion regions of the various primitive chondrites and components and the origin of the Solar System’s N and H isotope variability.

Polygonal patterned ground is a dominant geomorphic feature in the McMurdo Dry Valleys, Antarctica. In central Beacon Valley, polygons are considered to be ancient expressions of stagnant surface processes and are thought to be stable for millions of years. However, the actual rate at which these polygons form and alter is not known. We describe a detailed cross section of a polygon, make several depth profiles of meteoric 10Be along this cross section, and use optically stimulated luminescence to date a few key samples at the center of this polygon. While confirming conclusions of previous studies that the polygon shoulders are stable on a 100 k.y. time scale and experience little vertical sediment mixing, our results also give clear evidence of eolian transport to the polygon center, leading to a sediment accumulation rate of 3 cm k.y.−1 over the past 15 k.y. Moreover, the data suggest that the accumulated material is locally derived, and, hence, surface erosion of the polygon shoulders must exist that cannot be recorded by meteoric 10Be. We conclude that polygon modification and soil accumulation under the apparent frozen conditions of Beacon Valley is an active and ongoing process.

In this chapter, we summarize the evidence for high temperature Ca isotope fractionation in natural and experimental samples including minerals, igneous and metamorphic rocks and silicate melts and discuss the underlying isotope fractionation mechanisms. Furthermore, we outline the evidence for primordial nucleosynthetic variability of Ca isotopes in meteorites and their components, suggesting a diverse stellar origin of the Ca isotopes that make up our Solar System. We also present a brief synopsis of observed mass-dependent Ca stable isotope fractionation in meteoritic materials, which allow insights into the local conditions during the formative stage of our Solar System. Lastly, we provide an overview of the 40K–40Ca decay system and its use to track the evolution of Earth’s mantle, continental and oceanic crust over time as well as its potential to date igneous and sedimentary rocks.

A long-standing debate concerning Neogene Antarctic climate in the McMurdo Dry Valleys relies largely on evidence from landscape evolution, glacial modeling and stratigraphy. We provide new evidence from meteoric 10 Be for the onset of frozen, hyper-arid conditions on a high elevation (1840 m) interfluve at Table Mountain. A simple decay model for the co-occurrence of meteoric 10 Be and illuviated clay in cores of ice-cemented glacial sediments indicates that the clays were actively migrating down from the surface in a warm climate until the system froze between 6 and 9 Ma. Although this age range may be sensitive to possible interference by in situ produced 10 Be, the implied minimum age of 6 Ma for the Sirius Group indicates that the Dry Valleys were permanently frozen down to this elevation at this time. The model also suggests denudation rates of 1–6 cm Myr −1 since freezing. These data provide an independent test of glacial-stratigraphic evidence used to determine Antarctic paleoclimate.

Meteorites with significantly sub-chondritic Al/Mg that formed in the first 2 million years of the Solar System should be characterised by deficits in the abundance of 26Mg (δ26Mg∗) due to the absence of in-growth of 26Mg from the decay of short-lived 26Al (t1/2 = 0.73 Myr). However, these 26Mg deficits will be small (δ26Mg∗ >−0.037‰) even for material that formed at the same time as the Solar System’s oldest solids – calcium–aluminium-rich inclusions – and thus measurement of these deficits is analytically challenging. Here, we report on a search for 26Mg deficits in three types of ultramafic meteorites (pallasites, ureilites and aubrites) by multiple-collector inductively coupled plasma mass spectrometry. A range of analytical tests were carried out including analysis of: (1) a range of synthetic Mg solution standards; (2) Mg gravimetrically doped with a high purity 26Mg spike; (3) Mg cuts collected sequentially from cation exchange separation columns with fractionated stable Mg isotope compositions; (4) Mg separated from samples that was bracketed by analyses of both DSM-3 and Mg separated from a natural olivine sample subjected to the same chemical processing as the samples. These tests confirm it is possible to resolve differences in δ26Mg∗ from the terrestrial materials that are ⩽0.005‰. However, if Mg yields from chemical separation are low or an inappropriate equilibrium-isotopically fractionated standard is used this will generate analytical artefacts on δ26Mg∗ when this is calculated with the kinetic/exponential mass fractionation law as is the case when correcting for instrumental mass bias during mass spectrometric analysis. Olivine from four different main group pallasites and four bulk ureilites have small deficits in the abundance of 26Mg with δ 26 Mg DSM- 3 ∗ = - 0.0120 ± 0.0018 ‰ and δ 26 Mg DSM- 3 ∗ = - 0.0062 ± 0.0023 ‰ , respectively, relative to terrestrial olivine ( δ 26 Mg DSM- 3 ∗ = + 0.0029 ± 0.0028 ‰ ). Six aubrites have δ 26 Mg DSM- 3 ∗ = + 0.0015 ± 0.0020 ‰ , which is identical to terrestrial olivine. Model ages from these deficits can be calculated by assuming that 26Al was homogeneously distributed in the planetesimal-forming regions of the proto-planetary disc at the same level as calcium–aluminium-rich inclusions (CAIs). The absence of 26Mg deficits in aubrites, means these can only be constrained to have formed relatively late >2.9 Myr after CAI formation. Model ages calculated from pallasite olivine deficits would suggest that pallasite olivine crystallised and was diffusively isolated on its parent body 1.24 - 0.28 + 0.40 Myr after formation of CAIs. Similarly, ureilites would have experienced silicate partial melting and lowering of their bulk Al/Mg ratios 1.9 - 0.7 + 2.2 Myr after CAI formation. The model ages for silicate differentiation on the main group pallasite parent body are intermediate between those for metal-silicate fractionation for core formation obtained from magmatic iron meteorites and those for asteroidal silicate magmatism obtained from basaltic meteorites.

We report on mineralogy, petrography, and whole-rock 26 Al- 26 Mg systematics of eight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. The AOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al-rich inclusions (CAIs) of different types, and show evidence for alteration to varying degrees. Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel is enriched in FeO; opaque nodules are replaced by Fe,Ni-sulfides, ferroan olivine and Ca,Fe-rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown by ferroan olivine. The AOAs are surrounded by fine-grained, matrix-like rims composed mainly of ferroan olivine and by a discontinuous layer of Ca,Fe-rich silicates. These observations indicate that AOAs experienced in situ elemental open-system iron-alkali- halogen metasomatic alteration during which Fe, Na, Cl, and Si were introduced, whereas Ca was removed from AOAs and used to form the Ca,Fe-rich silicate rims around AOAs. The whole-rock 26 Al- 26 Mg systematics of the Allende AOAs plot above the isochron of the whole-rock Allende CAIs with a slope of (5.23 ± 0.13) · 10 )5 reported by Jacobsen et al. (2008). In contrast, whole-rock 26 Al- 26 Mg isotope systematics of CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochron with a slope of (5.25± 0.01) · 10 )5 (Larsen et al. 2011). We infer that the excesses in 26 Mg* present in Allende AOAs are due to their late-stage open-system metasomatic alteration. Thus, the 26 Al- 26 Mg isotope systematics of Allende CAIs and AOAs are disturbed by parent body alteration processes, and may not be suitable for high-precision chronology of the early solar system events and processes.

Variations of the mass-independent abundance of 26 Mg ( δ 26 Mg*) and stable Mg ( δ 25 Mg) isotope composition of chondrites are important because they constrain the homogeneity of 26 Al and Mg isotopes in the proto-planetary disc and the validity of the short-lived 26 Al-to- 26 Mg chronometer applied to meteorites. We present high-precision Mg isotope data and Al/Mg ratios of chondrites representing nearly all major chondrite classes, including a step-leaching experiment on the CM2 chondrite Murchison. δ 26 Mg* variations in leachates of Murchison representing acid soluble material are ≤ 30 times smaller than reported for neutron-rich isotopes of Ti and Cr and do not reveal resolvable deficits in δ 26 Mg* (−0.002 to + 0.118‰). Very small variations in δ 26 Mg* anomalies in bulk chondrites (−0.006 to + 0.019‰) correlate with increasing 27 Al/ 24 Mg ratios and δ 50 Ti, reflecting the variable presence of calcium–aluminium-rich inclusions (CAIs) in some types of carbonaceous chondrites. Similarly, release of radiogenic 26 Mg produced by 26 Al decay from CAI material in the step-leaching of Murchison best explains the high δ 26 Mg* observed in the last, aggressive, leaching steps of this experiment. Overall, the observed variations in δ 26 Mg* are small and potential differences beyond that which result from the presence of CAI-like material cannot be detected within the analytical uncertainties of this study (± 0.004‰). The results do not allow radical heterogeneity of 26 Al (≥±30%) or measurable Mg nucleosynthetic heterogeneity (≥±0.005‰) to have existed on a planetesimal scale in the proto-planetary disc. Combined with published δ 26 Mg* data for CAIs, the bulk chondrite data yield a precise initial ( 26 Al/ 27 Al) 0 = (5.21 ± 0.06) × 10 − 5 and δ 26 Mg* = −0.0340 ± 0.0016‰ for the Solar System. However, it is not possible with the currently available data to determine with certainty whether CAIs and the material from which planetesimals accreted including chondrite parent bodies had precisely the same initial levels of 26 Al, although planetesimals and planets appear to have accreted from material with a mean initial ( 26 Al/ 27 Al) 0 in the range of 2.1 to 6.7 × 10 − 5 . The average stable Mg isotope composition of all analysed chondrites, with the exception of a chondrule from the CBa chondrite Gujba ( δ 25 Mg DSM - 3 = −0.032 ± 0.035‰), is δ 25 Mg DSM - 3 = −0.152 ± 0.079‰ (2 sd) and is indistinguishable from that of the Earth's mantle.

We present high-precision Mg isotope data for most classes of basaltic meteorites including eucrites, mesosiderite silicate clasts, angrites and the ungrouped Northwest Africa (NWA) 2976 measured by pseudo-high-resolution multiple-collector inductively coupled plasma mass spectrometry and utilising improved techniques for chemical purification of Mg. With the exception of the angrites Angra dos Reis, Lewis Cliff (LEW) 86010, NWA 1296 and NWA 2999 and the diogenite Bilanga, which have either been shown to have young ages by other dating techniques or have low Al/Mg ratios, all bulk samples of basaltic meteorites have 26Mg excesses ( δ 26 Mg ∗ = + 0.0135 to +0.0392‰). The 26Mg excesses cannot be explained by analytical artefacts, cosmogenic effects or heterogeneity of initial 26Al/27Al, Al/Mg ratios or Mg isotopes in asteroidal parent bodies as compared to Earth or chondrites. The 26Mg excesses record asteroidal melting and formation of basaltic magmas with super-chondritic Al/Mg and confirm that radioactive decay of short-lived 26Al was the primary heat source that melted planetesimals. Model 26Al–26Mg ages for magmatism on the eucrite/mesosiderite, angrite and NWA 2976 parent bodies are 2.6–3.2, 3.9–4.1 and 3.5 Myr, respectively, after formation of calcium–aluminium-rich inclusions (CAIs). However, the validity of these model ages depends on whether the elevated Al/Mg ratios of basaltic meteorites result from magma ocean evolution on asteroids through fractional crystallisation or directly during partial melting. Mineral isochrons for the angrites Sahara (Sah) 99555 and D’Orbigny, and NWA 2976, yield ages of 5.06 - 0.05 + 0.06 Myr and 4.86 - 0.09 + 0.10 Myr , respectively, after CAI formation. Both isochrons have elevated initial δ 26 Mg ∗ values. Given the brecciated and equilibrated texture of NWA 2976 it is probable that its isochron age and elevated initial δ 26 Mg ∗ ( + 0.0175 ± 0.0034 ‰ ) reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites the marginally elevated initial δ 26 Mg ∗ ( + 0.0068 ± 0.0058 ‰ ) may reflect either δ 26 Mg ∗ ingrowth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic Al/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. 26Al–26Mg model ages and an olivine + pyroxene + whole rock isochron for the angrites Sah 99555 and D’Orbigny are in good agreement with age constraints from 53Mn–53Cr and 182Hf–182W short-lived chronometers, suggesting that the 26Al–26Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. Even when age constraints from the 26Al–26Mg angrite model ages or the mafic mineral + whole rock isochron are considered, the relative time difference between Sah 99555/D’Orbigny crystallisation and CAI formation cannot be reconciled with Pb–Pb ages for Sah 99555/D’Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that 26Al was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disc as compared to CAIs, or that CAI Pb–Pb ages may not accurately date CAI formation, which might be better dated by the 182Hf–182W and 26Al–26Mg chronometers as 4568.3 ± 0.7 (Burkhardt et al., 2008) and 4568.5 ± 0.3 Ma (herein), respectively, when mapped onto an absolute timescale using Pb–Pb ages for angrites.

article The mantle is Earth's largest reservoir of Mg containing N99% of Earth's Mg inventory. However, no consensus exists on the stable Mg isotope composition of the Earth's mantle or how variable it is and, in particular, whether the mantle has the same stable Mg isotope composition as chondrite meteorites. We have determined the Mg isotope composition of olivine from 22 mantle peridotites from eastern Australia, west Antarctica, Jordan, Yemen and southwest Greenland by pseudo-high-resolution MC-ICP-MS on Mg purified to N99%. The samples include fertile lherzolites, depleted harzburgites and dunites, cryptically metasomatised ('dry') peridotites and modally metasomatised apatite±amphibole-bearing harzburgites and wehrlites. Olivine from these samples of early Archaean through to Permian lithospheric mantle have δ 25 MgDSM-3= −0.22 to −0.08‰. These data indicate the bulk upper mantle as represented by peridotite olivine is homogeneous within current analytical uncertainties (external reproducibility ≤±0.07‰ (2 sd)). We find no systematic δ 25 Mg variations with location, lithospheric age, peridotite fertility, or degree or nature of mantle metasomatism. Although pyroxene may have slightly heavier δ 25 Mg than coexisting olivine, any fractionation

Refractory inclusions [calcium–aluminum-rich inclusions, (CAIs)] represent the oldest Solar System solids and provide information regarding the formation of the Sun and its protoplanetary disk. CAIs contain evidence of now extinct short-lived radioisotopes (e.g., 26 Al, 41 Ca, and 182 Hf) synthesized in one or multiple stars and added to the protosolar molecular cloud before or during its collapse. Understanding how and when short-lived radioisotopes were added to the Solar System is necessary to assess their validity as chronometers and constrain the birthplace of the Sun. Whereas most CAIs formed with the canonical abundance of 26 Al corresponding to 26 Al/ 27 Al of ∼5 × 10 −5 , rare CAIs with fractionation and unidentified nuclear isotope effects (FUN CAIs) record nucleosynthetic isotopic heterogeneity and 26 Al/ 27 Al of −6 , possibly reflecting their formation before canonical CAIs. Thus, FUN CAIs may provide a unique window into the earliest Solar System, including the origin of short-lived radioisotopes. However, their chronology is unknown. Using the 182 Hf– 182 W chronometer, we show that a FUN CAI recording a condensation origin from a solar gas formed coevally with canonical CAIs, but with 26 Al/ 27 Al of ∼3 × 10 −6 . The decoupling between 182 Hf and 26 Al requires distinct stellar origins: steady-state galactic stellar nucleosynthesis for 182 Hf and late-stage contamination of the protosolar molecular cloud by a massive star(s) for 26 Al. Admixing of stellar-derived 26 Al to the protoplanetary disk occurred during the epoch of CAI formation and, therefore, the 26 Al– 26 Mg systematics of CAIs cannot be used to define their formation interval. In contrast, our results support 182 Hf homogeneity and chronological significance of the 182 Hf– 182 W clock.

[1] Concentrations of 10Be, 9Be, and salt in a soil profile from the lower Wright Valley reveal two distinct climatic regimes. In the upper horizon of the soil profile, a thin gravel lag overlies the Hart ash (3.9 ± 0.3 Ma), and this sits on the surface of a well-developed paleosol, which makes up the lower horizon of the profile. The surface lag has a smaller inventory of 10Be than predicted from the fallout rate at nearby Taylor Dome. Below the gravel, the Hart ash is virtually devoid of 10Be and the calculated erosion rate for the gravel-ash horizon is 2.8 m Ma−1. The paleosol, which has been sealed by the overlying ash, has a relatively high inventory of 10Be, and this gives it a significantly lower erosion rate (0.5 m Ma−1) than the gravel-ash horizon. Beryllium-10 migrates into a soil profile either by solution transport or by fine-particle translocation. The lack of 10Be in the Hart ash indicates that neither of these processes have been active at this site for the past 3.9 Ma. However, enrichment of 10Be in the paleosol indicates they were active prior to deposition of the ash. The most likely reason these processes were active is that there was more water, which could translocate fine particles with attached 10Be. Thus, the gravel-ash horizon developed in climatic conditions similar to the present, while the paleosol developed in wetter and possibly warmer conditions during the early Pliocene and possibly before that time.