144 research outputs found
Evolution of oxygen isotopic composition in the inner solar nebula
Changes in the chemical and isotopic composition of the solar nebula with
time are reflected in the properties of different constituents that are
preserved in chondritic meteorites. CR carbonaceous chondrites are among the
most primitive of all chondrite types and must have preserved solar nebula
records largely unchanged. We have analyzed the oxygen and magnesium isotopes
in a range of the CR constituents of different formation temperatures and ages,
including refractory inclusions and chondrules of various types. The results
provide new constraints on the time variation of the oxygen isotopic
composition of the inner (<5 AU) solar nebula - the region where refractory
inclusions and chondrules most likely formed. A chronology based on the decay
of short-lived 26Al (t1/2 ~ 0.73 Ma) indicates that the inner solar nebula gas
was 16O-rich when refractory inclusions formed, but less than 0.8 Ma later, gas
in the inner solar nebula became 16O-poor and this state persisted at least
until CR chondrules formed ~1-2 Myr later. We suggest that the inner solar
nebula became 16O-poor because meter-size icy bodies, which were enriched in
17,18O due to isotopic self-shielding during the ultraviolet photo dissociation
of CO in the protosolar molecular cloud or protoplanetary disk, agglomerated
outside the snowline, drifted rapidly towards the Sun, and evaporated at the
snowline. This led to significant enrichment in 16O-depleted water, which then
spread through the inner solar system. Astronomical studies of the spatial
and/or temporal variations of water abundance in protoplanetary disks may
clarify these processes.Comment: 27 pages, 5 figure
Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets
Oxygen isotopes were measured in four chondritic hydrated interplanetary dust particles (IDPs) and five chondritic anhydrous IDPs including two GEMS-rich particles (Glass embedded with metal and sulfides) by a combination of high precision and high lateral resolution ion microprobe techniques. All IDPs have isotopic compositions tightly clustered around that of solar system planetary materials. Hydrated IDPs have mass-fractionated oxygen isotopic compositions similar to those of CI and CM carbonaceous chondrites, consistent with hydration of initially anhydrous protosolar dust. Anhydrous IDPs have small 16O excesses and depletions similar to those of carbonaceous chondrites, the largest 16O variations being hosted by the two GEMS-rich IDPs. Coarse-grained forsteritic olivine and enstatite in anhydrous IDPs are isotopically similar to their counterparts in comet Wild 2 and in chondrules suggesting a high temperature inner solar system origin. The small variations in the 16O content of GEMS-rich IDPs suggest that most GEMS either do not preserve a record of interstellar processes or the initial interstellar dust is not 16O-rich as expected by self-shielding models, although a larger dataset is required to verify these conclusions. Together with other chemical and mineralogical indicators, O isotopes show that the parent-bodies of carbonaceous chondrites, of chondritic IDPs, of most Antarctic micrometeorites, and comet Wild 2 belong to a single family of objects of carbonaceous chondrite chemical affinity as distinct from ordinary, enstatite, K- and R-chondrites. Comparison with astronomical observations thus suggests a chemical continuum of objects including main belt and outer solar system asteroids such as C-type, P-type and D-type asteroids, Trojans and Centaurs as well as short-period comets and other Kuiper Belt Objects
Determining the Elemental and Isotopic Composition of the preSolar Nebula from Genesis Data Analysis: The Case of Oxygen
We compare element and isotopic fractionations measured in solar wind samples
collected by NASA's Genesis mission with those predicted from models
incorporating both the ponderomotive force in the chromosphere and conservation
of the first adiabatic invariant in the low corona. Generally good agreement is
found, suggesting that these factors are consistent with the process of solar
wind fractionation. Based on bulk wind measurements, we also consider in more
detail the isotopic and elemental abundances of O. We find mild support for an
O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A
stronger conclusion must await solar wind regime specific measurements from the
Genesis samples.Comment: 6 pages, accepted by Astrophysical Journal Letter
Evidence for Life on Earth before 3,800 Million Years Ago
It is unknown when life first appeared on Earth. The earliest known microfossils (approx. 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than approx. 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences a approx. 3,800 Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present
Tracking the Martian Mantle Signature in Olivine-Hosted Melt Inclusions of Basaltic Shergottites Yamato 980459 and Tissint
The Martian shergottite meteorites are basaltic to lherzolitic igneous rocks that represent a period of relatively young mantle melting and volcanism, approximately 600-150 Ma (e.g. [1,2]). Their isotopic and elemental composition has provided important constraints on the accretion, evolution, structure and bulk composition of Mars. Measurements of the radiogenic isotope and trace element concentrations of the shergottite meteorite suite have identified two end-members; (1) incompatible trace element enriched, with radiogenic Sr and negative epsilon Nd-143, and (2) incompatible traceelement depleted, with non-radiogenic Sr and positive epsilon 143-Nd(e.g. [3-5]). The depleted component represents the shergottite martian mantle. The identity of the enriched component is subject to debate, and has been proposed to be either assimilated ancient martian crust [3] or from enriched domains in the martian mantle that may represent a late-stage magma ocean crystallization residue [4,5]. Olivine-phyric shergottites typically have the highest Mg# of the shergottite group and represent near-primitive melts having experienced minimal fractional crystallization or crystal accumulation [6]. Olivine-hosted melt inclusions (MI) in these shergottites represent the most chemically primitive components available to understand the nature of their source(s), melting processes in the martian mantle, and origin of enriched components. We present trace element compositions of olivine hosted melt inclusions in two depleted olivinephyric shergottites, Yamato 980459 (Y98) and Tissint (Fig. 1), and the mesostasis glass of Y98, using Secondary Ionization Mass Spectrometry (SIMS). We discuss our data in the context of understanding the nature and origin of the depleted martian mantle and the emergence of the enriched component
The effect of a startle-eliciting device on the foraging success of individual harbor seals (Phoca vitulina)
Pinniped predation on commercially and ecologically important prey has been a source of conflict for centuries. In the Salish Sea, harbor seals (Phoca vitulina) are suspected of impeding the recovery of culturally and ecologically critical Pacific salmon (Oncorhynchus spp.). In Fall 2020, a novel deterrent called Targeted Acoustic Startle Technology (TAST) was deployed at Whatcom Creek to deter harbor seals from preying on fall runs of hatchery chum (O. keta) and Chinook (O. tshawytscha) salmon in Bellingham, Washington, USA. Field observations were conducted in 2020 to compare the presence and foraging success of individual harbor seals across sound exposure (TAST-on) and control (TAST-off) conditions. Observations conducted the previous (2019) and following (2021) years were used to compare the effects observed in 2020 to two control years. Using photo-identification, individual seals were associated with foraging successes across all 3 years of the study. Generalized linear mixed models showed a significant 45.6% reduction in the duration (min) individuals remained at the creek with TAST on, and a significant 43.8% reduction in the overall foraging success of individuals. However, the observed effect of TAST varied across individual seals. Seals that were observed regularly within one season were more likely to return the year after, regardless of TAST treatment. Generalized linear models showed interannual variation in the number of seals present and salmon consumed. However, the effect of TAST in 2020 was greater than the observed variation across years. Our analyses suggest TAST can be an effective tool for managing pinniped predation, although alternate strategies such as deploying TAST longer-term and using multi-unit setups to increase coverage could help strengthen its effects. Future studies should further examine the individual variability found in this study.Peer reviewe
The <i>Castalia</i> mission to Main Belt Comet 133P/Elst-Pizarro
We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBCâs activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESAâs highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these
In situ ÎŽ7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q03001, doi:10.1029/2010GC003322.In situ secondary ion mass spectrometry (SIMS) analyses of ÎŽ7Li, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of ÎŽ7Li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasmaâmass spectrometry (MC-ICP-MS). Both techniques yielded similar ÎŽ7Li in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors α7Li/6Li range from 0.9895 to 0.9923, which translates to a fractionation between aragonite and fluid from â10.5â° to â7.7â°. The within-sample ÎŽ7Li range determined by SIMS is up to 27â°, exceeding the difference between bulk ÎŽ7Li analyses of different aragonite precipitates. Moreover, the centers of aragonite hemispherical bundles (spherulites) are enriched in Li/Ca and Mg/Ca relative to spherulite fibers by up to factors of 2 and 8, respectively. The Li/Ca and Mg/Ca ratios of spherulite fibers increase with aragonite precipitation rate. These results suggest that precipitation rate is a potentially important consideration when using Li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature.SIMS analyses were supported by U.S. NSF, EAR, Instrumentation and Facilities Program. The development of
the method for bulk d7Li analysis and the MCâICPâMS measurements
were covered by NSF grant EAR/IFâ0318137. Precipitation
experiments were supported by NSF through grants
OCEâ0402728, OCEâ0527350, and OCEâ0823527 to Glenn
Gaetani and Anne Cohen and through grant EARâ0337481 to
Bruce Watson
The composition of the protosolar disk and the formation conditions for comets
Conditions in the protosolar nebula have left their mark in the composition
of cometary volatiles, thought to be some of the most pristine material in the
solar system. Cometary compositions represent the end point of processing that
began in the parent molecular cloud core and continued through the collapse of
that core to form the protosun and the solar nebula, and finally during the
evolution of the solar nebula itself as the cometary bodies were accreting.
Disentangling the effects of the various epochs on the final composition of a
comet is complicated. But comets are not the only source of information about
the solar nebula. Protostellar disks around young stars similar to the protosun
provide a way of investigating the evolution of disks similar to the solar
nebula while they are in the process of evolving to form their own solar
systems. In this way we can learn about the physical and chemical conditions
under which comets formed, and about the types of dynamical processing that
shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both
cometary volatiles and the composition, structure and evolution of protostellar
disks.Comment: To appear in Space Science Reviews. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11214-015-0167-
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