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Carbon and Nitrogen Isotopic Anomalies in an Anhydrous Interplanetary Dust Particle

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Science  27 Feb 2004:
Vol. 303, Issue 5662, pp. 1355-1358
DOI: 10.1126/science.1093283

Abstract

Because hydrogen and nitrogen isotopic anomalies in interplanetary dust particles have been associated with carbonaceous material, the lack of similar anomalies in carbon has been a major conundrum. We report here the presence of a 13C depletion associated with a 15N enrichment in an anhydrous interplanetary dust particle. Our observations suggest that the anomalies are carried by heteroatomic organic compounds. Theoretical models indicate that low-temperature formation of organic compounds in cold interstellar molecular clouds can produce carbon and nitrogen fractionations, but it remains to be seen whether the specific effects observed here can be reproduced.

Interstellar molecular clouds are the principal formation sites of organic matter in the Milky Way galaxy. A variety of simple molecules, such as CH4, CH3OH, and H2CO, are produced in dense cold (10 to 30 K) clouds (1). At such low temperatures, where the difference in chemical binding energy exceeds thermal energy, mass fractionation produces molecules with isotopic ratios that can be anomalous relative to terrestrial values (13). Such anomalous ratios potentially provide a fingerprint for abiotic interstellar organic matter that was incorporated into the solar system and survives today in cosmically primitive materials such as interplanetary dust particles (IDPs).

IDPs collected in Earth's stratosphere are complex assemblages of primitive solar system material and carry various isotopic anomalies (2, 4, 5). Deuterium enhancements result from extreme chemical fractionation in cold molecular clouds (6), and D/H ratios in some IDPs approach the values observed in interstellar molecules (7), suggesting the intact survival of some molecular cloud material. Nitrogen isotopic anomalies, in the form of 15N enrichments, are also common and are postulated to result from low-temperature interstellar chemistry (2, 8). This origin is complicated because N isotopic fractionation has not been observed in the interstellar medium and because anomalous N can also have a nucleosynthetic origin. However, recent work shows that chemical reactions in dense molecular gases can produce elevated 15N/14N ratios (9, 10), although the maximum enhancements fall short of observed 15N enrichments in IDPs (11, 12). Organic compounds appear to be the source of many of the D and 15N enrichments (1315); it is puzzling, therefore, that C isotopic anomalies have not been observed, despite numerous measurements (5).

We carried out simultaneous C and N isotopic imaging measurements of an anhydrous noncluster particle (L2036-G16), using the NanoSIMS, a new generation secondary ion mass spectrometer that allows isotopic imaging at a spatial scale of 100 nm. The particle, nicknamed Benavente (16), was pressed into a high-purity Au substrate along with isotopic standards, which were measured together with the IDP on the same sample mount (17). The bulk C isotopic composition of Benavente is normal, with a 12C/13C ratio of 89.3 ± 1.0 (δ13C = +8 ± 11‰), but the IDP is enriched in 15N, with an average 14N/15N ratio of 224.3 ± 1.7 (δ15N = +213 ± 9‰). Benavente also contains a large (0.6 × 1.8 μm2) region that is strongly enriched in 15N (14N/15N = 119.8 ± 1.3; δ15N = +1270 ± 25‰) and depleted in 13C (12C/13C = 96.6 ± 1.3; δ13C = –70 ± 13‰) (Fig. 1) (18). Although the 13C depletion is not great, its importance is emphasized by the large size of the region and its association with the 15N enrichment (Fig. 2) (19). Furthermore, both anomalies are consistently present in the 25 successively measured layers. Several studies (2022) indicate a solar N isotopic composition that is ∼30% lighter than the terrestrial one. Thus, the 15N enrichments in Benavente relative to solar composition may be considerably higher than the values noted here. Moreover, recent work (23) suggests that solar C may be ∼10% lighter than terrestrial C, a value that is similar to the magnitude of the 13C depletion in our anomalous region.

Fig. 1.

Isotopic composition of the anomalous region compared with similar-sized areas of Benavente. Nitrogen isotopic compositions spread toward subterrestrial ratios in the “bulk” of the particle, indicating an overall enrichment in 15N.

Fig. 2.

δ15N (left) and δ13C (right) images of Benavente showing the 15N-enriched and 13C-depleted anomalous region. Field of view is 10 μm.

In addition to isotopic information, our analysis provides clues to the nature of the carrier phase(s) of the C and N anomaly in Benavente. The anomalous region has a higher C yield than most of the IDP and a CN/C ratio of ∼0.9, suggesting that the C and N anomalies are carried by organic matter. If this material is carbonaceous, the calibration curve of CN/C ratios versus N concentrations in organic material established by (24) suggests a N concentration of ∼3.0 ± 1.5 weight % (wt %). This is at the upper end of the range of N concentrations of insoluble organic matter in carbonaceous chondrites (15) and falls within the field observed for CHON grains from comet Halley (25). Recent measurements of the 14N/15N ratios of the CN radical in the coma of two comets give a value of 140 ± 30 (26), within errors the same as that of the 14N/15N ratio of our anomalous region.

Using a focused ion beam (FIB) technique (27), we extracted a section (∼5 by 1 by 0.1 μm) from an area about 5 μm from the anomalous region. This area has lower C and CN signals than the anomalous region, and C and N isotopic compositions similar to the bulk IDP. Using a transmission electron microscope (TEM), we employed energy-filtered imaging to investigate the mineralogy of the section (17). The region is rich in GEMS (glass with embedded metal and sulfides) (28) and also contains forsterite (Mg2SiO4), iron-rich sulfides (pyrrhotite, Fe1–xS), and abundant amorphous carbonaceous material that is clumped into distinct regions and coats some grains, such as the GEMS.

After the TEM investigation, we mapped the C and N isotopic compositions in the FIB slice with the NanoSIMS and reconfirmed the 15N enrichment of ∼200‰ observed in the bulk IDP. Moreover, we found a 250-nm region with a strong 15N enrichment of +1110 ± 98‰ that was not seen in the original NanoSIMS measurement because it is located below the original surface of the IDP. Spatial correlation of this anomaly with the TEM images (Fig. 3) shows that it consists of amorphous carbonaceous material immediately surrounded by GEMS and other silicates. Infrared (IR) spectra (17) show a prominent C-H stretch feature at ∼3.4 μm (Fig. 4), similar to that observed in meteoritic kerogen (4). The positions of the bands within the feature are consistent with those of aliphatic hydrocarbons, confirming the organic nature of carbonaceous material. Aromatic hydrocarbons are likely to be present too, but are probably dominated by the strong resonance of the aliphatic C-H feature. Nitrogen abundances in the FIB section are low (1 to 2 wt %, based on electron energy loss spectroscopy), but the N is associated with the carbonaceous material and, thus, has an organic origin. No other elements are associated with the N that might indicate the presence of a different carrier phase. The 15N-rich region in the FIB slice is not 13C-depleted (probably because of the small size of the area, leading to dilution effects from surrounding isotopically normal C), but the similarity of the magnitude of the N anomaly to that of the C- and N-anomalous region (and the proximity of the FIB slice to the original anomalous area) suggests that the same type of amorphous carbon is the carrier of the correlated C and N anomaly. Although presolar graphite in meteorites is 15N-enriched and 13C-depleted (29), it is unlikely to be the carrier phase of the C and N anomaly in Benavente: Our TEM and spectroscopic work shows that only amorphous carbon is present in this IDP. Moreover, in contrast to carbonaceous chondrites, graphitic carbon is conspicuously absent from chondritic IDPs (4, 30).

Fig. 3.

FIB section extracted from Benavente showing, in the circle, the 15N-rich region. (A) Bright-field TEM image (300 keV). GEMS, glass with embedded metal and sulfides. (B) Energy-filtered carbon jump-ratio image. (C) Energy-filtered oxygen jump-ratio image. (D) Composite image showing silicates (green), carbonaceous material (blue), and iron or iron sulfides (red).

Fig. 4.

IR spectrum of the Benavente FIB section, showing the C-H stretch feature at 3.4 μm indicative of hydrocarbons.

Without C isotopic anomalies, it is not possible to distinguish whether the carbonaceous material in IDPs is itself presolar or simply a more “recent” (e.g., solar system) host substrate for presolar D and 15N-enriched species. Previous studies have suggested that some IDPs contain carbon compounds with heteroatoms (e.g., N in –CN attached to aromatic chromophores) (31), but without correlated C and N isotopic data, the origin of such molecules has remained uncertain (13, 31, 32). Our observation of associated C and N anomalies establishes that IDPs contain heteroatomic organic compounds of presolar interstellar origin that are more complex than the simple deuterated compounds implied by earlier measurements (7). During its prebiotic period, Earth may have accreted as much as a centimeter of abiotic carbonaceous matter every million years, much of it settling to the surface within small (<25 μm diameter), high-surface-area IDPs (3335). This constant flux of particulate organic matter continues to be delivered to the surfaces of terrestrial planets today and includes interstellar molecules such as those found in Benavente.

Gas-phase reactions are expected to produce C isotopic fractionations, but different processes produce fractionation effects in opposite directions (1, 3638). Thus, it has been suggested that the lack of C isotopic anomalies in IDPs is due to the existence of multiple reaction pathways that cancel out any anomalies produced (1, 38). Others have suggested that isotopic fractionation in C is inhibited through the condensation of CO onto grain surfaces and its participation in grain chemistry (2, 5). Our observation of a 13C depletion associated with a 15N enrichment in Benavente shows that C isotopic fractionation does occur and requires processes that can produce both effects in the same material. Gas-phase ion-molecule reactions can enhance the 12C/13C ratios of organic species (36, 37) to the level observed in Benavente, but whether these reactions also result in depleted 14N/15N ratios has not been studied. Low-temperature interstellar chemistry as the source of the 15N enrichments seen in IDPs has only recently been investigated theoretically. A study of ion-molecule exchange reactions involving the most abundant N-bearing species in interstellar clouds (9) indicated a maximum enhancement in 15N of +250‰. Another recent model, investigating NH3 formation in dense molecular clouds, suggests a maximum enrichment of +800‰ (10). These models are consistent with the modest enrichment in 15N seen in the bulk IDP but fall short of the values needed to account for the +1270‰ enrichment in 15N observed in the anomalous region. Moreover, it is not clear whether 15N-rich ammonia can pass on its anomalous N to the organic hosts thought to be responsible for N isotopic anomalies in IDPs. If future investigations of interstellar chemistry cannot account for the C and N isotopic fractionations observed in IDPs, circumstellar origins may need to be considered.

Supporting Online Material

www.sciencemag.org/cgi/content/full/303/5662/1355/DC1

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