Molecular and isotopic compositions of hydrocarbons at DSDP Site 76-533

In an investigation of gas hydrates in deep ocean sediments, gas samples from Deep Sea Drilling Project Site 533 on the Blake Outer Ridge in the northwest Atlantic were obtained for molecular and isotopic analyses. Gas samples were collected from the first successful deployment of a pressure core barrel (PCB) in a hydrate region. The pressure decline curves from two of the four PCB retrievals at in situ pressures suggested the presence of small amounts of gas hydrates. Compositional and isotopic measurements of gases from several points along the pressure decline curve indicated that (1) biogenic methane (d13C = -68 per mil; C1/C2 = 5000) was the dominant gas (>90%); (2) little fractionation in the C1/C2 ratio or the C carbon isotopic composition occurred as gas hydrates decomposed during pressure decline experiments; (3) the percent of C3, i-C4, and CO2 degassed increased as the pressure declined, indicating that these molecules may help stabilize the hydrate structure; (4) excess nitrogen was present during initial degassing; and (5) C1/C2 ratios and isotopic ratios of C gases were similar to those obtained from conventional core sampling. The PCB gas also contained trace amounts of saturated, acyclic, cyclic, and aromatic C5-C14 hydrocarbons, as well as alkenes and tetrahydrothiophenes. Gas from a decomposed specimen of gas hydrate had similar molecular and isotopic ratios to the PCB gas (d13C of -68 per mil for methane and a C1/C2 ratio of about 6000). Regular trends in the d13C of methane (about -95 to -60 per mil) and C1/C2 ratios (about 25000 to 2000) were observed with depth. Capillary gas chromatography (GC) and total scanning fluorescence measurements of extracted organic material were characteristic of hydrocarbons dominated by a marine source, though significant amounts of perylene were also present.

(Table 1) Strontium isotopic ratios of interstitial waters from ODP Leg 125 holes

The strontium isotopic data presented here are from interstitial waters squeezed from unconsolidated serpentine, an unusual type of substrate that was recovered from Mariana and Bonin forearc seamounts and has not been previously drilled by the Deep Sea Drilling Project or Ocean Drilling Program. The texture and composition of some of these serpentine deposits from Conical Seamount, located on the Mariana forearc, indicate emplacement as low- or high-viscosity, cold gravitational flows, which are therefore neither sediment nor igneous rock. The strontium isotopic ratios of the interstitial waters from the unconsolidated serpentine range from 0.70912 to 0.70525 and trend toward a relatively less radiogenic composition with increasing sub-bottom depth. These strontium isotopic ratios are derived from at least two strontium sources: seawater and igneous. The strontium isotopic gradients from the interstitial waters from the Leg 125 sites are probably the result of diffusive transport of strontium from an igneous source deep within the lithosphere that may be contaminated with subducted or underplated sediment.

(Table 3) Oxygen and carbon isotopic record of living (Rose Bengal Stained) benthic foraminifera of North Atlantic surface sediments

Variation of the d13C of living (Rose Bengal stained) deep-sea benthic foraminifera is documented from two deep-water sites (~2430 and ~3010 m) from a northwest Atlantic Ocean study area 275 km south of Nantucket Island. The carbon isotopic data of Hoeglundina elegans and Uvigerina peregrina from five sets of Multicorer and Soutar Box Core samples taken over a 10-month interval (March, May, July, and October 1996 and January 1997) are compared with an 11.5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa. Carbon isotopic data of Hoeglundina elegans at 3010 m show 0.3 per mil lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This d13C change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of d13C in the pore waters within the phytodetritus and overlying the sediment surface. Carbon isotopic data of H. elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a d13C enrichment following the spring bloom. This different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the March 1996 cruise. The d13C of Uvigerina peregrina at 2430 m shows variation over the 10 month interval, but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes. The isotopic analyses are grouped into 100 µm size classes on the basis of length measurements of individual specimens to evaluate d13C ontogenetic changes of each species. The data show no consistent patterns between size classes in the d13C of either H. elegans or U. peregrina. These results suggest that variation in organic carbon flux does not preferentially affect particular size classes, nor do d13C ontogenetic changes exist within the >250 to >750 µm size range for these species at this locality. On the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct d13C in paleoceanographic studies. The prediction standard deviation, which is composed of cruise, core, subcore, and residual (replicate) variability, provides an estimate of the magnitude of variability in fossil d13C data; it is 0.27 per mil for H. elegans at 3010 m and 0.4 per mil for U. peregrina at the 2430 m site. Since these standard deviations are based on living specimens, they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses. Most paleoceanographic reconstructions are based on the analysis of multiple specimens, and as a result, the standard error would be expected to be reduced for any particular sample. The reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records.

Geochemistry and isotopic ratios of Neogene basalts from Spitzbergen Island, Svalbard archipelago

Petrological and geochemical data obtained on Neogene magmatism restricted to a deep fault in Andree Land at Spitsbergen Island, which was related to the overall restyling of the Arctic territory at 25-20 Ma, indicate that the derivation of the Neogene magmas was significantly affected by the continental pyroxenite mantle. The Neogene basalts are noted for a radiogenic isotopic composition of Pb (207Pb/204Pb= 15.5-15.55, 206Pb/204Pb = 18.4-18.6, 208Pb/204Pb = 38.4-38.6) and Sr (87Sr/86Sr = 0.7038-0.7048) at low 143Nd/144Nd = 0.5129. Melts of this type are the extremely enriched end member of the isotopic mixing of a depleted and enriched sources and determine a geochemical trend that passes through the compositions of alkaline magmas from Quaternary volcanoes at Spitsbergen and weakly enriched tholeiites of the Knipovich Ridge, which started to develop simultaneously with the onset of Neogene magmatism in the western part of Spitsbergen. The composition of the liquidus olivine (which is rich in NiO) indicates that melting occurred in the olivine-free mantle. Our data thus testify that a significant role in the genesis of the Neogene magmas was played by continental pyroxenite mantle.

(Table 1) Composition of interstitial waters from ODP Leg 125 sites

Pore waters were collected from nine sites during Leg 125 of the Ocean Drilling Program (ODP). The first four sites (778-781) were drilled in the Mariana forearc on and near Conical Seamount, an active serpentine "mud volcano" located about 80 km behind the trench axis and 120 km in front of the active island arc. The last five sites (782-786) were drilled in the Izu-Bonin forearc between the trench and the outer arc high. Pore waters from the five sites from both areas that penetrated serpentine silts (Sites 778,779,780,783, and 784) are discussed in detail by Mottl (this volume). Here we report analyses of the pore waters from all nine sites for Li, Rb, Sr, Ba, Mn, B, and the sulfur isotopic ratio of dissolved sulfate. Sampling methods and results of analyses for major and minor species determined aboard ship were presented by Fryer, Pearce, Stokking, et al. (1990, doi:10.2973/odp.proc.ir.125.1990).

Calculation of the oxygen isotope discrimination factor for studying plant respiration

Measurement of discrimination against 18O during dark respiration in plants is currently accepted as the only reliable method of estimating the partitioning of electrons between the cytochrome and alternative pathways. In this paper, we review the theory of the technique and its application to a gas-phase system. We extend it to include sampling effects and show that the isotope discrimination factor, D, is calculated as –dln(1 + δ)/dlnO*, where δ is isotopic composition of the substrate oxygen and O*=[O2]/[N2] in a closed chamber containing tissue respiring in the dark. It is not necessary to integrate the expression but, if the integrated form is used, the resultant regression should not be constrained through the origin. This is important since any error in D will have significant effects on the estimation of the flux of electrons through the two pathways.

Determination of management units for grey mackerel fisheries in northern Australia.

The requirement for Queensland, Northern Territory and Western Australian jurisdictions to ensure sustainable harvest of fish resources and their optimal use relies on robust information on the resource status. For grey mackerel (Scomberomorus semifasciatus) fisheries, each of these jurisdictions has their own management regime in their corresponding waters. The lack of information on stock structure of grey mackerel, however, means that the appropriate spatial scale of management is not known. As well, fishers require assurance of future sustainability to encourage investment and long-term involvement in a fishery that supplies lucrative overseas markets. These management and fisher-unfriendly circumstances must be viewed in the context of recent 3-fold increases in catches of grey mackerel along the Queensland east coast, combined with significant and increasing catches in other parts of the species' northern Australian range. Establishing the stock structure of grey mackerel would also immensely improve the relevance of resource assessments for fishery management of grey mackerel across northern Australia. This highlighted the urgent need for stock structure information for this species. The impetus for this project came from the strategic recommendations of the FRDC review by Ward and Rogers (2003), "Northern mackerel (Scombridae: Scomberomorus): current and future research needs" (Project No. 2002/096), which promoted the urgency for information on the stock structure of grey mackerel. In following these recommendations this project adopted a multi-technique and phased sampling approach as carried out by Buckworth et al (2007), who examined the stock structure of Spanish mackerel, Scomberomorus commerson, across northern Australia. The project objectives were to determine the stock structure of grey mackerel across their northern Australian range, and use this information to define management units and their appropriate spatial scales. We used multiple techniques concurrently to determine the stock structure of grey mackerel. These techniques were: genetic analyses (mitochondrial DNA and microsatellite DNA), otolith (ear bones) isotope ratios, parasite abundances, and growth parameters. The advantage of using this type of multi-technique approach was that each of the different methods is informative about the fish’s life history at different spatial and temporal scales. Genetics can inform about the evolutionary patterns as well as rates of mixing of fish from adjacent areas, while parasites and otolith microchemistry are directly influenced by the environment and so will inform about the patterns of movement during the fishes lifetime. Growth patterns are influenced by both genetic and environmental factors. Due to these differences the use of these techniques concurrently increases the likelihood of detecting different stocks where they exist. We adopted a phased sampling approach whereby sampling was carried out at broad spatial scales in the first year: east coast, eastern Gulf of Carpentaria (GoC), western GoC, and the NW Northern Territory (NW NT). By comparing the fish samples from each of these locations, and using each of the techniques, we tested the null hypothesis that grey mackerel were comprised of a single homogeneous population across northern Australia. Having rejected the null hypothesis we re-sampled the 1st year locations to test for temporal stability in stock structure, and to assess stock structure at finer spatial scales. This included increased spatial coverage on the east coast, the GoC, and WA. From genetic approaches we determined that there at least four genetic stocks of grey mackerel across northern Australia: WA, NW NT (Timor/Arafura), the GoC and the east Grey mackerel management units in northern Australia ix coast. All markers revealed concordant patterns showing WA and NW NT to be clearly divergent stocks. The mtDNA D-loop fragment appeared to have more power to resolve stock boundaries because it was able to show that the GoC and east coast QLD stocks were genetically differentiated. Patterns of stock structure on a finer scale, or where stock boundaries are located, were less clear. From otolith stable isotope analyses four major groups of S. semifasciatus were identified: WA, NT/GoC, northern east coast and central east coast. Differences in the isotopic composition of whole otoliths indicate that these groups must have spent their life history in different locations. The magnitude of the difference between the groups suggests a prolonged separation period at least equal to the fish’s life span. The parasite abundance analyses, although did not include samples from WA, suggest the existence of at least four stocks of grey mackerel in northern Australia: NW NT, the GoC, northern east coast and central east coast. Grey mackerel parasite fauna on the east coast suggests a separation somewhere between Townsville and Mackay. The NW NT region also appears to comprise a separate stock while within the GoC there exists a high degree of variability in parasite faunas among the regions sampled. This may be due to 1. natural variation within the GoC and there is one grey mackerel stock, or 2. the existence of multiple localised adult sub-stocks (metapopulations) within the GoC. Growth parameter comparisons were only possible from four major locations and identified the NW NT, the GoC, and the east coast as having different population growth characteristics. Through the use of multiple techniques, and by integrating the results from each, we were able to determine that there exist at least five stocks of grey mackerel across northern Australia, with some likelihood of additional stock structuring within the GoC. The major management units determined from this study therefore were Western Australia, NW Northern Territory (Timor/Arafura), the Gulf of Carpentaria, northern east Queensland coast and central east Queensland coast. The management implications of these results indicate the possible need for management of grey mackerel fisheries in Australia to be carried out on regional scales finer than are currently in place. In some regions the spatial scales of management might continue as is currently (e.g. WA), while in other regions, such as the GoC and the east coast, managers should at least monitor fisheries on a more local scale dictated by fishing effort and assess accordingly. Stock assessments should also consider the stock divisions identified, particularly on the east coast and for the GoC, and use life history parameters particular to each stock. We also emphasise that where we have not identified different stocks does not preclude the possibility of the occurrence of further stock division. Further, this study did not, nor did it set out to, assess the status of each of the stocks identified. This we identify as a high priority action for research and development of grey mackerel fisheries, as well as a management strategy evaluation that incorporates the conclusions of this work. Until such time that these priorities are addressed, management of grey mackerel fisheries should be cognisant of these uncertainties, particularly for the GoC and the Queensland east coast.

Environmental and climatic dependences of stable isotope ratios in tree rings on different temporal scales

This work examines stable isotope ratios of carbon, oxygen and hydrogen in annual growth rings of trees. Isotopic composition in wood cellulose is used as a tool to study past climate. The method benefits from the accurate and precise dating provided by dendrochronology. In this study the origin, nature and the strength of climatic correlations are studied on different temporal scales and at different sites in Finland. The origin of carbon isotopic signal is in photosynthetic fractionation. The basic physical and chemical fractionations involved are reasonably well understood. This was confirmed by measuring instantaneous photosynthetic discrimination on Scots pine (Pinus sylvestris L.). The internal conductance of CO2 was recognized to have a significant impact on the observed fractionation, and further investigations are suggested to quantify its role in controlling the isotopic signal of photosynthates. Isotopic composition of the produced biomass can potentially be affected by variety of external factors that induce physiological changes in trees. Response of carbon isotopic signal in tree ring cellulose to changes in resource availability was assessed in a manipulation experiment. It showed that the signal was relatively stable despite of changes in water and nitrogen availability to the tree. Palaeoclimatic reconstructions are typically based on functions describing empirical relationship between isotopic and climatic parameters. These empirical relationships may change depending on the site conditions, species and timeframe studied. Annual variation in Scots pine tree ring carbon and oxygen isotopic composition was studied in northern and in central eastern Finland and annual variation in tree ring latewood carbon, oxygen and hydrogen isotopic ratio in Oak (Quercus robur L.) was studied in southern Finland. In all of the studied sites at least one of the studied isotope ratios was shown to record climate strongly enough to be used in climatic reconstructions. Using the observed relationships, four-century-long climate reconstructions from living Scots pine were created for northern and central eastern Finland. Also temporal stability of the relationships between three proxy indicators, tree ring growth and carbon and oxygen isotopic composition was studied during the four-hundred-year period. Isotope ratios measured from tree rings in Finland were shown to be sensitive indicators of climate. Increasing understanding of environmental controls and physiological mechanisms affecting tree ring isotopic composition will make possible more accurate interpretation of isotope data. This study also demonstrated that by measuring multiple isotopes and physical proxies from the same tree rings, additional information on tree physiology can be obtained. Thus isotopic ratios measured from tree ring cellulose provide means to improve the reliability of climate reconstructions.

Diurnal variation of atmospheric CO2 concentration and delta C-13 in an urban atmosphere during winter-role of the Nocturnal Boundary Layer

The paper presents the importance of the Nocturnal Boundary Layer in driving the diurnal variability of the atmospheric CO2 mixing ratio and the carbon isotope ratio at ground level from an urban station in India. Our observations are the first of their kind from this region. The atmospheric CO2 mixing ratio and the carbon isotopic ratio were measured for both the morning (05:30-07:30 IST) and afternoon time (16:00-18:00 IST) air samples at 5 m above ground level in Bangalore city, Karnataka State (12 degrees 58' N, 77 degrees 38' E, masl = 920 m) for a 10 day period during the winter of 2008. We observed a change of similar to 7% the in CO2 mixing ratio between the morning and afternoon time air samples. A stable isotope analysis of CO2 from morning samples showed a depletion in the carbon isotope ratio by similar to 2 parts per thousand compared to the afternoon samples. Along with the ground-based measurement of air samples, data of radiosonde measurements were also obtained from the Indian Meteorological Department to identify the vertical atmospheric structure at different time in a day. We proposed the presence or absence of the NBL as a controlling factor for the observed variability in the mixing ratio as well as its isotopic composition. Here we used the Keeling model approach to find out the carbon isotope ratio for the local sources. The local sources have further been characterized as anthropogenic and biological respiration (in %) using a two-component mixing model. We also used a vertical mixing model based on the concept of the mixing of isotopically depleted (carbon isotope) ``polluted air'' (PA) with isotopically enriched ``free atmospheric air'' (FA) above. Using this modeling approach, the contribution of FA at ground level is being estimated for both the morning and afternoon time air samples.

Tracing the source of bottled water using stable isotope techniques

The Indian Summer Monsoon (ISM) precipitation recharges ground water aquifers in a large portion of the Indian subcontinent. Monsoonal precipitation over the Indian region brings moisture from the Arabian Sea and the Bay of Bengal (BoB). A large difference in the salinity of these two reservoirs, owing to the large amount of freshwater discharge from the continental rivers in the case of the BoB and dominating evaporation processes over the Arabian Sea region, allows us to distinguish the isotopic signatures in water originating in these two water bodies. Most bottled water manufacturers exploit the natural resources of groundwater, replenished by the monsoonal precipitation, for bottling purposes. The work presented here relates the isotopic ratios of bottled water to latitude, moisture source and seasonality in precipitation isotope ratios. We investigated the impact of the above factors on the isotopic composition of bottled water. The result shows a strong relationship between isotope ratios in precipitation (obtained from the GNIP data base)/bottled water with latitude. The approach can be used to predict the latitude at which the bottled water was manufactured. The paper provides two alternative approaches to address the site prediction. The limitations of this approach in identifying source locations and the uncertainty in latitude estimations are discussed. Furthermore, the method provided here can also be used as an important forensic tool for exploring the source location of bottled water from other regions. Copyright (C) 2011 John Wiley & Sons, Ltd.

Precision and long-term stability of clumped-isotope analysis of CO2 using a small-sector isotope ratio mass spectrometer

RATIONALE The ratio of the measured abundance of 13C18O bonding CO2 to its stochastic abundance, prescribed by the delta 13C and delta 18O values from a carbonate mineral, is sensitive to its growth temperature. Recently, clumped-isotope thermometry, which uses this ratio, has been adopted as a new tool to elucidate paleotemperatures quantitatively. METHODS Clumped isotopes in CO2 were measured with a small-sector isotope ratio mass spectrometer. CO2 samples digested from several kinds of calcium carbonates by phosphoric acid at 25 degrees C were purified using both cryogenic and gas-chromatographic separations, and their isotopic composition (delta 13C, delta 18O, Delta 47, Delta 48 and Delta 49 values) were then determined using a dual-inlet Delta XP mass spectrometer. RESULTS The internal precisions of the single gas Delta 47 measurements were 0.005 and 0.02 parts per thousand (1 SE) for the optimum and the routine analytical conditions, respectively, which are comparable with those obtained using a MAT 253 mass spectrometer. The long-term variations in the Delta 47 values for the in-house working standard and the heated CO2 gases since 2007 were close to the routine, single gas uncertainty while showing seasonal-like periodicities with a decreasing trend. Unlike the MAT 253, the Delta XP did not show any significant relationship between the Delta 47 and delta 47 values. CONCLUSIONS The Delta XP gave results that were approximately as precise as those of the MAT 253 for clumped-isotope analysis. The temporal stability of the Delta XP seemed to be lower, although an advantage of the Delta XP was that no dependency of delta 47 on Delta 47 was found. Copyright (c) 2012 John Wiley & Sons, Ltd.

The variations and climatic significance of D/H ratios in the carbon-bound hydrogen of cellulose in trees

The σD values of nitrated cellulose from a variety of trees covering a wide geographic range have been measured. These measurements have been used to ascertain which factors are likely to cause σD variations in cellulose C-H hydrogen.

It is found that a primary source of tree σD variation is the σD variation of the environmental precipitation. Superimposed on this are isotopic variations caused by the transpiration of the leaf water incorporated by the tree. The magnitude of this transpiration effect appears to be related to relative humidity.

Within a single tree, it is found that the hydrogen isotope variations which occur for a ring sequence in one radial direction may not be exactly the same as those which occur in a different direction. Such heterogeneities appear most likely to occur in trees with asymmetric ring patterns that contain reaction wood. In the absence of reaction wood such heterogeneities do not seem to occur. Thus, hydrogen isotope analyses of tree ring sequences should be performed on trees which do not contain reaction wood.

Comparisons of tree σD variations with variations in local climate are performed on two levels: spatial and temporal. It is found that the σD values of 20 North American trees from a wide geographic range are reasonably well-correlated with the corresponding average annual temperature. The correlation is similar to that observed for a comparison of the σD values of annual precipitation of 11 North American sites with annual temperature. However, it appears that this correlation is significantly disrupted by trees which grew on poorly drained sites such as those in stagnant marshes. Therefore, site selection may be important in choosing trees for climatic interpretation of σD values, although proper sites do not seem to be uncommon.

The measurement of σD values in 5-year samples from the tree ring sequences of 13 trees from 11 North American sites reveals a variety of relationships with local climate. As it was for the spatial σD vs climate comparison, site selection is also apparently important for temporal tree σD vs climate comparisons. Again, it seems that poorly-drained sites are to be avoided. For nine trees from different "well-behaved" sites, it was found that the local climatic variable best related to the σD variations was not the same for all sites.

Two of these trees showed a strong negative correlation with the amount of local summer precipitation. Consideration of factors likely to influence the isotopic composition of summer rain suggests that rainfall intensity may be important. The higher the intensity, the lower the σD value. Such an effect might explain the negative correlation of σD vs summer precipitation amount for these two trees. A third tree also exhibited a strong correlation with summer climate, but in this instance it was a positive correlation of σD with summer temperature.

The remaining six trees exhibited the best correlation between σD values and local annual climate. However, in none of these six cases was it annual temperature that was the most important variable. In fact annual temperature commonly showed no relationship at all with tree σD values. Instead, it was found that a simple mass balance model incorporating two basic assumptions yielded parameters which produced the best relationships with tree σD values. First, it was assumed that the σD values of these six trees reflected the σD values of annual precipitation incorporated by these trees. Second, it was assumed that the σD value of the annual precipitation was a weighted average of two seasonal isotopic components: summer and winter. Mass balance equations derived from these assumptions yielded combinations of variables that commonly showed a relationship with tree σD values where none had previously been discerned.

It was found for these "well-behaved" trees that not all sample intervals in a σD vs local climate plot fell along a well-defined trend. These departures from the local σD VS climate norm were defined as "anomalous". Some of these anomalous intervals were common to trees from different locales. When such widespread commonalty of an anomalous interval occurred, it was observed that the interval corresponded to an interval in which drought had existed in the North American Great Plains.

Consequently, there appears to be a combination of both local and large scale climatic information in the σD variations of tree cellulose C-H hydrogen.

Observations of hydrogen and helium isotopes in solar cosmic rays

The isotopic composition of hydrogen and helium in solar cosmic rays provides a means of studying solar flare particle acceleration mechanisms since the enhanced relative abundance of rare isotopes, such as ²H, ³H and ³He, is due to their production by inelastic nuclear collisions in the solar atmosphere during the flare. In this work the Caltech Electron/Isotope Spectrometer on the IMP-7 spacecraft has been used to measure this isotopic composition. The response of the dE/dx-E particle telescope is discussed and alpha particle channeling in thin detectors is identified as an important background source affecting measurement of low values of (³He/⁴He).

The following flare-averaged results are obtained for the period, October, 1972 - November, 1973: (²H/¹H) = 7⁺¹⁰_-6 X 10^-6 (1.6 - 8.6 MeV/nuc), (³H/¹H) less than 3.4 x 10^-6 (1.2 - 6.8 MeV/nuc), (³He/⁴He) = (9 ± 4) x 10^-3, (³He/¹H) = (1.7 ± 0.7) x 10^-4 (3.1 - 15.0 MeV/nuc). The deuterium and tritium ratios are significantly lower than the same ratios at higher energies, suggesting that the deuterium and tritium spectra are harder than that of the protons. They are, however, consistent with the same thin target model relativistic path length of ~ 1 g/cm² (or equivalently ~ 0.3 g/cm² at 30 MeV/nuc) which is implied by the higher energy results. The ³He results, consistent with previous observations, would imply a path length at least 3 times as long, but the observations may be contaminated by small ³He rich solar events.

During 1973 three "³He rich events," containing much more ³He than ²H or ³H were observed on 14 February, 29 June and 5 September. Although the total production cross sections for ²H,³H and ³He are comparable, an upper limit to (²H/³He) and (³H/³He) was 0.053 (2.9-6.8 MeV/nuc), summing over the three events. This upper limit is marginally consistent with Ramaty and Kozlovsky's thick target model which accounts for such events by the nuclear reaction kinematics and directional properties of the flare acceleration process. The 5 September event was particularly significant in that much more ³He was observed than ⁴He and the fluxes of ³He and ¹H were about equal. The range of (³He/⁴He) for such events reported to date is 0.2 to ~ 6 while (³He/¹H) extends from 10^-3 to ~ 1. The role of backscattered and mirroring protons and alphas in accounting for such variations is discussed.

Observations of nitrogen and oxygen isotopes in the low energy cosmic rays

The isotopic composition of the enhanced low energy nitrogen and oxygen cosmic rays can provide information regarding the source of these particles. Using the Caltech Electron/Isotope Spectrometer aboard the IMP-7 satellite, a measurement of this isotopic composition was made. To determine the isotope response of the instrument, a calibration was performed, and it was determined that the standard range-energy tables were inadequate to calculate the isotope response. From the calibration, corrections to the standard range-energy tables were obtained which can be used to calculate the isotope response of this and similar instruments.

The low energy nitrogen and oxygen cosmic rays were determined to be primarily ^(14)N and ^(16)O. Upper limits were obtained for the abundances of the other stable nitrogen and oxygen isotopes. To the 84% confidence level the isotopic abundances are: ^(15)N/N ≤ 0.26 (5.6- 12.7 MeV/nucleon), ^(17)0/0 ≤ 0.13 (7.0- 11.8 MeV/nucleon), (18)0/0 ≤ 0.12 (7.0 - 11.2 MeV/nucleon). The nitrogen composition differs from higher energy measurements which indicate that ^(15)N, which is thought to be secondary, is the dominant isotope. This implies that the low energy enhanced cosmic rays are not part of the same population as the higher energy cosmic rays and that they have not passed through enough material to produce a large fraction of ^(15)N. The isotopic composition of the low energy enhanced nitrogen and oxygen is consistent with the local acceleration theory of Fisk, Kozlovsky, and Ramaty, in which interstellar material is accelerated to several MeV/nucleon. If, on the other hand, the low energy nitrogen and oxygen result from nucleosynthesis in a galactic source, then the nucleosynthesis processes which produce an enhancement of nitrogen and oxygen and a depletion of carbon are restricted to producing predominantly ^(14)N and ^(16)O.