Distinct Isotope Ratios Research Articles

Contemporaneous formation of chondrules in distinct oxygen isotope reservoirs

We investigated the 26Al–26Mg systematics of type I (FeO-poor) chondrules from one of the most primitive carbonaceous chondrites, Acfer 094. The inferred initial 26Al/27Al ratios of chondrules from Acfer 094 ranged from (4.2±2.0)×10−6 to (9.0±1.5)×10−6. These chondrules have distinct oxygen isotope ratios (Δ17O: −5‰, −2‰, and 0‰), though no correlation between inferred initial 26Al/27Al ratios and oxygen isotope ratios were observed. These results indicate the regional oxygen isotope heterogeneity in the solar nebula over the duration of chondrule formation.A few low-Ca pyroxene grains in chondrules have small but resolvable 26Mg excesses at the level of ∼0.1‰, which may be the result of a partial melting of chondrules by reheating events. If this is the case, the true inferred initial 26Al/27Al ratios of these particular chondrule could be slightly lower (<10%) than estimates using Mg isotope ratios of coexisting plagioclase and olivine.Stable isotope ratios of Mg (25Mg/24Mg) of many olivine grains in chondrules are positively fractionated by ∼1‰ relative to those in other phases within the same chondrules. This suggests that olivine preserved a positively fractionated Mg isotope ratios of the chondrule-forming melt prior to significant condensation during the cooling stage of chondrule formation.

Explaining the Phanerozoic Ca isotope history of seawater

A new geochemical budget for the modern marine carbonate sink helps to explain the major features of the Phanerozoic Ca isotope record. A large compilation of Ca isotope ratios for modern carbonates, incorporating more than 50 new measurements, represents the quantitatively important components of the system. With this data set, distinct Ca isotope ratios are identified for different types of marine carbonate, the balance of which has changed over time with shifts between calcite and aragonite seas and with the development of pelagic calcification during the Mesozoic. It is suggested that large-scale changes in the Ca isotope ratio of seawater, as exemplified by that in the Carboniferous, were no longer possible after Jurassic time because of the generation of a deep-sea calcite sink expressed by deposition of foraminiferal–coccolith ooze across the world ocean. This work demonstrates the close connection between isotopic cycling, carbonate sedimentation, seawater chemistry, and evolutionary trends.

Ion microprobe analyses of oxygen three‐isotope ratios of chondrules from the Sayh al Uhaymir 290 CH chondrite using a multiple‐hole disk

Abstract– The ion microprobe is the only technique capable of determining high‐precision stable isotope ratios in individual tiny extraterrestrial particles (≤100 μm in diameter), but these small samples present special analytical challenges. We produced a new sample holder disk with multiple holes (three holes and seven holes), in which epoxy disks containing a single unknown sample and a standard grain are cast and polished. Performance tests for oxygen two‐isotope analyses using San Carlos olivine standard grains show that the new multiple‐hole disks allow accurate analysis of tiny particles if the particles are located within the 500 μm and 1 mm radius of the center of holes for seven‐hole and three‐hole disks, respectively. Using the new seven‐hole disk, oxygen three‐isotope ratios of eight magnesian cryptocrystalline chondrules (approximately 100 μm in diameter) from the Sayh al Uhaymir (SaU) 290 CH chondrite were analyzed by ion microprobe at the University of Wisconsin. Five out of eight chondrules have nearly identical oxygen isotope ratios (Δ17O = −2.2 ± 0.6‰; 2SD), which is consistent with those of magnesian cryptocrystalline chondrules in CH/CB and CB chondrites, suggesting a genetic relationship, i.e., formation by a common (possibly impact) heating event. The other three chondrules have distinct oxygen isotope ratios (Δ17O values from −6.4‰ to +2.2‰). Given that similar variation in Δ17O values was observed in type I porphyritic chondrules in a CH/CB chondrite, the three chondrules may have formed in the solar nebula, similar to the type I porphyritic chondrules.

Evaluation of a stable-isotope labelling technique for mass marking fin rays of age-0 lake sturgeon

The effectiveness of marking age-0 lake sturgeon, Acipenser fulvescens Rafinesque, pectoral fin rays with a stable strontium isotope was evaluated. Age-0 lake sturgeon were reared in water spiked with 0 (control), 25, 50 or 100 μg L−1 86SrC03 for 10 and 24 days; fish from each treatment group were retained for up to 120 days post-labelling to assess mark retention. Enriched-isotope marks imparted to fin rays were distinct from fin ray 88Sr/86Sr ratios of control fish immediately following marking, with the 100 μg L−1 86SrCO3 treatments consistently yielding the highest rate of marking success (83–92%). Lower marking success (25–69%) was observed with the 25 and 50 μg L−1 86SrCO3 treatments. Isotopic marks in fin rays were retained for 120 days post-labelling. Immersion marking of juvenile fish pectoral fin rays with distinct strontium isotope ratios is possible and does not require sacrificing fish to check for marks.

Stable Isotopes Trace Estuarine Transformations of Carbon and Nitrogen from Primary- and Secondary-Treated Paper and Pulp Mill Effluent

Stable isotope analysis of a novel combination of carbon and nitrogen pools traced inputs and processing of primary-treated (PE) and secondary-treated effluent (SE) from a paper and pulp mill (PPM) in a temperate Australian estuary. Distinct carbon stable isotope ratios of dissolved organic carbon (DOC) near the PPM outfall indicated large PE and reduced SE inputs of DOC. DOC was remineralized to dissolved inorganic carbon regardless of season, but rates were lower in winter. PE discharge in winter elevated DOC concentrations along much of the estuary. Distinct stable isotope ratios confirmed particulate organic matter (POM) input from PE and SE to the water column and into the sediment. This was relatively localized, indicating rapid POM settlement regardless of season. SE discharge increased nutrient inputs and enhanced algal productivity, particularly in summer when chlorophyll-a concentrations were elevated throughout the estuary. SE discharge reduced pCO(2) from levels associated with PE discharge. However, the estuary remained heterotrophic as subsequent respiration or decomposition of algal material offset reductions in PPM organic matter input. The influence of the PPM was apparent throughout the estuary, demonstrating the ability of anthropogenic inputs, and changes to these, to affect ecosystem functioning.

Crystallization Dynamics of Granite Magma Chambers in the Absence of Regional Stress: Multiphysics Modeling with Natural Examples

Numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection, during which wall-rock contamination and magma mixing are possible, followed by a long period of no convective cooling, during which melt segregation occurs. Convection is driven by the negative density gradient generated in the upper cooling zone by melt-to-solid phase transformation. Convection breaks the upper mushy zone and drags the fragments downwards with descending Rayleigh^Taylor fingers. Such fragments can be preserved as microgranular enclaves. The descending Rayleigh^Taylor fingers split low aspect-ratio (sill-like) magma chambers into nearly isolated convection cells. If the magma is initially heterogeneous, this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios, and finally results in a pluton with marked lateral compositional variations, easily misinterpreted as different intrusive batches. Convective heat-loss quickly leads most of the magma chamber to critical crystallinity, independently of the vertical coordinate, so that a chamber-wide three-dimensional skeleton of crystals with uniform initial porosity c .0 ·4^0·5 is formed. This configuration is gravitationally unstable; therefore, it spontaneously compacts towards an equilibrium vertical variation of porosity approaching Atty’s Law. In the absence of regional stress, the upwards migration of the inter-crystalline melts, as a result of compaction, is the most effective way of melt^solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates. Granite magma chambers fractionated by these mechanisms will produce short-range differentiation series, from a composition slightly less silicic than the initial magma to high-silica segregates. In the presence of regional stress, tectonic squeezing and shearing during the post-convective stage can expel residual fluid more efficiently and lead to wide-range granite differentiation series, from rocks notably less silicic than the initial magma to high-silica leucogranites.

Isotope ratios of lead in Japanese women’s hair of the twentieth century

Isotope ratios of lead ((207)Pb/(206)Pb and (208)Pb/(206)Pb) in Japanese women's hair of the twentieth century were measured to evaluate lead contamination of human proximate environment of those days. The historic hair samples (n = 40) were collected in 1980s by staffs of Department of Human Ecology, University of Tokyo, from Japanese women who cut their hair in 1910s to 1968 by themselves for hair piece or needle pad and who had stored it by the time of collection. Additional five contemporary hair samples (one from male and four from females) were also included. The hair samples were digested with HNO(3), diluted with water to Pb concentration of 10 microg/kg, and isotope ratios were measured by inductively coupled plasma mass spectrometry. Isotope ratios as well as Pb concentration in the hair samples of the twentieth century varied to a considerable extent depending on the period of hair cut. The oldest hair samples (1910-1920s) had the highest concentration and the most distinct isotope ratios from those of Japanese domestic Pb indicating serious contamination of proximate environment of people of those days with Pb originated from mines in other regions of the world, probably through the usage of Pb-containing face powder. The concentration and isotope ratios of Pb decreased thereafter indicating less serious contamination of proximate environment of Japanese which, in turn, should have affected by environmental contamination of mid-twentieth century due to industrial activities and leaded gasoline. Contemporary Pb level was quite low (<1 mg/kg); however, isotope ratios were close to those in 1960s when hair concentration was assumed to be strongly affected by leaded gasoline.

Neodymium isotopes in Archean seawater and implications for the marine Nd cycle in Earth's early oceans

Published neodymium (Nd) isotopic data for Archean iron-formations (IF) suggest that, overall, seawater throughout the Archean typically displayed 143Nd/ 144Nd close to bulk Earth values, with Є Nd( t) between − 1.5 and + 2.5. Neodymium isotopic ratios in seawater during deposition of the ~ 3.8 Isua (Greenland) IF likely displayed positive Є Nd(3.8 Ga) of + 2.5, as suggested by IF-G, an Isua reference IF that is considered the best archive for Early Archean seawater. Seawater 143Nd/ 144Nd ratios dominated by radiogenic Nd (positive Є Nd( t)) seem to have persisted for much of the Archean, as IF from the Pietersburg greenstone belt, South Africa, suggest seawater Є Nd(2.95 Ga) ≥ + 1. However, similarly aged (~ 2.9 Ga) IFs from South Africa indicate that significant variations in seawater 143Nd/ 144Nd occurred, and clearly show influences from isotopically distinct crustal sources. These variations are apparently related to depositional environment, with cratonic margin, shallow-water IFs possessing a continental Є Nd( t) of − 3, while IFs associated with sub-aqueous mafic volcanics display more radiogenic, positive Є Nd( t) values. Such variation in seawater 143Nd/ 144Nd is not possible in an isotopically well-mixed ocean, and similar to today, it appears that marine Nd cycling in the Archean produced water masses with distinct Nd isotopic ratios. Since the presence of banded iron-formations requires a reducing Archean ocean capable of transporting Fe, metal-oxide precipitation and scavenging processes near deep sea hydrothermal vent systems would not have scavenged mantle Nd, i.e., Nd sourced from alteration of oceanic crust. We propose that bulk anoxic seawater prior to 2.7 Ga possessed relatively constant positive Є Nd( t) of + 1 to + 2, whereas local shallow-water masses associated with exposed evolved crust could possess distinctly different, lower Є Nd( t).

Hadrosaurid migration: inferences based on stable isotope comparisons among Late Cretaceous dinosaur localities

Stable carbon and oxygen isotope ratios were measured for carbonate in samples of hadrosaurid tooth enamel and dentine, and gar scale ganoine and dentine from five geologically “contemporaneous“ (two-million-year resolution) and geographically distant late Campanian formations (Two Medicine, Dinosaur Park, Judith River, Kaiparowits, and Fruitland) in the Western Interior Basin. In all cases, isotopic offsets were observed between enamel and dentine from the same teeth, with dentine being characterized by higher and more variable carbon and oxygen isotope ratios. Isotopic offsets were also observed between gar ganoine and hadrosaur enamel in all sites analyzed. Both of these observations indicate that diagenetic overprinting of enamel isotope ratios did not entirely obfuscate primary signals. Decreases in carbon and oxygen isotope ratios were observed in hadrosaur enamel from east to west, and overlap in isotope ratios occurred only between two of the sampled sites (Dinosaur Park and Judith River Formations).The lack of isotopic overlap for enamel among localities could be due to diagenetic resetting of isotope ratios such that they reflect local groundwater effects rather than primary biogenic inputs. However, the large range in carbon isotope ratios, the consistent taxonomic offsets for enamel/ganoine data, and comparisons of enamel-dentine data from the same teeth all suggest that diagenesis is not the lone driver of the signal. In the absence of major alteration, the mostly likely explanation for the isotopic patterns observed is that hadrosaurids from the targeted formations were eating plants and drinking waters with distinct isotopic ratios. One implication of this reconstruction is that hadrosaurids in the Late Cretaceous of the Western Interior did not migrate to an extent that would obscure local isotopic signatures.

Reconstructing Neolithic groups in Sarawak, Malaysia through lead and strontium isotope analysis

Heavy stable isotope analysis of mid-late Holocene Neolithic burials from Sarawak (Malaysia) identifies groups not apparent in mortuary treatment or inferred subsistence. Isotope ratios of strontium ( 87Sr/ 86Sr) and lead ( 208Pb/ 204Pb, 207Pb/ 204Pb, 206Pb/ 204Pb) from adult tooth enamel show distinct groups at Niah Cave's West Mouth organized by dietary catchment. Two roughly contemporary sites, Gua Sireh and Lobang Angin, are included to assess regional heterogeneity in northern Borneo. Heavy stable isotope ratios for humans differ from sampled modern plants and archaeological fauna from West Mouth, and suggest a non-local origin for some of the West Mouth human remains. Distinct heavy isotope ratios characterize all three sites and show regional differentiation broadly consistent with geological expectations. Results supplement paleodiet and morphological analyses and have implications for clarifying Niah Cave mortuary use during the mid-late Holocene. Further and more precise geographic placement will follow through additional landscape sampling for strontium and lead isotope values.

Distinctive stable isotope ratios in important zooplankton species in relation to estuarine salinity gradients: Potential tracer of fish migration

To assess the potential of stable isotope ratios as an indicator of fish migration within estuaries, stable isotope ratios in important zooplankton species were analyzed in relation to estuarine salinity gradients. Gut contents from migratory juveniles of the euryhaline marine fish Lateolabrax japonicus were examined along the Chikugo River estuary of the Ariake Sea, which has the most developed estuarine turbidity maximum (ETM) in Japan. Early juveniles in March and April preyed primarily on two copepod species; Sinocalanus sinensis at lower salinities and Acartia omorii at higher salinities. Late juveniles (standard length > 40 mm) at lower salinities preyed exclusively on the mysid Acanthomysis longirostris until July and complementarily on the decapod Acetes japonicus in August. These prey species were collected along the estuary during the spring–summer seasons of 2003 and 2004, and their carbon and nitrogen stable isotope ratios ( δ 13C and δ 15N) were evaluated. The δ 13C values of prey species were distinct from each other and were primarily depleted within and in close proximity to the ETM (salinity < 10); S. sinensis (−26.6‰) < Acanthomysis longirostris (−23.3‰) < Acartia omorii (−21.1‰) < Acetes japonicus (−18.5‰). The overall gradient of δ 13C with salinity occurred for all prey species and showed minor temporal fluctuations, while it was not directly influenced by the δ 13C values in particulate organic matter along the estuary. In contrast to δ 13C, the δ 15N values of prey species did not exhibit any clear relationship with salinity. The present study demonstrated that δ 13C has the potential for application as a tracer of fish migration into lower salinity areas including the ETM.

Identification of Pb sources in Yellow Sea sediments using stable Pb isotope ratios

Rapid economic developments in East Asian countries have inevitably resulted in environmental degradation in the surrounding seas, and concern for both the environment and protection from pollutants is increasing. Identification of sources of contaminants is essential to environmental pollution management. In this study, the provenance of anthropogenic lead (Pb), a major pollutant of Yellow Sea sediments, was determined for river mouth sediments, including those of the Changjiang, Huanghe, Han, and Geum Rivers, and for age-determined shelf core sediments through the measurement of Pb isotope ratios in the HCl-leached fraction using multi-collector inductively coupled plasma-mass spectrometry (MC ICP/MS). Anthropogenic Pb has accumulated in shelf core sediments since 1910, and its isotope ratios were estimated as 0.863–0.866 and 2.119–2.125 for 207Pb/ 206Pb and 208Pb/ 206Pb, respectively, from the mixing relationships of the two endmembers. River mouth sediments exhibited enough distinction in anthropogenic Pb isotope ratios to be discriminated: 0.874 (2.144) in the Huanghe, 0.856 (2.129) in the Han, 0.857 (2.122) in the Geum, and 0.854 (2.101) in the Changjiang for 207Pb/ 206Pb ( 208Pb/ 206Pb), respectively. Although isotope ratios of geogenic Pb in sediments dating before 1910 showed narrow ranges (0.842–0.845 and 2.088–2.100 for 207Pb/ 206Pb and 208Pb/ 206Pb, respectively), distinct isotope ratios in each core permitted source identification of sediments in the Yellow Sea based on geographic locations and the geogenic Pb of each river. By comparing the isotope ratios of the estimated anthropogenic Pb to source-related materials, the provenances of anthropogenic Pb in Chinese river sediments were presumed to be Chinese coal or ore, which is also a major source of atmospheric particulate Pb. The anthropogenic Pb in the shelf core sediments in the northern Yellow Sea originated from northern Chinese cities such as Beijing and Tianjin through atmospheric pathways. Pb isotope ratios indicated that Pb in Korean river sediments was characteristic of local Korean ores.

Mantle plumes and geochemistry

There is considerable interest in the extent to which mantle plumes exist, how many there may be, and how best they can be recognized. It has proved unexpectedly difficult to image them consistently from seismology, and it has been suggested that they may be recognized from the geochemistry of rocks erupted at the Earth's surface. The difficulty is that plumes are physical features, and they can only be identified geochemically if they are either derived from, or entrain material that has a distinctive geochemical fingerprint. Too often plumes have been invoked because the erupted basalts have a within plate signature, irrespective of whether the volumes of magma or the rates of eruption can be estimated. U-series isotopes in basalts are sensitive to variations in the upwelling rates of their mantle source rocks, and those can be used to invoke upward movements that may be associated with mantle plumes. A contribution from the Earth's core might provide a possible geochemical tracer of deep-seated mantle plumes, although it is difficult to assess whether core material may be readily entrained in the mantle. Given the marked compositional differences between the core and the mantle, it should be possible to identify material that contains a contribution from the core and hence may be inferred to be from the core–mantle boundary. The core should have high Os and W abundances, and distinctive isotope ratios. Arguments for the recognition of core material using Os and W isotopes are therefore reviewed, as are those relying on rare gases to identify contributions from the lower mantle.

Sulfur stable isotopes separate producers in marine food-web analysis.

Ecological applications of stable isotope analysis rely on different producers having distinct isotopic ratios to trace energy and nutrient transfer to consumers. Carbon (C) and nitrogen (N) are the usual elements analysed. We tested the hypothesis that producers unable to be separated using C and N would be separated by sulphur (S), by reviewing estuarine and marine food web studies using all three elements (total of 836 pairwise comparisons between producers). S had a wider range of values across all producers than C and N (S: 34.4, C: 23.3, N: 18.7 per thousand ), and a higher mean difference among producers (S: 9.3, C: 6.5, N: 3.3 per thousand ). We varied from 1 to 10 per thousand the distance producers must be apart to be considered separate. For each of these gap distances, S-separated producers tied on C and N in 40% or more of cases. Comparing the three elements individually, S had fewer tied pairs of producers for any gap distance than C or N. However, S also has higher within-producer variability. Statistical tests on simulated data showed that this higher variability caused S to be less effective than C for analysing differences among mean producer values, yet mixing models showed that S had the smallest confidence intervals around mean estimates of source contributions to consumers. We also examined the spatial and temporal scales over which S isotope signatures of the saltmarsh plant Spartina alterniflora varied. Differences between samples taken within tens of metres were smallest, but between samples hundreds of metres apart were as different as samples thousands of kilometres apart. The time between samples being taken did not influence S signatures. Overall, the use of S is recommended because it has a high probability of distinguishing the contribution of different producers to aquatic food webs. When two elements are employed, the combination of S and C separates more producers than any other combination.

Spatial variation in the [formula omitted] signature of Ruppia megacarpa (Mason) in coastal lagoons of southwestern Australia and its implication for isotopic studies

Ruppia megacarpa was collected from three coastal lagoons in southwestern Australia: Nornalup Inlet, Peel Inlet and Oyster Harbour. In each case, plants were sampled from four sites and δ 13 C values were determined for leaves, rhizomes and roots of replicate plants to assess the variability between lagoons, between sites in lagoons and between plant parts at sites. The δ 13 C values of tissue ranged from −6.6 to −17.8‰ with a mean of −11.15‰. In Nornalup Inlet, there were significant differences in the δ 13 C values of different plant parts, but there was also a significant interaction between plant part and site. In Peel Inlet significant differences were found between plant parts and between sites. Oyster Harbour displayed no significant differences in δ 13 C values between any of these factors. The δ 13 C values of R. megacarpa were highly variable, with only one other marine angiosperm having a higher recorded range in values. Significant differences in δ 13 C values occurred between sites within lagoons and between plant parts, and the pattern of spatial variability was different in each lagoon. The spatial differences within lagoons appear to relate to differences in water quality resulting from proximity to river and ocean inputs. Differences between plant parts may be related to the storage of biochemical products with distinct isotope ratios. Trophic studies, therefore, need to be conducted in a way that explicitly identifies the spatial scales that capture the variability in δ 13 C values relevant to the aims of the study.

The oxygen isotope composition of nitrate generated by nitrification in acid forest floors

The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ18O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ18O values of nitrate formed by microbial nitrification in acid forest floors.Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ18O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ18O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ18O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.

The purification of chlorins from marine particles and sediments for nitrogen and carbon isotopic analysis

Methods are presented for the separation of chlorins from marine particles and sediments for nitrogen and carbon isotopic analysis. Carbon and nitrogen purities of 91 and 96%, respectively, were obtained for particulate chlorins, with recoveries of 88%. Sedimentary chlorins were obtained with carbon and nitrogen purities of 88 and 89%, respectively, and recoveries of 18%. The reproducibility of carbon and nitrogen isotopic analyses on particulate and sedimentary chlorins was 0.2‰. The low recovery of chlorins from sediments is primarily a result of their complex distribution, requiring that only the one or two most abundant chlorins from a sample are purified. The procedure for particulate samples includes ultrasonic extraction with solvents, two phase separations, and reversed- and normal-phase high-performance liquid chromatographic separations (HPLC). The procedure for sedimentary chlorins includes ultrasonic extraction by solvent, solid-phase extraction on silica gel, reversed-phase HPLC, size-exclusion chromatography, and normal-phase HPLC. The purification procedures require ca. 4 h for particulate samples, and ca. 8 h for sediment samples when two samples are processed simultaneously. The application of these methods to surficial Black Sea sediments demonstrated that different chlorins have distinct nitrogen and carbon isotopic ratios. The δ 15N differences are interpreted in terms of changes in the seasonal flux of material out of the euphotic zone, while the δ 13C differences are thought to derive from the presence or absence of the chlorin phytyl side-chain.

Stable isotopes reveal strong marine and El Niño effects on island food webs

Stable isotope analysis is a powerful tool for unravelling the complex structure of food webs1,2,3. This technique is particularly well suited for studies at ecosystem boundaries, where physical processes and mobile consumers link the dynamics of seemingly disparate systems4,5,6. In coastal and insular environments, seabirds play a crucial role in transporting marine-based energy and nutrients to islands7,8,9. Here we show using stable isotopes that nutrients from the ocean drive the dynamics of terrestrial food webs on small islands. The indirect effects of seabird-derived nutrients on plant productivity are particularly prominent during wet El Nino Southern Oscillation years on our Gulf of California study sites. During dry years that characterize the region, many terrestrial consumers are subsidized by carrion and prey from the ocean. Shifts in trophic structure related to El Nino Southern Oscillation could only be elucidated because of the distinct nitrogen isotope ratios associated with seabird islands. The contributions of seabirds and other marine sources are reflected in the isotope signatures of terrestrial consumers in ways that challenge conventional interpretations of stable isotope results in studies of food webs.

Rb−Sr and Sm−Nd isotopic study of the Seoul granitic batholith in middle Korea

We report Rb−Sr and Sm−Nd isotope data to better constrain the origin of the Seoul granitic batholith in Uijongbu-Dongduchon-Pocheon area. The dominant rocks of the Seoul granitic batholith intruding the Precambrian Gyonggi massif are biotite granite and garnet biotite granite. Rb−Sr whole rock data define three subparallel lines in an isochron diagram, which is related to geography rather than to lithology. Biotite granite and garnet biotite granite from the northeastern part of the study area give an isochron age of 170.0±5.2 Ma with an initial Sr isotopic ratio of 0.71669±0.00043, while those from the central part give an isochron age of 167±19 Ma with an initial ratio of 0.71280±0.00075. On the other hand, three biotite granites from the northern part, having low Rb/Sr ratio, plot below the isochron of the central samples with a similar slope (∼168 Ma) and have calculated initial Sr isotopic ratios of 0.7102–0.7109. Whole rock Sm−Nd data do not show isochron relationship, nor do they show distinct initial isotopic ratios for different areas as defined by initial Sr isotope ratio. Calculated eNd (T=170 Ma) values range from −15.6 to −19.9 and are generally higher than those of basement gneisses (an average of −23.4) around the batholith. These isotope data indicate that three distinct magmas with different Sr but similar Nd isotope ratios intruded during middle Jurassic time to form the Seoul batholith in the study area. The Sr and Nd isotope data from the basement rocks of the Gyeonggi massif suggest that the initial Sr and Nd isotope data of the Seoul batholith can be explained by mixing of three components: the mantle component and two crust components with high and low87Sr/86Sr, which rules out the possibility of its origin from direct partial melting of basement rocks, but requires significant mantle input. The isotope data also suggest that geochemical differences (MnO, Rb, Sc, Nb, Y, and Yb contents) between biotite granite and garnet biotite granite in our previous major and trace element study might be related to some sort of fractionation in each magma.

A mass‐ and isotope‐balance model of DOC mixing in estuaries

Expanding knowledge of degradation kinetics coupled with significant improvements in measurement precision of dissolved organic carbon (DOC) necessitates a review of factors that influence its cycling in estuaries. A one dimensional, time‐dependent estuarine model was used to ascertain how sinks and sources of DOC produced conservative and nonconservative property‐salinity distributions. DOC from riverine and coastal mixing members was assumed to have labile (k = 0.1 d−1) and refractory (k = 0.001 d−1) components. The magnitude of other inputs (i.e., marshes and phytoplankton) varied with respect to riverine loading (20 mol C s−1). Also, marsh (δ13C = − 12) and phytoplankton (δ13C = −20) DOC had distinct isotopic ratios compared with riverine (δ13C = −28) and coastal (δ13C = −23) DOC. Diffusive mixing times were modified by adjusting the dispersion coefficient (velocity was constant). As expected, conservative profiles were obtained when time‐scales (half‐life, turnover time) associated with sources and sinks were significantly greater than estuarine mixing times. Degradation of coastal DOC, however, had little impact on property‐salinity relationships at all mixing times studied. Moreover, nonconservative profiles were detected when at least 10% of riverine DOC was labile. In turn, the magnitude of marsh or phytoplankton inputs compared to riverine loading had a greater impact on property‐salinity distributions than estuarine residence time. Reexamination of literature data (Parker River and Ipswich River estuaries, Massachusetts) indicated that DOC concentration and isotope data can, coupled with this modeling effort, be used to estimate the magnitude and origin of DOC in estuaries.