Trends In Isotopic Composition Research Articles

Long-term ecosystem nitrogen limitation from foliar δ15 N data and a land surface model.

The effect of nutrient availability on plant growth and the terrestrial carbon sink under climate change and elevated CO2 remains one of the main uncertainties of the terrestrial carbon cycle. This is partially due to the difficulty of assessing nutrient limitation at large scales over long periods of time. Consistent declines in leaf nitrogen (N) content and leaf δ15 N have been used to suggest that nitrogen limitation has increased in recent decades, most likely due to the concurrent increase in atmospheric CO2 . However, such data sets are often not straightforward to interpret due to the complex factors that contribute to the spatial and temporal variation in leaf N and isotope concentration. We use the land surface model (LSM) QUINCY, which has the unique capacity to represent N isotopic processes, in conjunction with two large data sets of foliar N and N isotope content. We run the model with different scenarios to test whether foliar δ15 N isotopic data can be used to infer large-scale N limitation and if the observed trends are caused by increasing atmospheric CO2 , changes in climate or changes in sources and magnitude of anthropogenic N deposition. We show that while the model can capture the observed change in leaf N content and predict widespread increases in N limitation, it does not capture the pronounced, but very spatially heterogeneous, decrease in foliar δ15 N observed in the data across the globe. The addition of an observation-based temporal trend in isotopic composition of N deposition leads to a more pronounced decrease in simulated leaf δ15 N. Our results show that leaf δ15 N observations cannot, on their own, be used to assess global-scale N limitation and that using such a data set in conjunction with an LSM can reveal the drivers behind the observed patterns.

Potential of Clumped Isotopes in Constraining the Global Atmospheric Methane Budget

AbstractAtmospheric methane (CH4) and its isotopic composition trends over the last decades are explained by various flux scenarios, from tropical wetland emission increases through to reductions in global hydroxyl (OH). In this study, we develop a modeling framework to assess the potential usefulness of clumped isotope measurements to distinguish between the main drivers of change in the CH4 burden. We model interhemispheric differences of 0.12‰ and 0.38‰ and seasonal cycles of 0.02‰–0.04‰ and 0.21‰–0.32‰ for Δ13CH3D and Δ12CH2D2, respectively, which is insignificant relative to the uncertainty of measurements that could eventually be made. We show, however, that measurements of Δ12CH2D2 specifically could provide constraints for understanding trends in the global total source and sink magnitudes, which has not been possible with the current sets of observables. Changes in OH concentration of 10% developed across three decades result in a difference of up to 2‰ in Δ12CH2D2, which would be observable given current measurement uncertainty limits. For this type of global‐scale analysis, we show that measurements of Δ13CH3D would be unlikely to provide additional useful information. We suggest an emphasis should now be on developing the methods to make measurements from ambient air samples, followed by measurements of Δ13CH3D and Δ12CH2D2 from sampling at clean Northern and Southern Hemisphere sites, combined with more accurate and precise laboratory measurements of the clumped kinetic isotope effects relevant for the atmospheric sinks.

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400–16,000 mg/L), Br (6–70 mg/L), I (0.4–72 mg/L), Sr (0.1–200 mg/L), B (3–130 mg/L), Li (0.1–13 mg/L), and Na (33–6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ37Cl, δ11B, and δ7Li values range from −1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and −3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ37Cl = 0‰; δ11B = 40‰) and sedimentary pore fluids (δ37Cl ≈ −8‰ to 0‰; δ11B > ∼17‰). Br/Cl (0.0025–0.005) and I/Cl (0.00005–0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ7Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (

Behavior of Stable Carbon and Stable Nitrogen Isotopes during Hydrothermal Carbonization of biomass

This work studied the changes of stable isotope compositions (C and N) of diverse biomass feedstocks (loblolly pine, cow manure, and sewage biosolids) as a result of hydrothermal carbonization (HTC), and provides insight into degradation pathways based on these changes. HTC was conducted at different times (5 and 30 min) and temperatures (180, 220, and 260 °C) and produced hydrochars were characterized in terms of ultimate, proximate, stable carbon isotope (δ13C), and stable nitrogen isotope (δ15N) compositions. In addition to reaction conditions, results showed that the differences in feedstock composition, closely related to reactivity, determined in large part isotope behavior. In general, the carbon densification associated with HTC corresponded to a small but consistent decrease of δ13C values. In contrast, there were significant increases of δ15N values during HTC, with larger shifts associated with increased HTC reaction severity. These trends in isotopic compositions, related to changes of characteristic elemental indices (O/C, H/C, O/N, H/N), allowed for the identification of particular reaction pathways for individual feedstocks.

Impacts of changes in groundwater recharge on the isotopic composition and geochemistry of seasonally ice-covered lakes: insights for sustainable management

Abstract. Lakes are under increasing pressure due to widespread anthropogenic impacts related to rapid development and population growth. Accordingly, many lakes are currently undergoing a systematic decline in water quality. Recent studies have highlighted that global warming and the subsequent changes in water use may further exacerbate eutrophication in lakes. Lake evolution depends strongly on hydrologic balance, and therefore on groundwater connectivity. Groundwater also influences the sensitivity of lacustrine ecosystems to climate and environmental changes, and governs their resilience. Improved characterization of groundwater exchange with lakes is needed today for lake preservation, lake restoration, and sustainable management of lake water quality into the future. In this context, the aim of the present paper is to determine if the future evolution of the climate, the population, and the recharge could modify the geochemistry of lakes (mainly isotopic signature and quality via phosphorous load) and if the isotopic monitoring of lakes could be an efficient tool to highlight the variability of the water budget and quality. Small groundwater-connected lakes were chosen to simulate changes in water balance and water quality expected under future climate change scenarios, namely representative concentration pathways (RCPs) 4.5 and 8.5. Contemporary baseline conditions, including isotope mass balance and geochemical characteristics, were determined through an intensive field-based research program prior to the simulations. Results highlight that future lake geochemistry and isotopic composition trends will depend on four main parameters: location (and therefore climate conditions), lake catchment size (which impacts the intensity of the flux change), lake volume (which impacts the range of variation), and lake G index (i.e., the percentage of groundwater that makes up total lake inflows), the latter being the dominant control on water balance conditions, as revealed by the sensitivity of lake isotopic composition. Based on these model simulations, stable isotopes appear to be especially useful for detecting changes in recharge to lakes with a G index of between 50 and 80 %, but response is non-linear. Simulated monthly trends reveal that evolution of annual lake isotopic composition can be dampened by opposing monthly recharge fluctuations. It is also shown that changes in water quality in groundwater-connected lakes depend significantly on lake location and on the intensity of recharge change.

Spring response to precipitation events using δ18O and δ2H in the Tanour catchment, NW Jordan

ABSTRACTThe Tanour spring is one of the several karst springs located in the northern part of Jordan. Water samples from the Tanour spring and precipitation were collected in the area of Ajloun in NW Jordan for the analysis of stable oxygen and hydrogen isotopes to evaluate the spring response to precipitation events. Rainwater and snow samples were collected from different elevations during winters of 2013–2014 and 2014–2015. In addition, spring samples were collected between December 2014 and March 2015. δ18O values in rainwater vary from −3.26 to −17.34 ‰ (average: −7.84 ± 3.23 ‰), while δ2H values range between −4.4 and −110.4 ‰ (average: −35.7 ± 25.0 ‰). Deuterium excess ranges from 17.8 to 34.1 ‰ (average: 27.1 ± 4.0 ‰). The Local Meteoric Water Line for the study area was calculated to be δ2H = 7.66*δ18O + 24.43 (R2 = 0.98). Pre-event spring discharge showed variation in δ18O (range −6.29 to −7.17 ‰; average −6.58 ± 0.19 ‰) and δ2H values (range −28.8 to −32.7 ‰; average: −30.5 ± 1.0 ‰). In contrast, δ18O and δ2H rapidly changed to more negative values during rainfall and snowmelt events and persisted for several days before returning to background values. Spring water temperature, spring discharge, and turbidity followed the trend in isotopic composition during and after the precipitation events. The rapid change in the isotopic composition, spring discharge, water temperature, and turbidity in response to recharge events is related to fast water travel times and low storage capacity in the conduit system of the karst aquifer. Based on the changes in the isotopic composition of spring water after the precipitation events, the water travel time in the aquifer is in the order of 5–11 days.

Increased water use efficiency but contrasting tree growth patterns in Fitzroya cupressoides forests of southern Chile during recent decades

AbstractLittle is known about how old‐growth and massive forests are responding to environmental change. We investigated tree‐ring growth and carbon isotopes of the long‐lived and high biomass Fitzroya cupressoides in two stands growing in contrasting environmental conditions in the Coastal Range (~300 years old) and Andean Cordilleras (>1500 years old) of southern Chile. The interannual variability in δ13C was assessed for the period 1800–2010, and changes in discrimination and intrinsic water use efficiency (iWUE) were evaluated in relation to changes in climate and tree‐ring growth during the last century. 13C discrimination has significantly decreased, and iWUE has increased since the 1900s in both sites. However, these trends in isotopic composition have been accompanied by different growth patterns: decreasing growth rates in the Coastal Range since the 1970s and increasing growth rates in the Andes since the 1900s. Trees growing in the Coastal Range have become more efficient in their use of water, probably due to reduced stomatal conductance caused by increases in CO2 and warming. Trees growing in the Andes have also become more water use efficient, but this has been likely due to increased photosynthetic rates. Fitzroya forests, including particularly old‐growth stands, are responding to recent environmental changes, and their response has been site dependent. The growth of forests under a more Mediterranean climate influence and restrictive soil conditions in the Coastal Range has been more negatively affected by current warming and drying; while the growth of old stands in the wet Andes has been positively affected by changes in climate (decreasing cloudiness) and increasing CO2. Permanent monitoring of these endangered forests under ongoing environmental changes is needed in order to reassure the long‐term preservation of this millennial‐aged species.

Isotopic analysis (C, N) and species composition of rodent assemblage as a tool for reconstruction of climate and environment evolution during Late Quaternary: A case study from Biśnik Cave (Częstochowa Upland, Poland)

Rodent remains are potentially a powerful tool in paleoecological reconstructions. Rodents are a worldwide group of mammals inhabiting all climatic zones, from arctic tundra to tropical forests. Some of their representatives have narrow ranges of environmental tolerance, while others have cosmopolitan distributions. Additionally, rodent remains are well preserved and numerous in cave sediments, and are easily identified to the species level.The species composition of rodent assemblage from the specific cave gives valuable data on ecosystem status around the site. Moreover, isotopic composition (C, N) of rodent remains provides information on some environmental factors, e.g. type of vegetation, soil activity, precipitation, and temperature.The material for this study was collected from the sediments of the Biśnik Cave (Poland). The cave is known from its sediment sequence deposited during the middle and late Pleistocene and Holocene, containing a rich collection of the Pleistocene fauna remains and archeological artifacts. The stratigraphy of the sediment sequence previously was based on archeological findings and several radiometric dates. In this study, we applied U–Th dating method to bone collagen, rodent tooth enamel, and cave bear tooth enamel. Finally, we revised the chronology of sediment sequence from the Biśnik Cave based on these new data.We found significantly higher variability of nitrogen and carbon isotopic composition in the lemming than in the common vole. However, both species show the same trends in isotopic composition of nitrogen in time. At the beginning of the record, δ15N values were relatively high, suggesting intensive soil activity and, indirectly, mild climate. After that, from a stratigraphic layer dated to the early Würm glaciation, lower δ15N values point to lower soil activity. Deterioration of ecosystem productivity and climate conditions were also confirmed by higher δ13C. This indicates lower canopy effect and suggests conversion of vegetation around the cave. The isotopic record suggests high instability of the environmental conditions during the Würm glaciations, with significantly warmer periods before and after the Last Glacial Maximum.

A Hf-isotope perspective on continent formation in the south Peruvian Andes

Abstract Convergent continental margins are the primary host of both growth and loss of continental crust. Continental growth largely occurs via subduction-driven magmatism, whereas continental loss largely occurs via subduction erosion and sediment subduction. Because the latter typically involves partial recycling into magmas, both growth and loss of continental crust can be represented in the magmatic record. The degree of crustal recycling can be estimated from the initial Hf isotope signatures in both magmatic and detrital zircon grains. Recent insights into the geodynamic evolution of the Peruvian margin, in combination with a new dataset of Hf isotopic data on zircon from the Carboniferous to Early Cretaceous, enable us to (1) compare the geodynamic history of the southern Peruvian margin with its Hf isotopic evolution, and (2) quantify the crustal growth between 500 and 135 Ma. The data exhibit a correlation with trends in isotope composition v. time and reflect the dominantly extensional regime that prevailed from the onset of subduction from 530 Ma to c. 135 Ma. This study demonstrates that the Peruvian margin experienced continental growth with juvenile input to arc magmatism of 30–45% on average, and illustrates the use of U–Pb and Hf isotopes in zircon as a tool to trace episodes of crustal growth through time. Supplementary material: Hf istopic analyses on zircon (A1 and A2) and new U–Pb zircon ages (A3) are available at http://www.geolsoc.org.uk/SUP18661 .

Spatial distribution and flux of terrigenic He dissolved in the sediment pore water of Lake Van (Turkey)

In this study, the largest ever carried out to measure noble gases in the pore water of unconsolidated sediments in lakes, the emission of terrigenic He through the sediment column of Lake Van was successfully mapped on the local scale. The main input of He to the water body occurs at the borders of a deep basin within the lake, which is probably the remains of a collapsed caldera. The 3 He / 4 He ratio identifies the He injected into the sedimentary column of Lake Van as a mixture of He released from a mantle source and radiogenic He of crustal origin ( 3 He / 4 He ∼ 2.6 - 4.1 × 10 - 6 ) . During passage through the pore space, terrigenic He seems to be further enriched in radiogenic He that is most likely produced in the sediment column. In fact, two distinct trends in isotopic composition can be distinguished in the He injected from the lake basement into the sediments. One of these characterizes samples from the shallow water, the other characterizes samples from the deep basin. However, both of these trends are related to the same source of terrigenic He. The He fluxes determined seem to be characteristic of each sampling location and might be considered as a proxy for the fluid permeability of the deep sediment column. These new findings provide insight into the process of fluid transport within the sediments and into the process of formation of the lake basin. Moreover, the isotopic signature of the He that emanates into the water column of Lake Van is strongly affected by the mixing conditions prevailing in the overlying water body. This fact misled previous studies to interpret the terrigenic He in Lake Van as being solely of mantle origin ( 3 He / 4 He ∼ 10 - 5 ) .

Temporal trends in the stable isotope composition of precipitation: a comparison between the eastern Mediterranean and central Europe

Stable isotopes in precipitation are recognized as a major tool for tracking the water cycle. The development of gridded datasets is a solution to the need for high-resolution isotopic data. The lack of measurement though, dictates the use of “long-term average” isotopic values, usually, calculated from short time series, of unequal durations and, sometimes, not even referring to the same time period. Thus, the influence of possible trends, that might be present in the isotopic data, should be accounted for. In the present work, we investigated the existence of temporal trends in the isotopic composition of precipitation of central Europe and eastern Mediterranean. Nine stations were selected, having time series extending over at least a 20-year-long period and fulfilling certain data completeness criteria. Possible trends were detected for three overlapping 20-year periods (1961–1980, 1971–1990, and 1981–2000) using linear regression, the Mann–Kendall test, and a partial Mann–Kendall test, to compensate for the influence of meteorological parameters. We found that very few cases present statistically significant trends. There is significant variability of the observed trends, both on a seasonal and on a station basis, in the central Europe and the eastern Mediterranean area alike. Overall, the insignificance of the observed isotopic trends, and the lack of any coherent spatial and temporal patterns, seems to be supporting the current practice for estimating the long-term average isotopic composition of precipitation.

Chemical and oxygen isotopic compositions of accretionary rim and matrix olivine in CV chondrites: Constraints on the evolution of nebular dust

A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca–Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and 16O-rich olivine (Fa 1–40 and Δ 17O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and 16O-depleted (Fa 15–50 and Δ 17O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and 16O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and 16O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ 17O values. However, not all fine-grained olivines are FeO-rich and 16O-poor; some small (<1 μm in Leoville and 5–10 μm in Vigarano and Allende) ferrous (Fa >20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in 16O (Δ 17O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an 16O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as 16O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an 16O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of 16O-rich forsterites with grain overgrowths or newly formed grains of 16O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of 16O-rich forsterite and subsequently exchanged Fe–Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).

Historical trends in the lead isotopic composition of archival Sphagnum mosses from Scotland (1838-2000).

The analysis of almost 200 Scottish Sphagnum moss samples collected over the past 170 years has revealed trends in the isotopic composition of lead similar to those previously established for dated Scottish lake sediments and peat bogs, lending credibility to these proxy records of atmospheric lead contamination and deposition. The effect of temporal variations in contributions from sources such as smelting of indigenous lead ores (206Pb/207Pb approximately 1.16-1.18), coal combustion (206Pb/207Pb approximately 1.17-1.19), and the use of imported Australian lead (206Pb/207Pb approximately 1.04) was clearly seen in the Scottish moss 206Pb/207Pb record. This showed some differences from the corresponding archival herbage record for the south of England, where the initial influence of Australian lead occurred earlier, at the end of the 19th century. A significant decline from a 206Pb/ 207Pb value of approximately 1.17 in the Scottish moss record began in the 1920s and continued until the 1980s (206Pb/207Pb approximately 1.12). The success of measures to reduce lead emissions to the atmosphere over the past 20 years in the U.K., in particular from petrol-engined vehicles using alkyl lead additives manufactured primarily from Australian lead, is evident in both the increasing 206Pb/207Pb ratio and falling lead concentration data for Scottish moss.

Isotopic compositions of different presolar silicon carbide size fractions from the Murchison meteorite

Abstract— We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr, and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of silicon carbide (SiC), ranging from 0.38 to 3.0μm in diameter, from the Murchison CM2 carbonaceous chondrite. Ratios of 12C/13C range from 37 to 42 and 14N/15N ratios from 370 to 520, within the range of single‐grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the singlegrain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain‐size fractions. All bulk samples have large 26Mg excesses attributed to the presence of short‐lived 26Al at the time of grain formation. Inferred 26Al/27Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their 26Al/27Al ratio. Our Ca‐isotopic measurements, the first made on presolar SiC grains, show excesses in 42Ca and 43Ca, which is in general agreement with theoretical expectations for AGB stars. Calcium‐44 excesses are much larger than expected and are probably because of X grains, which have high44Ca excesses because of the decay of short‐lived 44Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial 44Ti/48Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti‐isotopic ratios of the bulk samples show a V‐shaped pattern with excesses of all isotopes relative to 48Ti. Isotopes 46Ti, 47Ti, and 50Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s‐process nucleosynthesis in AGB stars. In contrast, 49Ti does not show any excess relative to the singlegrain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured 53Cr/52Cr and 57Fe/56Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.

Rapid magma production rates, underplating and remelting in the Andes: isotopic evidence from northern-central Peru (9–11 °S)

Combined strontium and neodymium isotope data on the Miocene-Pliocene Cordillera Blanca batholith (9–11 °S) are compared with acid plutonic and volcanic rocks of similar age and composition from the central Andes (ca. 16–23 °S). Although intruded through 50–60 km of continental crust the batholith rocks, which range in composition from quartz diorite to high silica leucogranodiorite, show little sign of contamination by mature continental basement. Initial 87Sr 86Sr ratios define a range of 0.7041 to 0.7057, with average ϵ Nd values close to bulk earth (−0.5), over a relatively large range in SiO 2. It is difficult to reconcile the isotopic composition of the batholith rocks with simple AFC models involving fractionation of either clinopyroxene (deep-level) or plagioclase (high-level) dominated assemblages, and the isotopic variation in these rocks is instead considered to be inherited from a primary subcontinental lithosphere source through a two-stage process of crustal underplating and subsequent partial melting. Estimated (mantle) magma production rates during underplating are 0.1–0.3 km 3 yr −1. Comparisons with Miocene plutonic rocks from the western and eastern Cordillera at ~ 11 °S show no clear trends in isotopic compositions in time or space. Furthermore, the isotopic compositions of Nd and Sr in the batholith rocks contrast strongly with basaltic to high silica volcanic and plutonic rocks of similar age exposed in the central Andes, where 143Nd 144Nd and 87Sr 86Sr are thought to reflect contamination of mantle-derived magmas and/or tectonic reworking of old basement material during crustal thickening. The lack of significant involvement of basement material in the petrogenesis of the batholith magmas may be due to differences in the mechanism of crustal thickening along the Andean chain during the Late Miocene, with thickening in the central Andes being caused predominantly by tectonic shortening, while in northern-central Peru thickening occurred mostly through crustal underplating of mantle-derived basalt. These two regions thus represent first-order crustal domains in the central Andean Cordilleras. The boundary between them is marked by the Abancay Deflection.

The Geochemical Evolution of Anorthosite Residual Magmas in the Laramie Anorthosite Complex, Wyoming

Olivine- and pyroxene-bearing Fe-enriched dioritic rocks in the 1434 Ma Laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite. Geochemical characteristics of this suite include the following: high contents of TiO2, Fe2O3T⁠, and P2O5; high incompatible trace element contents; rare earth element patterns with a large range of Eu anomalies; and isotopic compositions that reflect the geographic location of individual samples, with ISr increasing and ∈Nd decreasing from south to north. After extraction from anorthosite, fractionation of ferrodioritic residual magmas resulted in secondary residual monzodioritic melts and complementary oxide-rich ferrodiorite cumulates. Geographic trends in isotopic composition reflect an increasing Archean crustal component from south to north. Dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally. Large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle-derived plagioclase-rich diapirs, followed by subsequent extraction and isolation of Fe-enriched interstitial melt. Strong isotopic contrasts between anorthosite and associated Fe-enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis.

Yields and carbon isotopic composition of pyrolysis products from artificial maturation processes

Yields and isotopic composition of various pyrolysis products were investigated for a rich marine type source from the Upper Jurassic Kimmeridge Clay Formation onshore U.K. Three different artificial maturation processes were studied: closed, dry pyrolysis (350°C for different time intervals between 1 hr and 42 days); temperature-programmed open, dry pyrolysis (72 hr at different temperature levels between 150° and 500°C); and hydrous pyrolysis (isothermally heated for 72 hr at 280–365°C). The main difference in hydrocarbon generation is observed between the open and the two closed systems, with significantly lower yields from the open system. The presence of water during the hydrous pyrolysis enhances the generation of carbon dioxide and bitumen, with maximum yields of 4.7 mmol g −1 total organic carbon (TOC) of carbon dioxide and 977 mg g −1 TOC of bitumen in the hydrous system. All three maturation processes generate hydrocarbon gases that exhibit maturity-dependent trends in isotopic composition. The main difference in the carbon isotopic composition of the products is an enrichment of the heavy isotope in the open system compared to the closed systems, with hydrous pyrolysis giving gaseous products most depleted in the heavy isotope. This effect is most pronounced for methane and carbon dioxide. None of the three methods studied produced yield and isotopic compositions which precisely match the composition of natural gases. However, the relationship between the different components are the same on a general level, and with caution the results can be considered representative for real situations.

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.

Activities of the global biosphere as reflected in atmospheric CO2 records

A two‐dimensional global atmospheric diffusion simulation model is used to establish zonal and monthly average net surface fluxes of carbon dioxide (CO2) which are consistent with the variations in CO2 concentration observed at six monitoring stations distributed globally. These fluxes represent the zonally averaged net ecosystem production of primarily terrestrial biosphere. Annually, the global net ecosystem production during the photosynthetically more active season removed 6.8×1012 kg of carbon from the atmosphere, returning it during the less active winter months. This turnover represents about 14% of the annual continental net primary production of carbon and 0.8% of the total terrestrial biomass. During the growing season, net ecosystem production appears to be relatively independent of latitude in the northern hemisphere (∼6×10−9 kg carbon m−2 s−1). The model indicates that time correlations between the CO2 concentration at different altitudes of the northern hemisphere with the interhemispheric advection of air results in a net interhemispheric CO2 flux which becomes zero when the South Pole CO2 concentration is on average 0.92 ppmv above that at Mauna Loa, Hawaii. The influence of both the net and the gross atmospheric biospheric exchange of carbon on the atmospheric stable carbon isotope ratio is modeled. On time scales of a year the gross turnover of carbon has an insignificant effect, and all isotopic effects can be related to the net exchanges. It is argued that as with concentration, long‐term trends in isotopic composition will be best observed in the southern hemisphere.

Composição isotópica de carbono e oxigênio de conchas de pelecípodos do litoral brasileiro e seu significado ambiental

Carbon and oxygen isotopic compositions have been determined on modern pelecypod shells and ancient pelecypod shells whose ages had been measured by the radiocarbon method. The modern shells collected along the Brazilian coastline in order to identify possible isotopic composition trends in the shells of the pelecypods living along this coastline. The ancient pelecypod shells were collected mostly from the Cananéia-Iguape coastal plain (State of Sao Paulo). The isotopic composition ratios of these samples can be correlated with sea-level fluctuations during the Holocene, whose transgressive and regressive phases left their own typical isotopic compositions imprinted on these shells. Some samples of freshwater pelecypod shells were also analysed to establish the limites of &#948;C13 and &#948;O18 in fluviatile and lacustrine environments. The analytical results showed that isotopic composition can be used to distinguish between pelecypod shells in freshwater environments from those in saltwater environments, as previously has been verified elsewhere. This fact demonstrates that biological control of the isotopic fractionation mechanisms may be overriden by environmental controls.