Shift In Isotopic Composition Research Articles

Younger Dryas and early Holocene climate in south Greenland inferred from oxygen isotopes of chironomids, aquatic Moss, and Moss cellulose

Ice core records have long indicated that the Younger Dryas began and ended with large, abrupt climate shifts over Greenland. Key climatic features remain unknown, including the magnitude of warming during the Younger Dryas-Holocene transition along with the seasonality and spatial variability of Younger Dryas climate changes across Greenland. Here, we use geochemical and paleoecological proxies from lake sediments at Lake N14 in south Greenland to address these outstanding questions. Radiocarbon dating and diatom assemblages confirm early deglaciation and isolation of Lake N14 before ∼13,600 cal yr BP, consistent with previous work. Oxygen isotope ratios (δ18O) of chironomid head capsules, bulk aquatic moss, and aquatic moss-derived cellulose are used to reconstruct oxygen isotopes of past lake water and annual precipitation. Oxygen isotope proxies indicate annual precipitation δ18O values increased by 5.9–7.7‰ at the end of the Younger Dryas. Following the Younger Dryas, moss and cellulose δ18O values show a clear decline in precipitation δ18O values of 2–3‰ from ∼11,540–11,340 cal yr BP that may correspond with the Preboreal Oscillation. Reconstructed precipitation δ18O values then gradually increased from 11,300–10,100 cal yr BP. All three aquatic organic materials register similar shifts in precipitation δ18O values over time, and they closely parallel the δ18O shifts observed in ice cores. This evidence strongly supports the utility of these methods for reconstructing lake water δ18O, and furthermore precipitation δ18O values where lake water reflects precipitation. The relatively large shift in isotopic composition of precipitation at Lake N14 suggests that shifts in temperature, precipitation seasonality, and/or moisture sources at the end of the Younger Dryas were even larger in south Greenland than they were in central Greenland, most likely because of the proximity to major changes in North Atlantic Ocean circulation. The annual air temperature change estimated at Lake N14 at the end of the Younger Dryas is also very large (∼18 ± 7 °C) compared to the summer warming previously inferred from chironomid species assemblages there (∼6 °C). This indicates that the strongest warming at the end of the Younger Dryas occurred in the winter season, consistent with past observations of intensified Younger Dryas seasonality at Lake N14 and elsewhere in Greenland.

Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis

Abstract. The terrestrial water cycle partitions precipitation between its two ultimate fates: “green water” that is evaporated or transpired back to the atmosphere, and “blue water” that is discharged to stream channels. Measuring this partitioning is difficult, particularly on seasonal timescales. End-member mixing analysis has been widely used to quantify streamflow as a mixture of isotopically distinct sources, but knowing where streamwater comes from is not the same as knowing where precipitation goes, and this latter question is the one we seek to answer. Here we introduce “end-member splitting analysis”, which uses isotopic tracers and water flux measurements to quantify how isotopically distinct inputs (such as summer vs. winter precipitation) are partitioned into different ultimate outputs (such as evapotranspiration and summer vs. winter streamflow). End-member splitting analysis has modest data requirements and can potentially be applied in many different catchment settings. We illustrate this data-driven, model-independent approach with publicly available biweekly isotope time series from Hubbard Brook Watershed 3. A marked seasonal shift in isotopic composition allows us to distinguish rainy-season (April–November) and snowy-season (December–March) precipitation and to trace their respective fates. End-member splitting shows that about one-sixth (18±2 %) of rainy-season precipitation is discharged during the snowy season, but this accounts for over half (60±9 %) of snowy-season streamflow. By contrast, most (55± 13 %) snowy-season precipitation becomes streamflow during the rainy season, where it accounts for 38±9 % of rainy-season streamflow. Our analysis thus shows that significant fractions of each season's streamflow originated as the other season's precipitation, implying significant inter-seasonal water storage within the catchment as both groundwater and snowpack. End-member splitting can also quantify how much of each season's precipitation is eventually evapotranspired. At Watershed 3, we find that only about half (44±8 %) of rainy-season precipitation evapotranspires, but almost all (85±15 %) evapotranspiration originates as rainy-season precipitation, implying that there is relatively little inter-seasonal water storage supplying evapotranspiration. We show how results from this new technique can be combined with young water fractions (calculated from seasonal isotope cycles in precipitation and streamflow) and new water fractions (calculated from correlations between precipitation and streamflow isotope fluctuations) to infer how precipitation is partitioned on multiple timescales. This proof-of-concept study demonstrates that end-member mixing and splitting yield different, but complementary, insights into catchment-scale partitioning of precipitation into blue water and green water. It could thus help in gauging the vulnerability of both water resources and terrestrial ecosystems to changes in seasonal precipitation.

Dissolved Carbon Transport and Processing in North America’s Largest Swamp River Entering the Northern Gulf of Mexico

Transport and transformation of riverine dissolved carbon is an important component of global carbon cycling. The Atchafalaya River (AR) flows 189 kilometers through the largest bottomland swamp in North America and discharges ~25% of the flow of the Mississippi River into the Gulf of Mexico annually, providing a unique opportunity to study the floodplain/wetland impacts on dissolved carbon. The aim of this study is to determine how dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in the AR change spatially and seasonally, and to elucidate which processes control the carbon cycling in this intricate swamp-river system. From May 2015 to May 2016, we conducted monthly river sampling from the river’s inflow to its outflow, analyzing samples for concentrations and δ13C stable isotope composition of DOC and DIC. We found that DIC concentrations in the AR were three times higher than the DOC concentrations on average, and showed more pronounced downstream changes than the DOC. During the study period, the river discharged a total of 5.35 Tg DIC and a total of 2.34 Tg DOC into the Gulf of Mexico. Based on the mass inflow–outflow balance, approximately 0.53 Tg (~10%) of the total DIC exported was produced within the floodplain/wetland system, while 0.24 Tg (~10%) of the DOC entering the basin was removed. The AR’s water was consistently oversaturated with CO2 partial pressure (pCO2) above the atmospheric pCO2 (with pCO2 varying from 551 µatm to 6922 µatm), indicating a large source of DIC from river waters to the atmosphere as well as to the coastal margins. Largest changes in carbon constituents occurred during periods of greatest inundation of the swamp-river basin and corresponded with shifts in isotopic composition. This effect was particularly pronounced during the initial flood stages, supporting the hypothesis that subtropical floodplains can act as effective enhancers of the biogeochemical cycling of dissolved carbon.

Assessing geochemical reactions during CO2 injection into an oil-bearing reef in the Northern Michigan basin

The Dover 33 Reef, part of the Niagaran Reef Complex in Northern Michigan (USA), has been the focus of an enhanced oil recovery/carbon capture utilization and storage (EOR/CCUS) project as part of the Phase III-Midwest Regional Carbon Sequestration Partnership (Gupta et al., 2013a,b). The Dover 33 structure has experienced significant CO2 flooding in the past two decades, and over the course of the current injection study (between February 2013 and July 2016) has received approximately 100 to 1000 tonnes/day into the central injection well (L-M) 1–33). As part of the geochemical monitoring effort of the study, gas and fluid samples were collected from Dover 33 reef, and several other nearby reef structures, to assess the impact of CO2 injection on the geochemical processes occurring within in the reef.The injected gas is composed of approximately 95% CO2, with a δ13CCO2 of ∼20.5‰, which is consistent with previously published compositions of Antrim shale gas, the source of the CO2. The concentrations and isotopic compositions of higher pressure gas collected from the L-M 5–33 monitoring well were similar to those measured in the injection well, but did exhibit a small but systematic shift in isotopic composition towards lower values over the course of the study, suggesting mixing and dilution between the gas in the reservoir and the injected gas. In contrast the δ13CCO2 of gas samples from the monitoring well with the lower surface pressure, L-M 2–33, are consistently lower throughout the study, ∼18.5‰, indicating that reactions with the injected CO2 are occurring within the reef or with the well casing.Fluid samples were collected to assess the extent of interaction among the injected gas, the reservoir rock, and the brine. The brine samples are acidic (pH ∼ 4.1 to 4.9) with a total salt content of nearly 400 g/L. Analysis of the isotopic composition of dissolved inorganic carbon (DIC) in the Dover 33 brine shows that δ13C is higher than the injected gas (27–33‰) suggesting that the gas is not in equilibrium with DIC in the brine, and that there has been little isotopic exchange with carbonate minerals in the reef. The water isotope composition of the brine, δ18O and δD, plot below the meteoric water line, indicating that the water is not of recent meteoric origin and has undergone isotopic exchange with both gas and minerals within the reef structure. The 87Sr/86Sr ratios of the brine samples range from 0.70865 to 0.70869, consistent with Silurian seawater composition.Geochemical modelling of the brine composition shows that the predicted CO2 solubility as DIC is much greater than the measured DIC, and that the brines are supersaturated with respect to carbonate minerals, suggesting the potential for significant trapping of CO2 in both dissolved and mineral form.

When to conduct an isotopic survey for lake water balance evaluation in highly seasonal climates

AbstractOne‐time or short‐term lake water isotopic surveys are often employed to evaluate regional lake water balance. However, it can be difficult to determine the optimal time‐window for sampling to obtain a representative long‐term perspective of lake water balance in settings influenced by seasonal variations in precipitation, evaporative loss, glacial/snow meltwater, and larger seasonal shifts in isotopic composition of precipitation. This is especially true for areas of the Tibetan Plateau that are influenced by the summer Indian monsoon. Although high‐frequency sampling is always preferred as the most rigorous approach to characterize the water budget of lakes or watersheds, this may be impractical in remote regions and over large spatial scales. To assess the potential sensitivity of isotope balance characterization to seasonal variability, we used a weekly lake water isotope data set acquired over a period of 3 years on the Tibetan Plateau to evaluate the potential inaccuracies that might have arisen from using isotopic data collected during narrower time‐windows. For this assessment, we use weekly isotopic data collected during the study and assume that these sampling events were stand‐alone one‐time surveys. We then demonstrate the sensitivity of the isotope balance method in this setting, particularly for the rainy season that significantly underestimated the evaporation/inflow. In contrast, isotopic composition of the lake water was found to be more representative of long‐term conditions when sampled in October on the Tibetan Plateau. To broaden our evaluation of seasonality effects over a range of climatic zones, published high‐frequency isotopic data were also compiled, and a similar assessment was carried out for selected regions of the world. The synthesized data and model outputs, which confirm pronounced variations in lake water isotopic composition and evaporation/inflow across a range of seasonal climates, were used to determine optimal sampling windows for these specific regions.

Shift in benthic assemblages and organisms’ diet at salmon farms: community structure and stable isotope analyses

The extent of the influence of salmon farming on the environment and on the uptake of particulate and dissolved effluents by benthic organisms was assessed using community structure and stable isotope analyses. Sediment cores were collected in 2 directions: perpendicular and parallel to the main residual current, 0, 25 and 200 m from 2 salmon farms (Millstone and Cranford) located in Mulroy Bay, Ireland. In addition, artificial substrates were placed for 2 mo at 1 m depth 0, 25 and 200 m from one farm to trace the uptake of farm-related nutrients by fouling organisms. The extent of measurable change in benthic communities (abundance, diversity, structure, trophic composition) depended on residual current direction. Intraspecific variation in isotopic values in benthic invertebrates was mostly explained by distance from cages. Organisms collected at impacted sites exhibited a shift in isotopic composition towards that of farm wastes. A shift in δ13C was observed in several invertebrates, including the polychaetes Malacoceros fuliginosus and Nephtys hombergii, Nematoda and the anemone Anthopleura balii. Fouling communities collected on artificial structures, mainly composed of the tunicate Ascidiella aspersa, showed higher δ15N values at fish cage sites compared to sites 200 m away. The study demonstrated that fish effluents were assimilated and became food sources for several organisms.

Baseline shifts in coral skeletal oxygen isotopic composition: a signature of symbiont shuffling?

Decades-long records of the stable isotopic composition of coral skeletal cores were analyzed from four sites on the Mesoamerican Reef. Two of the sites exhibited baseline shifts in oxygen isotopic composition after known coral bleaching events. Changes in pH at the calcification site caused by a change in the associated symbiont community are invoked to explain the observed shift in the isotopic composition. To test the hypothesis that changes in symbiont clade could affect skeletal chemistry, additional coral samples were collected from Belize for paired Symbiodinium identification and skeletal stable isotopic analysis. We found some evidence that skeletal stable isotopic composition may be affected by symbiont clade and suggest this is an important topic for future investigation. If different Symbiodinium clades leave consistent signatures in skeletal geochemical composition, the signature will provide a method to quantify past symbiont shuffling events, important for understanding how corals are likely to respond to climate change.

Cenozoic Volcanism on the Hangai Dome, Central Mongolia: Geochemical Evidence for Changing Melt Sources and Implications for Mechanisms of Melting

Cenozoic volcanism within Mongolia forms part of a large central Asian province of intra-plate magmatism. Numerous small-volume volcanic cones and alkali basalt lava flows have been formed since c. 30 Ma; from c. 12 Ma activity has been focused on the uplifted Hangai dome. A mechanism for melting beneath the dome has, however, thus far remained enigmatic. Some of the oldest basalts on the Hangai dome erupted at its centre at ∼6 Ma and their geochemistry suggests a garnet lherzolite source region at 90–100 km depth. These lavas have Pb isotope compositions similar to those of depleted Indian mid-ocean ridge basalts (MORB) ( 206 Pb/ 204 Pb = 17·822, 207 Pb/ 204 Pb = 15·482, 208 Pb/ 204 Pb = 37·767), which may be indicative of the involvement of ambient asthenospheric mantle in their petrogenesis. Younger basalts exhibit a gradual shift in isotopic composition towards a source that has less radiogenic Pb and more radiogenic Sr, evidenced by the eruption of lavas with 206 Pb/ 204 Pb = 16·991 and 87 Sr/ 86 Sr = 0·704704. The youngest lavas, dated as younger than ∼8 ka, have the highest K 2 O contents (up to 5·2 wt %) and are characterized by the most enriched trace-element signatures; they are interpreted to represent melting of a metasomatically altered sub-continental lithospheric mantle containing phlogopite. Concurrent with progressive melting of the lithosphere, melting appears to propagate outwards from the centre of the dome to its margins; by 0·7 Ma the marginal magmatism is interpreted to result from melting of a depleted MORB-source mantle component with a smaller contribution from the lithospheric mantle. The spatial and temporal variations in melting beneath the Hangai dome may be explained by either lithospheric delamination or the presence of a small-scale thermal anomaly in the upper mantle. Although it is not possible to distinguish between these models on the basis of geochemistry alone, the lack of a viable mechanism to generate small-scale upwelling lends support to a model involving delamination of the lithospheric mantle beneath the Hangai dome.

Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt, Namibia

Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with B-isotope ratios typical for continental crust (δ11B average −8.4‰ ± 1.4, n = 11; one sample at −13‰, n = 2). Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This “main-stage” tourmaline is alkali-deficient schorl (20–30% X-site vacancy, Fe/(Fe + Mg) 0.8–1), with uniform B-isotope compositions (δ11B −8.7‰ ± 1.5, n = 49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about −9‰, like the main-stage tourmaline, and a second at −2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na–B–Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.

Expression of maternal isotopes in offspring: implications for interpreting ontogenetic shifts in isotopic composition of consumer tissues

When evaluating ontogenetic shifts in isotopic composition of consumer tissues within the context of a dietary analysis, the isotopic starting point of consumers in the population should not be ignored. Neonate isotopic composition may be different from that of juveniles and adults; in general, neonate tissues are built from maternal resources rather than food resources. Thus, the range of isotope values observed within a population of consumers may be significantly impacted by consumer isotope ratios at birth. Long-term goals of my research involve the use of stable isotopes to assess the role diet plays in driving population level differences in life history and demography observed among three pigmy rattlesnake (Sistrurus miliarius; family Viperidae) populations. For meaningful interpretation of field-collected data, it was important to determine starting (i.e., at birth) isotopic compositions of rattlesnakes from the study populations. We quantified isotopic composition of pregnant pigmy rattlesnake scale tissue, isotopic composition of neonate scale tissue and the degree that neonate scale tissue isotopic composition reflected the isotopic composition of maternal scale tissue. Collectively, neonate isotopic composition was highly variable among-litters; average litter δ13C values spanned 7 ‰ and average litter δ15N values spanned 2.8 ‰. Over 95 % of the variation in offspring isotopic composition was expressed among litters. Thus, high levels of among-litter isotopic variation were largely due to the retention of a maternal signal. Results of the enclosure study suggest that highly variable isotopic signatures in young animals within field populations could easily reflect the retention of a maternal signal rather than differences in resource utilisation among younger snakes.

13C and 15N natural abundance of the soil microbial biomass

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is 13C, and especially 15N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was 15N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and 13C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also 13C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but 13C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.

Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc

Geologic, chronologic, and U–Th isotope data from Pleistocene–Recent basaltic to rhyolitic lavas and their phenocrysts constrain the long-term evolution of magmatic processes at Seguam Island, Aleutian Island arc, and suggest a model in which erupted magmas were derived from a single, deep seated reservoir for ∼130 kyrs of its eruptive history. The monotonic evolution in ( 230Th / 232Th) 0 ratios is consistent with radiogenic ingrowth of 230Th in a long-lived magma reservoir between 142 and 9 ka. Internal U–Th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40Ar / 39Ar dating, which implies a short period (i.e., 10 3 yrs or less) of crystal residence in the magma prior to eruption. These results can be reconciled if small batches of magma are repeatedly extracted from a deep, thermally buffered, basaltic reservoir, followed by decompression-driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption. Stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering ∼12 km 2. The ignimbrite, as well as subsequent rhyolitic and basaltic lava flows, signal a decrease in ( 230Th / 232Th) 0 and 87Sr / 86Sr ratios. The abrupt change in magma composition at 9 ka most likely reflects disruption of the long-lived reservoir by rapid ascent of basaltic magma with low ( 230Th / 232Th) and 87Sr / 86Sr ratios and excess 226Ra. While the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored, the shift in isotopic composition and evidence for a protracted > 100 kyr period of magma storage, would have been missed had the focus been on only the most recent historical eruptions. Our results suggest that: not all arc basalt produced in the mantle wedge separates and ascends rapidly, magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano, and U–Th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs, provided a sufficiently long period of volcanic activity is examined.

Some isotopic and hydrological changes associated with the 1999 Chi‐Chi earthquake, Taiwan

Abstract The changes in the isotopic composition of, and the groundwater level in, the Choshui River alluvial fan near the ruptured Chelungpu Fault during and following the 1999 (Mw = 7.5) Chi‐Chi earthquake in Taiwan are reported. Three aspects of the hydrological changes are noticed. First, following the Chi‐Chi earthquake, the lower aquifers beneath the Choshui River fan showed a significant shift in isotopic composition towards that of the surface water in the Choshui River, suggesting enhanced exchanges of water between the river and the groundwater. Second, in some wells, water levels and isotopic compositions in different aquifers converged to the same respective values during the Chi‐Chi earthquake, suggesting coseismic exchanges of water between the different aquifers, which implies enhanced permeability due perhaps to the fracturing and breaching of aquitards between the aquifers. Third, the pattern of the coseismic water‐level response is distinctly different from that of the shift in the isotopic composition, suggesting that they were produced by different mechanisms.

Facies, dissolution seams and stable isotope compositions of the Rohtas Limestone (Vindhyan Supergroup) in the Son valley area, central India

The early Mesoproterozoic Rohtas Limestone in the Son valley area of central India represents an overall shallowing-upward carbonate succession. Detailed facies analysis of the limestone reveals outer- to inner-shelf deposition in an open marine setting. Wave-ripples, hummocky cross stratifications and edgewise conglomerates argue against a deep marine depositional model for the Rohtas Limestone proposed earlier. Stable isotope analysis of the limestone shows that δ13C and δ18O values are compatible with the early Mesoproterozoic open seawater composition. The ribbon limestone facies in the Rohtas Limestone is characterized by micritic beds, each decoupled in a lower band enriched and an upper band depleted in dissolution seams. Band-wise isotopic analysis reveals systematic short-term variations. Comparative enrichment of the heavier isotopes in the upper bands is attributed to early cementation from sea water and water derived from the lower band undergoing dissolution because of lowering of pH at depth. The short-term positive shifts in isotopic compositions in almost every upward gradational transition from a seamed band to a non-seamed band support the contention that dissolution seams here are of early diagenetic origin, although their formation was accentuated under overburden pressure.

A coupled fluid-inclusion and stable isotope record of paleofluids in the Monterey Formation, California

Determining the history of fossil hydrologic systems that is recorded in vein minerals can be complicated by heterogeneity in the oxygen and carbon isotope compositions at millimeter and smaller scales. Several samples of diagenetic dolomite from two location in the Miocene Monterey Formation, California, have δ18O and δ13C values that vary over a few millimeters by ∼28‰ and 6.5‰, respectively (relative to the Pee Dee belemnite isotope standard). These variations reflect episodic flow and mixing of waters from different sources with distinct diagenetic histories. On the other hand, isotopic conpositions of dolomite in samples with gradual shifts in isotopic compositions and correlated δ18O and δ13C values may have been controlled by mass diffusion and thermal conduction between episodic flow events. Primary fluid-inclusion assemblages from samples collected at one location yielded homogenization temperatures of 87.8 ± 3.6 °C (2σ). These temperatures were combined with measured δ18O values very nearly collocated in each dolomite sample to show that δ18Owater values ranged between −0.8‰ and +2.5‰ (relative to standard mean ocean water). The samples from this Monterey Formation dolomite thus indicate their precipitation from water more enriched in 18O than late Miocene and younger seawater and must reflect modification of the δ18O values of the involved waters by mineral-dehydration reactions prior to dolomite precipitation. Samples from the other location contain no primary fluid- inclusion assemblages and few abrupt shifts in isotopic compositions, perhaps reflecting recrystallization. Small-scale variations in isotopic compositions of vein minerals could be interpreted as the result of a changing regional hydrologic pattern, but we think such an interpretation for the Monterey Formation samples would be incorrect. Instead, we infer that these variations provide important information about localized effects on the ancient hydrologic system and that these effects bear on the origins of the paleofluids responsible for the mineral precipitation.

Microbial influences on local carbon isotopic ratios and their preservation in carbonate.

Analysis of the carbon isotopic composition of carbonates is a valuable tool for studying microbial processes and looking for evidence of life. Microbial fixation of CO2 and conversion of organic carbon to CO2 can produce measurable delta 13C shifts in a microbial mat environment. Diffusion modeling demonstrates that substantial isotopic shifts can develop within the mat and in the diffusion boundary layer in the fluid when CO2 fixation is rapid and prolonged for several hours. Carbonates that precipitate during rapid CO2 fixation can preserve these microbially produced isotopic shifts. However, continued precipitation during intervals when respiration dominates or after the cessation of active microbial growth commonly dilutes autotrophic isotopic signatures. Thus, preserved isotopic signatures rarely reflect the magnitude of isotopic shifts within the mat. Interpretation of observed isotopic shifts in microbial mat carbonate depends on fully characterizing ambient delta 13C and eliminating other origins for isotopic shifts. The carbon isotopic composition of reservoirs can vary substantially, both on Earth and on other planets. Characterizing the reservoir composition and any changes through time is critical to evaluating microbially induced shifts. In addition, careful evaluation of non-microbial causes for shifts in isotopic composition is essential for a reliable interpretation. Complicating processes include recrystallization, calcite precipitation over extended periods of time, variable precipitation rates and water chemistry, and mixing of carbonates having different isotopic signatures.

Microbial processes and carbon‐isotope fractionation in tropical and temperate grassland soils

Abstract1. The carbon content and δ13C value of soil organic carbon (SOC), microbial biomass (Cmic) and respired CO2 were measured in a range of grassland soils from tropical and temperate biomes to determine if isotope effect of microbial degradation can induce a shift in isotope composition of SOC and CO2. The soil from a depth of 0–2 cm was analysed. Cmic was measured using the chloroform fumigation extraction method, while CO2 was measured in a closed system after 3 and 10 days of incubation. Two soils, temperate and tropical, were used for a long‐term experiment, in which measurements were performed after 3, 10 and 40 days of incubation.2. SOC and Cmic decrease exponentially with increasing mean annual temperature. Cmic decreases more slowly than SOC, resulting in a higher proportion of Cmic in the SOC of tropical soils relative to temperate soils.3. The δ13C value of Cmic and respired CO2 reflects gross changes in the δ13C value of SOC in the corresponding sample. On average, Cmic is 13C‐enriched by c. 2‰ compared with SOC, while respired CO2 is 13C‐depleted by c. 2·2‰ compared with Cmic. Thus, the observed 13C‐enrichment in Cmic is balanced by a corresponding 13C‐depletion in respired CO2 resulting in the δ13C value of respired CO2 being approximately similar to the δ13C of SOC.4. The isotope effect of microbial degradation is of importance in soil. It can be induced by selective utilization of SOC and isotope discrimination during metabolism. Metabolic isotopic discrimination is dependent on the growth stage of the soil microbial population.

Effects of northern pike (Esox lucius) additions on pollutant accumulation and food web structure, as determined by δ13C and δ15N , in a eutrophic and an oligotrophic lake

In May 1993 and 1994, northern pike (Esox lucius) were added to eutrophic Lake 227 and oligotrophic Lake 110 at the Experimental Lakes Area in northwestern Ontario. Both lakes were previously dominated by cyprinids, and northern pike additions significantly decreased cyprinid densities in both lakes. Food web relationships were determined pre- and post-manipulation using stable carbon (δ13C) and nitrogen (δ15N) isotope analyses of zooplankton, benthic invertebrates, and fishes. In Lake 110, fathead minnow (Pimephales promelas) shifted from a zooplanktivorous to a zoobenthivorous diet as indicated by a shift in isotopic composition to more depleted δ15N and enriched δ13C values after northern pike additions. It was not possible to direct predator-induced shifts in cyprinid diets in Lake 227. Concentrations of mercury and organochlorines (sum of polychlorinated biphenyls (ΣPCB), dichlorodiphenyltrichloroethane (ΣDDT), and hexachlorocyclohexane (ΣHCH)) in these fish did not change markedly after northern pike introductions despite the shifts in diet for fathead minnow from Lake 110. However, concentrations of all contaminants were lower in biota from eutrophic Lake 227 when compared with Lake 110, which is consistent with previous studies. Our results indicate that nutrient availability, rather than short-term changes in food web structure, determined contaminant concentrations in fish and other biota from these lakes.

Stable isotope evidence of heterogeneous fluid infiltration at the Ubehebe Peak contact aureole, Death Valley National Park, California

Stable isotope ratios of carbon and oxygen are used to define quantitatively the effects of magmatic fluid infiltration in marbles contact metamorphosed by the 173 Ma Ubehebe Peak quartz monzonite, Death Valley National Park, California. In previous studies of fluid infiltration, quantitative interpretation of aureole-wide isotopic data has been difficult due to small data sets. For this study, sampling strategies were developed to obtain a data set that was large and unbiased enough to be statistically representative of the carbonates north of the Ubehebe Peak intrusion. A total of 357 samples of marble were analyzed for bulk carbonate isotopic ratios. Separate analyses of coexisting calcite and dolomite were also obtained for an additional 31 samples. Unmetamorphosed samples (1900-3000 m from the intrusion) have delta ¹⁸ O (per mil SMOW) values of 25.5+ or -0.8 (1sigma ) and delta ¹³ C (per mil PDB) values of -0.4+ or -0.6 (1sigma ). Samples in the tremolite zone (750-1900 m) have delta ¹⁸ O values ranging from 19.4 to 27.7 per mil with a median value of 25.2 per mil; and delta ¹³ C ranges from -5.1 to 0.5 per mil with a median value of -0.9 per mil. Forsterite zone samples (0-750 m) have isotopic ratios shifted to values as low as 11.1 per mil (delta ¹⁸ O) and -9.1 per mil (delta ¹³ C). Despite this shift, most forsterite zone samples retain sedimentary isotopic compositions with median delta ¹⁸ O values of 25.0 per mil and delta ¹³ C values of -1.2 per mil. delta ¹⁸ O values for igneous minerals show no evidence for interaction with heated meteoric or metamorphic fluids. The shifts in isotopic compositions within the marbles are interpreted to be the result of magmatic infiltration. The effects of this infiltration were quantified by identifying samples with isotopic alteration that can only be attributed to infiltration. The results show that magmatic fluid infiltration was limited in extent and very heterogeneous. There is no evidence for infiltration of isotopically reactive fluids beyond 850 m from the intrusive contact, and within this 850 m zone only 28 percent of the samples have been infiltratively altered with respect to delta ¹⁸ O, and 20 percent are depleted in delta ¹³ C compositions. The isotopic data, when evaluated in conjunction with geostatistical and petrologic data, indicate that the geometry of the hydrothermal flow system was mainly vertical and away from the pluton. Infiltration was restricted to large, nearly vertical, "tube-like" zones of increased permeability. These higher permeability zones likely reflect an initial heterogeneity of the host rocks and show no significant evidence for reaction enhanced permeability. Given the heterogeneity of the system and a lack of knowledge about many basic parameters controlling fluid infiltration, it is shown that the best method of calculating the amount and composition of the infiltrating fluid may be a mass balance approach (fluid/rock ratio). The application of mass balance models is discussed and shown to be valid only under limited conditions. Since infiltration at Ubehebe Peak was largely vertical, the observed isotope alteration patterns represent an infiltration side and not a front. Because this precludes the use of traditional mass balance calculations, a new infiltration side (InSide) model is proposed that allows the isotopic data to be evaluated. The InSide model uses the ratio of the areal amounts of infiltrative alteration to calculate a fluid composition. Fluid amounts cannot be obtained from this model. Results for the Ubehebe Peak data show that the infiltrating fluid had an average X _CO2 of 0.3. Although not in agreement with estimates based on phase petrology (X _CO2 <0.05), such discrepancy in the carbon mass balance is not limited to the Ubehebe Peak aureole and is a common problem in many other aureoles. The statistically representative Ubehebe Peak data set provides the most accurate picture of aureole-scale fluid infiltration presently available. Although in many ways this study quantifies the heterogeneous nature of contact metamorphic fluid infiltration, it also highlights some serious problems in predicting the amount and composition of infiltrating fluids. Data gained from studies such as this, however, will lead to an increased understanding of fluid infiltration and contribute to the development of more accurate models.

Nd and Pb isotopes in Atlantic and Pacific water masses before and after closure of the Panama gateway

Two Nd and Pb isotope time series of hydrogenous ferromanganese crusts, one from the Tehuantepec Ridge in the deep eastern equatorial Pacific and the other from Blake Plateau in the shallow northwestern Atlantic, which cover the past 7‐8 m.y., show no variations coincident with the final closure of the Panama gateway, estimated as ca. 3.5 Ma. The record of the Atlantic crust located in the present-day Gulf Stream shows a shift in isotope composition from ca. 8 to 5 Ma that is explained by a diminishing supply of Pacific water. It is argued that the major restriction of water-mass exchange through the Panama gateway occurred before 5 Ma and thus cannot serve as a direct cause of the onset of Northern Hemisphere glaciation. The absence of a significant signature in the isotope records from the Pacific crust suggests that the volume of water exchanged with the Atlantic through shallow archipelagic straits of the gateway during the 3‐4 m.y. prior to closure was too small to influence the radiogenic isotope composition of Pacific deep water.