Stable Isotope Trends Research Articles

Spatial and temporal variability in ringed seal (Pusa hispida) stable isotopes in the Beaufort Sea.

Arctic ecosystem dynamics are shifting in response to warming temperatures and sea ice loss. Such ecosystems may be monitored by examining the diet of upper trophic level species, which varies with prey availability. To assess interannual variation in the Beaufort Sea ecosystem, we examined spatial and temporal trends in ringed seal (Pusa hispida) δ13C and δ15N in claw growth layers grown from 1964 to 2011. Stable isotopes were correlated with climate indices, environmental conditions, seal population productivity, and geographic location. Sex and age did not influence stable isotopes. Enriched 13C was linked to cyclonic circulation regimes, seal productivity, and westward sampling locations. Higher δ15N was linked to lower sea surface temperatures, a higher percentage of pups in the subsistence harvest, and sample locations that were eastward and further from shore. From the 1960s to 2000s, ringed seal niche width expanded, suggesting a diversification of diet due to expansion of prey and/or seal space use. Overall, trends in ringed seal stable isotopes indicate changes within the Beaufort Sea ecosystem affected by water temperatures and circulation regimes. We suggest that continued monitoring of upper trophic level species will yield insights into changing ecosystem structure with climate change.

Mercury and stable isotope (δ13C and δ15N) trends in decapods of the Beaufort Sea

To date, limited data are available on mercury (Hg) concentrations in Arctic benthos. It is important that we establish baseline Hg concentrations in benthic communities to improve our understanding of Hg within the Arctic marine ecosystem because they are food sources of key species, such as beluga whales, which are consumed by Inuit as a part of their traditional diet. In this study, we present Hg concentrations (total and monomethyl—THg and MMHg) in three shrimp species in the Canadian Beaufort Sea region: Eualus gaimardii, Sclerocrangon boreas and Sclerocrangon ferox. We compared the variability between the three species and investigated whether it could be accounted for by regional (depth, latitude, longitude) or biological indices (stable isotope, SI, ratios of nitrogen and carbon). E. gaimardii exhibited the lowest mean δ15N and δ13C values and mean THg and MMHg concentrations, whereas S. boreas had the highest mean δ15N value and S. ferox had the highest mean δ13C value and mean THg and MMHg concentrations. Differences in SI and Hg data among the three species were attributed to their different trophic functional groups, regional features and relative incidences of species collected at our sampling sites. The determination of baseline THg and MMHg concentrations and insight on how concentrations vary with geographical correlates reported here among three common Arctic decapod species will contribute to closing the Hg-benthos knowledge gaps within the Arctic food web.

Episodic fluid flow in an active fault

Abstract We present a reconstruction of episodic fluid flow over the past ∼250 k.y. along the Malpais normal fault, which hosts the Beowawe hydrothermal system (Nevada, USA), using a novel combination of the apatite (U-Th)/He (AHe) thermochronometer and a model of the thermal effects of fluid flow. Samples show partial resetting of the AHe thermochronometer in a 40-m-wide zone around the fault. Numerical models using current fluid temperatures and discharge rates indicate that fluid flow events lasting 2 k.y. or more lead to fully reset samples. Episodic fluid pulses lasting 1 k.y. result in partially reset samples, with 30–40 individual fluid pulses required to match the data. Episodic fluid flow is also supported by an overturned geothermal gradient in a borehole that crosses the fault, and by breaks in stable isotope trends in hydrothermal sinter deposits that coincide with two independently dated earthquakes in the past 20 k.y. This suggests a system where fluid flow is triggered by repeated seismic activity, and that seals itself over ∼1 k.y. due to the formation of clays and silicates in the fault damage zone. Hydrothermal activity is younger than the 6–10 Ma age of the fault, which means that deep (∼5 km) fluid flow was initiated only after a large part of the 230 m of fault offset had taken place.

Shell beaches to beachrock: Diagenetic stable isotopic trends

Modern shell accumulations on Padre Island, Texas Gulf Coast, are composed of pristine aragonite whereas laterally adjacent Pleistocene beachrock on the mainland shores of Laguna Madre are made up of the same taxa but display a range of mixtures of aragonite and diagenetic low-Mg calcite. Cements and pedogenic crusts associated with the beachrock are 100% low-Mg calcite. Stable isotopic values for shells composed of 0, 15, 20, 45, 75 and 90% calcite display high correlations between the δ18O values and mineralogy (R2 = 0.95) and δ13C values and mineralogy (R2 = 0.87). The highly correlated changes in the stable isotopic compositions and mineralogy of the skeletal material are also evident in the data from the analyses of the pure calcite cements and pedogenic crusts. Changes in stable isotopic composition of the shells are the result of the early diagenetic alteration of the original aragonitic composition to low Mg-calcite.

Stable isotope trends in gilgaied Vertosols – variability between mounds and depressions and implications for sampling

Stable isotopes (2H and 18O) are widely used in ecohydrological studies in Australia but their trends in Vertosols with microtopography (gilgai) is unknown. An understanding of short-distance variations in stable isotopes is important for designing cost-effective, sound sampling strategies in ecohydrological studies, but the knowledge can also further inform our understanding of infiltration and drainage processes in these soils. A comparison of mounds and depressions in sites with large and small gilgai revealed surprisingly little variation in stable isotope profiles between gilgai components or in relation to gilgai size. Variations in cracking, surface conditions, surface cover, solar radiation and wooded vegetation patterns could have potentially contributed to large variations in stable isotope profiles throughout the sites but the influence of these appears to have been minor and constrained to the upper 0.3m. Despite cracks being present to depths up to 1.5m, few samples are needed below 0.6m depth to characterise the isotope signature of the subsoil. Comparison of isotope profiles in closely spaced cores suggested that one core can sufficiently capture the profile trend, although bulking of multiple cores is recommended to minimise the likelihood of sampling error.

The Impact of Mountain Range Geographic Orientation on the Altitude Effect of Precipitation δ18O in the Upper Reaches of the Heihe River Basin in the Qilian Mountains

The precipitation δ18O-elevation gradients are important for paleoclimate, hydrology, and paleoelevation studies. The field setting for this research was the upper reaches of the Heihe River Basin within the Qilian Mountains in the Northern Tibetan Plateau. Three study sites were established along the Heihe main river. These sites were the Yingluoxia and Qilian hydrological stations and the Yeniugou meteorological station. The Yingluoxia hydrological station was the dividing point between the upper and middle reaches of the Heihe River Basin. The altitudes of these sites range from 1600 m to 3300 m. Summer precipitation is predominant with regard to the annual precipitation amount. By analysis of variance (ANOVA), the precipitation δ18O data collected from the three sites were analyzed, spanning a year of precipitation data from 2007.10 to 2008.9. The results showed that the δ18O-elevation gradient was not significant (α = 0.05) at a seasonal or annual scale in this region and the precipitation-weighted annual mean δ18O was −7.1‰. Mechanisms that have been proposed to explain this result consider the role of two processes, including (1) mixing of moisture sources, a process common in an arid and semiarid region, and (2) the absence of a mechanism for water vapor to climb along slopes in the precipitation system. Atmospheric water vapor mainly travels along the trend of the Qilian Mountains range rather than climbing it because this region is dominated by the westerlies and the trend of the Qilian mountains is geographically aligned to the NWW (north-west-west) direction. We demonstrated that, aside from the water vapor source, the spatial relationship between the water vapor transport pathway and the trend of the mountain range are the main driving factors associated with the stable isotope trends in precipitation. As a result, it is important to re-recognize the timing and location of groundwater recharge in the Heihe River Basin.

Significance of stable carbon isotope trends in carbonate concretions formed in association with anaerobic oxidation of methane (AOM), Middle and Upper Devonian shale succession, western New York State, USA

Carbonate concretions are concentrated along discrete stratigraphic horizons within Middle and Upper Devonian carbonaceous black and organic-deficient gray shale of western New York State. Textural characteristics preserved throughout studied concretions, including a clay grain microfabric typical of flocculated clay and spherical algal cysts, are consistent with a model entailing the formation of low-density, compaction-resistant calcium carbonate masses at shallow burial depth, within the zone of bacterial sulfate reduction. Burial of nascent concretions to the sulfate methane transition zone (SMTZ) was accompanied by the nearly complete infilling of porosity, the diagenetic consequence of the anaerobic oxidation of methane (AOM). Each concretionary horizon, then, defines the position of the SMTZ stabilized during an episode of reduced sedimentation rate. Most analyzed concretions (75%) of the Middle Devonian Marcellus Formation into the Upper Devonian Gowanda Formation display center-to-edge profiles of increasing bulk δ13C; far fewer concretions are characterized by radial profiles of decreasing δ13C. The common radial profiles of increasing δ13C can be attributed to AOM-related kinetic carbon isotope fractionation under diagenetic conditions of limited methane, which favors the production of dissolved inorganic carbon of increasing δ13C. Less common concretions displaying center-to-edge profiles of diminishing δ13C may record the anaerobic oxidation of more strongly 13C-depleted methane produced by carbon dioxide reduction, methanogenesis related to the degradation of organic matter within and near the SMTZ, or AOM back flux, an enzyme-mediated equilibrium isotope effect. These processes are favored by sulfate-limited diagenetic conditions more typical of the lower SMTZ and immediately underlying upper methanogenic zone (MEZ). Contradictory isotope profiles of texturally similar concretions can be accounted for by diagenesis associated with an extended SMTZ (ESMTZ) comprising the methane-limited SMTZ and the underlying sulfate-deficient upper MEZ. Vertical shifts of the diagenetic horizons of the ESMTZ induced by variable methane flux would have juxtaposed concretions displaying seemingly incompatible isotopic histories.

Precipitation stable isotope records from the northern Hengduan Mountains in China capture signals of the winter India–Burma Trough and the Indian Summer Monsoon

This project reports results of the first precipitation stable isotope (δ18O and δD) time series produced for Qamdo in the northern Hengduan Mountains in the southeastern Tibetan Plateau. The data showed that the fluctuations of precipitation stable isotopes at Qamdo during the different seasons revealed various moisture sources. The westerlies and local recycling moisture dominated at the study area before the pre-monsoon and after the post-monsoon seasons, which resulted in similar trends of both precipitation stable isotopes and temperature. The marine moisture was transported to the northern Hengduan Mountains by the winter India–Burma Trough combined with convection. Consequently, stable isotopes in subsequent precipitation were occasionally observed to decrease suddenly. However, δ18O and δD values of precipitation at Qamdo were lower during the monsoon period and the duration of those low values was longer because of the effects of the Indian Summer Monsoon and the strengthening convection. Our findings indicate that the effects of seasonal precipitation differences caused by various climate systems, including the winter India–Burma Trough and Indian Summer Monsoon, need to be considered when attempting to interpret tree-ring and ice core records for the Hengduan Mountains.

Formation of Rotliegend Ca-Cl brines in the North German Basin compared to analogues in the geological record

The formation of terrestrial Ca-Cl brines is controlled by two major processes: chemical reactions between halite brines and igneous and metamorphic rocks and/or their debris in sedimentary basins, and the abstraction of H2O by evaporation, freezing or formation of hydrous minerals. Albitization dominates in environments of low H2O activity, i.e., halite saturated systems. Chloritization, serpentinization and formation of smectites seems to be related to less saline systems, i.e., enhanced water activity.The Rotliegend brines of the North German Basin evolved by interaction of infiltrating halite brines with volcanic debris of the Permian acid volcanic rocks by albitization and chloritization of plagioclase. At present their temperatures are between 130 and 150°C at 3000–4000m depth. Although they developed in a different environment they chemically and isotopically resemble the basinal brines from the Mississippi Salt Dome basin or even the deep brines from the Red Sea. All these brines have in common that halite brines interacted with plagioclase (An+SiO2+2Na+⇔2Alb+Ca2+) or possibly with clay minerals (Ca-clay+2Na+⇔Na2-clay+Ca2+) at enhanced temperatures. The trends of stable isotopes show slopes in the δD-δ18O diagram which are typical for hydrothermal reactions of evaporation brines with rock or sediments. The genesis of Rotliegend brines differ from Ca-Cl brines in the Canadian Shield, Siberian Platform, the Dry Valleys in Antarctica and even the local brines from the Chilean salars and the Californian Bristol Dry Lake. The Antarctic Dry Valley brines seem to be dominated by melting of ice-cements of their permafrost sections. The Ca-Cl brines of springs in the Chilean salars and the Bristol Dry Lake ascend due to thermal convection along tectonic faults. Although the Canadian Shield brines chemically resemble those of the Siberian Platform their stable isotope fractionation suggest that they developed by different processes of water abstraction. Exotic stable isotope values in brines are gained by freezing or ultrafiltration processes in the precursor sediments of the present crystalline rocks of the Canadian Shield and by small volume ratios of liquids and metamorphic rocks such as of KTB/VB.

Climate fluctuations recorded in phreatic and vadose calcretes of the Lower Carboniferous Clyde Sandstone Formation of Machrihanish, Kintyre Peninsula, SW Scotland

The Galdrings cliffs of Machrihanish, in the Kintyre Peninsula of Scotland, expose a Lower Carboniferous clastic succession that hosts a wide variety of calcretes, including thick and massive host-replacing phreatic calcrete hardpans (HRPCHs), which are geologically rare, and which are the products of a thorough replacement of host minerals by calcite in the mixing zone between fresh and evaporitic groundwaters. Isotopic values of 53 samples from various calcretes distributed in a c. 45 m thick succession provided well-defined trends of covariance between carbon and oxygen isotopes, thus delineating trends of aridification and humidification from the interplay of precipitation and evaporation rates. Although values from HRPCHs are more constrained, all other forms of calcrete in the succession (pedogenic nodules, pillar calcrete, laminar calcrete and invasive phreatic calcrete cement in sandstone) follow similar stable isotopic trends. An aridification trend preceded the formation of each HRPCH occurrence, corroborating studies on modern equivalents that suggest that they develop in the more arid range of calcrete formation. The better constrained isotopic values of HRPCHs compared with those of more common forms of calcrete are interpreted to be indicative of more specific environmental requirements, which may in part explain their rarity in the geological record.

Stable carbon isotopes (δ13C) of total organic carbon and long-chain n-alkanes as proxies for climate and environmental change in a sediment core from Lake Petén-Itzá, Guatemala

Sediment core PI-6 from Lake Peten Itza, Guatemala, possesses an ~85-ka record of climate and environmental change from lowland Central America. Variations in sediment lithology suggest large and abrupt changes in precipitation during the last glacial and deglacial periods, and into the early Holocene. We measured stable carbon isotope ratios of total organic carbon and long-chain n-alkanes from the core, the latter representing a largely allochthonous (terrestrial) source of organic matter, to reveal past shifts in the relative proportion of C3–C4 terrestrial biomass. We sought to test whether stable carbon isotope results were consistent with other paleoclimate proxies measured in the PI-6 core, and if extraction and isotope analysis of n-alkanes is warranted. The largest δ13C variations are associated with Heinrich Events. Carbon isotope values in sediments deposited during the last glacial maximum indicate moderate precipitation with little fluctuation. The deglacial was a period of pronounced climate variability, e.g. a relatively warm and moist Bolling–Allerod, but a cool and dry Younger Dryas. Arid periods of the deglacial were inferred from samples with high δ13C values in total organic carbon, which reflect times of greater proportions of C4 plants. These inferences are supported by stable isotope measurements on ostracod shells and relative abundance of grass pollen from the same depths in core PI-6. Similar trends in carbon stable isotopes measured on bulk organic carbon and n-alkanes suggest that carbon isotope measures on bulk organic carbon in sediments from this lake are sufficient to infer past climate-driven shifts in local vegetation.

Decline in methylmercury in museum-preserved bivalves from San Francisco Bay, California

There are ongoing efforts to manage mercury and nutrient pollution in San Francisco Bay (California, USA), but historical data on biological responses are limited. We used bivalves preserved in formalin or ethanol from museum collections to investigate long-term trends in methylmercury (MeHg) concentrations and carbon and nitrogen isotopic signatures. In the southern reach of the estuary, South Bay, MeHg in the Asian date mussel (Musculista senhousia) significantly declined over the study duration (1970 to 2012). Mean MeHg concentrations were highest (218ng/g dry weight, dw) in 1975 and declined 3.8-fold (to 57ng/g dw) by 2012. This decrease corresponded with closure of the New Almaden Mercury Mines and was consistent with previously observed declines in sediment core mercury concentrations. In contrast, across all sites, MeHg in the overbite clam (Potamocorbula amurensis) increased 1.3-fold from 64ng/g dw before 2000 to 81ng/g dw during the 2000s and was higher than in M. senhousia. Pearson correlation coefficients of the association between MeHg and δ13C or δ15N provided no evidence that food web alterations explained changing MeHg concentrations. However, isotopic composition shifted temporally. South Bay bivalve δ15N increased from 12‰ in the 1970s to 18‰ in 2012. This increase corresponded with increasing nitrogen loadings from wastewater treatment plants until the late 1980s and increasing phytoplankton biomass from the 1990s to 2012. Similarly, a 3‰ decline in δ13C from 2002 to 2012 may represent greater utilization of planktonic food sources. In a complimentary 90day laboratory study to validate use of these preserved specimens, preservation had only minor effects (<0.5‰) on δ13C and δ15N. MeHg increased following preservation but then stabilized. These are the first documented long-term trends in biota MeHg and stable isotopes in this heavily impacted estuary and support the utility of preserved specimens to infer contaminant and biogeochemical trends.

Late Cretaceous (late Campanian–Maastrichtian) sea-surface temperature record of the Boreal Chalk Sea

Abstract. The last 8 Myr of the Cretaceous greenhouse interval were characterized by a progressive global cooling with superimposed cool/warm fluctuations. The mechanisms responsible for these climatic fluctuations remain a source of debate that can only be resolved through multi-disciplinary studies and better time constraints. For the first time, we present a record of very high-resolution (ca. 4.5 kyr) sea-surface temperature (SST) changes from the Boreal epicontinental Chalk Sea (Stevns-1 core, Denmark), tied to an astronomical timescale of the late Campanian–Maastrichtian (74 to 66 Ma). Well-preserved bulk stable isotope trends and calcareous nannofossil palaeoecological patterns from the fully cored Stevns-1 borehole show marked changes in SSTs. These variations correlate with deep-water records of climate change from the tropical South Atlantic and Pacific oceans but differ greatly from the climate variations of the North Atlantic. We demonstrate that the onset and end of the early Maastrichtian cooling and of the large negative Campanian–Maastrichtian boundary carbon isotope excursion are coincident in the Chalk Sea. The direct link between SSTs and δ13C variations in the Chalk Sea reassesses long-term glacio-eustasy as the potential driver of carbon isotope and climatic variations in the Maastrichtian.

The use of stable carbon isotope trends as a correlation tool: an example from the Surat Basin, Australia

The Surat Basin’s Middle Jurassic Walloon Subgroup is a productive coal seam gas source in Queensland, Australia. The Walloon Subgroup can be subdivided into the Upper and Lower Juandah coal measures, the Tangalooma Sandstone, the Taroom Coal Measures, and the Eurombah/Durabilla Formation, from top to bottom. Correlation across the basin is challenging due to high lateral variability and lack of extensive stratigraphic markers. The Walloon Subgroup is also, in places, incised by the overlying Springbok Sandstone, sometimes interpreted as far down as the Tangalooma Sandstone. New age dates suggest that the Walloon Coal Measures are Oxfordian in age and mark a period of high rates of Corg production and burial, and an intermittent decrease of atmospheric pCO2. The un- or dis-conformable base of the Springbok Sandstone coincides with a turning point of this supposedly global phenomenon. This study uses organic stable carbon isotope trends as a correlation tool within the Surat Basin’s Walloon Subgroup and its overlying Springbok Sandstone. Analysis of a stratigraphic suite of coal samples from several wells across the Surat Basin shows a gradual enrichment in 13C up section from the Taroom to the Lower Juandah Coal Measures, with the most positive δ13C values within the Upper Juandah Coal Measures. Thereafter there is a rapid reversal to more negative δ13C values for coal samples of the Springbok Sandstone. The upward enrichment occurs well before the shift in maceral composition to increased inertinite content in the coals, suggesting more global allogenic processes are controlling the carbon isotopic trend. The consistency of these trends lends a more confident correlation for sub-units within the Walloon Subgroup, and assists in determining the level of incision disconformity of the Springbok Sandstone.

Origin and evolution of geothermal fluids from Las Tres Vírgenes and Cerro Prieto fields, Mexico – Co-genetic volcanic activity and paleoclimatic constraints

Major and trace elements, noble gases, and stable (δD, δ18O) and cosmogenic (3H, 14C) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Vírgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured 3He/4He (R) ratios normalized to the atmospheric value (Ra = 1.386 × 10−6) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U–Th/4He ages for CP fluids (0.7–7 Ma) suggest the presence of a sustained 4He flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5–3 Ma). Radiogenic in situ4He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 ± 0.11 pmC – 35.61 ± 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene – Early Holocene meteoric water (58–75%) with a fossil seawater component (25–42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. δD values for the LTV meteoric end-member, which are 20‰–44‰ depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9–13.6 °C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonic–volcanic activity in the Las Tres Vírgenes region. Fast hydrothermal alteration processes caused a secondary, positive δ18O-shift from 4‰ to 6‰ for LTV and from 2‰ to 4‰ for CP geothermal fluids since the Late Glacial infiltration.

Temporal dynamics of groundwater-dissolved inorganic carbon beneath a drought-affected braided stream: Platte River case study

Impacts of environmental changes on groundwater carbon cycling are poorly understood despite their potentially high relevance to terrestrial carbon budgets. This study focuses on streambed groundwater chemistry during a period of drought-induced river drying and consequent disconnection between surface water and groundwater. Shallow groundwater underlying vegetated and bare portions of a braided streambed in the Platte River (Nebraska, USA) was monitored during drought conditions in summer 2012. Water temperature and dissolved inorganic carbon (dominated by HCO3−) in streambed groundwater were correlated over a 3 month period coinciding with a decline in river discharge from 35 to 0 m3 s−1. Physical, chemical, and isotopic parameters were monitored to investigate mechanisms affecting the HCO3− trend. Equilibrium thermodynamic modeling suggests that an increase of pCO2 near the water table, coupled with carbonate mineral weathering, can explain the trend. Stronger temporal trends in Ca2+ and Mg2+ compared to Cl− are consistent with carbonate mineral reequilibria rather than evaporative concentration as the primary mechanism of the increased HCO3−. Stable isotope trends are not apparent, providing further evidence of thermodynamic controls rather than evaporation from the water table. A combination of increased temperature and O2 in the dewatered portion of the streambed is the most likely driver of increased pCO2 near the water table. Results of this study highlight potential linkages between surface environmental changes and groundwater chemistry and underscore the need for high-resolution chemical monitoring of alluvial groundwater in order to identify environmental change impacts.

Incorporating stable isotopes into a multidisciplinary framework to improve data inference and their conservation and management application

Through its ability to address complex ecological questions and the possibility of analysing large sample sizes to understand population‑level processes, the use of stable isotope analysis (δ13C and δ15N) has grown rapidly in recent years. Importantly, it is now becoming an accepted tool to derive data for conservation and management planning at the species, community and ecosystem levels. With this acceptance, however, the stable isotope research community faces new challenges to ensure that data are interpreted and presented effectively to maximise their potential for guiding management. We present a case study on stable isotope trends in the vertebrae of white sharks Carcharodon carcharias to show how multiple plausible explanations could be provided to explain the observed isotopic patterns, a point that is likely ubiquitous among isotope studies in ecology. Based on this, we promote that integrating stable isotope data in a multidisciplinary framework will generate the most reliable data for conservationists and resource managers. If this is not possible, we suggest that the isotope community should be more accepting of presenting multiple possible explanations for trends observed in data, rather than focusing on a single interpretation that could potentially misguide management.

Use of Stable Isotopes to Trace Municipal Wastewater Effluents into Food Webs within a Highly Developed River System

Many anthropogenic inputs, such as municipal wastewater effluents (MWWEs), affect stable isotope signatures (δ13C and δ15N) at the base of exposed food webs creating spatial patterns reflecting their incorporation into aquatic food webs. The Grand River in southern Ontario, Canada, is a heavily modified, rapidly urbanizing river that assimilates wastewater from 30 municipal wastewater treatment plants. Stable isotope analysis was applied to resident aquatic invertebrates and fish influenced by three different wastewater outfalls in early, middle, and late summer to determine how values shifted seasonally and with differing effluent quality. There was a slight increase in δ13C in both invertebrates and fish in late summer downstream from the three outfalls, but it is difficult to separate effects of the effluents from downstream gradients. Downstream of two of the three outfalls, the δ15N tended to increase relative to upstream, while the remaining effluent, of the poorest quality, decreased δ15N values of both invertebrates and fish. Spatial trends in stable isotopes became more pronounced as the summer progressed with the greatest between-site differences occurring in late summer. This study reflects the complex nutrient dynamics associated with MWWE inputs to rivers and contributes to our understanding and application of stable isotope analysis in impacted lotic ecosystems. Copyright © 2014 John Wiley & Sons, Ltd.

Meteoric diagenesis and fluid-rock interaction in the Middle Permian Capitan backreef: Yates Formation, Slaughter Canyon, New Mexico

This study examines the geochemical record of diagenesis in the Middle Permian Yates shelf, exposed in Slaughter Canyon, New Mexico. This diagenetic history significantly modified lithologies, depositional fabrics, and pore systems. Early diagenesis was dominated during sea level highstands by marine cementation and reflux dolomitization, and during sea level lowstands by meteoric cementation and stabilization—the focus of this study. This early diagenesis variably overprinted primary marine isotopic signatures, potentially leading to erroneous chemostratigraphic correlations or paleoclimate reconstructions. Four correlative sections through one m-scale cycle were analyzed for their and values. They show significant (2–4‰) and variability in coeval, texturally well-preserved calcites. The and values of marine cements, brachiopods, bulk carbonate, micritic matrix, and the first generation of meteoric spar (from high to low values) delineate an “inverted J curve,” indicating the variable alteration of components by diagenetic fluids. Numerical models indicate that the observed stable isotope trend is most consistent with diagenetic alteration in a partially closed system by meteoric fluids mixed with a progressively diminishing contribution of recycled marine waters. In the Yates shelf, marine cements provide a more robust primary isotopic record than micritic matrix; however, neither preserves primary seawater isotopic values. Furthermore, common criteria used to diagenetically screen samples proved inadequate (e.g., textural preservation, staining, luminescence, depletion near sequence boundaries). Instead, diagenetic resetting is resolved by analyzing multiple, closely spaced, independently correlated sections, and by delineating trends between primary and later diagenetic components in populations of isotopic data.

Red calcite: an indicator of paleo‐karst systems associated with bauxitic unconformities

AbstractA unique red calcite generation, which fills fractures/cavities, is hosted by Mesozoic carbonates in the Transdanubian Range, Hungary. Solid inclusions are located along growth zones of calcite. Hematite, the most abundant solid inclusion, gives the red colour of it. Outcrop‐scale geometry, mineralogical features and detrital mineral assemblage (hematite, gibbsite, goethite, kaolinite, smectite, illite, Cr‐spinel, monazite, xenotime, zircon, apatite and Ti‐oxide) of calcite precipitates suggest strong correlation between the calcite and nearby karst bauxite deposits. Fluid inclusion petrography and microthermometry (T &lt; 50°C; salinity from 0 to 0.17 NaCl eq. w%) of primary fluid inclusions, and the stable isotope trend of the calcite, following the meteoric water line, clearly indicate vadose and phreatic meteoric origin in a near‐surface karst system. The late Cretaceous to mid‐Eocene unconformity‐related cavity‐filling deposits occur close to the surface; indicating that the most recent Quaternary exhumation re‐exposed those surfaces that existed at the time of calcite mineralization. Thus, red calcite precipitates are interpreted as being speleothems, vestiges of the subterranean part of the pre‐Middle Eocene karst. The infiltrated, fine bauxite particles enclosed by the calcite are the witnesses of the once areally extensive pre‐Middle Eocene bauxitic blanket that became partially eroded by the time of the deposition of the cover beds. Red calcite when found in core samples may provide good evidence on bauxite formation associated with the overlying unconformity, even if it was later removed by erosion. Therefore, presence or absence of red calcite may be used as distinguishing criteria between karst episodes with or without bauxite formation.