Increase In δ15N Research Articles

The effect of salinization and freshening events in coastal aquifers on nutrient characteristics as deduced from column experiments under aerobic and anaerobic conditions

Summary This study experimentally quantified the effect of seawater intrusion (salinization) and freshening events in coastal aquifers on nutrient (N, P and DSi) dynamics across the fresh–saline groundwater interface. Laboratory column experiments were conducted under aerobic and anaerobic conditions in order to simulate the processes occurring in the fresh–saline interface. They were performed with aquifer sediments, simulating the natural conditions during alterations of natural fresh groundwater to seawater and vice versa. The salinization and freshening experiments showed that NH4+ and PO43− and DSi were affected mainly by ion exchange processes while microbial activity controlled the nitrogen species NO3− and NO2−. Due to the cation exchange, salinization generated enrichment (above the expected conservative behavior) of NH4+, up to 80 μmol L−1 (an order of magnitude higher than in seawater or fresh groundwater). Under anaerobic conditions NO3− was removed by denitrification, as demonstrated by the decrease in NO3− concentrations, the increase in NO2− concentrations, and the increase in δ15N by 15–25‰. Clear evidence was shown for anion exchange of PO43−, which competes with HCO3− and boron on adsorption sites. DSi seems to take part in the exchange process, similar to PO43−.

Regional nitrogen dynamics in the TERENO Bode River catchment, Germany, as constrained by stable isotope patterns

Interactions between hydrological characteristics and microbial activities affect the isotopic composition of dissolved nitrate in surface water. Nitrogen and oxygen isotopic signatures of riverine nitrate in 133 sampling locations distributed over the Bode River catchment in the Harz Mountains, Germany, were used to identify nitrate sources and transformation processes. An annual monitoring programme consisting of seasonal sampling campaigns in spring, summer and autumn was conducted. δ15N and δ18O of nitrate and corresponding concentrations were measured as well as δ2H and δ18O of water to determine the deuterium excess. In addition, precipitation on 25 sampling stations was sampled and considered as a potential input factor. The Bode River catchment is strongly influenced by agricultural land use which is about 70 % of the overall size of the catchment. Different nitrogen sources such as ammonia (NH4) fertilizer, soil nitrogen, organic fertilizer or nitrate in precipitation show partly clear nitrate isotopic differences. Processes such as microbial denitrification result in fractionation and lead to an increase in δ15N of nitrate. We observed an evident regional and partly temporal variation of nitrate isotope signatures which are clearly different between main landscape types. Spring water sections within the high mountains contain nitrate in low concentrations with low δ15NNO3 values of −3 ‰ and high δ18ONO3 values up to 13 ‰. High mountain stream water sub-catchments dominated by nearly undisturbed forest and grassland contribute nitrate with δ15NNO3 and δ18ONO3 values of −1 and −3.5 ‰, respectively. In the further flow path, which is affected by an increasing agricultural land use and urban sewage, we recognized an increase in δ15NNO3 and δ18ONO3 up to 22 and 18 ‰, respectively, with high variations during the year. A correlation seems to exist between the percentage of agricultural land use area and the corresponding δ15NNO3 values for sub-catchments. A shift towards heavier isotope values in stream water samples taken in July 2012 is significant (p-value = 6 · 10−6) compared to samples from March and October 2012. We also see a season-depending impact of microbial denitrification. Denitrification, especially evident in the lowlands, predominantly takes place in the riverbeds. In addition, mixing processes of different nitrate sources and temperature-depending biological processes such as nitrification have to be taken into consideration. Constant-tempered groundwater does not play a noticeable role in the processes of the stream water system. As constrained from oxygen isotope signatures, precipitation associated with low nitrate concentrations does not have an obvious impact on stream water nitrate in the high mountain region.

Calculating a statistically robust δ13C and δ15N offset for charred cereal and pulse seeds

This study measured δ13C and δ15N of fresh and charred seeds from six different taxa of cereals and pulses. For each taxon, 12 different batches of seeds, all from the same growing context or field, were charred for 4, 8, or 24 hours at 215, 230, 245, or 260°C, with the thirteenth batch left uncharred. The least charred samples showed no average offset compared to their uncharred counterparts, but among the charred seeds, every 15°C increase resulted in an increase in δ15N by 0.12‰, while increasing the duration of charring by 4 hours resulted in an average increase of 0.04‰. Differences in δ13C among the charred samples were only minimally dependent on time (+0.016‰/4 hours duration), but not temperature. The average offset for all batches and all taxa was minimal for δ13C, at +0.11‰ (95% confidence interval (CI) 0.003–0.22‰). The average offset for δ15N of 0.31‰ (95% CI 0.05–0.57‰) is smaller than the conservative estimate of 1‰ used in previous studies. We demonstrate that using this new, smaller charring offset for δ15N makes a significant difference in paleodietary models, one that further emphasizes the potentially high δ15N of manured crops relative to animal forage.Statement of significance Archaeologists and scientists measure carbon and nitrogen isotope ratios in ancient crop remains that have been preserved by charring to understand how crops were grown in the past. The charring process preserves the seed's physical form but results in a slight alteration to the carbon and nitrogen isotope ratios. Our study systematically measured this offset within a window of charring temperatures and times: a window in which the seeds and grains have been completely chemically converted into a stable form, but where this process has been gentle enough that the physical form of the seed has not been distorted. From this study, we understand more precisely the size of the small adjustment we need to make to our measurements of ancient charred crop remains so that we can work back to the original values that reflect the conditions in which these crops were once grown.Data availability The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are contained within the paper.

First ice core records of NO3− stable isotopes from Lomonosovfonna, Svalbard

AbstractSamples from two ice cores drilled at Lomonosovfonna, Svalbard, covering the period 1957–2009, and 1650–1995, respectively, were analyzed for NO3−concentrations, and NO3− stable isotopes (δ15N and δ18O). Post‐1950 δ15N has an average of (−6.9 ± 1.9)‰, which is lower than the isotopic signal known for Summit, Greenland but agrees with values observed in recent Svalbard snow and aerosol. Pre‐1900 δ15N has an average of (4.2 ± 1.6)‰ suggesting that natural sources, enriched in the 15 N isotope, dominated before industrialization. The post‐1950 δ18O average of (75.1 ± 4.1)‰ agrees with data from low and polar latitudes, suggesting similar atmospheric NOy (NOy = NO + NO2 + HNO3) processing pathways. The combination of anthropogenic source δ15N and transport isotope effect was estimated as −29.1‰ for the last 60 years. This value is below the usual range of NOx (NOx = NO + NO2) anthropogenic sources which is likely the result of a transport isotope effect of −32‰. We suggest that the δ15N recorded at Lomonosovfonna is influenced mainly by fossil fuel combustion, soil emissions, and forest fires; the first and second being responsible for the marked decrease in δ15N observed in the post‐1950s record with soil emissions being associated to the decreasing trend in δ15N observed up to present time, and the third being responsible for the sharp increase of δ15N around 2000.

Linking Population Declines with Diet Quality in Vaux's Swifts

Over the past 50 years, a range of environmental stressors has resulted in aerial insectivores throughout North America experiencing sharp population declines. Vaux’s swifts (Chaetura vauxi) are small, long-distance migratory birds that are currently experiencing range-wide population declines, especially at the their northern range limit. Many studies attribute the Vaux’s swift’s population decline to a loss in nesting habitat; however, other potentially exacerbating factors, such as changes in insect composition and diet, remain unstudied. Here, we examine a ~26 year dietary archive of Vaux’s swift guano to ask whether diet composition has changed over time. Vaux’s swifts roost communally at the same roost sites each year when migrating, often within decommissioned brick chimneys. As a result, guano accumulates at the base of these roost sites, providing a chronostratified snapshot of the bird’s historic diet. We obtained a vertical core sample from a Vaux’s swift guano deposit in a chimney on Vancouver Island, BC, Canada. We symmetrically stratified the guano and assessed diet composition by visually analyzing egested arthropod exoskeletons, identifying them to order, and measuring δ15N signatures at each layer. Our 26-year data set revealed an increase in the ratio of Hemiptera to Coleoptera corresponding with an increase in δ15N, suggesting a possible decline in diet quality through time because Hemipterans tend towards higher trophic status than Coleopterans, but are of less caloric value per capita. In addition, δ15N was significantly negatively correlated with an annual population index (Breeding Bird Survey). We suggest a reduction in diet quality may be contributing to the decline of Vaux’s swift populations, and could be contributing to more declines in other aerial insectivore species.

Use of stable isotope composition variability of particulate organic matter to assess the anthropogenic organic matter in coastal environment (Istra Peninsula, Northern Adriatic)

Nitrogen and carbon stable isotope analysis of particulate organic matter (POM) was used to assess the impact of anthropogenically derived organic matter in coastal parts of the Istra Peninsula (Northern Adriatic). The investigation was conducted in areas potentially impacted and enriched with different amounts of organic matter. Elevated δ15N values in POM reflect enrichment of organic matter near the coast due to inadequate municipal infrastructure in cities and local septic systems as well as inputs from riverine terrestrial material. On the contrary, negative δ15N values reflect the depleting effect of purification plants. Significant differences in nitrogen stable isotope ratios were observed between western and southeastern coasts. The δ13C values show small depletion at sites with potentially greater anthropogenic impact. A weak temporal increase of δ15N from spring to late summer was observed. The results for nitrogen and carbon isotope ratios of POM and their comparisons with other areas suggest diverse amounts and sources of organic matter as well as differences in movement of floating POM within the water column. The δ15N and δ13C values reflect the ratios of marine and terrigenous organic matter, the latter being mainly anthropogenically influenced. The results finally suggest precaution in applicability of POM as tracers for detecting anthropogenic organic matter in marine coastal ecosystem.

Stable isotope ratios of carbon, nitrogen and oxygen in killer whales (Orcinus orca) stranded on the coast of Hokkaido, Japan

We analyzed δ13C, δ15N and δ18O in the muscle and liver from killer whales stranded on the coast of Japan. The δ15N values in the muscle samples from calves were apparently higher than those in their lactating mothers, suggesting that nursing may result in the higher δ15N values in the muscle samples of calves. The δ15N value in the muscle samples of male and female whales, except for the calves, were positively correlated with the δ13C values and body length, suggesting that the increases in δ15N were due to the growth of the whales and increase in their trophic level. In contrast, the δ18O values in the muscle samples of female whales except for the calves were negatively correlated with the δ13C and δ15N values. The δ18O may be lower in whales occupying higher trophic positions (δ15N), although it might also be affected by geographic and climatic conditions.

Ecological tracers reveal resource convergence among prey fish species in a large lake ecosystem

Summary We measured stable isotopes of carbon (δ13C) and nitrogen (δ15N) and fatty acid profiles in Lake Ontario alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), slimy sculpin (Cottus cognatus) and round goby (Neogobius melanostomus) collected from 1982 to 2008 to investigate how temporal variability in these ecological tracers can relate to ecosystem‐level changes associated with the establishment of highly invasive dreissenid mussels. Prey fish δ15N values remained relatively constant, with only slimy sculpin exhibiting a temporal increase in δ15N. In contrast, δ13C values for alewife, rainbow smelt and, especially, slimy sculpin became less negative over time and were consistent with the benthification of the Lake Ontario food web associated with dreissenids. Principal components analysis revealed higher contributions of 14:0 and 16:1n‐7 fatty acids and increasingly negative δ13C values in older samples in agreement with the greater historical importance of pelagic production for alewife, rainbow smelt and slimy sculpin. Temporal declines in fatty acid unsaturation indices and Σn‐3/Σn‐6 ratios, and also increased 24:0/14:0 ratios for alewife, rainbow smelt and slimy sculpin, indicated the increasing importance of nearshore production pathways for more recently collected fish and resulted in values more similar to those for round goby. These results indicate a temporal convergence of the food niche, whereas food partitioning has historically supported the coexistence of prey fish species in Lake Ontario. This convergence is consistent with changes in food‐web processes associated with the invasion of dreissenid mussels.

Partitioning between benthic and pelagic nitrate reduction in the Lake Lugano south basin

We evaluated the seasonal variation of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) rates in the sediments and the integrative N (and O) isotopic signatures of dissolved inorganic nitrogen (DIN) compounds in the overlying water column of the monomictic Lake Lugano south basin. Denitrification was the dominant NO−3 reduction pathway, whereas the contribution of anammox and DNRA to total benthic NO−3 reduction was < 6% and < 12%, respectively. Sedimentary denitrification rates were highest (up to 57.2 ± 16.8 µmol N m−2 h−1) during fully oxic bottom water conditions. With the formation of seasonal bottom water anoxia, NO−3 reduction was partitioned between water column and sedimentary processes. Total benthic NO−3 reduction rates determined in 15N‐label experiments and sediment—water interface N2 fluxes as calculated from water column N2 : Ar gradients revealed that sedimentary denitrification still accounted for ∼ 40% of total N2 production during bottom water anoxia. The partitioning between water column and sedimentary denitrification was further evaluated by the natural abundance stable N isotope composition of dissolved NO−3 in the water column. With anaerobic bottom water conditions, water column NO−3 concentrations gradually decreased, paralleled by an increase in δ15N— and δ18O— NO−3 from approximately 7‰ to 20‰ and from 2‰ to 14‰, respectively. Using a closed‐system (Rayleigh) model, the N and O isotope effects associated with community NO−3 consumption were 15ε ≈ 13.7‰ and 18ε ≈ 11.3‰, respectively. With the assumptions of a relatively low net N isotope effect associated with sedimentary denitrification (i.e., 15εsed = 1.5–3‰) vs. a fully expressed biological N isotope fractionation during water column denitrification (i.e., 15εwater = 20–25‰), our results confirm that 36–51% of reduction occurred within the sediment. The general agreement between the indirect (isotopic) approach and the flux and rate measurements suggests that water column nitrate isotope measurements can be used to distinguish between benthic and pelagic denitrification quantitatively.

Less means more: nutrient stress leads to higher δ15N ratios in fish

Summary Isotopic ratios of nitrogen are often used in food‐web studies to determine trophic position (including food chain length) and food sources, with greater ratios of 15N/14N (δ15N) usually considered indicative of higher trophic position. However, fasting and starving animals may also show a progressive increase in δ15N over time as they catabolise their own tissues. To determine the importance of starvation, we conducted a 4‐month laboratory experiment testing effects of starvation on body condition and isotope ratios in the muscle tissue of freshwater guppies (Poecilia reticulata). We also compared laboratory results and conclusions with analyses of body condition and isotope ratios in various small species of fish collected in four seasons from the Kansas River in north‐eastern Kansas, U.S.A. Fish starved in our laboratory experiment had significantly higher 15N values and poorer body condition than those fed more regularly. The diverse group of fish species collected in summer (July) from the Kansas River had higher weight‐to‐length ratios and lower 15N values than those retrieved in other seasons. Overall body condition resulting from reduced food consumption explained 44 and 53% of the variability in 15N for field and laboratory fish, respectively. These results are applicable to a wide variety of food‐web research but are especially pertinent to studies of organisms that undergo large changes in life history, dormancy, extended fasts or periods of significant nutritional allocation to young.

Elevated 15N/14N in particulate organic matter, zooplankton, and diatom frustule-bound nitrogen in the ice-covered water column of the Bering Sea eastern shelf

We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ15NDB), and zooplankton from water column material collected with net tows across the eastern Bering Sea shelf in late winter of 2007 and 2008, to investigate the N dynamics of primary and secondary production in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1–14.7‰; frustule-bound N: 4.9–20.7‰; zooplankton: 6.4–18.0‰), with POM δ15N reaching ~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28‰ and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic–pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.

Biological nitrate utilization in south Siberian lakes (Baikal and Hovsgol) during the Last Glacial period: the influence of climate change on primary productivity

Stable nitrogen isotope ratios (δ15N) in sediment cores recovered from two Siberian lakes (Baikal and Hovsgol) are determined in order to evaluate the biologically mediated nitrogen cycle from the Last Glacial period to the postglacial period. The ranges of δ15N values from the two lakes are similar, varying from +2.8‰ to +6.7‰. The high values of δ15N (around +6‰) during the Last Glacial Maximum (LGM, 26.5–19 cal ka BP) indicate that a larger fraction of the available nitrate in both lakes was utilised by phytoplankton. A cool and dry climate during the LGM may have limited the input of terrestrial nitrate, resulting in the use of a large fraction of the nitrate, and low productivity, in both lakes. The use of a smaller fraction of the nitrate by phytoplankton in both lakes after the LGM (19–12 cal ka BP) is reflected in the lower δ15N values (down to +3.0‰). The increase in summer insolation after the LGM caused increased vertical mixing of the lake water column, which transported nitrate upwards from the deeper waters and into the photic zone. The reduced rate of nitrogen consumption after the LGM indicates that nitrate was more readily available to phytoplankton at this time and did not limit their activities. This change suggests that other factors limited phytoplankton activity during this period. At the climatic transition between Oxygen Isotope Stages 2 and 1 (around 11.5 cal ka BP), the rapid increase in δ15N (from +3.0‰ to +6.0‰) reflected a rapid increase in nitrate utilization. At this transition, increases in precipitation and lake levels, which were associated with the increased intensity of the East Asian Monsoon, may have brought a large influx of nutrients required by the phytoplankton into the lakes from the watershed. This hydrological change led to increased phytoplankton activity in both lakes, resulting in increased nitrate consumption. The δ15N data obtained here from the sediments of lakes Baikal and Hovsgol suggest that the nitrogen cycle in the Siberian region is strongly linked to hydrological changes across the Asian continent, which are related to global climate change.

Freshwater Reservoir Offsets Investigated Through Paired Human-Faunal14C Dating and Stable Carbon and Nitrogen Isotope Analysis at Lake Baikal, Siberia

Thirty-three paired accelerator mass spectrometry (AMS) radiocarbon dates on human and terrestrial faunal remains from the same Neolithic and Early Bronze Age graves are used to develop a correction for the freshwater reservoir effect (FRE) at Lake Baikal, Siberia. Excluding two outliers, stable nitrogen isotope (δ15N) values show a positive correlation(r2= 0.672,p< 0.000) with offsets in14C yr between paired human and fauna determinations. The highest offset observed in our data set is 622 yr, which is close to the value of ∼700 yr suggested for endemic seals in the lake. For each per mil increase in δ15N, the offset increases by 77 ± 10 yr in the overall data set. However, there are indications that different regression models apply in each of two microregions of Cis-Baikal. In the first, sites on the southwest shore of the lake and along the Angara River show a strong positive correlation between δ15N values and offsets in14C yr (r2= 0.814,p< 0.000). In the other, the Little Sea, both δ13C and δ15N values make significant contributions to the model (adjustedr2= 0.878; δ13Cp< 0.001; δ15Np< 0.000). This can be related to the complex13C ecology of the lake, which displays one of the widest ranges of δ13C values known for any natural ecosystem. The results will be important in terms of refining the culture-history of the region, as well as exploring the dynamic interactions of hunter-gatherer communities both synchronically and diachronically.

Community N and O isotope fractionation by sulfide-dependent denitrification and anammox in a stratified lacustrine water column

We investigated the community nitrogen (N) and oxygen (O) isotope effects of fixed N loss in the northern basin of Lake Lugano, where sulfide-dependent denitrification and anammox are the main drivers of suboxic N2 production. A decrease in nitrate (NO3−) concentration toward the redox transition zone (RTZ) at mid-water depth was paralleled by an increase in δ15N and δ18O from approximately 5‰ to >20‰ and from 0‰ to >10‰, respectively. Ammonium (NH4+) concentrations were highest in the near-bottom water and decreased toward the RTZ concomitant with an increase in δ15N–NH4+ from ∼7‰ to >15‰. A diffusion-reaction model yielded N and O isotope enrichment factors that are significantly smaller than isotope effects reported previously for microbial NO3− reduction and NH4+ oxidation (15εNO3≈10‰, 18εNO3≈7‰, and 15εNH4≈10−12‰). For the Lake Lugano north basin, we constrain the apparent under-expression of the N isotope effects to: (1) environmental conditions (e.g., substrate limitation, low cell specific N transformation rates), or (2) low process-specific (chemolithotrophic denitrification and anammox) isotope fractionation. Our results have confirmed the robust nature of the co-linearity between N and O isotope enrichment during microbial denitrification beyond its organotrophic mode. However, the ratio of 18O to 15N enrichment (18εNO3:15εNO3) associated with NO3− reduction in the RTZ was ∼0.89, which is lower than observed in marine environments and in most culture experiments. We propose that chemolithotrophic NO3− reduction in the Lake Lugano north basin was partly catalyzed by the periplasmic dissimilatory nitrate reductase (Nap) (rather than the membrane-bound dissimilatory Nar), which is known to express comparably low 18εNO3:15εNO3 ratios in the ambient NO3− pool. However, NO2− re-oxidation, e.g., during anammox or microaerobic nitrification, could have contributed to the lowered 18O to 15N enrichment ratios. Although we do not yet understand the exact controls on the observed N (and O) isotope fractionation in the Lake Lugano north basin, our study implies that caution is advised when assuming canonical (i.e., high) N isotope effects for NO3− reduction and NH4+ oxidation in natural environments. In Lake Lugano, the community N (and O) isotope effects by sulfide-dependent denitrification and anammox in a natural ecosystem appear to be significantly lower than for organotrophic denitrification and aerobic ammonium oxidation.

Amino acid nitrogen isotopic composition patterns in lacustrine sedimenting matter

Amino acids (AAs) comprise a large fraction of organic nitrogen (N) in plankton and sedimenting matter. Aquatic studies of organic N compounds in general and of AAs in particular, mostly concentrate on marine environments. In order to study the cycling and fate of organic N and AAs in lakes, we measured the N isotopic composition (δ15N) of bulk organic matter (OM) and of single hydrolysable AAs in sediment trap and sediment samples from two Swiss lakes with contrasting trophic state: Lake Brienz, an oligotrophic lake with an oxic water column, and Lake Zug a eutrophic, meromictic lake. We also measured the N isotopic composition of water column nitrate, the likely inorganic N source during biosynthesis in both lakes. The δ15N–AA patterns found for the sediment trap material were consistent with published δ15N–AA data for marine plankton. The AA composition and primary δ15N–AA signatures are preserved until burial in the sediments. During early sedimentary diagenesis, the δ15N values of single AAs appear to increase, exceeding those of the bulk OM. This increase in δ15N–AA is paralleled by a decreased contribution of AAs to the total OM pool with progressed degradation, suggesting preferential AA degradation associated with a significant N isotope fractionation. Indicators for trophic level based on δ15N–AAs were determined, for the first time in lacustrine systems. In our samples, the trophic AAs were generally enriched in 15N compared to source AAs and higher trophic δ15N–AA values in Lake Zug were consistent with a higher trophic level of the bulk biomass compared to Lake Brienz. Especially the difference between average trophic δ15N–AAs and average source δ15N–AAs was sensitive to the trophic states of the two lakes. A proxy for total heterotrophic AA re-synthesis (ΣV), which is strongly associated with heterotrophic microbial reworking of the OM, was calculated based on δ15N values of trophic AAs. Higher ΣV in Lake Brienz indicate enhanced heterotrophic bacterial reworking of AAs under oligotrophic conditions. Despite changes in the δ15N–AA values within the sediments, the proxies based on these values were consistent over the studied sediment profile, indicating the preservation of trophic signatures; therefore, our results underscore that δ15N–AA analysis of sedimentary records represents a promising tool to assess trophic levels and bacterial re-synthesis in lakes.

Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean

Nitrogen cycling has been evaluated across a depth transect in the late Paleoproterozoic Animikie Basin (North America), spanning the end of Earth’s final period of global iron precipitation, and a major transition to euxinic conditions in areas of high productivity. Sediments from near shore, where productivity was highest, have δ15N compositions up to ∼3‰ higher than at more distal sites. This suggests that as NH4+ mixed vertically upward into the oxic photic zone from the anoxic ocean interior, it was either assimilated by organisms or oxidized. Subsequent enhanced production of N2 by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in δ15N close to shore. Any deficit in biologically available N was overcome by N2-fixing organisms, but the input of N with low δ15N from this process did not overwhelm the increase in δ15N from denitrification. Because there is no evidence for conditions of severe N stress arising from trace metal limitation (particularly Mo) of N fixation during the transition to euxinic conditions, losses of N were either very small (potentially because low O2 levels limited NH4+ oxidation), or alternative pathways that retained N were important. The fact that Mo appears to have remained bioavailable for N fixation, either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester Mo on a global scale, or that rivers directly delivered Mo to surface waters on the inner shelf. The effects of N2 fixation on δ15N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.

Influence of bottom water anoxia on nitrogen isotopic ratios and amino acid contributions of recent sediments from small eutrophic Lonar Lake, central India

Lonar Lake is a eutrophic, saline soda lake with permanently anoxic deep water. The high pH and deoxygenation result in very elevated δ15N of suspended particulate matter (SPM) and sediments due to denitrification and pH‐related loss of gaseous ammonium. SPM and sinking particles are predominantly aquatic in origin, whereas surface sediments are of mixed terrestrial plant and planktonic source. An indicator of degradation intensity was derived from a principal component analysis of the spectral distribution of amino acids and named Lonar degradation index (LI). A ratio of individual amino acids (Ox : Anox ratio) was additionally used to determine the relative degree of aerobic vs. anaerobic degradation. These two biogeochemical indicators can be used to detect changes in degradation intensity and redox conditions in the geological history, and thus the paleoclimatic interpretation of Lonar sediments. Surface sediments can be divided into three zones: (1) a nearshore, oxic zone of predominantly aquatic organic matter, in which oxidation leads to a strong diagenetic increase of δ15N; (2) an alluvial zone with a predominance of isotopically depleted land plant and soil organic matter degraded under oxic conditions; and (3) an anoxic, deep zone, which receives aquatic organic matter and land plant‐derived material transported near the bottom and in which organic matter is well preserved due to anoxic diagenetic conditions.

Mid- to late-Holocene reservoir-age variability and isotope-based palaeoenvironmental reconstruction in the Limfjord, Denmark

Palaeoenvironmental and 14C reservoir age variability in the Limfjord, a sound through northern Jutland, Denmark, was investigated for the period 7300 to 1300 cal yr BP. Shells and bulk sediment samples from a core from a former inlet, Kilen, were analysed by radiocarbon dating and stable isotope (C/N) measurements. A strong correlation between the C/N ratios and δ13C values verifies that these are good carbon source indicators and thus allow environmental reconstructions. Furthermore, δ13C values are correlated with salinity in the photic zone, inferred quantitatively from diatom assemblages. They are therefore used to differentiate between brackish and marine palaeo-conditions. 14C reservoir ages of shells vary from Δ R=−140 to +300 14C years. Between 7300 and 5400 cal. yr BP, reservoir age and stable isotope values are highly variable and indicate mixing of marine water and brackish surface waters with hard water effects. After 5400 cal. yr BP, the Δ R values stabilise and show an increasingly marine environment, with 14C reservoir ages close to 400 years (Δ R=0). After 2000 cal. yr BP, Kilen becomes brackish. Reservoir ages and stable isotope values are again highly variable and δ13C and C/N values are no longer correlated. Before 4000 cal. yr BP, δ15N values vary by only 1‰ and do not reflect the changes in the marine environment. A δ15N increase between 3500 and 2000 cal. yr BP signifies enhanced organic productivity or a change in agricultural practices.

Assessing natural variation and the effects of charring, burial and pre-treatment on the stable carbon and nitrogen isotope values of archaeobotanical cereals and pulses

The aim of this study is to assess the potential of charred archaeobotanical cereal grain and pulse seed δ13C and δ15N values to provide evidence of crop growing conditions and as a potential component of palaeodietary studies. In order to reliably interpret archaeobotanical δ13C and δ15N values it is necessary to take into account the impact of charring, burial and laboratory pre-treatment procedures. We examine the effects of charring and burial on bulk δ13C, δ15N, %C, %N and C:N ratios in modern cereal and pulse material, and of cleaning by acid–base–acid (ABA) pre-treatment on modern and archaeobotanical charred material. Our study utilised bulk grain and seed samples to help account for within-ear/pod and between-plant variability in δ13C and δ15N values. Heating at relatively low temperatures and for prolonged times (230 °C for up to 24 h) is conducive to the formation of well preserved, undistorted charred cereal grain and pulse seed. Heating for 24 h has a systematic and predictable effect on δ15N values, with increases of around 1‰ on average in cereal grains and pulse seeds, and no consistent impact on δ13C values. Increases in δ15N are likely due to the loss of lighter 14N via N-containing volatiles. Burial (for up to 2 years) and ABA pre-treatment have no significant effects on δ13C or δ15N values. After pre-treatment, however, the %C and %N contents of the archaeobotanical material more closely resembles that of the modern charred grains and seeds, suggesting that archaeobotanical remains accumulate non-structural material during burial but retain their original carbon and nitrogen content. Therefore %C, %N contents and C:N ratios can provide useful criteria for assessing archaeobotanical preservation.

Assessing the food web impacts of an anadromousArctic charr introduction to a sub‐Arctic watershed using stable isotopes

AbstractAnadromousArctic charr,Salvelinus alpinus(L.), was introduced to a sub‐Arctic river–lake system near the village ofKujjuuaq,Nunavik, and the stable isotope values and diets of key resident fish species were used to assess changes in feeding patterns. Stable isotope values for most species did not differ significantly between the pre‐ and post‐introduction periods, with observed shifts being within the bounds of expected natural variation. Lake chub,Couesius plumbeus(Agassiz), were the single species to show a difference between study periods, with a small but significant increase in δ15N. No significant post‐introduction changes were seen in lake trout,Salvelinus namaycush(Walbaum), omnivory or in any of the assessed quantitative food web metrics. Gut contents of major fish species similarly showed significant temporal overlap between the pre‐ and post‐introduction periods, and there was no significant change in species' weight–length relationships. The minor ecological impact was interpreted in relation to the availability of open niches exploitable by ecological generalists such asArctic charr. The explanation accords with the known habitat and feeding flexibility ofArctic charr and the ecological immaturity of sub‐Arctic lakes known to have driven adaptive variation amongArctic charr. Findings suggest that anadromousArctic charr may be introduced at moderate densities to other sub‐Arctic watersheds without major negative food web consequences for other resident fish species.