Nitrogen Isotope Measurements Research Articles

Australopithecus at Sterkfontein did not consume substantial mammalian meat.

Incorporation of animal-based foods into early hominin diets has been hypothesized to be a major catalyst of many important evolutionary events, including brain expansion. However, direct evidence of the onset and evolution of animal resource consumption in hominins remains elusive. The nitrogen-15 to nitrogen-14 ratio of collagen provides trophic information about individuals in modern and geologically recent ecosystems (<200,000 years ago), but diagenetic loss of this organic matter precludes studies of greater age. By contrast, nitrogen in tooth enamel is preserved for millions of years. We report enamel-bound organic nitrogen and carbonate carbon isotope measurements of Sterkfontein Member 4 mammalian fauna, including seven Australopithecus specimens. Our results suggest a variable but plant-based diet (largely C3) for these hominins. Therefore, we argue that Australopithecus at Sterkfontein did not engage in regular mammalian meat consumption.

Tidal Flushing Rather Than Non-Point Source Nitrogen Pollution Drives Nutrient Dynamics in A Putatively Eutrophic Estuary.

The effects of nonpoint source nutrients on estuaries can be difficult to pinpoint, with researchers often using indicator species, monitoring, and models to detect influence and change. Here, we made stable isotope measurements of nitrogen and carbon in sediment, water column particulates, primary producers, and consumers at 35 stations in the reportedly eutrophic Barnegat Bay (New Jersey) to assess N sources and processing pathways. Combined with water quality and hydrological data, our C and N isoscapes revealed four distinct geographic zones with diverging isotopic baselines, indicating variable nutrient sources and processing pathways. Overall, the carbon stable isotopes δ13C) reflected the terrestrial-marine gradient with the most depleted values in the urban and poorly flushed north of the estuary to the most enriched values in the salt marsh-dominated south. In contrast, the nitrogen stable isotope values δ15N) were most enriched near the oceanic inlets and were consistent with offshore δ15N values in particulate organic matter. Several biogeochemical processes likely alter δ15N, but the relatively lower δ15N values associated with the most urbanized area indicate that anthropogenic runoff is not a dominant N source to this area. Our findings stand in contrast to previous studies of similar estuaries, as δ15N signatures of biota in this system are inversely correlated to population density and nutrient concentrations. Further, our analyses of archival plant (Spartina sp., Phragmites australis) and shell (Geukensia demissa, Ilyanassa obsoleta) samples collected between 1880 and 2020 indicated that δ15N values have decreased over time, particularly in the consumers. Overall, we find that water quality issues appear to be most acute in the poorly flushed parts of Barnegat Bay and emphasize the important role that oceanic exchange plays in water quality and associated estuarine food webs in the lagoon.

Vertical Differences of Nitrate Sources in Urban Boundary Layer Based on Tower Measurements

Reducing nitrogen oxide (NOx) emissions is key to limiting fine particulate pollution globally. Nitrogen isotopes are powerful tools for exploring sources of atmospheric nitrate and its precursor NOx. This work performed simultaneous vertical measurements of nitrogen isotopes of nitrate (δ15N(NO3–)) at 8 and 260 m heights on a meteorological tower in Beijing. Although δ15N(NO3–) of the two heights were close, a significant difference (P < 0.01) in the δ15N of nitrogen oxides (NOx) was found in winter after considering the nitrogen isotope fractionation between NOx and the nitrate constrained by the oxygen anomaly (Δ17O) of nitrate. Combining the estimated δ15N of NOx and a Bayesian model, it was found that the proportion of coal combustion to nitrate aerosols was higher at 260 m (54% ± 9%) than that at the ground surface (31% ± 10%) in winter, suggesting that the air masses carrying coal-derived aerosols reached the upper air of Beijing through regional transport. In summer, the vertical variations of nitrate sources were not obvious. Our findings suggest that conventional ground-based observations may underestimate the contribution of coal combustion to atmospheric aerosols in the urban boundary layer as air pollutants are influenced by a combination of local emissions, meteorology, and regional transport.

The “post-weanling’s conundrum”: exploring the impact of infant and child feeding practices on early mortality in the Bronze Age burial cave of Moro de Alins, north-eastern Iberia, through stable isotope analysis

The relationship between infant and child feeding practices and early mortality is difficult to address in past societies. Here, stable carbon (δ13C) and nitrogen (δ15N) isotope measurements of bulk bone and sequential dentine samples of deciduous second and/or permanent first molars of four younger children, one older child, one late adolescent, and two young adults (n = 8) from Moro de Alins cave, north-eastern Iberia, are used to explore the potential impact of early-life nutrition on mortality in the Bronze Age. Isotope results are compatible with generally short exclusive breastfeeding and standard weaning periods compared to other pre-modern populations. However, there are differences in exclusive breastfeeding mean δ13C values and in Δ13C trophic shifts between exclusive breastfeeding and immediate post-weaning isotope values for those individuals who survived into adolescence and adulthood and those who did not. While the former seem to be consistent with trophic distances published for modern mother–infant pairs, the latter are above most of them. This may suggest that individuals who consumed similar foods to their mothers or suffered from less physiological stress during or after weaning had greater chances of survival during early childhood and beyond. Post-weaning seems to have been a particularly stressful period of life, where a number of instances of patterns of opposing isotopic covariance compatible with catabolic changes, often preceding death among non-survivors, are detected. This outcome shows the key role of nutritional and/or physiological status in early-life morbidity and mortality among partially and especially fully weaned children from pre-antibiotic, pre-vaccination, and poor sanitation contexts and proposes that adult survival is rooted in early life experiences, in keeping with the developmental origins of health and disease.

Combining Nitrogen Isotopes and Redox Proxies Strengthens Paleoenvironmental Interpretations: Examples From Neoproterozoic Snowball Earth Sediments

The history of the nitrogen cycle on Earth is linked to the redox evolution of the surface environment. Many nitrogen cycle fluxes are microbially mediated, and the particular fluxes operating at any given time in an ecosystem depend on the presence, absence or abundance of oxygen. However, interpreting this relationship is complicated as several isotopic fractionations associated with N-cycling are not diagnostic of a particular redox state. Thus, linking nitrogen isotopic analyses with redox-sensitive proxies is essential when interpretating past environments. Specifically, we use concentrations of U, V and Mo, along with Fe-speciation, to augment and contextualize nitrogen isotopic measurements. As an example, we consider samples from the Neoproterozoic Cryogenian period to suggest that there was oxygenated water, with associated aerobic N cycle fluxes. This interpretation is based on positive δ15N values between 4 to 80/00, Fe-speciation data consistent with anoxic bottom water during the Snowball ocean and oxygenated after, and redox-sensitive trace metals indicative of oxic weathering and surface water. Typically, high δ15N values are interpreted to reflect enhanced denitrification. We propose potential causes including a post-Snowball freshwater melt lid that suppressed deep water ventilation and that denitrification occurred more rapidly at high temperatures after the Snowball. These interpretations are buttressed by combined N isotope and redox analyses. This approach is especially useful during times of dynamic redox in the ocean-atmosphere system to interpret biologic isotopic signals.

Improved quality control criteria for stable carbon and nitrogen isotope measurements of ancient bone collagen

The carbon (δ13C) and nitrogen (δ15N) isotopic compositions of bone and tooth collagen provide a powerful tool for studying past biological, environmental, and cultural phenomena. Collagen has a well-understood chemical composition that has enabled the development of invaluable quality control (QC) criteria for isotopic data – something that is extremely rare among biogeochemical research on ancient biomolecules as a whole. The most important of these collagen QC criteria is atomic C:N ratio (ratio of carbon to nitrogen atoms), which provides an indicator of the extent to which the amount of carbon and nitrogen present in a sample matches the known composition of pure collagen, thereby indicating whether contamination or diagenesis may be influencing a sample's isotopic compositions. We present a model describing the relationship between the carbon and nitrogen isotopic and elemental compositions that accounts for the isotopic composition of the endogenous collagen and exogenous contaminants as well as taxon-specific information about the collagen's amino acid composition. In some cases the traditional C:N QC parameters are applicable, while in others they can result in the inclusion of unreliable (altered) isotopic data primarily due to contamination from humic substances. Using new and previously published data on taxa commonly encountered in ancient studies, we further illustrate how using traditional C:N QC parameters may lead to the inclusion of altered isotopic compositions in real archaeological and paleontological scenarios. We argue that the traditional ‘one size fits all’ approach to the C:N QC criterion should be avoided and we outline new collagen QC criteria specific to certain taxa and environments on the basis of the results of our model. These revised criteria will help to improve the interpretation of isotopic data by more accurately identifying samples with isotopic compositions altered by contamination.

Ice Age‐Holocene Similarity of Foraminifera‐Bound Nitrogen Isotope Ratios in the Eastern Equatorial Pacific

AbstractBulk sediment δ15N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ15N, then a deglacial δ15N maximum, followed by a gradual decline to a late Holocene δ15N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ15N of the LGM has been interpreted to reflect an ice age reduction in water column denitrification. We report foraminifera shell‐bound nitrogen isotope (FB‐δ15N) measurements for the two species Neogloboquadrina dutertrei and Neogloboquadrina incompta over the last 35 ka in two sediment cores from the eastern equatorial Pacific (EEP), both of which have the typical LGM‐to‐Holocene increase in bulk sediment δ15N. FB‐δ15N contrasts with bulk sediment δ15N by not indicating a lower δ15N during the LGM. Instead, the FB‐δ15N records are dominated by a deglacial δ15N maximum, with comparable LGM and Holocene values. The lower LGM δ15N of the bulk sediment records may be an artifact, possibly related to greater exogenous N inputs and/or weaker sedimentary diagenesis during the LGM. The new data raise the possibility that the previously inferred glacial reduction in ETP water column denitrification was incorrect. A review of reconstructed ice age conditions and geochemical box model output provides mechanistic support for this possibility. However, equatorial ocean circulation and nitrate‐rich surface water overlying both core sites allow for other possible interpretations, calling for replication at non‐equatorial ETP sites.

Climate‐Modulated Nutrient Conditions Along the Labrador Shelf: Evidence From Nitrogen Isotopes in a Six‐Hundred‐Year‐Old Crustose Coralline Alga

AbstractThe impacts of climate change on north Atlantic nutrient chemistry remain poorly understood, as there exist a multitude of rapidly changing biological and physical drivers of nutrient conditions throughout the region. Here, we present nitrogen isotope measurements derived from a six‐hundred‐year‐old crustose coralline alga (δ15Nalgal) to elucidate historical and contemporary trends in nitrate utilization and circulation patterns along the Labrador Shelf. Prior to the early 1900s, we argue that intervals during which utilization approached completion were controlled by reduced nitrate advection linked to an increased proportion of nitrate‐poor polar waters and subdued Atlantic influence, as expected from concurrent negative modes of the Atlantic multidecadal oscillation. While nitrate conditions should have recovered in recent years, our record suggests that high utilization persisted since ∼1870, which we also attribute to reduced Atlantic advection, likely associated with the twentieth‐century anthropogenic weakening of the Atlantic meridional overturning circulation. These results highlight the role of ongoing climate‐induced circulation changes in modulating nutrient distributions throughout the subpolar north Atlantic, which may have implications for other environmental phenomena such as fisheries and oceanic carbon storage.

First sample of N2H+ nitrogen isotopic ratio measurements in low-mass protostars

Context. The nitrogen isotopic ratio is considered an important diagnostic tool of the star formation process, and N2H+ is particularly important because it is directly linked to molecular nitrogen N2. However, theoretical models still do not provide an exhaustive explanation for the observed 14N/15N values. Aims. Recent theoretical works suggest that the 14N/15N behaviour is dominated by two competing reactions that destroy N2H+: dissociative recombination and reaction with CO. When CO is depleted from the gas phase, if the N2H+ recombination rate is lower with respect to that for N15NH+, the rarer isotopologue is destroyed more quickly. In prestellar cores, due to a combination of low temperatures and high densities, most CO is frozen in ices onto the dust grains, leading to high levels of depletion. On the contrary, in protostellar cores, where temperature are higher, CO ices evaporate back to the gas phase. This implies that the N2H+ isotopic ratio in protostellar cores should be lower than that in prestellar cores, and consistent with the elemental value of ≈440. We aim to test this hypothesis, producing the first sample of N2H+∕N15NH+ measurements in low-mass protostars. Methods. We observe the N2H+ and N15NH+ lowest rotational transition towards six young stellar objects in the Perseus and Taurus molecular clouds. We model the spectra with a custom python code using a constant Tex approach to fit the observations. We discuss in the Appendix the validity of this hypothesis. The derived column densities are used to compute the nitrogen isotopic ratios. Results. Our analysis yields an average of 14N/15N|pro = 420 ± 15 in the protostellar sample. This is consistent with the protosolar value of 440, and significantly lower than the average value previously obtained in a sample of prestellar objects. Conclusions. Our results are in agreement with the hypothesis that, when CO is depleted from the gas-phase, dissociative recombinations with free electrons destroy N15NH+ faster than N2H+, leading to high isotopic ratios in prestellar cores where carbon monoxide is frozen onto dust grains.

Quantification and Carbon and Nitrogen Isotopic Measurements of Heme B in Environmental Samples.

Heme B is an iron-coordinated tetrapyrrole molecule that acts as a cofactor in hemoproteins. It is expected to be ubiquitous in the environment, as b-type hemoproteins catalyze a variety of essential biochemical reactions. In this study, we developed an analytical method to quantify heme B in biological and environmental samples using high-performance liquid chromatography (HPLC) coupled to a photodiode array detector. The applicability of our method was further extended by the use of liquid chromatography/mass spectrometry (LC/MS; detection limit: ∼1 fmol), which enabled the quantification of a trace amount of dissolved heme B in filtered seawater and sedimentary heme B coexisting with an abundant interfering organic matrix. For compound-specific carbon and nitrogen isotopic measurements, heme B was successfully isolated and purified from biological and environmental samples by a combination of anion-exchange column chromatography, methyl esterification, and dual-step HPLC. While carbon and nitrogen isotopic compositions of heme B in phototrophs were mostly comparable to those of chlorophyll a, heme B in suspended particulate materials in coastal water and an intertidal sediment was 13C-depleted and 15N-enriched relative to chlorophyll a, suggesting that nonphototrophic microorganisms are also a significant source of heme B in natural environments.

Quality control for modern bone collagen stable carbon and nitrogen isotope measurements

Abstract Isotopic analyses of collagen, the main protein preserved in subfossil bone and tooth, has long provided a powerful tool for the reconstruction of ancient diets and environments. Although isotopic studies of contemporary ecosystems have typically focused on more accessible tissues (e.g. muscle, hair), there is growing interest in the potential for analyses of collagen because it is often available in hard tissue archives (e.g. scales, skin, bone, tooth), allowing for enhanced long‐term retrospective studies. The quality of measurements of the stable carbon and nitrogen isotopic compositions of ancient samples is subject to robust and well‐established criteria for detection of contaminants and diagenesis. Among these quality control (QC) criteria, the most widely utilized is the atomic C:N ratio (C:NAtomic), which for ancient samples has an acceptable range between 2.9 and 3.6. While this QC criterion was developed for ancient materials, it has increasingly being applied to collagen from modern tissues. Here, we use a large survey of published collagen amino acid compositions (n = 436) from 193 vertebrate species as well as recent experimental isotopic evidence from 413 modern collagen extracts to demonstrate that the C:NAtomic range used for ancient samples is not suitable for assessing collagen quality of modern and archived historical samples. For modern tissues, collagen C:NAtomic falling outside 3.00–3.30 for fish and 3.00–3.28 for mammals and birds can produce systematically skewed isotopic compositions and may lead to significant interpretative errors. These findings are followed by a review of protocols for improving C:NAtomic criteria for modern collagen extracts. Given the tremendous conservation and environmental policy‐informing potential that retrospective isotopic analyses of collagen from contemporary and archived vertebrate tissues have for addressing pressing questions about long‐term environmental conditions and species behaviours, it is critical that QC criteria tailored to modern tissues are established.

Resource utilisation and regional interaction in protohistoric Cambodia – The evidence from Angkor Borei

Angkor Borei is a protohistoric (ca. 500 BCE − 500 CE) site in southern Cambodia (Takeo Province), on the western edge of the Mekong Delta. Cambodia’s protohistoric period, concurrent with the Iron Age elsewhere in mainland Southeast Asia, is a period characterised by major socio-political transformation: early state formation, incorporation into the South China Sea network, and urbanisation. First occupied in the mid-first millennium BCE, Angkor Borei became the delta’s largest regional centre during the Funan period (c. 1st-6th century CE). This study builds on previous skeletal chemistry research, increasing the sample set by additional 15 individuals, to refine our understanding of the residential behaviour and exploitation strategies of the Angkor Borei mortuary sample. Using strontium, oxygen, carbon, and nitrogen isotope measurements from tooth enamel and bone, and incorporating bioavailable baseline strontium isotope data, we find that the majority of individuals have a childhood 87Sr/86Sr signature consistent with locally acquired food resources. For those individuals with outlier 87Sr/86Sr values, utilisation of the broader regional environment is suggested without the need to infer long-distance migration. The evidence for population stability at Angkor Borei during this dynamic period of increasing regional societal complexity indicates that the catalysts for change are manifold. Many factors are likely to have contributed to the genesis of early state society including social differentiation, cultural exchange, mercantile activity, residential mobility, and settlement growth, rather than one ‘external’ prime causative factor.

A Review of the Techniques Used for Measurements of Nitrogen Isotopes in Atmospheric Aerosols

This review of the methods used for measurements of stable nitrogen isotopes in gases and aerosol particles also summarizes some of the latest research on nitrate, ammonium, nitrogen oxides and ammonia in the atmosphere. The main methods used for the determination of N isotopes in nitrate and nitrogen oxides are one that makes use of denitrifying bacteria and a two-step chemical reduction method (cadmium + hydrazoic acid). A commonly used method for measuring nitrogen isotopes in ammonia and ammonium ion is a chemical conversion that uses sodium azide in an acetic acid buffer or hydroxylamine hydrochloride. A common characteristic of these methods is that nitrous oxide is the final analysis product, and the isotopes are determined by mass spectrometry. The precision of the analyses are typically very high (~ 0.25‰), and the methods require a small amount of sample, usually tens of nmols is sufficient. Some improvements to these methods and collection methods have been made in recent years, and these advances will facilitate research on nitrogen isotopes in the atmosphere.

Economic Diversification Supported the Growth of Mongolia\u2019s Nomadic Empires

Populations in Mongolia from the late second millennium B.C.E. through the Mongol Empire are traditionally assumed, by archaeologists and historians, to have maintained a highly specialized horse-facilitated form of mobile pastoralism. Until recently, a dearth of direct evidence for prehistoric human diet and subsistence economies in Mongolia has rendered systematic testing of this view impossible. Here, we present stable carbon and nitrogen isotope measurements of human bone collagen, and stable carbon isotope analysis of human enamel bioapatite, from 137 well-dated ancient Mongolian individuals spanning the period c. 4400 B.C.E. to 1300 C.E. Our results demonstrate an increase in consumption of C4 plants beginning at c. 800 B.C.E., almost certainly indicative of millet consumption, an interpretation supported by archaeological evidence. The escalating scale of millet consumption on the eastern Eurasian steppe over time, and an expansion of isotopic niche widths, indicate that historic Mongolian empires were supported by a diversification of economic strategies rather than uniform, specialized pastoralism.

Radiocarbon measurements using new automated graphite preparation laboratory coupled with stable isotope mass-spectrometry at Birbal Sahni Institute of Palaeosciences, Lucknow (India)

Conventional beta counting technique based radiocarbon dating facility at Birbal Sahni Institute of Palaeosciences- Lucknow was established in 1974 (Rajagopalan,1978). In 2017–18, BSIP received an upgrade with installation of an Automated Graphitization Equipment (AGE) coupled with an Elemental Analyser, a Carbonate Handling System (CHS) along with an in-line stable isotope mass-spectrometer (IRMS). Using this combo, stable Carbon (C), Nitrogen (N) and Sulfur (S) isotopic measurements could be carried out in both organic and inorganic type samples followed by graphite preparation (~1 mg) for 14C measurement by Accelerator Mass Spectrometry (AMS). This communication addresses details of pre-processing, processing, and quality checks adopted for achieving acceptable and demonstrable accuracy and precision of measured Δ14C, δ13C, δ15N, and δ34S measurements. Information regarding chemical preparation of samples for aforesaid stable and radio isotopic analysis is provided in succinct manner. Overall, average coefficient of variation determining precision of our graphite powders for 14C measurements is ~2.4%. The mean age of blank (anthracite) processed using established EA-IRMS-AGE unit comes as 42,100 ± 300 years.

The reactivity of lattice nitrogen within the Ni2Mo3N and NiCoMo3N phases

In this study, the reactivity of bulk lattice nitrogen within the filled β-Mn structured Ni2Mo3N phase has been investigated by application of powder neutron diffraction and heterolytic nitrogen isotopic exchange measurements. In contrast to Co3Mo3N, despite the similarity in the N immediate local environment comprising NMo6 octahedra, its reactivity is found to be limited and this lower reactivity was maintained upon the introduction of a significant proportion of cobalt to yield its filled β-Mn structured CoNiMo3N quaternary nitride counterpart.

Impact of abrupt sea ice loss on Greenland water isotopes during the last glacial period

Greenland ice cores provide excellent evidence of past abrupt climate changes. However, there is no universally accepted theory of how and why these Dansgaard-Oeschger (DO) events occur. Several mechanisms have been proposed to explain DO events, including sea ice, ice shelf buildup, ice sheets, atmospheric circulation, and meltwater changes. DO event temperature reconstructions depend on the stable water isotope ([Formula: see text]O) and nitrogen isotope measurements from Greenland ice cores: interpretation of these measurements holds the key to understanding the nature of DO events. Here, we demonstrate the primary importance of sea ice as a control on Greenland ice core [Formula: see text]O: 95% of the variability in [Formula: see text]O in southern Greenland is explained by DO event sea ice changes. Our suite of DO events, simulated using a general circulation model, accurately captures the amplitude of [Formula: see text]O enrichment during the abrupt DO event onsets. Simulated geographical variability is broadly consistent with available ice core evidence. We find an hitherto unknown sensitivity of the [Formula: see text]O paleothermometer to the magnitude of DO event temperature increase: the change in [Formula: see text]O per Kelvin temperature increase reduces with DO event amplitude. We show that this effect is controlled by precipitation seasonality.

Osteological and stable isotope (δ13C and δ15N) analysis of faunal remains from Khami, Zimbabwe

ABSTRACTThis paper presents distribution patterns and stable carbon and nitrogen isotope measurements of recently excavated faunal remains from two middens at Khami, a UNESCO World Heritage site in Zimbabwe. The middens are dated to c. AD 1475–1650. The results of the analysis indicate that food practices may have differed between high- and low-lying areas of the site, as reflected in the two excavated contexts studied here. δ13C values of serial samples of tooth dentine show that cattle and wild grazers consumed C4 grasses year-round. The availability of rich natural grazing would have been a considerable attraction to the builders of the site.

How the distribution of anthropogenic nitrogen has changed in Narragansett Bay (RI, USA) following major reductions in nutrient loads.

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ15N) and carbon (δ13C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ15N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1-2‰, on average. In contrast, the δ13C values measured in particulate matter and hard clams have declined by about the same amount. The δ15N results indicated that, even after the N-reductions, sewage N still plays an important role in supporting primary and secondary production throughout the Bay. However, the δ13C suggest that overall net production in Narragansett Bay has decreased. In the five years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.

Climate and anthropogenic controls on the carbon cycle of Xingyun Lake, China

The organic and inorganic carbon cycles in lakes are responsive to both natural climate variability and human-induced environmental changes. Here we present an 8600-year sediment record from Xingyun Lake in Yunnan, China that provides insight into carbon cycling and lake primary productivity using stable isotope measurements of organic carbon and nitrogen, carbon to nitrogen ratio, inorganic carbon isotopes, and mass accumulation rates of carbon, nitrogen, and calcite. The early to middle Holocene is characterized by relative stability in most proxies although some variation in organic carbon isotopes reflects fluctuating terrestrial organic matter inputs. The middle to late Holocene, from 5300 to 3300 years BP, is marked by amplified variability in nearly all proxies and suggests either increased C4 vegetation or increased primary productivity. These changes are coincident with declining lake levels and increased aridity throughout the Indian Summer Monsoon region. After 3300 years BP, the greatest variability is in nitrogen isotopes and coincides closely with human settlement in the region. These changes are accompanied by further indications of increased primary productivity, suggesting eutrophic lake conditions. While we cannot definitively link human activity to these changes, the balance of evidence suggests that other mechanisms are unlikely. This study supports previous work that anthropogenic activities began measurably impacting the landscape in southwestern China within the last few millennia.