Source Signature Research Articles

Evaluating the applicability of the ratio of PM2.5 and carbon monoxide as source signatures

Air pollution is among the top risk faced by people around the world, and therefore combating it is among the top priorities. It begins with identifying the sources that contribute the most to local air pollution to prioritize their control. There are advanced methods for source identification and apportionment, but such methods are not available in many low-income countries and not everywhere in all high-income countries. We propose a simplified method by using source the signatures to help obtain information about the local source contribution if no other methods are available. Using low-cost monitors, particle mass (PM2.5) and carbon monoxide (CO) concentrations were measured and the ratio of CO/PM2.5 was determined. We investigated outdoor and indoor sources, including vehicular exhaust, combustion of biomass, incense and mosquito coil burning, and cigarette smoking. The results show that the ratios differed significantly between certain pollutant sources. Compressed natural gas (CNG) engines have a high ratio (mean value of 972 ± 419), which is attributed to relatively low PM2.5 emissions, while ship emissions and cigarette smoke recorded a relatively low ratio. Most traffic emissions recorded higher ratios than those of bushfire emissions, and ratios of most outdoor pollutant sources were much higher than those of indoor pollutant sources. There is a clear trend for ratios to decrease from high to low for CNG, petrol, diesel for buses, and fuel for ships. Our results suggest that the ratio of CO/PM2.5 can be used as an effective method to identify pollution sources.

Year-round observations of stable carbon isotopic composition of carboxylic acids, oxoacids and α-Dicarbonyls in fine aerosols at Tianjin, North China: Implications for origins and aging

To better understand the origins and photochemical processing (aging) of organic aerosols (OA), we studied fine aerosols (PM2.5) collected at urban (Nankai District (ND)) and suburban (Haihe Education Park (HEP)) Tianjin, North China over a one-year period (2018–2019) for stable carbon isotopic composition (δ13C) of water-soluble diacids, oxoacids, α-dicarbonyls and fatty acids. Maleic (M, −18.3 ± 10.9‰ at ND and −23.5 ± 10.2‰ at HEP) and fumaric (F, −22.0 ± 12.1‰ at ND and −22.5 ± 10.5‰ at HEP) acids were found to be most enriched with 13C followed by oxalic acid (C2, −24.7 ± 3.9‰ at ND and −25.9 ± 4.7‰ at HEP) during the campaign. Based on seasonal changes in δ13C of selected marker species: C6 and C9 diacids, phthalic, glyoxylic and pyruvic acids and glyoxal, and their comparison with the source signatures, we found that water-soluble OA in Tianjin were mainly originated from fossil fuel combustion and biomass burning emissions and were subjected for significant aging. The contribution from fossil fuel combustion including coal combustion was high in autumn and winter, especially at ND. Considering the enrichment of 13C in specific species together with linear relations of δ13C of selected species with their concentrations, with mass ratios and with the relative abundance of C2 diacid, we inferred that the photochemical transformations of longer-chain diacids, oxidation of α-dicarbonyls (Gly and mGly), preferably in gas phase, were important in warm period (March–September), whereas the oxidation of Gly, mGly and other precursors in aqueous phase were major in cold period (October–February).

Measurements of Atmospheric Methane Emissions from Stray Gas Migration: A Case Study from the Marcellus Shale.

Understanding emissions of methane from legacy and ongoing shale gas development requires both regional studies that assess the frequency of emissions and case studies that assess causation. We present the first direct measurements of emissions in a case study of a putatively leaking gas well in the largest shale gas play in the United States. We quantify atmospheric methane emissions in farmland >2 km from the nearest shale gas well cited for casing and cementing issues. We find that emissions are highly heterogeneous as they travel long distances in the subsurface. Emissions were measured near observed patches of dead vegetation and methane bubbling from a stream. An eddy covariance flux tower, chamber flux measurements, and a survey of enhancements of the near-surface methane mole fraction were used to quantify emissions and evaluate the spatial and temporal variability. We combined eddy covariance measurements with the survey of the methane mole fraction to estimate total emissions over the study area (2,800 m2). Estimated at ∼6 kg CH4 day-1, emissions were spatially heterogeneous but showed no temporal trends over 6 months. The isotopic signature of the atmospheric CH4 source (δ13CH4) was equal to -29‰, consistent with methane of thermogenic origin and similar to the isotopic signature of the gas reported from the nearest shale gas well. While the magnitude of emissions from the potential leak is modest compared to large emitters identified among shale gas production sites, it is large compared to estimates of emissions from single abandoned wells. Since other areas of emissions have been identified close to this putatively leaking well, our estimate of emissions likely represents only a portion of total emissions from this event. More comprehensive quantification will require more extensive spatial and temporal sampling of the locations of gas migration to the surface as well as an investigation into the mechanisms of subsurface gas migration. This work highlights an example of atmospheric methane emissions from potential stray gas migration at a location far from a well pad, and further research should explore the frequency and mechanisms behind these types of events to inform careful and strategic natural gas development.

Identifying and estimating the sources of river flow in the cold arid desert environment of Upper Indus River Basin (UIRB), western Himalayas

A reliable water supply in different Himalayan River basins is increasingly important for domestic, agriculture, and hydropower generation. These water resources are under serious threat due to climate change, with the potential to alter the economic stability of 237 million people living in the Indus River Basin alone. In the present study, we used new stable water isotope data set to identify and estimate the different sources of streamflow and their controlling factors in the Upper Indus River Basin (UIRB), India. The data set presented wide spatial and temporal variability without the distinct isotopic signature of various sources of river flow. However, variable but distinct signatures of sources of river/stream flow exist at the sub-basin or catchment scale. These variabilities are ascribed to changing physiographical, meteorological, and local climatic conditions. Further, the distinct microclimatic conditions including altitudinal variability, aspect slope, etc. govern the spatio-temporal variability of sources and streamflow, hence different lapse rates at sub-basin/catchment scale. The study suggested that the contribution of snowmelt and glacier melt to river flow varies spatially and temporally. The Bayesian mixing model results suggested that snowmelt contribution is higher in Indus (63 ± 1.2%) and Shyok (58 ± 1.7%) while as, glacier melt contribution is higher in Nubra 64 ± 2.3% and Suru 60 ± 2.7% sub-basins/catchments. The groundwater contribution (baseflow) sustains and regulates the flow in rivers/streams during winter and spring, which is very vital for the local water supply. The study suggests that the spatially diverse rugged topography and microclimate in UIRB dominantly control the differential contribution from various sources of river flow. The warming climate, which has resulted in a decrease in solid precipitation, continuous glacier mass loss, early melting of snow cover, etc., would have an inconsistent impact on the perennial flow of rivers with the potential to alter the economic and political stability in the region.

Odds and ends of atmospheric mercury in Europe and over the North Atlantic Ocean: temporal trends of 25 years of measurements

Abstract. The global monitoring plan of the Minamata Convention on Mercury was established to generate long-term data necessary for evaluating the effectiveness of regulatory measures at a global scale. After 25 years of monitoring (since 1995), Mace Head is one of the atmospheric monitoring stations with the longest mercury record and has produced sufficient data for the analysis of temporal trends of total gaseous mercury (TGM) in Europe and the North Atlantic. Using concentration-weighted trajectories for atmospheric mercury measured at Mace Head as well as another five locations in Europe, Amderma, Andøya, Villum, Waldhof and Zeppelin, we identify the regional probabilistic source contribution factor and its changes for the period of 1996 to 2019. Temporal trends indicate that concentrations of mercury in the atmosphere in Europe and the North Atlantic have declined significantly over the past 25 years at a non-monotonic rate averaging 0.03 ngm-3yr-1. Concentrations of TGM at remote marine sites were shown to be affected by continental long-range transport, and evaluation of reanalysis back trajectories displays a significant decrease in TGM in continental air masses from Europe in the last 2 decades. In addition, using the relationship between mercury and other atmospheric trace gases that could serve as a source signature, we perform factorization regression analysis, based on positive rotatable factorization to solve probabilistic mass functions. We reconstructed atmospheric mercury concentration and assessed the contribution of the major natural and anthropogenic sources. The results reveal that the observed downward trend in the atmospheric mercury is mainly associated with a factor with a high load of long-lived anthropogenic species.

Mapping the spatial distribution of NO<sub>2</sub> with in situ and remote sensing instruments during the Munich NO<sub>2</sub> imaging campaign

Abstract. We present results from the Munich Nitrogen dioxide (NO2) Imaging Campaign (MuNIC), where NO2 near-surface concentrations (NSCs) and vertical column densities (VCDs) were measured with stationary, mobile, and airborne in situ and remote sensing instruments in Munich, Germany. The most intensive day of the campaign was 7 July 2016, when the NO2 VCD field was mapped with the Airborne Prism Experiment (APEX) imaging spectrometer. The spatial distribution of APEX VCDs was rather smooth, with a horizontal gradient between lower values upwind and higher values downwind of the city center. The NO2 map had no pronounced source signatures except for the plumes of two combined heat and power (CHP) plants. The APEX VCDs have a fair correlation with mobile multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations from two vehicles conducted on the same afternoon (r=0.55). In contrast to the VCDs, mobile NSC measurements revealed high spatial and temporal variability along the roads, with the highest values in congested areas and tunnels. The NOx emissions of the two CHP plants were estimated from the APEX observations using a mass-balance approach. The NOx emission estimates are consistent with CO2 emissions determined from two ground-based Fourier transform infrared (FTIR) instruments operated near one CHP plant. The estimates are higher than the reported emissions but are probably overestimated because the uncertainties are large, as conditions were unstable and convective with low and highly variable wind speeds. Under such conditions, the application of mass-balance approaches is problematic because they assume steady-state conditions. We conclude that airborne imaging spectrometers are well suited for mapping the spatial distribution of NO2 VCDs over large areas. The emission plumes of point sources can be detected in the APEX observations, but accurate flow fields are essential for estimating emissions with sufficient accuracy. The application of airborne imaging spectrometers for studying NSCs is less straightforward and requires us to account for the non-trivial relationship between VCDs and NSCs.

Biogeochemical impacts of sewage effluents in predominantly rural river catchments: Are point source inputs distinct to background diffuse pollution?

Discharge of treated sewage effluent to rivers can degrade aquatic ecosystem quality, interacting with multiple stressors in the wider catchment. In predominantly rural catchments, the river reach influence of point source effluents is unknown relative to complex background pressures. We examined water column, sediment and biofilm biogeochemical water quality parameters along river transects (200 m upstream to 1 km downstream) during summer at five wastewater treatment works (WWTW) in Scotland.Treated sewage effluent (subset, n = 3) pollutant concentrations varied between sites. Downstream concentration profiles of water and sediment biogeochemical parameters showed complex spatial changes. A hypothesised point source signature of elevated concentrations of pollution immediately downstream of WWTW then a decaying pollution ‘plume’ did not commonly occur. Instead, elevated soluble reactive phosphorus (SRP), ammonium and coliforms (maximum 0.23 mgP/l, 0.33 mgN/l and >2 × 106 MPN/100 ml) occurred immediately downstream of two WWTW, whereas some downstream pollutant concentrations decreased. Microbial substrate respiration responses only differed 1 km downstream. Significantly greater concentrations of sediment metal occurred >500 m downstream, likely due to the redeposition of historic contaminated sediments. Significantly lowered chlorophyll-a downstream of one WWTW coincided with elevated metals, despite water SRP and sediment P increases. Overall, stress caused to microbes and algae by effluent contaminants outweighed the subsidy effect of WWTW nutrients.We observed variable effluent flows to the rivers limited localised pollution downstream of WWTW and overall influence of arable land cover on river water quality. Together, this challenges views of consistently discharging point sources impacting low dilution sensitive rivers in summer contrasting with ‘diffuse’ sources. Thus, river water column and benthic compartments are altered at varying scales by point source effluents in combination with rural catchment pollution sources, both discrete (e.g. farmyards and septic tanks) and diffuse.

Gas geochemistry of geothermal fluids from the Hatchobaru geothermal field, Japan

We analyzed the chemical and isotopic composition of the geothermal fluids from the Hatchobaru geothermal field in the Kyushu Island, Japan. Fluid chemistry showed similarity with that reported in earlier studies. Chemical geothermometry provided an estimated reservoir temperature of 250–300 °C. Helium and carbon isotopic ratios of the steam is likely to reflect the signature of the magmatic heat source. The apparent equilibrium temperature for carbon isotope exchange between CO2 and CH4 was 375–430 °C. The sulfur isotope data were in accordance with the idea that an acidic Cl-SO4-type fluid was modified from a neutral Cl-type fluid.

Analysis of reduced and oxidized nitrogen-containing organic compounds at a coastal site in summer and winter

Abstract. Nitrogen-containing organic compounds, which may be directly emitted into the atmosphere or which may form via reactions with prevalent reactive nitrogen species (e.g., NH3, NOx, NO3), have important but uncertain effects on climate and human health. Using gas and liquid chromatography with soft ionization and high-resolution mass spectrometry, we performed a molecular-level speciation of functionalized organic compounds at a coastal site on the Long Island Sound in summer (during the 2018 Long Island Sound Tropospheric Ozone Study – LISTOS – campaign) and winter. This region often experiences poor air quality due to the emissions of reactive anthropogenic, biogenic, and marine-derived compounds and their chemical transformation products. We observed a range of functionalized compounds containing oxygen, nitrogen, and/or sulfur atoms resulting from these direct emissions and chemical transformations, including photochemical and aqueous-phase processing that was more pronounced in summer and winter, respectively. In both summer and winter, nitrogen-containing organic aerosols dominated the observed distribution of functionalized particle-phase species ionized by our analytical techniques, with 85 % and 68 % of total measured ion abundance containing a nitrogen atom, respectively. Nitrogen-containing particles included reduced nitrogen functional groups (e.g., amines, imines, azoles) and common NOz contributors (e.g., organonitrates). Reduced nitrogen functional groups observed in the particle phase were frequently paired with oxygen-containing groups elsewhere on the molecule, and their prevalence often rivaled that of oxidized nitrogen groups detected by our methods. Supplemental gas-phase measurements, collected on adsorptive samplers and analyzed with a novel liquid chromatography-based method, suggest that gas-phase reduced nitrogen compounds are possible contributing precursors to the observed nitrogen-containing particles. Altogether, this work highlights the prevalence of reduced nitrogen-containing compounds in the less-studied northeastern US and potentially in other regions with similar anthropogenic, biogenic, and marine source signatures.

Stable isotopic signatures of methane from waste sources through atmospheric measurements

This study aimed to characterize the carbon isotopic signatures (δ13C-CH4) of several methane waste sources, predominantly in the UK, and during field campaigns in the Netherlands and Turkey. CH4 plumes emitted from waste sources were detected during mobile surveys using a cavity ring-down spectroscopy (CRDS) analyser. Air samples were collected in the plumes for subsequent isotope analysis by gas chromatography isotope ratio mass spectrometry (GC-IRMS) to characterize δ13C-CH4. The isotopic signatures were determined through a Keeling plot approach and the bivariate correlated errors and intrinsic scatter (BCES) fitting method. The δ13C-CH4 and δ2H-CH4 signatures were identified from biogas plants (−54.6 ± 5.6‰, n = 34; −314.4 ± 23‰ n = 3), landfills (−56.8 ± 2.3‰, n = 43; −268.2 ± 2.1‰, n = 2), sewage treatment plants (−51.6 ± 2.2‰, n = 15; −303.9 ± 22‰, n = 6), composting facilities (−54.7 ± 3.9‰, n = 6), a landfill leachate treatment plant (−57.1 ± 1.8‰, n = 2), one water treatment plant (−53.7 ± 0.1‰) and a waste recycling facility (−53.2 ± 0.2‰). The overall signature of 71 waste sources ranged from −64.4 to −44.3‰, with an average of −55.1 ± 4.1‰ (n = 102) for δ13C, −341 to −267‰, with an average of −300.3 ± 25‰ (n = 11) for δ2H, which can be distinguished from other source types in the UK such as gas leaks and ruminants. The study also demonstrates that δ2H-CH4 signatures, in addition to δ13C-CH4, can aid in better waste source apportionment and increase the granularity of isotope data required to improve regional modelling.

Cosmogenic gamma-ray and neutrino fluxes from blazars associated with IceCube events

Context.Blazars constitute the vast majority of extragalacticγ-ray sources. They can also contribute a sizable fraction of the diffuse astrophysical neutrinos detected by IceCube. In the past few years, the real-time alert system of IceCube has led to the multiwavelength follow-up of very high-energy neutrino events of plausible astrophysical origin. Spatial and temporal coincidences of a number of these neutrino events withγ-ray blazars provide a unique opportunity to decipher cosmic-ray interactions in the relativistic jets.Aims.The aim of this work is to test if theγ-ray blazars associated with the IceCube neutrino events are also sources of ultra-high-energy cosmic rays (UHECRs;E > 1018eV).Methods.Assuming that blazars accelerate UHECRs, we calculate the “guaranteed” contribution to the line-of-sight cosmogenicγ-ray and neutrino fluxes from four blazars associated with IceCube neutrino events. We compare these fluxes with the sensitivities of the upcomingγ-ray imaging telescopes, such as the CTA, and with the planned neutrino detectors, such as IceCube-Gen2.Results.We find that detection of the cosmogenic neutrino fluxes from the blazars TXS 0506+056, PKS 1502+106, and GB6 J1040+0617 would require UHECR luminosity ≳10 times the inferred neutrino luminosity from the associated IceCube events, with the maximum UHECR proton energyEp, max ≈ 1020eV. Cosmogenicγ-ray emission from blazars TXS 0506+056, 3HSP J095507.9 +355101, and GB6 J1040+0617 can be detected by the CTA if the UHECR luminosity is ≳10 times the neutrino luminosity inferred from the associated IceCube events and forEp, max ≳ 1019eV.Conclusions.Detection of cosmogenic neutrino and/orγ-ray flux(es) from blazars associated with IceCube neutrinos may lead to the first direct signature(s) of UHECR sources. Given their relatively low redshifts and hence total energetics, TXS 0506+056 and 3HSP J095507.9+355101 should be the prime targets for upcoming large neutrino andγ-ray telescopes.

Palaeoredox reconstruction in the eastern Arabian Sea since the late Miocene: Insights from trace elements and stable isotopes of molybdenum (δ98/95Mo) and tungsten (δ186/184W) at IODP Site U1457 of Laxmi Basin

The present study investigates the oxygenation history of the northeastern Arabian Sea since the late Miocene using redox sensitive elemental and metal stable isotopic signatures in the deep-sea sediments. To achieve this, the sediment core samples collected at Site U1457 (67°55.80′E, 17°9.95′N, water depth 3534 m) of Laxmi Basin in the northeastern Arabian Sea during the International Ocean Discovery Program (IODP) Expedition 355 were analysed for a suite of elemental (Mo, W, U, V, Ba, Cd and P) and stable molybdenum (Mo) isotope (δ 98/95 Mo relative to NIST SRM 3134 lot No. 130418) as well as stable tungsten (W) isotope (δ 186/184 W relative to NIST SRM 3163 lot No. 080331) composition. Sedimentary δ 98/95 Mo values (−0.70‰ to +1.18‰) at IODP Site U1457 in the northeastern Arabian Sea indicated partial authigenic Mo component. In contrast, the sedimentary δ 186/184 W values (−0.02‰ to +0.21‰) were in the range similar to that of lithogenous material suggesting dominance of detrital composition. The study reveals that the water column in the eastern Arabian Sea was oxic during the late Miocene and Pliocene while oxic to suboxic condition prevailed during the Pleistocene. The study also explores that under oxic to suboxic condition with limited particle shuttling, the W isotopes do not undergo significant fractionation, and their isotope ratios reflect the detrital source signature. This study reports the first results on isotopic compositions of Mo and W in sediments of the northeastern Arabian Sea since the late Miocene to investigate the palaeoredox conditions on a million-year time scale. • First results of δ 98/95 Mo and δ 186/184 W in the late Miocene Arabian Sea sediments. • Oxic condition during late Miocene & Pliocene, and suboxic during Pleistocene. • δ 98/95 Mo values (−0.70 to +1.18‰) exhibits partial authigenic component. • δ 186/184 W values (−0.02 to +0.21‰) exhibit dominant detrital control. • Under oxic setting with limited particle shuttling, tungsten isotopes do not fractionate.

Silver isotope fractionation in ore-forming hydrothermal systems

The behavior of silver in hydrothermal ore-forming systems has been the subject of numerous studies. Equilibrium and Rayleigh-type isotope fractionation of silver during transport- and deposition-related processes in silver-bearing hydrothermal ore-forming systems are investigated experimentally and theoretically in this study. The results of open-system evaporation experiments in a simple Ag+–H2O system show that the transfer kinetics of silver from liquid to vapor follows a second-order kinetic reaction model with activation energy (Ea) values of 131.20 ± 14.63 kJ·mol−1 and 145.38 ± 2.93 kJ·mol−1 in neutral and acidic solutions, respectively. No loss of silver from the solution was observed during surface evaporation, and the rate constant, ka, increased exponentially with temperature above 343 K at ambient pressure. During vapor–liquid separation, the vapor is enriched in 107Ag, whereas the liquid is enriched in 109Ag. The silver isotope fractionation follows a Rayleigh-type fractionation model with αvapor-liquid factors of 0.99936–0.99985 at 373 K under neutral and acidic conditions, respectively, whereas the temperature-dependent equilibrium isotope fractionation follows the relationship, 1000lnαvapor-liquid = −0.0039 × 106/T2 – 0.0037 based on density functional theory (DFT) calculations. This explains the relatively narrow interval of δ109Ag values (from −0.3 to +0.4‰) analyzed in Ag-bearing veins from hydrothermal ore-deposits. Based on the bond-strength of Ag to the nearest atoms in minerals, the 109Ag enrichment decreases in the order pyrargyrite, argentite, proustite, stephanite, native silver, iodargyrite, chlorargyrite. The equilibrium silver isotope fractionation between fluid and mineral during ore formation can be considerable depending on the temperature, the nature of the silver-bearing mineral deposited, whether or not there is vapor–liquid phase separation, and whether there is a reduction of oxidized silver (Ag(+1)) to the reduced form Ag(0). Consequently, the source signatures of δ109Ag are altered significantly by transport- and deposition-related processes during ore formation (e.g., boiling, precipitation and reduction). A new silver isotope-geothermometer is proposed based on the equilibrium silver isotope fractionation between argentite and stephanite. This geothermometer, which employs the relationship 1000lnαargentite-stephanite = 0.0128 × 106/T2 + 0.0014, can be used to constrain temperature in silver-bearing hydrothermal systems provided that argentite and stephanite are in equilibrium.

Zero-offset data estimation using CNN for applying 1D full waveform inversion

Abstract Full waveform inversion (FWI) in the time domain has limitations due to large computing time and memory requirements. Some studies have addressed this problem by using machine learning techniques. Most FWI studies using machine learning directly estimate the subsurface velocity structure by training the seismic data generated through various synthetic models to obtain the subsurface velocity structure. In this study, we propose a method to convert the common midpoint (CMP) gather to zero-offset data at a CMP location using a convolutional neural network (CNN) to increase the computing efficiency for the FWI. As the training data, we use synthetic data generated by the seismic exploration geometry and the source signature of field data. Since the proposed method performs FWI using the converted zero-offset data, it can be performed more efficiently than FWI using the existing multichannel data. However, it is difficult to apply a seismic exploration geometry and a source signature that has not been used for training. To verify the proposed method, it is applied to a synthetic model not used for learning as well as focused field data. It is confirmed that a proper subsurface velocity structure was obtained.

Evaluating the connectivity of the Yamuna and the Sarasvati during the Holocene: Evidence from geochemical provenance of sediment in the Markanda River valley, India

In the northwest Indo-Gangetic plains, the linkage of the Ghaggar-Hakra paleochannel with the Sarasvati River of early Indian literature and the Harappan (or Indus) civilization poses an important case for understanding drainage reorganization and human-landscape interactions. Among the major paleochannels of the Ghaggar-Hakra, sediment provenance studies on the paleo-Sutlej branch show its connectivity with the Sutlej catchment during the Holocene. However, evaluation of the role of fluvial morphodynamics on the Harappan settlements remains restricted because of limited effort to identify drainage reorganization of the paleo-Yamuna and its connectivity to the Ghaggar-Hakra paleochannel. Here we focus on the sediment deposits in the Markanda Valley, upstream of the northernmost purported paleochannel (Y1) of the river Yamuna, to explore the possible connectivity of the Yamuna catchment with the Ghaggar-Hakra paleochannel through Y1. We analyzed sediment samples from fan surfaces, fluvial terraces, and the modern riverbed for trace element and Sr-Nd isotope geochemistry to identify source signatures. Chronology of the samples, available through Singh et al. (2021), shows aggradation of the fan (57.7 ka) and T-1 to T-5 fluvial terraces during 13.1 to 6.0 ka, and deposition of paleoflood sediment during 3.9–3.8 ka. Results of provenance analysis suggest that sediment in the Markanda Valley was primarily derived from the silicate rocks of the Subathu-Dagshai-Kasauli Formations and the Siwalik Group of the Sub-Himalayas, thus implying that the river Yamuna was not connected to the ancient Ghaggar-Hakra river system (the Sarasvati) through the paleochannel Y1 during the Holocene. The Late Holocene sediment in the Ghaggar-Hakra plains (<4.5 ka), like the sediment in the Markanda Valley, was derived from the sub-Himalaya and transported through Y1, whereas the depositional pathway for the older sediment (>4.5 ka), having sources in the Himalayan hinterland, was likely the southernmost paleochannel (Y2) of the Yamuna.

A systematic approach to understand hydrogeochemical dynamics in large river systems: Development and application to the River Ganges (Ganga) in India

Large river systems, such as the River Ganges (Ganga), provide crucial water resources for the environment and society, yet often face significant challenges associated with cumulative impacts arising from upstream environmental and anthropogenic influences. Understanding the complex dynamics of such systems remains a major challenge, especially given accelerating environmental stressors including climate change and urbanization, and due to limitations in data and process understanding across scales. An integrated approach is required which robustly enables the hydrogeochemical dynamics and underpinning processes impacting water quality in large river systems to be explored. Here we develop a systematic approach for improving the understanding of hydrogeochemical dynamics and processes in large river systems, and apply this to a longitudinal survey (> 2500 km) of the River Ganges (Ganga) and key tributaries in the Indo-Gangetic basin. This framework enables us to succinctly interpret downstream water quality trends in response to the underpinning processes controlling major element hydrogeochemistry across the basin, based on conceptual water source signatures and dynamics. Informed by a 2019 post-monsoonal survey of 81 river bank-side sampling locations, the spatial distribution of a suite of selected physico-chemical and inorganic parameters, combined with segmented linear regression, reveals minor and major downstream hydrogeochemical transitions. We use this information to identify five major hydrogeochemical zones, characterized, in part, by the inputs of key tributaries, urban and agricultural areas, and estuarine inputs near the Bay of Bengal. Dominant trends are further explored by investigating geochemical relationships (e.g. Na:Cl, Ca:Na, Mg:Na, Sr:Ca and NO3:Cl), and how water source signatures and dynamics are modified by key processes, to assess the relative importance of controls such as dilution, evaporation, water-rock interactions (including carbonate and silicate weathering) and anthropogenic inputs. Mixing/dilution between sources and water-rock interactions explain most regional trends in major ion chemistry, although localized controls plausibly linked to anthropogenic activities are also evident in some locations. Temporal and spatial representativeness of river bank-side sampling are considered by supplementary sampling across the river at selected locations and via comparison to historical records. Limitations of such large-scale longitudinal sampling programs are discussed, as well as approaches to address some of these inherent challenges. This approach brings new, systematic insight into the basin-wide controls on the dominant geochemistry of the River Ganga, and provides a framework for characterising dominant hydrogeochemical zones, processes and controls, with utility to be transferable to other large river systems.

Defining isotopic signatures of potential procurement sources: A case study in the Mesa Verde region of the US Southwest

Combining strontium and oxygen isotope analyses has proved useful in determining animal procurement sources in archaeological case studies. In this paper, we analysed 87Sr/86Sr and δ18O of 55 rodents from archaeological contexts and 94 plants of the Mesa Verde and McElmo Dome regions (US Southwest) to estimate their regional isotopic signatures. We asked to what extent the new isotopic data would allow us to isolate one region from another, thus providing a background for interpreting fauna acquisition strategies. The results clearly show trends in bioavailable Sr across the Mesa Verde landscape. The lower 87Sr/86Sr values are synonymous with areas composed of igneous rock, while the highest values correspond, for the most part, to the San Juan Mountains, a region that likely provided large game hunting opportunities. Moreover, the McElmo Dome area associated with the sites under study is represented by a uniquely narrow range of Sr values suggesting that prey acquired outside a 10 km foraging range from such sites will be identifiable. Although plant oxygen isotope data did not further differentiate specific zones within the studied area, the significant correlation of plant δ18O with elevation will be useful for archaeologists to examine large game hunting strategies. Furthermore, this study expands the existing database of isotopic signatures for the American Southwest and, in particular, the San Juan Basin and its surroundings.

Climate-driven drainage reorganization of small mountainous rivers in Taiwan (East Asia) since the last glaciation: The Zhuoshui River example

As an important component in East Asia sediment source-to-sink systems, small mountainous rivers in Taiwan deliver disproportionately large amounts of sediments to oceans. Although the modern fluvial sediment transport processes, discharge fluxes and sediment compositions have been well investigated, the drainage evolution of these mountainous rivers remains understudied and sediment fluxes are expected to vary greatly in glacial-interglacial cycles due to the tremendous climatic fluctuations. To define how the drainage of Taiwan mountainous rivers has evolved since the last glaciation, we target a sediment core (98 m in length) from the Zhuoshui River delta, western Taiwan and conduct sediment petrography, heavy mineral analysis, detrital zircon U Pb geochronology and clay mineralogy to investigate provenance variations and river basin evolution since 60 ka. Sediments of the last glaciation show comparatively high illite crystallinity index values, low metasedimentary lithic fragment and stable heavy mineral contents and similar detrital zircon U Pb ages with downstream signatures, indicating prominent sediment contributions from the Coastal Plain and Western Foothills regions (elevation <1 km) during the glaciation. However, characteristics of the deglacial and Holocene sediments indicate high contributions from the higher Hsueshan Range and Central Range regions (elevation >1 km). We suggest that headward extension and drainage capture since the deglaciation, which was most likely due to the increasing monsoon rainfall, account for the provenance variations in the discharged sediments. This implies a climate-driven drainage reorganization of the small mountainous rivers in Taiwan since the last glaciation. Our findings highlight the previously-overlooked, variable provenance information from Taiwan in glacial-interglacial cycles, and the dynamic source signatures are important to East Asia sediment source-to-sink studies. • The provenance of Zhuoshui River sediments has changed since the last glaciation. • The provenance variation was caused by climate-driven drainage reorganization. • The river system of Zhuoshui River was not developed during the last glaciation. • The Zhuoshui River headward extended and captured other basins in the Holocene.

Demystifying mercury geochemistry in contaminated soil-groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses.

Interpretation of mercury (Hg) geochemistry in environmental systems remains a challenge. This is largely associated with the inability to identify specific Hg transformation processes and species using established analytical methods in Hg geochemistry (total Hg and Hg speciation). In this study, we demonstrate the improved Hg geochemical interpretation, particularly related to process tracing, that can be achieved when Hg stable isotope analyses are complemented by a suite of more established methods and applied to both solid- (soil) and liquid-phases (groundwater) across two Hg2+-chloride (HgCl2) contaminated sites with distinct geological and physicochemical properties. This novel approach allowed us to identify processes such as Hg2+ (i.e., HgCl2) sorption to the solid-phase, Hg2+ speciation changes associated with changes in groundwater level and redox conditions (particularly in the upper aquifer and capillary fringe), Hg2+ reduction to Hg0, and dark abiotic redox equilibration between Hg0 and Hg(II). Hg stable isotope analyses play a critical role in our ability to distinguish, or trace, these in situ processes. While we caution against the non-critical use of Hg isotope data for source tracing in environmental systems, due to potentially variable source signatures and overprinting by transformation processes, our study demonstrates the benefits of combining multiple analytical approaches, including Hg isotope ratios as a process tracer, to obtain an improved picture of the enigmatic geochemical behavior and fate of Hg at contaminated legacy sites.

Noise suppression using a near-source wavelet

Denoising becomes a nontrivial task when the noise and signal overlap in multiple domains, such as time, frequency, and velocity. Fortunately, signal and noise waveforms, in general, tend to remain morphologically different and such differences can be used to separate body-wave signals from other waveforms such as ground roll and cultural noise. The key in denoising using a near-source wavelet is to find a wavelet that is a close approximation of the true source signature and remains uncontaminated by the Green’s function in any significant manner. An inverse filter designed using such a wavelet selectively compresses the body waves that can be extracted using median and low-pass filters. The overall phenomenon is explained with a synthetic example. The idea is also tested on a land data set generated using a large weight-drop source in which the wavelet recorded approximately 3 m from the source location fulfills the criteria set in our method. The results suggest that the incremental effort of recording an extra trace close to the source location during acquisition may provide previously unavailable denoising opportunities during processing, although the trace itself may be redundant for imaging.