Isotope Mass Balance Research Articles

Magnitude and Drivers of Oxic Methane Production in Small Temperate Lakes

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH4) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH4 production (OMP) have remained ambiguous. Based on the summer CH4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008-0.44 km2), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L-1 day-1) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH4 (δ13C-CH4) indicates direct photoautotrophic release as the most likely source of oxic CH4. Furthermore, we show that the oxic CH4 contribution to the SML CH4 saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km2. Given that global lake CH4 emissions are dominated by small lakes (SA of <1 km2), the large contribution of oxic CH4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH4 emissions.

Lacustrine diatom oxygen isotopes as palaeo precipitation proxy - Holocene environmental and snowmelt variations recorded at Lake Bolshoye Shchuchye, Polar Urals, Russia

The diatom oxygen isotope composition (δ18Odiatom) from lacustrine sediments helps tracing the hydrological and climate dynamics in individual lake catchments, and is generally linked to changes in temperature and δ18Olake. Lake Bolshoye Shchuchye (67°53′N; 66°19′ E; 186 m a.s.l) is the largest and deepest freshwater reservoir in the Polar Urals, Arctic Russia. The diatom oxygen isotope interpretation is supported by modern (isotope) hydrology, local bioindicators such as chironomids, isotope mass-balance modelling and a digital elevation model of the catchment.The Bolshoye Shchuchye δ18Odiatom record generally follows a decrease in summer insolation and the northern hemisphere (NH) temperature history. However, it displays exceptional, short-term variations exceeding 5‰, especially in Mid and Late Holocene. This centennial-scale variability occurs roughly contemporaneously with and similar in frequency to Holocene NH glacier advances. However, larger Holocene glacier advances in the Lake Bolshoye Shchuchye catchment are unknown and have not left any significant imprint on the lake sediment record. As Lake Bolshoye Shchuchye is deep and voluminous, about 30–50% of its volume needs to be exchanged with isotopically different water within decades to account for these shifts in the δ18Odiatom record. A plausible source of water with light isotope composition inflow is snow, known to be transported in surplus by snow redistribution from the windward to the leeward side of the Polar Urals. Here, we propose snow melt variability and associated influx changes being the dominant mechanism responsible for the observed short-term changes in the δ18Odiatom record. This is the first time such drastic, centennial-scale hydrological changes in a catchment have been identified in Holocene lacustrine diatom oxygen isotopes, which, for Lake Bolshoye Shchuchye, are interpreted as proxy for palaeo precipitation and, on millennial timescales, for summer temperatures.

Isotopic evidence for sources of dissolved carbon and the role of organic matter respiration in the Fraser River basin, Canada

Sources of dissolved and particulate carbon to the Fraser River system vary significantly in space and time. Tributaries in the northern interior of the basin consistently deliver higher concentrations of dissolved organic carbon (DOC) to the main stem than other tributaries. Based on samples collected near the Fraser River mouth throughout 2013, the radiocarbon age of DOC exported from the Fraser River does not change significantly across seasons despite a spike in DOC concentration during the freshet, suggesting modulation of heterogeneous upstream chemical and isotopic signals during transit through the river basin. Dissolved inorganic carbon (DIC) concentrations are highest in the Rocky Mountain headwater region where carbonate weathering is evident, but also in tributaries with high DOC concentrations, suggesting that DOC respiration may be responsible for a significant portion of DIC in this basin. Using an isotope and major ion mass balance approach to constrain the contributions of carbonate and silicate weathering and DOC respiration, we estimate that up to 33 ± 11% of DIC is derived from DOC respiration in some parts of the Fraser River basin. Overall, these results indicate close coupling between the cycling of DOC and DIC, and that carbon is actively processed and transformed during transport through the river network.

Identifying NOx Sources in Arequipa, Peru Using Nitrogen Isotopes in Particulate Nitrate

We reported on the first time series of δ15N in aerosol nitrate from South America. Particulate matter less than 2.5 microns in diameter (PM2.5) was collected at four sites located in Arequipa, a major city in southern Peru. The δ 15N values for nitrate in PM2.5 ranged from -1.7–15.9‰ and averaged 5.3 ± 3.0‰, with no significant difference between the four study sites and no discernable seasonal trend. These values are significantly higher than those in aerosol nitrate from southern hemisphere marine environments and those from the northern hemisphere. We explain the elevated values using an isotope mass balance mixing model that estimates a source NOx δ15N of -8 ± 3‰, derived mainly from anthropogenic sources (vehicles, industry). An isotope enabled 0-D photochemical box model was used to estimate the isotope enrichment of nitrate relative to NOx due to kinetic, equilibrium, and photolysis isotope effects occurring during NOx oxidation. This “source plus photochemistry” approach resulted in general agreement with the observations. This suggests that if the photochemistry effect can be accounted for, nitrate δ15N can be used to assess the relative importance of NOx sources and could be a new tool to validate NOx emission inventories.

Developing Nitrogen Isotopic Source Profiles of Atmospheric Ammonia for Source Apportionment of Ammonia in Urban Beijing

Atmospheric ammonia (NH3) is the key precursor in secondary particle formation, which is identified as the most abundant components of haze in Beijing in most cases. It is critical to understand the characteristics of NH3 from various emission sources and quantify each source contribution to NH3 in ambient atmosphere. Stable nitrogen (N) isotope composition (δ15N) is an effective tool to study NH3 source. However, this tool cannot be effectively applied in Beijing due to the lack of comprehensive N nitrogen isotope source profiles. Reliable source profiles are the basis of source apportionment of NH3 using the isotope mixing model. In this study, multiple NH3 source samples were collected at sites, representing six major NH3 source types in Beijing from 2017 to 2018 in four seasons. The δ15N values of 212 NH3 source samples were determined to build a local source profiles database of δ15N. NH3 from traffic source presents significantly higher δ15N values (−14.0 ± 5.4‰), distinguished from other sources. The δ15N values of other sources besides traffic were more depleted and did not clear differences (solid waste, sewage, human feces, fertilizer, and livestock for −33.6 ± 4.5‰, −34.1 ± 4.8‰, −32.2 ± 3.8‰, −35.0 ± 3.9‰, and −34.9 ± 4.4‰, respectively). These sources were classified into non-traffic source in this study. From March 2018 to March 2019, ambient NH3 samples were collected at an urban site in Beijing. With the newly developed source profiles in this study, the contribution of traffic and non-traffic sources to ambient NH3 in an urban site in Beijing was calculated using 15N isotope mass balance equations. Traffic and non-traffic sources contributed 8% and 92% to ambient NH3 in urban Beijing, respectively. The highest seasonal average contribution of traffic to ambient NH3 was found in winter (22%). Our results reveal the importance of traffic source and provide evidence for the need to control NH3 emission from traffic in urban Beijing in winter.

Decoupled oxygenation of the Ediacaran ocean and atmosphere during the rise of early animals

The Ediacaran Period (∼635 to 541 Ma) witnessed the early diversification and radiation of metazoans, in the form of the Ediacaran Biota. This biological revolution, beginning at ∼575 Ma, has been widely attributed to a temporally restricted episode of deeper ocean oxygenation, potentially caused by a contemporaneous rise in atmospheric oxygen levels. However, quantitative geochemical-record-driven estimates of Ediacaran atmospheric and oceanic redox evolution are lacking, and hence possible links between oceanic and atmospheric oxygenation remain speculative. Here, after screening for possible post-depositional alteration, we utilize paleogeographically-diverse carbon and sulfur isotope records from South China, Oman and USA-Mexico, to develop a biogeochemical isotope mass balance model to quantify Ediacaran atmospheric oxygen and oceanic sulfate evolution. Model results from all three continents indicate that Ediacaran atmospheric oxygen levels rose monotonically between ∼630 Ma and ∼590 Ma, and subsequently remained relatively stable at around 0.6 present atmospheric level for the remainder of the Ediacaran. By contrast, the marine sulfate reservoir appears to have remained relatively stable before ∼575 Ma, with a subsequent large pulse where sulfate concentrations rose to ∼8 mM. These quantitative results indicate that Ediacaran oceanic and atmospheric oxygenation were decoupled, which is consistent with published geochemical records. We propose that the early Ediacaran rise of atmospheric oxygen levels, driven by increased net burial of organic carbon and pyrite, may not have established widespread deep-ocean oxygenation. Instead, later pulsed input of oxidizing power (mainly sulfate) from the continents drove transient episodes of seafloor oxygenation that accompanied radiations of the Ediacaran Biota.

2.0 Ga orogenic graphite deposits and associated 13C-enriched meta-carbonate rocks from South China Craton: Implications for global Lomagundi event

The Lomagundi (-Jatuli) event, characterized by extremely high positive global inorganic carbon isotope excursion at about 2.2 billion years ago, is pivotal in investigating the causes and consequences of great oxygenation event, inventory and sequestration of carbon on the Earth’s surface, evolution of life, and more profoundly tectonic control on Earth’s environment. However, the reasons that caused the isotopic excursion are not resolved yet. Herein, we report the discovery of meta-carbonate rocks with distinct positive carbon isotopic excursion from the Paleoproterozoic continental collision zone of the Kongling Complex, South China Craton. The δ13CV-PDB values for meta-carbonate rocks show positive values in the range from +5.5‰ to +11.6‰, whereas the δ13CV-PDB values of associated graphite deposits range from −25.8‰ to −9.5‰. Zircon U-Pb-Hf isotopes from zircon-bearing meta-carbonate sample yielded weighted average 207Pb/206Pb age of 2001.3 ± 9.5 Ma, with corresponding εHf(t) range from −7.05 to −3.16, comparable to the values of local 2.9–2.6 Ga basement rocks. Geochemical characteristics of meta-carbonate rocks, such as their rare earth element patterns and the trace element parameters of La, Ce, Eu, and Gd anomalies and Y/Ho ratio, suggest that the carbonate deposition took place in passive continental margin in association with large volumes of organic carbon. The extensive graphite deposits from Kongling Complex in South China Craton, their equivalents in the North China Craton and elsewhere across the globe prove that the burial of 12C-enriched organic carbon has eventually resulted in the global enrichment of 13C in the atmospheric CO2, which is recorded in the marine carbonate rocks. Isotopic mass balance estimates indicate that more than half of the organic carbon was buried during the oceanic closure. Hence, the observed global shift could be directly related to the continent collision event in greater China, thus resolving the long-standing paradox of the Lomagundi global positive carbon isotope excursion. Moreover, the present results suggest that orogenesis play a significant role in sequestration of carbon into the continental crust.

Methods for the Determination of Stable Isotope Ratios of Multiple Nitrogen Species in Rainwater Using Distillation and Evaporation

Nitrogen (N) isotope ratios (δ15N) of multiple N species including ammonium (NH4+), nitrate (NO3−), dissolved organic N (DON), and total dissolved N (TDN) can serve as indicators of the sources of wet N deposition. However, methods for sequential analysis of these multiple N species are not well developed. In this study, methods for the determination of δ15N of these multiple N species were proposed through a traditional distillation method for NH4+ and NO3−, direct evaporation for TDN, and an isotope mass balance equation for δ15N-DON. A series of experiments were conducted 1) to find evaporation conditions including pH (&amp;lt;3.5 vs. 5.4) and evaporation methods (oven drying at 60 and 100°C, infra-red chamber, and freeze-drying), 2) to investigate precision and accuracy of distillation for δ15N of NH4+ and NO3−, and 3) to determine δ15N-TDN by direct evaporation and δ15N-DON with mass balance equation. (NH4)2SO4 (‒4.0 ± 0.03‰), KNO3 (‒4.2 ± 0.03‰), and CO(NH2)2 (‒5.4 ± 0.04‰) were used as reference materials for NH4+, NO3−, and DON, respectively. In this study, for the analysis of δ15N of NH4+ and NO3−, the conventional distillation method was tested in an effort to save cost for laboratories equipped with the distillation system. Evaporation using oven at 60°C (but not 100°C), infra-red chamber, or freeze-drier after acidification to &amp;lt; pH 3.5 prevented 14NH3 loss. Analysis of the δ15N-NO3− (‒4.4 ± 0.1‰ to ‒3.9 ± 0.5‰) was reliable for a wide range of N content (0.1–0.5 mg), but analytical errors for δ15N-NH4+ were as high as 2.1‰ when N content was small (e.g., 0.1–0.3 mg N) due to background contamination and potential interference by co-existing DON. Direct evaporation of solution containing NH4+, NO3−, and DON to dryness produced reliable δ15N-TDN with accuracy &amp;lt;0.15‰ and precision &amp;lt;0.21‰. However, the analytical errors of δ15N-DON were highly dependent on the content of co-existing NH4+ as well as DON content. Therefore, the proposed protocol can be applied for rainwater containing a high NH4+ concentration (&amp;gt;2.0 mg N L−1 assuming that 200 ml of sample is used for distillation).

A multi-stable isotopic constraint on water column oxygen sinks in the Pearl River Estuary, South China

Bottom water oxygen depletion is a central concern in estuaries and coastal oceans worldwide. However, a mechanistic understanding and quantitative diagnosis of different oxygen-consuming processes are less clear. In this study, a multi-stable isotope approach is developed to delineate the role of oxygen respiration and nitrification contributing to total oxygen consumption in the Pearl River Estuary (PRE), a large eutrophic estuary in south China. The approach highly couples with analysis of the carbon isotope composition of dissolved inorganic carbon (δ13C-DIC) and with stable nitrogen isotope analysis in ammonium (δ15N-NH4+) and nitrate (δ15N-NO3−). In all seasons, relatively low DO concentrations were observed in the upper reach and, to some extent, in the outer estuary during summer, while high concentrations of DO were found in the transition zone between the inner and outer estuary. On the basis of isotopic differentiation, our data reveal that much more depleted δ13C-DIC is coincident with DIC additions and low oxygen in the upper reach and inner estuary during most seasons. This is most likely a consequence of organic carbon (OC) degradation via aerobic respiration. Based on the carbon isotopic mass balance of DIC and the stoichiometry ratio of −ΔDO/ΔDIC, we found that the OC degradation dominates the total oxygen consumption in the upper reach, as well as in the inner estuary during summer (48.3%–93.5%). In addition, nitrification is another key process in contributing to total oxygen loss in the upper reach, as supported by the well-coupled variations of δ15N of NH4+ and NO3− and apparent oxygen utilization (AOU). Using the formerly determined N isotopic fractionation and observed δ15N variation, we estimated that nitrification could account for 35.3%–44.1% and 28.5%–31.6% of the total oxygen consumption in the upper reach during winter and summer, respectively, while its contribution to total oxygen loss is minor in the inner and outer estuary. Overall, this study demonstrates the potential of the multi-stable isotopic approach to assess oxygen sink partitioning in large human-perturbed estuaries.

Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (δ30Si) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26–0.52 wt% or 92–185 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 μM, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

Oceanic anoxia and extinction in the latest Ordovician

The Late Ordovician (Hirnantian) mass extinction (LOME) was marked by two discrete pulses of high species turnover rates attributed to glacial cooling (LOME-1) and subsequent expansion of anoxic marine conditions (LOME-2). However, the mechanisms and extent of global marine anoxia remain controversial. In this study, we present uranium isotope (δ238U) data from a new Ordovician-Silurian (O-S) boundary carbonate section in the Southwest China to explore the extent/duration of the global marine anoxia, and links to the LOME. This section was found to continuously record the characteristic Hirnantian paired carbon isotope excursion (HICE) and complete benthic faunal turnover across the O-S boundary based on detailed stratigraphic constraint. The poor correlations between δ238Ucarb data, local redox proxy (Ce anomalies), and diagenetic indicators (e.g., Mn and Sr contents, Mn/Sr ratio, δ18O), as well as microscopic petrographic examinations of O-S carbonates suggest that most of the δ238U data are not produced by either local redox conditions or post-depositional diagenetic processes. By coupling our new uranium isotope data with a stochastic U isotope mass balance model, we suggest there were two episodes of widespread marine anoxia over this time interval revealed by systematic changes of δ238U with two negative shifts punctuated by one positive one in between. The former took place in the late Katian, predating the LOME-1 episode and the pronounced climatic cooling; while the latter occurred synchronously with the LOME-2 in the mid-late Hirnantian, separated by a potential oceanic oxygenation event. This presents a more complicated picture of anoxia—as inferred from U isotope records—as a driver of the end-Ordovician mass extinction. The insignificant biotic response to late Katian anoxia is consistent with the idea that the effects of anoxia on marine ecosystem can be highly variable. On the other hand, the presence of anoxia in mid-late Hirnantian strengthen the connection between the oceanic deoxygenation and fauna turnover in LOME-2.

A nitrate budget of the Bohai Sea based on an isotope mass balance model

Abstract. The Bohai Sea (BHS) is a semi-closed marginal sea impacted by one of the most populated areas of China. The supply of nutrients, markedly that of reactive nitrogen, via fluvial and atmospheric transport has strongly increased in parallel with the growing population. Therefore, it is crucial to quantify the reactive nitrogen input to the BHS and understand the processes and determine the quantities of nitrogen eliminated in and exported from the BHS. The nitrogen budget and in particular the internal sources and sinks of nitrate were constrained by using a mass-based and dual stable isotope approach based on δ15N and δ18O of nitrate (NO3-). Samples of water, suspended matter, and sediments were taken in the BHS in spring (March and April) and summer (July and August) 2018. The Yellow River (YR) was sampled in May and July to November, and Daliao River, Hai River, Luan River, and Xiaoqing River were sampled in November of 2018. In addition to nutrient, particulate organic carbon, and nitrogen concentrations, the dual isotopes of nitrate (δ15N and δ18O), δ15N of suspended matters, and sediments were determined. Based on the available mass fluxes and isotope data an updated nitrogen budget is proposed. Compared to previous estimates, it is more complete and includes the impact of interior cycling (nitrification) on the nitrate pool. The main nitrate sources are rivers contributing 19.2 %–25.6 % and the combined terrestrial runoff (including submarine fresh groundwater discharge of nitrate) accounting for 27.8 %–37.1 % of the nitrate input to the BHS, while atmospheric input contributes 6.9 %–22.2 % to total nitrate. An unusually active interior nitrogen cycling contributes 40.7 %–65.3 % to total nitrate via nitrification. Nitrogen is mainly trapped in the BHS and mainly removed by sedimentation (70.4 %–77.8 %), and only very little is exported to the Yellow Sea (YS) (only 1.8 %–2.4 %). At present denitrification is active in the sediments and removes 20.4 %–27.2 % of nitrate from the pool. However, a further eutrophication of the BHS could induce water column hypoxia and denitrification, as is increasingly observed in other marginal seas and seasonally off river mouths.

CaCO3 dissolution in carbonate-poor shelf sands increases with ocean acidification and porewater residence time

Carbonate-poor sandy sediments comprise much of the shelf area, and—despite their low CaCO3 content—contain a significant pool of CaCO3 base available to neutralize ocean acid. Here, we conducted flow-through column experiments on permeable, carbonate-poor sand obtained from Catalina Island, CA, to quantify CaCO3 dissolution across a range of current and future seawater conditions. Using 13C isotope mass balance, we show that dissolution depends both on the CaCO3 saturation state (Ω) of the inflowing seawater, as well as porewater residence time. At current ocean conditions (Ωaragonite = 2.4 and Ωcalcite = 3.7 at our field site), dissolution was negligible for porewater residence times < 1.8 h, but increased thereafter, following sufficient production of CO2 from aerobic respiration. As Ω of inlet water was lowered, simulating future ocean conditions, dissolution began earlier and rates increased. The response to acidification was similar to previously reported observations in carbonate-rich shelf environments, suggesting that carbonate-poor sediments have the potential to support enhanced dissolution in an acidifying ocean, given sufficient CaCO3 substrate. With continued acidification projected to occur this century, these sediments could transition from a net source of acid to the overlying seawater (production of alkalinity to dissolved inorganic carbon, ΔAlk/ΔDIC < 1) to net source of buffering capacity (ΔAlk/ΔDIC > 1) when overlying seawater Ωaragonite reaches 0.96 to 0.69 (Ωcalcite = 1.50 and 1.07), depending on porewater residence time. In some areas with naturally acidic water, this threshold has already been reached.

&lt;i&gt;Corrigendum to&lt;/i&gt;: Constraining nitrogen sources to a seagrass-dominated coastal embayment by using an isotope mass balance approach

Nitrogen (N) is often the key nutrient limiting primary production in coastal waters. Quantifying sources and sinks of N is therefore critical to understanding the factors that underpin the productivity of coastal ecosystems. Constraining nitrogen inputs can be difficult for some terms such as N fixation and marine exchange as a consequence of uncertainties associated with scaling and stochasticity. To help overcome these issues, we undertook a N budget incorporating an isotope and mass balance to constrain N sources in a large oligotrophic coastal embayment (Western Port, Australia). The total N input to Western Port was calculated to be 1400Mg Nyear&lt;sup&gt;&amp;#x2212;1&lt;/sup&gt;, which is remarkably consistent with previous estimates of sedimentation rates within the system. Catchment inputs, N fixation, marine sources and atmospheric deposition comprised 44, 28, 28 and 13% of N inputs respectively. Retention of marine-derived N equated to ~3 and ~10% of total N and NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; flushed through the system from the marine end-member. The relatively high contribution of N fixation compared with previous studies was most likely to be due to the high proportion of nutrient-limited intertidal sediments where N is mediated by seagrasses and sediment cyanobacteria.

Seasonal, weathering and water use controls of silicon cycling along the river flow in two contrasting basins of South India

We present the first study of river water silicon isotopic composition from two contrasting basins in South India, the east flowing Kaveri river and the west flowing Netravathi river. Both rivers originate from the Western Ghats. River water samples were collected from mainstream, tributaries and reservoirs at different locations along the river flow during dry and wet (monsoon) seasons, with an additional post-monsoon sampling in Netravathi. High rainfall in Netravathi basin and upper reaches of Kaveri induce intense weathering in the region, with superposing contribution from anthropogenic controls in downstream Kaveri. The δ30SiDSi values range from +0.42 to +1.65‰ for Netravathi river basin and + 0.32 to +2.85‰ for Kaveri river basin. Silicate weathering index (Re) shows intense weathering associated with monosiallitization (kaolinite-gibbsite formation) in Netravathi basin and relatively moderate weathering with bisiallitization (smectite-kaolinite formation) in Kaveri basin. The seasonal changes in δ30SiDSi and Re in each basin shows similar patterns, with a likely higher and heavier contribution of soil water from deeper soil profiles closer to weathering front and bedrock, associated with significant secondary mineral formation during dry season and leaching of superficial soil profiles during the high discharge periods of monsoon. We provide a theoretical framework to estimate relative contribution of silicate weathering vs. anthropogenic processes on riverine δ30SiDSi. Re is broadly correlated with δ30SiDSi and an isotopic mass balance involving the whole rock composition (excluding the stable quartz and sericite) shows that the δ30Si in river water are well explained by silicate weathering and gibbsite-kaolinite formation in the Netravathi and upper upper reaches of Kaveri. However, silicate weathering explains only partially the heavier δ30SiDSi signatures in the middle and lower reaches of the Kaveri, and the additional enrichment of about +1.06‰ can be attributed to uptake of silicon and Si-depleted return flow through irrigated agriculture in the basin. This study confirms the major control of pedoclimatic conditions on the δ30Si of rivers and provides for the first time an estimation of the impact of human activities on the silicon isotopic signature of rivers.

Refining the contribution of riverine particulate release to the global marine Nd budget

The release of neodymium (Nd) from particles along continental margins may contribute to losses in the global of Nd budget. The Changjiang River, which carries a heavy load of total suspended matter, empties into the East China Sea, and a strong particulate–seawater interaction process occurs along the salinity gradient. In the low-salinity region (S < 2.0), strong removal of dissolved rare earth elements (dREEs) occurs, but the Nd isotope values are uniform. At mid- and high-salinity (S = 2.0–28.0 and S > 28.0) areas, the dREE concentrations increase slightly. An Nd isotope mass balance indicates that the release of particulate matter is a source of dREEs in the Changjiang estuary. The release rate of particulate Nd (NdSPM) to the dissolved Nd pool in Changjiang estuary is higher than other estuaries, such as Amazon estuary. Composite all river data available from the previous studies indicate that 5800–9200 Mg per year of Nd is released to global marine waters from riverine particles. This estimated quantity is on the same order of magnitude as the calculated global Nd release flux based on the case study in the Amazon estuary. Our study indicates that to better constrain the global Nd budget, it is required to consider the release rate of NdSPM in different rivers due to the significant difference among various rivers, but with very limited available data as of now.

Construction of a hydrogeochemical conceptual model and identification of the groundwater pollution contribution rate in a pyrite mining area

To effectively restore the groundwater environment of the Shiping Mine (SPM) area, which is contaminated by acid mine drainage (AMD), a hydrogeochemical conceptual model was constructed based on groundwater chemistry and environmental stable isotopes. The contribution rate of various pollution sources in the groundwater environment was quantitatively analyzed using an optimized stable isotope mass balance model. A total of 68 groups of water samples were collected. The sampling period covered the dry, intermediate, and wet periods of a complete hydrological year. Samples were taken from rain, springs, mine drainage, tailings leachate, and surface water; and the detection and analysis indicators included 24 parameters, such as inorganic salts, heavy metals, and isotopes. A hydrogeochemical and statistical data analysis was performed. The main source of groundwater replenishment was found to be atmospheric precipitation, with the water-rock interaction of calcite and pyrite, and mining activities being the main controlling factors of hydrogeochemical processes. Acid mine drainage significantly enhanced the dissolution of various minerals, and the detection rate of Zn, Cu, As, Cd, and Pb increased from 0–30%–100% when compared with groundwater in the area upstream of the mines. The optimized mass balance model results revealed that the contribution rates of upstream groundwater, mine water and leachate were 0.78–0.86, 0.08–0.18, and 0.04–0.06 for Heidong underground river, respectively; were 0.27–0.36, 0.62–0.68, and 0.03–0.05 for Tiantang underground river, respectively. Furthermore, based on the water balance analysis, 34–70% of the mine water was found to infiltrate directly through karst fissures and karst pipes and could not be collected at the mine entrance. Acid mine drainage that directly infiltrated through runoff could easily be ignored due to the hidden migration path, which may cause the groundwater environment to be remediated less effectively than expected.

Reconstructing postglacial hydrologic and environmental change in the eastern Kenai Peninsula lowlands using proxy data and mass balance modeling

AbstractDespite extensive paleoenvironmental research on the postglacial history of the Kenai Peninsula, Alaska, uncertainties remain regarding the region's deglaciation, vegetation development, and past hydroclimate. To elucidate this complex environmental history, we present new proxy datasets from Hidden and Kelly lakes, located in the eastern Kenai lowlands at the foot of the Kenai Mountains, including sedimentological properties (magnetic susceptibility, organic matter, grain size, and biogenic silica), pollen and macrofossils, diatom assemblages, and diatom oxygen isotopes. We use a simple hydrologic and isotope mass balance model to constrain interpretations of the diatom oxygen isotope data. Results reveal that glacier ice retreated from Hidden Lake's headwaters by ca. 13.1 cal ka BP, and that groundwater was an important component of Kelly Lake's hydrologic budget in the Early Holocene. As the forest developed and the climate became wetter in the Middle to Late Holocene, Kelly Lake reached or exceeded its modern level. In the last ca. 75 years, rising temperature caused rapid changes in biogenic silica content and diatom oxygen isotope values. Our findings demonstrate the utility of mass balance modeling to constrain interpretations of paleolimnologic oxygen isotope data, and that groundwater can exert a strong influence on lake water isotopes, potentially confounding interpretations of regional climate.

Molybdenum isotope-based redox deviation driven by continental margin euxinia during the early Cambrian

Oceanic redox changes may be a key environmental trigger for the “Cambrian explosion”. However, a hypothesized predominantly oxygenated early Cambrian ocean, inferred from high sedimentary molybdenum (Mo) isotope ratios, conflicts with other evidence for euxinic continental margins. Here, we demonstrate that seawater Mo isotope ratios like those of today’s predominantly well-oxygenated oceans can also occur in less oxygenated early Cambrian oceans. New and existing Mo isotope data from early Cambrian black shales in South China span a range of greater than 3‰, which can only be explained by multi-stage fractionation of Mo isotopes. The lowest Mo isotopes ratios likely reflect a Fe-Mn (oxyhydr)oxide shuttle that preferentially transferred the lighter Mo isotopes into weakly euxinic bottom waters where Fe-Mn (oxyhydr)oxide dissolution released isotopically light Mo that was then scavenged by organic matters and sulfide minerals. We use a novel isotope mass balance model to show that Fe-Mn (oxyhydr)oxide shuttle operating on euxinic continental margins would preferentially bury lighter-mass Mo isotopes to such an extent that early Cambrian seawater can become enriched in heavier Mo isotopes at levels comparable to modern seawater. Hence, widespread ocean oxygenation may not have occurred during the early Cambrian as previously thought. Based on this paradigm shift, reconsideration of sedimentary Mo isotope temporal trends suggests a stepwise and protracted oxidation of the oceans until the Late Devonian. Sporadic intervals with Mo isotope ratios spanning over 3‰ and often accompanied by dramatic change in biodiversity (e.g., Ediacaran-Cambrian and Permian-Triassic transitions) probably had euxinic continental margins. Links between fossil records and marine redox structures disclose the importance of spatiotemporal fluctuations in continental margin euxinia on biodiversity changes.

Soil Organic Carbon Isotope Tracing in Sorghum under Ambient CO2 and Free-Air CO2 Enrichment (FACE)

As atmospheric carbon dioxide concentrations, [CO2Air], continue their uncontrolled rise, the capacity of soils to accumulate or retain carbon is uncertain. Free-air CO2 enrichment (FACE) experiments have been conducted to better understand the plant, soil and ecosystem response to elevated [CO2], frequently employing commercial CO2 that imparts a distinct isotopic signal to the system for tracing carbon. We conducted a FACE experiment in 1998 and 1999, whereby sorghum (C4 photosynthetic pathway) was grown in four replicates of four treatments using a split-strip plot design: (i) ambient CO2/ample water (365 μmol mol−1, “Control–Wet”), (ii) ambient CO2/water stress (“Control–Dry”), (iii) CO2-enriched (560 μmol mol−1, “FACE–Wet”), and (iv) CO2-enriched/water stressed (“FACE–Dry”). The stable-carbon isotope composition of the added CO2 (in FACE treatments) was close to that of free atmosphere background values, so the subsequent similar 13C-enriched carbon signal photosynthetically fixed by C4 sorghum plants could be used to trace the fate of carbon in both FACE and control treatments. Measurement of soil organic carbon content (SOC (%) = gC/gdry soil × 100%) and δ13C at three depths (0–15, 15–30, and 30–60 cm) were made on soils from the beginning and end of the two experimental growing seasons. A progressive ca. 0.5‰–1.0‰ δ13C increase in the upper soil SOC in all treatments over the course of the experiment indicated common entry of new sorghum carbon into the SOC pools. The 0–15 cm SOC in FACE treatments was 13C-enriched relative to the Control by ca. 1‰, and according to isotopic mass balance, the fraction of the new sorghum-derived SOC in the Control–Wet treatment at the end of the second season was 8.4%, 14.2% in FACE–Wet, 6.5% in Control–Dry, and 14.2% in FACE–Dry. The net SOC enhancement resulting from CO2 enrichment was therefore 5.8% (or 2.9% y−1 of experiment) under ample water and 7.7% (3.8% y−1 of experiment) under limited water, which matches the pattern of greater aboveground biomass increase with elevated [CO2Air] under the Dry treatment, but no parallel isotopic shifts were found in deeper soils. However, these increased fractions of new carbon in SOC at the end of the experiment do not necessarily mean an increase in total SOC content, because gravimetric measurements of SOC did not reveal a significant increase under elevated [CO2Air], at least within the limits of SOC-content error bars. Thus, new carbon gains might be offset by pre-experiment carbon losses. The results demonstrate successful isotopic tracing of carbon from plants to soils in this sorghum FACE experiment showing differences between FACE and Control treatments, which suggest more dynamic cycling of SOC under elevated [CO2Air] than in the Control treatment.