Radiocarbon Values Research Articles

동해 용존무기탄소 중 방사성탄소의 분지별 비교 및 시간에 따른 변화

This study examined the spatial and temporal variation of dissolved inorganic radiocarbon in the East Sea. Five vertical profiles of radiocarbon values were obtained from samples collected in 1999 in three basins (Japan Basin, Ulleung Basin, Yamato Basin) of the East Sea. Radiocarbon values decreased from 63-85‰ at the surface to about -50‰ with increasing depth (up to 2,000 m) and were nearly constant in the layer deeper than 2,000 m in all basins. Radiocarbon values did not show significant basin-to-basin differences in the surface and the bottom layers. In the intermediate layer (200-2,000 m), however, they decreased in the order of Japan Basin > Ulleung Basin > Yamato Basin, which is consistent with the suggested circulation pattern in the intermediate layer of the East Sea. Radiocarbon was found to have decreased at ~2%/year in the surface water of the East Sea. In contrast, in the interior of the East Sea, radiocarbon values have increased with time in all three basins. In the Central Water, the annual increase rate was about 3.3‰, which is faster than the rates in the Deep and Bottom Waters. The radiocarbon in the Deep and Bottom Waters had increased until mid-1990s, after which time it has been almost constant.

CONTRIBUTIONS OF RADIOCARBON ANALYSIS IN HUMAN RIGHTS INVESTIGATIONS

Estimation of time since death represents an essential, but difficult component of the analysis of human remains in the context of human rights investigations. Although morphological indicators of postmortem change can be influenced by many variables, radiocarbon analysis has proven to represent a useful approach. Radiocarbon values clearly reveal if the remains are ancient (radioactive decay) or modern (more recent than 1950 A.D.). If the latter, analysis of the radiocarbon value of specific tissues in consideration of documented atmospheric bomb‐pulse values potentially can clarify both the birth date and death date. Frequently in human rights cases, it is important to establish if recovered remains relate to a particular death date or date range. In these cases, hypotheses can be formulated for testing with radiocarbon analysis.

Deep South Atlantic carbonate chemistry and increased interocean deep water exchange during last deglaciation

Carbon release from the deep ocean at glacial terminations is a critical component of past climate change, but the underlying mechanisms remain poorly understood. We present a 28,000-year high-resolution record of carbonate ion concentration, a key parameter of the global carbon cycle, at 5-km water depth in the South Atlantic. We observe similar carbonate ion concentrations between the Last Glacial Maximum and the late Holocene, despite elevated concentrations in the glacial surface ocean. This strongly supports the importance of respiratory carbon accumulation in a stratified deep ocean for atmospheric CO2 reduction during the last ice age. After ∼9 μmol/kg decline during Heinrich Stadial 1, deep South Atlantic carbonate ion concentration rose by ∼24 μmol/kg from the onset of Bølling to Pre-boreal, likely caused by strengthening North Atlantic Deep Water formation (Bølling) or increased ventilation in the Southern Ocean (Younger Drays) or both (Pre-boreal). The ∼15 μmol/kg decline in deep water carbonate ion since ∼10 ka is consistent with extraction of alkalinity from seawater by deep-sea CaCO3 compensation and coral reef growth on continental shelves during the Holocene.Between 16,600 and 15,000 years ago, deep South Atlantic carbonate ion values converged with those at 3.4-km water depth in the western equatorial Pacific, as did carbon isotope and radiocarbon values. These observations suggest a period of enhanced lateral exchange of carbon between the deep South Atlantic and Pacific Oceans, probably due to an increased transfer of momentum from southern westerlies to the Southern Ocean. By spreading carbon-rich deep Pacific waters around Antarctica for upwelling, invigorated interocean deep water exchange would lead to more efficient CO2 degassing from the Southern Ocean, and thus to an atmospheric CO2 rise, during the early deglaciation.

An investigation of size-fractionated organic matter from Lake Superior and a tributary stream using radiocarbon, stable isotopes and NMR

This study investigated the concentration and isotopic composition of different size fractions of organic matter (OM) in Lake Superior and in one of its many tributary streams and rivers (Amity Creek, Duluth, Minnesota, USA). Structural compositional drivers of the Δ14C of high molecular weight (HMW, >1kDa) dissolved organic carbon (DOC) in the lake were also evaluated. Low molecular weight (LMW, <1kDa) DOC was the fraction containing the largest proportion (68–88%) of organic carbon (OC) in the lake. Particulate organic carbon (POC, >0.7μm) was generally 13C-depleted (−29±1.2‰) relative to “bulk” (<0.7μm) DOC (−26.4±0.7‰), “init” (<0.2μm) DOC (−26.6±0.8‰), HMW DOC (−26.9±0.3‰) and LMW DOC (−26.5±0.9‰), and had more variable 14C content (Δ14C of −94‰ to 53‰; 735years BP to modern) than the other size fractions. Init DOC (Δ14C of 17–59‰), HMW DOC (Δ14C of 23–64‰) and LMW DOC (Δ14C of 16–62‰) all reflected contemporary (modern) radiocarbon signatures. Bulk DOC (Δ14C of −19‰ to 57‰; 90years BP to modern) had modern radiocarbon values in the offshore sites (Δ14C of 2–57‰) with pre-aged samples (Δ14C of −8‰ to −19‰) seen at the nearshore site. HMW DOM was relatively N-poor (C:N of 12–19) compared to particulate organic matter (POM, C:N of 8–10) revealing either a more diagenetically altered state or contrasting sources. 13C NMR data showed that biochemical composition of HMW DOC in Lake Superior was dominated by carbohydrates (53–65%) with only trace aromatic components (2–4%). Structurally complex components such as heteropolysaccharides (HPS), amide/peptides and amino sugars (AMS) constitute 75–84% of HMW DOC whereas carboxylic-rich alicyclic molecules (CRAM) made up 16–25% of HMW DOC in the lake. Combined HPS and AMS, O-alkyl carbohydrate carbon, and total carbohydrate carbon contents were significantly positively correlated to the Δ14C of HMW DOC suggesting they contribute a contemporary 14C-enriched component to the HMW DOC in the lake. In contrast, CRAM and aliphatic carbons were significantly inversely correlated with Δ14C of HMW DOC implying that these represent 14C-depleted (older) components of HMW DOC in the lake. At Amity Creek, storm events led to larger loads of bulk DOC and POC, which were both 14C-enriched (modern). However, in baseflow conditions 14C-depleted bulk DOC and POC were exported. LMW DOC from the creek was considerably 14C-depleted compared to concurrent HMW DOC.

Carbon isotopic evidence for microbial control of carbon supply to Orca Basin at the seawater–brine interface

Abstract. Orca Basin, an intraslope basin on the Texas-Louisiana continental slope, hosts a hypersaline, anoxic brine in its lowermost 200 m in which limited microbial activity has been reported. This brine contains a large reservoir of reduced and aged carbon, and appears to be stable at decadal time scales: concentrations and isotopic composition of dissolved inorganic (DIC) and organic carbon (DOC) are similar to measurements made in the 1970s. Both DIC and DOC are more "aged" within the brine pool than in overlying water, and the isotopic contrast between brine carbon and seawater carbon is much greater for DIC than DOC. While the stable carbon isotopic composition of brine DIC points towards a combination of methane and organic carbon remineralization as its source, radiocarbon and box model results point to the brine interface as the major source region for DIC, allowing for only limited oxidation of methane diffusing upwards from sediments. This conclusion is consistent with previous studies that identify the seawater–brine interface as the focus of microbial activity associated with Orca Basin brine. Isotopic similarities between DIC and DOC suggest a different relationship between these two carbon reservoirs than is typically observed in deep ocean basins. Radiocarbon values implicate the seawater–brine interface region as the likely source region for DOC to the brine as well as DIC.

Isotopic (δ13C, Δ14C) analysis of organic acids in marine samples using wet chemical oxidation

We present a method for the isolation and off‐line isotope analysis of formate and acetate in marine samples. Organic acids are separated by high performance liquid chromatography and collected in glass Exetainer® screw‐capped vials that have been prespiked with an oxidant and flushed with helium. The vials are subsequently heated to convert the organic compounds to CO2 for radiocarbon and δ13C analysis. Small aliquots are sampled for measurement of δ13C by isotope ratio mass spectrometry, whereas the majority of the CO2 is saved for radiocarbon analysis by accelerator mass spectrometry using a gas ion source. Accurate δ13C and radiocarbon values were obtained for formate and acetate standards spiked into deep seawater and saline Milli‐Q water at concentrations of 25 to 1000 µM C. The process blank associated with the isolation of formate for radiocarbon analysis was ~1.5 µg C and stable over time. Accurate results could be obtained for marine samples with only 25 µM formate. The radiocarbon analysis of acetate showed significantly higher and more variable extraneous carbon contributions, particularly for samples spiked into seawater. Potential improvements that may make the method appropriate for the radiocarbon analysis of acetate in seawater are discussed. The blanks associated with the wet chemical oxidation were assessed independently and found to be small and consistent (&lt;0.3 µg C), potentially making this approach feasible for a broader set of compounds separated by HPLC for radiocarbon analysis.

Radiocarbon Variability in Groundwater in an Extremely Arid Zone—The Arava Valley, Israel

Radiocarbon values of groundwater in main aquifers of the extremely arid Negev Desert and the Arava Valley, southern Israel, are used for studying the underground flow regime, particularly the complex connections between different aquifers and mixing of water bodies. The study shows that 14C can serve as a hydrological tracer in arid environments and that groundwater dating may be possible (although not very accurate) even in this extremely arid environment (precipitation, &lt;50 mm/yr), where there is almost no vegetation. There are several aquifers in this region, some of which are deep (deeper than 500 m) and regional and contain mainly fossil water, while others are local and restricted to the Arava, much shallower (50–200 m) and are thought to contain historical to recent waters. Most of the current recharge to these shallow unconsolidated aquifers comes from flash floods that flow from the mountains rising on both sides of the valley. The groundwater in the deep aquifers has low 14C values (usually &lt;5 pMC), implying old ages (preliminary ages &gt;26,000 yr). Groundwater in the shallow aquifers characterized by higher 14C values (up to 60–70 pMC) imply younger ages and faster groundwater flow (recent recharge). This is also supported by the presence of tritium in some of the samples. A few exceptional values are explained by the unique mixing of water from different sources; another is due to a technical failure in the well.

Radiocarbon Variability in Groundwater in an Extremely Arid Zone—The Arava Valley, Israel

Radiocarbon values of groundwater in main aquifers of the extremely arid Negev Desert and the Arava Valley, southern Israel, are used for studying the underground flow regime, particularly the complex connections between different aquifers and mixing of water bodies. The study shows that 14C can serve as a hydrological tracer in arid environments and that groundwater dating may be possible (although not very accurate) even in this extremely arid environment (precipitation, &lt;50 mm/yr), where there is almost no vegetation. There are several aquifers in this region, some of which are deep (deeper than 500 m) and regional and contain mainly fossil water, while others are local and restricted to the Arava, much shallower (50–200 m) and are thought to contain historical to recent waters. Most of the current recharge to these shallow unconsolidated aquifers comes from flash floods that flow from the mountains rising on both sides of the valley. The groundwater in the deep aquifers has low 14C values (usually &lt;5 pMC), implying old ages (preliminary ages &gt;26,000 yr). Groundwater in the shallow aquifers characterized by higher 14C values (up to 60–70 pMC) imply younger ages and faster groundwater flow (recent recharge). This is also supported by the presence of tritium in some of the samples. A few exceptional values are explained by the unique mixing of water from different sources; another is due to a technical failure in the well.

Profiles of geochemical and isotopic signatures from the Helan Mountains to the eastern Tengger Desert, northwestern China

We used environmental tracers to provide insights into groundwater evolution and flow processes on the eastern margin of China's Tengger Desert and determine recharge sources and recharge timing along the flow paths. The chemical composition was generally Na+ enriched, with no dominant anions. The chemistry is strongly influenced by evaporation and subsequent dissolution of minerals during recharge in the rainy season. Other processes, including cation exchange and weathering, also contribute to the water composition. The median groundwater δ18O value was around −9.1‰, and most groundwater in the basin was depleted in heavy isotopes. δ13C in deep groundwater ranged from −4.73 to −9.56‰, indicating that carbonate mixing is common but that isolated carbonate still exists. Radiocarbon values in groundwater ranged from 5.4 to 63.5 pmc. We estimated a residence time of 17.9 and 19.6 kyr at two desert sites, revealing that some replenishment of desert aquifers occurred in the late Pleistocene and early Holocene, when some of the upper reaches of rivers were characterized as modern water.

Carbon Dating Gets A Boost

For more than 50,000 years, an ancient lake called Suigetsu on Japan’s Honshu island has been accumulating alternating layers of fossilized algae and leaves that settle at its bottom every season. Now, an international team of researchers from Japan, Germany, and the U.K. reports that a 40-meter core of Lake Suigetsu’s sediment provides a geochemical record that can dramatically improve the accuracy of radiocarbon dating (Science, DOI: 10.1126/science.1226660). Dating based on the radioactive decay of 14C is essential to many fields, from climate research to archaeology. In particular, the Lake Suigetsu sediment core improves the accuracy of radiocarbon dating for the period between 12,000 and 52,800 years ago, says team member Christopher Bronk Ramsey of Oxford University. “These beautifully preserved leaves give direct access to past radiocarbon values,” he says. To date objects of unknown age through the radioactive decay of 14C, researchers need reference materials, such as tree rings ...

Groundwater recharge and evolution in the Dunhuang Basin, northwestern China

Groundwater recharge and evolution in the Quaternary aquifer beneath the Dunhuang Basin was investigated using chemical indicators, stable isotopes, and radiocarbon data to provide guidance for regional water management. The quality of groundwater and surface water is generally good with low salinity and it is unpolluted. The dissolution of halite and sylvite from fine-grained sediments controls concentrations of Na+ and K+ in the groundwater, but Na+/Cl- molar ratios >1 in all samples are also indicative of weathering of feldspar contributing to excess Na+. The dissolution of carbonate minerals yields Ca2+ to the groundwater, thereby exerting a strong influence on groundwater salinity. The delta O-18 and delta H-2 values in unconfined groundwater are enriched along the groundwater flow path from SW to NE. In contrast, confined groundwater was depleted in heavy isotopes, with mean values of -10.4 parts per thousand delta O-18 and -74.4 parts per thousand delta H-2. Compared with the precipitation values, all of the groundwater samples were strongly depleted in heavy isotopes, indicating that modern direct recharge to the groundwater aquifers in the plains area is quite limited. The unconfined water is generally young with radiocarbon values of 64.9-79.6 pmc. In the northern basin, radiocarbon content in the confined groundwater is less than 15 pmc and an uncorrected age of similar to 15 ka, indicates that this groundwater was recharged during a humid climatic phases of the late Pleistocence or early Holocene. The results have important implications for inter-basin water allocation programmes and groundwater management in the Dunhuang Basin. (C) 2012 Elsevier Ltd. All rights reserved.

Radiocarbon and stable‐isotope geochemistry of organic and inorganic carbon in Lake Superior

We present a lake‐wide investigation of Lake Superior carbon and organic matter biogeochemistry using radiocarbon, stable isotope, and carbon concentrations. Dissolved inorganic carbon (DIC) abundance in the lake was 121–122 Tg C, with offshore concentration andδ13C values being laterally homogenous and tightly coupled to the physical and thermal regime and biochemical processes. Offshore Δ14C of DIC (50–65‰) exhibited lateral homogeneity and was more 14C enriched than co‐occurring atmospheric CO2 (∼38‰); nearshore Δ14C of DIC (36–38‰) was similar to atmospheric CO2. Dissolved organic carbon (DOC) abundance was 14.2–16.4 Tg C. DOC's concentration and δ13C were homogenous in June (mixed lake), but varied laterally during August (stratification) possibly due to spatial differences in lake productivity. Throughout sampling, DOC had modern radiocarbon values (14–58‰) indicating a semilabile nature with a turnover time of ≤60 years. Lake particulate organic carbon (POC, 0.9–1.3 Tg C) was consistently 13C depleted relative to DOC. The δ15N of epilimnetic particulate organic nitrogen shifted to more negative values during stratification possibly indicating greater use of nitrate (rather than ammonium) by phytoplankton in August. POC's radiocarbon was spatially heterogeneous (Δ14C range: 58‰ to −303‰), and generally 14C depleted relative to DOC and DIC. POC 14C depletion could not be accounted for by black carbon in the lake but, because of its spatial and temporal distribution, is attributed to sediment resuspension. The presence of old POC within the epilimnion of the open lake indicates possible benthic‐pelagic coupling in the lake's organic carbon cycle; the ultimate fate of this old POC bears further investigation.

Rapid radiocarbon (14C) analysis of coral and carbonate samples using a continuous‐flow accelerator mass spectrometry (CFAMS) system

Radiocarbon analyses of carbonate materials provide critical information for understanding the last glacial cycle, recent climate history and paleoceanography. Methods that reduce the time and cost of radiocarbon (14C) analysis are highly desirable for large sample sets and reconnaissance type studies. We have developed a method for rapid radiocarbon analysis of carbonates using a novel continuous‐flow accelerator mass spectrometry (CFAMS) system. We analyzed a suite of deep‐sea coral samples and compared the results with those obtained using a conventional AMS system. Measurement uncertainty is &lt;0.02 Fm or 160 Ryr for a modern sample and the mean background was 37,800 Ryr. Radiocarbon values were repeatable and in good agreement with those from the conventional AMS system. Sample handling and preparation is relatively simple and the method offered a significant increase in speed and cost effectiveness. We applied the method to coral samples from the Eastern Pacific Ocean to obtain an age distribution and identify samples for further analysis. This paper is intended to update the paleoceanographic community on the status of this new method and demonstrate its feasibility as a choice for rapid and affordable radiocarbon analysis.

Radiocarbon analysis of dental enamel and bone to evaluate date of birth and death: Perspective from the southern hemisphere

Radiocarbon analysis was conducted on dental enamel, cortical bone and trabecular bone from four human adults from Andean Peru with known birth dates and death dates, the latter all falling within the modern bomb-curve period. Radiocarbon results were compared with known tropospheric values from the southern hemisphere. The results demonstrate that dental enamel can provide useful information regarding birth dates in consideration of known formation times of the specific teeth examined if the radiocarbon values fall within the modern bomb-curve period. Trabecular bone radiocarbon values are closer to the tropospheric values at the date of death than are values from cortical bone reflecting relative differences in the rate of bone remodeling in those tissues. Age at death also represents an important factor.

Quantification of Extraneous Carbon during Compound Specific Radiocarbon Analysis of Black Carbon

Radiocarbon ((14)C) is a radioactive isotope that is useful for determining the age and cycling of carbon-based materials in the Earth system. Compound specific radiocarbon analysis (CSRA) provides powerful insight into the turnover of individual components that make up the carbon cycle. Extraneous or nonspecific background carbon (C(ex)) is added during sample processing and subsequent isolation of CSRA samples. Here, we evaluate the quantity and radiocarbon signature of C(ex) added from two sources: preparative capillary gas chromatography (PCGC, C(PCGC)) and chemical preparation of CSRA of black carbon samples (C(chemistry)). We evaluated the blank directly using process blanks and indirectly by quantifying the difference in the isotopic composition between processed and unprocessed samples for a range of sample sizes. The direct and indirect assessment of C(chemistry+PCGC) agree, both in magnitude and radiocarbon value (1.1 +/- 0.5 microg of C, fraction modern = 0.2). Half of the C(ex) is introduced before PCGC isolation, likely from coeluting compounds in solvents used in the extraction method. The magnitude of propagated uncertainties of CSRA samples are a function of sample size and collection duration. Small samples collected for a brief amount of time have a smaller propagated (14)C uncertainty than larger samples collected for a longer period of time. CSRA users are cautioned to consider the magnitude of uncertainty they require for their system of interest, to frequently evaluate the magnitude of C(ex) added during sampling processing, and to avoid isolating samples < or = 5 microg of carbon.

Validation of yellowedge grouper, Epinephelus flavolimbatus, age using nuclear bomb-produced radiocarbon

Age validation and estimates of longevity of yellowedge grouper (Epinephelus flavolimbatus) from the Gulf of Mexico (GOM) are needed to inform fishery management decisions. Yellowedge grouper sagittal otoliths (n = 100) were collected, aged using conventional means, and cores were submitted for radiocarbon (14C) measurement. Radiocarbon values of yellowedge grouper otoliths were compared to established radiocarbon chronologies in the region to validate the age and ageing methodology of this species. The yellowedge grouper chronology displayed a similar sigmoidal trend as previously published chronologies. In addition to the core analysis, multiple areas on otolith sections from eight specimens were analyzed for Δ14C to validate age estimates for fish born prior to the 14C increase. Our results indicate that yellowedge grouper live longer than previously reported (minimum of 40 years based on radiocarbon measurements). The validated ageing methodology supported an estimated maximum longevity of 85 years and established that yellowedge grouper have the longest lifespan currently known for any species of grouper in the GOM. Results also indicate a depth-age interaction in that material extracted from adult otolith sections assigned to post-bomb dates exhibited lower Δ14C values than cores (juvenile material) assigned to the same post-bomb dates. This finding is likely explained by lower 14C levels reported from water masses at deeper depths (>100 m) which are inhabited by adults.

Environmental isotopic and hydrochemical study of groundwater in the Ejina Basin, northwest China

The study investigates the groundwater evolution and its residence time in the Ejina Basin, northwest China according to isotope and hydrochemical analyses results. The groundwater chemistry is mainly controlled by the dissolution of halite, Glauber’s salt, gypsum, dolomite and calcite, also influenced by other processes such as evaporation, ion exchange, and deposition. Based on tritium content in atmospheric precipitation and by adopting a model with exponential time distribution function, the mean residence time of the unconfined aquifer groundwater with fairly high tritium activities (21–49 TU) is evaluated. The results show that these groundwaters have low residence time (5–120 years) and are renewable. In contrast, the deep confined groundwaters are tritium-free and radiocarbon values range from 18.3 to 26.7 pmc. According to the most commonly used 14C correction models, the radiocarbon groundwater ages were calculated which yield ages of approximately 4,087–9,364 years BP. Isotopic signatures indicate formation of deep confined groundwaters in a colder and wetter climate during the late Pleistocene and Holocene. It is suggested that long-term, rational water usage guide should be set up for the Heihe River Basin as a whole to permit a considerable discharge to the Ejina Basin.

Changes in the soil C cycle at the arid‐hyperarid transition in the Atacama Desert

We examined soil organic C (OC) turnover and transport across the rainfall transition from a biotic, arid site to a largely abiotic, hyperarid site. With this transition, OC concentrations decrease, and C cycling slows precipitously, both in surface horizons and below ground. The concentration and isotopic character of soil OC across this transition reflect decreasing rates of inputs, decomposition, and downward transport. OC concentrations in the arid soil increase slightly with depth in the upper meter, but are generally low and variable (∼0.05%; total inventory of 1.82 kg m−2); OC‐Δ14C values decrease from modern (+7‰) to very 14C‐depleted (−966‰) with depth; and OC‐δ13C values are variable (−23.7‰ to –14.1‰). Using a transport model, we show that these trends reflect relatively rapid cycling in the upper few centimeters, and spatially variable preservation of belowground OC from root inputs, possibly during a previous, wetter climate supporting higher soil OC concentrations. In the driest soil, the OC inventory is the lowest among the sites (0.19 kg m−2), and radiocarbon values are 14C‐depleted (−365‰ to –696‰) but show no trend with depth, indicating belowground OC inputs and long OC residence times throughout the upper meter (104 y). A distinct depth trend in δ13C values and OC/ON values within the upper 40 cm at the driest site may reflect photochemical alteration of organic matter at the soil surface, combined with limited subsurface decomposition and downward transport. We argue that while root inputs are preserved at the wetter sites, C cycling in the most hyperarid soil occurs through infrequent, rapid dissolved transport of highly photodegraded organic matter during rare rain events, each followed by a pulse of decomposition and subsequent prolonged drought. These belowground inputs are likely a primary control on the character, activity, and depth distribution of small microbial populations. While the lack of water is the dominant control on C cycling, very low C/N ratios of organic matter suggest that when rainfall occurs, hyperarid soils are effectively C limited. The preservation of fossil root fragments in the sediment beneath the driest soil indicates that wetter climate conditions preceded formation of this soil, and that vadose zone microbial activity has been extremely limited for the past 2 My.

Carbon sources and signals through time in an Alpine groundwater Basin, Sagehen California

In 2003, chlorofluorocarbon (CFC) apparent ages, major ion chemistry and C isotopes were determined in nine springs from Sagehen Basin, a high elevation watershed in the eastern Sierra Nevada. Springs with similar apparent ages, which ranged between 15 and 45 a, had very similar chemistry despite being found in different areas of the watershed. In agreement with earlier studies, concentrations of rock-derived cations (Ca 2+ and Na + ), conductivity, temperature and pH increase with apparent age, documenting the chemical evolution of this groundwater system. In contrast with the cation data, δ 13 C and ΣCO 2 show no correlation with apparent age. δ 13 C displays a strong linear relationship with 1/ΣCO 2 ( R 2 = 0.91). This is consistent with results from a previously developed soil respiration/diffusion model. Spring radiocarbon content ranged between 85 and 110 pmc and varied with apparent age, whereby the youngest groundwater has the highest radiocarbon values. The spring radiocarbon is set by the soil p CO 2 and its trend can be best described assuming the soil CO 2 is composed of a mixture of 50–66% fast- (15–25 a) and 33–50% slow- (4 ka) cycling components. These results are consistent with previous soil C studies. The C isotope data indicate that in Sagehen Basin the groundwater ΣCO 2 is inherited from the soil zone with little, if any, contribution from the dissolution of disseminated calcite.

The hydrochemical characteristics and evolution of groundwater and surface water in the Heihe River Basin, northwest China

A combination of isotopic and chemical indicators has been used to characterize rainfall, surface water and groundwater in the Heihe River Basin, China. Surface- vs. groundwater chemistry data enabled geographical zones and chemical types to be differentiated. The dissolution of halite, Glauber’s salt, gypsum, dolomite and calcite determine Na+, Cl−, Mg2+, Ca2+, \( {\text{SO}}^{{2 - }}_{4} \) and \( {\text{HCO}}^{ - }_{3} \) chemistry, but other processes such as evaporation, ion exchange, and deposition also influence the water composition. The majority of deep confined groundwaters are tritium-free and less mineralized than in the shallow aquifer. Radiocarbon values in the deeper groundwaters range from 18.8 to 38.9 pmc, and 80 pmc probably represents the upper limit of initial 14C activity; this yields ages of approximately 5,960–11,971 years BP, which is about 3,000 years older than those calculated by models in previous work. The shallow aquifer exhibits fairly high and variable tritium activities (4–75 TU), evidence of recent recharge and low residence time (<60 years), which is in line with estimates from previous work. Isotopic signatures indicate formation of deeper groundwaters in a colder and wetter climate during the late Pleistocene and Holocene. The results suggested that significant changes are urgently needed in water-use strategy to achieve sustainable development.