Radiocarbon measurements on bulk subaqueous sediments typically provide ages significantly older than actual time of deposition. This is generally caused by the presence of reworked organic compounds, which are depleted in 14C. To explore this issue of age heterogeneity, we collected 4 organic-rich samples from varying depths in a lake sediment core at the Gemündener Maar (Eifel, Germany), a lake of volcanic origin. We divided each sample into 5 standard grain-size fractions: gravel, sand, silt, clay, and 1 fraction smaller than 0.45 μm. These were cleaned separately using a standard acid-alkali-acid treatment. The highly organic gravel-size fraction provided the youngest 14C ages of all grain-size fractions and seems to be associated most closely with the time of deposition. By contrast, the silt and clay fractions show significantly older ages. If the investigated limnic sediment layer does not contain any identifiable terrestrial macrofossils, extracting and measuring coarser grain-size fractions instead of measuring bulk sediment samples will provide a better approximation of the time of sedimentation.

The assessment of construction sites for the new Visaginas Nuclear Power Plant (Visaginas NPP), including groundwater characterization, took place over the last few years. For a better understanding of the groundwater system, studies on radiocarbon; tritium; stable isotopes of hydrogen, oxygen, and carbon; and helium content were carried out at the location of the new NPP, at the Western and Eastern sites, as well as in the near-surface repository (NSR) site. Two critical depth zones in the Quaternary aquifer system were characterized by different groundwater residence times and having slightly different stable isotope features and helium content. The first shallow interval of the Quaternary multi-aquifer system consists of an unconfined aquifer and semiconfined aquifer. The second depth interval of the system is related to the lower Quaternary confined aquifer. Groundwater residence time in the first flow system was mainly based on tritium data and ranges from 6 to 60 yr. These aquifers are the most important in terms of safety assessment and are considered as a potential radionuclide transfer pathway in safety assessment. Groundwater residence time in the lower Quaternary aquifers based on 14C data varies from modern to several thousand years and in some intervals up to 10,500 yr.

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, <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 <5 pMC), implying old ages (preliminary ages >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.

Dating sedimentary series spanning the past few tens of thousands of years is often problematic due to the quality of radiocarbon data obtained from organic matter (OM), including bulk OM. This problem recently arose when establishing the chronology of a sediment infill at the Sarliève paleolake (French Massif Central). In the studied section of the cores that covers the Neolithic, Ruppia seeds yielded consistent ages for the lower part (7195 ± 75 to 6050 ± 60 yr BP). A reservoir age of 82 ± 42 14C yr was estimated through the comparison of ages derived from charcoal, Ruppia seeds, and charophyte oogonia sampled on a single level. The upper part of the cores lacks macrofossils and bulk OM dating yields unusable data because of a significant contribution of aged OM derived from the Oligocene substratum in the catchment. We therefore performed dating of lipids extracted from the sediments. The age of the lipids was 2880 ± 30 yr BP near the top of the section, i.e. much younger than the age estimated from previous correlations based on pollen assemblages. These new data call into question previous paleoenvironmental interpretations. The combined dating methodology used for the Neolithic series of Sarliève is a rather uncommon approach that may help to refine chronologies of Holocene sedimentary series.

Radiocarbon decay is rarely used to assess the residence time of modern groundwater due to the low resolution of its long half-life in comparison to the expected range of ages. Nonetheless, the modern 14C peak induced by the nuclear bomb tests traces efficiently the impacts of recent human activities on groundwater recharge, as well as for tritium. A simple lumped parameter model (LPM) was implemented in order to assess the interest of 14C and 3H nuclear peaks in a highly anthropized aquifer system of southeastern Spain under intense agricultural development. It required i) to assess a correction factor for modern 14C activities and ii) to reconstruct the 3H recharge input function, affected by irrigation. In such a complex hydrogeological context, an exponential model did not provide satisfying results for all samples. A better solution was reached by taking into account the qualitative recent variation of the recharge rates into a combined exponential flow and piston flow model. Apart from presenting an uncommon approach for 14C dating of modern groundwater, this study highlights the need of considering not only the variation of the tracer but also the variability of recharge rates in LPMs.

Characteristics of dissolved humic and fulvic acids in river waters were studied during 2003–2005 at 4 sites located in the headwaters and in the upper and lower Tokachi River, including a lowland tributary site. Fulvic acids from the headwaters to downstream areas have similar elemental composition and 13C-NMR spectra. Humic acids have similar characteristics in the Tokachi River system. In contrast, δ13C and Δ14C values exhibit a decreasing trend from the upper to the lower and tributary sites, although the headwater site has heavier δ13C and lower Δ14C values than the upper site. Fulvic acids had similar δ13C values from the upper to lower sites, but 123′ higher in Δ14C than those of humic acids on average. The δ13C and Δ14C values exhibited differences in downward variation for humic and fulvic acids. In the Tokachi River system, these results suggest that differences in transport pathways and residence times of humic and fulvic acids reflect differences in the δ13C and Δ14C values in a single river basin.

Horizontal and vertical variations in the distribution of 14C, δ13C, δ18O, and δ2H in groundwater of Žitný Island (Rye Island) have been studied. Žitný Island, situated in the Danube River Basin, is the largest island in Europe that is formed by interconnected rivers. It is also the largest groundwater reservoir in central Europe (∼1010 m3 of drinking water). The δ2H vs. δ18O plot made from collected groundwater samples showed an agreement with the Global Meteoric Water Line. In the eastern part of the island, it was found that subsurface water profiles (below 10 m water depth) showed enriched δ18O levels, which were probably caused by large evaporation losses and the practice of irrigating the land for agriculture. The core of the subsurface 14C profile represents contemporary groundwater with 14C values >80 pMC, indicating that the Danube River during all its water levels feeds most of the groundwater of Žitný Island. However, on the eastern part of the island a small area was found where the δ13C and 14C data (down to ∼30 pMC) helped to identify a groundwater aquifer formed below the Neogene clay sediments. This is the first time that vertical distributions of isotopes in different groundwater horizons have been studied.

Lac Pavin is a meromictic maar lake for which the interpretation of sediment radiocarbon dates is complicated by the existence of a largely undefined reservoir effect resulting from degradation of carbon stored in the bottom layer of the water column. A data set of the contemporary 14C distribution of dissolved and particulate organic pools in the water column is presented to address this issue. Dissolved inorganic carbon (DIC) and organic carbon (DOC), plankton, suspended particulate organic carbon (POCsusp), sinking POC (POCsink), and bottom sediment organic carbon (SOC) were analyzed. Present-day Δ14C values of DIC were measured ranging from −750′ in the monimolimnion to atmospheric values in lake surface waters and in spring inlet waters. A range of Δ14C values between −200 and −300′ was observed for superficial POCsusp, POCsink, SOC, and DOC. This relatively uniform 14C offset of the exported organic production from the surface waters to the bottom represents a contemporary reservoir effect of ∼2500 yr. Laminated buried sediment samples and terrestrial vegetal macro-remains were used to evaluate temporal reservoir effect variations since the formation of the crater lake (7 ka cal BP). Buried sediment layers presented a similar offset or showed larger differences between Δ14C values of bulk sediment and terrestrial plant remains (–400 to −500′). Furthermore, an almost 0-yr reservoir effect was inferred from the sediment layers deposited just above the volcanic bedrock at the early flooding of the crater, and increasing slightly within the first centuries of the lake's history. A second objective was to tentatively model a defined scenario of the cycling of carbon in the lake capable of predicting a modern reservoir effect. Alternative scenarios were then tested for which a larger contribution of deeper DIC would provide a model compatible with larger past reservoir effects. It is concluded that using Δ14C SOC variation in laminated lake sediments as a proxy of paleolimnological conditions may be valuable provided that more data on the dynamics of the 14C composition of plankton and more detailed sampling of laminated sediment layers are available.

We present here the new line installed at the LMC14 laboratory (Saclay, France) for dissolved inorganic carbon (DIC) extraction from marine and freshwater samples. The operating system and extraction process are described. The efficiency of the line design was checked, and the background (0.42 ± 0.11 pMC) and the reproducibility on artificial samples obtained by dissolution of IAEA-C1, IAEA-C2, and commercial bicarbonate in water were evaluated. An intercomparison with an independent lab (IDES) was also carried out on a natural sample. The line processes 3 samples a day under a helium flow and is able to run samples up to 40,000 ka.

A continuous and high-resolution record of the radiocarbon reservoir age (RA) has been recovered from the primary aragonites that were deposited from the last glacial Lake Lisan. The RA is calculated as the difference between the measured 14C “apparent” age in the aragonite and the atmospheric age at any particular time. The RA shows temporal decreases during the time interval of ≃28 to ≃18 ka cal BP. This behavior is attributed to a continuous addition of low RA-high bicarbonate freshwater into the high RA-Ca-chloride (low bicarbonate) brine solution filling the lake. The mixing of the brine with freshwater drives the precipitation of CaCO3 in the form of aragonite from the lake epilimnion (surface layer). The runoff-brine mixture in Lake Lisan is also reflected by the Sr/Ca ratios that are positively correlated with the RA. Nevertheless, the 14C content in the epilimnion did not drop at the same rate as the atmospheric value but rather remained nearly constant. We suggest that turbulent mixing with the much saltier hypolimnion (lower layer) across the hypolimnion/epilimnion interface at a depth of about 390 m below sea level, buffered the 14C content as well as the Sr and Ca concentrations in the aragonite precipitating solution. The RA-Sr/Ca related limnological model developed here opens the way to determine the reservoir-age-corrected atmospheric ages of Lisan Formation aragonites beyond 28 ka cal BP.

Rice paddies are highly important agricultural soils in view of their relevance as major staple food provider in the world and their key role in the global carbon cycle, caused by special management practices. A soil chronosequence, consisting of paddy and upland soils, developed on reclaimed estuarine sediments in the Province of Zhejiang, China, was sampled to investigate the influence of duration of agricultural use (50 to 2000 yr) on soil composition. The uniform composition of the parent material provides the unique opportunity to compare the effects of different land management practices (paddy and non-paddy) on soil carbon dynamics and the origin of organic carbon (OC) in top- and subsoils, using 14C measurements by accelerator mass spectrometry (AMS). The total soil organic carbon (TOC) was split into chemically defined pools of different mobility, namely the acid- and water-soluble fulvic acids (FA), the alkali-soluble humic acids (HA), and insoluble humin fraction. The more mobile HA and FA fractions contain significantly more 14C than the corresponding TOC and humin, indicating a downward transport of OC in the subsoil. Plant roots with 14C concentrations up to 128% of the modern standard, found far below the plough pan, reveal plant roots and root exudates as other direct sources of subsoil OC in paddy soils.

Time-series radiocarbon measurements have substantial ability to constrain the size and residence time of the soil C pools commonly represented in ecosystem models. 14C remains unique in its ability to constrain the size and turnover rate of the large stabilized soil C pool with roughly decadal residence times. The Judgeford soil, near Wellington, New Zealand, provides a detailed 11-point 14C time series enabling observation of the incorporation and loss of bomb 14C in surface soil from 1959–2002. Calculations of the flow of C through the plant-soil system can be improved further by combining the known constraints of net primary productivity (NPP) and 14C-derived C turnover. We show the Biome-BGC model provides good estimates of NPP for the Judgeford site and estimates NPP from 1956–2010. Synthesis of NPP and 14C data allows parameters associated with the rapid turnover “active” soil C pool to be estimated. This step is important because it demonstrates that NPP and 14C can provide full data-based constraint of pool sizes and turnover rates for the 3 pools of soil C used in nearly all ecosystem and global C-cycle models.

Radiocarbon analysis is an important tool in quantifying soil organic matter (SOM) dynamics within the terrestrial carbon cycle. However, there is increasing appreciation that representing SOM as a single, homogeneous pool with a single, mean 14C concentration is inadequate. We investigate whether the differing patterns in CO2 release during ramped-temperature oxidation reflect organic matter of different ages, and hypothesize that thermally labile SOM (combusting at low temperatures) consists of younger carbon than thermally resistant organic matter. Topsoil samples under contrasting land uses (native vegetation and long-term cultivation) were selected for 14C analysis before and after acid fumigation for the removal of carbonates. Results of bulk 14C analyses showed a significant shift in 14C age from 0.944 Fm under native vegetation to 0.790 Fm under cultivation. Four to 5 “fractions” associated with different CO2-evolution regions were identified by thermal analysis and analyzed for 14C via modifications to NOSAMS' established “programmed temperature pyrolysis system,” in which discrete CO2 fractions evolved during ramped-temperature oxidation were isotopically characterized by a microwave gas ion source (GIS) continuous-flow AMS (CFAMS) system. Results showed that while acid fumigation removed soil carbonates, the treatment also significantly altered the thermograms and inferred SOM composition. While direct attribution of 14C values to individual peaks is somewhat confounded by overlapping temperature ranges for oxidation of unique populations of carbon, in general, thermally stable fractions of SOM appear to be 14C-depleted compared to thermally reactive (low temperature) fractions regardless of pretreatment.