Radiocarbon: A chronological tool for the recent past

The past few hundred years have seen large fluctuations in atmospheric ¹⁴C concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age, and the Spörer and Maunder solar activity minima. In addition, however, changes in human activity since the middle of the 19th century have released ¹⁴C-free CO₂ to the atmosphere. Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapon testing resulted in a dramatic increase in the concentration of ¹⁴C in the atmosphere. This was followed by a significant decrease in atmospheric ¹⁴C as restrictions on nuclear weapon testing began to take effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The large fluctuations in atmospheric ¹⁴C that occurred prior to 1955 mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. This difficulty may be overcome by obtaining a series of ¹⁴C dates from a sequence and either wiggle-matching these dates to a radiocarbon calibration curve or using additional information on dated materials and their surrounding environment to narrow the calibrated age ranges associated with each ¹⁴C date. For the period since 1955 (the bomb-pulse period), significant differences in atmospheric ¹⁴C levels between consecutive years offer the possibility of dating recent samples with a resolution of from one to a few years. These approaches to dating the recent past are illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms and speleothems.     

Seasonal radiocarbon reservoir ages for the 17th century James River,Virginia estuary

This study utilizes a combined stable isotope and ¹⁴C dating approach to determine the radiocarbon reservoir age correction, ΔR, for the James River, Virginia estuary from 17th century Crassostrea virginica shells of known collection dates. ΔR, which can vary spatially and temporally, is a locality-specific adjustment applied to the global ocean reservoir, R, to further account for the offset between the atmospheric and marine ¹⁴C calibration curves. To assess the temporal variability in ΔR, continuous δ¹⁸O sampling along the oyster shell hinge provides a seasonal record throughout the oyster's life. This is then used to identify sampling locations for ¹⁴C measurements based on calcite precipitated during the Summer (>19 °C) and Fall through Spring (F-Sp, <15 °C) months. The resulting seasonal ΔR values range from −151 ± 46 to +109 ± 55 ¹⁴C years (260 years) due to changes in the contribution and age of dissolved inorganic carbon (DIC) from marine and freshwater sources in the James River estuary. The F-Sp samples display a larger ΔR range than the Summer samples, as do the shells precipitated during drought conditions (1606–1612) when compared to shells from the remainder of the 17th century. The largest intrashell ΔR variability, 195 ¹⁴C years, is similarly found in a drought shell and is attributed to variability caused by the extreme regional 1606–1612 drought. Early land use changes related to European development and farming practices also altered the age of DIC in the James River estuary. We estimate that the soil inorganic carbon (SIC) contributing to freshwater DIC ranged from 0 to ∼1800 years old and reflected both the drought and land use changes that occurred during the 17th century. Using only the Summer samples, which represent the majority of shell calcite, we obtain a mean ΔR = −32 ± 11 ¹⁴C years (1σ) for 17th century James River estuary ΔR at the very onset of European colonization. Employing a seasonally resolved sampling method will provide the greatest constraint on ¹⁴C measurements in an estuarine environment where multiple carbon sources can fluctuate on seasonal timescales and as a result of large scale environmental change.     

40Ar/39Ar step heating of thermally overprinted biotite,hornblende and potassium feldspar from Eldora,Colorado

⁴⁰Ar/³⁹Ar incremental heating experiments were applied to hornblendes, coarse-grained biotites and K-feldspars from 1400 m.y. old rocks near the contact with the ～60 m.y. old Eldora stock in the Front Range of Colorado. The aim was to distinguish, on the basis of argon isotopic data alone, a partially re-set K-Ar date from an undisturbed or a completely overprinted K-Ar date. In the laboratory heating of biotites the radiogenic argon (⁴⁰Ar^*) and potassium-derived³⁹Ar (³⁹Ar^*) were released in two stages — in the range ～600–900°C and above ～900°C. The two biotites furthest from the contact and the one adjacent to the contact give well-defined apparent-age plateaus at ～1230 m.y. and 63 m.y. respectively for all argon released above ～600°C. The 1230-m.y. date may represent a thermal event or the end of a long cooling while the 63-m.y. date essentially represents the time of reheating. Partially overprinted biotites at intermediate distances have significantly anomalous plunges in apparent ages for argon released above ～900°C, thus distinguishing them from undisturbed and completely outgassed biotites.The bulk of the⁴⁰Ar^* and³⁹Ar^* in the hornblendes was released in the range ～950–1100°C. The hornblende furthest from the contact gives a well-defined plateau at 1400 m.y. for the 98% of the argon that was released above ～950°C. A partially overprinted hornblende from near the contact gives an apparent plateau at ～1050 m.y. The existence of such a false plateau precludes the distinction of partially overprinted K-Ar hornblende dates from undisturbed K-Ar hornblende dates without independent evidence. Reasonable estimates of the time of reheating are not recovered in the age spectra for partially overprinted hornblende and biotites.For the feldspars the bulk of the⁴⁰Ar^* and³⁹Ar^* was released in the laboratory heating between about 900°C and 1200°C, probably reflecting phase changes near these temperatures. The argon released below about 900°C records reasonable maximum dates for the time of the thermal overprinting. For the microcline 22500 (the sample number specifies the distance, in feet, from the contact) this effect is slight — a minimum date of 147 m.y. occurs in 2.3% of the total³⁹Ar^*. For samples 2400, 1070, and 85 the respective minimum dates are similar at 72, 81, and 68 m.y. and dramatically improve on the total or integrated dates of 238, 358 and 211 m.y. The high-temperature (>900°C) apparent ages for these three feldspars do not define plateaus and are geologically meaningless. The high-temperature apparent ages for the last 50% of the³⁹Ar^* released from 22500 do define a plateau, but the 1060-m.y. date is also probably geologically meaningless.     

New marine ΔR values for Arctic Canada

For more than four decades, the reporting of ¹⁴C dates on marine molluscs from Arctic Canada has been notable for the lack of consistently applied marine reservoir corrections. We propose that the common approach of reporting Canadian Arctic marine ¹⁴C dates using presumed time-invariant reservoir corrections be abandoned in favour of calibration of ¹⁴C dates, using the current standard protocol. This approach best facilitates inter- and intra-regional correlation, and correlation with other geochronometers. In order to enable the consistent calibration of marine ¹⁴C dates from Arctic Canada, we analysed a ¹⁴C database of 108 marine mollusc samples collected live between 1894 and 1956, and determined regional reservoir offset values (ΔR) for eight oceanographically distinct regions. The following new ΔR values should be used for ¹⁴C calibration: NW Canadian Arctic Archipelago, 335 ± 85 yrs; Foxe Basin, 310 ± 90 yrs; NE Baffin Island, 220 ± 20 yrs; SE Baffin Island, 150 ± 60 yrs; Hudson Strait, 65 ± 60 yrs; Ungava Bay, 145 ± 95 yrs; Hudson Bay, 110 ± 65 yrs; and James Bay, 365 ± 115 yrs.     

Accurate chronological construction for two young stalagmites from the tropical South Pacific

Modern to Holocene tropical Pacific stalagmites are commonly difficult to date with the U-series, the most commonly used dating method for speleothems. When U-series does not provide robust age models, due to multiple sources of ²³⁰Th or little U, radiocarbon is, potentially, the best alternative. The ¹⁴C content of two stalagmites (Pu17 and Nu16) collected from Pouatea and Nurau caves in the Cook Island Archipelago of the South Pacific were measured to obtain accurate chronology for their most modern parts. The bomb-pulse soil continuum modelling indicates that bomb radiocarbon in Pu17 onsets in 1956 and reaches its maximum in 1966 CE, suggesting a fast transfer of atmospheric carbon to the stalagmite of <1 year. The modelling for Pu17 suggests a 20% contribution from C₁ - an instantaneous carbon source, which renders possible an immediate transfer of atmospheric signal into the cave. Nu16 shows a slower transfer of atmospheric carbon to the stalagmite than Pu17, with bomb radiocarbon onsetting in 1957 CE and peaking in 1972 CE. The less negative δ¹³C values in Nu16 than Pu17, and also the modelling corroborated this, which points out no contribution from the instantaneous carbon source. The radiocarbon age models and laminae counting age models were then spliced to achieve a single master chronology for the top part of each stalagmite. This study is an example of ¹⁴C age modelling combined with visible physical and chemical laminae counting and how it can improve the accuracy and precision of dating for otherwise hard-to-date tropical Pacific speleothems. Such accurate and precise age models pave the way to obtain sub-annually resolved paleoclimate records by further improving the calibration of climate proxy data with the current and instrumental weather parameters.     

古土壤中胡敏酸和胡敏素14C测年值统计分析

文中使用统计方法对44个不同气候条件下形成的古土壤中胡敏酸、胡敏素的14C测年结果进行分析,探讨不同组分14C年龄的可靠性。对比和统计分析结果表明,从总体上胡敏素可能是古土壤中最古老的惰性组分,其14C年龄可代表古土壤形成(平均)年龄;形成于晚更新世的地质样品胡敏酸与胡敏素的14C年龄差异并不明显,但全新世尤其晚全新世的沉积物中胡敏酸与胡敏素的14C年龄差异比较显著,可能因埋藏较浅而受到更多的污染影响;对全新世(尤其晚全新世)古土壤样品开展不同组分14C测年是保证地质事件年龄准确可靠的有效手段     

The construction of a reliable absolute chronology for the last two millennia in an anthropogenically disturbed peat bog: Limitations and advantages of using a radio-isotopic proxy and age–depth modelling

The main aim of this paper is to present the pitfalls connected with the construction of reliable chronologies for anthropogenically disturbed peatlands over the last two millennia based on ²¹⁰Pb and ¹⁴C dating, i.e. the period of the strongest human impact on these ecosystems. The following hypotheses have been formulated: i) parts of peatlands suspected to be affected by peat extraction may possess traces of mechanical disturbances undetectable using different analyses based on biota proxy; ii) failure to consider information included in radionuclide date inversions may contribute to the establishment of misleading chronologies. To test these hypotheses, different scenarios of chronology based on high resolution ²¹⁰Pb and ¹⁴C dating from a peat core retrieved from the Puścizna Krauszowska bog (southern Poland) have been analysed. Nowadays, this mire is intensively exploited by humans; however, it contains remains of dome considered undisturbed, from which the core presented in this paper was collected. The set of dates revealed the presence of marked ¹⁴C date inversions (mechanical disturbances) which, if inappropriately interpreted before the age–depth modelling process, may lead to the establishment of misleading chronologies, and thus an incorrect interpretation of biota proxy records, e.g. pollen. Those sections of peat profiles with prominent age inversions and/or strong discrepancies between the peat accumulation rate and bulk density should be rejected from age–depth modelling, even if interpretable chronologies can be obtained.     

Hydrological control of the dead carbon fraction in a Holocene tropical speleothem

Over the past decade, a number of speleothem studies have used radiocarbon (¹⁴C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse ¹⁴C to anchor chronologies over the last 60 years, the combination of U-Th and ¹⁴C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric ¹⁴C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use ¹⁴C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ¹⁸O, δ¹³C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with ¹⁴C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO₂, and ultimately led to a greater contribution of dead carbon from the bedrock.     

Robust chronological reconstruction for young speleothems using radiocarbon

We have studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean) and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia). Attempts to date these speleothems by the Th/U method have proved unsuccessful with some age reversals for SC4 due to multiple sources of non-authigenic Th. This method has also resulted in imprecise ages for WM7 because of low U concentrations (<10 ppb) and consequently very low levels of authigenic ²³⁰Th relative even to the very low levels of detrital ²³⁰Th present. Here, we present an alternative method for reliable dating of these young speleothems using radiocarbon. Approximately 100 carbonate samples from SC4 and WM7 were analysed for ¹⁴C by accelerator mass spectrometry (AMS). The AMS results indicate that bomb ¹⁴C was evident in the youngest parts of both stalagmites. Two different approaches were used to estimate dead carbon fraction (DCF) values for these stalagmites for the pre-bomb period. For SC4, the DCF values were estimated based on the timing of ¹⁴C dates for that period determined by high-resolution δ¹⁸O recorded in the speleothem, and the timing of the onset of bomb ¹⁴C. For WM7, a “maximum” range of pre-bomb DCF was determined. Chronologies of these speleothems were built based on a dense sequence of DCF-corrected ages using three different age-depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach). Good agreement between these age-depth models were observed indicating that the top 170 mm of SC4 and the top 50 mm of WM7 grew during the past 550–750 years and 1360–1740 years, respectively.     

Soil organic carbon storage and soil CO<Subscript>2</Subscript> flux in the alpine meadow ecosystem

High-resolution sampling, measurements of organic carbon contents and ¹⁴C signatures of selected four soil profiles in the Haibei Station situated on the northeast Tibetan Plateau, and application of ¹⁴C tracing technology were conducted in an attempt to investigate the turnover times of soil organic carbon and the soil-CO₂ flux in the alpine meadow ecosystem. The results show that the organic carbon stored in the soils varies from 22.12×10⁴ kg C hm⁻² to 30.75×10⁴ kg C hm⁻² in the alpine meadow ecosystems, with an average of 26.86×10⁴ kg C hm⁻². Turnover times of organic carbon pools increase with depth from 45 a to 73 a in the surface soil horizon to hundreds of years or millennia or even longer at the deep soil horizons in the alpine meadow ecosystems. The soil-CO₂ flux ranges from 103.24 g C m⁻² a⁻¹ to 254.93 gC m⁻² a⁻¹, with an average of 191.23 g C m⁻² a⁻¹. The CO₂ efflux produced from microbial decomposition of organic matter varies from 73.3 g C m⁻² a⁻¹ to 181 g C m⁻² a⁻¹. More than 30% of total soil organic carbon resides in the active carbon pool and 72.8%281.23% of total CO₂ emitted from organic matter decomposition results from the topsoil horizon (from 0 cm to 10 cm) for the Kobresia meadow. Responding to global warming, the storage, volume of flow and fate of the soil organic carbon in the alpine meadow ecosystem of the Tibetan Plateau will be changed, which needs further research. Supported by the National Natural Science Foundation of China (Grant Nos. 40231015, 40471120 and 40473002) and the Guangdong Provincial Natural Science Foundation of China (Grant No. 06300102)     

Mid‐Holocene sub‐blanket‐peat alluvia and sediment sources in the upper Liffey valley,Co. Wicklow,Ireland

A recently exposed section across a ?rst‐order valley buried beneath the regional blanket peat on hillside slopes in the upper Liffey valley, Co. Wicklow, is described. The section shows two alluvia within a shallow valley form underlain by an extensive boulder and stone line over regional till and weathered granite. ¹⁴C dates from wood in the alluvia indicate the older alluvium to have formed between 4324 ± 53 BP and 4126 ± 45 BP and the younger between 3217 ± 53 BP and 2975 ± 53 BP . The basal layer of the overlying peat yielded a date of 2208 ± 61 BP . The younger alluvium shows the effects of soil paludi?cation prior to the peat expansion. Dated pollen analyses elsewhere in the upper catchment con?rm the spread of blanket peat over most areas above 350 m after 4000–3600 BP . The buried valley was contributing sediments to the mid‐Holocene ?oodplains in the upper Liffey valley prior to the extension of blanket peat over the catchment after which sediment yields from it and the other catchment slopes declined. Copyright © 2003 John Wiley & Sons, Ltd.     

Late Cenomanian to Middle Turonian high-resolution carbon isotope stratigraphy: New data from the Münsterland Cretaceous Basin,Germany

New high resolution carbon isotope stratigraphies from two basinal pelagic carbonate successions in northern Germany (Halle and Oerlinghausen, Münsterland Cretaceous Basin) resolve late Cenomanian to early Mid-Turonian carbon cycle variations at timescales of less than 100 kyr. Beside the major carbon isotope excursion of the late Cenomanian oceanic anoxic event (OAE 2), 11 small-scale distinct features are precisely resolved in the δ¹³C carbonate curve and related to boreal macrofossil zonations. The small-scale carbon isotope events correspond to secular δ¹³C carbonate variations identified previously in the English Chalk. The boreal high-resolution δ¹³C carbonate curve shows a detailed coincidence with two Tethyan δ¹³C curves from Italy, what demonstrates the interregional significance of the δ¹³C dates and allows their correlation within error limits of ± 40 kyr. Furthermore, the new δ¹³C curve enables the calibration of boreal and tethyan macro- and microfossil zonations. Accordingly, the Tethyan calcareous nannoplankton boundary NC13/NC14 corresponds to the boreal FO of C. woollgari, the index taxon for the Lower-Middle Turonian boundary. The cyclic appearance and the temporal spacing of the small-scale carbon isotope events suggest that orbital forcing exerted control on surface water productivity and organic matter preservation at the sea floor.     

Ecosystem engineering by hummock-building earthworms in seasonal wetlands of eastern South Africa: Insights into the mechanics of biomorphodynamic feedbacks in wetland ecosystems

This paper resolves the origin of clay hummock micro-topography in seasonal wetlands of the Drakensberg Foothills, providing a review and appraisal of previously-suggested mechanisms of hummock formation in the context of new field and laboratory data. Field surveys revealed neo-formation of clay hummocks in a river channel that had been abandoned in c.1984. Fresh earthworm castings were located atop hummocks protruding from inundated abandoned channel margins. Earthworm castings, and sediment cores taken in hummocks and adjacent hollows, were analysed for soil-adsorbed carbon and nitrogen using an HCN analyser, and for ²¹⁰Pb activity using alpha-geochronology. ²¹⁰Pb activity profiles suggest relative enrichment of the isotope in hummocks, and relative depletion in adjacent hollows. Earthworm castings are characterised by very high ²¹⁰Pb activity, as well as high C and N contents. Hummocks have significantly higher C and N contents than adjacent hollows. Results suggest that it is the foraging activity of earthworms in litter-rich seasonal wetland hollows, and repeated excretion of castings atop adjacent hummocks, that is responsible for the elemental enrichment observed. The paper presents a conceptual model of hummock formation in wetlands through interactions between hydrogeomorphology and earthworm activity, and illustrates a mechanism of biogeomorphic inheritance through which ordered patterns of preferential flow can emerge in ecosystems. Further implications of hummock formation and nodal accumulation of nutrients are considered in relation to wetland resilience and regulatory ecosystem service provision.© 2018 John Wiley & Sons, Ltd.     

Testing the ABOx-SC method: Dating known-age charcoals associated with the Campanian Ignimbrite

Over the past decade several studies have shown the improvements to radiocarbon chronologies that arise when Acid Base Oxidation-Stepped Combustion (ABOx-SC, Bird et al., 1999) pretreatment methods are applied to the dating of charcoal thought to be >30 ka BP. However, few studies have examined whether the use of ABOx-SC produces dates that are not only older, but accurate on known-age charcoal samples that could not be decontaminated using the routine Acid–Base–Acid (ABA) pretreatment protocol. In this study we date 9 charcoal fragments found below the Campanian Ignimbrite (CI) tephra layer, dated by ⁴⁰Ar/³⁹Ar to 39,230 ± 45 years (De Vivo et al., 2001, Rolandi et al., 2003), from three Palaeolithic sites. When treated with the ABOx-SC pretreatment protocol, the radiocarbon dates provide an accurate terminus post quem for the CI. In contrast, the ABA protocol consistently underestimates the age of the tephra. These results serve as a warning against the use of consistency as an indicator for reliability, demonstrate that the routine ABA method is not sufficient to decontaminate charcoal samples from sites of Palaeolithic age, and show that ABOx-SC produces not only older, but accurate age estimates.     

Reduction of thermohaline circulation during deglaciation: the effect on atmospheric radiocarbon and CO2

A two-? ocean reservoir model is employed to examine the combined effects of vertical ocean circulation, organic matter extraction and cosmic ray production on the¹⁴C/¹²C in the atmosphere, ocean and sediment. In this model, dissolution of deep-sea calcium carbonate sediment is assumed to respond to the supply of particulate carbonate from the surface ocean and to the dissolved carbonate-ion concentration of the deep-sea.If the vertical ocean circulation decreased by 50% during the maximum rate of deglaciation, the atmospheric¹⁴C/¹²C would have increased concurrently by 10% relative to the pre-bomb present. Further, if cosmic ray production of¹⁴C was 50% greater than present at about 7800 years B.P. as suggested by archeomagnetic measurements, a double maximum of 10% occurs. The first is at about 10.5 ka and the second at 6.5 ka. This result is similar to the variation of¹⁴C/¹²C over the last 10,000 years calculated from the¹⁴C dates measured by Stuiver on the varved Lake of the Clouds. The result is not sensibly altered if 10¹⁷ moles of organic carbon is extracted from the ocean during sea-level rise.Reduction of the thermohaline ocean circulation by 50% over a one- to two-thousand-year interval would not be sensibly detected in the dating of deep-sea sediment. If Broecker's organic nutrient extraction occurs in conjunction with reduction of vertical circulation, a decrease in the atmospheric P_CO2 will precede the eventual rise.     

Comparison between luminescence and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia

Dust depositions are critical archives for understanding interior aridification and westerly climatic changes in Central Asia. Accurate and reliable dating of loess is very important for interpreting and correlating environmental records. There remains a disparity between luminescence ages and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia. In this study, we establish a closely spaced quartz optically stimulated luminescence (OSL) chronology for the 20.5-m-thick Nilka loess section in the Ili Basin. Based on OSL ages, two intervals of higher mass accumulation rate occurred at 49–43 ka and 24–14 ka. We further compare these OSL ages with 23 accelerator mass spectrometry (AMS) ¹⁴C ages of bulk organic matter. The results indicate that the OSL and radiocarbon ages agree well for ages younger than ca. 25 ¹⁴C cal ka BP. However, beyond 30 cal ka BP, there is no consistent increase in AMS ¹⁴C age with depth, while the OSL ages continue to increase. These differences confirm the observation that the AMS ¹⁴C ages obtained using conventional acid–base–acid (ABA) pretreatment are severely underestimated in other terrestrial deposits in Central Asia, which could be due to 2–4% modern carbon contamination. However, OSL dating is applicable for constructing an accurate chronology beyond 30 cal ka BP. We suggest caution when interpreting paleoenvironmental changes based on radiocarbon ages older than 25 cal ka BP.     

Applying a Bayesian approach for refining the chronostratigraphy of the Yumidong site in the Three Gorges region,central China

Yumidong (Corn Cave) is a newly discovered Paleolithic site in the Three Gorges region of central China. Numerous Paleolithic artifacts have been excavated from the sedimentary deposits of the cave in association with faunal remains attributed to the Middle-Late Pleistocene Ailuropoda-Stegodon fauna of southern China. To establish the chronology of the sedimentary sequence (>5 m thick), ¹⁴C dating was applied to bone and charcoal samples (n = 6); the U-series method was used to date in situ precipitated speleothems (n = 12), transported speleothem samples (n = 6) and 18 subsamples of a fossil tooth; and the coupled ESR/U-series method was used to date fossil teeth (n = 6). The derived dates were combined using a hierarchical Bayesian approach to generate a unified chronostratigraphy for the Yumidong sequence. In our Bayesian analyses, the ¹⁴C and coupled ESR/U-series dates were considered to provide direct age estimates for the target layers, while the U-series dates of the in situ precipitated speleothems and fossil tooth were used as minimum age constraints and those of the transported speleothem fragments as maximum age constraints. The Bayesian analyses provided robust time intervals for the archeological layers: L2-Upper (14–23 ka), L2-Lower (27–63 ka), L3 (106–171 ka), L4 (140–192 ka), L10 (157–229 ka), L11 (181–256 ka), and L12 (214–274 ka) with a probability of 95%, allowing the establishment of a ∼300 ka long geological and archeological history for the Yumidong site and placing it as a reference site for Paleolithic cultural evolution in the Three Gorges region from the late Middle Pleistocene to Late Pleistocene.     

Characterization of organic matter in marine sediments to estimate age offset of bulk radiocarbon dating

Radiocarbon dating of Arctic marine sediment is often challenging due to the low availability of calcareous fossils. Consequently, bulk organic matter dating has at times been used to establish sediment core chronologies. Yet, radiocarbon dates based on bulk organic matter often appear to deviate vastly from dates based on fossils, mainly caused by input of allochthounous carbon, including terrigenous organic matter. In this study, we aim to examine the link between the composition of the bulk organic matter and the age offsets between the bulk radiocarbon dates and those obtained from calcareous foraminiferal tests. All samples are taken from the marine sediment core AMD14-204C from offshore Upernavik (eastern Baffin Bay). The radiocarbon dates for bulk organic matter are on average ∼3000 years older than the radiocarbon dates based on foraminifera, but with changing age offsets throughout the record. To investigate the cause of this age offset and its variations over time, we applied core scanning, X-ray Fluorescence analysis, stable isotopes, organic pyrolysis and microscopic organic petrology to examine the distribution and characterization of the organic matter. The results show that the older organic matter includes clastic input of reworked sedimentary rocks potentially originating from West Greenland and/or the Canadian Arctic Archipelago. Changes in the input of contemporary marine algal produced organic matter versus both terrigenous input and reworked ancient organic matter appear to control the age offsets between the bulk and foraminifera dates. A low Hydrogen Index and low δ¹³C_org values together with a high Oxygen Index, indicative of high influence of terrigenous organic matter, seem to correspond to samples with the largest age offsets; 1000–2000 years greater than in other samples. To examine the cause of the variations in the age offsets, a new quantification of the autochthonous organic matter as a fraction of the TOC was calculated. This shows that samples with the largest age offsets contained the lowest fraction (as low as ∼12%) of autochthonous organic matter in the TOC.     

Using a transient infiltrometric technique for intensively sampling field‐saturated hydraulic conductivity of a clay soil in two runoff plots

The point measurement of soil properties allows to explain and simulate plot scale hydrological processes. An intensive sampling was carried out at the surface of an unsaturated clay soil to measure, on two adjacent plots of 4 × 11 m² and two different dates (May 2007 and February–March 2008), dry soil bulk density, ρ_b, and antecedent soil water content, θ_i, at 88 points. Field‐saturated soil hydraulic conductivity, K_fs, was also measured at 176 points by the transient Simplified Falling Head technique to determine the soil water permeability characteristics at the beginning of a possible rainfall event yielding measurable runoff. The ρ_b values did not differ significantly between the two dates, but wetter soil conditions (by 31%) and lower conductivities (1.95 times) were detected on the second date as compared with the first one. Significantly higher (by a factor of 1.8) K_fs values were obtained with the 0.30‐m‐diameter ring compared with the 0.15‐m‐diameter ring. A high K_fs (> 100 mm h^?1) was generally obtained for low θ_i values (< 0.3 m³m^?3), whereas a high θ_i yielded an increased percentage of low K_fs data (1–100 mm h^?1). The median of K_fs for each plot/sampling date combination was not lower than 600 mm h^?1, and rainfall intensities rarely exceeded 100 mm h^?1 at the site. The occurrence of runoff at the base of the plot needs a substantial reduction of the surface soil permeability characteristics during the event, probably promoted by a higher water content than the one of this investigation (saturation degree = 0.44–0.62) and some soil compaction due to rainfall impact. An intensive soil sampling reduces the risk of an erroneous interpretation of hydrological processes. In an unstable clay soil, changes in K_fs during the event seem to have a noticeable effect on runoff generation, and they should be considered for modeling hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Mercury cycling during acid rain recovery at the forested Lesní potok catchment,Czech Republic

From 2011 to 2019, mercury (Hg) stores and fluxes were studied in the small forested catchment Lesní potok (LES) in the central Czech Republic using the watershed mass balance approach together with internal measurements. Mean input fluxes of Hg via open bulk deposition, beech throughfall and spruce throughfall during the periodwere 2.9, 3.9 and 7.6 μg m⁻² year⁻¹, respectively. These values were considerably lower than corresponding deposition Hg fluxes reported in the early years of the 21st century from catchments in Germany. Current bulk precipitation inputs at unimpacted Czech mountainous sites were lower than those in Germany. The largest Hg inputs to the catchment were via litterfall, averaging 22.6 and 17.8 μg m⁻² year⁻¹ for beech and spruce stands. The average Hg input, based on the sum of mean litterfall and throughfall deposition, was 23.0 μg m⁻² year⁻¹, compared to the estimated Hg output in runoff of 0.5 μg m⁻² year⁻¹, which is low compared to other reported values. Thus, only ~2% of Hg input is exported in stream runoff. Stream water Hg was only weakly related to dissolved organic carbon (DOC) but both concentrations were positively correlated with water temperature. The estimated total soil Hg pool averaged 47.5 mg m⁻², only 4% of which was in the O-horizon. Thus Hg in the O-horizon pool represents 72 years of deposition at the current input flux and 3800 years of export at the current runoff flux. Age-dating by ¹⁴C suggested that organic soil contains Hg from recent deposition, while mineral soil at 40–80 cm depth contained 4400-year old carbon, suggesting the soil had accumulated atmospheric Hg inputs through millennia to reach the highest soil Hg pool of the soil profile. These findings suggest that industrial era intensification of the Hg cycle is superimposed on a slower-paced Hg cycle during most of the Holocene.