Chronology building using objective identification of annual signals in trace element profiles of stalagmites

Determination of annual lamination provides important additional constraints to radiometric dates on speleothems, both for dating the duration of specific growth intervals and optimizing growth models. In the absence of visible laminae, however, speleothem age models are reliant upon curve fitting through discretely dated points and are therefore inherently more uncertain than annual chronologies from laminae. Given that the impact of seasonality on speleothems is expected to be strong enough to generate an annual pulse in trace element chemistry regardless of whether or not visible or fluorescent growth laminae are visible, we demonstrate the potential for deriving high-resolution stalagmite chronologies from non-laminated samples using annual chemical variations in stalagmites from two Alpine caves (Obir, Austria and Ernesto, NE Italy). Trace element data were obtained by ion microprobe analyses for H, P, Mg, Na, Sr and Ba and the annual signal was sought using spectral and wavelet analysis. An automated chemical peak-counting software tool was developed in MATLAB^©. It counts significant annual peaks using criteria of minimum amplitude in relation to the local standard deviation of signal variation and minimum separation between peaks determined by the thickness of the preceding layers. Verification of the tool using visibly laminated samples suggests the software is a reliable and accurate method of chronology building, with hit ratios greater than 0.93 and less than 0.75% false alarm occurrences. Used in conjunction with other dating methods such as radiocarbon, U–Th and sulphur peak dating, the automated chemical laminae chronology-building approach provides a more meaningful alternative to simple age-depth curve fitting for non-laminated samples.     

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.     

Bomb C time history recorded in two modern stalagmites — importance for soil organic matter dynamics and bomb C distribution over continents

Carbon 14 activity measurements made by Accelerator Mass Spectrometry on two modern stalagmites from the Han-sur-Lesse cave (Belgium) and from the Postojna Cave (Slovenia) permit the construction of ¹⁴C activity (a¹⁴C) time series over the last 50 years. A high precision chronology is given by annual laminae in the first stalagmite and by a specific mark (explosion in the Postojna Cave in 1944) in the second one. In both stalagmites, ¹⁴C activity increase due to nuclear tests in the atmosphere is remarkable. However, instead of a sharp peak like the one observed in the atmosphere around 1963–1964, the ¹⁴C activities of the stalagmite CaCO₃ show an abrupt increase, with an offset of 1–10 years, followed by a high activity plateau for the Han-sur-Lesse sample and a slight decrease for the Postojna sample. For both stalagmites, the variation of the a¹⁴C amplitude between pre- and post-bomb period is much lower than the atmospheric record, which demonstrates the damping effect of the soil carbon reservoir. We have modeled the CaCO₃ activities using fractionation processes between atmosphere CO₂, soil CO₂ and organic matter (OM), dissolved inorganic carbon and stalagmite CaCO₃. In both cases studied, the model and former soil studies suggest that CO₂ from soil organic matter (SOM) decomposition, which has a slow turnover (i.e. >1 y), is of major importance in winter, when the development of speleothem is the most important. Combined with the fact that 80–90% of the stalagmite carbon comes from soil CO₂, this produces a damping effect on the speleothem a¹⁴C. Consequently, the ‘geochemical time resolution', at least for speleothem carbon, is much lower than the structural resolution given by annual laminae alternations and is mainly controlled by soil carbon dynamics: a¹⁴C and δ¹³C are smoothed over several years. Differences between the ¹⁴C time series of the Han-sur-Lesse and Postojna stalagmites are likely to be due to the double amount of precipitation in Postojna, which produces a faster soil OM turnover and thus a ‘system' which is more sensitive to atmospheric changes.     

A method to anchor floating chronologies in annually laminated speleothems with U–Th dates

Stalagmites occasionally present laminae which, when demonstrated to be annual, may be used to construct an annually resolved chronology. Such annually laminated records provide an opportunity to improve the precision of age models based on other dating techniques. Since annually laminated stalagmites sometimes present a complex stratigraphy with lateral variations in lamina thickness associated with changing macroscopic growth shapes, a procedure for lamina counting is developed here, which complements other methods of speleothem lamina counting. Regardless of the complexity of laminae, when the exact date of a laminated section is unknown, lamina counting provides a floating chronology. This paper describes a method to anchor floating chronologies in speleothems using the least-squares fit of the lamina counting to the radiometric dates (typically U–Th). The estimation of uncertainties in the age model is also considered, which accounts for uncertainties in the lamina counting as well as the fit of the lamina count to the radiometric dates. The uncertainty of this fit does not depend on the analytical uncertainty of the radiometric dates or the precision in the lamina counting, and simply considers all the available dates and their distance to the proposed age model. As an example, the method was applied to a stalagmite from Northern Spain and its accuracy was compared with the annually resolved and cross-dated Greenland chronology during the 8.2 ka event. Although this method has been described for stalagmites, it could be applied to other records in which cross-dating techniques would not be suitable.     

洞穴石笋的^14C年代学研究——石花洞研究系列之二

由于受“死碳”的影响,年轻洞穴碳酸盐的＾１４Ｃ测年始终受到限制,为此,建立了＾１４Ｃ法测定石笋生长速率及校正“死碳”影响的理论模式,从而将石笋的测年时限和精度大大提高。这一理论成功地运用于北京石花洞年轻石笋的定年。利用＾１４Ｃ法与石笋生长光性年际纹层推算的平均生长速率十分吻合,表明石笋Ｓ３１２的生长速率为０．０４２ｍｍ／ａ。＾２１０Ｐｂ法测定石笋顶部的年龄小于１００ａ,与＾１４Ｃ结果的外推年龄基本     

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.     

Age models for long lacustrine sediment records using multiple dating approaches – An example from Lake Bosumtwi,Ghana

The continuous 300-m long drill cores obtained from Lake Bosumtwi, Ghana represent one of the longest, continuous lacustrine sequences obtained from an extant lake, and contain an unprecedented record of late Quaternary climate change in West Africa. However, one of the main challenges associated with generating long paleoclimate time series from terrestrial records such as this is the development of accurate age-depth relationships because unlike marine records, lacustrine sequences cannot be tuned to global ice volume records via δ¹⁸O stratigraphy. The Lake Bosumtwi record thus offers an excellent case study for examining the potential and the challenges associated with different geochronological techniques in lacustrine systems. In the present study, we use a combination of radiocarbon, optically stimulated luminescence and U-series dating and paleomagnetic excursions to generate a chronology for the upper ca. 150 ka of sedimentation at Lake Bosumtwi and employ a Bayesian approach to generate a continuous age-depth relationship. The resultant chronology is then used to test the effectiveness of tuning of an environmental magnetic proxy for dust against a well-dated record of high latitude dust. Our approach highlights the advantages of using multiple dating approaches, and the dangers of relying on too few age constraints when dating long sedimentary sequences. However, the excellent agreement between the different approaches over most of the record suggest that well-constrained age-depth models for long sedimentary sequences can be produced using this combination of approaches. Furthermore, our data provide support for extending the chronology beyond the limit of radiocarbon, U-series and OSL in the future using paleomagnetic excursions/reversals and tuning against well-dated high latitude paleoclimate records.     

Optimizing radiocarbon chronologies in peat profiles with examples from Xinjiang,China

Reliable chronological frameworks are crucial to paleoenvironmental studies, and high precision ¹⁴C dating is the foundation, but many factors, such as dating materials, surficial deposition (influenced by nuclear bomb), and the ¹⁴C age plateau, will affect the reliability of the ¹⁴C ages and chronology frameworks. In this paper, we present 87 ¹⁴C dates of different peat fractions from three peat sites in Xinjiang, China. Plant macrofossils, rootlets, the fine fraction of <90 μm, the mid-size fraction of 90–250 μm and the coarse fraction of >250 μm from selected peat samples were measured to investigate the alternative suitable fraction for dating except for plant macrofossils. We discovered that the 90–250 μm component of peat can provide alternative and reliable results in case of plant macrofossils are not available. Additionally, more dating samples from surficial peat deposition were collected, and accurate surface chronological control points were produced by comparing ¹⁴C results of plant macrofossils with atmospheric ¹⁴C bomb data. Furthermore, multiple data sets with wiggle matching were used along the radiocarbon age plateau to minimize calibrated errors when dates on the ¹⁴C age plateau were shown. Finally, radiocarbon chronology frameworks in peat profiles were optimized. In conclusion, we not only focus on the reliable dating materials, but also highlight that the importance of surficial deposition (after 1950AD) and the anomalous ¹⁴C dates when establishing the dating framework in peat profiles. Furthermore, we propose that the obtaining chronological control points of surficial peat is an important part of the establishing and improving of peat chronological framework in future research.     

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.     

Variability of natural and bomb-produced radionuclide distributions in abyssal red clay sediments

The nature of sedimentation and mixing are examined in abyssal red clay sediments from the North Central Pacific using three types of indicators: ²³⁰Th/²³²Th, organic¹⁴C, and¹³⁷Cs and ^239.240Pu.²³⁰Th/²³²Th analysed revealed that the clay sedimentation rate in three box cores collected within a 50 km radius was less than 1.0 mm/10³ yr. However, analyses of the organic carbon in thin layers of sediment revealed that radiocarbon was present much deeper in the cores (down to 20 cm) than was expected from the ²³⁰Th/²³²Th distribution. In addition, both the stratigraphy and inventory of radiocarbon was significantly different between box cores. The distributions and inventories of¹³⁷Cs and^239.240Pu were similar to that found for radiocarbon, further illustrating the spatial variability of radionuclides in oligotrophic North Pacific red clays. These data suggest that bioturbational processes are important for transporting organic carbon down into the sediment column.     

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.     

Variability in luminescent lamination and initial 230Th/232Th activity ratios in a late Holocene stalagmite from northern Norway

Luminescent lamination in a stalagmite from northern Norway is used to construct a ～2780-year long, floating record of annual growth rate. Thermal ionisation mass-spectrometric (TIMS) U–Th ages (n = 12) were determined along the growth axis and three subsample locations and ages (corrected and uncorrected for initial ²³⁰Th/²³²Th activity) were selected as anchor points for the floating chronology. On the basis of these anchor points, termination of growth occurred between AD 1729 and AD 1826. The annual banding records are used to evaluate the initial ²³⁰Th/²³²Th activity ratio adopted for correction of the U–Th ages. To achieve a reasonable fit between U–Th ages and estimates predicted by the anchored annual band age models, mean initial ²³⁰Th/²³²Th activity ratios of between 0.44 and 1.47 must be invoked. However, there remains a reasonable degree of scatter about the expected linear relationship between annual bands and U–Th chronology for individual subsamples indicating that the use of a single correction factor for Holocene stalagmites should be applied with caution.Stalagmite growth rate fluctuates on annual to centennial scale. The growth termination of the stalagmite presented here could have been a result of environmental change associated with the Little Ice Age, or, possibly local percolation pathway changes after an M_s ～ 6 earthquake in the region in AD 1819. Stable-isotope data from the same axis of growth show a pattern similar to the large-scale growth rate variations, and these combined proxy records are interpreted as showing gradual cooling and/or shortening of the vegetation growth season for the last 3000 years.     

Temporal and spatial variations in the discharge and dissolved organic carbon of drip waters in Beijing Shihua Cave,China

To detect the causal relationship between cave drip waters and stalagmite laminae, which have been used as a climate change proxy, three drip sites in Beijing Shihua Cave were monitored for discharge and dissolved organic carbon (DOC). Drip discharges and DOC were determined at 0 to 14‐day intervals over the period 2004–2006. Drip discharges show two types of response to surface precipitation variations: (1) a rapid response; and (2) a time‐lagged response. Intra‐annual variability in drip discharge is significantly higher than inter‐annual variability. The content of DOC in all drip waters varies inter‐ and intra‐annually and has good correlation with drip water discharge at the rapid response sites. High DOC was observed in July and August in the three years observed. The flushing of soil organic matter is dependent upon the intensity of rain events. The DOC content of drip water increases sharply above a threshold rainfall intensity (>50 mm d^?1) and shows several pulses corresponding with intense rain events (>25 mm d^?1). The DOC content was lower and less variable during the dry period than during the rainy period. The shape of DOC peak also varies from year to year as it is influenced by the intensity and frequency of rainfall. The different drip sites show marked differences in DOC response, which are dominated by hydrological behaviour linked to the recharge of the soil and karst micro‐fissure/porosity network. The results explain why not all stalagmite laminae are consistent with climate changes and suggest that the structure of the rainy season events could be preserved in speleothems. Copyright © 2008 John Wiley & Sons, Ltd.     

A possible Younger Dryas-type event during Asian monsoonal Termination 3

1 Introduction The Younger Dryas (YD) cold spell is one of the most prominent abrupt climate change events in the past and a key feature of the last glacial termination. A great agreement has been attained in terms of the tim- ing and geographic extent of…     

Towards radiocarbon calibration beyond 28 ka using speleothems from the Bahamas

We present a new speleothem record of atmospheric Δ¹⁴C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS ¹⁴C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ¹⁴C was calculated by correcting ¹⁴C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite ¹⁴C ages with the IntCal04 (Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ¹⁴C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology (Svensson et al., 2008). These data are however in conflict with a previously published ¹⁴C data set for a stalagmite record from the Bahamas — GB-89-24-1 (Beck et al., 2001), which likely suffered from ¹⁴C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ?¹⁴C data do not show the extreme shifts between 44 and 40 ka reported in the previous study (Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ¹⁴C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ?¹⁴C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.     

Transferring radiometric dating of the last interglacial sea level high stand to marine and ice core records

In order to derive a radiometric age marker for the end of the penultimate glacial–interglacial transition, we compiled published U-series isotope measurements on corals from the period extending from stage 6 to the middle of the last interglacial, and computed the corresponding open-system ages using Thompson et al. model (Thompson, W.G., Spiegelman, M.W., Goldstein, S.L., Speed, R.C., An open-system model for U-series age determinations of fossil corals. Earth Planet. Sci. Lett. 210 (2003) 365–381). We obtain a global mean age of 126 calendar kyr BP (ka) ± 1.7kyr (2σ) for the beginning of the last interglacial sea level high stand. After showing that the phase relationships observed between changes in sea level, North Atlantic benthic and planktonic foraminifera oxygen isotopic records, and atmospheric methane over the last deglaciation were likely also valid over the penultimate deglaciation, we derive an age of 131.2ka ± 2kyr (2σ) for the abrupt increase in atmospheric CH₄ and North Atlantic surface temperature marking the end of the penultimate glacial–interglacial transition. This age is consistent with U–Th dates of the penultimate glacial–interglacial transition recorded in speleothems from sites where speleothems isotopic records are synchronous with North Atlantic temperature records over the last deglaciation. Finally, we show that the phase obtained between the climatic response and northern hemisphere summer insolation is not constant from Termination II to Termination I, implying that northern hemisphere summer insolation alone cannot explain the timing of terminations.     

Synchrotron radiation applications to past volcanism archived in speleothems: An overview

Precise dating and correlation of past key volcanic eruptions over a wide geographic area in archives of past climate variability is necessary to support a direct causality between volcanism and climate changes. Research has mostly focused on ice cores and varved sediments, which capture a record of volcanic eruptions in geochemistry and the presence of tephra and criptotephra. Precisely dated cave carbonate deposits, collectively known as speleothems are other valuable palaeoclimate archives, and encode information on past volcanism in their sulphate concentration variability. Due to the physical characteristic of speleothems, detection of sulphate concentration variability requires techniques capable of high spatial resolution, very low limit of detection (ppm to ppb) and low background noise. Synchrotron radiation-based (SR) micro X-ray fluorescence (µXRF) and X-ray absorption near-edge spectrometry prove to be one of the most effective techniques to detect short-lived pulses of sulphate concentration increase, which may be interpreted as being related to atmospheric load due to volcanic eruptions. Here, we provide an overview of existing work as well as a novel interpretation of a SR µXRF-based sulphate series in an annually laminated stalagmite with robust chronology. Sulphate concentration peaks in the years 1815–1816, 1844 and 1947, possibly coinciding with Tambora, Krakatau, and Hekla eruptions. It is concluded that sulphate concentration in speleothems expand the potential to correlate volcanic eruption events at a global scale.     

An automated method for varve interpolation and its application to the Late Glacial chronology from Lake Suigetsu,Japan

The Lake Suigetsu sediment has been recognised for its potential to create a wholly terrestrial (i.e. non-reservoir-corrected) ¹⁴C calibration dataset, as it exhibits annual laminations (varves) for much of its depth and is rich in terrestrial leaf fossils, providing a record of atmospheric radiocarbon. Microscopic analysis revealed that the varve record is curtailed due to the incomplete formation or preservation of annual laminae, necessitating interpolation. The program for varve interpolation presented here analyses the seasonal layer distribution and automatically derives a sedimentation rate estimate, which is the basis for interpolation, and applies it to complement the original varve count. As the interpolation is automated it largely avoids subjectivity, which manual interpolation approaches often suffer from. Application to the Late Glacial chronology from Lake Suigetsu demonstrates the implementation and the limits of the method. To evaluate the reliability of the technique, the interpolation result is compared with the ¹⁴C chronology from Lake Suigetsu, calibrated with the tree-ring derived section of the IntCal09 calibration curve. The comparison shows that the accuracy of the interpolation result is well within the 68.2% probability range of the calibrated ¹⁴C dates and that it is therefore suitable for calibration beyond the present tree-ring limit.     

StalAge – An algorithm designed for construction of speleothem age models

Here we present a new algorithm (StalAge), which is designed to construct speleothem age models. The algorithm uses U-series ages and their corresponding age uncertainty for modelling and also includes stratigraphic information in order to further constrain and improve the age model. StalAge is applicable to problematic datasets that include outliers, age inversions, hiatuses and large changes in growth rate. Manual selection of potentially inaccurate ages prior to application is not required. StalAge can be applied by the general, non-expert user and has no adjustable free parameters. This offers the highest degree of reproducibility and comparability of speleothem records from different studies. StalAge consists of three major steps. Firstly, major outliers are identified. Secondly, age data are screened for minor outliers and age inversions, and the uncertainty of potential outliers is increased using an iterative procedure. Finally, the age model and corresponding 95%-confidence limits are calculated by a Monte-Carlo simulation fitting ensembles of straight lines to sub-sets of the age data.We apply StalAge to a synthetic stalagmite ’sample’ including several problematic features in order to test its performance and robustness. The true age is mostly within the 95%-confidence age limits of StalAge showing that the calculated age models are accurate even for very difficult samples. We also apply StalAge to three published speleothem datasets. One of those is annually laminated, and the lamina counting chronology agrees with the age model calculated by StalAge. For the other two speleothems the resulting age models are similar to the published age models, which are both based on smoothing splines. Calculated uncertainties are in the range of those calculated by combined application of Bayesian chronological ordering and a spline, showing that StalAge is efficient in using stratigraphic information in order to reduce age model uncertainty.The algorithm is written in the open source statistical software R and available from the authors or as an electronic supplement of this paper.     

Advances and limitations in establishing a contiguous high-resolution atmospheric radiocarbon record derived from subfossil kauri tree rings for the interval 60–27 cal kyr BP

Radiocarbon dating is the most widely applied and reliable dating technique for providing chronological control during Marine Isotope Stage 3 (MIS3; ∼60–27 cal kyr BP). Past variations in the atmospheric concentration of radiocarbon mean a calibration curve is required. IntCal20 and SHCal20 calibration curves covering MIS3 are presently largely based on non-atmospheric records which, in combination with larger radiocarbon (¹⁴C) dating uncertainties, results in significant smoothing and reduced resolution in calibration curve structure. Floating tree ring radiocarbon chronologies that are wiggle-matched to other palaeo records (particularly to Hulu Cave speleothems) have the potential to provide detailed structure to the MIS3 portion of extant calibration curves. New Zealand subfossil kauri (Agathis australis) trees are long-lived and are useful for constructing temporally-floating MIS3 atmospheric radiocarbon datasets. This paper presents extant and emerging data from several important Northland subfossil kauri locations (Omaha, Babylon Coast, Bream Bay, Kai Iwi Lakes, Mangawhai). We show the span of seven floating MIS3 kauri sequences (individual trees and chronologies) from which sequential radiocarbon series covering a total of 7556 years is now in development (representing 23% of the period 60–27 cal kyr BP). We also report radiocarbon dates for an additional 34 ancient kauri from MIS3 that can provide additional coverage. After these floating subfossil wood sequences have been produced, close to 40% of MIS3 (12,420 years) will be covered by contiguous subfossil kauri radiocarbon measurements. Based on our findings, we discuss the prospects and limitations for obtaining a highly resolved and precise atmospheric radiocarbon calibration curve comprehensively covering MIS3 using subfossil kauri.