Description: Available online 21 December 2012 Publication year: 2012 Source: Quaternary Geochronology

Description: Available online 21 December 2012 Publication year: 2012 Source: Quaternary Geochronology New Ar/Ar data for alkaline basaltic rocks on the West African island of São Tomé document a Pleistocene eruptive episode that saw the formation of cinder cones and lava flows between 0.036±0.004 and 0.86±0.02 Ma (2σ). The cinder cones trend north-northeast along the long axis of the island, interpreted as a rift zone. This rift zone appears to be the source of several lava flows that reach the coastline, including one that underlies the airport of São Tomé. Estimates of magma production rates, both from recent lava flows and from consideration of the total edifice history suggest low magma production rates (10 −4 to 10 −2 m 3 /s), compared to other ocean island settings. Assuming a background of low overall volcanic activity, it is expected that any future eruptions would most likely occur along the north-northeast rift, resulting in cinder cone and lava flow emplacements that may impact the densely populated northeast region of the island. Highlights ► Ar/Ar data for 9 samples are presented, 6 gave meaningful ages. ► Basalts from São Tomé give ages of 0.036±0.004 and 0.86±0.02 Ma (2s). ► Magma production rates suggest low activity (10 -4 to 10 -2 m 3 /s). ► Future volcanic activity may be centered on the long axis of the island. ► Future volcanic activity may generate cinder cones, ash and lava flows.

Description: Available online 19 December 2012 Publication year: 2012 Source: Quaternary Geochronology The continuous 300-meter 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 δ 18 O 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. Highlights ► New age model developed for the uppermost ∼150 ka of sediments from Lake Bosumtwi. ► The age model compares ages from 14 C, OSL, U-series, paleomagnectic excursions. ► Agreement between independent dating methods supports the age-depth model. ► Paleomagnetic excursions and tuning offer opportunities for a longer chronology

Description: Available online 17 December 2012 Publication year: 2012 Source: Quaternary Geochronology A block of sulfide crust collected from an active hydrothermal mound in an Archaean site (12°56.4'N, 143°37.9'E; depth ca. 3,000 m) of the South Mariana Trough was dated using both 230 Th/ 234 U disequilibrium and electron spin resonance (ESR) methods to establish the growth duration. Eight subsamples from the sulfide crust were separated further into magnetic and non-magnetic fractions using a Franz isodynamic separator. Thirteen sulfide samples, soluble in nitric acid, yielded 230 Th/ 234 U ages of 0.3 -2.2 ka . The magnetic fractions had significantly lower Th/U ratios, which enabled age determinations as precise as ± 2% (2σ). The age distribution obtained for the section of sulfide crust analysed is consistent with deposition of sulfide minerals from the upper surface of the crust to the inner side. The 230 Th/ 234 U ages of the sulfide minerals were compared with ESR ages of barites separated from 12 subsamples of the same sulfide crust. ESR ages of 0.27 to 1.3 ka show a spatial pattern broadly resembling that observed in 230 Th/ 234 U dating method. While there are some significant offsets, these results illustrate the potential of the two methods for use in investigation of the evolutional history of a hydrothermal system. Highlights ► U-Th and ESR dating were applied for hydrothermal samples from South Mariana. ► The samples yieled U-Th ages from 250 to 2000 a and broadly consistent with ESR ages. ► The potential of the two methods in dating hydrothermal deposits has been revealed. ► Sulfide deposits can record the history of hydrothermal activity of ► 1 ka.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14 Speleothems, such as stalagmites and flowstones, can be dated with unprecedented precision in the range of the last 650,000 a by the 230 Th/U-method, which is considered as one of their major advantages as climate archives. However, a standard approach for the construction of speleothem age models and the estimation of the corresponding uncertainty has not been established yet. Here we apply five age modelling approaches ( StalAge , OxCal, a finite positive growth rate model and two spline-based models) to a synthetic speleothem growth model and two natural samples. All data sets contain problematic features such as outliers, age inversions, large and abrupt changes in growth rate as well as hiatuses. For data sets constrained by a large number of ages and not including problematic sections, all age models provide similar results. In case of problematic sections, the algorithms provide significantly different age models and uncertainty ranges. StalAge , OxCal and the finite positive growth rate model are, in general, more flexible since they are capable of modelling hiatuses and account for problematic sections by increased uncertainty. The spline-based age models, in contrast, reveal problems in modelling problematic sections. Application to the synthetic data set allows testing the performance of the algorithms because the ‘true’ age model is available and can be compared with the age models. OxCal and StalAge generally show a good performance for this example, even if they are inaccurate for a short section in the area of a hiatus. The two spline-based models and the finite positive growth rate model show larger inaccurately modelled sections. Highlights ► Five different age models are applied to different speleothem data sets. ► For well constrained and unproblematic sections, all age models are similar. ► For problematic sections, the age models and uncertainties are very different. ► StalAge, OxCal and the finite positive growth rate model are more flexible than splines.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14 Over the past decade, a number of speleothem studies have used radiocarbon ( 14 C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse 14 C to anchor chronologies over the last 60 years, the combination of U-Th and 14 C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric 14 C 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 14 C, 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 δ 18 O, δ 13 C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with 14 C 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 CO 2 , and ultimately led to a greater contribution of dead carbon from the bedrock. Highlights ► We examine the dead carbon fraction (DCF) of a late Holocene tropical speleothem. ► We find a strong association between the DCF and above-cave hydrology. ► More dead carbon delivered to the speleothem during a stronger monsoon and vice versa . ► Soil-karst model reveals that dissolution of host-rock governs DCF variability. ► higher (lower) DCF corresponds with more closed (open)-system conditions.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14 Speleothem proxy records, coupled with U–Th chronologies, are widely used in studies of palaeoclimate, landscape and faunal evolution. The prospect of extending such studies beyond ∼500 ka has fuelled considerable interest in the U–Pb dating method. While theoretical mechanisms for the transport and incorporation of both U and Pb in speleothem calcite remains a matter of debate, it is clear that natural samples span a broad spectrum of compositional variation, and likely contain Pb components with multiple origins and/or transport pathways. Based upon these observations, optimal methods for sample selection are proposed and a variety of techniques for age estimation are explored. Laser ablation ICPMS and autoradiography are ideal rapid screening methods for reconnaissance studies. Common Pb isotopic variability is a key parameter in the choice of analytical strategy for age determination. In large U–Pb studies where invariant common Pb can be established, common Pb corrected ages can be produced for individual sample aliquots promoting rapid throughput and permitting the establishment of detailed age-depth models. Even with these technologies, however, age resolution becomes an issue beyond a few million years, at which time alternative strategies for developing internal speleothem chronologies, such as lamina counting, must be considered in unison with U–Pb dating. Highlights ► Natural variations in U and Pb content in speleothems. ► Optimal strategies for sample screening. ► Alternative methods for isochron construction. ► Limitations on age models through time.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14 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 230 Th relative even to the very low levels of detrital 230 Th 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 14 C by accelerator mass spectrometry (AMS). The AMS results indicate that bomb 14 C 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 14 C dates for that period determined by high-resolution δ 18 O recorded in the speleothem, and the timing of the onset of bomb 14 C. 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. Highlights ► We had problems in dating two young speleothems using the Th/U method. ▶ We built 14 C-based chronologies for these spels using different age-depth models. ▶ Excellent agreement between the outputs of these models was observed. ▶ Reliable 14 C-based chronologies for these speleothems were achieved. ▶ Radiocarbon can be used to build robust chronologies for young spels.

Description: December 2012 Publication year: 2012 Source: Quaternary Geochronology, Volume 14 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. Highlights ► A method to measure laminas in stalagmites with a complex stratigraphy is proposed. ► We propose how to anchor floating chronologies in speleothems using U–Th dates. ► The uncertainties for the lamina counting and its anchor to the dates are estimated. ► The method was applied to a stalagmite that grew during the 8.2 ka event in Spain. ► This method may be applied to floating chronologies of other records but stalagmites.