Constraints on the nature of the subvolcanic reservoir at South Sister volcano,Oregon from U-series dating combined with sub-crystal trace-element analysis of plagioclase and zircon

We present sub-crystal-scale ²³⁸U–²³⁰Th zircon ages and ²³⁸U–²³⁰Th–²²⁶Ra plagioclase ages of bulk mineral separates from the Holocene (2.0–2.3 ka) eruptions of the Rock Mesa (RM) and Devil's Hills (DH) rhyolites at South Sister volcano, Oregon. We link these age data with sub-crystal trace-element analyses of zircon and plagioclase to provide insight into the subvolcanic system at South Sister, as an example of a small-volume continental arc volcano. Our results document the presence of coeval yet physically-distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed. Zircons from the RM and DH dominantly record ages from 20 to 80 ka, with some grains recording ages > 350 ka, whereas plagioclase records ²³⁰Th–²²⁶Ra ages of 2.3–6.8 ka (RM) and 4.0–9.6 ka (DH-3) and a ²³⁸U–²³⁰Th age of 10 ± 34 ka (DH-3). We interpret zircons with ages < 350 ka as antecrysts inherited from a longer lived upper-crustal magma reservoir from which the rhyolites were generated, based on crystallization ages coeval with earlier periods of silicic volcanism at South Sister, the undersaturated nature of the RM and DH magmas with respect to zircon, and Ti-in-zircon temperatures consistent with low-temperature (< 815 °C) crystallization. In contrast, plagioclase ages are near the eruption age and dominantly preserve information about the recent (< 10 ka), higher-temperature evolution of the host magmas. Although zircon and plagioclase record different crystallization ages, each phase crystallized over the same time period in the RM compared to DH rhyolites. Linking these crystal age data with sub-crystal trace-element analyses demonstrates that zircon and plagioclase have distinct trace-element characteristics between eruptions, which require that the RM and DH crystals (and therefore magmas) were derived from distinct regions that had evolved independently for > 50 ka within a heterogeneous magmatic system and coexisted as physically-distinct, dominantly-liquid bodies prior to eruption. Thus, we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous, largely crystalline reservoir. Similarities in crystal age and chemical data to that at other young silicic systems (e.g., Mount St. Helens, Okataina Caldera Complex) suggest that this model may be more generally applicable to silicic magmas.     

Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using 238U–230Th disequilibria and Os isotopes

A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (²³⁰Th/²³⁸U) > 1 and span a large range in (²³⁰Th/²³²Th) ratios, and most basalts have higher (²³⁰Th/²³²Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate ²³⁰Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by ¹⁸⁷Os/¹⁸⁸Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large ²³⁰Th excesses (25–48%) and high ¹⁸⁷Os/¹⁸⁸Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization.     

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.     

High-precision U-series measurements of more than 500,000 year old fossil corals

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ～ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the ²³⁸U–²³⁴U–²³⁰Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of ²³⁰Th and ²³⁴U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.     

Assessing the use of U–Th methods to determine the age of cold-water calcareous algae

Calcitic crusts of calcareous red algae could be suitable material for age determination of raised marine deposits and palaeothermometry at annual to sub-annual resolution. We examined the potential of U–Th dating of cold-water calcareous algae by analysing fossil specimens (n=10) from Kapp Ekholm (Svalbard) and recent specimens from Norway (n=3) and Scotland (n=1). After initial measurements using α-spectrometry, thermal ionisation mass spectrometry (TIMS) was used to study the material in more detail. Recent specimens contain 0.19–1.55 μg g⁻¹ U, and the measured (²³⁴U/²³⁸U) activity ratios vary between 1.12 and 1.30. Fossil specimens contain 1–168 μg g⁻¹ U, and display variable and highly elevated initial (²³⁴U/²³⁸U) activity ratios. In general, the TIMS results show increasing (²³⁴U/²³⁸U) activity ratios and concentrations of U and Th with increasing stratigraphic age. From this it is evident that marine calcareous red algae contain U in-vivo and they experience substantial post-mortem uptake of U. We conclude that direct U–Th dating of fossil calcareous algae from raised marine deposits is not viable without further geochemical understanding of the in-vivo uptake and post-depositional pathways of U and Th in such deposits. Despite the convincing open-system behaviour of the material, comparison with previously published chronostratigraphy from the site shows that the calcareous algae generally yield ages that are too old. This is in contrast to the expected result based on simple continuous post-mortem U accumulation and calls for a complex model comprising migration of U and multi-component addition of Th (detrital/colloidal) to explain the observed trends.     

U–Pb dating of speleothems from Spannagel Cave,Austrian Alps: A high resolution comparison with U–series ages

U–Pb dating is increasingly used to date speleothems that are too old for precise U–Th disequilibrium dating; however there is little data that can independently validate its application to such material. This study presents U–Pb ages for speleothems from the Spannagel Cave in the Austrian Alps including a detailed comparison with U–Th ages from an unusually U–rich sample that yields precise ages by both methods. Sample SPA4 is a flowstone with three growth phases separated by distinct hiatuses. For the youngest growth phase the U–Pb and U–Th ages are 267 ± 1 ka and 267 ± 5 ka respectively; the middle growth phase is 291 ± 1 versus 295 ± 11 ka while for the oldest growth phase a single sub-sample, assuming the same initial Pb composition as for the younger phases, yields an age of 340 ± 2 ka compared to 353 ± 9 ka by U–Th. Correlation of these ages with the marine isotope stages confirms that these speleothems grew during glacial stages as suggested by previous work on the same sample. Sample SPA 15 has U–Th isotopic compositions indistinguishable from secular equilibrium; the U–Pb data on the main growth phase of this sample give an age of 551 ± 10 ka, whereas a single analysis from the oldest phase suggests it may be on the order of 40 ka older. This detailed comparison of U–Pb and U–Th ages provides important support for the potential validity of the U–Pb method in older samples beyond the range of U–Th.     

Thermal history and origin of the Tanzanian Craton from Pb isotope thermochronology of feldspars from lower crustal xenoliths

Common and radiogenic Pb isotopic compositions of plagioclase and anti-perthitic feldspars from granulite-facies lower crustal xenoliths from the Labait Volcano on the eastern margin of the Tanzanian Craton have been measured via laser ablation MC-ICP-MS. Common Pb in plagioclase and a single stage Pb evolution model indicate that the lower crust of the Tanzanian Craton was extracted from mantle having a ²³⁸U/²⁰⁴Pb of 8.1 ± 0.3 and a ²³²Th/²³⁸U of 4.3 ± 0.1 at 2.71 ± 0.09 Ga (all uncertainties are 2σ). Since 2.4 Ga, some orthoclase domains within anti-perthites have evolved with a maximum ²³⁸U/²⁰⁴Pb of 6 and ²³²Th/²³⁸U of 4.3. The spread in Pb isotopic composition in the anti-perthitic feldspars yields single crystal Pb–Pb isochrons of ～ 2.4 Ga, within uncertainty of U–Pb zircon ages from the same sample suite. The Pb isotopic heterogeneities imply that these granulites resided at temperatures < 600 °C in the lower crust of the Tanzanian Craton from ca. 2.4 Ga to the present. In concert with the chemistry of surface samples, mantle xenoliths, and lower crustal xenoliths, our data imply that the cratonic lithosphere in Tanzania formed ca. ～ 2.7 Ga, in a convergent margin setting, and has remained undisturbed since 2.7 Ga.     

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.     

Partitioning of U and Th during garnet pyroxenite partial melting: Constraints on the source of alkaline ocean island basalts

Uranium series disequilibria in ocean island basalts (OIB) provide evidence for the presence of garnet in their source region. It has been suggested that enriched OIB signatures derive from mantle lithologies other than peridotite, such as eclogite or pyroxenite, and, in particular, that silica-poor garnet pyroxenite is the source lithology for alkali basalts. To test the ability of such a source to produce the U–Th disequilibria observed in alkali OIB, we determined experimentally clinopyroxene-melt and garnet-melt partition coefficients for a suite of trace elements, including U and Th, at 2.5 GPa and 1420–1450 °C. The starting composition for the experiments was a 21% partial melt of a silica-poor garnet pyroxenite. Experimentally determined clinopyroxene-melt partition coefficients range from 0.0083 ± 0.0006 to 0.020 ± 0.002 for Th and from 0.0094 ± 0.0006 to 0.024 ±0.002 for U, and garnet-melt partition coefficients are 0.0032 ± 0.0004 for Th and 0.013 ± 0.002 for U. Comparison of our experimental results with partition coefficients from previous experimental studies shows that the relative compatibilities of U and Th in both garnet and clinopyroxene are different for different mineral compositions, leading to varying degrees of U/Th fractionation with changing lithology. For a given melting rate and extent of partial melting, mafic lithologies tend to produce larger ²³⁰Th excesses than peridotite. However, this effect is minimized by the greater overall extents of melting experienced by eclogites and pyroxenites relative to peridotite. Results from chromatographic, batch, and fractional melting calculations with binary mixing between partial melts of pyroxenite and peridotite, carried out using our new partitioning data for the pyroxenite component and taking into account variable productivities and different solidus depths for the two lithologies, suggest that OIB are not the product of progressive melting of a source containing a fixed quantity of garnet pyroxenite. Melting a peridotite with enriched signatures, and mixing those melts with melts of a depleted, “normal” peridotite, is an alternative explanation for the trends seen in Hawaiian, Azores and Samoan lavas.     

Uranium loss and aragonite–calcite age discordance in a calcitized aragonite stalagmite

We analyzed uranium-series concentrations and isotopic ratios in a mixed aragonite and calcite stalagmite from Juxtlahuaca Cave, from the Sierra Madre del Sur of Mexico. The U-series data for the aragonite layers return highly precise and stratigraphically correct ages over the past ca. 4300 years. In contrast, age determinations from calcite layers are too old by several hundred years relative to the precise aragonite ages, have analytical uncertainties an order of magnitude larger than aragonite ages, and yield ages that do not overlap the aragonite ages within analytical uncertainties. Based on geochemical and petrographic observations, we interpret the calcite layers to have formed from recrystallization of aragonite soon after primary aragonite deposition. Calcite occurs as discontinuous lenses on and off the growth axis, and laminae can be traced between aragonite and calcite layers, demonstrating that visible growth banding is not effaced in the recrystallization process. Paired aragonite-calcite U-series data from coeval stratigraphic layers demonstrate that uranium concentrations decrease by two orders of magnitude during calcitization, and result in decreased (²³⁴U/²³⁸U). Uranium loss during diagenesis mimics a need for an age correction using an initial ²³⁰Th/²³²Th ratio one to two orders of magnitude larger than the bulk Earth ratio of 4.4 ± 2.2 × 10⁻⁶. A need for apparent high initial ²³⁰Th/²³²Th ratios results from ingrowth of ²³⁰Th during ²³⁴U decay.     

Natural radioactivity assessment of surface sediments in the Yangtze Estuary

The activities of the natural radionuclides (²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K) of the surface sediments in the Yangtze Estuary were determined and used to evaluate radiation hazards in the study area. The of activities of ²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K ranges from 14.1 to 62.3, 26.1 to 71.9, 13.7 to 52.3, and 392 to 898 Bq kg^? 1, respectively, which were comparable to values of other regions in China. The activities of ²³²Th, ⁴⁰K and ²²⁶Ra were clearly different from the global recommended values. The radium equivalent activity was less than the recommended limit of 370 Bq kg^? 1; therefore, the sediment in this area can be safely used for reclamation. The external hazard index values were less than one. The average absorbed gamma dose rate and annual effective dose equivalent values were slightly greater than the world average value. ²²⁶Ra/²³⁸U and ²³²Th/²³⁸U ratios could potentially be applied for tracing sediment source.     

Interpretation of the 231Pa/230Th paleocirculation proxy: New water-column measurements from the southwest Indian Ocean

Measurements of ²³¹Pa, ²³⁰Th and ²³²Th concentrations have been made on five water-column profiles along the western margin of the Madagascar and Mascarene Basins in the southern Indian Ocean. These measurements help to fill a significant gap in the global coverage of water-column ²³²Th, ²³⁰Th and ²³¹Pa data. ²³²Th concentrations vary, but generally increase with depth, suggesting higher particle loading in deeper waters, and the presence of a significant dissolved fraction of ²³²Th. ²³⁰Th concentrations increase with depth, and profiles are similar to the average of existing data from other regions. ²³¹Pa concentrations, on the other hand, show significant depth structure, apparently reflecting the various water masses sampled at this location. The modified remnants of North Atlantic Deep Water are found at a depth of ≈ 2000 m and exhibit elevated ²³¹Pa concentrations exported from the South Atlantic. Antarctic Intermediate and Bottom Waters have lower ²³¹Pa, probably due to scavenging onto opal particles during transit from the Southern Ocean. The differences between water masses raises a question: which water mass is important in controlling the ²³¹Pa/²³⁰Th ratio in underlying sediments? A simple one-dimensional model is used to demonstrate that the ²³⁰Th and ²³¹Pa exported to sea-floor sediments last equilibrates with waters close to the seafloor (within ≈ 1000 m), rather than averaging the whole water column. These findings suggest that ²³¹Pa_xs/²³⁰Th_xs in sediments provides information primarily about deep-water masses. In this region, sedimentary records will therefore provide information about the past flow of Antarctic Bottom Water into the Indian Ocean. Interpretation of data from other regions, such as the North Atlantic where this proxy has most successfully been applied, requires careful consideration of regional oceanography and knowledge of the composition of the water masses being investigated.     

Radium-226 chronology of Holocene travertine from Esanatoglia (Central Italy)

Here we investigate the feasibility of using alpha-spectrometric ²²⁶Ra methodology to date low U freshwater carbonate deposits (<0.1 μg g⁻¹) by analysing Holocene freshwater travertine deposits from Esanatoglia, Umbro-Marchean Apennines, Central Italy. Previously, such methods have been tested on hydrothermal deposits, which are characterised by high Ra and U. We calculate a ²²⁶Ra age estimate of 2.9±0.4 ka (1σ) for Holocene travertine that is in reasonable agreement with archaeological evidence by assuming that the ²²⁶Ra initial activity of the Holocene travertine is comparable with that of present-day carbonate deposits in the same area. We also investigate the use of Ba to normalise ²²⁶Ra activities. Comparison of age estimates based on Ra methods with ages derived from alpha-spectrometric ²³⁰Th/²³⁴U isochron techniques for the same co-eval sub-samples indicates that, where only alpha-spectrometric methods are available, the former are likely to be much more useful for Holocene travertine material, especially if there is a significant detrital component.     

Evaluation of the 87Rb decay constant by age comparison against the U–Pb system

The Rb–Sr decay system is one of the most widely used geochronometers for obtaining ages and cooling rates of terrestrial magmatic, metamorphic, and hydrothermal events. It has also been extensively applied to date extraterrestrial, early solar system events. The accuracy of Rb–Sr ages, however, strongly depends on the accuracy of the ⁸⁷Rb decay constant (λ⁸⁷Rb). We determined λ⁸⁷Rb relative to the decay constants of ²³⁵U and ²³⁸U by comparing Rb–Sr ages of minerals with U–Pb ages obtained from the same intrusion. Comparison of U–Pb emplacement ages with high-precision Rb–Sr mineral ages from three rapidly cooled igneous rocks covering an age range of ca. 2.5 Ga yields an unweighted mean λ⁸⁷Rb of 1.393 ± 0.004 × 10^?11 yr^?1 (i.e., ± 0.3%), corresponding to a half-life of 49.76 × 10⁹ years. Because this decay constant is 2% lower than the presently recommended one, many previously published ages are 2% too young and the resulting geologic interpretations may need revision.     

Validating a Swedish varve chronology using radiocarbon,palaeomagnetic secular variation,lead pollution history and statistical correlation

We use statistical correlation of palaeomagnetic secular variation (PSV) curves from a varved Holocene lake sediment sequence in west central Sweden (Lake Kälksjön) against those of a Fennoscandian master stack (FENNOSTACK) to correct for an apparent error in the varve chronology. Additional correlation between a lead pollution-derived chronology for the last 2000 years corroborates the PSV results. Use of the FENNOSTACK palaeomagnetic master curve reveals no significant difference in duration between large-scale features from ～2500 to ～8000 cal. yrs BP. Statistical correlation, however, implies that 270 years are missing from the younger part (<1000 cal. yrs BP) of the varve chronology, and that there is an overestimation by approximately 230 years in the number of varves counted in the early Holocene (>8000 cal. yrs BP). A similar comparison between the PSV-determined ages and calibrated bulk radiocarbon ages suggests that the sediments of mid-Holocene age contain substantial amounts of old carbon, probably of soil origin, which causes bulk sediment-calibrated mean ¹⁴C ages to be up to 850 years older than the corrected varve chronology, which extends to 9193 ± 186 cal. yrs BP. This study highlights both the use of statistical correlation as a technique for detecting errors between chronologies, and the importance of validating incremental chronologies with more than one independent method.     

U–Th radioactive disequilibrium and ESR dating of a barite-containing sulfide crust from South Mariana Trough

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. 3000 m) of the South Mariana Trough was dated using both ²³⁰Th/²³⁴U 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 ²³⁰Th/²³⁴U 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 analyzed is consistent with deposition of sulfide minerals from the upper surface of the crust to the inner side. The ²³⁰Th/²³⁴U ages of the sulfide minerals were compared with ESR ages of barites separated from 12 subsamples of the same sulfide crust. ESR ages of 0.27–1.3 ka show a spatial pattern broadly resembling that observed in ²³⁰Th/²³⁴U 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.     

Late slowdown of the Atlantic Meridional Overturning Circulation during the Last Glacial Inception: New constraints from sedimentary (231Pa/230Th)

Our study gives new constraints on the response of Atlantic Meridional Overturning Circulation (AMOC) export to various forcings during the Last Glacial Inception. The decay corrected excess sedimentary (²³¹Pa/²³⁰Th) activity ratio (hereafter referred to as (Pa/Th)) has been measured over that period in two deep cores from the Western (SU90-11, 44°04′N, 40°01′W, 3645 m) and Eastern (MD01-2446, 39°03′N, 12°37′W, 3547 m) basins of the North Atlantic. Both records display significant changes despite the relatively short half-life of ²³¹Pa (～ 32 kyr) compared to the period we investigate. The (Pa/Th) variability does not correlate to changes in local opal flux normalized to ²³⁰Th. Moreover, the (Pa/Th) profiles display a high degree of coherency with indirect proxies of AMOC activity such as the benthic foraminifera δ¹³C and the mid-latitude summer Sea Surface Temperature in nearby reference cores. These additional pieces of evidence support our interpretation of the (Pa/Th) as reflecting AMOC export. The (Pa/Th) repeatedly underwent rapid changes during the Last Glacial Inception associated with the extension of ice rafted detritus in the North Atlantic, highlighting the control of ice-sheet dynamics through freshwater forcing on AMOC export. AMOC export remains large during periods of ice-sheet growth and its decreases lag the Northern Hemisphere summer insolation forcing. AMOC modulation appears driven by ice-sheet dynamics, itself driven by the seasonal insolation gradient between low and high Northern Hemisphere latitudes and the associated intensity of the meridional oceanic and atmospheric circulation.     

U–Pb dating of calcite–aragonite layers in speleothems from hominin sites in South Africa by MC-ICP-MS

Speleothems are found in association with hominin fossil-bearing cave deposits in South Africa and can be used to provide valuable chronological constraints. Such material is generally too old for U–Th dating and, although U–Pb geochronology presents a suitable alternative, bulk U concentrations are typically too low (<0.05 μg/g) to provide useful ages. For this reason, we used a simple non-invasive beta-scanner imaging screening step to identify U-rich (≥1 μg/g) domains that could be analyzed with MC-ICP-MS techniques to provide U–Pb ages. We demonstrate the technique using samples from Sterkfontein cave that exhibit infrequent <1 cm-thick layers with U concentrations ≥1 μg/g. Relict aragonite needles are found exclusively in these U-rich layers. We analyzed material from the same flowstone suite as Walker et al. (2006) and obtained a U–Pb age of ～2.3 Ma that agrees well with their estimate of 2.24 ± 0.09 Ma. We also obtained similar U–Pb (0.164 ± 0.026 to 0.200 ± 0.052 Ma) and U–Th (0.148 ± 0.003 Ma) ages for another sample exhibiting U-rich layers. We recognize that open-system behaviour during the partial transformation of aragonite to calcite is a potential problem and argue, on the basis of geochemistry and age consistencies, that recrystallization took place rapidly after speleothem formation and did not significantly affect the U–Pb ages.     

A geochemical perspective on Parisian urban history based on U–Th dating,laminae counting and yttrium and REE concentrations of recent carbonates in underground aqueducts

Two laminated carbonate deposits from the wall of an underground aqueduct, built between the 13th and 15th century, in Paris, France, were studied to provide historical information about nearby human occupation and urban development. To obtain a robust chronology for these small (29 and 45 mm thick) and very young deposits (< 800 a based on the probable date of aqueduct construction), we adopted two methods: laminae counting and U–Th analysis. For laminae, we tested the hypothesis that their deposition is bi-annual. Concerning U–Th dating, the influence of inherited ²³⁰Th on calculated ages is discussed and detrital ²³⁰Th/²³²Th values for each speleothem was determined using stratigraphic constraints and found to be significantly different despite their close proximity. In these speleothems, that precipitated over approximately 300 a according to U–Th dating, the number of laminae is similar, and two laminae were deposited each year. The lamina-counting chronology was used to interpret the rare earth elements and yttrium content (REE + Y) variations measured by ICP-QMS as historical variations in water quality in relation with the anthropogenic use of these elements. There is a marked increase of the REE + Y concentrations since the second half of the twentieth century. A marked negative Ce anomaly in the NASC-normalized REE trend is evidenced, and is more pronounced for post 1850s. This anomaly could be due to prior precipitation of Ce⁴⁺ before the water reached the aqueduct, related to a high organic matter content of the water in oxidizing conditions. The increase in the Ce anomaly could indicate a second source of water, from a wastewater collection system or a modern water conveyance system. We thus demonstrate the interest of using these annual carbonate deposits in urban areas as a proxy for the history of urbanization or human activities.     

Constraining the long-term evolution of the slip rate for a major extensional fault system in the central Aegean,Greece, using thermochronology

The brittle/ductile transition is a major rheologic boundary in the crust yet little is known about how or if rates of tectonic processes are influenced by this boundary. In this study we examine the slip history of the large-scale Naxos/Paros extensional fault system (NPEFS), Cyclades, Greece, by comparing published slip rates for the ductile crust with new thermochronological constraints on slip rates in the brittle regime. Based on apatite and zircon fission-track (AFT and ZFT) and (U–Th)/He dating we observe variable slip rates across the brittle/ductile transition on Naxos. ZFT and AFT ages range from 11.8 ± 0.8 to 9.7 ± 0.8 Ma and 11.2 ± 1.6 to 8.2 ± 1.2 Ma and (U–Th)/He zircon and apatite ages are between 10.4 ± 0.4 to 9.2 ± 0.3 Ma and 10.7 ± 1.0 to 8.9 ± 0.6 Ma, respectively. On Paros, ZFT and AFT ages range from 13.1 ± 1.4 Ma to 11.1 ± 1.0 Ma and 12.7 ± 2.8 Ma to 10.5 ± 2.0 Ma while the (U–Th)/He zircon ages are slightly younger between 8.3 ± 0.4 Ma and 9.8 ± 0.3 Ma. All ages consistently decrease northwards in the direction of hanging wall transport. Most of our new thermochronological results and associated thermal modeling more strongly support the scenario of an identical fault dip and a constant or slightly accelerating slip rate of ∼ 6–8 km Myr^− 1 on the NPEFS across the brittle/ductile transition. Even the intrusion of a large granodiorite body into the narrowing fault zone at ∼ 12 Ma on Naxos does not seem to have affected the thermal structure of the area in a way that would significantly disturb the slip rate. The data also show that the NPEFS accomplished a minimum total offset of ∼ 50 km between ∼ 16 and 8 Ma.