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Abstract: Large excesses of 238U and 226Ra relative to 230Th characterize many arc magmas and are commonly interpreted to represent recent addition of slab-derived fluid to the mantle wedge beneath the arc. A significant proportion of arc magmas are, however, in 238U–230Th radioactive equilibrium. This is generally thought to result from “buffering” of the young slab fluid U-series signal by a sediment component in secular equilibrium. Here we present new 238U–230Th–226Ra and 235U–231Pa measurements for historic andesites from Volcán de Colima, Mexico. In all lavas (230Th/238U) are in equilibrium, whereas (231Pa/235U) and (226Ra/230Th) are significantly greater than one. These data demonstrate that arc magmas with (230Th/238U) equilibrium can have significant 231Pa and 226Ra excesses, precluding ageing of the magmas in the crust as the cause of 230Th/238U equilibrium. Quantitative modeling of metasomatic and melting processes further indicates that addition of sediment melts to a depleted mantle wedge produces significant 230Th excesses and that 238U excesses induced by recent addition of fluids derived from the altered oceanic crust are not sufficient to compensate these 230Th excesses. U-series activity ratios in Colima magmas are best explained by models in which the metasomatised mantle returns to secular equilibrium before melting, implying a time lag ⩾350 kyr, with subsequent production of 231Pa and 226Ra excesses by in-growth during melting rather than by addition of slab fluids. Investigation of a global compilation of U-series data in arc magma indicates that our model proposed for Colima applies to most arc magmas in or near (230Th–238U) equilibrium. The time lag between mantle metasomatism and melting appears to vary between hundreds years to more than 350 kyr in subduction zones. We posit that the absence of U/Th elemental fractionation during melting of arc sources in (230Th/238U) equilibrium reflects a higher compared to MORB sources that yield magmas with 230Th excesses. [Copyright &y& Elsevier]

Abstract: Four soil profiles of known age (40, 250, 600 and 3000ka) from the Merced soil chronosequence in California were analysed for U and Th isotopes, and for major- and trace-element compositions, to test the assumption that leaching of U-series isotopes is a first order process with an invariant rate constant, as frequently assumed when applying this isotopic tool to determine weathering duration and sediment residence time. Average (230Th/238U) values for each profile decrease from a high of 1.28 in the youngest soil (40ka) to 1.09, 0.99 and 0.98 in the 250ka, 600ka and 3000ka soils respectively. (234U/238U) values also show variation across the 4 soils. The simultaneous variation in (230Th/238U) and (234U/238U) cannot be explained by a first-order kinetic model with invariant rate constants and instead requires that leaching coefficients for the U-series isotopes change with time. The observed data is consistent with a two-stage model that reflects an initial period of fast leaching of U and Th from a readily weathered soil pool (such as grain boundaries or defects), followed by slow leaching from a more resistant soil pool. Application of such a two-stage model may lead to significantly shorter calculated timescales of weathering than those obtained by the widely applied single-stage model of U-series weathering, which assumes no change in leaching coefficients over time. The results of this study have wide implications for published and future use of U-series tools in weathering studies. [Copyright &y& Elsevier]

We have developed techniques for measuring 234U and 230Th on Faraday cups with precisions of 1–3 epsilon units (1 ε-unit=1 part in 104) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Using a Thermo-Scientific Neptune with desolvation nebulization, we obtained ionization/transmission efficiencies of 1–2% for both U and Th. We set up protocols to correct for tailing, prepared U and Th gravimetric standards, tested a Th mass fractionation correction procedure based on U isotopes, and identified natural calcite samples likely to be in U–Th isotopic secular equilibrium. The measured atomic ratios, 234U/238U=54.970 (±0.019)×10−6 and 230Th/238U=16.916 (±0.018)×10−6, for these calcite samples were identical within errors (quoted 2σ uncertainties calculated combining all sources of error). Half-life values calculated from these ratios are consistent with previous values, but have much smaller errors: 245,620±260a for 234U and 75,584±110a for 230Th (quoted 2σ uncertainties calculated using all sources of error). In calculating a 230Th age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the 238U half-life value, uncertainty in our gravimetric uranium and thorium standards, and uncertainty in the absolute isotopic composition of the uranium standard), yields effective uncertainties for the purposes of 230Th dating of ±70a for the 234U half-life value and ±30a for the 230Th half-life value. Under ideal circumstances, with our methods, the 2σ uncertainty in age, including uncertainty in half-life values is ±10a at 10ka, ±100a at 130ka, ±300a at 200ka, ±1ka at 300ka, ±2ka at 400ka, ±6ka at 500ka, and ±12ka at 600ka. The isotopic composition of a sample with an age <800ka can clearly be resolved from the isotopic composition of a sample in secular equilibrium, assuming closed system behavior. Using these techniques, we analyzed a Sanbao Cave (Hubei, China) stalagmite that formed between 510 and 640ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite δ18O and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values. [ABSTRACT FROM AUTHOR]

Abstract: No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238U–230Th–226Ra–210Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report 210Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222Rn from 226Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies. [Copyright &y& Elsevier]

Abstract: The Cascade arc is the warm-slab subduction zone global end member, where a broad variety of primitive magmas with highly variable slab fluid signatures have erupted in close spatial and temporal proximity. A number of petrogenetic models have been proposed to explain the occurrence of such diverse magmas, but the source(s) of these magmas and the timing of fluid addition to the sub-arc mantle remain controversial. We present uranium-series isotope data (238U–230Th–226Ra) for eighteen mafic lavas from the Three Sisters region of the central Oregon Cascades, and for a further six lavas from the rear-arc Newberry Volcano. The majority of these samples have geochemical characteristics (e.g. Nb/Zr, Ba/Zr, 87Sr/86Sr) consistent with previously described calc-alkaline basalts from this region of the arc, and indicative of limited fluid involvement at some stage in their genesis. Trace element and long-lived radiogenic isotope modeling suggests that this fluid was derived from dehydration of subducting sediments, and was added to an enriched, garnet-bearing mantle wedge source. The trace element systematics of the lavas are consistent with small degree (<10%) melts of this fluid-modified source. All samples display (230Th/238U) and (226Ra/230Th)≥1, similar to values measured in fresh MORB and other parts of the arc. Results of a dynamic melting model support the interpretation that these lavas are small degree melts of an asthenospheric source, and do not allow for a lithospheric mantle source. However, the U-series data do not permit us to determine whether fluid addition was the trigger for melting, or whether the lavas were generated from a secular equilibrium source that had experienced fluid addition >350ka prior to melting. Regardless, modern fluid input is limited and melting is dominantly occurring in response to upwelling and decompression. [Copyright &y& Elsevier]

Abstract: U-series disequilibria provide important constraints on the processes and time scales of melt production, differentiation, and transport in subduction settings. Such constraints, which are essential for understanding the chemical evolution of the continental crust, are conventionally based on the assumption that the U-series disequilibria measured in mafic lavas are produced during mantle metasomatism and melting, and that intracrustal differentiation and assimilation have limited impacts. Here we show that mantle-derived U-series disequilibria in mafic lavas erupted at Volcán Llaima, Chile are significantly diminished by assimilation of plutonic rocks forming Llaima''s subvolcanic basement. This contamination process is extremely subtle in terms of “classical” indicators of crustal assimilation like Sr, Nd or Pb isotopes because it is a manifestation of assimilative recycling of the plutonic roots of the arc. This process results in variations in U-series disequilibria and incompatible trace element ratios that are significant compared to regional and global variability in arc magmas. Furthermore, it yields linear correlations between U-series excesses and incompatible trace element ratios that are generally interpreted as slab-fluid indicators and chronometers, or tracers of sediment recycling in subduction zone. Cannibalization of ancestral magmas by ascending melts warrants careful evaluation when considering the components and chemical fluxes in subduction zones. Linear arrays defined by activity ratios of U-series nuclides with different half-lives may be the most reliable indicators of assimilative recycling of ancestral intrusive magmas. [Copyright &y& Elsevier]

Abstract: This study presents uranium and thorium concentrations and activity ratios for all riverine phases (bedload, suspended load, dissolved load and colloids) from basaltic terrains in Iceland and the Azores. Small basaltic islands, such as these, are thought to account for ~25% of CO2 consumed by global silicate weathering, and for ~45% of the flux of suspended material to the oceans. These data indicate that [U] and [Th] in the dissolved and colloidal fractions are strongly controlled by pH, and to a much lesser extent by levels of dissolved organic carbon (which are low in these environments). At high pH, basalt glass dissolution is enhanced, and secondary mineral formation (e.g. Fe-oxyhydroxides and allophane) is suppressed, resulting in high dissolved [U], and low colloidal [U] and [Th], indicating a direct chemical weathering control on elemental abundances. When the dissolved (234U/238U) activity ratio is >~1.3 (i.e. when physical weathering, groundwater contribution or soil formation are high), there is little isotope exchange between dissolved and colloidal fractions. At lower activity ratios, the dissolved load and colloids have indistinguishable activity ratios, suggesting that when chemical weathering rates are high, secondary clay formation is also high, and colloids rapidly adsorb dissolved U. Many of the suspended sediment samples have (234U/238U) activity ratios of >1, which suggests that uptake of U onto the suspended load is important. Identical (230Th/232Th) in suspended, dissolved and colloidal samples suggests that Th, like U, is exchanged or sorbed rapidly between all riverine phases. This particle-reactivity, combined with poorly constrained contributions from groundwater and hydrothermal water, and short-term variations in input to soils (volcanic and glacial), suggests that U-series nuclides in riverine material from such basaltic terrains are unlikely to reflect steady state erosion processes. [Copyright &y& Elsevier]

Abstract: Recent studies document the synchronous nature of shifts in North Atlantic regional climate, the intensity of the East Asian monsoon, and productivity and precipitation in the Cariaco Basin during the last glacial and deglacial period. Yet questions remain as to what climate mechanisms influenced continental regions far removed from the North Atlantic and beyond the direct influence of the inter-tropical convergence zone. Here, we present U-series calibrated stable isotopic and trace element time series for a speleothem from Moaning Cave on the western slope of the central Sierra Nevada, California that documents changes in precipitation that are approximately coeval with Greenland temperature changes for the period 16.5 to 8.8ka. From 16.5 to 10.6ka, the Moaning Cave stalagmite proxies record drier and possibly warmer conditions, signified by elevated δ 18O, δ 13C, [Mg], [Sr], and [Ba] and more radiogenic 87Sr/86Sr, during Northern Hemisphere warm periods (Bølling, early and late Allerød) and wetter and possibly colder conditions during Northern Hemisphere cool periods (Older Dryas, Inter-Allerød Cold Period, and Younger Dryas). Moaning Cave stable isotope records indicate that wet conditions persisted in this area well beyond 11.5ka, suggesting the effects of the Younger Dryas event may have been longer lived in the western Sierra Nevada than in Greenland. However, a shifting drip center and corresponding change in seepage water routing may have influenced the trace element records between 10.6 and 9.6ka. Linkages between northern high-latitude climate and precipitation in the Sierra Nevada suggested here could indicate that, under conditions of continued global warming, this drought-prone region may experience a reduction in Pacific-sourced moisture. [Copyright &y& Elsevier]

Abstract: In historical basalts from a wide range of tectonic settings, 210Pb is often found to have an activity deficit relative to its predecessor 226Ra. Several processes have been hypothesized as causes of 210Pb deficits in basalts. In subduction zone and ocean island environments, 210Pb deficits have often been attributed to shallow magmatic degassing. At mid-ocean ridges, 210Pb deficits have been inferred to result from mantle melting, limiting the time between melt production and eruption to 100 years or less. Here we present an alternative mechanism for producing 210Pb deficits in magmas, by diffusive exchange between a melt and cumulate minerals (plagioclase and/or clinopyroxene) in the crust. The deficit in 210Pb develops in response to its decay toward secular equilibrium with 226Ra within the mineral grains; decay provides an internal sink for 210Pb that drives continuous diffusive exchange with the melt. Deficits in 210Pb develop under a broad range of conditions, in enriched and depleted melts and during interaction with young or old cumulate minerals. The magnitude of the deficit depends mainly on the equilibrium mineral/melt partition coefficients for Pb and Ra and on the melt/rock ratio during diffusive interaction, and is only weakly dependent on the relative diffusivities of Ra and Pb in the minerals or the trace element disequilibrium between the melt and cumulate minerals. Plagioclase in the crust has greater leverage on the 210Pb–226Ra system than any silicate mineral present during mantle melting, and is capable of inducing significant 210Pb deficits in the melt even at melt fractions above 50%. Its influence on the melt is also rapid, with a substantial 210Pb deficit developing in less than a year and approaching a steady state value after several decades or less. The strong control crustal cumulates are capable of exerting on 210Pb–226Ra fractionation in melts indicates that they may have a significant role in a wide range of tectonic environments, and suggests caution in interpreting 210Pb deficits as a signature of mantle melting, or as a product of 222Rn degassing. [Copyright &y& Elsevier]

Abstract: U-series systematics as well as Sr isotopes were measured on young seamount lavas from the Pitcairn hotspot collected during the Polynaut cruise. The combined U-series and Sr isotope data reveal typical mixing relationships between two endmembers. One typical ‘plume’ endmember with radiogenic 87Sr/86Sr and relatively low 230Th/238U and a ‘lithosphere’ endmember with less radiogenic 87Sr/86Sr and relatively larger 230Th/238U. Remarkably, all the lavas, except for a few arguably older samples, are characterized by 226Ra deficits relative to 230Th. On the basis of water content and trace element systematics, we argue that this is due to melting in the presence of phlogopite, which is only stable at lithosphere temperatures. A melting model including the diffusive exchange of elements among phlogopite, garnet and melt is used to constrain melting conditions of the lithosphere. These unusual 226Ra–230Th signatures can be explained by relatively slow melting rates at low matrix porosity. Our model also demonstrates that the effective partitioning behavior is dependent on the melting rate. A simple thermal model for lithosphere heating and melting is in good agreement with predicted melting rates. [Copyright &y& Elsevier]

Abstract: The uranium-series isotope signatures of the suspended and dissolved load of rivers have emerged as an important tool for understanding the processes of erosion and chemical weathering at the scale of a watershed. These signatures are a function of both time and weathering-induced fractionation between the different nuclides. Provided appropriate models can be developed, they can be used to constrain the residence time of river sediment. This chronometer is triggered as the bedrock starts weathering and the inferred timescale encompasses the residence time in the weathering profile, storage in temporary sediment deposits (e.g. floodplain) and transport in the river. This approach has been applied to various catchments over the past five years showing that river sediments can reside in a watershed for timescales ranging from a few hundreds of years (Iceland) to several hundreds of thousands of years (lowlands of the Amazon). Various factors control how long sediment resides in the watershed: the longest residence times are observed on stable cratons unaffected by glacial cycles (or more generally, climate variability) and human disturbance. Shorter residence times are observed in active orogens (Andes) or fast-eroding, recently glaciated catchments (Iceland). In several cases, the residence time of suspended sediments also corresponds to the time since the last major climate change. The U-series isotope composition of rivers can also be used to predict the river sediment yield assuming steady-state erosion is reached. By comparing this estimate with the modern sediment yield obtained by multi-year sediment gauging, it is clear that steady-state is seldom reached. This can be explained by climate variability and/or human disturbance. Steady-state is reached in those catchments where sediment transport is rapid (Iceland) or where the region has been unaffected by climate change and/or human disturbance. U-series are thus becoming an important tool to study the dynamics of erosion. [Copyright &y& Elsevier]

Abstract: The riverine flux of U that enters the deep oceans is not well constrained since the net losses during estuarine mixing are difficult to quantify. Riverine-dissolved U normally has a higher 234U/238U activity ratio (234U/238Uar) than the uniform value that characterizes open ocean seawater and could be used as a tracer of riverine inputs if one could resolve subtle variations in seawater composition. Using new mass spectrometry techniques we achieve a long-term reproducibility ±0.3‰ on 234U/238Uar which permits the tracing of riverine U in seawater samples from the Arctic – a partially restricted basin that is ideal for such a study. We find that surface waters from the Arctic basins carry elevated 234U/238Uar when compared with deep ocean seawater. Samples from the Canada Basin have a significant freshwater component and provide evidence that the Mackenzie River loses ∼65% of its U in the Mackenzie shelf/estuary zone before entering the deeper basin. This is in contrast to samples from the Makarov Basin, which provide evidence that all of the freshwater input is derived from the major Yenisey River alone, despite the proximity of the Lena and Ob Rivers. The differing behaviour of U between the Mackenzie and Yenisey Rivers is most likely a consequence of the strong binding of U to dissolved organic matter (DOC) or secondary phases in these rivers. The Yenisey River appears to transport the majority of the DOC through the shelf and into the Makarov Basin. In contrast, the Mackenzie River appears to lose a significant amount of DOC (>50%) in the estuary/shelf zone, which may lead to loss of associated U. These findings offer a more detailed picture of the fresh riverine water flow patterns in the Arctic Ocean when compared to other geochemical proxies. The non-conservative behaviour of U in the Mackenzie River through the shelf/estuaries has important implications for U input into oceans and the total marine budget. [Copyright &y& Elsevier]

Abstract: We report U–Th disequilibria data for a suite of 13 young basaltic samples from the Samoan Islands, which represent the end-member mantle component EM2, and 4 historic lavas from Mt. Erebus, typifying young HIMU. The Samoan samples have low 230Th/232Th and 238U/232Th, consistent with the enriched nature (EM2) of the Samoan mantle source, whereas the Mt. Erebus samples have high 206Pb/204Pb and intermediate (230Th/232Th) and (238U/232Th). When considered in the context of the global oceanic basalt database, the Samoan samples'' low 230Th/232Th and 238U/232Th greatly extend the global correlations between 230Th/232Th and 238U/232Th with 87Sr/86Sr and 143Nd/144Nd and change the functional form of these correlations from linear to hyperbolic. Using a maximum likelihood non-linear inversion method, we show that these correlations of 230Th/232Th and 238U/232Th with 87Sr/86Sr and 143Nd/144Nd can be approximated by two-component mixing. However, the global oceanic basalt data also show considerable scatter about the best-fit mixing curves. This scatter is attributed to additional source components and melting processes influencing the lavas (230Th/232Th) and (238U/232Th). Additional source components are supported by the Pb isotope data, which clearly require more than two endmember mantle components. We also show that the extent and variability in (230Th/238U) decreases as a function of source enrichment, with “depleted” MORB showing the largest extents and greatest variability in (230Th/238U) and “enriched” OIB, like Samoa, showing smaller extents and less variability of (230Th/238U). We interpret this observation in terms of differences in the melting regimes beneath mid-ocean ridges and ocean islands. [Copyright &y& Elsevier]

Abstract: Although the majority of young volcanic rocks in island arcs typically have 238U excesses, continental arc rocks display both 238U and 230Th excesses. In fact, there is a global correlation between the sense of U-series disequilibria and crustal thickness that suggests that crustal thickness may somehow influence the processes that fractionate U from Th. At Cotopaxi Volcano, Ecuador, (238U)/(230Th) values of 1.03–1.14 in rhyolites are attributed to accessory phase fractionation, whereas (238U)/(230Th) values of 0.96–1.07 in andesites can be explained by several potential processes, including melting of garnet-bearing lower crust. The Cotopaxi rocks have non-fractionated HFSE ratios and La/Yb values that are consistent with melting of a garnet-bearing lithology, and we suggest a model of lower crustal melting and assimilation to account for the range of U-series data in the Cotopaxi andesites. Mantle like 87Sr/ 86Sr and 143Nd/ 144Nd values indicate that the assimilant was a relatively juvenile and/or mafic component. The rhyolites contain apatite and allanite, fractionation of which can generate 238U excesses during crystallization, and modeling shows that 70–90% crystallization of an assemblage containing these phases could generate the observed 238U excesses. These data suggest that multi-level AFC processes contribute to magma evolution at Cotopaxi Volcano as magma traverses the continental crust of the Northern Volcanic Zone. On the basis of the 238U–230Th–226Ra data, the time for assimilation and ascent of the andesites was <8000 yr, whereas the rhyolites may have resided in the crust for 70–100ky. The modification of U–Th isotopic signatures may be a common feature of differentiated magmas in continental arc settings and such potential effects should be considered along with interpretations involving variable mantle sources and melting regimes. [Copyright &y& Elsevier]

Abstract: U-series studies in oceanic basalts are critical for understanding melting and melt extraction in the Earth''s mantle. The combined results of a detailed geochemical study of melting and melt extraction at Theistareykir, northern Iceland, provide a strong case for melt extraction via channeled melt flow at an active spreading ridge. It has often been argued, however, that widely used melting and melt extraction models, which simulate channeled melt extraction (i.e. fractional and/or dynamic melting), can only partially explain the global U-series systematics in oceanic basalts. As a consequence, more complicated models have been invoked, which suggest different styles of melt extraction at different depths/pressures in the mantle, so-called “two-porosity models”. Alternatively, diffusion-controlled mechanisms have been proposed. Here we show that U–Th–Pa–Ra systematics in oceanic basalts can indeed be explained by models where melt transport occurs without chemical equilibrium between melt and solid when variations in all three critical melting parameters (residual porosity, upwelling rate of the solid mantle and melt velocity) are taken into account. Melting at ridges requires systematic variation of at least two critical melting parameters, most likely upwelling and melt extraction rate. Melts generated with increasing lateral distance to the ridge axis are generated with slower upwelling rates and are extracted with lower velocities than melts created closer to the ridge axis. Melting at ocean islands, on the other hand, can successfully be explained by variations in upwelling rate only. Global U-series systematics in OIB originate from superimposed global variations in upwelling velocity due to different buoyancy fluxes and from local variation in upwelling velocity as a function of radial distance to the plume center. The model proposed here is consistent with other geochemical data for oceanic basalts and strongly supports melt extraction via high-porosity channels as a general means of melt extraction from the Earth''s upper mantle. [Copyright &y& Elsevier]

Abstract: We report U-series systematics (238U-230Th-231Pa-226Ra) for basalts from the Azores islands and the nearby mid-Atlantic ridge with the aim of constraining melting processes in this region of plume–ridge interaction. Both 230Th and 231Pa excess show rough negative trends with indices of source enrichment (Sr, Nd and Pb isotopes) which cannot be explained by melting of a source variably enriched by mafic components (e.g. pyroxenite). However, there is evidence for a distinct source component, possibly sediment, beneath Sao Miguel which may give rise to these trends. 231Pa excess and Sm-Nd fractionation are positively and negatively correlated (respectively) with distance from the centre of the plume, while 230Th excesses show no clear trend. This suggests that mantle upwelling velocity exerts a strong control on U-Pa fractionation and that melting is initiated deeper in the centre of the plume. These observations are used to infer melting conditions beneath the mid-Atlantic ridge and Azores islands. The mantle upwelling velocity is constrained to be 3–4 cm/a which is smaller than that inferred for Hawaii or Iceland. Predicted 226Ra-excesses are similar to, or lower than those measured, suggesting high velocity melt ascent in channels and rapid differentiation prior to eruption. The effect of the presence of volatiles in the source is also investigated and we show that the presence of water could explain the 230Th and 231Pa trends as a function of distance across the plume. A 3-D model for the Azores plume can reproduce the U-series isotope observations. [Copyright &y& Elsevier]

The frequency of oscillations in global ice volume as determined by dating of coral reef deposits provides an important test of the Milankovitch model of orbital climate forcing. The timing of two episodes of high sea level during marine isotope substages (MIS) 5a and 5c, here determined to have occurred at ∼84 and ∼101 ka BP are broadly consistent with the Milankovitch model. However, new high precision U-series dating of uplifted morphologically distinct coral-reef terraces at Barbados indicate that MIS-5a was both longer and more complex than previously established, with an additional sea-level highstand occurring at ∼77 ka BP. The U-series analysis of a large number of samples for each coral reef deposit reveals a well-defined correlation between measured age and initial δ234U. This correlation is evident even within the uncertainty band of ±8‰ commonly used to exclude diagenetically altered samples and allows a more precise and accurate constraint of the age of these deposits by evaluating where the trend line intersects the modern δ234U value. Other proxy climate records also indicate suborbital-period variability during MIS-5a and 5c and together imply that, as for the last ice age, orbital forcing alone cannot account for the observed changes in climate. [Copyright &y& Elsevier]

Along the main boundary fault of the central Tibetan Shuang Hu graben, two well-preserved fluvial terrace surfaces are vertically offset by ∼1.3 m (terrace I) and ∼14.8 m (terrace II). Using thermal ionization mass spectrometry (TIMS), we have determined 48 230Th–234U–238U ages of small (10–40 mg) samples from pedogenic carbonate rinds on clasts in the terrace deposits. Based on textural, microstratigraphic and geochemical criteria, we conclude that the U-series ages of innermost-rind samples provide reliable ages of 16.4±1.9 ka for terrace I, and 233.1±9.3 ka for terrace II. This constrains the average rate of vertical displacement along the normal fault to be 0.079±0.011 mm/yr during the past ∼16 kyr, and 0.064±0.007 mm/yr during the past ∼233 kyr. Combining these results with slip-rate estimates for other normal faults along the graben margins indicates that the cumulative vertical displacement on all normal faults did not exceed ∼0.3 mm/yr during the late Quaternary. This new rate from central Tibet is distinctly lower than the rate of 1.9±0.6 mm/yr inferred for normal faults bounding the Yadong-Gulu graben system in southern Tibet, consistent with more pronounced and common graben development in southern Tibet. We therefore suggest that normal faulting in southern Tibet is largely controlled by local processes, and that the age of its initiation is not a valid proxy for uplift of the Tibetan plateau, as has been widely assumed. Our data also place chronologic constraints on the termination of periglacial conditions in central Tibet during the last three interglaciations. Rind formation appears to have been relatively continuous on clasts in terrace I from ∼16 ka. Clast rinds from terrace II, in contrast, are characterized by three main episodes of rind growth that are widely separated in time. The earliest and least well-preserved episode started at 233.1±9.3 ka, a second episode occurred at 131.8±7.8 ka, and a third episode was approximately coeval with rind formation in terrace I after 16.4±1.9 ka. In each case, the age of onset of carbonate rind accumulation coincides with independently estimated ages of global deglaciation. Accordingly, we conclude that periglacial conditions in the central Tibetan plateau terminated synchronously with early stages of global warming following the last three glacial maxima, and that fluvial aggradation, terrace formation, and pedogenic carbonate accumulation in terrace soils ensued rapidly. [Copyright &y& Elsevier]

The source of excess 234U in fossil corals and its relationship to U-series age determinations has been an outstanding problem in geochronology for more than 20 years. With increasing numbers of U-series isotope measurements in corals, and significant improvements in analytical precision through mass spectrometry, it is increasingly apparent that a substantial fraction of observed isotope ratios cannot be reasonably explained by closed-system decay. Moreover, observations of a positive correlation between 234U/238U and 230Th/238U ratios in corals from the same terrace are difficult to explain. However, the decay of dissolved uranium and α-recoil mobilization of uranium daughters produce particle-reactive 234Th and 230Th, and the coupled addition of these Th isotopes could simultaneously increase coral 234U/238U and 230Th/238U. Here we present a quantitative model, based on decay-dependent redistribution of 234Th and 230Th, permitting calculation of open-system coral ages. These equations provide a general solution to the α-recoil redistribution problem, applicable to any alpha decay series. While measured isotope ratios of corals from the three youngest stratigraphically defined Barbados terraces are inconsistent with closed-system decay, they fall in broadly linear arrays agreeing with model predictions. Isotopic arrays of older Barbados corals, and corals from terraces around the world, are also consistent with model predictions suggesting the open-system model is generally applicable. Corals with extreme isotopic compositions that are impossible to produce by closed-system decay are consistent with the limited range of isotopic compositions predicted by the model at ages older than 600 ka. For corals from a single terrace, 234Th and 230Th redistribution appears to be a source of systematic conventional age error, even for corals with slightly elevated 234U. However, open-system ages are consistent, even for corals with extremely elevated 234U. For the youngest three Barbados terraces, mean open-system terrace ages are consistent with mean conventional terrace ages calculated from pristine samples. If the most accurate conventional ages are from corals with an initial 234U/238U identical to modern seawater, then the open-system model will improve the accuracy of coral U-series age determinations and dramatically increase the number of reliable ages. [Copyright &y& Elsevier]