Search

Dust plays important roles in the environment, and there has been much interest in the formation, provenance, and age of the world's dust deposits. Ongoing debates are concerned with the importance of glacial grinding versus eolian abrasion and fluvial transport in the formation of silt-sized particles. Short-lived uranium-series isotopes afford new insights because they can be used both for provenance fingerprinting and for constraining the integrated age of chemical and physical weathering and subsequent transport and storage of sediments. Here we present trace element and Sr, Nd, and U-series isotope analyses from a number of Australian dusts and suspended river sediments remobilized during floods a year later. The inferred ages of the Australian dust appear to be linked to aridification and the loss of inland megalakes ∼30–120 k.y. ago. This provides preliminary evidence that the age of dust may provide a new hydrological indicator in arid environments.

Tributary creeks of the southern Sierra Nevada have pronounced knickpoints that separate the landscape into an alternating sequence of gently sloped treads and steeply sloped risers. These knickpoints and the surrounding “stepped topography” suggest that the landscape is still responding to Pleistocene changes in base level on main-stem rivers. We tested this hypothesis using cosmogenic nuclides and uranium isotopes measured in stream sediment from widely distributed locations. Catchment-scale erosion rates from the cosmogenic nuclides suggest that the treads are relict surfaces that have adjusted to a previous base level. Nevertheless, erosion rates of relict interfluves are similar to canyon incision rates, implying that relief is unchanging in the lower Kings and San Joaquin Rivers. In addition, our results suggest that much of the southern Sierra Nevada is in a state of arrested development: the landscape is not fully adjusted to—and moreover is not responding to— changes in base-level lowering in the canyons. We propose that this can be explained by a paucity of coarse sediment supply, which fails to provide sufficient tools for bedrock channel incision at knickpoints. We hypothesize that the lack of coarse sediment in channels is driven by intense weathering of the local granitic bedrock, which reduces the size of sediment supplied from hillslopes to the channels. Our analysis highlights a feedback in which sediment size reduction due to weathering on hillslopes and transport in channels is both a key response to and control of bedrock channel incision and landscape adjustment to base-level change.

Flores represents a unique insular environment with an extensive record of Pleistocene fossil remains and stone artefacts. In the So'a Basin of central Flores these include endemic Stegodon, Komodo dragons, giant tortoises, rats, birds and hominins, and lithic artefacts that can be traced back to at least one million years ago (1 Ma). This comprehensive review presents important new data regarding the dating and faunal sequence of the So'a Basin, including the site of Mata Menge where Homo floresiensis-like fossils dating to approximately 0.7 Ma were discovered in 2014. By chemical fingerprinting key silicic tephra originating from local and distal eruptive sources we have now established basin-wide tephrostratigraphic correlations, and, together with new numerical ages, present an update of the chronostratigraphy of the So'a Basin, with major implications for the faunal sequence. These results show that a giant tortoise and the diminutive proboscidean Stegodon sondaari last occurred at the site of Tangi Talo ∼1.3 Ma, and not 0.9 Ma as previously thought. We also present new data suggesting that the disappearance of giant tortoise and S. sondaari from the sedimentary record occurred before, and/or was coincident with, the earliest hominin arrival, as evidenced by the first records of lithic artefacts occurring directly below the 1 Ma Wolo Sege Tephra. Artefacts become common in the younger layers, associated with a distinct fauna characterized by the medium-sized Stegodon florensis and giant rat Hooijeromys nusatenggara. Furthermore, we describe a newly discovered terrace fill, which extends the faunal record of Stegodon in the So'a Basin to the Late Pleistocene. Our evidence also suggests that the paleoenvironment of the So'a Basin became drier around the time of the observed faunal transition and arrival of hominins on the island, which could be related to an astronomically-forced climate response at the onset of the Mid-Pleistocene Transition (MPT; ∼1.25 Ma) leading to increased aridity and monsoonal intensity. publishedVersion

Reversing the field Do terrestrial geomagnetic field reversals have an effect on Earth's climate? Cooper et al. created a precisely dated radiocarbon record around the time of the Laschamps geomagnetic reversal about 41,000 years ago from the rings of New Zealand swamp kauri trees. This record reveals a substantial increase in the carbon-14 content of the atmosphere culminating during the period of weakening magnetic field strength preceding the polarity switch. The authors modeled the consequences of this event and concluded that the geomagnetic field minimum caused substantial changes in atmospheric ozone concentration that drove synchronous global climate and environmental shifts. Science , this issue p. 811

For over half a century, uranium (U) and thorium (Th) decay series isotopes have been used as a chronometer and tracer of a range of geological and biological processes. In volcanic rocks, they can be used to help determine how magma is produced in a range of environments, how fast it is transported, and minerals that are precipitated in the process. They have also been used to date volcanic eruptions. In soils, U-series isotopes can estimate how fast parent material is weathered and converted into regolith. In fluvial sediments, they allow a determination of how long sediments have resided in a catchment. Uranium-series isotopes have also been applied to the study of groundwater, where they inform on the dissolution rate of the aquifer rock and the groundwater flow rate. In marine sediments, U- and Th-series isotopes have played a key role in improving our understanding of marine processes. For instance, they can quantify particular organic matter flux (with implications for the carbon cycle), and inform on particular transport and palaeo-productivity. Finally, U-series dating of carbonates such as corals has been instrumental in our understanding of past sea levels. The use of these isotopes in the dating of phosphates such as fossil bones and teeth has allowed us to directly date human fossils, with major implications for human evolution.

Uranium (U) isotopes can be used to estimate the comminution age of sediments, i.e. the time elapsed from sediment production on continents, via weathering and physical erosion, to deposition in the sedimentary record. The calculation of this comminution age is based on measured (234U/238U) activity ratios in river sediments, and inferred time-dependent recoil effect, which leads to the preferential release of 234U from mineral lattices during erosion processes. In this study, we report on a large-scale (234U/238U) investigation of modern river sediments worldwide, with the aim to determine the extent to which parameters such as grain size, lithology, weathering, climate and geomorphology may influence the distribution of U isotopes in fine-grained sediments. Our extensive dataset (N=64) includes U isotopic measurements for many of the world’s largest rivers, but also rivers draining particular climatic and geological settings. Our results indicate that sediments collected from river basins draining mostly igneous, metamorphic or volcanic rocks often display (234U/238U) ratios >1, with clay-size fractions (

Understanding and quantifying the processes and geochemical cycles associated with catchment erosion, the development of soils and weathering horizons, and terrestrial habitat change beyond the scales of modern observations remain challenging. Such research, however, has become increasingly important to help predict future landscape change in light of increasing land use and rapid global warming. We herein review organic and inorganic geochemical tools applied to depositional archives to better understand various aspects of landscape evolution on geological time scales. We highlight the potentials and limitations of inorganic geochemical analytical methods, such as major element geochemistry, metal and radiogenic isotopes, and in-situ cosmogenic nuclides, as qualitative, semi-quantitative, and quantitative proxies for the transformation of bedrock material via regolith and soils to sediments. We also show how stable isotope geochemistry applied to lacustrine endogenic carbonates can be used to infer rock-water interactions, vegetation change, and soil development in limestone-rich catchments. Proxies focusing on the silicilastic element of sediment formation, transport and deposition are ideally combined with organic geochemical proxies for vegetation change and soil organic matter evolution in a catchment to gain a comprehensive picture of the Critical Zone’s evolution over time. Multi-proxy and multidisciplinary research combining organic and inorganic geochemical techniques from several sedimentary archives in the same catchment have high potential to provide comprehensive information on Quaternary landscape evolution and thus improve the robustness of associated forecasting models.

Uranium (U) isotopes are useful for constraining the timescales of weathering and erosion processes. The (234U/238U) activity ratio (parentheses denote activity ratio) of fine-grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to

For several decades, uranium–thorium (U–Th) dating has allowed geochronologists to precisely date geological materials, providing invaluable geochronological constraints on Quaternary processes. Open-system dating of bones and teeth has also provided ages of human and faunal remains of archaeological significance. To facilitate access to closed- and open-system U–Th dating to the broad scientific community, here we provide an R package, named UThwigl. Description of input and output parameters is given, as well as a guide for running the model. The package can be used three different ways: (i) as a web application, (ii) through a web browser with an internet connection, or (iii) in R (most efficiently with RStudio). Examples of application of the model are also provided, showing that it yields ages within error of previously published values.

The Payenia Basaltic Province (PBP) is the largest and the northernmost of the Quaternary back-arc Patagonian basaltic provinces in South America. In the last 10 years, several studies have investigated either, the geochemistry or the geochronology of this basaltic province. However, only a few investigations have focused on the two aspects simultaneously in order to reconstruct its Quaternary volcanic history. Consequently, this study aims to provide new Quaternary ages and to contribute in understanding how its geochemistry evolved though time. In the current study nine basaltic flows from the PBP in central west Argentina were dated using a combination of cosmogenic surface exposure, 40Ar/39Ar, and thermoluminescence dating methods. Seven flows have Middle to Late Pleistocene ages and two erupted in the Holocene. Using the new ages here inferred and the previously published Quaternary geochronology, together with the available geochemical data, maps of Pleistocene geochemical evolution have been generated. These maps indicate that two geochemically distinct magma types erupted over the same time interval (ca. 1.5 Ma) within the PBP: In the north-eastern part (Nevado volcanic field) of the province, an arc-like signature is distinguishable, whereas the southern part of the PBP (Rio Colorado volcanic field) exhibits an intraplate, Ocean Island Basalt (OIB)-like signature. The arc-like signature decreases in the Early to Middle Pleistocene as indicated by a reduction in Ba/La and La/Ta values in the Nevado volcanic field. At ca. 0.25 Ma a similar disparity has been inferred between two volcanic fields on the western part of the PBP, one erupting lavas with arc-like characteristics and the other with OIB-like signatures, despite being only tens of kilometres apart. Holocene volcanism is restricted to the western side of the Payun Matru volcanic field and is dominated by OIB-like signatures, notably high Ta/Hf and low Ba/La and La/Ta values.

Yorke Peninsula in southern Australia is an important region for reconstructing relative sea-level histories due to its location on the eastern margin of the tectonically stable Gawler Craton and in one of the world's geographically most remote far-field locations from the Pleistocene ice sheets. Richly fossiliferous, skeletal carbonate sands of the last interglacial (125 ka) Glanville Formation crop out in the coastal cliffs along large sectors of southern Yorke Peninsula. Sedimentary facies include deepening-upward intertidal to shallow subtidal facies, relict storm beach facies and cobble and boulder beach deposits in more exposed, higher energy locations. During deposition of the Glanville Formation, southern Yorke Peninsula had a different coastal geography with two prominent marine corridors extending across the southern-most portion of the entire peninsula. In a 3 km long coastal cliff section in southern Hardwicke Bay, the Glanville Formation crops out as an upward-deepening intertidal-subtidal succession capped by supratidal and subaerially-exposed sediments with pervasive calcrete development. The sedimentary succession passes upwards from a basal unit of intertidal sand flat facies with abundant gastropods (Batillaria diemenensis) near the upper bounding (disconformity) surface, upwards into richly fossiliferous shelly sands (coquina) representing a shallow subtidal facies formed by sediment aggradation in response to a relative sea-level rise. The subtidal facies is dominated by the bivalve molluscs Katelysia sp. and Amesodesma angusta, signifying a water deepening event. The subtidal facies is in turn overlain by pedogenically modified skeletal carbonate sands with pervasive calcrete development signifying a relative sea level fall at the end of the Last Interglacial Maximum. The upper-bounding surface of the shallow-water subtidal facies ranges from 2.4 to 3.0 m Australian Height Datum (AHD) and by analogy with modern sedimentary environments suggests a maximum palaeo-sea level of 4.8 ± 1.0 m during the Last Interglacial Maximum. Uranium-series ages of 127.3 ± 2.1 to 115.0 ± 5.4 ka on specimens of the solitary coral Plesiastrea versipora from the subtidal facies confirm that the succession was deposited during the Last Interglacial Maximum, and are consistent with the independent stratigraphical evidence that the highstand event was represented by a single phase of relative sea-level rise. Correlation with other occurrences of the Glanville Formation in southern Australia has also been confirmed by aminostratigraphy.

The uranium isotope composition (234U and 238U) of detrital matter has become an essential tool for evaluating the response of erosion and catchment dynamics to climate variability on geological time scales. Relative variability of the (234U/238U) activity ratio can be used as an estimate of the time that has elapsed since physical and chemical weathering has formed grains Combined U isotopes and gas adsorption surface area analyses on two sediment samples from the Mediterranean Sea yield comminution ages up to 5 times older after sample treatment, which highlights the importance of a comprehensive removal of non-detrital matter from the bulk sediment composition. Moreover, gas sorption analysis allowed determining whether a fractal correction for calculation of the recoil fraction should be applied. Precise estimates of the recoil fraction are crucial for calculating the comminution ages, as it governs the loss of 234U from detrital matter. Samples analyzed in this study display Type II isotherms suggesting a non-porous or macroporous surface. Micro- and mesopores, which increase the surface area during gas adsorption analyses but do not contribute to the loss of 234U are absent. Thus, a fractal correction to account for micro- and mesopores is not required.

Records of past sea levels, storms, and their impacts on coastlines are crucial for forecasting and managing future changes resulting from anthropogenic global warming. Coastal barriers that have prograded over the Holocene preserve within their accreting sands a history of storm erosion and changes in sea level. High-resolution geophysics, geochronology, and remote sensing techniques offer an optimal way to extract these records and decipher shoreline evolution. These methods include light detection and ranging (lidar) to image the lateral extent of relict shoreline dune morphology in 3-D, ground-penetrating radar (GPR) to record paleo-dune, beach, and nearshore stratigraphy, and optically stimulated luminescence (OSL) to date the deposition of sand grains along these shorelines. Utilization of these technological advances has recently become more prevalent in coastal research. The resolution and sensitivity of these methods offer unique insights on coastal environments and their relationship to past climate change. However, discrepancies in the analysis and presentation of the data can result in erroneous interpretations. When utilized correctly on prograded barriers these methods (independently or in various combinations) have produced storm records, constructed sea-level curves, quantified sediment budgets, and deciphered coastal evolution. Therefore, combining the application of GPR, OSL, and Lidar (GOaL) on one prograded barrier has the potential to generate three detailed records of (1) storms, (2) sea level, and (3) sediment supply for that coastline. Obtaining all three for one barrier (a GOaL hat-trick) can provide valuable insights into how these factors influenced past and future barrier evolution. Here we argue that systematically achieving GOaL hat-tricks on some of the 300+ prograded barriers worldwide would allow us to disentangle local patterns of sediment supply from the regional effects of storms or global changes in sea level, providing for a direct comparison to climate proxy records. Fully realizing this aim requires standardization of methods to optimize results. The impetus for this initiative is to establish a framework for consistent data collection and analysis that maximizes the potential of GOaL to contribute to climate change research that can assist coastal communities in mitigating future impacts of global warming.

Trophic structures, i.e., the diets, food chain positions, and feeding relationships of species in an ecological community, are a fundamental yet understudied avenue of investigation into Australian megafauna extinction. Calcium (Ca) isotope analysis has been developed as a tool to reconstruct trophic levels in vertebrate palaeobiology and palaeoecology. A baseline of modern marsupial Ca isotope signatures in a single trophic level and dietary niche is required to successfully apply this tool in order to reconstruct Australian faunal food webs. We present Ca isotope data from dental enamel of modern Tasmanian bare-nosed wombats (Vombatus ursinus tasmaniensis) to constrain the Ca isotope composition of this species, an opportunistic grazer, and assess whether sex and environmental variables influence such composition. We compare these data to complementary strontium isotope data from the same samples as a proxy for geographical location. Elodont (ever-growing) wombat teeth allow for a sequential sampling strategy and thus provide high resolution intra-individual variability on a seasonal scale, creating the largest intra-individual and intra-species set of Ca isotope data in a single species so far. We show that the Ca isotope composition of Tasmanian bare-nosed wombats falls within the range for herbivores and is independent of geological substrate. Comparison with carbon and oxygen isotope data from sequential samples within the same individuals highlights that sub-monthly intra-individual variations in Ca isotope compositions may result from change in the proportions in the wombat's diet of C3 and C4 plants, or monocots and dicots. The Ca isotope compositions of male and female wombats suggest that Ca isotopes in marsupials could be used to trace nursing and weaning behaviour. The characterisation of Ca isotope values in marsupial herbivore enamel presented contributes to a modern reference set for reconstructing diet from fossil remains of extinct Australian megafauna herbivores.

In optical dating of feldspars, it is commonly supposed that potassium (K)-rich feldspar grains have been isolated for luminescence measurements by means of density separation. The K and rubidium (Rb) concentrations of these grains deliver a significant internal beta dose rate. K and Rb concentrations of 12.5 ± 0.5 wt% and 400 ± 100 ppm, respectively, are typically assumed for dose rate and age calculation, but this assumption may be wrong. Thorium (Th) and uranium (U) also contribute to the beta and alpha components of the internal dose rate. We investigated the K, Rb, Th and U concentrations of individual luminescent feldspar grains in four sediment samples from Liang Bua in Indonesia to determine single-grain internal dose rates and appropriate sample-average internal dose rates. The composition of the ‘K-rich’ feldspar fraction was investigated using X-ray diffraction spectrometry (XRD), which found that this fraction was heavily contaminated in each sample by quartz and plagioclase feldspars. Grains with detectable post-infrared infrared stimulated luminescence (pIRIR) signals were characterised using a range of techniques, including energy-dispersive spectroscopy (EDS), wavelength-dispersive spectroscopy (WDS) and quantitative evaluation of minerals using EDS (QEM-EDS). The grains were found to be composed of a range of minerals, including feldspars, quartz, clay minerals and heavy minerals, as well as volcanic glass. The K concentrations of these composite grains were determined using QEM-EDS, and their Rb, Th and U concentrations were determined using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The calculated internal dose rates were very low for the majority of grains in each sample (

In contrast to the detection and resolution of Late Pleistocene interstadials (Marine Isotope Stages; MIS 5c, 5a and 3) in marine and ice core-based oxygen isotope records, accurately defining palaeosea-levels for these events from sea level indicators remains a challenge. Commonly, such investigations have been undertaken in tectonically active, subduction-related settings, on emergent marginal marine-coastal successions above present sea level, given that the higher ice volumes during these Marine Isotope Stages resulted in ice-equivalent sea levels significantly below present, particularly in far-field settings. Delineation of palaeosea-levels for this period, has accordingly involved an assumption about the long-term rate of coastal uplift. Core SV#23 collected from the tectonically highly stable, remote far-field setting of southern Gulf St Vincent, southern Australia, in a present water depth of 40 m obtained a 3.89 m undisturbed sedimentary record of Late Pleistocene to Holocene age. The basal 2.39 m of the sediment record is of Late Pleistocene interstadial age, as it overlies the Last Interglacial Glanville Formation (128–118 ka; MIS 5e) and occurs beneath the Holocene St Kilda Formation. The extent of aspartic acid and glutamic acid racemization in the benthic foraminifer Elphidium macelliforme sampled at 2 cm intervals continuously from the interstadial succession in Core SV#23, confirms the relative age interpreted from lithostratigraphy. A mean amino acid racemization (AAR) age of 75 ± 13 ka indicates a correlation with MIS 5a, and highlights the utility of E. macelliforme as a species for AAR geochronology. As the foraminifers more closely define the timing of sedimentation, the interstadial succession is regarded as beyond the range of radiocarbon dating (>50 kyr), as revealed in a comparison of radiocarbon ages of fossil molluscs with AAR ages for E. macelliforme from the same depth intervals within the interstadial succession. All of the radiocarbon ages for fossil marine molluscs from the interstadial succession represent minimum ages, resulting from contamination by trace levels of modern radiocarbon. The foraminiferal faunal assemblages within the interstadial succession reveal changes of water depth at the time of deposition, pointing to millennial-scale sea level variability. In particular, the ratio of Elphidium crispum to E. macelliforme indicates several oscillations in relative sea level within a bandwidth of 26 to 24 ± 4.6 m below present sea level (BPSL). Notwithstanding the large uncertainty associated with palaeosea-level estimation based on the Elphidium ratio, the inferred relative sea level record is in accord with the sea level records of Barbados and the Red Sea, and points to Sub-Milankovitch interstadial sea level variability as noted in Greenland and Antarctic ice cores. Refereed/Peer-reviewed

Soil sustainability implies maintaining the balance between soil erosion and production. While it is known how to assess soil erosion, only recently we have been able to estimate rates of soil and saprolite (namely regolith) production using uranium-series isotopes. This method assesses the time elapsed since rock-forming minerals start fractionating the U-series isotopes. In this study, we assess a sample pre-treatment protocol that has the potential to improve the method used to estimate regolith production rates. We propose that removal (or partial removal) of secondary phases precipitated from solution during pedogenesis (solution-derived phases) and organic matter from regolith may improve the accuracy of this method. This is tested using sequential extraction followed by etching as sample pre-treatment. Here, we assess their effect on the U-series isotopic composition of regolith and infer whether they minimize the presence of solution-derived phases and organic matter. We applied sequential extraction and etching to a basaltic weathering profile (bedrock, saprolite and soil) and compared the U-series isotopic composition before and after treatment. We also measured major elements concentrations and assessed mineralogy. The bedrock was in secular equilibrium and sequential extraction resulted in unchanged (234U/238U) activity ratios, while increased (230Th/238U). In contrast, etching resulted in increased (234U/238U) and (230Th/238U) activity ratios, which is attributed to the removal of primary minerals. Relative to the untreated bedrock, the untreated saprolite showed no changes in U and Th concentrations, and activity ratios. We hypothesise that during the conversion of bedrock into saprolite U and Th budgets are unaffected. Moreover, major element and mineralogical analyses suggest that during this process rock-forming minerals are converted into secondary phases (clays). We hypothesise that during this conversion the U-series isotopes are not fractionated; therefore, the removal of these secondary phases is not necessary. Relative to the saprolite, the soil showed gains of U and Th, (234U/238U) > 1 and (230Th/238U) 1. To minimize the presence of solution-derived phases and organic matter in basaltic weathering profiles we suggest that only soil samples should undergo sequential extraction, because only these are significantly affected by solution-derived phases and organic matter. Additionally, our experiments show the existence of fractionation processes that are often overlooked in U-series isotopic studies, i.e. implantation of 234U and 230Th recoiled from U-rich mineral (i.e. glass) into adjacent, U-poor phases (e.g. pyroxene and feldspar).

Knowledge of river sediment recycling provides important constraints on continent weathering and earth surface processes. In this study, we estimate the “comminution age” of sediments from the Changjiang (Yangtze River) and two small mountainous rivers in Taiwan based on their lithogenic (234U/238U) ratio. The (234U/238U) distributions in the Changjiang catchment are overall related to sediment grain size and chemical weathering regime, while (234U/238U) ratios in Taiwan rivers mainly depend on erosion/denudation processes. The comminution age constrains the time scale of sediment source-to-sink processes in catchments from sediment weathering/denudation to transportation, and finally deposition. Our results indicate that the comminution ages vary from 250 to 600 kyr for the Changjiang sediments and ~ 110 kyr for the Taiwan sediments. Different comminution ages are associated with contrasting erosion and weathering regimes and diverse topography between the large Changjiang catchment and small mountainous Taiwan basins. The longer comminution age of the Changjiang sediment is an interacting effect of a longer erosion/weathering history and sediment trapping effect (and thus slow transfer rate) created by broad floodplains and lakes in the middle and lower reaches. The shorter comminution age of the Taiwan sediment results from fast sediment denudation and transport associated with strong tectonic uplift, typhoon climate and steep topography. As these two major river systems dominate the sedimentology in East Asia continent margin, the distinct geological and topographical settings between the Changjiang and Taiwan river systems result in different sediment “source to sink” transport processes. This work presents a systematic and quantitative constraint on the time-scale of river sediment transfer process in East Asia, and also provides new insight into weathering regimes and sediment transport in monsoon climate-dominated continent and Island.

The last interglacial maximum (Marine Isotope Substage 5e [MIS 5e], 128–116 ka) is a distinctive event in recent Earth history. Shoreline successions of this age are important for calibrating climate models and defining the overall behaviour of the crust–mantle system to fluctuating ice and ocean-water volumes. In a global context, the recently intensified interest in last interglacial shoreline successions has revealed considerable variability in the magnitude of sea-level rise during this time interval and highlighted the need to examine paleosea-level evidence from tectonically stable, far-field settings. Situated in the far-field of continental ice sheets and on the tectonically stable Gawler Craton, the 300 km coastal sector of western Eyre Peninsula between Fowlers Bay and Lake Newland in southern Australia represents an important region for defining the glacio-eustatic (ice-equivalent) sea-level attained during the last interglacial maximum based on the relative sea-level observations from ...

Concentrations of cosmogenic (10)Be, measured in quartz from chert and river sediment around the Cradle of Humankind (CoH), are used to determine basin-averaged erosion rates and estimate incision rates for local river valleys. This study focusses on the catchment area that hosts Malapa cave with Australopithecus sediba, in order to compare regional versus localized erosion rates, and better constrain the timing of cave formation and fossil entrapment. Basin-averaged erosion rates for six sub-catchments draining the CoH show a narrow range (3.00 ± 0.28 to 4.15 ± 0.37 m/Mega-annum [Ma]; ±1σ) regardless of catchment size or underlying geology; e.g. the sub-catchment with Malapa Cave (3 km(2)) underlain by dolomite erodes at the same rate (3.30 ± 0.30 m/Ma) as the upper Skeerpoort River catchment (87 km(2)) underlain by shale, chert and conglomerate (3.23 ± 0.30 m/Ma). Likewise, the Skeerpoort River catchment (147 km(2)) draining the northern CoH erodes at a rate (3.00 ± 0.28 m/Ma) similar to the Bloubank-Crocodile River catchment (627 km(2)) that drains the southern CoH (at 3.62 ± 0.33 to 4.15 ± 0.37 m/Ma). Dolomite- and siliciclastic-dominated catchments erode at similar rates, consistent with physical weathering as the rate controlling process, and a relatively dry climate in more recent times. Erosion resistant chert dykes along the Grootvleispruit River below Malapa yield an incision rate of ∼8 m/Ma at steady-state erosion rates for chert of 0.86 ± 0.54 m/Ma. Results provide better palaeo-depth estimates for Malapa Cave of 7-16 m at the time of deposition of A. sediba. Low basin-averaged erosion rates and concave river profiles indicate that the landscape across the CoH is old, and eroding slowly; i.e. the physical character of the landscape changed little in the last 3-4 Ma, and dolomite was exposed on surface probably well into the Miocene. The apparent absence of early Pliocene- or Miocene-aged cave deposits and fossils in the CoH suggests that caves only started forming from 4 Ma onwards. Therefore, whilst the landscape in the CoH is old, cavities are a relatively young phenomenon, thus controlling the maximum age of fossils that can potentially be preserved in caves in the CoH.

Studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates, but also predict the consequences of future climate change on soil resource availability. Recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion. This work reviews the principles of the application of in situ-produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates. Results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10-kyr scale. At the million-year scale, it is more difficult to untangle the role of climate and tectonics. A strong coupling exists at the 10-kyr to 100-kyr scales between climatic cycles and the transfer time of regolith from source to sink. This coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale, or by the storage of sediments on continental shelves, at a larger scale. Although further analytical developments are required for these tools to reach their full potential, existing works suggest that in the near future, they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations (climatic, tectonic and/or anthropic).

Dougherty, A.J.; Choi, J-H., and Dosseto, A., 2016. Prograded Barriers + GPR + OSL = Insight on coastal change over intermediate spatial and temporal scales. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 368–372. Coconut Creek (Florida), ISSN 0749-0208. Sea level is predicted to rise ∼1m by the next century but the response of sandy shorelines is unknown. Understanding past centennial-scale coastal change is crucial to forecast erosion and prepare vulnerable communities/infrastructure for the impact of climate change. To predict intermediate-scale shoreline behavior, models of short-term morphodynamics along beaches and longer-term coastal landscape evolution are integrated. However, limitations exist as process-based engineering models depend on wave climate and beach profile data restricted to historical records (decadal at best), whi...

Understanding landscape evolution on geological time scales has become increasingly important in the light of recent climate warming and intensified land use. Uranium isotopes ((234U/238U) activity ratios) can be used to reconstruct catchment-wide erosion from fine-grained detrital matter, as the lighter nuclide 234U is lost from grains Loss of 234U is mainly attributed to recoil of 234Th during α-decay of 238U. However, mobilisation of 234U is also controlled by preferential oxidation and leaching of 234U from detrital grains. Further considerations are required concerning the sediment mineralogy and uranium bound to authigenic or endogenic matter. Here, we extend the discussion about the controls on previously published (234U/238U) activity ratios and uranium concentrations of detrital matter along a 5.43 m-long, Late Glacial to Holocene sedimentary record from Lake Ohrid (North Macedonia, Albania). Lake Ohrid is chosen to study how erosion responds to anthropogenic disturbances and climate variability, which are common through the Late Pleistocene and Holocene in the Mediterranean Region. We compared uranium concentration and isotope data to redox-sensitive, mineralogical and biogeochemical proxies. Mineralogical and biogeochemical data show no control on uranium concentration and (234U/238U) activity ratios. Rock magnetic redox proxies indicate uranium mobilisation from detrital matter in more oxic environments. No correlation between (234U/238U) activity ratios and rock magnetic data is observed, which implies that the redox environment has no control on the isotope signal. Monte Carlo simulations reveal that post-depositional preferential mobilisation of 234U has only negligible impact on the calculated sediment residence times. Our model implies that pre-depositional leaching can results in shorter sediment residence times but low (234U/238U) activity ratios during cold and dry intervals imply that recoil is likely the main processes explaining loss of 234U from the detrital grain. This is explained by the time scales studied herein in (>10,000 years), during which preferential leaching is seen to be not significant.

The ridgelines of mountain ranges are a source of geomorphic information unadulterated by the arrival of sediment from upslope. Studies along ridgecrests, therefore, can help identify and isolate the controls on important regolith properties such as thickness and texture. A 1.5 km section of ridgeline in the Sierra Nevada (CA) with a tenfold decrease in erosion rate (inferred from ridgetop convexity) provided an opportunity to conduct a high-resolution survey of regolith properties and investigate their controls. We found that regolith along the most quickly eroding section of the ridge was the rockiest and had the lowest clay concentrations. Furthermore, a general increase in regolith thickness with a slowing of erosion rate was accompanied by an increase in biomass, changes in vegetation community, broader ridgeline profiles, and an apparent increase in total available moisture. The greatest source of variation in regolith thickness at the 10–100 m scale, however, was the local topography along the ridgeline, with the deepest regolith in the saddles and the thinnest on the knobs. Because regolith in the saddles had higher surface soil moisture than the knobs, we conclude that the hydrological conditions primarily driven by local topography (i.e. rapid vs. slow drainage and water-storage potential) provide the fundamental controls on regolith thickness through feedbacks incorporating physical weathering by the biota and chemical weathering. Moreover, because the ridgeline saddles are the uppermost extensions of first-order valleys, we propose that the fluvial network affects regolith properties in the furthest reaches of the watershed

This study was supported by an ARC Discovery project grant DP1093708, and ARC Future Fellowship FT0990447 to AD and a University of Wollongong postgraduate scholarship award to ANM.

Historical patterns of diversity, biogeography and faunal turnover remain poorly understood for Wallacea, the biologically and geologically complex island region between the Asian and Australian continental shelves. A distinctive Quaternary vertebrate fauna containing the small-bodied hominin Homo floresiensis , pygmy Stegodon proboscideans, varanids and giant murids has been described from Flores, but Quaternary faunas are poorly known from most other Lesser Sunda Islands. We report the discovery of extensive new fossil vertebrate collections from Pleistocene and Holocene deposits on Sumba, a large Wallacean island situated less than 50 km south of Flores. A fossil assemblage recovered from a Pleistocene deposit at Lewapaku in the interior highlands of Sumba, which may be close to 1 million years old, contains a series of skeletal elements of a very small Stegodon referable to S. sumbaensis , a tooth attributable to Varanus komodoensis , and fragmentary remains of unidentified giant murids. Holocene cave deposits at Mahaniwa dated to approximately 2000–3500 BP yielded extensive material of two new genera of endemic large-bodied murids, as well as fossils of an extinct frugivorous varanid. This new baseline for reconstructing Wallacean faunal histories reveals that Sumba's Quaternary vertebrate fauna, although phylogenetically distinctive, was comparable in diversity and composition to the Quaternary fauna of Flores, suggesting that similar assemblages may have characterized Quaternary terrestrial ecosystems on many or all of the larger Lesser Sunda Islands.

The Pampas Onduladas flow in southern Mendoza, Argentina, is one of the four longest Quaternary basaltic flows on Earth. Such flows (> 100 km) are relatively rare on Earth as they require special conditions in order to travel long distances and there are no recent analogues. Favourable conditions include: a gentle topographic slope, an insulation process to preserve the melt at high temperature, and a large volume of lava with relatively low viscosity. This study investigates the rheological and geochemical characteristics of the ~ 170 km long Pampas Onduladas flow, assessing conditions that facilitated its exceptional length. The study also reports the first geochronological results for the Pampas Onduladas flow. 40 Ar/ 39 Ar step-heating analyses of groundmass reveal an eruption age of 373 ± 10 ka (2σ), making the Pampas Onduladas flow the oldest Quaternary long flow. The methods used to assess the rheological properties include the application of several GIS tools to a digital elevation model (DEM) to determine the length, width, thickness, volume and topographic slope of the flow as well as algorithms to determine its density, viscosity and temperature. The slope of the Pampas Onduladas flow determined from the initial part of the flow on the eastern side of La Carbonilla Fracture to its end point in the province of La Pampa is 0.84% (0.29°), the steepest substrate amongst long Quaternary flows. The rheological properties, such as density viscosity and temperature from the Pampas Onduladas flow are similar to values reported for other long Quaternary flows. However, the minimum volume calculated is relatively low for its length compared with other long Quaternary flows. Therefore, the extension of the Pampas Onduladas flow was probably controlled by a steep slope, combined with an insulating mechanism, which helped in providing optimal conditions for a travel length of almost 170 km.

The Payenia Basaltic Province (PBP) is located 450 km east of the Chile–Peru trench in central west Argentina, behind the Andean arc front, constituting the back-arc. In order to evaluate the influence of the subducting slab as well as the magmatic source of this region, two volcanic fields located at comparable distance to the trench, having abundant basaltic products and similar eruptive timeframes were chosen. The Llancanelo (LLVF) and the Payun Matru (PMVF) volcanic fields are part of the PBP and exhibit abundant basaltic activity during the Pleistocene. The geochemical data suggest that the LLVF has some arc signatures which have been described as weak as they are not as pronounced as in the Andean arc. The weak arc signature is not derived from slab dehydration as high Th enrichment relative to U cannot be explained by this process. We relate the Th enrichment as well as the lack of large residual garnet signatures, to slab sediments in the source. In the case of the PMVF, no arc signature has been inferred despite being only 30 km south of the LLVF. However the PMVF has a composition similar to that of the local intraplate end member, represented by the Rio Colorado volcanic field. The two volcanic fields, LLVF and PMVF, show indications of lower crustal assimilation as they trend towards the lower continental crust end member in Nb/U vs Ce/Pb and Nb/Yb vs Th/Yb diagrams. The geochemical differences between the LLVF and the PMVF as well as between several volcanic fields are illustrated using spatial distribution maps of geochemical ratios. Using this new approach, the decrease in arc signature can be traced in the back-arc and the higher enrichment in high field strength elements (HFSE) relative to large ion lithophile elements (LILE) in the PMVF compared to the LLVF is explicitly shown. These geospatial maps provide a graphical manner to illustrate the presence of two distinct types of volcanism (OIB-like and arc-like) occurring in the same Quaternary basaltic province.

The uranium-series isotopic compositions of soils and sediments evolve in response to time and weathering conditions. Therefore, these isotopes can be used to constrain the timescales of river sediment transport. Catchment evolution depends on the sediment dynamic timescales, on which erosion imparts a major control. Erosion rates in tectonically stable catchments are expected to be lower than those in tectonically active catchments, implying longer sediment residence times in tectonically stable catchments. Mineralogical, elemental and isotopic data are presented for modern channel sediments, alluvial and colluvial deposits from the Murrumbidgee River, a large catchment in the passive margin highlands of south-eastern Australia and three of its tributaries from the headwaters to the alluvial plain. Low variability in Si-based Weathering Index indicates that there is little chemical weathering occurring in the Murrumbidgee River during sediment transport. However, quartz content increases and plagioclase content decreases downstream, indicating progressive mineralogical sorting and/or physical comminution with increasing transport distance. U-series isotopic ratios in the Murrumbidgee River trunk stream sediments show no systematic downstream variation. The weathering ages of sediments within the catchment were determined using a loss–gain model of U-series isotopes. Modern sediments from a headwater tributary, the Bredbo River at Frogs Hollow, have a weathering age of 76 ± 30 kyr but all other modern channel sediments from the length of the Murrumbidgee River and its main tributaries have weathering ages ∼400 ± 180 kyr. The two headwater colluvial deposits have weathering ages of 57 ± 13 and 47 ± 11 kyr, respectively. All the alluvial deposits have weathering ages similar to those of modern sediments. No downstream trend in weathering age is observed. Together with the soil residence time of up to 30 kyr for ridge-top soils at Frogs Hollow in the upper catchment area of the Murrumbidgee River (Suresh et al., 2013), the current results indicate, for the first time, that sediments in the Murrumbidgee catchment are stored in hill slope for long time (∼200 kyr) before carried by the river. The long residence times of sediments indicate a low erosion rate from the catchment. The sediment transport timescales estimated are up to two orders of magnitude higher than those reported for tectonically active catchments in Iceland (Vigier et al., 2006) and in the Himalayas (Granet et al., 2007), indicating the influence of tectonism on catchment erosion.

The conversion of bedrock to regolith marks the inception of critical zone processes, but the factors that regulate it remain poorly understood. Although the thickness and degree of weathering of regolith are widely thought to be important regulators of the development of regolith and its water-storage potential, the functional relationships between regolith properties and the processes that generate it remain poorly documented. This is due in part to the fact that regolith is difficult to characterize by direct observations over the broad scales needed for process-based understanding of the critical zone. Here we use seismic refraction and resistivity imaging techniques to estimate variations in regolith thickness and porosity across a forested slope and swampy meadow in the Southern Sierra Critical Zone Observatory (SSCZO). Inferred seismic velocities and electrical resistivities image a weathering zone ranging in thickness from 10 to 35 m (average = 23 m) along one intensively studied transect. The inferred weathering zone consists of roughly equal thicknesses of saprolite (P-velocity

Three terrestrial climate proxies are used to investigate the evolution of Holocene palaeoenvironments in southern central Australia, all of which present a coherent record of palaeohydrology. Single-grain optically stimulated luminescence from sediments supplemented by 14C from charcoal and lacustrine shells was obtained to date shoreline deposits (Lake Callabonna) and the adjacent Mt Chambers Creek alluvial fan. Our findings are complemented by a U/Th-based record of speleothem growth in the Mt Chambers Creek catchment, which we interpret to reflect increased precipitation. Together, these archives shed light on the timing of, and possible sources of water for, Holocene pluvial intervals. We identified several phases of elevated lake levels dated at ~5.8–5.2, 4.5, 3.5–2.7 and 1 kyr, most of which correspond to fluvial activity resulting from increased precipitation in the adjacent ranges. The enhanced hydrology during phases of the late Holocene likely increased the reliability of resources for regional human populations during a time of reduced winter rainfall. When considered within the framework of the current understanding of Holocene palaeoclimate in central Australia, our data suggest that the pattern of landscape response was broadly synchronous with larger scale climatic variability and punctuated by pluvial periods greater than today.

The sustainability of soil resources is determined by the balance between the rates of production and removal of soils. Samples from four weathering profiles at Frogs Hollow in the upper catchment area of the Murrumbidgee River (southeastern Australia) were analyzed for their uranium-series (U-series) isotopic composition to estimate soil production rates. Sequential leaching was conducted on sample aliquots to assess how U-series nuclides are distributed between primary and secondary minerals. Soil is increasingly weathered from bottom to top which is evident from the decrease in ( 234 U/ 238 U) ratios and increase in relative quartz content with decreasing soil depth. One soil profile shows little variation in mineralogy and U-series geochemistry with depth, explained by the occurrence of already extensively weathered saprolite, so that further weathering has minimal effect on mineralogy and geochemistry. Al 2 O 3 is mobilized from these soils, and hence a silicon-based weathering index treating Al 2 O 3 as mobile is introduced, which increases with decreasing soil depth, in all profiles. Leached and unleached aliquots show similar mineralogy with slight variation in relative concentrations, whereas the elemental and isotopic composition of uranium and thorium show notable differences between leached and unleached samples. Unleached samples show systematic variations in uranium-series isotopic compositions with depth compared to leached samples. This is most likely explained by the mobilization of U and Th from the samples during leaching. Soil residence times are calculated by modeling U-series activity ratios for each profile separately. Inferred timescales vary up to 30 kyr for unleached aliquots from profile F1 to up to 12 kyr for both leached and unleached aliquots from profile F2. Muscovite content shows a linear relationship with U-series derived soil residence times. This relationship provides an alternative method to estimate residence timescales for profiles with significant U-series data scatter. Using this alternative approach, inferred soil residence times up to 33 kyr for leached samples of profile F1 and up to 34 kyr for leached samples of profile F3 were determined. A linear relationship between soil residence times and WIS (Si-based Weathering Index) exists and is used to estimate soil residence times for profile F3 (up to 28 kyr) and F4 (up to 37 kyr). The linear relationship between soil depth and calculated residence time allows determination of soil production rates, which range from 10 to 24 mm/kyr and are comparable to the rates determined previously using cosmogenic isotopes at the same site ( Heimsath et al., 2001b ). This implies that at this site, on the highland plateau of southeastern Australia, soil thickness has reached steady-state, possibly as a result of stable tectonic conditions but despite variable climatic conditions over the timescale of soil development. Soil-mantled landscapes are the geomorphic expression of this balance between soil production and denudation, and our results show that in tectonically quiescent regions, this landscape can be achieved in less than 30 kyr.

The Australian region spans some 60° of latitude and 50° of longitude and displays considerable regional climate variability both today and during the Late Quaternary. A synthesis of marine and terrestrial climate records, combining findings from the Southern Ocean, temperate, tropical and arid zones, identifies a complex response of climate proxies to a background of changing boundary conditions over the last 35,000 years. Climate drivers include the seasonal timing of insolation, greenhouse gas content of the atmosphere, sea level rise and ocean and atmospheric circulation changes. Our compilation finds few climatic events that could be used to construct a climate event stratigraphy for the entire region, limiting the usefulness of this approach. Instead we have taken a spatial approach, looking to discern the patterns of change across the continent. The data identify the clearest and most synchronous climatic response at the time of the Last Glacial Maximum (LGM) (21 ± 3 ka), with unambiguous cooling recorded in the ocean, and evidence of glaciation in the highlands of tropical New Guinea, southeast Australia and Tasmania. Many terrestrial records suggest drier conditions, but with the timing of inferred snowmelt, and changes to the rainfall/runoff relationships, driving higher river discharge at the LGM. In contrast, the deglaciation is a time of considerable south-east to north-west variation across the region. Warming was underway in all regions by 17 ka. Post-glacial sea level rise and its associated regional impacts have played an important role in determining the magnitude and timing of climate response in the north-west of the continent in contrast to the southern latitudes. No evidence for cooling during the Younger Dryas chronozone is evident in the region, but the Antarctic cold reversal clearly occurs south of Australia. The Holocene period is a time of considerable climate variability associated with an intense monsoon in the tropics early in the Holocene, giving way to a weakened monsoon and an increasingly El Niño-dominated ENSO to the present. The influence of ENSO is evident throughout the southeast of Australia, but not the southwest. This climate history provides a template from which to assess the regionality of climate events across Australia and make comparisons beyond our region.

Uranium-series isotopes can be used to determine constraints on the timescale of slab dehydration and melt production at subduction zones. However, interpretations of U–Th–Ra data suggest very different timescales of slab dehydration. Here, we present new U–Th–Ra data from Kamchatka along with a number of alternative models for production of radioactive disequilibrium. Variations in (226Ra/230Th) and (231Pa/235U) activity ratios are best explained by crystal fractionation with host rock assimilation for a duration of less than c. 6000 years. The association of the largest 226Ra excesses with high Sr/Th in the most primitive lavas suggests that Ra–Th fractionation is controlled by slab dehydration less than 10 ka ago. We show that U–Th data can be explained by dynamic melting of a recently ( ka) metasomatized mantle wedge. Dynamic melting of an oxidized source metasomatized several hundreds of thousands of years ago cannot produce significant 231Pa excess. Because 238U–230Th disequilibrium is inferred to be controlled by partial melting, there is no requirement for multi-stage slab dehydration commencing ∼150 ka. We suggest that Ra–Th disequilibria constrain the timing of slab dehydration, whilst U–Th fractionation is dominated by partial melting, at least at the Kamchatka arc.

Quantifying the rates of landscape evolution in response to climate change is inhibited by the difficulty of dating the formation of continental detrital sediments. We present uranium isotope data for Cooper Creek palaeochannel sediments from the Lake Eyre Basin in semi-arid South Australia in order to attempt to determine the formation ages and hence residence times of the sediments. To calculate the amount of recoil loss of 234U, a key input parameter used in the comminution approach, we use two suggested methods (weighted geometric and surface area measurement with an incorporated fractal correction) and typical assumed input parameter values found in the literature. The calculated recoil loss factors and comminution ages are highly dependent on the method of recoil loss factor determination used and the chosen assumptions. To appraise the ramifications of the assumptions inherent in the comminution age approach and determine individual and combined comminution age uncertainties associated to each variable, Monte Carlo simulations were conducted for a synthetic sediment sample. Using a reasonable associated uncertainty for each input factor and including variations in the source rock and measured (234U/238U) ratios, the total combined uncertainty on comminution age in our simulation (for both methods of recoil loss factor estimation) can amount to ±220–280 ka. The modelling shows that small changes in assumed input values translate into large effects on absolute comminution age. To improve the accuracy of the technique and provide meaningful absolute comminution ages, much tighter constraints are required on the assumptions for input factors such as the fraction of α-recoil lost 234Th and the initial (234U/238U) ratio of the source material. In order to be able to directly compare calculated comminution ages produced by different research groups, the standardisation of pre-treatment procedures, recoil loss factor estimation and assumed input parameter values is required. We suggest a set of input parameter values for such a purpose. Additional considerations for calculating comminution ages of sediments deposited within large, semi-arid drainage basins are discussed.

Uranium isotope ratios have been determined for the fine-grained detrital fraction of Pleistocene Wilkawillina valley-fill sediments, four local Proterozoic bedrock samples and fine-grained aeolian material from a sand dune deposit of the Flinders Ranges, South Australia. The aim was to quantify the comminution age, i.e. the time elapsed since physical weathering of the bedrock, and residence time of the valley-fill sediments and to place tighter constraints on input parameters for the comminution age calculation. Despite using two independent approaches for determination of the recoil lost fraction of 234 U from the sediment (weighted geometric and surface area estimates), samples fail to produce realistic comminution ages and hence, residence times. The issues involved in the ability to determine sediment comminution ages are discussed. The ( 234 U/ 238 U) activity ratio of the local bedrock is not in secular equilibrium, despite the bedrock being much older than 1 Ma, i.e. the timeframe for 234 U and 238 U to reach secular equilibrium in a closed system. Using the average Flinders Ranges bedrock ( 234 U/ 238 U) ratio instead of an assumed ( 234 U/ 238 U) activity ratio of unity for the source would significantly reduce calculated residence times. This result warrants concern for future studies using the comminution approach for which a secular equilibrium source ( 234 U/ 238 U) activity ratio is assumed. Significant input of aeolian material may modify the measured ( 234 U/ 238 U) activity ratios. Such input may be more tightly constrained in future studies using rare earth element and radiogenic isotopic data. Future comminution studies would benefit from further consideration of the importance of 1) leaching lost 234 U from source rock and bulk sediment samples, 2) wind deposition of fine-grained material and 3) the appropriateness and robustness of sample pre-treatment procedures.

A 2m-thick spheroidal weathering profile, developed on a quartz-diorite in the Rio Icacos watershed (Luquillo Mountains, eastern Puerto Rico), was analysed for major and trace element concentrations, Sr and Nd isotopic ratios and U-series nuclides (238U-234U-230Th-226Ra). In this profile a 40 cm thick soil horizon is overlying a 150 cm thick saprolite which is separated from the basal corestone by a ∼40 cm thick rindlet zone. The Sr and Nd isotopic variations along the whole profile imply that, in addition to geochemical fractionations associated to water-rock interactions, the geochemical budget of the profile is influenced by a significant accretion of atmospheric dusts. The mineralogical and geochemical variations along the profile also confirm that the weathering front does not progress continuously from the top to the base of the profile. The upper part of the profile is probably associated with a different weathering system (lateral weathering of upper corestones) than the lower part, which consists of the basal corestone, the associated rindlet system and the saprolite in contact with these rindlets. Consequently, the determination of weathering rates from 238U-234U-230Th-226Ra disequilibrium in a series of samples collected along a vertical depth profile can only be attempted for samples collected in the lower part of the profile, i.e. the rindlet zone and the lower saprolite. Similar propagation rates were derived for the rindlet system and the saprolite by using classical models involving loss and gain processes for all nuclides to interpret the variation of U-series nuclides in the rindlet-saprolite subsystem. The consistency of these weathering rates with average weathering and erosion rates derived via other methods for the whole watershed provides a new and independent argument that, in the Rio Icacos watershed, the weathering system has reached a geomorphologic steady state. Our study also indicates that even in environments with differential weathering, such as observed for the Puerto Rico site, the radioactive disequilibrium between the nuclides of a single radioactive series (here 238U-234U-230Th-226Ra) can still be interpreted in terms of a simplified scenario of homogeneous weathering. Our work also highlights the viability of coupling 230Th-234U and 226Ra-230Th disequilibrium to determine weathering rates in a differential weathering context. Incidentally, the U-Th-Ra disequilibrium in the corestone samples confirms that the outermost part of the corestone is already weathered.

The Payun Matru Volcanic Field constitutes part of the continental back-arc in Argentina. This volcanic field has been the focus of several regional investigations; however, geochemical analysis of recent volcanoes (

The ability to quantify how fast weathering profiles develop is crucial to assessing soil resource depletion and quantifying how landscapes evolve over millennia. Uranium-series isotopes can be used to determine the age of the weathering front throughout a profile and to infer estimates of regolith production rates, because the abundance of U-series isotopes in a weathering profile is a function of chemical weathering and time. This technique is applied to a weathering profile in Puerto Rico developed over a volcaniclastic bedrock. U-series isotope compositions are modelled, revealing that it takes 40–60 kyr to develop an 18 m-thick profile. This is used to estimate an average regolith production rate of 334±46 mm/kyr. This value is higher by a factor of up to 30 when compared to production rates estimated for weathering profiles developed over granitic or shale lithologies. This quantitatively underpins the lithological control on rates of regolith production (in a neighbouring watershed but over a granitic bedrock, production rates are only ∼30–40 mm/kyr). Moreover, by comparing these results to a compilation of soil erosion rates, it is clear that landscapes are controlled by the balance (or imbalance) between regolith production and erosion: soil-mantled landscapes are the result of a relative balance between production and erosion, whereas in cratonic areas, thicker weathering profiles are generated because erosion fails to match regolith production rates.

As climate is changing rapidly, there is an increasing need to understand how water and soil resources respond to climate change. Soil and sediment dynamics are sensitive to several external factors such as climate, vegetation type and distribution, human activity, and tectonic activity. However, the relationship between erosion and changes in these factors is difficult to constrain with current available approaches. Here we show that uranium isotopes in sediments from river paleochannels can be used to reconstruct variations in the residence time of sediments in a catchment over the past 100 k.y. We find that sediment residence times increase by an order of magnitude during interglacials compared to glacial periods. This is interpreted as a change in sediment stores in the landscape that are tapped by catchment erosion: young, upland soils during glacial periods, reworking of old alluvial sediments during interglacials. A direct correlation is found between the sediment residence time and climatic parameters (sea-surface temperature, atmospheric carbon dioxide content, and paleorainfall estimates), suggesting that during a glacial cycle, sediment dynamics closely follow variations in climate. However, this relationship is not simple because there is no correlation between sediment residence time and paleodischarge estimates. Because sediment residence time variations correlate with changes in vegetation inferred from pollen data, it is hypothesized that the influence of climate on erosion over a glacial cycle may be indirect, and operates via the influence of climate on the type of plant ecosystems within a catchment. If verified elsewhere, this conclusion would emphasize the important role of biology in the physical evolution of Earth's surface, here observed over a 100 k.y. time scale.

Determining the timing and source of gas transfer during intermittent intracaldera volcanism can aid in our understanding of degassing in these large systems. Using ( 210 Pb/ 226 Ra) ratios, (parentheses denote activity ratios) as a time-sensitive tracer, injections of 222 Rn and the subsequent time scales of gas accumulation and loss can be determined. Variations in ( 210 Pb/ 226 Ra) have been measured for 15 volcanic products erupted at Rabaul Caldera over the period 1994 to 2001. In addition, one basaltic enclave from the 1937 eruption was also analyzed. Water and carbon dioxide contents determined from olivine hosted melt inclusions erupted in 1997 are 210 Pb excesses and deficits are found in andesites and dacites, whereas the basaltic enclave displays an ( 210 Pb/ 226 Ra) 0 ratio of 7. Between 1994 and 1997 three samples with ( 210 Pb/ 226 Ra) deficits were erupted which indicate open system gas loss since 1992 and 1994. No correlation exists between ( 210 Pb/ 226 Ra) and lava chemistry, eruptive style or date. 210 Pb excesses are more common than deficits in Rabaul samples but cannot be explained by plagioclase feldspar accumulation, Pb sublimate accumulation or differentiation. Instead, a model of intra-magma 222 Rn transfer can produce 210 Pb excesses of the appropriate magnitude if gas transfer occurs over 1–5 years from an underlying body of magma that is 2–10 times larger than the volume of erupted material and that is consistent with geophysical estimates. Although intermittent gas transfer events can be inferred by the development of 210 Pb excess, there is no evidence at Rabaul for a direct link between eruptive style, gas flux and ( 210 Pb/ 226 Ra).

Intra-caldera eruptions have been speculated to sample the last batches of magma remaining from earlier caldera-forming eruptions. Rabaul Caldera, New Britain, Papua New Guinea has erupted several times since the last caldera-forming eruption in AD 640, with the most recent intra-caldera eruptions in 1878, 1937^1941 and 1994^ present from the Tavurvur and Vulcan vents. U-series isotopes, in conjunction with Sr/Sr and Nd/Nd, were analyzed on 16 samples collected from 1994 to 2001 to monitor short-term changes in magma composition to model magmatic processes and to test whether there is evidence of recent fresh magma input. Inflections on MgO diagrams imply that fractional crystallization is an important process in long-term magma evolution, and the homogeneity in Sr/Sr and Nd/Nd shows that assimilation of isotopically distinct material has not occurred. A vertical array on a Th^U isochron diagram requires open-system behavior and could support a model of differentiation of multiple magma batches over 26 kyr. However, the presence of (Ra/Th) excesses requires introduction of new magma within the past 8000 years and is permissible of a model in which the currently erupting magmas were emplaced at or since the last caldera-forming event. Other than the presence of mafic enclaves in the 1878 and 1937 eruptions, no evidence exists to suggest open-system magma injection. Systematic variation in U-series disequilibria between 1994 and 2001 is lacking, which may indicate that the system is broadly in steady state or that the processes acting to produce the limited compositional variation have time scales that are too short to be resolved by Ra isotopes (i.e. are less than a few hundred years).

The sustainability of soil is a major issue for society. In principle, the evolution of soil resources can be constrained by comparing the rates of soil erosion and production. Cosmogenic isotopes provide one measurement of soil erosion rates. They can also be used to estimate soil production rates but only if erosion is assumed to be balanced by production. This implies that the evolution of soil resources (thinning, thickening or constant) since soil thickness is assumed to be constant with time. Here we utilise an independent method to estimate soil production rates, using uranium-series (U-series) isotopes. The study of a site in temperate Australia undisturbed by human activity shows that soil production rates inferred from U-series isotopes are similar to erosion rates derived from beryllium-10 (10Be) measurements, implying that at this site there is no net accumulation or loss of soil. Saprolite production rates (the migration rate of the weathering front into the bedrock) are also similar to erosion rates so the thickness of the entire weathering profile is effectively in steady-state. This study demonstrates that the combination of U-series and cosmogenic isotopes can be used to quantitatively assess soil evolution and the development of weathering profiles. Preliminary observations suggest that the rate of bedrock weathering (i.e. saprolite production) in temperate Australia is of the same order of magnitude as that inferred for laterites in tropical climates. This may suggest that, for thick weathering profiles, although the extent of weathering strongly differs between temperate and tropical climates, the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present-day climatic conditions.

The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism. We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (