Anthony Dosseto

Amphibole “sponge” in arc crust?

Pressure-temperature-time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes, and hence the compositions of liquid fractionates. Here we use La/Yb and Dy/Yb versus SiO 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma. Production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid-lower crust, where amphibole is stable. Because amphibole is rarely a phenocryst phase, we term this “cryptic amphibole fractionation.” If this process is as widespread as our investigation suggests, then (1) amphibole cumulates may act as an effective filter for water dissolved in mantle-derived magmas; (2) amphibole cumulates may act as a fertile source of intracrustal melts and fluids; and (3) recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle.

U-Th-Pa-Ra study of the Kamchatka arc : New constraints on the genesis of arc lavas

Abstract The 238U-230Th-226Ra and 235U-231Pa disequilibria have been measured by mass spectrometry in historic lavas from the Kamchatka arc. The samples come from three closely located volcanoes in the Central Kamchatka Depression (CKD), the most active region of subducted-related volcanism in the world. The large excesses of 226Ra over 230Th found in the CKD lavas are believed to be linked to slab dehydration. Moreover, the samples show the uncommon feature of (230Th/238U) activity ratios both lower and higher than 1. The U-series disequilibria are characterized by binary trends between activity ratios, with (231Pa/235U) ratios all >1. It is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting. We suggest that they are more likely to reflect mixing between two end-members: a high-magnesia basalt (HMB) end-member with a clear slab fluid signature and a high-alumina andesite (HAA) end-member reflecting the contribution of a slab-derived melt. The U-Th-Ra characteristics of the HMB end-member can be explained either by a two-step fluid addition with a time lag of 150 ka between each event or by continuous dehydration. The inferred composition for the dehydrating slab is a phengite-bearing eclogite. Equilibrium transport or dynamic melting can both account for 231Pa excess over 235U in HMB end-member. Nevertheless, dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid-derived 226Ra/230Th. A continuous flux melting model is also tested. In this model, 231Pa-235U is quickly dominated by fluid addition and, for realistic extents of melting, this process cannot account for (231Pa/235U) ratios as high as 1.6, as observed in the HMB end-member. The involvement of a melt derived from the subducted oceanic crust is more likely for explaining the HAA end-member compositions than crustal assimilation. Melting of the oceanic crust is believed to occur in presence of residual phengite and rutile, resulting in no 226Ra-230Th disequilibrium and low 231Pa excess over 235U in the high-alumina andesites. Consequently, it appears that high-alumina andesites and high-magnesia basalts have distinct origins: the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle. It seems that there is no genetic link between these two magma types, in contrast with what was previously believed.

Revised stratigraphy and chronology for Homo floresiensis at Liang Bua in Indonesia

Homo floresiensis, a primitive hominin species discovered in Late Pleistocene sediments at Liang Bua (Flores, Indonesia), has generated wide interest and scientific debate. A major reason this taxon is controversial is because the H. floresiensis-bearing deposits, which include associated stone artefacts and remains of other extinct endemic fauna, were dated to between about 95 and 12 thousand calendar years (kyr) ago. These ages suggested that H. floresiensis survived until long after modern humans reached Australia by ~50 kyr ago. Here we report new stratigraphic and chronological evidence from Liang Bua that does not support the ages inferred previously for the H. floresiensis holotype (LB1), ~18 thousand calibrated radiocarbon years before present (kyr cal. bp), or the time of last appearance of this species (about 17 or 13–11 kyr cal. bp). Instead, the skeletal remains of H. floresiensis and the deposits containing them are dated to between about 100 and 60 kyr ago, whereas stone artefacts attributable to this species range from about 190 to 50 kyr in age. Whether H. floresiensis survived after 50 kyr ago—potentially encountering modern humans on Flores or other hominins dispersing through southeast Asia, such as Denisovans—is an open question.

Climatic and vegetation control on sediment dynamics during the last glacial cycle

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 Earths surface, here observed over a 100 k.y. time scale.

The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa

New ages for flowstone, sediments and fossil bones from the Dinaledi Chamber are presented. We combined optically stimulated luminescence dating of sediments with U-Th and palaeomagnetic analyses of flowstones to establish that all sediments containing Homo naledi fossils can be allocated to a single stratigraphic entity (sub-unit 3b), interpreted to be deposited between 236 ka and 414 ka. This result has been confirmed independently by dating three H. naledi teeth with combined U-series and electron spin resonance (US-ESR) dating. Two dating scenarios for the fossils were tested by varying the assumed levels of 222Rn loss in the encasing sediments: a maximum age scenario provides an average age for the two least altered fossil teeth of 253 +82/–70 ka, whilst a minimum age scenario yields an average age of 200 +70/–61 ka. We consider the maximum age scenario to more closely reflect conditions in the cave, and therefore, the true age of the fossils. By combining the US-ESR maximum age estimate obtained from the teeth, with the U-Th age for the oldest flowstone overlying Homo naledi fossils, we have constrained the depositional age of Homo naledi to a period between 236 ka and 335 ka. These age results demonstrate that a morphologically primitive hominin, Homo naledi, survived into the later parts of the Pleistocene in Africa, and indicate a much younger age for the Homo naledi fossils than have previously been hypothesized based on their morphology. DOI: http://dx.doi.org/10.7554/eLife.24231.001

Continuous adsorption and biotransformation of micropollutants by granular activated carbon-bound laccase in a packed-bed enzyme reactor

Laccase was immobilized on granular activated carbon (GAC) and the resulting GAC-bound laccase was used to degrade four micropollutants in a packed-bed column. Compared to the free enzyme, the immobilized laccase showed high residual activities over a broad range of pH and temperature. The GAC-bound laccase efficiently removed four micropollutants, namely, sulfamethoxazole, carbamazepine, diclofenac and bisphenol A, commonly detected in raw wastewater and wastewater-impacted water sources. Mass balance analysis showed that these micropollutants were enzymatically degraded following adsorption onto GAC. Higher degradation efficiency of micropollutants by the immobilized compared to free laccase was possibly due to better electron transfer between laccase and substrate molecules once they have adsorbed onto the GAC surface. Results here highlight the complementary effects of adsorption and enzymatic degradation on micropollutant removal by GAC-bound laccase. Indeed laccase-immobilized GAC outperformed regular GAC during continuous operation of packed-bed columns over two months (a throughput of 12,000 bed volumes).

Late-Holocene climatic variability indicated by three natural archives in arid southern Australia:

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.

Assessment of a sequential phase extraction procedure for uranium-series isotope analysis of soils and sediments

The study of uranium-series (U-series) isotopes in soil and sediment materials has been proposed to quantify rates and timescales of soil production and sediment transport. Previous works have studied bulk soil or sediment material, which is a complex assemblage of primary and secondary minerals and organic compounds. However, the approach relies on the fractionation between U-series isotopes in primary minerals since they were liberated from the parent rock via weathering. In addition, secondary minerals and organic compounds have their own isotopic compositions such that the composition of the bulk material may not reflect that of primary minerals. Hence, there is a need for a sample preparation procedure that allows the isolation of primary minerals in soil or fluvial sediment samples. In this study, a sequential extraction procedure to separate primary minerals from soils and sediments was assessed. The procedure was applied to standard rock sample powders (TML-3 and BCR-2) to test whether it introduced any artefactual radioactive disequilibrium. A new step was introduced to remove the clay-sized fraction (<2 µm). Significant amounts (5-14%) of U and Th were removed from the rock standards during the procedure. No significant alteration in ((234)U/(238)U) and ((230)Th/(238)U) activity ratios of the rock standards occurred during the procedure. Aliquots of soil sample were subjected to the sequential extraction process to test how each step modifies the uranium-series activity ratios and mineralogy. Although no secondary minerals were detected in the unleached soil aliquots, the sequential leaching process removed up to 17% of U and Th and modified their activity ratios by up to 3%. The modification of the activity ratios poses a demand for careful means to avoid redistribution of isotopes back to the residual phase during phase extraction.

Last interglacial (MIS 5e) sea-level determined from a tectonically stable, far-field location, Eyre Peninsula, southern Australia

ABSTRACT 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 this region. Low-energy, shoaling upward, peritidal bioclastic carbonate successions of the last interglacial (locally termed Glanville Formation) formed within back-barrier, estuarine–lagoonal environments in the lee of eolianite barrier complexes (locally termed Bridgewater Formation) along this coastline. The well-preserved shelly successions (coquinas) contain diverse molluscan fossil assemblages including species no longer living in the coastal waters of South Australia (e.g. the Sydney cockle Anadara trapezia and the benthic foraminifer Marginopora vertebralis). The extent of amino acid racemisation (a measure of fossil age based on increasing d/l value) in a range of species, and in particular A. trapezia and Katelysia sp., confirms the time equivalence of the isolated embayment-fill successions, correlated with the informal type section of the Glanville Formation at Dry Creek, north of Adelaide. Preliminary U-series analyses on A. trapezia also suggest a correlation with the last interglacial maximum, but further highlight the complexity in dating fossil molluscs by the U-series method in view of their open-system behaviour. The shelly successions of the Glanville Formation occur at elevations higher than attained by sea-level in the current, Holocene interglacial. A higher sea-level of between 2.1 ± 0.5 and 4 ± 0.5 m above present sea-level is inferred for the last interglacial maximum (MIS 5e) along this coastline based on the elevation of sedimentary successions host to the shallow subtidal–intertidal fossil molluscs Katelysia sp., and Anadara trapezia. The paleosea-level observations place a lower limit on the sea-level attained during the last interglacial maximum and suggest that caution be exercised in the definition of the upper limit of sea-level during this interglacial.

Uranium-series isotope and thermal constraints on the rate and depth of silicic magma genesis

Abstract Uranium-series isotopes provide important constraints on the timescale of magma differentiation and this can be used to identify where in the crust and silicic magmas acquire their geochemical characteristics. Timescales of differentiation can be inferred from the observed co-variations of U-series disequilibria with differentiation indexes. When crustal assimilation of secular equilibrium material is involved, inferred timescales will generally decrease. In turn, they will increase if periodical recharge (>20 wt% relative volume) of the magma body occurs. If crustal assimilation and magma recharge occur concurrently, inferred timescales for differentiation can be similar to that of closed system differentiation. We illustrate the approach with data from Mount St Helens which suggest that dacitic compositions are produced in c. 2000 years. Combining this with recent evidence for an important role for amphibole fractionation suggests that differentiation of a c. 10 km3 magma body at this volcanic centre occurs at 8–10 km depth in the crust.