Geochemical constraints on the role of tuffisite veins in degassing at the 2008–09 Chaitén and 2011–12 Cordón Caulle rhyolite eruptions

Hybrid activity during the rhyolitic eruptions of Chaitén (2008-09) and Cordón Caulle (2011-2012) in Chile has offered unprecedented insights into the enigmatic and complex degassing processes occurring during eruptions of silicic magma. Highly permeable, transient fracture networks within the conduit can act as outgassing channels. Their interaction with deeper volatile-rich melt can account for both punctuated explosive activity and large-scale degassing of the system, leading towards predominantly effusive behaviour. In this study we characterise trace element concentrations and 210Pb-226Ra systematics within pyroclastic material from the recent eruptions at Chaitén and Cordón Caulle volcanoes. Results reveal how gas fluxing from deep, volatile-rich reservoirs to the surface, within magmatic conduits, can be recorded by trace elements and 210Pb-226Ra disequilibria in tuffisite veins. Tuffisite veins (particle-filled fracture networks) are present in volcanic bombs from both eruptions. Trace element heterogeneity associated with tuffisites preserves evidence for degassing. At Chaitén, enrichments (e.g. Cu) and depletions (e.g. Mo, Li and Bi) are identified in vein material and clasts transported within veins, and record multiple degassing events. At Cordón Caulle, enrichments of volatiles in an early vein (e.g. Tl and Bi) and depletions in a later vein (e.g. Cd, In, Pb and Tl) reflect interactions between glassy clasts and the carrier gas phase that transported them. In contrast, 210Pb and 226Ra, which can be fractionated during degassing, are mostly in secular equilibrium. Modelling suggests that the disparity between the signals preserved in these two types of chemical signatures reflects the brevity of degassing events and the relative volumes of tuffisite veins and the bodies of degassing magma that they source gas from. The lack of preserved 210Pb enrichments in tuffisite veins at both volcanoes places an upper limit on the mass of deeper, bubble-rich magma outgassed via tuffisites during their lifetime. This study shows that both the presence, and absence, of sample-scale geochemical heterogeneity can be used to place constraints on syn-eruptive physical processes and underlines the value of analysing a wide suite of trace element species

Studying the past of Mediterranean outflow based on 230th excess inventories and contourites

The Mediterranean Outflow water (MOW) comes out from the Mediterranean Sea and then contours the northern slope of the Cadiz Gulf. Along its way to the southern Portuguese Margin, it divides itself into three levels flowing at different depths, 400 m, 800 m and 1200 m, respectively. These different pathways induce a series of contourites along the Cadiz slope as well as some sedimentary drifts, such as the Faro Drift. Based on the assumption that the sedimentologic characteristics of these contourites should give some light on the history of MOW velocity and intensity variability, two long sedimentary cores collected during the Marion Dufresnes 114/Images cruise in 1999 have been studied. The sampling sites of these two cores, MD99-2336 and MD99-2339, located in the Cadiz Gulf at 690 and 1177 m water column depths respectively, are thus, actually, below the first level and in the main core of the MOW third level. Along time, variations in these current levels, parallel to the slope, should then influence the existence and characteristics of contourites in both sedimentary records. For this purpose, thorium-230 (230Th) as well as granulometric and micropaleontologic analysis have been undergone at high resolution on the 4 uppermost meters spanning MIS1 to LGM times. The referred current prints can be detected by analysing surface and down core sediment for its 230Th content. This radioisotope is produced by the radioactive decay of uranium-234 which content in oceanic waters is known. Therefore, its production rate in the water column can be estimated as a linear function of the water depth (~ 2.6 dpm/cm2.ka for 1 km water depth). As 230Th is almost insoluble, it will sink to the oceanic floor together with the settling particles. This vertical flux to the underlying sediment is considered, in a first order approximation, equal to its production rate in the water column. On this basis, the 230Th excess in the sediment becomes a proxy for sedimentation versus erosion processes accordingly to the sign of the difference between the total and the vertical 230Th flux, i.e. if it is, respectively, positive or negative. With this method it is then possible to extrapolate on the location of the high velocity core area and whether its intensity changed or not looking at the inventory of excess 230Th in the contourite units. We acknowledge FEDER and OE that financed this study through the Portuguese Foundation for Science and Technology (PDCTM/PP/MAR/15297/1999)

MIS 5e at San Giovanni di Sinis (Sardinia, Italy): Stratigraphy, U/Th dating and "eustatic" inferences

AbstractThe most representative Late Pleistocene ("Tyrrhenian") outcrop in Sardinia (San Giovanni di Sinis) was re-visited based on detailed sedimentological and stratigraphical analysis supported by U-series dating of fossil corals. The stratigraphy shows shoreface–backshore sandstones overlying an erosional surface cut on vertebrate-bearing layers. Facies analysis and sequence-stratigraphic approaches suggest "eustatic" changes during the overall depositional interval of the marine sequence. Disconformities distinguish two coastal units suggesting a lateral shifts of the depositional environments driven by millennial-scale frequency sea level fluctuations, never exceeding a maximum height +1 to +3 m and by a subsequent rise to +5 ÷ +5.5 m asl.These deposits have historically been assigned to MIS 5, on the basis of i) their palaeontological content (warm "Senegalese" fauna), ii) lithostratigraphic interpretations and iii) amino-acid, geochronological data and generally attributed to the high sea-level episode of MIS 5e sub-stage, through sequential interpretation. U-series measurements in colonies of Cladocora caespitosa from the main outcrop of San Giovanni di Sinis and other minor deposits from the eastern side of the isthmus and from Capo San Marco, yielded 230Th-ages confirming an assignment of the embedding sediments to MIS 5e.Similar measurements in a pelecypod valve (Cerastoderma sp.) and a vermetid colony at San Marco yielded much younger apparent ages, tentatively attributed to late "diagenetic" U-uptake processes. Both the first minor pulses and the most important "eustatic" pulse recorded at San Giovanni di Sinis cannot be assigned unequivocally to one of the MIS 5e sea-level oscillations proposed in the recent literature, due to dating inaccuracies and uncertainties, but it seems probable that it occurred during an early part of the interval

Évolution récente du delta de la Yamachiche (Québec) : processus naturels et impacts anthropiques

Le lac Saint-Pierre, un élargissement du fleuve Saint-Laurent, pourrait connaître une baisse importante de niveau d’eau dans les cinquante prochaines années, conséquence du réchauffement climatique global. Sept rivières débouchent dans ce lac fluvial peu profond qui risquerait d’enregistrer une sédimentation importante. Nous avons étudié le delta de l’un de ces tributaires, la Yamachiche, pour évaluer les processus de sédimentation et les réponses de la rivière aux changements environnementaux passés. L’évolution du delta de la Yamachiche a été reconstituée par des photographies aériennes, des cartes anciennes, des analyses sédimentologiques et stratigraphiques de six coupes et de quatorze forages, et la datation par luminescence optique, 210Pb et 14C. Le delta est caractérisé par deux types de dépôts. La base est constituée de dépôts sableux de lit mineur du chenal, mis en place dans des conditions de sédimentation et d’érosion fréquentes. Ils sont surmontés par des dépôts limoneux de plaine d’inondation deltaïque, comprenant des lits sableux locaux. Ces dépôts présentent une variabilité latérale et longitudinale expliquée par les variations de niveau d’eau du lac et de la rivière à une échelle annuelle et décennale, par les migrations du chenal et par l’action érosive des vagues du lac Saint-Pierre. Le delta s’est formé depuis 150 ans, comme le montre la carte de 1859 où le delta n’était que très peu développé. La luminescence optique attribue des âges de 140 et de 280 ans aux dépôts sableux à la base. Les datations révèlent des taux d’accumulation verticale moyens compris entre 0,5 et 1,5 cm/an. Cette accumulation importante serait liée aux perturbations anthropiques affectant les sources sédimentaires du bassin versant depuis le début de la colonisation il y a 200 ans. L’âge récent du delta serait également expliqué par la stabilisation tardive du lac Saint-Pierre à son niveau actuel, qui est intervenue entre 1000 et 150 ans.Lake St. Pierre, being an enlargement of the St. Lawrence River, could experience a significant base level drop in the next fifty years as a consequence of global climate warming. Seven tributaries flow into this shallow lake, and base level changes could lead to increased sedimentation rates. We have studied the delta of the Yamachiche River, a tributary of Lake St. Pierre. The objective of the study is to document the active geomorphic processes and the response of the river to past environmental changes. We have reconstructed the evolution of the Yamachiche delta from aerial photos, historical maps, sedimentological and stratigraphic analysis of six sections and fourteen boreholes, and by sediment dating using optically stimulated luminescence, 210Pb and 14C. The subsurface deposits are divided into two facies : sandy low-water channel deposits at the base, and silty delta plain deposits at the top. Sedimentary sequences show changes along a longitudinal gradient driven by the level of the lake and the river dynamics on annual and decadal scales, and a high lateral variability driven by migration of the channel and by high-energy waves from Lake St. Pierre, which erode the downstream eastern part of the delta plain. Rapid progradation of the delta has occurred in the last 150 years, as demonstrated by the poor development of the delta on the 1859 map. Optically stimulated luminescence dating provides ages of 140 and 280 years at the base of the depositional sequence. These results are consistent with the chronological sequence deduced from the other dating methods. This framework gives mean accumulation rates between 0.5 and 1.5 cm/year. These high rates of sedimentation are likely linked to human activities in the watershed, which affected sedimentary sources since colonization of the St. Lawrence Lowlands 200 years ago. The young age of the delta could also be explained by the late stabilization of Lake St. Pierre at its present level, between 1000 and 150 years ago

Potential and limitation of 230Th-excess as a chronostratigraphic tool for late Quaternary Arctic Ocean sediment studies: An example from the Southern Lomonosov Ridge

Recently, the use of “extinction ages” of excesses in U-series isotopes (230Thxs, 231Paxs) has been proposed for the setting of benchmark ages of up to ~350 and ~150 ka, respectively, in late Quaternary marine records from the Arctic Ocean. However, the use of such U-series-based chronostratigraphic approaches has some limitations. These limitations are illustrated by U-series measurements in a cored sequence from the southern Lomonosov Ridge (PS2757). In this core, the final measurable excess in 230Th (230Thxs), strictly linked to the sedimentary flux of this isotope from the overlying water column (230Thxs-marine), is observed at a depth of ~590 cm downcore. An “extinction age” of ~230 ka can be estimated for the residual 230Thxs at this depth. It approximately matches the Marine Isotope Stage 7/8 transition. Below this transition, strong redox gradients constrained by a layer enriched in organic carbon resulted in a late-diagenetic relocation of uranium leached from detrital minerals in the over- and underlying oxidized layers. This uranium relocation resulted in large amplitude radioactive disequilibria within a core section otherwise characterized by near secular equilibria between inventories of 238U-series isotopes, implying an age greater than the “230Thxs-marine extinction age” for the whole section. In the overlying part of the core, the 230Thxs distribution correlates with other 230Thxs-documented sequences from the Central Arctic Ocean. 230Thxs can be thus used for stratigraphic correlations between the relatively low-sedimentation rate marine sequences of this basin, over the last two or three glacial cycles, but special attention to potential diagenetic effects is recommended. Moreover, as for a given 230Thxs-marine flux at the seafloor, initial 230Thxs-values are broadly inversely-proportional to the sedimentation rate, the resulting estimates of 230Thxs “extinction age” vary accordingly. This variability restricts the chronostratigraphic use of 230Thxs to sequences with relatively low sedimentation rates, such as those where the initial 230Thxs-marine significantly exceeds the 230Th-fraction carried by detrital minerals.publishedVersio

A first last glacial maximum to younger dryas stalagmite record from southern Portugal

A newly launched research program permitted the sampling of speleothem deposit in the Algarve area (Southern Portugal) with the primary objective of constraining the ages of past humid intervals in the area from U-series measurements.Os sedimentos estuarinos constituem valiosos registos das variações das condições climáticas e ambientais regionai

A first last glacial maximum stalagmite record from southern Portugal

A newly launched research program permitted the sampling of speleothem deposits in the Algarve area (Southern Portugal). Unfortunately, this stalagmite yielded very small amounts of U (~ 20 ppb), making the setting of a U-series chronology difficult due to both the relative importance of a detrital contaminating fraction and the lack of precision on overall 230Th measurements. Nevertheless, based on the top and most recent sample, we extracted a first order estimate for the 230Th/232Th ratio of the contaminating fraction and for its 234U/232Th and 238U/232Th ratios as well, assuming a secular equilibrium between 234U and 238U in this fraction

Quaternary coastal uplift along the Talara Arc (Ecuador, Northern Peru) from new marine terrace data

Marine Geology, v. 228, n. 1-4, p. 73-91, 2006. http://dx.doi.org/10.1016/j.margeo.2006.01.004International audienceMarine terrace sequences have been investigated along the Talara Arc, a 1000-km-long stretch of the coast of Ecuador and northern Peru, characterized by subduction with a concave plan-view. Seven areas were investigated, evidencing flights of up to seven marine terraces with elevations reaching up to 360 m above mean sea level (amsl). Dating of the terraces was made using the Infra Red Stimulated Luminescence (IRSL) technique on sands as old as MIS 9 (∼330 ka). 14 C and U-series dates were obtained from fossil shells for geochronological cross control. Mean uplift rates along the Talara Arc range from about 0.10 up to 0.50 mm/ yr. The strongest uplift is observed in the Manta Peninsula of Ecuador in front of the subduction of the Carnegie Ridge. The uplift rate tends to slow down towards the northern and southern ends of the Talara Arc and then the transition toward the stable or subsiding coasts of central Peru and northern Ecuador and Colombia is sharp. The uplift appears to be homogeneous and related to 1) the map view curvature of the Arc, 2) the concave subduction pattern and 3) the Carnegie Ridge subduction

First isotopic records from stalagmites in Algarve (South Portugal)

Os espeleotemas representam registos paleoclimáticos continentais de alta resolução e estão a ser cada vez mais estudos para fins de reconstrução climática de períodos não afectados por acções antropogénicas. Na região do Algarve (Sul de Portugal), a existência de formações carbonatadas do Jurássico e Cretácico permitiu um desenvolvimento de sistemas cársicos onde se encontram actualmente estruturas estalagmíticas. Neste estudo preliminar realizado sobre estas estalagmites da região do Algarve, foram realizadas datações U/Th e análises de isótopos estáveis que demonstram pela primeira vez a possibilidade de obter registos isotópicos continentais representativos do paleoclima do extremo Sudoeste da Península Ibérica