Goldschmidt Abstracts最新文献

Although the marginal seas represent only 7% of the total ocean area, the CO₂ fluxes are intensive and important for the carbon budget, exposing to an intense process of anthropogenic ocean acidification (OA). A decline in pH, especially in the estuarine waters, results also from the eutrophication-induced acidification. The Adriatic Sea is currently a CO₂ sink with an annual flux of approximately -1.2 to -3 mol C m^-2 yr^-1 which is twice as low compared to the net sink rates in the NW Mediterranean (-4 to -5 mol C m^-2 yr^-1). Based on the comparison of two winter cruises carried out in in the 25-year interval between 1983 and 2008, acidification rate of 0.003 pH_T units yr⁻¹ was estimated in the northern Adriatic which is similar to the Mediterranean open waters (with recent estimations of −0.0028 ± 0.0003 units pH_T yr⁻¹) and the surface coastal waters (-0.003 ± 0.001 and -0.0044 ± 0.00006 pH_T units yr⁻¹). The computed Revelle factor for the Adriatic Sea, with the value of about 10, indicates that the buffer capacity is rather high and that the waters should not be particularly exposed to acidification. Total alkalinity (TA) in the Adriatic (2.6-2.7 mM) is in the upper range of TA measured in the Mediterranean Sea because riverine inputs transport carbonates dissolved from the Alpine dolomites and karstic watersheds. The Adriatic Sea is the second sub-basin (319 Gmol yr^-1), following the Aegean Sea (which receives the TA contribution from the Black Sea), that contribute to the riverine TA discharges into the Mediterranean Sea. About 60% of the TA inflow into the Adriatic Sea is attributed to the Po river discharge with TA of ~3 mM and TA decreases with increasing salinity. Saturation state indicates that the waters of the Adriatic are supersaturated with respect to calcite (Ω_Ca) and aragonite (Ω_Ar) throughout the year. However, saturation states are considerably lower in the bottom water layers, due to the prevalence of benthic remineralization processes in the stratification period. The seasonal changes of the chemical and environmental conditions and relatively small size of the Adriatic Sea area the microbial community composition, function (growth, enzymatic activity) and carbon and nitrogen biogeochemical cycles. Significant effects on calcifying organisms and phytoplankton are expected while the effects of possible OA on microbially-driven processes are not known yet.

Basaltic eruptions are the most common form of volcanism on Earth and planetary bodies. The low viscosity of basaltic magmas generally favours effusive and mildly explosive volcanic activity. Highly explosive basaltic eruptions occur less frequently and their eruption mechanism still remains subject to debate, with implications for the significant hazard associated with explosive basaltic volcanism. Particularly, highly explosive eruptions require magma fragmentation, yet it is unclear how basaltic magmas can reach the fragmentation threshold.

In volcanic conduits, the crystallisation kinetics of an ascending magma are driven by degassing and cooling. So far, the crystallisation kinetics of magmas have been estimated through ex situ crystallization experiments. However, this experimental approach induces underestimation of crystallization kinetics in silicate melts. The   crystallization experiments reported in this study were performed in situ at Diamond Light Source (experiment EE12392 at the I12 beamline), Harwell, UK, using basalt from the 2001 Etna eruption as the starting material. We combined a bespoke high-temperature environmental cell with fast synchrotron X-ray microtomography to image the evolution of crystallization in real time. After 4 hours at sub-liquidus conditions (1170 °C and 1150 °C) the system was perturbed through a rapid cooling (0.4 °C/s), inducing a sudden increase of undercooling. Our study reports the first in situ observation of exceptionally rapid plagioclase and clinopyroxene crystallisation in trachybasaltic magmas. We combine these constraints on crystallisation kinetics and viscosity evolution with a numerical conduit model to show that exceptionally rapid syn-eruptive crystallisation is the fundamental process required to trigger basaltic magma fragmentation under high strain rates. Our in situ experimental and natural observations combined with a numerical conduit model allow us to conclude that pre-eruptive temperatures <1,100°C can promote highly explosive basaltic eruptions, such as Plinian volcanism, in which fragmentation is induced by fast syn-eruptive crystal growth under high undercooling and high decompression rates. This implies that all basaltic systems on Earth have the potential to produce powerful explosive eruptions.

Long Integration Dual Inlet (LIDI) is an established technology which enabled improved accuracy and precision of Δ47 analysis from carbonate samples by utilizing sequential measurement of the full sample and reference, rather than alternating between sample and reference on shorter time periods, as it is done in the classical Dual Inlet method. As of today, there are two key challenges that were limiting further improvements to Δ47 determination: the IRMS must be in a stable temperature environment during long measurement of sample and reference gas, and the crimping of the sample and reference capillaries must be precisely matched, otherwise the produced data will be inaccurate and have reduced precision.

Here we present the improvements made on the sample gas measurement and data evaluation, which we define as LIDI 2.

By applying the LIDI 2 method, sample bracketing is possible following a four-step approach, resulting in fully corrected temperature drift (i.e. eliminated from the data), decreasing the standard deviation by factor of 2. This is a substantial improvement for acquiring clumped isotope data as reaching a very stable temperature of ±0.1°C/h is a challenge for most laboratories.

Alongside eliminating variation in the Δ47 data caused by unstable laboratory air temperature, LIDI 2 also improves the overlap of sample and reference gas signals due to non-perfect crimping of the capillaries. The crimping procedure is laborious and rarely delivers perfect results. Additionally, the pressure adjustment before reference measurement must ensure there is no significant offset between sample and reference intensities. LIDI 2 delivers perfect sample versus reference intensity matching, which results in significantly higher precision on each sample gas analyzed. Standard error of a single sample measurement is improved by up to factor of 2.

The LIDI 2 method delivers improved accuracy and precision on Δ47 measurement from small Carbonate samples, which in combination with the latest advancements in inert capillaries coating and automated contaminant trapping contributes to enhanced clumped isotopes data quality.

Most magmatism occurring on Earth is conventionally attributed to passive mantle upwelling at mid-ocean ridges, slab devolatilization at subduction zones, and mantle plumes. However, the widespread Cenozoic intraplate volcanism in northeast China and the peculiar petit-spot volcanoes offshore the Japan trench cannot be readily associated with any of these mechanisms. Furthermore, the seismic tomography images show remarkable low velocity zones (LVZs) sit above and below the mantle transition zone which are coincidently corresponding to the volcanism. Here we show that most if not all the intraplate/petit-spot volcanism and LVZs present around the Japanese subduction zone can be explained by the Cenozoic interaction of the subducting Pacific slab with a hydrous transition zone. Numerical modelling results indicate that 0.2-0.3 wt.% H₂O dissolved in mantle minerals which are driven out from the transition zone in response to subduction and retreat of a stagnant plate is sufficient to reproduce the observations. This suggests that critical amounts of volatiles accumulated in the mantle transition zone due to past subduction episodes and/or delamination of volatile-rich lithosphere could generate abundant dynamics triggered by recent subduction event. This model is probably also applicable to the circum-Mediterranean and Turkish-Iranian Plateau regions characterized by intraplate/petit-spot volcanism and LVZs in the underlying mantle.

Estuaries are key ecosystems from economical and ecological points of view. This is especially true for the Seine Estuary, its watershed representing 12% of the France area (78 600 km²) in which 30% of the French population, 40% of the industry and 25% of the agriculture are concentrated. Estuaries transfer material from the continent to the oceans, including organic matter (OM), for which they are highly reactive zones. Elucidating the estuarine OM dynamics remains challenging, due to (i) the high variability of environmental parameters, such as salinity, light penetration and tidal range, (ii) the intrinsic heterogeneity and molecular diversity of OM and (iii) the permanently changing nature of this material. Estuarine OM can originate from various sources (transported from rivers or coastal ocean or be produced within the estuary itself) with a different composition, and thus a different behaviour in the ecosystem.

The aim of this work was to better constrain the sources of OM in the Seine Estuary. In order to take into account the spatiotemporal variability of OM characteristics, water and sediment samples (10 cm-long cores) were collected all along the estuary, i.e. in the upstream, maximum turbidity and downstream zones, during 5 campaigns with different tidal intensities and river flows. Elemental (C, N) and isotopic composition (δ¹³C and δ¹⁵N) as well as lipid biomarkers were analyzed in both particulate (POM) and sediment OM. This allows comparing the bulk and molecular composition as well as sources of OM in the particulate and sediment pools.

Several lipid biomarkers (n-alkanes, fatty acids, n-alcohols, sterols/stanols, GDGTs) were investigated in this study, as they provide complementary information of the sources and degradation degree of OM. Lipids from terrigenous sources were predominant in all samples, even though the concentrations of these compounds as well as those of anthropogenic origin were shown to decrease towards the mouth of the Seine Estuary. In addition, significant differences in bulk and molecular composition were observed between the particulate and sediment pool, especially with a higher abundance of aquatic (i.e. algal/bacterial) vs. terrigenous lipids in POM than sediment OM. Last, bulk and molecular analyses both showed the strong seasonal and spatial variability (along the estuary and with depth) of OM composition in the water column and sediment, which has to be taken into account when investigating estuarine OM dynamics.

The global Shuram anomaly records the longest and most negative carbonate carbon isotopic excursion in Earth history. It took place during the late Ediacaran (c. 570 – c. 551 Ma) with δ¹³C_carbvalues down to −12‰. In South China, Doushantuo Formation Member IV (c. 555-551 Ma) consists mainly of organic-rich black shales and records the recovery of this anomaly, with values going from –6‰ to +0.5‰. The origin of this anomaly is thought to be related to the existence of a vast pool of dissolve organic carbon (DOC) in the ocean that was episodically oxidized thereby providing a source of ¹³C-depleted inorganic carbon. However, the main processes that ultimately drove to its recovery remain elusive. Here, we present new δ¹³C_organd δ¹⁵N dataset along a shelf-to-basin transect of the Nanhua basin (South China) as robust organic proxies to reconstruct the spatial and temporal evolution of paleoproductivity at basin scale. In addition, Raman spectroscopy is used to assess the thermal maturity of the samples. These new results define areas of high primary productivity and suggest the existence of an oxygen minimum zone (OMZ) together with other reduced oxic areas. From base to top of Member IV, the observed increasing and covariant trends in δ¹³C_carb and δ¹³C_orgdata together with a decreasing drift in δ¹⁵N values in platform and mid-lower slope environments are interpreted as areas where primary productivity became the main source of organic matter. Conversely, decreasing trends in δ¹³C_carb and δ¹³C_org data together with invariant δ¹⁵N values in the upper slope and deep basin environments are interpreted as areas where reduced DOC dominated as the principal source or organic carbon. Based on that, we propose that a new balance was established between primary and secondary paleoproductivity, whereby the former succeeded to the latter as one of the principal contributors that led to the carbon isotope recovery in carbonates. This new model represents a plausible solution to the enigmatic negative δ¹³C_carbisotopic excursion of the late Ediacaran.

The most significant consequence of prograde metamorphism for orogenic evolution is the melting of high-grade metamorphic rocks, resulting in a dramatic decrease in their mechanical strength, the activation of shear zones and consequent exhumation. Granitic bodies emplaced within the highest metamorphic grades of the Himalayan orogen form by the melting of amphibolite-grade pelitic rocks, either due to the presence of aqueous fluid or through the dehydration of hydrous phases such as muscovite. Across the Himalayas, these granites, and partially melted source migmatites, are found in the Greater Himalayan Sequence (GHS), bounded by the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). Many of these granites formed during the Miocene when decompression of the unit during rapid exhumation triggered melting; however, exact timings and reaction pathways appear to vary laterally across the orogen. The timescales of anatexis, amalgamation, migration, and emplacement are the focus of active research and have implications for orogenic tectonic development. Recent studies of granite pluton formation suggest a series of pulsed melting events with protracted periods of crystallisation under low melt-fraction conditions. These studies show that grain-scale variations in age can be linked with trace element data in both monazite and zircon, spanning millions of years of crystallisation. It is, therefore, important to recognise the geochemical signatures that these processes leave in granites, migmatites, and melt-extracted restite and to delineate more precisely the relevant processes and timescales leading to magma genesis. We present a preliminary dataset that aims to constrain the source, melt reactions, and timescales of melting episodes that form the migmatites and leucogranites of the upper GHS. We sampled leucogranites, migmatites, and their host metasediments along the Rishi Ganga (Badrinath) and Alaknanda valleys in the Garhwal region of the Indian Himalaya. Zircon from these samples were analysed for their crystallisation age (U-Pb), Hf-isotopic ratios, oxygen isotope and trace element composition using LA-ICPMS.  Rim domains identified using cathodoluminescence (CL) imaging were preferentially targeted, with the aim of collecting data that related to Himalayan melting processes. Preliminary findings suggest that the leucogranites crystallised from 22 Ma to ~13 Ma, with punctuated zircon crystallisation occurring throughout this timespan. Zircon rim ages from migmatites are generally older, ranging from 34 Ma to ~15 Ma. Integration of Hf-isotopic and trace elemental data, combined with petrographic observations allow mineral age data to be linked to changes in geological processes.