Lithium isotopic composition of soil pore water: Responses to evapotranspiration

Lithium isotopes show great potential to trace Earth surface processes due to the large mass discrimination between 6Li and 7Li associated with clay uptake. However, factors controlling the Li isotopic composition (δ7Li) of river water, especially those with a water-bedrock δ7Li difference higher than that of the equilibrium fractionation associated with clay formation (ΔW-C), have not yet been fully resolved. Traditional interpretation involves the Rayleigh distillation, but it unrealistically separates the stage of clay formation from that of silicate dissolution using fractionation factors that are much lower than laboratories can constrain. We propose an in situ mechanism that simulates high δ7Li by evapotranspiration. A model with coupled mineral dissolution and clay precipitation shows that evaporative enrichment of pore-water Li progressively increases the incorporation of Li into clays with light δ7Li, resulting in higher δ7Li values in the residual water than ΔW-C. We also provide evidence from the Chinese Loess Plateau, where an evaporative effect readily explains the observed δ7Li. The influence of evapotranspiration on riverine δ7Li implies that changes in aridity may partly explain the variations of seawater δ7Li. The same principle may also apply to other stable isotopic systems whereby incorporation into secondary precipitates controls the isotopic fractionation.

Water heavily fractionated as it ascends on Mars as revealed by ExoMars/NOMAD

Isotopic ratios and, in particular, the water D/H ratio are powerful tracers of the evolution and transport of water on Mars. From measurements performed with ExoMars/NOMAD, we observe marked and rapid variability of the D/H along altitude on Mars and across the whole planet. The observations (from April 2018 to April 2019) sample a broad range of events on Mars, including a global dust storm, the evolution of water released from the southern polar cap during southern summer, the equinox phases, and a short but intense regional dust storm. In three instances, we observe water at very high altitudes (>80 km), the prime region where water is photodissociated and starts its escape to space. Rayleigh distillation appears the be the driving force affecting the D/H in many cases, yet in some instances, the exchange of water reservoirs with distinctive D/H could be responsible.

Drivers of the variability of the isotopic composition of water vapor in the surface boundary layer

Measurements of the isotopic composition of water vapor, δv, as well as measurements of the isotopic composition of evaporation and transpiration provide valuable insights in the hydrological cycle. Here we present measurements of δv in the surface boundary layer (SBL) in combination with eddy covariance (EC) measurements of the isotopic composition of evapotranspiration δET for both δD as well as δ18O over a full growing season above a managed beech forest in central Germany. Based on direct measurements of isoforcing IF and the height h of the planetary boundary layer (PBL), we provide an estimate of isoforcing-related changes in δv, revealing the influence of local evapotranspiration (ET) on δv. At seasonal time scales we find no evidence for a dominant control of δv by local ET. Rayleigh distillation could at most explain 35 % of the observed variability and we did not find indications for the influence of entrainment at seasonal time scales. Instead, we obtain a strong significant correlation (R2 ≈ 0.52; p

Significant Zr isotope variations in single zircon grains recording magma evolution history

Zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history. Here, we presented in situ high-precision (2SD <0.15‰ for δ94Zr) and high–spatial-resolution (20 µm) stable Zr isotope compositions of magmatic zircons in a suite of calc-alkaline plutonic rocks from the juvenile part of the Gangdese arc, southern Tibet. These zircon grains are internally zoned with Zr isotopically light cores and increasingly heavier rims. Our data suggest the preferential incorporation of lighter Zr isotopes in zircon from the melt, which would drive the residual melt to heavier values. The Rayleigh distillation model can well explain the observed internal zoning in single zircon grains, and the best-fit models gave average zircon–melt fractionation factors for each sample ranging from 0.99955 to 0.99988. The average fractionation factors are positively correlated with the median Ti-in-zircon temperatures, indicating a strong temperature dependence of Zr isotopic fractionation. The results demonstrate that in situ Zr isotope analyses would be another powerful contribution to the geochemical toolbox related to zircon. The findings of this study solve the fundamental issue on how zircon fractionates Zr isotopes in calc-alkaline magmas, the major type of magmas that led to forming continental crust over time. The results also show the great potential of stable Zr isotopes in tracing magmatic thermal and chemical evolution and thus possibly continental crustal differentiation.

No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

Carbonatites are unusual, carbon-rich magmas thought to form either by the melting of a carbon-rich mantle source or by low-degree partial melting of a carbon-poor (&lt;80 ppm C) mantle followed by protracted differentiation and/or immiscibility. Carbonate-bearing mantle xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatites, have provided key support for a C-rich mantle source. Here, we report unique microscale O and C isotopic analyses of those carbonates, which are present as interstitial grains in the silicate host lava, veins in the xenoliths, and pseudo-inclusions in olivine xenoliths. The δ18O values vary little, from 19‰ to 29‰, whereas δ13C values are more variable, ranging from –23‰ to +0.5‰. We show that such carbonate δ18O values result from the low-temperature precipitation of carbonate in equilibrium with meteoric water, rather than under mantle conditions. In this framework, the observed δ13C values can be reproduced by Rayleigh distillation driven by carbonate precipitation and associated degassing. Together with petrological evidence of a physical connection between the three types of carbonates, our isotopic data support the pedogenic formation of carbonates in the studied xenoliths by soil-water percolation and protracted crystallization along xenolith cracks. Our results refute a mechanism of C enrichment in the form of mantle carbonates in the mantle beneath the Natron Lake magmatic province and instead support carbonatite formation by low-degree partial melting of a C-poor mantle and subsequent protracted differentiation of alkaline magmas.

Supplemental Material: No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

Additional information on sample location, sample description, carbon and oxygen analytical methods, and the Rayleigh distillation model.<br>

Supplemental Material: No evidence for carbon enrichment in the mantle source of carbonatites in eastern Africa

Additional information on sample location, sample description, carbon and oxygen analytical methods, and the Rayleigh distillation model.<br>

Atmospheric circulation and the differentiation of precipitation sources during the Holocene inferred from five stalagmite records from Demänová Cave System (Central Europe)

Five stalagmites from the Demänová Cave System (DCS, Western Carpathians, Slovakia), spanning the period from 13,000 to 500 a BP, were analyzed for their oxygen and carbon stable isotopic composition of the calcite. The isotopic data obtained from several stalagmites located in one cave system allow us to separate the changes of regional/global importance from the local changes. Oxygen isotope ratios point to dynamic changes in the environment at the onset of the Holocene. Despite the local differences, carbon isotope data express the gradual and steady development of vegetation on the surface above the cave from the beginning of the Holocene until 6,000 a BP. The oxygen isotope values in the DCS stalagmites are higher than that derived from the Rayleigh distillation model until approximately 9,000 a BP, suggesting (1) an increase in the isotopic gradient to the east of Europe, probably caused by a different seasonality in precipitation amount or (2) different sources of meteoric water, transported from the Mediterranean and Black Sea region, in Central and Eastern Europe compared to the Western, circum-Atlantic part of the continent. The younger part of the DCS records falls in the range described by the model and points to the increasing role of the westerlies in the determination of the climatic conditions of Central Europe during middle- and late-Holocene.

Precipitation and ice core δD-δ¹⁸O line slopes and their climatological significance

The meteoric water line, defined by the correlation of hydrogen (δD) and oxygen (δ18O) values, is one of the earliest described characteristics of precipitation isotopic variations. However, spatial and temporal variations in the slope of this line are less studied. The slope of the δD-δ18O relationship is coupled with how d-excess covaries with δD or δ18O, and may provide an integrated tool for inferring hydrologic processes from the evaporation to condensation site. We present a study of δD-δ18O relationships on seasonal and annual timescales for event-based precipitation and a 15-meter ice core (Owen) at Summit, Greenland. Seasonally, precipitation δD-δ18O slopes are less than eight (summer = 7.71; winter = 7.77), while the annual slope is greater than eight (8.27). We suggest intra-season slopes result primarily from Rayleigh distillation, which, under prevailing conditions, produces slopes less than eight. The summer line has a greater intercept (higher d-excess) than the winter line. This separation causes annual slopes to be greater than seasonal ones. We attribute high summer d-excess to contributions of vapor sublimated from the Greenland Ice Sheet. Higher sublimated moisture proportions in summer cause larger separations between seasonal δD-δ18O lines, and thus higher annual slopes. Intra-seasonal distributions of precipitation amount also influence annual slopes because slopes are weighed by the number of storms each season. We generate indices to quantify sublimation proportion (SPI) and precipitation distribution (PDI), and find that annual Owen core slope measurements are significantly related to these indices, demonstrating that sublimation and precipitation distribution represent important climate conditions recorded in ice cores.

Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor

Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales.