Paolo A. Sossi

Zn isotope evidence for immediate resumption of primary productivity after snowball Earth

The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during post-glacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity. Here we report the first Zn isotopic data, together with carbon and oxygen isotopic profiles from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly ^(66)Zn-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove δ^(66)Zn (^(66)Zn/^(64)Zn, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in δ^(66)Zn records a vigorous increase in primary production and export from a nutrient-laden surface ocean.

Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites

X-ray emission in young stellar objects (YSOs) is orders of magnitude more intense than in main sequence stars1,2, suggestive of cosmic ray irradiation of surrounding accretion disks. Protoplanetary disk irradiation has been detected around YSOs by HERSCHEL3. In our solar system, short-lived 10Be (half-life = 1.39 My4), which cannot be produced by stellar nucleosynthesis, was discovered in the oldest solar system solids, the calcium-aluminium-rich inclusions (CAIs)5. The high 10Be abundance, as well as detection of other irradiation tracers6,7, suggest 10Be likely originates from cosmic ray irradiation caused by solar flares8. Nevertheless, the nature of these flares (gradual or impulsive), the target (gas or dust), and the duration and location of irradiation remain unknown. Here we use the vanadium isotopic composition, together with initial 10Be abundance to quantify irradiation conditions in the early Solar System9. For the initial 10Be abundances recorded in CAIs, 50V excesses of a few per mil relative to chondrites have been predicted10,11. We report 50V excesses in CAIs up to 4.4 per mil that co-vary with 10Be abundance. Their co-variation dictates that excess 50V and 10Be were synthesised through irradiation of refractory dust. Modelling of the production rate of 50V and 10Be demonstrates that the dust was exposed to solar cosmic rays produced by gradual flares for less than 300 years at about 0.1 au from the protoSun.

Refined separation of combined Fe-Hf from rock matrices for isotope analyses using AG-MP-1M and Ln-Spec chromatographic extraction resins

Graphical abstract

Pb and Zn Behaviour during LP/HT Metamorphism: Implications for Pb‐Zn Ore Genesis

Studies on zinc and lead mobility during regional metamorphism are rare and contentious (e.g., Haack et al., 1984; Pitcairn et al., 2006) as we currently lack information on controlling factors for Zn and Pb enrichment or depletion during regional metamorphism. Better understanding of Pb and Zn behaviour will help to shed light on the long-standing discussion on the original source of base metals that feed Pb-Zn ore systems. In this study, we systematically studied Zn and Pb behaviour on a whole-rock and mineral scale during prograde metamorphism using a set of well-characterised psammopelite samples. The combination of bulk–rock and mineral geochemistry with Zn isotope data allows a comprehensive understanding of Pb and Zn migration in metamorphic systems. The study site is the Eastern Mount Lofty Ranges, South Australia, metamorphosed during the Delamerian orogeny at ~ 500 Ma (e.g. Hammerli et al., 2014; Fig. 1). Metamorphic conditions range from ~350 ̊C to the onset of partial melting in the presence of excess aqueous fluid at ~ 650–700 ̊C (3 to 5 kbar). Stable isotope studies indicate widespread up-temperature fluid flow during metamorphism, which may have triggered significant element mobility (e.g., Oliver et al., 1998).