Abstract Unusually high δ 15 N values in the Neoarchean sedimentary record in the time period from 2.8 to 2.6 Ga, termed the Nitrogen Isotope Event (NIE), might be explained by aerobic N cycling prior to the Great Oxidation Event (GOE). Here we report strongly positive δ 15 N values up to +42.5 ‰ in ~2.75 – 2.73 Ga shallow-marine carbonates from Zimbabwe. As the corresponding deeper-marine shales exhibit negative δ 15 N values that are explained by partial biological uptake from a large ammonium reservoir, we interpret our data to have resulted from hydrothermal upwelling of 15 N-rich ammonium into shallow, partially oxic waters, consistent with uranium isotope variations. This work shows that anomalous N isotope signatures at the onset of the NIE temporally correlate with extensive volcanic and hydrothermal activity both locally and globally, which may have stimulated primary production and spurred biological innovation in the lead-up to the GOE.
Ureilites are meteorites that represent mantle restites of a planetesimal likely disrupted before the magma ocean stage and then reaccreted. Historically, it was speculated that evaporation shifts the Zn isotope ratios in ureilites toward heavier compositions. The fact that the ureilite parent body (UPB) is depleted in some moderately volatile elements (MVEs) makes ureilites an appealing target to study isotopic fractionation by evaporation in the early Solar System. Here, we show that Fe and Zn isotope ratios of bulk ureilites and their metal and silicate components rather record metal melting and extraction of Fe-FeS melts in the UPB, which also resulted in isotopic disequilibrium between the silicate and metal parts. This finding underlines that the isotopic evolution of MVEs in the early Solar System is not only affected by evaporation, but also by planetary differentiation processes due to the chalcophile and/or siderophile behaviour of many MVEs. It shows that to avoid interpretational bias due to undersampling of planetesimal reservoirs in meteorite collections, and to distinguish planetary differentiation from evaporation, isotopic compositions of MVEs should be combined with common geochemical proxies.
Abstract. Short-term hypoxia in epeiric water masses is a common phenomenon of modern marine environments and causes mass mortality in coastal marine ecosystems. Here, we test the hypothesis that during the early Aptian, platform-top hypoxia temporarily established in some of the vast epeiric seas of the central Tethys and caused, combined with other stressors, significant changes in reefal ecosystems. Potentially interesting target examples include time intervals characterized by the demise of lower Aptian rudist–coral communities and the establishment of microencruster facies, as previously described from the central and southern Tethys and from the proto-North Atlantic domain. These considerations are relevant as previous work has predominantly focused on early Aptian basinal anoxia in the context of Oceanic Anoxic Event (OAE) 1a, whereas the potential expansion of the oxygen minimum zone (OMZ) in coeval shallow-water environments is underexplored. Well-known patterns in the δ13C record during OAE 1a allow for a sufficiently time-resolved correlation with previously studied locations and assignment to chemostratigraphic segments. This paper presents and critically discusses the outcome of a multi-proxy study (e.g., rare earth elements (REEs), U isotopes, and redox-sensitive trace elements) applied to lower Aptian shallow-water carbonates today exposed in the Kanfanar quarry in Istria, Croatia. These rocks were deposited on an extensive, isolated high in the central Tethys surrounded by hemipelagic basins. Remarkably, during chemostratigraphic segment C2, the depletion of redox-sensitive trace elements As, V, Mo, and U in platform carbonates, deposited in normal marine oxic waters, record the first occurrence of basinal, organic-rich sediment deposition in which these elements are enriched. During the C3 segment, seawater oxygen depletion established on the platform top as indicated by the patterns in Ce/Ce* and U isotopes. Shifts in redox-sensitive proxies coincide with the expansion of microencruster facies. Segment C4 witnesses the return to normal marine reefal faunas on the platform top and is characterized by patterns in redox-sensitive proxies typical of normal marine dissolved oxygen levels. It remains unclear, however, if platform-top hypoxia resulted from the expansion and upwelling of basinal, oxygen-depleted water masses or if spatially isolated, shallow hypoxic water bodies formed on the platform. Data shown here are relevant as they shed light on the driving mechanisms that control poorly understood faunal patterns during OAE 1a in the neritic realm and provide evidence on the intricate relation between basinal and platform-top water masses.
The application of multiple collector inductively coupled plasma source mass spectrometry (MC‐ICPMS) to 176 Lu‐ 176 Hf and 92 Nb‐ 92 Zr chronometry has been hampered by complex Zr‐Hf purification procedures that involve multiple ion exchange column steps. This study presents a single‐column separation procedure for purification of Hf and Lu by ion exchange using Eichrom® Ln‐Spec resin. The sample is loaded in pure HCl, and element yields are not dependent on the sample matrix. For 92 Nb‐ 92 Zr chronometry, a one‐column procedure for purification of Zr using Biorad® AG‐1‐× 8 resin is described. Titanium and Mo are completely removed from the Zr, thus enabling accurate 92 Zr measurements. Zirconium and Nb are quantitatively separated from rock samples using Eichrom Ln‐Spec resin, allowing measurements of Zr/Nb with a precision of better than ±5% (2 σ ). The Ln‐Spec and anion resin procedures may be combined into a three‐column method for separation of Zr‐Nb, Hf, Ta, and Lu from rock samples. For the first time, this procedure permits combined isotope dilution measurements of Nb/Ta, Zr/Hf, and Lu/Hf using a mixed 94 Zr‐ 176 Lu‐ 180 Hf‐ 180 Ta tracer. Analytical protocols for Zr and Hf isotope measurements using the Micromass Isoprobe, a second generation, single‐focusing MC‐ICPMS, are reported. Using the Isoprobe at Münster, 2 σ external precisions of ±0.5ɛ units for Hf and Zr isotope measurements are achieved using as little as 5 ng (Hf) to 10 ng (Zr) of the element. The 176 Hf/ 177 Hf and Lu/Hf for rock reference materials agree well with other published MC‐ICPMS and thermal ionization mass spectrometry (TIMS) data.
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