Pj McGoldrick

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

Low oxygen limited the rise of animals Oxygen levels in Earths early atmosphere had an important influence on the evolution of complex life. Planavsky et al. analyzed the isotopic signature of chromium in sedimentary rocks from across the globe—a proxy for past oxygen levels. Oxygen levels in the mid-Proterozoic (1.6 billion to 900 million years ago) were very low: less than 0.1% of the modern atmosphere. These low levels were probably below the minimum oxygen requirements for the earliest animals, delaying their emergence and diversification. Science, this issue p. 635 Oxygen levels in Earth’s early atmosphere were often less than 1% of modern levels. The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

Widespread iron-rich conditions in the mid-Proterozoic ocean.

The chemical composition of the ocean changed markedly with the oxidation of the Earth’s surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean–atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth’s middle age (1.8–1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe2+-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

Routine quantitative multi-element analysis of sulphide minerals by laser ablation ICP-MS: Standard development and consideration of matrix effects

ABSTRACT The paper describes a calibration standard for quantitative in-situ multi-element analysis of sulphide minerals by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Standard STDGL2b2 is a mixture of 25% Zn concentrate and 75% pyrrhotite doped with a number of additional trace elements and fused into an X-ray fluorescence (XRF) glass disk. The homogeneity of the disk has been tested for the 55 elements of interest. All elements except Se, Tl, Au and Pt are homogenous (< 5% variation). Accurate analysis for the above four elements requires averaging multiple analyses of the standard. Element concentrations in STDGL2b2 were quantified by XRF and standard solution ICP-MS using a Finnigan Element and an Agilent 4500 mass-spectrometers. For the analysis of pyrite, pyrrhotite, chalcopyrite, galena and sphalerite, analytical errors caused by matrix-dependent fractionation have been evaluated by analysing five pressed-powder pellets.. The compositions of the powders have been analysed by XRF and solution ICP-MS. When Fe or Pb can be used as the internal standard, errors for most elements are < 15%, but reach up to 50% for W, Zn and Cd, requiring correction factors to be introduced. However, when Zn is used as the internal standard, significant correction factors are required for most elements. Comparison of the results obtained with two different laser microprobes, a solid state 213 nm and an excimer 193 nm, indicates that either is well suited for LA-ICP-MS analysis of sulphide minerals using STDGL2b2 as the calibration standard. Use of STDGL2b2 significantly improves accuracy of sulphide analysis by LA-ICP-MS compared to silicate reference materials, such as the NIST 600 series.

Lithogeochemical halos and geochemical vectors to stratiform sediment hosted Zn–Pb–Ag deposits: Part 2. HYC deposit, McArthur River, Northern Territory

The giant stratiform Zn-Pb-Ag HYC deposit displays a broad Zn, Pb and Tl halo which extends laterally along the favourable pyritic black shale facies of the Barney Creek Formation for at least 15 krn west of the deposit. A ferroan dolomite/ankerite halo overlaps with the Zn-Pb-Tl halo extending up to 250 m into the immediate stratigraphic hangingwaD, and 50 to 100 m into the footwall sediments close to the deposit. A manganese carbonate halo is offset from the ferroan dolomite/ankerite halo, being concentrated in the immediate footwall of the deposit and extending laterally along the W-Fold Shale Member. Manganiferous carbonate forms the most pronoW1ced and laterally extensive halo at RYC extending well beyond the Zn-Pb-TI and ankerite halos. The geometry and extent of the halos described above are based on sampling and analyses from two drill holes repolted in this study and a further six drill holes reported previously by I.B. Lambert and K.M. Scott [1. Geochem. Explor. 2, 307-330, 1973]. The SEDEX alteration index previously proposed as a vector for the Lady Loretta deposit [R.R. Large and P.I. McGoldrick, 1998, I. Geochem. Explor. 63, 37-56] has been shown to have applications in the McArthur Basin for defining the favourable stratigraphic unit hosting stratiform Zn-Pb-Ag mineralisation. A modified alteration index (AI Mark 3) has also been developed which eliminates the effect of the shale/dolomite ratio on the index and thus highlights the control of carbonate chemistry and its relationship to stratiform mineralisation. The application of a group of indices including Zn, Pb, Tl, SEDEX alteration index, AI Mark 3 and manganese content of dolomite (MnOd) is proposed for defining the most favourable stratigraphic units for stratiform Zn-Pb-Ag mineralisation within carbonate-bearing sedimentary basins.

Precise lead isotope ratios in Australian galena samples by high resolution inductively coupled plasma mass spectrometry

High resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) was used to measure Pb isotope ratios in standard solutions and Pb mineral digests. The RSD values obtained for208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 208Pb/206Pb and 207Pb/206Pb were 0.13, 0.11, 0.11, 0.046 and 0.048%, respectively (values as 1σ). These values were obtained from 30 analyses of three different standard sample types (multi-element standard, NIST SRM 981 and a Broken Hill galena digest). Based on 39 analyses of 11 galena samples from different locations around Australia, the difference between HR-ICP-MS and conventional TIMS values for 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 208Pb/206Pb and 207Pb/206Pb ratios was generally better than±0.2%. This paper outlines a very simple and rapid analytical method for the measurement of Pb isotope ratios, and is one of the first studies to use HR-ICP-MS to measure Pb isotope ratios in galena and galena-bearing ores.