In situ trace metal analysis of Neoarchaean – Ordovician shallow‐marine microbial‐carbonate‐hosted pyrites

Abstract

Pre-Cambrian atmospheric and oceanic redox evolutions are expressed in the inventory of redox-sensitive trace metals in marine sedimentary rocks. Most of the currently available information was derived from deep-water sedimentary rocks (black shale/banded iron formation). Many of the studied trace metals (e.g. Mo, U, Ni and Co) are sensitive to the composition of the exposed land surface and prevailing weathering style, and their oceanic inventory ultimately depends on the terrestrial flux. The validity of claims for increased/decreased terrestrial fluxes has remained untested as far as the shallow-marine environment is concerned. Here, the first systematic study of trace metal inventories of the shallow-marine environment by analysis of microbial carbonate-hosted pyrite, from ca. 2.65-0.52 Ga, is presented. A petrographic survey revealed a first-order difference in preservation of early diagenetic pyrite. Microbial carbonates formed before the 2.4 Ga great oxygenation event (GOE) are much richer in pyrite and contain pyrite grains of greater morphological variability but lesser chemical substitution than samples deposited after the GOE. This disparity in pyrite abundance and morphology is mirrored by the qualitative degree of preservation of organic matter (largely as kerogen). Thus, it seems that in microbial carbonates, pyrite formation and preservation were related to presence and preservation of organic C. Several redox-sensitive trace metals show interpretable temporal trends supporting earlier proposals derived from deep-water sedimentary rocks. Most notably, the shallow-water pyrite confirms a rise in the oceanic Mo inventory across the pre-Cambrian-Cambrian boundary, implying the establishment of efficient deep-ocean ventilation. The carbonate-hosted pyrite also confirms the Neoarchaean and early Palaeoproterozoic ocean had higher Ni concentration, which can now more firmly be attributed to a greater proportion of magnesian volcanic rock on land rather than a stronger hydrothermal flux of Ni. Additionally, systematic trends are reported for Co, As, and Zn, relating to terrestrial flux and oceanic productivity.

Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere

“…Hence, these data provide insight into the small degrees of redox change that led, perhaps in fits and starts, to full oceanic oxygenation. A Cryogenian–Ediacaran increase in oxygenation agrees with other redox-sensitive elements 10 62 63 and biomarker evidence 64 . Thus, the significance of the Se isotope record is not only that it adds to growing evidence that the late Proterozoic and Cambrian ocean and atmosphere reached a progressively more oxic state, coinciding with the diversification of animal life, but also that the process of oxidation was protracted, and not ultimately triggered by the Gaskiers deglaciation, as other data suggest 5 .…”

Trace elements at the intersection of marine biological and geochemical evolution

“…The use of the pyrite records has recently found support in the work of Gallagher et al (2015) who reported a suite of trace element data (Mo, Ni, As, Co, Zn) from Precambrian to Ordovician carbonate-hosted pyrite deposited in shallow marine environments. Their data were centered on the Archean-…”

The Neoarchaean surficial sulphur cycle: An alternative hypothesis based on analogies with 20th‐century atmospheric lead

“…The resulting tuffs are present above and below SrKu34 (Figure ) and these have been used for U/Pb zircon geochronology (e.g., Sumner & Bowring, ), providing strong evidence for relatively proximal, explosive volcanism from evolved magmas (that permitted crystallisation of zircon), possibly in the adjacent “Hamersley” arc‐retro arc system (Zeh et al., ). Finally, there is evidence that the Gamohaan basin was sulphate‐limited; not only is the range in δ 34 S limited (Domagal‐Goldman, Kasting, Johnston, & Farquhar, ; Guo et al., ; Papineau et al., ; Whitehouse, ) but several Gamohaan Formation spheroidal pyrites also preserve δ 34 S isotopic gradients as a function of their spherical segment radius, suggesting a limit on sulphate availability during BSR (Gallagher et al., ; Kamber & Whitehouse, ). Thus, sulphur fallout could easily overwhelm the ambient isotopic signal.…”

“…Namely, very delicate microbial kerogen is preserved in exquisite detail in the limestones below the host of SrKu34 (e.g., Kamber et al., ; Sumner, ) and many of these limestones contain intricately distributed micron‐scale pyrite grains with no evidence for dissolution. Furthermore, the pyrite chemistry itself (Gallagher et al., ) as well as the REE systematics of the hosting limestones (e.g., Kamber et al., ) argues strongly in favour of reducing conditions during the Gamohaan Fm. 's deposition and diagenesis.…”

“…However, a significant irreversible decline in oceanic Ni concentration at ca. 2.6 Ga is observed (e.g., Gallagher, Turner, & Kamber, 2015;Konhauser et al, 2009Konhauser et al, , 2015Large et al, 2014), likely reflecting the disappearance of komatiite and high-Mg basalts on land (Kamber, 2010), a direct consequence of cooling mantle plume temperatures and a concomitant end of very high-Mg volcanism (Konhauser et al, 2009). Once plume magmas became less magnesian, the crust too was being reorganised in terms of radioactive heat production, and the geology started to acquire a more familiar look to that of the Phanerozoic (Kamber, 2015).…”