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Changes in Arsenic Levels in the Precambrian Oceans in Relation to the Upcome of Free Oxygen

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Changes in Arsenic Levels in the Precambrian Oceans in Relation to the Upcome of Free Oxygen Examensarbete vid Institutionen för geovetenskaper ISSN 1650-6553 Nr 269 Changes in Arsenic Levels in the Precambrian Oceans in Relation to the Upcome of Free Oxygen Emma H.M. Arvestål Supervisor: Ernest Chi Fru 2 Abstract Life on Earth could have existed already 3.8 Ga ago, and yet, more complex, multicellular life did not evolve until over three billion years later, about 700 Ma ago. Many have searched for the reason behind this apparent delay in evolution, and the dominating theories put the blame on the hostile Precambrian environment with low oxygen levels and sulphide-rich oceans. There are, however, doubts whether this would be the full explanation, and this thesis therefore focuses on a new hypothesis; the levels of the redox sensitive element arsenic increased in the oceans as a consequence of the change in weathering patterns that followed the upcome of free oxygen in the atmosphere at about 2.4 billion years ago. Given its toxicity, this could have had negative effects upon the life of the time. To test the hypothesis, 66 samples from drill cores coming from South Africa and Gabon with ages between 2.7 and 2.05 Ga were analysed for their elemental composition, and their arsenic content were compared with carbon isotope data from the same samples. These confirmed that a rise in arsenic concentration following the upcome of free oxygen in the atmosphere and the onset of oxidative weathering of continental sulphides. Arsenic, which is commonly found in sulphide minerals, was weathered together with the sulphide and delivered into the oceans, where it in the Palaeoproterozoic increased to over 600% compared to the older Archaean levels, at least locally. Iron had the strongest control over the arsenic levels in the anoxic (ferruginous and sulphidic) oceans, probably due to its ability to remove arsenic through adsorption. During oxygenated conditions, sulphur instead had the strongest influence upon arsenic, likely because of the lack of dissolved iron. The highest arsenic levels were found in samples recognised as coming from oxygenated conditions, although this might be due to the oxygenation state of arsenic affecting its solubility. Arsenic is toxic already at low doses, especially if the necessary arsenic detoxification systems had not yet evolved. However, the lack of correlation between arsenic and changes in δ13C indicated that the increase of arsenic did not affect the primary production between 2.7 and 2.05 Ga. Thus, whether arsenic could have affected the evolution of life during the Mesoproterozoic remains to be shown. 3 4 Sammanfattning Redan för 3,8 miljarder år sedan kan det första primitiva livet på jorden ha uppstått. Det tog dock ytterligare tre miljarder år för mer komplext flercelligt liv att utvecklas. Denna fördröjning har enligt den kanske mest vedertagna teorin berott på de låga syrenivåerna under större delen av jordens historia, särskilt då miljön samtidigt var mycket ogästvänlig med svavelhaltiga och näringsfattiga vatten. Teorin har dock kommit att ifrågasättas eftersom det visat sig att djur kan leva i miljöer med betydligt lägre syrehalt än vad som tidigare varit känt. I denna studie testades därför en ny hypotes, nämligen att uppkomsten av fritt syre i atmosfären för 2,4 miljarder år sedan ledde till att koncentrationerna av det redoxkänsliga grundämnet arsenik ökade i haven som en följd av de förändrade vittringsprocesserna. Arsenik är mycket giftigt redan i låga mängder, och en ökning av ämnet kan därför ha lett till att livet kom att påverkas negativt. Hypotesen testades genom att analysera det kemiska innehållet i 66 prover från Sydafrika och Gabon med åldrar mellan 2,7 och 2,05 miljarder år och jämföra förändringarna i arsenikinnehåll med δ13C-värden i samma prover, analyserade av en annan forskargrupp. Proverna bekräftade att en ökning av arsenik skedde för runt 2,25 miljarder år sedan, och att ökningen åtminstone lokalt var över 600% jämfört med arkeiska nivåer. Trots detta verkar det enligt δ13C som att den biologiska aktiviteten under denna tid inte påverkades. Järn hade störst inflytande över arsenikkoncentrationerna under de tider då haven var syrefria och rika på antingen järn eller svavel. Detta beror troligen på järnmineralens förmåga att adsorbera arsenik. I vatten där det fanns syre var det istället svavel som utövade den största kontrollen, förmodligen på grund av bristen på järn. Högst arsenikhalter uppmättes i de prover som kom från syrehaltiga vatten, vilket troligen beror på att arsenik har lägre löslighet under dessa förhållanden. Trots att arsenikens påverkan på livet inte kunde styrkas så kan det inte uteslutas att en ökning av ämnet bidrog till att ha försenat evolutionen av multicellulärt liv under i första hand Mesoproterozoikum, i synnerhet om de gener som ger viss resistans mot arsenik ännu inte hade utvecklats. 5 6 Content Abstract ................................................................................................................................................................................. 3 Sammanfattning ................................................................................................................................................................ 5 Introduction ........................................................................................................................................................................ 9 The biogeochemistry of the Precambrian oceans ............................................................................................. 10 The Archaean ferruginous oceans ...................................................................................................................... 11 Definition of ferruginous waters and BIFs ................................................................................................. 11 Formations of BIFs ............................................................................................................................................... 12 Spatial and stratigraphic distribution of BIFs .......................................................................................... 13 The oceans after the BIFs .................................................................................................................................. 14 The oxygenation of the oceans and atmosphere .......................................................................................... 14 The emergence of free oxygen ........................................................................................................................ 14 The role of cyanobacteria .................................................................................................................................. 15 The role of O2 sinks and nutrients ................................................................................................................. 16 Effects on the atmosphere ................................................................................................................................ 17 The sulphidic oceans of the Proterozoic .......................................................................................................... 18 Definition of euxinic waters ............................................................................................................................. 18 The effects of H2S .................................................................................................................................................. 19 The effect of sulphur upon other elements ................................................................................................ 21 The end of the euxinic conditions .................................................................................................................. 22 Arsenic ................................................................................................................................................................................. 24 The distribution of arsenic in nature ................................................................................................................. 24 Arsenic cycling ............................................................................................................................................................ 25 The toxicity of arsenic .............................................................................................................................................. 27 Arsenic detoxification mechanisms ................................................................................................................... 29 Evolution of life ................................................................................................................................................................ 31 Signs of early life ........................................................................................................................................................ 31 δ13C as a proxy of biological activity .................................................................................................................. 32 Geological background.................................................................................................................................................. 33 South Africa .................................................................................................................................................................. 33 Gabon .............................................................................................................................................................................
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