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The Coevolution of Life and Arsenic in Precambrian Oceans

Final Report Summary - CLAPO (The Coevolution of Life and Arsenic in Precambrian Oceans)

The aim of CLAPO was to reconstruct the geological record and speciation of arsenic through Earth history. This was grounded on the proposition that the rise of atmospheric oxygen changed the flux dynamics of arsenic from land to the oceans and that these changes affected the evolutionary history of marine life. To answer these questions, CLAPO sampled thousands of organic carbon-rich rocks and banded iron formations deposited at the bottom of prehistoric oceans. The chemistry of these rocks was linked to experimental studies and modern analogue systems to deduce the impact of arsenic on ancient seawater chemistry and life. These studies revealed that marine arsenic dynamics have fluctuated dramatically through Earth history and that extreme snowball Earth climates were periods of seawater arsenic minima, while postglacial periods were intervals of seawater arsenic maxima, due to changes in continental weathering patterns, impacted by the presence or absence of ice cover on land, respectively. It was found that the rise of atmospheric oxygen after 2500 million years ago ushered in the first global appearance of arsenate and arsenic sulphides in seawater, as was hypothesised. These increases are related to oxidative weathering of the continental crust, releasing arsenate and sulphate into seawater. The sulphate-reducing bacteria in the anoxic bottom waters and sediments reduced the sulphate to produce the hydrogen sulphide that interacted with arsenic to deposit arsenic sulphides. The accumulation of arsenate in seawater is linked to the rise of strong arsenic oxidants like oxygen, nitrate and manganese oxides in the ocean-atmosphere system. It is proposed that the global rise of arsenate in seawater marked the onset of global arsenate detoxification and widespread competition of arsenate with phosphate for uptake into cells, which would have interfered with efficient primary production. Evidence from a modern natural analogue demonstrates that the sulphide-rich settings would have provided a refuge from severe arsenic toxicity because of the rapid precipitation of arsenic sulphides, thereby curtailing arsenic bioavailable. These sulphidic environments were phosphate-rich relative to the phosphate-limiting settings actively precipitating Fe(III)(oxyhydr)oxides. The Fe(III)(oxyhydr)oxide-rich settings were characterised by elevated arsenic toxicity, probably related to increase competition between phosphate and arsenate for the Fe(III) minerals, leading to phosphate being preferentially removed from seawater at the expense of arsenate. The preferential removal of phosphate from solution, while increasing seawater arsenic toxicity, would have in turn negatively impacted primary productivity. Ongoing work for the completion of a PhD degree has demonstrated that seawater arsenate and phosphate content impacted the fractionation of silicon isotopes under Precambrian seawater conditions. This PhD study is also on the path of providing estimates of dissolved seawater arsenic concentration as had been proposed. Data showed for the first time the deposition of a modern arsenic-rich Precambrian type banded iron formation on Milos Island, Greece, which would serve as a modern undisturbed analogue for understanding global biogeochemical processes in deep time. Collaborative work described ancient microbial communities and the earliest evidence of complex motility 2.1 billion years ago, together with the biodiversity and taphonomy of the oldest purported macrofossils in Earth history, in the Francevillian Series in Gabon