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CLAPO Résumé de rapport

Project ID: 336092
Financé au titre de: FP7-IDEAS-ERC
Pays: Sweden

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

CLAPO aims at understanding how changing marine arsenic concentrations have evolved over time, related to atmospheric oxygenation dynamics and whether the observed changes may have played a crucial role in biological evolution. The first half of the project has focused on providing definitive proof for the basic hypothesis that was put forward; namely that marine arsenic concentrations have covaried with atmospheric oxygenation dynamics and changing weathering regimes through Earth history. Results indeed reveal that prior to 2.45 billion years ago, marine arsenic content was low but that with the onset of the Great Oxidation Event (GOE), which coincided with when Earth experienced its first global glaciation (Snowball Earth), marine arsenic concentrations experienced an all time low. Immediately after the glaciations had retreated, a recorded elevated sedimentary arsenic content is recorded across a number of geological formations at ~2.1 billion years ago.
A similar cyclic arsenic trend again reappears about 1.35 billion years later, during and after the global Neoproterozoic glaciations that began sometime around 0.75 billion years ago and ended at ~0.58 billion years ago. These cyclicities are intimately linked to oxygen dynamics at these different times, where increase in atmospheric oxygen content coincides with low marine arsenic concentration and vice versa. Through our data we proposed that the low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. However, interpreted atmospheric oxygen increase after 0.58 Ga, despite exceedingly high marine arsenic content, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.
Through two studies, one published, another in review, we suggest that arsenic may have induced acute phosphate limitation in surface oceans, after 2.45 billion years ago. These studies provide new important evidence for the role of hydrothermal activity in the ancient oceans, aimed to satisfy models seeking to explain the maintenance of generally low atmospheric oxygen content throughout the Proterozoic. Intriguingly, and despite the fact that the GOE occurred sometime after 2.45 billion years, it was not until after 0.58 billion years ago that atmospheric oxygen content rose to present day levels. In searching for ways to explain how elevated marine arsenic content might have affected biological activity, the arsenic-CO2-rich shallow marine hydrothermal systems on Milos Island are being investigated as natural laboratories to explain how marine organisms respond to arsenic in a low phosphate marine environments. Here we find an interesting situation where communities are driven by a carbon fixation pathway that was likely more prevalent in the CO2-rich Precambrian world. Here, high affinity phosphate genes are abundant, compensating for arsenic stress on phosphate metabolism.

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