Billion-year-old lake deposit yields clues to Earth's ancient biosphere

[rangerrebew]

Billion-year-old lake deposit yields clues to Earth's ancient biosphere

7/18/2018 02:00:00 PM 
A sample of ancient oxygen, teased out of a 1.4 billion-year-old evaporative lake deposit in Ontario, provides fresh evidence of what the Earth's atmosphere and biosphere were like during the interval leading up to the emergence of animal life.
 
 
The findings, published in the journal Nature, represent the oldest measurement of atmospheric oxygen isotopes by nearly a billion years. The results support previous research suggesting that oxygen levels in the air during this time in Earth history were a tiny fraction of what they are today due to a much less productive biosphere.

https://archaeologynewsnetwork.blogspot.com/2018/07/billion-year-old-lake-deposit-yields.html

[truth_seeker]

Geology is fascinating. Science is fascinating.

[Suppressed]

The paper:
https://www.nature.com/articles/s41586-018-0349-y

Triple oxygen isotope evidence for limited mid-Proterozoic primary productivity
Peter W. Crockford, Justin A. Hayles, Huiming Bao, Noah J. Planavsky, Andrey Bekker, Philip W. Fralick, Galen P. Halverson, Thi Hao Bui, Yongbo Peng & Boswell A. Wing
Nature (2018)

Abstract
The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today¹. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆¹⁷O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆¹⁷O values (down to −0.88‰) observed in sulfates, except for those from the terminal Cryogenian period². This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO₂ and O₂ and the photosynthetic O₂ flux at this time³. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O₂ flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2 (2–30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O₂ was between 0.1–1% or 1–10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean^4,5,6 and Phanerozoic primary production⁷, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O₂ over geologic time⁸.

(References at link)