Uncovering a decline in Earth's adolescent biosphere
In the microbial world that characterizes the majority of Earth history, the size of the biosphere remains mysterious. Our paper documents the exit from Earth’s Great Oxidation Event which may be one of the greatest sustained reductions in the size of the biosphere in all of Earth history.
Our paper recently published in PNAS can be found here
2.4 billion years ago micro-organisms drove the first significant oxygenation of Earth’s atmosphere. Through oxygenic photosynthesis, these organisms reshaped the surface environment of the Earth and drastically changed the makeup of the biosphere. Across the next few hundred million years of Earth history it is debated exactly how much oxygen was in the atmosphere and whether this interval (termed Earth’s Great Oxidation Event, or GOE) was truly unique.
While the beginning of Earth’s GOE has received a large amount of attention, far less has been paid to the interval of time that separates it from the following billion years of Earth history. A possible reason for this is that there are few locations in the world that provide an archive of events across this end-GOE interval approximately two billion years ago. Scouring the literature, myself and co-lead author Malcolm Hodgskiss identified one remote location that may provide some answers.
The Belcher Islands are located in eastern Hudson Bay Canada and exposes kilometers of sediments across one of the most fascinating archipelagos in the world. These sediments record the events immediately after Earth’s Great Oxidation, and motivated our study of this area (Hodgskiss et al., 2019). During field work in 2017, Malcolm found an unusual occurrence of barite (barium sulfate) minerals in the stratigraphy of the Belcher Islands. Subsequent geochemical analyses revealed that these barite crystals bore signatures that may record the productivity of the ancient biosphere.
Barites from the Belcher islands displayed anomalous oxygen isotopic signatures that would be near-impossible to produce on the modern Earth. By combining a suite of isotopic tools (oxygen, sulfur, and barium) we were able to provide evidence that these signatures are a reflection of the conditions of the Earth immediately after the GOE approximately two billion years ago. Our results suggested that Earth’s biosphere after the GOE was much less productive than the biosphere during the GOE. Combining our results with previous measurements (Crockford et al., 2018; 2019) together with estimates for the composition of the ancient atmosphere suggested that across this end-GOE interval the biosphere reduced its overall productivity by at least 5-fold but possibly by over 200-fold.
Across this end-GOE transition, or what Paul Hoffman has termed ‘Oxit' (Exit from Oxygenation) it is likely that the oxygenic photosynthesizers responsible for oxygenation were not so different than modern cyanobacteria. Under this assumption, our record of productivity decline directly translates to a reduction in the amount of life, or size of the biosphere across the end-GOE transition. While extinction is not the appropriate term for this event, given that we will likely never know what, if any, actual species went extinct, it appears there was certainly a dramatic reduction in the amount of life on the planet. Our combined isotopic measurements and calculations suggest this reduction may have been even more severe than the great extinctions that came billions of years later during the Phanerozoic. While this work brings slightly more clarity to the Earth two billion years ago, much more work is needed to fully realize the history of our planet across this critical interval.
*The Author thanks Malcolm Hodgskiss for help in preparing this post as well as Paul F. Hoffman for inspiring conversations on the shores of Great Slave Lake
Hodgskiss, M.S.W., Crockford, P.W., Peng, Y., Horner, T., A Productivity Collapse to end Earth’s Great Oxidation. PNAS https://doi.org/10.1073/pnas.1900325116
Hodgskiss, M.S., Dagnaud, O.M., Frost, J.L., Halverson, G.P., Schmitz, M.D., Swanson-Hysell, N.L. and Sperling, E.A., 2019. New insights on the Orosirian carbon cycle, early Cyanobacteria, and the assembly of Laurentia from the Paleoproterozoic Belcher Group. Earth and Planetary Science Letters, 520, pp.141-152. https://doi.org/10.1016/j.epsl.2019.05.023
Crockford, P.W., Kunzmann, M., Bekker, A., Hayles, J., Bao, H., Halverson, G.P., Peng, Y., Bui, T.H., Cox, G.M., Gibson, T.M. and Wörndle, S., 2019. Claypool continued: Extending the isotopic record of sedimentary sulfate. Chemical Geology 513 200-225. https://doi.org/10.1016/j.chemgeo.2019.02.030
Crockford, P.W., Hayles, J.A., Bao, H., Planavsky, N.J., Bekker, A., Fralick, P.W., Halverson, G.P., Bui, T.H., Peng, Y. and Wing, B.A., 2018. Triple oxygen isotope evidence for limited mid-Proterozoic primary productivity. Nature, 559(7715), p.613. https://doi.org/10.1038/s41586-018-0349-y