From left to right: Peter Crockford holding up some sulfates in preparation for analysis. Huiming Bao’s lab at LSU we could be confident in our measurements by blasting our samples with laser beams under an incredibly toxic atmosphere of bromine pentaflouride and then carefully collecting the product oxygen. ![]() In order to reveal the isotopic characteristics buried in the sulfate samples we needed to be as precise as possible and minimize any potential contamination, especially from the atmosphere. ![]() Shortly after my arctic expedition I received a package in the mail and booked my ticket to Baton Rouge Louisiana to use some lasers. One such mineral is gypsum (calcium sulfate), and my hope was that the sulfate within these gypsum samples would retain a record of ancient atmospheric oxygen in its isotopes – my hidden signal. I was excited because when seas or lakes become restricted and evaporation outpaces precipitation, minerals start to form out of the leftover brine. I was sharing my thoughts on the notion of testing the productivity of the ancient biosphere using sulfates and how capturing one of these signals would be helpful when, serendipitously, he mentioned he had some samples of 1.4 billion-year-old sulfate deposited in an ancient evaporating lake. While on an expedition on Northern Baffin Island I had the good fortune of engaging in conversation with co-author Noah Planavsky from Yale University. Therefore, sulfates could be the window into deep time I was looking for, but in order to be successful in exploring the ancient biospheres’ influence on the ancient atmosphere I needed to find the right samples, samples that would capture a signal, hidden within ancient sulfate. It had been demonstrated that some of the oxygen within sulfate retains its isotopic characteristics from when it was in the atmosphere. fools gold) left on the surface of the Earth will react with atmospheric oxygen to produce sulfates. To do so, we relied on the knowledge that pyrites (a.k.a. Looking through deep time we needed another archive of atmospheric oxygen. Although this seems like a long time, a million years is a tiny fraction of Earth’s 4.6 billion-year old history. But was this the case over a billion years ago with a purely microbial biosphere? While it is likely true that throughout Earth history primary producers have played an enormous role in shaping the environment as they do today, quantifying their influence has only been possible over the relatively recent past with ice-core records extending back about a million years. While the majority of them do this, they also produce oxygen, and today they are so good at it that oxygen makes up about 20% of the Earth’s atmosphere. Primary producers (plants, bacteria, algae, etc.) suck carbon dioxide out of the atmosphere to form organic carbon for the rest of the biosphere (including us) to ingest. ![]() Our paper recently published in Nature can be found here
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